Spitfire 'Tailplane Protection' and Spinning Trials

Journal of Aeronautical History 2017/02 Spitfire 'Tailplane Protection' and Spinning Trials

Brian Brinkworth Waterlooville, Hants, UK

Summary

A comparison made in 1940 of the drag of the Mk I Hurricane and Spitfire aircraft included a contribution from an item on the Spitfire listed as for 'tailplane protection', for which no explanation had been given.

It is apparent that this would have referred to a device actually fitted to the fin, to prevent jamming of the rudder by the cable of an anti-spin parachute. There had been indications that both types might be difficult to recover from a spin, so a parachute system was provided when the early aircraft were presented for official acceptance trials.

The procedure at the time for comparing the drag of aircraft in standard conditions is outlined and the place of the anti-spin parachute is reviewed in the context of the history of spinning in the UK.

1 Introduction

A comparison of the drag of the Hurricane and Spitfire fighter aircraft was made in 1940 at the request of the Aerodynamics Sub-Committee of the Aeronautical Research Committee (ARC) (1, 2). It was seeking an explanation for the top speed of the Spitfire being 40mph higher, though the size and weight of the aircraft were roughly the same and they were fitted with the same type of engine and similar propellers. The tests giving these results were made at an altitude of 18,000ft in both cases.

The enquiry was remitted to (Arthur) Roderick Collar, then working in the Aerodynamics Department at the National Physical Laboratory (NPL). Combining theoretical estimates of the various contributions to the drag with flight and wind-tunnel data, he concluded in his report of June 1940 that the overall drag of the Spitfire was about 28% less than that of the Hurricane, which would substantially account for the difference in speed (1). This study was classified as 'Strictly Confidential'. Like much other sensitive work of the Establishments reporting to the ARC in wartime, it was not selected for publication in the Committee's Reports and Memoranda series (R&Ms), though these continued to be issued on a limited scale. But it seems also not to have been thought suitable for inclusion in the special 'catch-up' volumes published after the war, and so it has remained in the form of an internal committee paper.

Collar's study was reviewed in a recent contribution to this journal by John Ackroyd (2), who noted that among the contributions to the profile drag listed for the two aircraft, there is an item for 'Tailplane protection' included for the Spitfire, but not for the Hurricane. That is not a familiar term, and he considered the item to be 'something of a mystery'. The likely origin for it is given below, together with some observations on its place in the development of the testing of aircraft prior to entry into service, as practised at the time. 16

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2 'Tailplane protection'

A possible need for protection to the tailplane of service aircraft readily comes to mind. Debris thrown up by the main wheels had long been the cause of damage to under-surfaces during take- off and landing on unprepared ground. The risk was greater when aircraft were fitted with a skid or tailwheel, at forward speeds below a value at which the tail could be raised. Peter Amos has drawn the writer's attention to a feature of the tailplane construction of the Spitfire Mk I that might have been a response to that (3). This was that the lower skin was attached by screws to wooden formers, which could suggest that it was then to be more easily replaceable if subject to damage (4). However, that would not make any additional contribution to the drag, so the source must lie elsewhere.

On the basis of available evidence of equipment used at the time on early Spitfire and Hurricane aircraft during spin testing, the present writer believes that it is extremely likely that in his report Collar had mistakenly assigned to the tailplane a device which had been mounted on the fin of the early aircraft of both types, when they were used in test work. This was fitted to prevent the jamming of the rudder by the cable of an anti-spin parachute, provided for emergency use during tests on recovery from a spin. Model tests for the Hurricane and Spitfire had been done in the RAE Vertical Spinning Tunnel before the first flights of the prototypes had taken place, and these had indicated that recovery from a spin would be problematic in both cases (5).

The spin is a stable aerodynamic state, which to be disrupted so as to recover to normal flight requires moments to be applied to the aircraft in the lateral and normal planes. At the time being considered, the cockpit procedure for spin recovery by moments provided through the rudder and elevators had long been standardised. In a test, the spin would be started from a deep stall, with the rudder centralised and the control column held fully back. Sometimes a yaw deflection was applied at the entry to the stall to encourage the start of rotation. Then when the spin had stabilised, the first action would be to apply full opposite rudder and, as the rotation was felt to be stopping, to ease the control column slowly forward towards the neutral position. If recovery was difficult (and sometimes it became impossible) it was usually because the configuration of a 'flat spin' had been taken up. This is characterised by a slower rate of rotation, with the aircraft deeply o stalled and having a high incidence to the direction of motion, typically of 60 or more. The rudder and elevators are then rendered ineffective through being blanketed by the separated wake from the rear fuselage and tailplane.

An anti-spin parachute is an emergency device fitted to act if this occurs in initial spin testing. It consists of a small canopy at the end of a long cable, attached to a strong point, often on the rear fuselage. If this is deployed, the parachute and cable generally take up an orientation represented in Figure 1. As the motion of the aircraft is along a downward helical path, the lag of the parachute behind it produces both a displacement above the plane of the wings and a lateral displacement towards the side of the inner wing. The cable has to be long enough to ensure that the canopy flies in relatively undisturbed air, but sometimes it can move around in a more-or-less complicated way. Even a small parachute can apply greater moments to the aircraft than can be exerted through its controls. The lateral displacement of the cable enables a component of moment to be applied that acts against the spin, as normally provided by the rudder in the standard procedure for recovery. And the component of

Journal of Aeronautical History 2017/02 the moment from the vertical displacement causes the tail to be pulled up and the nose lowered, as in the contribution from the elevators on moving the control column forward. Thus, once deployed, the parachute can usually disrupt the spinning motion quite promptly. Then the pilot must quickly release the assembly by moving a toggle in the cockpit, and begin a normal pull-out from the ensuing steep dive.

3 Spinning trials

The Supermarine test pilot Jeffrey Quill reports that nervousness about Figure 1 Application of moments by an anti-spin making the first contractor's spin trials parachute to an aircraft in a flat spin with a Spitfire led to the installation of (B J Brinkworth) a primitive anti-spin parachute arrangement when those were to be undertaken (6). The parachute pack was stowed in the cockpit and if required it would have to be thrown out by the pilot. It would then tear away the cable, which had been just taped onto the skin of the rear fuselage, back to the strong point from which it could deploy and perform its task. The cable fitting was in the form of a release, to be operated by the pilot after the spin had stopped.

When the first company tests were made, the spin was found to be accompanied by a large and disagreeable oscillation in pitch. But despite that, Quill found that recovery could be effected readily by the usual actions through the controls, and the parachute did not have to be used.

It is not certain when it was first realised that with this arrangement there was a risk that the cable could become jammed between the rudder balance horn and the fin. This would be greatest if the attachment point of the cable was ahead of the empennage, and the rudder was still fully deflected in the standard procedure for spin recovery, its projection then forming an inviting V- shaped slot. And so for official acceptance trials before entry into service, a proper anti-spin parachute installation was to be provided, together with a guard mounted on the fin in front of the rudder horn balance and covering its full range of movement. In what follows, this will be called the 'rudder guard'. An illustration from a later RAE report on anti-spin parachute practice is reproduced in Figure 2. This shows the outline of a Spitfire with this installation (though the aircraft shown is a later mark and the guard is of a different design, more like that used earlier on the Hurricane).

That the rudderguard isand seen aft inend Figure of the 3, fuselage, fitted to thatHurricane had been L1547 made for to its the spinning production trials Hurricane at the Aeroplane to and improve spin recovery, as suggested after the model tests in the RAE Vertical Spinning Tunnel (8) Armament(5) Experimental Establishment (A&AEE) at Martlesham Heath in the winter of 1938/39 . It takes. the form of a curved bar, supported by triangulated struts from each side of the fin, and lying a short distance forward, clear of the path of the rudder horn. The later trials were joined by Hurricane L1696, but that had not been fitted with the guard. Both aircraft had the modifications to 18

Journal of Aeronautical History 2017/02 the rudder and aft end of the fuselage, that had been made to the production Hurricane to improve spin recovery, as suggested after the model tests in the RAE Vertical Spinning Tunnel (5).

The Hurricane was tested with the original wooden propeller and with the 2-pitch metal one fitted in 1940. It was found to be 'simple and easy to fly, with no apparent vices' (8). Figure 2 Anti-spin parachute installation on a later Spitfire, showing the rudder horn balance guard (Reference 7)

Figure 3 Hurricane Mk I L1547, showing rudder guard fitted for spinning trials (permission The National Archives)

Spinning was readily produced, both from a straight stall and with a gentle turn at entry. The spin, with a time of about 3 seconds per turn, was described as 'pleasant'. Recovery required one or two turns by the standard procedure, and the anti-spin parachute did not need to be deployed. Attention was however drawn to the greater loss of height by the new monoplane aircraft compared with earlier biplane types, taking in this case about 2,000ft for recovery and return to level flight. At that time the full test procedure required eight turns to be made before beginning recovery action, and overall that would entail a loss of 5,800ft.

The Spitfire acceptance trials, in October 1939, were among the last to be made at Martlesham Heath before the relocation of A&AEE to a more defensible position at Boscombe Down for the duration of the war. The first production aircraft K9787 was used, and it was reported that its behaviour in spinning was 'satisfactory'. It came 'easily' out of the spin in one or two turns after recovery action was initiated (9). As with the Hurricane, a height loss of 2,000ft was recorded for recovery and return to level flight by the standard procedure. It is not reported that the anti-spin parachute was ever deployed in these tests, but it was considered that if it had been used a loss of more than 6,000ft was to 'be expected'.

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Although several images of Spitfires with the rudder guard in position are extant, the details of the arrangement are not generally well shown. It was evidently fitted to several of the early production aircraft, as it can be made out on K9795 in Figure 4, having remained in position when the aircraft was hurriedly entered into RAF service with No.19 Squadron at Duxford. The form is simpler than on the Hurricane, with the curved bar looping rearwards to become a transverse member passing through the fin ahead of the rudder hinge. The front of the bar is further supported by a short strut joining it to the leading edge of the fin, which can just be made out in this view.

Figure 4 Spitfire Mk I K9795 in service, with rudder guard still in position (permission The Imperial War Museum)

4 Drag and thrust estimation.

Reporting his estimates of the components of drag of these aircraft, Collar acknowledges that he had 'not examined more than two or three machines of each type'. In 1940, it was unlikely that he could have seen a Hurricane with the fin guard still in place, but it must be concluded that one or more of the Spitfires he viewed had been fitted with one. He must have then included it in the items contributing to his drag assessments, as if it had been a standard component.

Among other points arising from his examinations, he remarked that 'the Spitfire has a smoother surface and the riveting and general assembly give a cleaner finish than on the Hurricane'. He noted also that the wing-root/fuselage fairing was shorter on the Spitfire.

Most of the data and the procedure on which his comparison is based had been supplied by staff of the Aerodynamics Department at the Royal Aircraft Establishment (RAE) (to which he was to

Journal of Aeronautical History 2017/02 be seconded himself shortly afterwards (10)). The method had already been worked out by P A Hufton of that Department, who reported in April 1940 a comparison he had made of the straight- and-level performances of a standard Spitfire and the 'high-speed' Spitfire that had been modified by Supermarine shortly before the war for a possible attempt on the world speed record (11). Details of the procedure and results of the studies can be obtained from Refences 1, 2 and 11, so the observations here should be set in the historical context of the time.

The basis of the method was to make two estimates of the overall drag of the aircraft at the speed and altitude of the test at which the maximum speed had been measured. One estimate was obtained from the sum of the calculated profile drag of its component parts, plus the induced (lift- dependent) drag of the wings. Corrections to the sum were then made for the effect of the 'interference' between the flows over the fuselage and the roots of the wings in the region where they came together, plus that of the 'washout' of the wings on the induced drag in the case of the Spitfire. [This is a twist introduced to reduce slightly the incidence at the tips. Its aim is to ensure that in a stall the separation of the flow would occur first at the roots. In combat, this feature allowed a turn to be tightened to the very onset of a high-speed stall, while still allowing the ailerons in their outboard positions to remain effective. Washout was not used on the Hurricane].

The second method was to estimate the thrust of the propeller (still called the 'airscrew' at this time), which in steady, straight and level flight must be equal to the overall drag with a correction for any thrust from the cooling system. This required values to be known of the shaft power of the engine and of the efficiency of the propeller when converting that into thrust. Engine data were available for the Merlin II engine used in both types (some obtained in simulated altitude conditions), and Collar could bring to bear his own expertise on the characteristics of propellers, for which he was already recognised(10). Further calculations were made of the contributions to the thrust from the gain in momentum of the gases discharged rearward from the engine exhaust ducts and from the air heated in passing through the 'radiator' and oil cooler. The total of the latter thrust contributions and that of the propeller must then be equal to the overall drag in steady, straight and level flight.

Both estimates of the drag would be subject to uncertainties. One factor was the effect of the swirling outflow from the propeller on the profile and interference drag, for which reliable methods of assessment had not yet been developed. In his calculations Collar assumed that this would cause the flow in the boundary layers, where the profile drag is generated, to be turbulent all the way rearward from the leading edge of the wings and the nose of the fuselage.

It was assumed that the thrust estimate would be the more reliable of the two, a margin of about 5% being given for that by Collar. In both Hufton’s and Collar's studies the thrust figure was the larger, and the difference between them was then distributed between the various sources of drag in the other estimate to give equal final figures. For Collar's results the adjustments were made to the items that were the most uncertain, particularly those for leaks at the control hinges and interference around the wing roots. He adds smoothly that these changes were 'of course not so great as to remove the contributions outside the range of probability'.

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5 Scaling to standard conditions

The final stage in the estimation was to scale the results to standard conditions of a speed of 100ft/s at sea level density. This process had been in use for some time previously, intended to provide a common basis for comparisons to be made with values for other aircraft and from wind- tunnel model tests. In this scaling, adjustment was made for the drag being proportional to the density of the air and the square of the speed. For instance, scaling from the maximum speed of the test at 18,000ft to 100ft/s at sea level would require division of the drag by a factor of about 13.0 for the Hurricane and 16.3 for the Spitfire.

It should be noted that this standard value involves just a change of scale and does not give an estimate of the actual drag of an aircraft if it was being flown in the conditions specified (if indeed it could be flown in those conditions) (2). The value remains proportional to the overall drag, and the distribution of drag between the various sources that contributed to it, as they were for the conditions of the test at altitude, including the orientation taken up by the aircraft in those conditions. Smaller aircraft could be tested in the actual scaled condition in the RAE 24ft wind- tunnel, if mounted at the correct orientation. Collar went on to show that for the aircraft in his study, the results from conventional wind tunnel tests with models could be reconciled with those at full size.

The scaled overall drag estimates for the Spitfire in the standard conditions were found to be 60.2lb by Hufton and 59.0lb by Collar respectively, in good agreement. For comparison, Collar's value for the Hurricane was 82.0lb and Hufton's for the 'high-speed' Spitfire was 53.3lb. (The latter had not been considered low enough to justify continuing with preparations for a speed record attempt).

Beyond citing Hufton's methods, Collar gave no details of how he had made his estimates of the various components of drag at the time, though he could hardly have anticipated that they would become of historical interest. His value for the drag of the 'tailplane protection' for the Spitfire was in any case only 0.3lb, an insufficient contribution to give rise to any concern about its accuracy. However, Hufton's figures include one of only 0.2lb - for the 'aerial post', an item which was not given separately by Collar, having been rolled up with other minor ones in his estimates. More usually called the 'aerial mast', this was the tubular rod seen behind the cockpits in Figures 3 and 4. At this time the aircraft were fitted with the TR9 HF radio, requiring an aerial wire, running between the mast and a fitting at the top of the rudder hinge. During 1940, squadrons were being equipped with the TR1133 VHF set, for which the mast alone could contain the aerial, and that was given a more streamlined form.

Perhaps a better perspective for these seemingly small quantities would be given by the original value of drag in the test conditions of the maximum level speed of 365mph at altitude for the Mark I Spitfire. Reversing the scaling shows that Collar's estimate for the rudder guard would have been about 5lb. At this speed, the power expended in propelling this small component would be nearly 5HP, so a collection of such little things could have significant effects if overlooked.

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6 Conclusion

Roderick Collar made many further contributions to the development of aeronautics (10). His 1940 review of the drag of the two leading British fighter aircraft would relate to just one among the many anxieties of that critical time. That he had attributed the rudder horn balance guard for the Spitfire to the tailplane was a slip that could readily be understood. His inclusion of the small contribution from it to the overall drag shows attention to detail, and today also helps to place the device more firmly in the history of spinning and spin recovery testing in the UK.

Acknowledgements

Permission was granted for inclusion of the following : Photographs of Figures 3 and 4 by The National Archives and The Imperial War Museum respectively. Citations from Reference 10 by The Librarian, The Royal Society

References 1 COLLAR, A R The performance of the 'Hurricane' and 'Spitfire' aeroplanes ARC Current Paper Ae1674, June 1940. National Archives DSIR 23/7847 2 ACKROYD, J A D The aerodynamics of the Spitfire Journal of Aeronautical History, 2016/03, 59 - 86 3 AMOS, P Private communication, Dec 2016 4 MORGAN, E B and SHACKLADY, E. Spitfire - The History Key Books Ltd, Stamford, Lincolnshire, 1987 5 BRINKWORTH, B J On the early history of spinning and spin research in the UK - Part 2: 1930 - 1940. Journal of Aeronautical History, 2015/03, 168 - 240 6 QUILL, J Spitfire John Murray (Publishers) Ltd, London, 1983 7 MITCHELL, J R The design of anti-spin parachute installations RAE Tech Note Mech Eng 61, Jan 1951. National Archives AVIA 6/15623 8 - Hurricane (Merlin II) Spinning and diving trials with a 2-pitch metal airscrew and night flying trials with different airscrews and exhausts (Aircraft Nos L1547, L1574 & L1696) A&AEE Martlesham Heath, Part of Report No M.689a, April 1939. National Archives AVIA 18/635 9 - Spitfire K-9787 (Merlin II). Spinning and diving trials A&AEE Martlesham Heath, Part of Report No M.692a, Oct 1939. National Archives AVIA 18/636 23

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10 BISHOP, R E D Arthur Roderick Collar, 22 February 1908 - 12 February 1986 Biographical Memoirs of Fellows of the Royal Society, 33, 1987, pp.164 - 185. 11 HUFTON, P A An analysis of the performance of the high speed Spitfire and a standard Spitfire. RAE Departmental Note – Performance No.12, April 1940. National Archives AVIA 6/13752

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.