On the Early History of Spinning and Spin Research in the UK Part 2.Pdf

Journal of Aeronautical History Paper No. 2015/03 On the Early History of Spinning and Spin Research in the UK Part 2 : The period 1930 - 1940 B J Brinkworth Waterlooville, Hants, UK Abstract In Part 1 (Journal of Aeronautical History, Paper 2014/03) a continuous thread of experience and research since WW1 was traced, which had provided a growing understanding of the propensity of an aircraft to spin and a standard procedure that gave the pilot a good chance of being able to recover if one developed. The 1930s were a period of rapid change in air defence policy and technology, leading to the emergence of the monoplane as the preferred configurat- ion for fighters. During this time there was an expansion of work on spinning, with particular reference to progression to the fast flat spin, from which recovery was usually impossible. A major step forward came with the opening of the RAE vertical Free Spinning Tunnel. Systematic investigations of spinning behaviour could now be made with models that were correctly scaled dynamically. From the results of these and the growing body of tests at full scale, a procedure was devised that distinguished between physical characteristics of aircraft that could be recovered from a spin and of those that could not. Although empirical, this was firmly founded in theory, as the work had been from the beginning. This procedure allowed designers to include for the first time routine checks at all stages of the design to estimate the likelihood that a new type could be recovered from a spin. The procedure would be refined and become standardised in later work, but the stage reached around the beginning of WW2 marks a distinctive end-point for the early part of the history of spinning in the UK. 1. Introduction The scientific study of spinning had started in 1917, with a theoretical account of the dynamics of an aircraft in a steady spin, together with the first reported observations of the motion in flight (see Part 1, paper 2014/03). Reviewed at the start of the 1930s, it could be said that work during the intervening years had materially consolidated that position. The critical influence of the moments of the aerodynamic forces acting on the aircraft had soon been recognised. These moments could not yet be calculated, but measurements with models, latterly using the ‘rotating balance’ in the wind tunnel, had begun to give more relevant values of their magnitudes and a general understanding of their effects. In particular, it was revealed that any displacement in yaw of the aircraft relative to its helical path was of first importance in determining the required moments, though that had been neglected in earlier work. Flight-testing at full scale would always be required, to expose new problems and to validate conclusions reached from theory and testing with models. A standard instrument package had been developed for this, which could now record automatically all the measurements 168 Journal of Aeronautical History Paper No. 2015/03 required to define the behaviour of an aircraft in the spin. It had also been understood that the outcome of research needed to be communicated more readily to personnel in the industry, where designers were becoming increasingly receptive and better placed to implement advice when it became available. Yet in its Technical Report for 1929-30, the ARC was obliged to state that ‘Existing knowledge is insufficient to prevent the occasional appearance of an aeroplane upon which it is difficult, or impossible, to check a fully developed spin’ (1). And in the Supplement to that report, it had gone on to endorse the opinion of its Air Ministry member that ‘the spinning of aircraft presents one of the most important practical problems yet to be solved’. Accordingly, the Committee had recommended that research on the subject should ‘take first priority’. From a purely practical viewpoint, it was necessary that much of the work on spinning would continue to be done at RAE and NPL with models. It had been realised that the aerodynamic environment of an aircraft in a spin was unique, and for measurements on models to be useful the situations in which they had been obtained had to represent that environment closely. This would be a leading theme in the development of experimental methods in the following years. There were continuing concerns about recovery from spinning, particularly from the flat spin, characterised by very high angles of incidence and high rates of rotation. Unexpectedly large moments were being required from the controls in both the pitch and yaw directions to effect recovery. The spinning trials of service aircraft at A&AEE Martlesham Heath and MAEE Felixstowe would continue. In this Annual Report there was a specific mention of further tests to be made on the Fairey IIIF aircraft, for which the spin had ‘sometimes been a very unsteady motion’. This single-engined aircraft was a versatile machine that served in a variety of roles for both the RN and RAF over a service life of 14 years. Unsteady motion usually consisted of nutation (a nodding movement) imposed on the rotation, and there was a belief that this indicated that the aircraft was close to moving on to a flat spin. The Fairey IIIF could be fitted interchangeably with wheels or floats. There was concern about the possible effects of the proportionately large contributions made by floats to the moments of inertia and the base and side areas, which are apparent in the photograph of Figure 1. The case shows the complexities likely to be faced in reaching reliable judgments about the spinning characteristics of a given aircraft type at the time. It had not yet been possible to do that fully before the aircraft had been required to spin during trials for acceptance into service, and modifications made to it afterwards could also be problematic. Figure 1 The Fairey IIIF multi-role aircraft, with floats fitted (National Aerospace Library Collection) 169 Journal of Aeronautical History Paper No. 2015/03 The wings of the Fairey IIIF had been set at zero stagger, a feature known to be implicated in progression to the flat spin, but the gap between the two wings was large enough to be a signif- icant mitigating factor in countering that. To establish a numerical base for an investigation, initial measurements were made of the rolling and yawing moments due to roll with models at 1/15th scale, both with and without floats, in the rotating balance at NPL (2). The rolling moment about the spin axis due to the fin and rudder was negative (against the spin) at all incidences tested, except for the highest value of 61o, a value that could be reached in a flat spin, when the moment became positive. However, the moment due to the addition of floats was positive at all incidences. The yawing moment about the body axis due to the floats was also negative over most of the incidence range, but it too became positive at the highest incidence tested. Measurements without rotation were made of the forces and couples for all body axes, reflecting the considerable variations with incidence and sideslip that had been noted regularly in other cases previously. The opportunity was taken in these tests to build up further basic information on the effects of changes to the tailplane shape and position and effects of using differential and floating ailerons and ‘interceptors’ (shallow strips that could be raised from the upper surface of wings, usually in association with tip slots, although the slots themselves were not represented on the models tested on this occasion). When used in calculations, these results gave quite good agreement between the calculated values of incidence and spin rate and the full-scale spin results already available for the landplane version of the aircraft. Though the margin of safety was lower for the seaplane version, it was indicated that there should be no great difficulty in recovering it from the spin. However, during the trials made at MAEE at Felixstowe the testers had been unable to do so at one point and had to abandon the aircraft, so in service the crews were advised to do that at once if a spin developed or continued below 1,500ft. This would be a region in which seaplanes would commonly be operating, but it appears that no further work was done on spinning with that version of the type. More generally, progress in understanding spinning was a result of interaction between theoretical analysis, experiments in wind tunnels and flight tests. As such, it is an excellent example of the value of this combined approach to aeronautical problems. 2 Communication with designers During the 1920s, UK industry had not followed the US in raising its productivity sufficiently to be fully competitive in world markets. This situation was to continue into the 1930s due to national monetary policies which over-valued the currency, while at home it was difficult to obtain the finance necessary to modernise plant and equipment. Stringent economy was necessary for aircraft firms to remain in business, but some changes did occur that contributed to an increasing awareness of the importance of what was being found in the various aspects of aeronautical research and to apply that to design. 170 Journal of Aeronautical History Paper No. 2015/03 At this time the number of persons in the industry with full professional qualifications was still quite low. The design team for a new project would continue to be a small group, affording few opportunities for ambitious newcomers to enter. Although a modest proportion of senior people were members of the RAeS, many had risen under the pressures of WW 1, largely on a basis of the hard lessons of experience.

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