On the Early History of Spinning and Spin Research in the UK Part 3: the Period 1940 to 1949
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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 133 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 134 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.