On the Early History of Spinning and Spin Research in the UK Part 3: the Period 1940 to 1949

Journal of Aeronautical History Paper 2019/05

On the early history of spinning and spin research in the UK Part 3: the period 1940 to 1949

Brian Brinkworth Waterlooville UK

Abstract

This third part of a study of the history of spinning and spin research in the UK covers the decade of the 1940s, which was dominated by almost five years of the Second World War. New types of aircraft were required to replace obsolete ones and to fill changing operational needs, though they were subject to essentially the same spin testing procedures as in the pre-war period. Testing with dynamic models continued in the vertical Free Spinning Tunnel at the Royal Aircraft Establishment, and at full-scale at the Aeroplane and Armament Experimental Establishment. In the later years of the war, the first squadrons of jet-propelled types were formed, followed by the appearance of aircraft with new configurations for flight in the compressible range.

Although little fundamental research on spinning could be undertaken in wartime conditions, progress continued, mainly through empirical developments in the model testing methods. These included refinement of the modelling by, for example, representing the angular momentum of engines and propellers, and of the test procedures to improve the agreement between the outcome of a model test and that of the corresponding aircraft test at full-scale. These were significant advances, which were made at the expense of greater complexity in the methods employed.

1. Introduction 1.1 Spinning and recovery The development in Britain of an understanding of the spinning of aircraft and of means of recovering from spins has been reviewed previously in this journal, covering the earlier periods from 1909 to 1929 (1) and from 1930 to 1939 (2). This is continued here for the decade of the 1940s, which include most of the years of World War Two (WW2). By way of introduction, a brief outline is given here of key elements of that understanding, and of the situation as it stood at the end of the 1930s.

The spin had been a known hazard to manned flight from its earliest days, generally following a stall, with one wing dropping. The aircraft then descends rapidly along a vertical helical path in a combination of falling and rotating, while remaining deeply stalled. Two distinct types of spin had been identified - the steep spin, in which the incidence of the aircraft to its path lies roughly in the range 30 o to 50 o, and the flat spin, where it can be 70 o or more. The rate of rotation is higher in the flat spin, sometimes taking less than 2 seconds per turn, and it is rarely possible to recover from it.

The spin is a steady state, with the inertia of the dynamic motion in equilibrium with the aerodynamic forces and moments caused by the airflow over the aircraft. A complete theoretical representation of this state had been established before the end of the 1920s. But in the deeply- stalled condition of the spin the airflow over the aircraft is separated, and the aerodynamics of that

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Journal of Aeronautical History Paper 2019/05 situation had not been investigated by the end of the 1930s. Thus, estimation of the applied forces and moments in the spin could be made only from empirical data.

Actions by the pilot to bring about recovery from the spin had been established by trial and error during the Great War (WW1), and were duly incorporated as standard procedures in pilot training. This was usually to centralise the controls, then apply opposite rudder to slow the rotation, followed by moving the stick forward to unstall the wing and begin a pull-out. The forward speed on emerging from a spin can be high, and care is needed in this phase to avoid inducing a high normal acceleration. When aircraft were small and light, recovery actions could be a sequence of independent measures, but as the mass and moments of inertia grew with aircraft development, they tended to merge into one progressive movement.

1.2 The position in the late 1930s By the end of the 1930s the body of measurements that had been gathered with models on rotating balances in wind tunnels and in flight at full-scale allowed some advice to be given to designers on features of an aircraft that could reduce its tendency to spin and increase the chance of recovery if a spin occurred.

For new aircraft to be considered for acceptance into service with the RAF and the Fleet Air Arm (FAA), prototypes were required to be evaluated by the Aeroplane and Armament Experimental Establishment (A&AEE), then at Martlesham Heath, or the Marine Aircraft Experimental Establishment (MAEE) at Felixstowe. The trial programmes conducted there included testing in the spin. The RAF requirement for acceptance of a fighter aircraft was that it should be coming out of the spin within two further turns after moving the controls to the positions specified for recovery. If the type was ordered into production, examples of the first aircraft to be completed were checked again by A&AEE for the Release to Service. It was not unusual for problems in handling, including irregularity in the spin, to appear at this stage. Advice on possible means of rectification was often offered to the manufacturer, or where the reasons for failure were not clear, aircraft could be sent to RAE at Farnborough for more detailed examination.

The direction of research on spinning had taken a new direction in the latter part of the decade, with the opening of the vertical Free Spinning Tunnel at the RAE (3). The tunnel is described in part 2 of this paper (2). It allowed models, correctly scaled geometrically and dynamically (in terms of inertia), to be set spinning in an up-going airstream matched to the rate of fall. Their motions could then be observed and measured, as the spin developed and in a prolonged spin. Then the controls were moved to represent the standard method of recovery. Models were observed to behave in ways that were sufficiently similar to those found in flight at full-scale for this procedure to become a major advance.

It was considered that there could be factors that could affect the scaling of model results to represent the behaviour of a given aircraft accurately. Accordingly, for model testing two measures were taken routinely to bias the situation and build a factor of safety into the procedure. One was to modify the model so that its moment of inertia in pitch was 10% larger than the value given by scaling the data for the full-scale aircraft, and to position the centre of gravity 6% of the mean chord aft of the normal rearward limit. By this the stability in pitch was reduced, a factor known to make a transition to the flat spin more likely. The second measure was to attach a vane to the tip of the

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Journal of Aeronautical History Paper 2019/05 wing that was to be innermost in the spin, to apply an additional turning moment in the pro-spin direction. An arbitrary unit had been adopted for moments, such that ten units would be roughly equivalent to that applied by a fully-deflected rudder at full-scale. Initially, the behaviour considered to be satisfactory was for recovery to take place within the full-scale equivalent of 10 seconds of the controls being activated, with an applied moment equivalent to 10 units at full-scale. Tests were also repeated with increasing pro-spin moments, to establish the value beyond which recovery became impossible.

The spinning characteristics of a new design could now be estimated as soon as its shape and mass distribution had been established sufficiently for a representative model to be made. If the spin and recovery in a model test was unsatisfactory, corrective measures could be tried on the model to advise the designers. From a combination of theory and experience, these were usually changes intended to increase the aerodynamic moment caused by side forces on the rear fuselage and the fin and rudder, which arose from the displacement in yaw experienced by the aircraft in a spin. In recovery, the moment produced by movement of the rudder was vital, as this was the first action to be undertaken in the standard procedure for recovery taught to pilots.

An important further advance in this direction was put forward by RAE at the turn of the 1930s (4, 5). In this the factors considered to have the greatest influence on the spinning behaviour and recovery of aircraft were represented approximately by three non-dimensional coefficients:

X, the Inertia Coefficient, based on the difference (C - A) between its moments of inertia about the normal (yaw) axis and the longitudinal (roll) axis respectively*, Y, the Body Damping Ratio, representing the restoring moment of forces on the projected side area of the rear fuselage and empennage in a displacement in yaw, and Z, the Unshielded Rudder Volume Coefficient, expressing the effectiveness of the rudder in applying a restoring moment to begin the recovery.

(Symbols representing these coefficients were not assigned originally. X, Y and Z were used in Part 2 of this study (2) and are continued in use here).

Coefficients similar to Y and Z were familiar to aerodynamicists from their use in estimation of stability and control, though in Z the term 'unshielded' referred to the part of the rudder that lay outside the estimated path of the wake shed from the tailplane at the incidence expected in a spin.

The coefficients are non-dimensional so that a model that is correctly scaled dynamically has the same numerical values for them as does the full-size aircraft. When values of Y and Z from full- scale and model tests were plotted against X, it was found that the points for aircraft with satisfactory and unsatisfactory spin behaviour were sufficiently separated to allow some provisional 'pass/fail' boundaries for Y and Z to be laid down for given values of X. With further experience, it was hoped that this approach would at last meet the objective of enabling designers to have an idea of the risk of a new type developing a dangerous spin and being unable to recover from a spin if one occurred. That could be checked routinely during the design and development once values for X, Y and Z could be estimated.

* The generation of inertia couples in pitch and yaw during a spin is explained and illustrated in sections 5.1.2 to 5.1.5 of part 2 of this paper (2).

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2. The early 1940s 2.1 A new situation When World War 2 opened on 3rd September 1939, plans that had been prepared for this eventuality were set in motion promptly. It had been recognised since WW1 that air power would be the major factor in any future conflict, and the means of projecting British capability in that area had been embodied in the War Potential plan for aircraft production that was revised periodically. By the 1930s this envisaged that a total monthly output of 2,000 aircraft of all types would be required to sustain a war if it was prolonged. After events in Europe showed that Germany was likely to become the enemy again, the initial stages of an expansion of industry to implement War Potential were begun in 1936 (6).

The strategic aim for the RAF would include the effective disruption of the enemy's arms production and infrastructure. Orders were placed for the development of the long-range heavy bombers needed for this role, but it was anticipated that, even with maximum effort, it would take several years to reach the rate of production required to sustain that. Meanwhile, an enemy was being faced which already had substantial air resources, strengthened by secret aircrew training schemes and operational experience in the Spanish Civil War. The immediate needs were for the protection of the homeland, requiring further development of the country's integrated air defence system by the inclusion of radar and emphasis on the output of fighters for the operational arm. Facilities would also have to be built up for the training of the great expansion in the number of aircrew required by the plan. Among many other factors, it would be necessary to ensure that spinning would not be a significant hazard for those involved.

Procedures for the procurement of a new aircraft that had taken years in peacetime were now telescoped, to the point of ordering 'off the drawing board'. It had been usual for rigorous testing to take place before any order was placed for production in quantity. The loss of a sole prototype in test flying had previously set back programmes considerably, so now two or more were required, and preparations for production would often be started before they had flown. The first spins were part of the contractor's trials, detailed in specifications and production contracts, and if difficulties were encountered at that stage, companies were encouraged to consult RAE for suggestions about modifications to correct them. The A&AEE and MAEE, where the final acceptance tests for release of a new type into service were carried out, had both been located on the East Coast. There they would be particularly vulnerable to enemy action, so they were moved in 1939 to Boscombe Down near Salisbury and Helensburgh on the Firth of Clyde respectively. RAE remained at Farnborough in northeast Hampshire.

Formerly, the practice in spin testing had been to let a spin continue for eight turns to ensure that it was fully developed before recovery action commenced. By 1940 this had been reduced for fighter aircraft to just two turns, the part known as the 'incipient spin' region. This was reckoned to be more representative of the current situation, when trained pilots were expected to recognise quickly that they had entered a spin and to begin recovery action promptly. For an aircraft to be cleared to go into service with the RAF, recovery had then to take place within two further turns after the controls had been moved to begin recovery (7). Standard spinning trials were just a small part of very wide-ranging evaluations of the suitability of a new type to be accepted into service. As well as all aspects of its performance and handling, these tests reviewed its suitability as a workplace, covering matters such as how the layout would help aircrew to carry out their duties efficiently, safety in operation, and in leaving the aircraft in emergencies. A new type or mark of aircraft could

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Journal of Aeronautical History Paper 2019/05 not be delivered to RAF operational units unless a certificate of Release to Service had been issued by A&AEE. The accompanying report of test results often included a list of modifications required to be made to the design of the aircraft concerned. The significance of this mechanism for the feedback of good practice to designers, based on the ever-growing experience of independent testers, has not been generally appreciated.

This vital safeguard of quality was exercised with rigour and professionalism despite the great pressures of the wartime period. During the six years of the war the A&AEE tested 1,500 different types and marks of British and American aircraft (7). Also tested were individual 'rogue' aircraft that were reported by training and operational stations to have shown irregular characteristics. Aircraft could be further remitted onward to RAE when more intensive investigations needed to be carried out.

The first part of the review that follows concerns aircraft that were in service during the early 1940s, with particular reference to types for which problems with spinning had been reported and which provide illustrations of the methods employed at that time to inquire into and counter those problems. A few types that were tested but not ordered into production are mentioned also if they had shown some deficiencies in their spinning characteristics.

2.2 Aircraft types in the early wartime period 2.2.1 Single-engined training aircraft The spinning characteristics of training aircraft were of particular concern, as they would be handled by pilots with little experience, who would be most likely to enter a spin inadvertently and to become confused about the recommended procedures for recovering from it, or slow in applying the recovery procedures. Accordingly, recovery action for these types was to be delayed during testing into the region where the spin had stabilised, known as 'prolonged' spinning. Where the behaviour was satisfactory relative to the required standard, but close to the acceptable limit, it was more likely in the case of trainers that there would be a comment in the subsequent report, indicating that a wider margin should be provided.

The basic (or ab initio) and intermediate stages of pilot training in the RAF proceeded in steps, generally taking place at training stations employing aircraft specifically ordered for those duties. Advanced training was taken on types that would be used in service, often in units located on stations where these were operational. Despite the ever-pressing demands for pilots, the training programme was substantially maintained throughout the war, with only minor reductions of hours at a few points.

Spinning and recovery were included in the training of pilots from the ab initio level onwards. The type generally employed for that stage was the de Havilland DH2 Tiger Moth. Originally a popular civil type, of largely wooden construction, it had been used widely in flying clubs for training and for sports flying generally. Adopted by the RAF as its principal basic trainer, it was employed in great numbers, both at training squadrons in the UK and throughout the Dominions participating in the Commonwealth Air Training Plan. It had first been tested at Martlesham Heath before the war and reckoned to be generally docile. On use of the standard procedure, it recovered normally from a spin, though it was noted that the response to movement of the controls had been slow. Subsequent events provide an example of how types that were well-established in service could display

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Journal of Aeronautical History Paper 2019/05 problematic features arising from minor changes of use or the introduction of modifications that would not have been expected to have troublesome consequences.

At the beginning of WW2 many of the Tiger Moths in service were of the Mark II variety, for which the official specification T.7/35 had been issued, as shown in Figure 1. Following reports of crashes at Training Schools, three aircraft of this type were sent to Boscombe Down in 1940 for investigation of its behaviour in the spin, that had been described as 'difficult' (7). Extensive trials there confirmed that all three were taking up to four turns to begin recovery, but no specific reason had been found for that. The aircraft were then remitted to RAE for more intensive study, with only an observation that the more dangerous spins had occurred when the entry to the spin had been somewhat mishandled.

Figure 1. de Havilland Tiger Moth Mk II ab initio trainer

The investigation of the spinning issue by RAE illustrates the thoroughness with which problems were followed up, even in the most critical stages of the war (8). Many full-scale spinning tests were made in this case, with the three aircraft from A&AEE and two more obtained from training squadrons. It was found that all five developed a conventional steep spin with the normal method of entry, but a flatter one could be induced if a small amount of opposite aileron had been applied at that time. This was recognised as something that might easily be done inadvertently by a trainee.

Standard recoveries could be obtained by the normal routine for all except one aircraft, where the incidence rose to 50 o, effectively into the region of the flat spin. Recovery was obtained for this case also, though only after 13 turns, and by use of (unspecified) 'emergency action'.

In pursuit of reasons for the behaviour observed, it was noted by RAE that the evolution from the Mark I (essentially the civil type) into Mark II had increased the moment of inertia coefficient X. The main contributors to that were the strengthening of parts of the structure and the addition of mass balance weights to the ailerons and rudder. It was also said that typical service navigation

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Journal of Aeronautical History Paper 2019/05 lights had been added at the wing tips, and that the mass and location of these had been enough to affect the inertia significantly (9). Some of the Mk II aircraft had been fitted with rails to carry eight small bombs, located below the fuselage at its junction with the lower wing. Training in targeting with these was a measure introduced in the months prior to the opening of the war, when any option was considered if it might contribute to thwarting an expected invasion. It had been retained afterwards, perhaps to provide an early assessment of the capacity of a trainee to maintain the precise tracking required in the run-up to the release of bombs.

RAE's next move was to procure a civil Tiger Moth and modify it to bring its moments of inertia and centre of gravity position up to the values of the Mark II version, by adding weights in the wings, fitting balance weights to the controls and adding bomb rack rails. In experiments with this aircraft, it was found that with application of full opposite aileron a flat spin could be induced, though recovery from it could be obtained by the standard procedure after eight turns. Systematic removal of the added items, with repeated testing in between, showed that the bomb rack rails had the greatest effect, followed by the balance weights for the control surfaces. Flight trials were then made with the rails and weights removed, and various palliative measures applied that previous experience had shown to be helpful in deterring the development of the flat spin. Fitting strakes to the top of the rear fuselage ahead of the tailplane generally had the most effect (2).

In conclusion, it was recommended that the rails and balance weights should be removed from all aircraft of that type in service and that strakes should be added to those in subsequent production. Figure 2 shows the form of the strakes applied for testing at RAE.

Figure 2. RAE drawing showing anti-spin strakes for tests of Tiger Moth Mk II

Another ab initio trainer that was in service at the beginning of the war, the Miles Magister, was a single-engined low-wing monoplane of wooden construction, supplied by Phillips & Powis Aircraft Ltd (later Miles Aircraft Ltd) (10). Like the Tiger Moth, it had been derived from a successful civil type, though it had some more modern features such as flaps and wheel brakes. It too had

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Journal of Aeronautical History Paper 2019/05 experienced sporadic episodes of unsatisfactory spinning behaviour. Modification of the shape of the rear fuselage and the fitment of strakes ahead of the tailplane had resolved the problem. Though the effectiveness of this measure had been found empirically some years previously, the actual mechanism of its action had not been fully researched. Details of such modifications were rarely reported, but the form of the strakes shown in Figure 2 for the trials of the Tiger Moth Mk II at RAE suggests that its action was probably effected by stabilising the flow separating at high incidence from the upper surface of the rear fuselage. This would take the form of two narrow sheets trailing from the outward-facing edges of the strakes. While they remained apart, these sheets would then have interfered less with the action of the rudder than had the broad wake formed behind the unmodified fuselage.

From 1939 Miles also supplied the Master trainer, that bridged the intermediate and advanced stages of preparation of pilots for operation in single-seat fighter aircraft, as shown in Figure 3 (10). Designed to resemble a fighter, and with a performance to match, this provided the trainee with a front cockpit laid out with the controls, instrumentation and other equipment that would be met on operational types. With a light weight, a Rolls-Royce Kestrel engine and three-bladed constant- speed propeller, it was claimed to be the fastest training aeroplane in the world. Produced in quantity, partly on the first moving track assembly line in Britain at Woodley near Reading, and later at other plants, it was followed by further marks using the Bristol Mercury and Pratt & Witney Twin Wasp engines (10, 11). The Mark 1A was fitted with flaps, retractable undercarriage and a reflector gunsight for its single Browning gun with a gun camera for training purposes. Having handling characteristics similar to those of the RAF's Hurricane and Spitfire fighters, the Master helped to ease the transition for thousands of pilots into operating these and other types.

Figure 3. Miles Master intermediate / advanced trainer (Miles Aircraft Collection)

Two Masters had been at Martlesham Heath in 1939 and they moved with A&AEE to Boscombe Down. In the acceptance trials the spin behaviour was found to be normal, perhaps helped by modifications made before the start of production, which included some of the measures to counter undesirable spins that were now being recognised. These included lengthening and deepening the rear fuselage and enlarging the fin and rudder (to increase the side forces in yaw and their moment arms) and raising the position of the tailplane from the top of the fuselage to a location on the fin (where in this case about half the rudder area now lay below it and would not be shielded by its wake at high incidence).

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Just prior to the war, Miles had also designed another ab initio trainer, the M.18, shown in Figure 4, which was intended to build on the experience obtained with the Magister. It was not ordered into service, and was not given a name, but it could be noted here because trials with one of the prototypes had raised concerns about its spinning characteristics, which again led to modifications of kinds now effectively becoming standard procedures. Though its spinning had been found to be 'satisfactory' at Martlesham Heath, it was also fitted subsequently with strakes on the rear fuselage, as for the Magister. However, in company tests, its spin was found to be still very flat, and although it could be recovered, the process was prolonged. When spinning-tunnel model tests were made for it at

Figure 4. Miles M.18 proposed successor to the Magister ab initio trainer (Miles Aircraft Collection) RAE, it could be recovered against a pro-spin moment of 14 units, but that was considered to be borderline for an ab initio trainer (12). Accordingly, another measure was recommended, that the fin and rudder be moved forward by 24 in relative to the tailplane, which would take most of the rudder clear of its wake in a spin. This was done on one of the prototypes (though actually by 22 in), moving the rudder post to a position at the leading edge of the tailplane. No further spinning trouble was reported. At A&AEE in May 1941 an M.18 was said to be 'impossible to spin' (7). The forward position of the fin and rudder is a very prominent feature, and as used for some other aircraft mentioned below, it was sometimes introduced during the design phase, specifically as a precaution against spinning.

In pre-war years Percival Aircraft Ltd had been a rival to Miles in producing aircraft of wooden construction for the fields of sports flying and air racing. The Proctor, a version of its Vega Gull machine, was produced to specification T.20/38 for radio training and general communication purposes (see Figure 5). With the growing importance of VHF transmission for ground control, landing aids and other purposes, this type became the main RAF trainer for radio operations throughout the war period. For the initial model testing in October 1940, the inertia coefficient X was found to be adequately low, but in relation to that the damping coefficient Y was very small, indicating that the spin recovery would be poor (13). On test, it failed to meet the required pass criterion 'by a large margin', recovery being possible against a pro-spin moment of four units but impossible against five. The prototype had flown a year previously, but the contractor's trials had not included spinning, so the recommendation was that they should 'not be carried out'. Unusually,

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Journal of Aeronautical History Paper 2019/05 modifications to obtain better spinning and recovery were not suggested, perhaps because its duties largely involved only point-to-point flying without manoeuvres, and so it remained a type for which spinning was simply prohibited. In service, it was developed into several Marks, with more than a thousand built in total.

Figure 5. Percival Procter

The North American Harvard was a 2-seat single-engined monoplane for the advanced training of fighter pilots, used in Britain by the RAF and the FAA and throughout the Commonwealth Air Training Plan. An order for Harvards had been placed by the British Purchasing Commission shortly before the war, and an early example was under test at Martlesham Heath in September 1939 (7) (see Figure 6). It was reported to have 'excessive propeller noise' (which would be endorsed by anyone who heard a Harvard in flight subsequently) and 'an undesirable wing drop at the stall'.

Figure 6. North American Harvard trainer

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The latter was a known precursor to a spin, so this aspect of performance was revisited by A&AEE as successive Marks of the type came for approval. The Mark I could be recovered, though with a loss of height that was considered to be marginal for a trainer aircraft, but the conclusion was that further spin testing would not be needed. Complaints were received from the RAF that the Mark III had a high rate of rotation in the spin. Although this was believed to be an indication of the propensity to develop a flat spin, the behaviour was considered at Boscombe Down to be 'only a little worse than normal'. No representations seem to have been made to the manufacturer on this issue, but in any case it was known that American firms were reluctant to make any modifications after the start of delivery of a type, unless they had arisen in items specifically required in the contract (2). At this time there was no equivalent of A&AEE in the US to carry out independent routine testing of American-designed aircraft.

2.2.2 Twin-engined trainers At the start of the war, the RAF had two versatile aircraft for training aircrew for multi-engined operation, the Avro Anson and the Airspeed Oxford, shown in Figure 7. Of roughly the same size and shape, both were low-wing monoplanes based on pre-existing commercial types, with twin air- cooled radial engines, and flaps and retractable undercarriage (initially hand-operated). The Anson had been ordered for land-based maritime reconnaissance duties, but it took on many other roles before being transferred to operations as a trainer just prior to the war. The Oxford, designed to specification T.23/36, entered service at about the same time. As well as serving the RAF these became preferred types for the Commonwealth Air Training Plan, more than 8,000 of each type being produced in Britain, and more built under licence in Canada and Australia.

Like the Master, both aircraft were provided with instrumentation and other fittings and equipment representing those of the (bomber) types currently in service, and, notably with the Oxford, with internal arrangements that could be quickly changed to suit particular training needs. Additions to the facilities for pilots, navigators and wireless operators were bomb-sights and racks for practice bombs for training bomb-aimers and dorsal turrets for air gunners. This versatility could enable an entire aircrew to be given initial experience in operating as a team, though the capacities of the interiors of the aircraft were rather cramped for that. Vertical camera installations provided training for photographic reconnaissance, and the Oxford could be rapidly converted into an air ambulance.

For all-purpose aircraft with multiple occupation, it might be expected that the possibility of spinning problems would be a concern. The Anson had cleared the acceptance tests for the maritime reconnaissance role at Martlesham Heath before the war and was in service at 26 squadrons by the outbreak of WW2. The Oxford, tested as a model at RAE, had failed to reach the standard required for spin recovery, despite some modifications being made to the design. A second model, made with a twin-finned empennage, met the requirements, probably due to the outer surfaces of the fins and rudders being clear of the wake of the tailplane in a spin. However, to avoid delay in the rearmament build-up, the Oxford was put into production with the single fin, with the proviso that deliberate spinning would be forbidden.

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Figure 7. Avro Anson (upper) and Airspeed Oxford (lower) advanced trainers

In 1940, Airspeed fitted a full-scale Oxford with twin fins as originally proposed, and this was submitted for spin testing at RAE, with an anti-spin parachute as an extra safeguard (14). This test aircraft is described throughout the RAE report as having 'twin rudders', not the usual term for one with twin fins, though strictly correct since each had its own rudder. The aircraft was flown by several pilots and spin recovery was found to be 'rapid and straightforward'. However, one pilot, flying solo, experienced a flatter spin than usual but obtained recovery by use of the engine on the inside of the spin, with the outer one idling. This differential use of the thrust of engines on opposite sides of the central axis provided an anti-spin yawing moment that was not affected in the same way as were the control surfaces by the deep stall experienced in the spin. It was recommended that this procedure be made part of the training for the Oxford, and perhaps was to be extended to all multi- engined aircraft. Though the twin-fin version was not put into production, this method of spin recovery became recognised as a regular procedure for larger aircraft.

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2.2.3 Fighter aircraft At the beginning of the War the RAF was only beginning to be equipped with fighter types that could be called modern in design. There had been differing opinions within the Air Ministry during the 1930s as to the future operational requirements for fighters and the technical features that they would need to meet those, including their armament. One outcome had been the concept of the 'turret fighter', exemplified by the Bolton-Paul Defiant. The presence of a 4-gun dorsal turret on a fighter hugely degraded its performance, but the RAF had to commit the squadrons of those that it had in 1940 to engage in ground attack, in which losses to ground fire and fast fighters were very great. There would be a similar story for the 'light bomber' the Fairy Battle.

Confusion and delays in initiating the processes that would normally lead to orders produced the perilous situation in which leading firms in the industry had clearer perceptions about what would be required than the Ministry did. Having been presented with specifications which they considered to be unrealistic, these firms proceeded with prototype designs of their own as private ventures. Fortunately, there were those in the Air Ministry who kept in contact with them and encouraged this. Orders for types based on these were eventually placed, though not until a time of such urgency that it was almost too late.

Thus it was that the eight-gun Hawker Hurricane and Supermarine Spitfire, shown in Figure 8, came to define the latest conception of the high-speed single-seat interceptor fighter of the late 1930s. When these types began entry into service in December 1937 and August 1938 respectively, there was great pressure to get effective numbers of them out to the squadrons, with enough trained pilots ready to take them into combat (6).

Model spinning tests of both aircraft were made before the prototypes had flown, and it was considered from those that their behaviour in this respect would be at best borderline in both cases (15). As recommended by RAE, production Hurricanes were fitted with a supplementary fin under the rear fuselage, beginning forward of the tailwheel and merging with a downward extension of the rudder. When tests of both types were made at A&AEE in 1938, their spin characteristics were judged to be acceptable and the anti-spin parachutes that had been fitted as a precaution against failure to recover had not needed to be deployed.

After further Spitfire model tests were made at RAE, it was suggested that its rear fuselage should be lengthened and the tailplane raised, though the manufacturers did not make these modifications. However, in contractor’s tests after the prototype had been fitted with armament and developed towards production standard, the test pilot Jeffrey Quill reported a disagreeable aspect to the spin in the form of 'a series of convulsive kicks' (16). Irregularities continued to be noted in various Marks of the Spitfire in later years of testing at A&AEE (7). In 1942, it was recorded of a Mk IX that spins had been accompanied by 'unpleasant pitching and buffeting', and of a Mk XII, the first with the Griffon engine, that spinning produced 'the usual pitching and buffeting'. By 1945 the spinning of the Seafire Mk XV was just described as 'acceptable'. Fortunately, for both Hurricane and Spitfire, all Marks had proved recoverable from spins in service.

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Figure 8. Hawker Hurricane (upper) and Supermarine Spitfire (lower) single-seat fighters

Also existing early in the war were prototypes under specification F.18/37 of a potential fighter to succeed the Hurricane. This would employ a new engine in the 2,000 HP range, with a maximum speed of 400 mph and ceiling of 35,000 ft. A contract had been given to Hawker, where design along similar lines was already in progress, for a fighter to be powered by the Rolls-Royce Vulture 24-cylinder engine. At Hawker's suggestion another prototype was also ordered, to be provided with a different engine of similar rating that was currently under development, the Napier Sabre. The two machines bore some resemblance to the Hurricane, but differed from each other in the wing position, span and area.

In model tests at RAE in October 1939, both versions had shown similar spin recovery, which exceeded the requirements in the clean condition, but failed to reach it with undercarriage and flaps down (17).

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Given the pressures of the time, orders were placed for the type with the Vulture engine, to be built at other factories in the Hawker Siddeley group, and the first production aircraft, to be known as the Tornado, flew in August 1941. But there were problems with the reliability of the Vulture engine and it was abandoned by Rolls-Royce, also bringing the production of the Tornado to an end. Work on the Sabre engine continued at Napier (D Napier & Son Ltd, from 1942 part of the English Electric group). In due course, the two Hawker types made their appearance in production, a revised Tornado with the Sabre engine, to be called Typhoon, and the Tempest in several marks using three different engines. Spin characteristics of these are reported in Section 3.3.

2.2.4 Twin-engined fighters As seen above, twin-engined trainers had been evaluated for spinning behaviour, but quite early in the wartime period twin-engined fighters made their appearance also. The first British type was the third in a sequence of Bristol aircraft, the Beaufighter, shown in Figure 9. Initially based on the Beaufort medium bomber, with many common parts and the use of jigs and fixtures of that type to speed production, it entered service in July 1940, at the height of the Battle of Britain. Being much larger than the single-engined fighters, and with a two-man crew, it was fitted by its greater range, endurance and armament to be developed subsequently for multi-role usage in RAF Fighter and Coastal Commands. It was seen that, having to be fast and manoeuvrable in these roles, it was more likely to experience conditions that might lead inadvertently to entry to a spin than other twin- engined types.

Figure 9. Bristol Beaufighter

A model of the Mk I version was tested at RAE in 1941 as being representative of the heavy fighter type (18). Although its inertia coefficient was satisfactory, there were concerns about its spin recovery, due to the damping and rudder volume coefficients being very low. This expectation was confirmed when at the simulated altitude of 15,000 ft it could not be recovered against a pro-spin moment of more than 12 units within the 10 seconds (full-scale) specified at the time. Following this failure, the opportunity was taken to extend the tests to cover variations in the model's overall

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Journal of Aeronautical History Paper 2019/05 weight, mass distribution, equivalent height, direction of spin and position of controls during spin and recovery. At the equivalent of 30,000 ft, recovery in 10 seconds full-scale could not be obtained against a moment of more than 7 units. Perhaps by way of encouragement, the report concluded that there should be no difficulty in recovery 'below 10,000 ft', and the potential for differential use of engine thrust to assist in recovery at altitude was mentioned again in connection with this type.

Although no remedial measures were suggested after the model tests, when full-scale Beaufighters were tested later at A&AEE the spin recovery was found to be satisfactory for all Marks (7). In service it proved to be a rugged and formidable weapon in various theatres and in many different roles. About 5,500 were built in Britain and under licence in Australia.

Another type tested in 1940 was a model of the Gloster F.9/40 (19). Described only as 'a twin-engined low wing monoplane intended for high speed fighter duties', this was to become the Meteor, the first British jet-propelled type to enter service. The data given showed that the inertia coefficient was favourable and although the fuselage damping coefficient was also low, the model was expected to make a good recovery from the spin, and this proved to be the case. With the worst conditions of loading, recovery was obtained within 10 seconds full-scale against an applied yawing moment of 17 units flaps up, and 15 units flaps down. Thus the design passed the model spinning standard for the time.

From November 1941 the RAF began to receive the de Havilland Mosquito, the two-man twin- engined type that became famous for its exceptional performance and versatility. When tested at

Figure 10. de Havilland Mosquito

A&AEE later, no adverse spin characteristics were reported. However, as can be seen in Figure 10 the design placed the fin in a forward position, with the rudder post about level with the leading edge of the tailplane. The tall rudder meant that the major part of its area was clear of the position assumed for the wake of the tailplane in the spin, giving a generous value of the unshielded rudder coefficient Z. With the further option to use differential engine thrust, the aircraft would be expected to be well placed for spin recovery, though no test results have been found. About 7,800 Mosquitos of many marks and varieties entered service, some built at DH subsidiary companies in Canada and Australia.

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Government papers for this period include inter-departmental correspondence on testing the Beaufighter and Mosquito, which indicate that there was reluctance by officials to cause concern to manufacturers about spin characteristics of heavy fighters (20). This centred on the potential wing loading due to rotation in the spin. It had not been shown then that these types experienced about the same normal acceleration as had been found in the types on which the spin criteria had long been based. But it was seen that specifying a possibly unnecessary loading case in contract documents might result in strengthening of the structure, requiring additional weight that would adversely affect performance. Evidently it was noted that these types had not been prone to spinning troubles in service, and no action was taken in this regard. In the last letter of the file, dated in 1945, the writer states that 'It is a requirement of twin-engine fighters that it should be possible to recover from incipient spins, but I do not think that any have ever been tested at A&AEE for this characteristic'.

2.2.5 Naval aircraft Among aircraft operated by the FAA at the beginning of WW2 was the remarkable Fairey Swordfish, shown in Figure 11. This biplane had been ordered originally to specification S.9/30 and entered service in 1936 for varied duties, with a three-man crew as a fleet spotter (hence category S.) for registering the fall of shot from naval gunnery, and for general maritime reconnaissance. With a crew reduced to two it could carry a torpedo for attack on shipping and surfaced submarines. Though slow and of an obsolete configuration, it remained in service throughout the war, operating with good effectiveness in a great diversity of theatres and operations, with RAF squadrons as well as the FAA. About 2,400 were built, between the parent company and Blackburn Aircraft Ltd. It was not referred to A&AEE or RAE for problems in the spin.

Figure 11. The Fairey Swordfish

Other Blackburn aircraft with an input from Fairey were the Skua and Roc, originally to specifications O.27/34 and O.30/35 (the duty under 'O.' had initially been observation, but later covered a variety of functions, mainly for carrier-borne operations).

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Model testing for the Skua at RAE in 1938 had shown satisfactory recovery from a spin, but there was concern that the body damping and rudder coefficients might be inadequate with the high value of the moment of inertia coefficient for this type. Because of space limitations in handling and stowage on carriers, the fuselage could not be lengthened, a measure often advised to enhance the body damping and rudder coefficients by extending their moment arms. It was recommended instead that the fin and rudder coefficients could be usefully enhanced if the fin was moved forward, which would take it out of the wake from the tailplane and would also clear most of the rudder. A supplementary fin was fitted below the rear fuselage, where it would not lie in a separated flow in the spin. This arrangement is apparent in the upper image in Figure 12.

The Roc was in effect a variant of the Skua, armed with a 4-gun dorsal turret as for the RAF Defiant, conceived at about the same time. After model tests on the Roc just prior to the war, RAE suggested that both types should be fitted with wing tip slats (to limit the liability to wing-dropping at the onset of the stall) and that deliberate spinning should be prohibited. This suggests that the conclusions from tests were pessimistic, for there seems to have been no indication in subsequent service that these aircraft were particularly prone to entering the spin or resistant to recovery from it. Further, from about this time, slats were becoming unpopular with pilots, due to their tendency to open unexpectedly when gusts were encountered and differentially during manoeuvres.

Figure 12. Blackburn Skua (upper) and Fairey Fulmer (lower)

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The Roc was handicapped by the dorsal turret and is generally considered to have been an unsatisfactory concept. A more successful Fairey naval aircraft becoming available near the beginning of the war was the Fulmar, also shown in Figure 12. A sturdy two-man carrier-borne aircraft with a long endurance for reconnaissance and shadowing, it also carried eight guns for interception as required. It had been commissioned to specification O.8/38, though the design was based on a fighter submitted for an earlier RAF requirement, for which the spin of the model had been described as 'vicious' (21). The fin was not placed forward as in the Skua, so in the spin the rudder was significantly exposed to the wake of the tailplane, giving a low value for the rudder volume coefficient. In the haste for production there had been no prototypes, so no spin trials were made until the first production aircraft arrived at A&AEE for the Release to Service review in May 1940. It was however generally well received, and, perhaps unexpectedly, the report refers to its 'instantaneous spin recovery' (7).

The Lockheed Hudson, intended as a replacement for the Anson for maritime patrol and attack with RAF Coastal Command, was the first American type ordered before the war by the British Purchasing Commission. This was a development of a pre-existing twin-engined civil type, to be fitted with a bomb-bay, a dorsal gun turret and forward-facing machine guns. In the inter-war years, involvement in foreign conflicts had been forbidden under the American Neutrality Act, so these aircraft had to be exported via Canada and fitted with their armament after arriving in Britain. All Marks from I to VI were tested by A&AEE between 1939 and 1943, mainly for clearance with various weapons, including bombs and rocket projectiles. The Mark III could also carry a large dinghy under the fuselage for Air/Sea Rescue work (7). There is a reference for this type to a violent stall with the left wing dropping, but no spin problems seem to have been investigated.

2.2.6 Bombers In the inter-war years there had been a class of 'light bombers', with the specification code P. The 3-man Fairey Battle, with a bomb load of 1,000 lb, exemplified this. It was a streamlined all-metal single-engined monoplane, with flaps and retractable undercarriage, considered an advanced design in the early 1930s. But by the time of its delayed entry into service in 1937 it proved to be seriously underpowered for its size and weight. When it was deployed in ground attack in support of the British Expeditionary Force, with minimal armament it suffered very heavy losses. The era of the light bomber effectively passed with the Battle, though it continued in production, serving in large numbers in the Commonwealth Air Training Plan, as a single-engined type able to carry an instructor as well as a pilot under training. For tactical purposes the light bomber was replaced by the new varieties of fast and manoeuvrable twin-engined fighter-type aircraft described in sub- section 2.2.4 above, equipped with heavy gun (and later rocket) armament and sometimes carrying bombs.

Aircraft generally described as 'medium bombers' were twin-engined, though still classified under specification code P. Some of this type designed in the early 1930s were in service at the beginning of the war and were in action from the first day. Among these, the Bristol Blenheim was also pressed into service in ground support for the BEF, but suffered heavily. They were followed in the bombing role by types such as the Bristol successor to the Blenheim, the Beaufort, and the Vickers Wellington. These carried the war against the enemy effectively at the time, with much lower losses by operating at night-time.

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The medium bombers had range enough to reach the concentrations of industry in the valleys of the Ruhr and Rhine and on to Berlin. But to serve fully the strategic aims of the RAF, the class of 4- engined 'heavy bombers', to specification code B were required. A large 4-engined type had been in service with Coastal Command since 1938, the Short Sunderland, a long-range reconnaissance and attack flying-boat to specification R.2/33. Experience in producing this type had been of value at Shorts when designing the Stirling, the first of the heavy bombers to be delivered, in 1938. Two others of this class, the Handley Page Halifax and Avro Lancaster, came into service with Bomber Command in 1940 and 1942 respectively.

Use of more than one engine had to be considered of potential significance for spinning behaviour, due to their contributions to the moments of inertia of the aircraft. The importance of these would show up in their effects on the inertia coefficient X. This involves the difference (C - A) between the moments of inertia about the normal and longitudinal axes of the aircraft respectively (See Part 2 sections 5.1.2 to 5.1.4 for details (2)). Though it had not seemed necessary to revise the working boundaries used in assessing the spin coefficients when applied to twin-engined types, the arrival of the heavy bombers brought a configuration that moved further from those of the types on which they were originally based.

A general treatment from this time of the effects on spin characteristics of having wing-mounted engines has not been found, but it can be readily illustrated by reference to Figure 13. This shows the location of an engine in relation to the three principal axes of inertia of the aircraft, centred on its overall centre of gravity G. For simplicity, it is supposed that the centre of gravity of an engine Ge lies within the plane xy. The perpendicular distances of Ge from the longitudinal and lateral axes are then y and x respectively, and its distance from the normal axis is shown as r.

Figure 13. Moment of inertia of a wing-mounted engine

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If the mass of the engine is Me, then in the standard nomenclature, its moments of inertia about the three axes are roll Ae = Mey2, pitch Be = Me x2 and yaw Ce = Me r2. The contribution Xe made by the engine to the inertia coefficient for spinning, proportional to the difference (Ce - Ae), is Me (r2 - y2). Further, r, y and x form a right-angled triangle, so this can be written as Mex2. This is equal to Be, the moment of inertia of the engine about the lateral axis, and the result is generally used in that form.

The engines are often the most massive individual components of a multi-engined aircraft, so for reasons of balance, their distance x ahead of the overall centre of gravity G cannot be large. As for the three RAF 4-engined bombers, their outer engines lay a short distance aft relative to the inner ones, so the distance x, and hence the contribution Be to the spin inertia coefficient, was somewhat smaller for those than for the inboard engines.

Though the contribution of the engines to the spin characteristics of twin and four-engined types is not negligible, it was soon realised from considerations such as this that it would not be as troublesome as had been feared. The need for long bomb-bays to cater for the loads required would mean that the moment arms for the fuselage damping and unshielded rudder volume coefficients would be ample for their purposes.

3. Developments during the early wartime period 3.1 The RAE Free Spinning Tunnel Immediate operational needs during the first part of the war left little room for developments in spinning theory, but there was further exploration of the relationship between the behaviour of a scaled model in the spinning tunnel and that of the corresponding aircraft in its full-scale operational situation. There would be associated refinements in the procedures for model testing to provide more confidence in the representation it gave, while preserving a margin of safety in what was essentially a pass/fail process. At this time the main responsibility for work on spinning in Aerodynamics Department fell to Dr G E Pringle.

Models built for spinning trials had to be scaled both geometrically and dynamically. The conditions for achieving this were reviewed in Part 2 of this study (2). Briefly, geometrical scaling is the usual procedure as for other wind tunnel models, requiring the dimensions of the external shape to be everywhere in a fixed proportion to that of the aircraft, say 1 to n. The RAE Free Spinning Tunnel had a diameter of 12 ft and models up to about 3 ft span could spin in it without their motion being influenced by the presence of the walls. In its earliest use, the linear scale n would be typically around 12 to 16, at which size models could be made accurately. The airspeed in the tunnel had to oppose the scaled value of the vertical rate of descent of the aircraft in the spin. For dynamic scaling it was shown that n should be proportional to the wing loading w of the aircraft. This quantity was increasing steadily with developments in aircraft technology, as could be illustrated by the front-line machines produced by the Hawker company. When the tunnel came into service in 1931, that was the Hart, with a wing loading of about 13 lb/ft2, but for the Hurricane in 1937 the wing loading was about 30 lb/ft2 and for the first Typhoon of 1941 it was 41 lb/ft2. This meant that models had to become progressively smaller. Skilled modellers could work to such scales, but it became more

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Journal of Aeronautical History Paper 2019/05 difficult to produce the accurate representation of the distribution of mass in the model that would correctly scale the moments of inertia.

The first development in this area was the fitting of a more powerful fan motor to the tunnel in 1938, by which the maximum air speed was increased from 35 to 56 ft/s (22). However, it was to be expected that the trend to higher wing loading was bound to continue, and it would not be long before representations were being made for the construction of a new vertical tunnel (23). The existing tunnel took in air at the bottom and discharged to the atmosphere at the top. To limit the required fan power, the tunnel now proposed would be of the closed return-flow type that recirculated the air. It would have a concentric form, as shown in Figure 14. At the working section the diameter would be 15 ft, and to cover a wide range of operating conditions, the air could be pressurised up to a maximum of 4 atmospheres. A fan power of 1,000 HP would be required to give a maximum tunnel speed of 140 ft/s at ground level pressure and 87 ft/s at 4 atm. These conditions would allow the testing of a model at 1/20 scale of an aircraft with a maximum wing loading of 55 lb/ft2 operating at altitudes up to 40,000 ft.

The exigencies of wartime did not allow further work to proceed at the time on the building of a new tunnel, so for the period covered by this Part, the operating conditions remained as in 1938. There was a return to the subject after the war for the construction of a vertical tunnel at the proposed National Aeronautical Establishment near Bedford, later to become RAE Bedford.

3.2 Developments in spin testing As experience was gained with model spinning the focus of attention turned increasingly to the effectiveness of relating the results obtained there to the behaviour of the aircraft at full-scale. The first aspect of this had been the extent to which the model itself could be representative, which became more questionable when models had to be made to quite small scales. The practice had been introduced of increasing two of the basic features of the model to reduce the possibility that minor failures to reproduce exactly the characteristics of the full-size aircraft might result in a dangerously optimistic test result. Accordingly, the changes were made in directions that would tend to worsen the spin behaviour.

One change was an increase in the inertial coefficient in pitch X by 10%, shown to be equivalent to making B, the moment of inertia about the lateral axis, 10% bigger than the scaled value quoted for the aircraft. The second adjustment was to the permitted range of the position of the centre of gravity G. This was extended rearwards by 6% of mean chord, and most of the spinning tests were done with G located at this extended aft limit. The effect of that is somewhat to reduce the stability of the model in pitch, which was known to be a factor in the tendency of an aircraft to move to the high incidence of the flat spin.

These changes had been made to allow for possible imperfections in producing the model. A more difficult uncertainty concerned the extent to which the airflow over the model might differ from that over the full-sized aircraft. This was the 'scale effect', a term already familiar in conventional wind- tunnel testing, but at that time no theoretical representation of the flow had been obtained for an aircraft in a spin as a basis for assessing it. For normal flight, the aerodynamic characteristics are largely governed by the development of boundary layers on the wings and empennage, and by that time, the scaling of this in terms of the Reynolds number was quite well understood. Conventional

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Journal of Aeronautical History Paper 2019/05 wind tunnel work had developed by establishing empirical correction factors to measured results when scaling them up to representative values, and adjustments along a similar line were expected to be needed here. But in the spin, the flow is fully separated from most or all parts of the aircraft, leaving it in a broad turbulent wake. There was a feeling that this pattern would not vary so much with scale, though practical difficulties had prevented the acquisition of data that would enable an understanding of it to be built up.

Pringle and Alston conducted a major review of the current arrangements for spin tunnel testing in 1941, with reference to cases where the results of model spinning tests had not agreed well with those at full-scale (24). One was that of the Vickers Wellesley single-engined light bomber. It came into service in 1937, eventually equipping six squadrons of Bomber Command. One squadron was allocated to the RAF Long-range Development Flight, and in November 1938 three aircraft of this type flew non-stop from Egypt to Australia, a record distance of over 7,000 miles.

The Wellesley had unusual proportions, as shown in Figure 15a. The wing span of over 74 ft was nearly twice the length of the fuselage. Since the span appeared in the denominator of the inertial coefficient X, it was rendered low (in a favourable direction), though the short length of the rear fuselage was detrimental to the other coefficients Y and Z. During a standard test of the aircraft for lateral stability before delivery, the company test pilot Jeffrey Quill experienced a flat spin which had not been encountered previously. Nothing that Quill tried disturbed the spin, and after many turns, he was obliged to abandon the machine at a height between three and four thousand feet (on the second attempt, having got back into the cockpit in the first, to switch off the engine when he realised that otherwise he might be hit by the propeller (24). Model tests at RAE had failed to reproduce this kind of spin, and in service the type had not been particularly prone to problems in that area.

In the new study, it was suspected on the basis of the known coupling between yaw and roll in stability theory that something similar might occur in the spin for an aircraft with so much of its mass in the wings. This had been neglected when the wing tip vane to apply an additional yawing moment had come into use in spin testing.

Figure 15a. Vickers Wellesley light bomber

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To investigate this, many more spins were carried out with the Wellesley model, now fitted with an additional vane to apply a series of moments in roll. These tests were filmed, to allow the angles of the orientation of the model to be recorded at every stage. A frame from one test is included in Figure 15b. The data obtained showed that there was a clear relation between rolling and yawing moments, such that in the original spinning test the roll had been accompanied by an additional yaw of significant magnitude. This acted in a direction that negated part of the yawing moment applied by the tip vane, with the result that the margin of safety was significantly lower than had been estimated in this case.

Figure 15b. A model of the Wellesley in the spinning tunnel

It was concluded however that the Wellesley was an extreme example, and a general adoption of a procedure using an additional rolling vane need not be proposed at this point. Rather, 'a watch would be kept for further anomalies resulting from the present routine methods and ultimately it may be possible to revise the standards of the test.'

Comparisons between model tests and full-scale experience continued to indicate that model testing under the prevailing procedure tended to give a more optimistic assessment of the behaviour than would be found when the type was tested at full-scale. An example was that of another private venture trainer developed by Percival Aircraft Ltd (25). In 1942 this passed the model test comfortably, with recovery in 6 seconds against 15 units of yawing moment, under the 'worst conditions of loading' at an equivalent altitude of 10,000 ft, appropriate for the duty. It was subsequently ordered to specification T.23/43 as the Prentice, but early aircraft showed poor directional response. The consequent modification to the empennage took an unusual form, with cut-outs at the inboard ends of the elevators that provided better flow to the rudder, which was also modified. More urgent requirements and production difficulties delayed the entry into service of this type until 1947, but several hundred were subsequently delivered, latterly after production was transferred to Blackburn Aircraft Ltd.

Pringle continued work in the area of spinning and drafted a significant proposal in 1943 (26), which

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Journal of Aeronautical History Paper 2019/05 was also combined with reference 24 and issued as an ARC R&M (27). This expressed the relationship between the performance of the model and that of the full-scale aircraft in terms of the 'threshold', the limiting value of the applied yawing moment beyond which recovery became impossible (Gates's earlier term for that had been the 'precipice' (2)). A new test procedure was proposed, in which the threshold would be determined for the model by a series of tests with vanes applying a sequence of increasing pro-spin yawing moments. This would be repeated for left and right hand spins and the average taken. The average would then be corrected for the effect of the associated rolling moment by a series of tests with a second vane added to the wing tips, as shown in Figure 16. A further correction for the effects of probable errors in the reproduction of the moments of inertia in the model was also suggested. If the corrected threshold was found to be 17 units or greater, the aircraft would be expected to pass the standard full-scale spin test, as applied by A&AEE.

It was recognised that the procedure for spin model testing would be considerably lengthened by the addition of extra tests using rolling vanes. Accordingly, an approximate formula was worked out

Figure 16. Twin vanes for spinning model tests for calculating the effect of the rolling moment on the moment applied by the yawing vane. This could be expressed in terms of a correction to the model threshold involving the ratio B/A of the moments of inertia about the lateral and longitudinal axes respectively. It had been formulated in the hope that it might be used in place of tests with rolling vanes, but consideration of the further uncertainty in the process caused by it led to the recommended lower limit of the threshold being raised to 21 units, and this alternative seems not to have been used in practice.

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3.3 The new test procedure Several aircraft bridged the change from the previous test standard to this new one. For example, two fighter types had been put forward by Hawker as reported in Section 2.2.3. Models of both versions were tested in the RAE vertical tunnel in October 1939 (17). They showed similar spinning characteristics, recovering in a clean condition against pro-spin yawing moments of around 20 units, but with flaps and undercarriage down, this fell to 13 for one and 10 for the other. Though this was technically unsatisfactory, under the urgency of the time the Air Ministry placed initial orders for the first of the types, to be named Typhoon, powered by the Napier Sabre engine.

In full-scale spin tests, the Typhoon was found to be borderline at 20,000 ft, and on one occasion the pilot deployed the spin parachute to obtain recovery. Further tests were carried out by the company test pilot P G Lucas, and an account of this was included in the R&M version of the report reviewed above (27). It was found that for this aircraft the spin was accompanied by violent pitching and yawing, though it could often be recovered by use of the standard procedure. But on applying full opposite rudder after 2½ turns of a spin to the left, the nose had risen suddenly and the stick came hard back. With use of both hands, the stick could be moved a short distance and by rocking it forwards and backwards an oscillation in pitch was built up, in which the nose was forced down and the aircraft recovered, though only after falling between six and seven thousand feet.

Models of the Typhoon were tested again in the spinning tunnel in the spring of 1943, when Pringle's new procedure was under consideration (28). Recovery was found to be smoother if begun early in the spin, though this depended on using the full range of control movements from the beginning, confirming the observations made at full-scale. RAE recommended enlarging the rudder or fitting a fin extension below the aft end of the fuselage.

More urgently, attention had been needed to intractable structural weakness of the rear fuselage of this type and unreliability of the engine, but the empennage was also modified, eventually by fitting a larger assembly that had been developed for the successor Hawker aircraft, the Tempest. Ultimately over 3,000 Typhoons were produced, and served very successfully, in particular becoming a formidable ground-attack aircraft, for which it was heavily armed with various guns, cannons, rockets and bombs.

Another requirement for this period was for a heavy fighter, designed to specification F.7/41, having a pressurised cabin to enable the engagement of bombers flying at 40,000 ft and above. The Luftwaffe had made sporadic sorties over Britain at this height with the modified Junkers Ju 86R, and the Air Ministry feared that there was to be a renewed bomber offensive, operating well above the height at which interception was possible at that time.

Two twin-engined aircraft submitted for this duty were the Westland Welkin and the Vickers 432. A model of the Welkin, a derivative of the Whirlwind fighter, was tested for spin in March 1942, showing excellent recovery within the specified 10 seconds full-scale against a pro-spin yawing moment of 31 units at a simulated 15,000 ft. and 19 units at 20,000 ft (29). A model of the Vickers Type 432, tested in June 1943 (30), recovered within 10 seconds full-scale against a moment of

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14½ units, and with an enlarged fin and rudder, this threshold was raised to 17½ units. At this later time, the new test procedure was being adopted; applying the corrections for the rolling moment and random errors in the moments of inertia, requiring a threshold of 17 units for a pass, but in the tests only 11 was reached. Several modifications of the model were investigated, though none was sufficient to obtain a satisfactory recovery.

An order for the Welkin was placed and a small number delivered (Figure 17). Meanwhile, however, high-altitude interceptions had been made by specially modified and lightened Spitfires. The German incursions then ended, and the threat of a new bomber offensive faded, so new aircraft for this duty were no longer required. Figure 17. Westland Welkin

3.4 Loading cases When setting the strength of an airframe, the formal requirements for the loading cases that designers must apply (arising in take-off, landing, manoeuvres, etc.) were laid down in the Air Ministry publication AP970, which was subject to revision as new information became available. It had been concluded in the earliest stages of spin investigation that the normal acceleration experienced in spinning would not represent a significant loading case for aircraft (1), and this had remained the accepted view. Some reconsideration was caused by the arrival of the heavy twin- engined fighters, and in 1943 Pringle wrote a Technical Note to provide an interim assessment of the rate of rotation to be expected in the spin of these types (31). The only data available were from observations on models in the vertical tunnel, covering just five types. It is of interest to note that this included the Gloster Meteor, that was to be the first British jet-propelled type to enter service.

The analysis was confined to the rate of rotation  about the (vertical) axis of the spin. This varied over the range from about 1.7 to 2.9 rad/s, roughly from 4 to 2 seconds per turn. The ratio of the rate of rotation to the rate of descent V was made non-dimensional as a coefficient  =  s/V, where s is the semi-span. It was found that upper limits of  did not vary much with altitude, being about 0.35 at the equivalent altitude of 15,000 ft and 0.30 at 30,000 ft. When combined with an existing expression for V, this gave a simple working relationship for determining .

Work was concluded at this stage, though two further steps would have been required to give the normal acceleration and hence a possible loading case. First, to include the radius of the path R of the centre of gravity of the aircraft, to give the acceleration  2 R towards its centre, then the 2 incidence  to give the component of that in the direction of the normal axis,  R sin  . Methods for obtaining typical values of these quantities from film had already been developed for the work

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Journal of Aeronautical History Paper 2019/05 on the Wellesley. Perhaps Pringle had written this Technical Note just for the record, as he was to return to the question of loading in the spin at a later date.

Another aspect of loading reviewed at this time was that applied when a tail parachute had to be deployed to obtain recovery in a spin (32). The design load to be used for this case was expressed in AP970 in terms of the rate of descent of the aircraft at the point of recovery, but when the parachute first opened there was generally a 'snap' or abrupt load, followed by a rise to a steadier load when it was fully inflated. This was the beginning of a dynamic process, in which the aircraft's speed was finally brought to a new equilibrium value. Backed by some measurements at full-scale and others in the spinning tunnel, Pringle developed an approximate calculation procedure for this process. Incidental observations arising from this were that if the parachute had not stopped the rotation within two or three turns, it was 'unlikely to succeed at all', and that during a successful recovery, the path was made steeper, so that the diving speed at exit was increased.

A suitable approximation for the maximum load for design purposes for this phase was given by 2 3.5 w d , where w is the wing loading of the aircraft and d the diameter of the parachute canopy when fully inflated. It was clear that parachutes designed for other purposes were being fitted for this duty, often of unnecessarily large diameter, and that loads could become dangerously high. Pringle argued that it was imperative that parachutes of approved design should be available for this means of spin recovery.

4 The later wartime period 4.1 Angular momentum in model testing Pringle's next contribution concerned differences noted between left and right-handed spins of the same aircraft (33). In acceptance tests at full-scale the times to recovery were obtained for spins in both directions, and commonly found to differ. This was also the case for the threshold values in model spin testing. Contributing factors would be those that produced pitching and yawing moments that were asymmetric, that is, acted in the same direction in spins of opposite rotation.

It would be expected that one cause of asymmetric moments would be the gyroscopic couple due to the angular momentum of an engine and propeller. This had the same sense relative to the aircraft whatever the direction of the spin. The largest effect was likely to be a pitching moment, which was favourable to recovery when the rotation was in the same sense as that of the spin, but adverse in the opposite case. The situation with respect to the yawing moment was more complex, as it depends on the direction of tilt of a spinning aircraft, which could be outward or inward. Calculations were performed for two single-engined fighter-type aircraft, one modern and the other the Bristol fighter, for which relevant data were available, both having 4-bladed propellers. One with a turbojet engine also made an early appearance here. [The latter was called a 'gyrone', Whittle's original name for a turbojet that persisted for a time.] The effects on recovery of using one or both engines of a twin-engined type with propellers were also examined. Estimated results were made for the effects of the propellers, based on the earlier values for a single-engined one. This involved further assumptions, and little of general application could be concluded beyond confirmation that the most important effect is the favourable yawing moment produced when thrust from the inner engine is brought into use. The lift component of the propeller thrust at the high incidence is also important. An item requiring to be investigated was the response of the constant- speed unit of the propeller of the inner engine to the reduced forward speed there, with the

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Comparative figures showed that despite the high rotational speed of the centrifugal jet engine, the angular momentum of the modern piston engine with propeller was nearly three times as great. Another factor recognised was the momentum given to the air, which is the means of propulsion in both cases. That passing through a propeller is external to the airframe, and in a spin is deflected away from it by the high incidence of the airflow. The flow through a jet engine is internal to the airframe and its momentum vector is carried with it round the spin axis, bringing a Coriolis acceleration into effect. With typical rates of spin, the magnitude of the force involved would not usually be important, but its moment could be significant, depending on the location of the engine relative to the centre of gravity of the aircraft.

Pringle presents some simplified expressions for obtaining the contribution of gyroscopic couples to the behaviour in pitch, roll and yaw, but this required having to assume values for many quantities, such as angles of orientation and aerodynamic derivatives, taken from measurements in spins of single-engined aircraft in the past. This suggested that representation of the gyroscopic effects of engines and propellers should be included in model testing.

The scaling laws show that if the rotor of a jet engine is to be correctly represented on a small model, its rate of rotation would have to be impracticably high. It was not necessary to represent the moment of inertia and angular velocity of a propeller or rotor separately, just their product, to give the requisite angular momentum. An apparatus was devised that would do this, as shown in Figure 18. A flywheel that would represent the effect of both engines and could be accommodated in the fuselage was designed to provide a wide range of conditions when the driving wheel was run in the range up to 2,000 rpm. The representation of moment of momentum was checked by suitably suspending the flywheel unit and measuring its rate of precession under a known applied moment. This was first used with the 1:32 scale model of the Meteor as shown. To represent both engines running at full thrust, the flywheel was first run up outside the tunnel to 17,700 rpm, to give time to launch the model and allow the spin to develop fully before it ran down to 15,000 rpm, the value required for correct simulation for the aircraft at 15,000 ft.

The rotation of the flywheel was left- handed, and as expected it was found that when the engines were represented as running the left-handed spin was steeper than if they had been idling. With engines idling it was flatter for both directions of spin. Though the difference between the thresholds for no recovery from right and left-handed Figure 18. Spinning rig for engine angular momentum

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It was clear that the contribution of the angular momentum of the engine was a significant factor in the difference between recovery from spins of opposite hands, but the apparatus shown in Figure 18 could not simulate a situation with just one engine running. It was estimated that the effect in that case would be equivalent to a change of 3.6 units of applied pro-spin moment, helpful or otherwise, according to the direction of the spin.

Brief considerations were given also to two aspects of the situation at the start of a spin. One is of the gyroscopic effects in a steep turn, from which a stall and spin might develop, and the other a more general view of how aircraft characteristics and conditions in the incipient stage might favour one or the other direction of spin. At this stage, these were mainly reminders that at some point there would have to be a consideration of the continuity of effects throughout the whole process of the spin, from conditions prior to entry to the fully developed state and recovery.

4.2 Model tests Tests in the later years of the war included models of other early jet-propelled types. An RAE report dated in January 1944 by Pringle and associates gives results of spin tests for the 'Gloster Tourist' (34). The juxtaposition of items in this report is very odd. 'Tourist' was one of the code names used for the E.28/39, the first aircraft to fly with Whittle's turbojet engine, when it was making test flights from Edge Hill in the spring of 1942. The drawings of the models used in the tests show four types, including two with twin fins. All of these come from the earliest stages of the design, that would have dated from the winter of 1939, but on one the fin is shown with the shape and location well forward on the tailplane, as in the final design of the aircraft, rolled out in April 1941. A model of the aircraft as built is not included. Further confusion is added by the reported fitting of a flywheel to represent the angular momentum of the engine, an arrangement first described much later, as shown above.

Having regard to the sequence of events in the preparation of this historic aircraft, any model spin tests would most likely have been made early in 1940 (35). The results given in Reference 34 are in fact in accord with the requirements as they were at that date (recovery within 10 seconds of moving the controls, against a pro-spin yawing moment of 10 units, applied by a wing tip vane). It is conjectured that these tests had been made then, but not formally reported at the time, perhaps due to the programme having been given the highest security classification of Most Secret. Other works done during wartime but considered to have been important enough to be recorded were published afterwards in special volumes of the R&M series of the ARC. These results were not included in those. It seems probable that the report of 1944 had been based on material in a file of miscellaneous aspects of the E.28/39 programme held at RAE Aerodynamics Department, and the anachronisms had been overlooked when they were written up.

Overall, the results had shown that the aircraft fell somewhat below the specified requirements, but it was not considered necessary to proceed with spin tests at full-scale. It is noted that the designer of the E.28, (Wilfred) George Carter, seems to have had the spin very much in mind, having positioned the fin well ahead of the tailplane, with a generously proportioned rudder, and had provided an anti- spin parachute, to be deployed from the rear fuselage. There were no reports of any spin problems

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The Gloster Meteor that followed it was now about to enter service with the RAF, and model tests were reported of the second turbine-powered aircraft for service, the de Havilland Type 100 to specification E.6/41, later to become the Vampire (36). Shown in Figure 19, this had twin booms carrying the empennage, and models with three variations of the fins and rudders were tested. It was uncertain whether the spin characteristics could be assessed by the coefficients currently used, which had been conceived for monoplanes with a conventional rear fuselage and empennage, but the value of the standard inertia coefficient so obtained was low enough to be encouraging. In the tests, all models passed the 17 unit threshold by small margins.

One model was fitted with a flywheel to represent the engine. With left rotation, the left-handed spin was steeper and recovery better, as expected. The induced yawing motion from the flywheel was assessed to be equivalent to about 3½ units full-scale and 1½ when idling. It was noted that as the cg of the engine was close to the overall cg position, it would experience gyroscopic couples but a negligible Coriolis force. This model was also tested in an inverted spin, but as the rudder was fully effective in that case the recovery was prompt.

Figure 19. de Havilland Vampire

Other aircraft with models tested in this period included the Hawker Tempest, now fitted with two types of engine, the Mk II with a Bristol Centaurus air-cooled radial engine and the Mk V with the Napier Sabre liquid-cooled engine. As shown in Figure 20, these differed considerably in appearance, due to the Centaurus having an annular intake around the engine for cooling air, and the Sabre requiring an intake and radiator in a prominent chin-mounted housing. The inertia coefficients for both marks were higher than for the Typhoon, so good spin recovery was not to be expected. The tests showed that it was slightly better for the Mk II than the Mk V, but that neither could be passed as satisfactory (37). Tests were made with miniature anti-spin parachutes, indicating that with those recovery should be acceptable for both in an emergency.

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Figure 20. Hawker Tempest Mk.II on left, Mk.V on right

At the end of 1944, tests were made to investigate recovery from a spin by parachutes attached to wing tips (38). This had become necessary because several British firms had been working on the design of tailless aircraft, which had no rear fuselage to which recovery parachutes could be attached as in previous practice. A Tailless Aircraft Advisory Committee had been set up in July 1943, and the model used as an example was of one of four unpowered machines of this type commissioned from the firm of General Aircraft Ltd, which had experimented with gliders from pre-war times.

Various individuals had obtained stable flight at low speeds from tailless aircraft since the early days of manned flight. The aircraft consisted principally of the wings, with no rear fuselage or empennage and at best a small cabin for the pilot (e.g. Geoffrey Hill's Pterodactyl). These generally had wings that were swept back, so that the lifting surfaces had some extension in the longitudinal direction. The wings were then twisted so that the incidence was progressively reduced towards the tips. With sweepback, this provided a forward position of the centre of lift, while the rear portion of the wings, with lower incidence and longer moment arm, acted to provide stability in pitch in a similar way to that of a tailplane in the orthodox arrangement.

The testers soon appreciated that it was best to fit parachutes to both wing tips for this variety of aircraft. These provided a high rate of retardation without a large asymmetry that could draw the aircraft into an undesirable attitude. During the spin, the cables were found to take up large angles fore-and-aft and sideways, as sketched in Figure 21. Methods for calculating the loads applied at the wing tips were given, and work recommended on ensuring that in use the two parachutes would be deployed and jettisoned simultaneously.

Attention to load determination during the spin was addressed in another report, in which a compilation of recommended expressions was set out for design calculations for aircraft of conventional configuration (39). These covered the determination of operating conditions in the spin for a representative flat spin and steep spin, required to obtain the drag forces and pitching moment. Tables are given for factors such as the incidence, indicated forward speed, rate of rotation, radius of spin path, wing drag coefficient and tailplane normal force coefficient. Angles that could be assumed for the towing cable after parachute deployment were tabulated, concluding with a value for the forward speed in the dive following recovery.

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Figure 21. Wing tip spin recovery parachutes

This practical support for strength calculation was very helpful, given the now-extensive array of research reports on spinning from which designers would otherwise have much labour in extracting what was needed. Nevertheless, this material was still to be regarded as provisional, until further evidence could be obtained from flight trials.

5. The early post-war period 5.1 A new era As WW2 drew to an end in 1945, a new era in aeronautics was opening as the results of research that had been classified during the hostilities became more widely known and applied. The falling efficiency of propellers as aircraft speeds approached the transonic region had been limiting, but that would be removed by the availability of turbojet propulsion. Captured German aircraft and records of research work done in Germany began to be revealed, with considerable impact in the countries of the Allies. This encouraged consideration of what were at first termed 'unorthodox configurations' to obtain better aerodynamic efficiency in the new operating conditions, including forms with sharply swept back and delta wings. But the wind tunnel, the standard research tool for aerodynamics of past decades, could not be made to function in the transonic region. A flow at high subsonic speeds could be produced, but the further acceleration of the air around a model would produce regions where it would locally reach the speed of sound. Disturbances in the flow would be carried downstream, accompanied by shock waves reflected form the tunnel walls. The effects caused the flow to 'choke' at the working section, preventing any further increase in speed. It would take almost another decade before ways had been found to circumvent that. New wind-tunnel designs were needed to enable models to be routinely tested throughout the transonic range. Meanwhile it was necessary to prepare for higher speeds largely by experimentation in flight.

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Though firms were planning the next generation of turbine-propelled aircraft to enter service after the Meteor and Vampire, the gestation period of types conceived earlier meant that most of those coming forward for spinning tests in this period were still driven by the combination of piston- engine and propeller.

5.2 More on wing tip parachutes One 'unorthodox' type that well represented the ambitions of the time was the de Havilland DH 108, shown in Figure 22 with the contemporary DH 103 Hornet. Commissioned to specifications in the Experimental category from 1945, successive developments of the 108 put up notable performances, including obtaining a record average speed of 605 mph over a 100 km closed course, and in a shallow dive becoming the first British aircraft to exceed the speed of sound. But crashes of all three of the type that were flown, fatal to the pilot in each case, were reminders of the harsh realities of advances at this time.

Figure 22. de Havilland Hornet (left) and 108 Swallow (right) A report on spinning model testing of the DH108 was not published until 1948, as reviewed later. (40) But the aircraft had featured before that in a further report on wing parachutes presented as Part II of Reference 39 and reviewed in Section 4.2 above. Both parts were later published together as an ARC R&M (41).

Wing parachutes had been installed on the aircraft at both tips and had been streamed on one occasion when it was accidentally taken into a spin. Although both parachutes were trailing at the limits of their cables, the pilot could see that their canopies had not opened, and the aircraft was subsequently recovered from the spin by use of the controls. As it recovered from the spin, one of the parachutes opened fully when the forward speed reached 310 ft/s. Both were then jettisoned and the machine dived away safely. Before being released, the canopies had been rotating in a coning motion, showing that there had been vortices in the regions of the wake of the stalled wings into which the parachutes were streamed.

This behaviour was investigated in the RAE 24ft tunnel with half-scale parachutes on cables of o varying length trailed from the tip of a half wing of 12 ft semi-span, swept back at 45 and mounted o at 45 incidence. These confirmed that the mean air speed in the wake of the stalled wing was substantially reduced below the free air speed in the tunnel, and that this had interfered with the opening of the parachutes. The effects diminished at the longer cable lengths employed.

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Accompanying tests were made in the vertical tunnel with a model DH108, using miniature parachute canopies made in paper.

The aerodynamicists were joined in this study by John Picken of the Parachute Section of Mechanical Engineering Department. That Section had a retired bomber aircraft for research into the behaviour of parachutes, which could be launched by an observer in the rear turret. The parachute was carried on a long cable, with the end fixed to a glider-towing point under the rear fuselage. The cable was progressively pulled out of a bag and the parachute deployed as the cable became taut. The opening behaviour was filmed over a range of speeds with a high-speed cine camera, and a record obtained of the force in the cable from a strain-gauged link at the towing point. These trials had shown that there were two critical forward speeds for a given design of parachute. One was the opening speed, below which the canopy merely fluttered, while its mouth remained closed. Over a range of speeds above the opening speed the canopy was fully inflated, but a second critical speed could be reached, at which it closed again. This was due to the growing inward radial components of the forces applied by the inclined rigging lines overcoming the outward forces on the canopy in the inflated state.

It was concluded that the events with the DH108 had been due to the air speed in the wake at the location of the parachutes during the spin being below their critical opening speed, so that the canopies trailed behind unopened, providing little drag. After recovery by use of the controls, the increasing forward speed and reducing incidence in the dive brought the air speed in the wake above the critical opening speed, allowing the canopies to inflate.

The recommendation was that the towing points should be placed close to the wing tips and the cable length should be as long as possible, so that the opening of the parachutes could occur where the air speed in the wake was higher. As shown in Figure 21, the helical path taken by the spinning aircraft and the inertia of the parachutes meant that they would lag behind the wings, displaced above and sideways. Their combined effect was to apply a drag to stabilise the descent and a yawing moment in the anti-spin direction, of particular value for recovery from a flat spin.

With due regard to the uncertainties of calculating the positioning of the parachutes, it was estimated that the cable lengths could be up to 1½ times the semi-span without risk of them becoming entangled. The ongoing work in the Parachute Section on design to reduce the opening speed and increase the closing speed of parachutes was endorsed in the report.

5.3 Other spin tests With the easing of the immediate pressures of wartime, it became possible to extend the testing of a model by varying the conditions being represented and making modifications to the models in ways that had been found to affect the behaviour in a spin. It was hoped that data could be gathered in this way that would help with the understanding of the scale effects that were believed to be the cause of the behaviour at full-scale sometimes being significantly different from that predicted on the basis of model tests. A representative model of a new type could not be made until the design had reached at least the stage at which the prototype could be defined. Towards the end of the decade there were more cases where model results were available, but there had not yet been any corresponding experience at full-scale with which the effects could be compared.

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The DH103 Hornet, a long-range fighter development of the Mosquito, which is also shown in Figure 22, was still of conventional design, but with unusual features. The forward fuselage was short, and the twin engine nacelles projected well ahead of the cabin, to accommodate Merlin engines with some components repositioned behind them so that the outer diameter could be reduced. It was wondered if the presence of the large nacelles forward would reduce the effect of the fuselage damping coefficient, which had been calculated on the basis of side area aft of the cg.

In the model spinning trials at a scaled altitude of 15,000 ft, the spin was found to be flat, with an o (42) incidence of 60 and rate of rotation of 3.1 rad/s . But recovery was straightforward, up to a threshold of 23 units of applied pro-spin moment. The effects of several variations were then examined. Increase in the overall weight made the rotation faster, with little change in recovery, but increase in the inertia difference coefficient made it slower, with adverse effect on the threshold. A dorsal fin, fitted with the intention of increasing the fuselage damping, had no effect, it was thought because it had been effectively shielded by the fuselage at the high incidence. Various propeller arrangements were represented. Relative to the case without any, two propellers of the same rotation changed the threshold by 3 or 4 units, but with opposite rotations (as they were to be on this aircraft in service), the threshold was essentially unchanged, as expected since the moments of momentum of the two engines and propellers cancelled.

The next prototype from Hawker to be tested was the Seahawk, to specification N.7/45, shown in Figure 23, which was to become the FAA's standard fighter and strike aircraft (43). This had straight wings and was powered by the Roll-Royce Nene turbine engine, with wing-root intakes and bifurcated tailpipes that discharged just aft of the wing roots. In standard conditions the rate of spin o was 2.1 rad/s with an incidence of 55 , and recovery was obtained against pro-spin moments of 14 - 15 units. Though increases in weight, moment of inertia difference and altitude were adverse, its spin and recovery were considered to be satisfactory for prototype flying.

Figure 23. Hawker Sea Hawk

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Air Ministry specification T.7/45 was for a 3-seat advanced trainer to replace the Harvard, and to be the first to be powered by a turboprop engine. Avro submitted the Athena and Boulton Paul the Balliol, which became competitors for a production order. Both were flown as prototypes, the Balliol with an Armstrong Siddeley Mamba engine, the first single-engined turboprop aircraft to fly. The specification was revised considerably under T.14/47, to be for a two-seater aircraft, with the Merlin engine, but the models for spin testing were of the first prototypes, designed to the earlier requirement.

Under the standard conditions, neither prototype showed satisfactory spin recovery, though the Athena was considered to be just acceptable with a small fin extension below the fuselage and an increase in rudder area below the tailplane (44). The Balliol could be brought up to borderline level by moving the tailplane back by 14.6 in full-scale. There was concern about the fast flat spin of this o (45) aircraft, at 4.2 rad/s with 70 incidence . The revised designs under T.14/47 were not tested as models during the 1940s, though the Balliol was chosen for production later, both for the RAF and for the FAA for carrier operation as the Sea Balliol.

The Westland Wyvern was a single-seat strike aircraft for carrier operations with the FAA, originally conceived to specification N.11/44, though in its ultimate form it did not enter service until the next decade. It was originally to be powered by the Rolls-Royce Eagle engine, but when this was withdrawn, it was fitted with an Armstrong Siddeley Python turbine engine, in both cases driving contra-rotating propellers. Tests were first carried out in April 1948 on a model of the TF.1 pre- production version (46). It was noted that the body damping and unshielded rudder coefficients were low, so it was expected that recovery would not be satisfactory, and this was confirmed when the threshold moment was found to be only 3½ units. Of several modifications tried, the most effective was raising the tailplane, but a borderline threshold level could be reached only by making an impracticable rise of 32 in at full-scale.

Two months later, a model was tested of the TF.2 version, shown in Figure 24. Although the type would not enter service in the decade covered here, the model was notable for the fitment of miniature contra-rotating propellers with internal electric motor drives (47), shown in the lower part of Figure 24. The 2-inch scale provides an indication of the continuing ingenuity and craftsmanship brought to bear at RAE on providing the most realistic testing conditions that could be obtained with very small models.

The spin coefficients for the TF.2 version were little changed from those of the model tested earlier. o Though the spin was mild, with incidence between 45 and 55 and rotation of 2 rad/s, recovery was possible only up to an applied moment of 5½ units. Tests with raised positions of the tailplane, as with the TF.1 model, could obtain a satisfactory recovery only with it near the top of the fin. A contra-rotating pair of propellers should not apply a nett moment to the aircraft, nor produce a rotation in the slipstream, and here it was indicated that the spin was not noticeably different if the drive was on or off. As reported below, the effect of propeller rotation was explored more thoroughly later, when the contribution of the engine could also be represented.

The main production model of the Wyvern was the TF.4 (later S.4), which entered service in 1953, so any further measures lie beyond the scope of this Part of the study.

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FIG3. POWER UNIT WITH TWO MOTORS

FIG4. POWER UNIT WITH SINGLE MOTOR

Figure 24. Westland Wyvern II and propeller drives for spinning models

Two models tested in July 1948 were of developments of the Hawker Tempest II, the Fury and Sea Fury, to specifications F.2/43 and N.7/43 for the RAF and FAA respectively. Figure 25 shows the Sea Fury. This differed from the Fury mainly by having folding wings and catapult and arrester

Figure 25. Hawker Sea Fury

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Journal of Aeronautical History Paper 2019/05 fittings with some local strengthening of the fuselage. Fitted with the Centaurus 2-row radial engine, these were the most powerful propeller-driven single-engined fighters produced in Britain. The o spins for both models were quite flat, with incidence around 60 and full-scale rotation of 2.7 rad/s. There was a difference of 10½ units of threshold between left hand and right hand spins due to the gyroscopic moment of the propeller. A feature of the spin tunnel tests was the inclusion of an inverted spin with the Sea Fury model. This could then be recovered up to a threshold of 74 units, showing the major improvement to the result in this position, when the rudder and fin were not being affected by the wake from the rear fuselage and tailplane. Clearance was given for spinning with both prototypes. The Fury order for the RAF was later cancelled, but the Sea Fury served with the RN, latterly with a five-bladed propeller.

Another naval aircraft modelled at this time was the Fairey Firefly, a substantial two-seat multi- purpose aircraft powered by a Rolls-Royce Griffon engine, as shown in Figure 26. Originally conceived early in the war, it was delayed in production, and the Mark I version entered service only in 1944. When model tests were made in 1948, aircraft up to Mark 4 had served well in many theatres, notably in the Far East. The model tested had been made for the Mark I, and in an unmodified state recovery could be obtained only up to 2 units of pro-spin moment. Production aircraft of all marks had been modified, by moving the tailplane 18in forward relative to the fin, and when the appropriate change had been made to the model the threshold had moved up to 9 units (49). For the fully-representative Mk4 version this was raised to 12 units. With the forward position of the tailplane, the fin was still largely in its wake, so the damping coefficient was lower than considered desirable, though the rudder was largely aft of the wake, giving a good value of the unshielded volume coefficient. On the basis of the accumulated experience, this combination of coefficients was expected to lead to a flat spin, with a fast rotation, and the measured values were an o incidence of 67 and rotation of about 2½ rad/s at full-scale. The conclusion was that it might be difficult for the pilot to meet the requirement for fighters of recovery in the incipient stage of the spin, but that it would be straightforward if the rotation was allowed to develop for five seconds before moving the controls.

Figure 26. Fairey Firefly

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It has been seen that direct comparisons between model spinning and full-scale experience with the type came only rarely. An opportunity now arose with the Percival Prentice trainer, shown in Figure 27. Designed to specification T.23/43, this resembled its predecessor the Proctor, shown in Figure 5, though now of all-metal construction, having side-by-side dual control with a third seat behind to serve a variety of duties. An early model spin test had indicated that spin recovery at full- scale should be satisfactory, but some difficulties in the spin were experienced when the prototype was taken through the prescribed trials by the contractor. For a training aircraft, these required at least eight turns to be made before the controls were moved to begin recovery. It was noted that over this period the spin had become flatter, and control movements became ineffective. After more turns the anti-spin parachute had been used, finally obtaining a successful outcome.

Figure 27. Percival Prentice ab initio trainer

An investigation with models was then undertaken at RAE (50). The model of the aircraft as originally designed was first tested, with and without strakes along the top of the rear fuselage, a popular and usually effective measure at that time, as reported earlier. The values of the inertial difference and unshielded rudder volume spin coefficients were not greatly changed, but as experience had shown, with strakes fitted the damping had been significantly raised. From recent work it had been concluded that an aircraft with acceptable damping but very low unshielded rudder volume would be likely to develop a flat spin. In this case, with strakes the incidence had risen from o o 49 to 65 , though with a similar rate of rotation of around 2½ rad/s. The model was then modified to more closely resemble the prototype, and when fitted with strakes it was found to have a very similar spin to that of the original model with strakes.

Tests were then made on the model with a series of modifications to provide data for analysis at RAE. These covered the original design and addition of the fuselage strakes, a dorsal fin extension, raising the tailplane and moving the fin rearwards so that the rudder would be clear of the tailplane wake. All had a useful effect on recovery, with the exception of the fin extension, thought to be due to its having been shielded by the wake from the quite broad rear fuselage. In the later tests the model developed a flatter spin than the original, though with a somewhat lower rate of rotation.

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Values for the incidence and rate of rotation estimated during the contractor's trials were considered by RAE to be insufficiently accurate to provide a direct comparison with the model results, so a production Prentice was obtained for full-scale trials at Farnborough. These clearly showed that it had two distinct spin states. Sometimes, after two or three turns there was a sudden change to a flatter spin, from which recovery was more difficult.

In the report, it was pointed out that a steady spin required that the aerodynamic and inertial moments must be in balance. Theoretically this could occur for more than one state, though that had rarely been observed. The inertial moment in pitch, which was generally the most important one, was proportional to the square of the rate of rotation and to sin 2α, where α is the incidence in the spin. The development leading to this result can be found in Part 1 of this study (1). When the damping was high the rate of rotation would be low, so α would then be expected to tend towards o 45 , where the sine term had its maximum value. This would be the case when strakes were fitted.

It was thought that a scale effect had been shown, such that when the Reynolds number was much higher at full-scale, the damping was less effective and the tendency was for the spin to settle at a higher rate of rotation where the moment would still be sufficient if the spin was flatter and the o incidence greater than 45 . The effect of scale was less when the strakes were in position, and the wake more turbulent, so there was little difference in that case between the original and the prototype.

It had not been possible to determine the cause of a change to a steeper spin that occurred suddenly, as noted at full-scale, but from a consideration of angles, it was thought that this could have happened when the tailplane first became fully stalled.

The Prentice entered service as a replacement for the RAF's Tiger Moths, with modifications including the fitting of fuselage strakes, enlarged fin and rudder, tailplane moved forward, and upturned wing tip sections, in which form it remained in production up to 1949. As to further work arising from the tests, it was recommended that there should be a reconsideration of the effect on the damping of the cross-sectional shape of the rear fuselage. It had been shown by Irving and others by 1935 (see Part 2 of this study) that a shape that was basically square, with a semi-circular fairing above it - one that was commonly chosen - had an undesirable effect on spin and recovery, but the effects of scale on this had not been investigated at that time.

Work done in 1946 on the experimental DH108, shown earlier in Figure 22, was for some reason not reported until the end of 1948. Reference has been made in Section 5.2 to the parts of this relating to the use of anti-spin parachutes, but there were also tests of the standard kind in the Free Spinning Tunnel (51).

The value of the inertia difference coefficient for the aircraft was within the range normally acceptable for single-engine monoplane fighters, but with the short fuselage the damping coefficient was quite inadequate and the unshielded rudder coefficient 'practically zero'. Even if the best control positions were used and response to the incipient spin made immediately, recovery of the model could be obtained only against applied moments up to 6½ units. As seen earlier, however, anti-spin parachutes streamed from the wing tips could be very effective, even for prolonged spins. Inverted spins were checked, to see the effect of the fin and rudder when being fully unshielded in that case. With the control directions appropriately reversed, the spin developed normally, with o o initial incidence of 40 rising to 68 . The rate of rotation ranged from 1.4 to 3.75 rad/s (1.7 seconds

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Journal of Aeronautical History Paper 2019/05 per turn). In the dive following recovery from the spin, the speed was higher than that found for the conventional configurations due to the low drag coefficient of the type. It had been designed to an experimental specification which did not include requirements with regard to spinning, so no recommendations were made in that respect.

The last RAE report on spinning issued in this decade gave a comprehensive cover of the range of Meteor variants that served with the RAF by that time - Marks 2, 3, 4, 6 & 7 - and the experimental turbopropeller test-bed aircraft, shown in Figure 28 (52). It had been possible to build a basic model that could be modified successively to provide results for all versions. Tests were run at the equivalent of 15,000 ft and 30,000 ft, indicating that all the full-size aircraft should be recovered from incipient spins at both altitudes and all except the Mark 6 from sustained spins at 15,000 ft. The rates of rotation and incidence in the spin were similar for all, lying between 2 and 2.5 rad/s o and 45 to 55 respectively at full-scale. Representation of the angular momentum of the engines produced a measurable difference between the thresholds in left and right-handed spins as expected, though this was not reckoned sufficient to be troublesome at full-scale. Finally, comparisons with full-scale spin tests made with Meteors of Marks 3, 4, and 7, confirmed that the predictions from the model tests were realistic.

Figure 28. Gloster Meteor variant with turboprop engines

6 Closure – the decade A continuous thread in the developing understanding of the spin and of recovery from it had been traced in earlier Parts of this study from 1909 onwards. In the decade of the 1940s reviewed here, wartime conditions left few opportunities for contributing to spin theory, so the leading topic of new work on spinning was of further improvements to the testing of models in the vertical Free Spinning Tunnel at RAE. This included the representation of the angular momentum of engines and propellers. Studies of the use of spin recovery parachutes, initially a single one attached to the rear fuselage and later pairs attached to the wing tips, also included work at model scale.

It had been expected in the past that there would be 'scale effects' that could cause the spin and recovery behaviour of models to differ from that of the corresponding aircraft at full-scale. Changes intended to render the tests more representative in this respect were introduced but also

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Journal of Aeronautical History Paper 2019/05 made them more elaborate and time-consuming to carry out. Although these measures remained largely empirical, they continued to be formulated with due regard to theoretical principles. It seemed likely that the effects of scale would not be understood fully unless more knowledge could be obtained of the generation of the aerodynamic forces and moments experienced by an aircraft in the deeply-stalled condition of the spin.

The later years of the 1940s were the start of the era of jet propulsion. The first aircraft entering squadron service with the RAF and the FAA were straight-winged and had spin characteristics similar to those of the late piston-engined era. But the arrival of the turbine engine had opened the way into the transonic region of flight, and the first aircraft with new configurations being considered for this regime began to appear. It was indicated that the effects of their 'unorthodox' shapes on spin characteristics were likely to be a leading area of concern in the next decade.

Acknowledgements Leslie Ruskell (Farnborough Air Sciences Trust) rendered much assistance in accessing RAE reports from the period on microfilm

Photographs are from the Mary Evans Picture Library/Royal Aeronautical Society Collection unless otherwise stated. Thanks to Tony Pilmer of the National Aerospace Library making them available.

References

1. Brinkworth, B J. On the early history of spinning and spin research in the UK. Part 1: The period 1909 - 1929 J Aero Hist, 4, 2014, 106 - 160

2. Brinkworth, B J. On the early history of spinning and spin research in the UK. Part 2: The period 1930 - 1940 J Aero Hist, 5, 2015, 168 - 240 3. Finn, E. Analysis of routine tests of monoplanes in the Royal Aircraft Establishment Free Spinning Tunnel, ARC R&M 1810, July 1937, HMSO 1939 4. Gates, S B. Note on model spinning standards, RAE Report BA 1436, Oct 1937 5. Tye, W and Fagg, S V. Spinning criteria for monoplanes, RAE Report AD 3131, May 1940 6. Brinkworth, B J. On the planning for British aircraft production for the Second World War and reference to James Connolly. J Aero Hist, 8, 2018, 233 - 298 7. Mason, T. The Secret Years: Flight Testing at Boscombe Down 1939-1945, Crecy Publishing Ltd, Manchester, 2010

8. Francis, R H and Lyons D J. Note on Tiger Moth spinning tests, RAE Report Aero 1716, Nov 1941 9. Wheeler, Air Commodore A H. Shuttleworth Collection. Letter to George Miles, 11 Sept 1981, Private communication via Peter Amos, Miles Aircraft Collection 10. Amos, P. Miles Aircraft - The Early Years, 1925 - 1939, Air Britain Historians Ltd, Tonbridge, 2009

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11. Amos, P. Miles Aircraft - The Post-War Years, 1945 - 1948, Air Britain Publishing Ltd, Tonbridge, 2016

12. Finn, E and Bigg, F J. Model Spinning Tests on the Phillips and Powis M.18, RAE Report BA 1611, July, 1940 1.3 Model spinning tests of Percival Proctor, RAE Report BA 1635, Oct 1940 14 Full-scale spinning tests on twin rudder Oxford N.6327, RAE Report BA 1638, Nov 1940 15. Brinkworth, B J. Spitfire 'tailplane protection' and spinning trials, J Aero Hist, 7, 16-24 16. Quill, J. Spitfire, John Murray (Publishers) Ltd, London, 1983 17. Model spinning tests on the Hawker F.18/37, RAE Report BA 1554a, Oct 1939 18. Model spinning tests on the Beaufighter I, RAE Report BA 1684, Oct 1941

19. Model spinning tests of the Gloster Interceptor, RAE Report BA 1636, Oct 1940 20. Spinning of twin-engined fighters, The National Archives AVIA15/2693, 6 Oct 1945

21. Addendum to Report BA 1311 - Model spinning tests of the Fairey P.4/34, RAE Report BA 1311(a), July 1938 22. RAE Free Spinning Tunnel, RAE Tech Note BA FSN69, Jan 1939 23. Pringle, G E. Note on requirements for a new free-spinning tunnel, RAE Tech Note Aero 1315, Nov 1943 24. Pringle, G E and Alston, H G. Note on the technique of model spinning in the RAE free spinning tunnel, RAE Report B A 1693, July 1941

25 Model spinning tests on Percival trainer, RAE Report Aero 1731, Feb 1942 26. Pringle, G E. The difference between model spinning and full-scale, RAE Report 1820, May 1943 27. Pringle, G E. The difference between spinning of model and full-scale aircraft, ARC R&M 1967, May 1943, HMSO 1952

28. Pringle, G E and Warren, V G. Further model spinning tests of the Typhoon, RAE Report Aero 1819, May 1943 29. Model spinning tests on the Westland F.4/40, RAE Report Aero 1739, March, 1942 30. Pringle, G E and Warren, V G. Model spinning test of the Vickers F.7/41, RAE Report Aero 1832, June 1943 31. Pringle, G E. Note on rates of turning in the spin of two twin-engined types, RAE Tech Note Aero 1317, Nov 1943 32. Pringle, G E. Note on recovery from spins by tail parachute, RAE Tech Note Aero 1323, Nov 1943 33. Pringle, G E. Factors affecting the asymmetry of left and right handed spins, RAE Report Aero 1915, Jun 1944

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34. Pringle, G E and Warren, V G. Model Spinning test of the Gloster Tourist (E.28/39), RAE Report Aero 1909, Jan 1944

35. Brinkworth, B J. The Gloster E.29/39 - Fin arrangement and spinning characteristics, J Aero Hist, 5, 2015, 65 - 82 36. Pringle, G E and Warren, V G. Model spinning tests of a twin boom fighter (DH100 E.6/41), RAE Report Aero 1939, Apr 1944 37. Model spinning tests on the Tempest, RAE Report Aero 1917, Feb 1944 38. Pringle, G E and Somerville, T V. Wing parachutes for recovery from the spin, RAE Tech Note 1559, Dec 1944 39. Pringle, G E. Model spinning data affecting strength requirements, RAE Tech Note Aero 1576, Jan 1945

40. Harper, D J, Mitchell, J R, Picken J and Pringle, G E. Wing parachutes for recovery from the spin. Part II: Wake phenomena, RAE Aero Tech Note 1881, Mar 1947 41. Pringle, G E and Somerville, T V. ARC R&M 2543 March 1947, HMSO 1959

42. Pringle, G E and Harper D J. Model spinning tests of a twin engined fighter - Hornet, RAE Report Aero 2203, Jun 1947 43. Harper, D J. Routine model spinning tests of a jet engined naval fighter (Hawker N.7/46), RAE Report Aero 2231, Nov 1947 44. Tatchell, J S and Pringle G E. Model spinning tests on an intermediate trainer [Avro Athena T.4/45], RAE Report Aero 2267, May 1948

45. Tatchell, J S and Pringle G E. Model spinning tests of an intermediate training aircraft (Boulton Paul Balliol T.7/45), RAE Report Aero 2253, Mar 1948 46. Harper, D J and Pringle, G E. Model spinning tests of a single engined naval strike aircraft (Westland Wyvern) - plus addendum - some measurements of the effects of applied rolling moments on the model recovery, RAE Report Aero 2262Apr 1948 47. Harper, D J. Model spinning tests including tests with a power unit driving the propeller of a single engined naval strike aircraft (Westland Wyvern 2), RAE Report Aero 2271, Jun 1948

48. Tatchell, J S and Harper D J. Model spinning tests on 2 single engined fighters (Hawker Fury F.2/43 and Sea Fury N.22/43), RAE Report Aero 2273, Jul 1948 49. Tatchell, J S. Model spinning tests on 2 versions of a single engined naval fighter (Fairey Firefly Mks 1 and 4) - plus amendment Oct 1948, RAE Report Aero 2286, Aug 1948

50. Harper D J, Comparison of model and full-scale spinning tests on a basic trainer (Percival Proctor), RAE Repot Aero 2298, Nov 1948 51. Harper, D J. Model spinning tests on an experimental tailless aircraft (D.H. 108, E.18/45)(with Addendum and Corrigendum), RAE Report Aero 2305, Dec 1948 52. Harper, D J and Dennis, D R. Routine spinning tests on several variants of a twin engined fighter (Gloster Meteor), RAE Report Aero 2343, Nov 1949

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The author

Brian Brinkworth read Mechanical Engineering at Bristol University. He worked first on defence research at the Royal Aircraft Establishment Farnborough during the 1950s. There, he was assigned part-time to be Secretary of the Engineering Physics Sub-Committee of the Aeronautical Research Council (ARC), and after moving into Academia in 1960, he was appointed an Independent Member and later Chairman of several ARC Committees and served on the Council itself. Thereafter he was appointed to committees of the Aerospace Technology Board.

At Cardiff University he was Professor of Energy Studies, Head of Department and Dean of the Faculty of Engineering. For work on the evaluation of new energy sources he was awarded the James Watt Gold Medal of the Institution of Civil Engineers. In 1990 he was President of the Institute of Energy and elected Fellow of the Royal Academy of Engineering in 1993.

Since retiring, he has pursued an interest in the history of aviation, contributing papers to the journals of the RAeS, which he joined in 1959. He holds a Private Pilot’s Licence.

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