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Aviation History
1955
1955 - 1735.PDF
858 FLIGHT R.A.E. rocket model of 1953. M=2.5. An R.Ae.S. Main Lecture Given at Boscombe Down PART I Free-flight High-speed Research AT the Royal Aeronautical Society's first "main" lecture to A\ be held at the Aeroplane and Armament Experimental •*• -*- Establishment, Boscombe Down, Mr. P. A. Hufton, M.Sc, and Mr. J. A. Hamilton, M.B.E., B.Sc, A.F.R.Ae.S., spoke on Free-flight Techniques for High-speed Aerodynamic Research. Before a large audience at Boscombe on Thursday, December 1st, Mr. Hufton, head of the supersonics section of the aerodynamics department, R.A.E., gave general introductory and final remarks to the paper, the main body of which was presented by Mr. Hamilton, who is in charge of free-flight investigations in Mr. Hufton's section at the R.A.E. Chairman of the meeting was Mr. N. E. Rowe, president of the Society, who was intro- duced by A. Cdre. R. A. Ramsay Rae, O.B.E., president of the Boscombe Down R.Ae.S. branch. The use of freely flying models to gain an understanding of the laws of flight (the paper began) dated from the earliest days of aerodynamic experiment. For the aeronautical pioneers simple flying models were one obvious means of putting their theoretical analyses to the test and of acquiring additional physical insight into the mechanics of flight. But the disadvantages of flying models for all but the simplest qualitative tests soon led to their almost complete extinction in favour of the wind-tunnel; this eclipse persisted until the problems raised by wind-tunnel block- age at near-sonic velocities led to a renaissance of free-flight tech- nique—now enabled, by developments in electronics and rocket propulsion, to provide quantitative information throughout the transonic Mach number range. With the advent of truly transonic working sections the tunnel had once more become the primary tool of the aerodynamicist at transonic speeds; but, unlike other stop-gap expedients, the free- flight technique had shown virtues which had gained it a place with full-scale flight and the tunnel, as a research facility in its own right, at high supersonic as well as at transonic speeds. Three main forms of free-flight* experiment had been tried in the United Kingdom. They were those employing: (a) powered, air-launched test vehicles; (b) unpowered, air-launched test vehicles; (c) powered, ground-launched test vehicles. The initial effort on free-flight development went into a powered air-launched test vehicle—the R.A.E. Vickers rocket model. This was a model aircraft with unswept wings, having a length of lift and a wing span of 8ft. Propulsion was by liquid-fuel rocket motor using a mixture of methyl alcohol and hydrazine hydrate as a fuel and concentrated hydrogen peroxide as an oxidant. The aim was to release the model from its parent aircraft (a Mosquito) at a height of 36,000ft and a speed of 400 m.p.h.: once released it was propelled by the rocket to supersonic velocity and controlled in straight and level flight by means of an autopilot. During its flight the model was tracked from a ground station by radar and its behaviour recorded by ground telemetry. The fate of this investigation was now well-known. Mechanical and electronic problems, raised by the complicated and advanced nature of the model and its rocket motor, engulfed the project from its inception; and to these problems were added the opera- tional difficulties of a complex ground and air organization when flight-test work began. After a long and troubled period of gestation a successful modus operandi was evolved, but by that time more economical methods of investigating transonic flight had been developed and the project was abandoned before it had produced any results of value. More success was achieved with unpowered airborne test vehicles. These were also equipped with telemetry and tracked by radar but obtained the required transonic velocities in free fall. This technique was still in use in the United States and to a lesser *In this context and elsewhere throughout the paper the term "freeflight" appertained to aerodynamic experiments at transonic and super- sonic speeds. Investigations into low-speed aerodynamic problems hadalso been made using freely flying models, e.g., for take-off and landing and spinning. These were not considered in the lecture. extent in this country for bomb ballistics; its major drawback, apart from the operational difficulties, was the limited Mach number range which could be covered by one model. For aircraft- model work it had been abandoned in the United Kingdom in favour of the powered ground-launched model. Many factors influenced this choice of technique, but the over- riding one was the operational simplicity of launching from the ground compared with air launching. The relative compactness of the range layout for ground-launched trials eased the problem of controlling the experiment and introduced that high degree of flexibility which was necessary when dealing with test vehicles of unconventional design—and often unconventional behaviour! In ground-launched free-flight tests the aircraft or component model was launched from a simple platform and accelerated to the maximum required velocity by a solid-fuel rocket motor. After the motor had ceased to burn—the propulsion period usually occupied no more than two or three seconds—the model deceler- ated in coasting flight to cover the necessary Mach number range. During the period of coasting the behaviour of the model was recorded externally by electronic and optical means and internally by radio telemetry. The remainder of the paper was concerned with the ground- launched model method of free-flight aerodynamic research. An average time of flight for a rocket-propelled aerodynamic test vehicle was 12 sec. Within this period the vehicle was accelerated from rest to a maximum Mach number of, say, 1.5 and then decelerated to subsonic velocity: the total distance covered was three or four miles and the maximum height achieved about 2,000ft. During its flight the model might be rolling con- tinuously at rates of up to 3,000 deg/sec, or oscillating continuously in pitch with frequencies of the order of 12 c.p.s. The problem of recording the behaviour of such a short-lived body was mani- festly a formidable one and its solution had given rise to a new and esoteric branch of instrument and electronic engineering. Associated with these developments was the establishment and operation of the complex ground organization at the trials range. The responsibility for this service, whether for aircraft-model or missile trials, rested with the Guided Weapons Trials Department of the R.A.E. Indeed, most of the recording methods described owed their inception to the need for guided-missile flight trials, and the aerodynamic free-flight technique was fortunate in having at its disposal not only the instrument and electronic develop- ments prompted by the guided missile, but also the associated production facilities. It was doubtful whether the free-flight technique would have been successful in its present form without this help from the missile groups. The three principal measurements were those of trajectory, velocity and position in roll. Trajectory was obtained optically by having several kine-theodolites spaced at suitable intervals along the proposed direction of flight. These gave simultaneous records of the bearing and elevation of the test vehicle at intervals of i sec throughout its flight. Simultaneity was ensured by having the shutter of each camera operated from a central timing source which also supplied timing to all other range facilities and thus eliminated errors owing to incorrect synchronization between the various forms of record involved. Velocity was measured by a radio-Doppler system which beat a constant-frequency radio signal against its echo from the moving test vehicle. Thus, the apparatus measured the change in the distance transmitter-vehicle-receiver, and compared this with a time base: the trajectory of the model being known, this informa- tion could be converted into line-of-flight velocity. If the theodo- lite and Doppler records were of reasonable quality, the line-of- flight velocity could be determined to within ±2ft/sec. ;••; Using a numerical double-differentiation process, the Doppler record could be made to yield longitudinal acceleration and hence drag. The accuracy obtained in drag measurement using Doppler was dependent upon the rate of change of drag with time: where
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