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Aviation History
1961
1961 - 1866.PDF
980 FLIGHT, 28 December 1961 Missiles and Space flight. . . Environmental of Bloodhound Structural resonance investigations, similar to those made during normal aircraft prototype proving programmes, in progress on a Bloodhound ENVIRONMENTAL engineering as a recognized and developedtechnology is largely a postwar development. As is so oftenthe case, its techniques principally originated in meeting military requirements, and its growth reflects the increasing aware-ness of the need for an organized attack on unreliability. In no field is reliability more vital than in aviation and, as theboundaries of flight have been advanced, the aircraft industry has become increasingly environment-conscious. With such factorsas kinetic heating and acoustically excited vibration to be taken into account, laboratory tests must reproduce service conditions sofaithfully that equipment likely to fail in service will fail in the laboratory. In guided-missiles, reliability is all-important. These complexweapons must lie quiescent for months, often in unfavourable conditions, and yet remain capable of instant operation under themost exacting environmental conditions. In manned aircraft, the failure of system components need not be disastrous; alternativeservices and emergency measures can be brought into play, and a crew is available to take corrective action. Missiles possess no suchsafeguards, and the failure or inconsistent operation of one part is usually failure complete. The guided-weapons manufacturers have therefore had to buildup considerable facilities, manned by specialist staff and employing specially designed equipment and advanced techniques, to simulate(often in combination) such physical effects as the near-vacuum conditions of high altitudes, the high temperatures resulting fromaerodynamic heating or the low temperatures of stratosphere conditions, the accelerations attendant upon launching or flightmanoeuvres, the static or dynamic loads resulting from flight aerodynamics, and mechanically or acoustically excited vibration. The environmental laboratory of Bristol Aircraft, establishdprimarily to develop and prove the Bristol/Ferranti Bloodhound surface-to-air missile, typifies the larger laboratories in being inBritain. It is grouped in relation to its principal test activities, which can be loosely classed as kinetic heating, vibration, naturalenvironment, and packaging and transportation. Of the environmental conditions peculiar to missiles and aircraft,one of the most difficult to synthesize on a large scale is kinetic Cold-chamber test of Bloodhound afterbody to prove the functioning of the co-axial cable connections in very low temperatures or aerodynamic heating. During sustained level flight, the airframewill slowly attain a steady elevated-temperature condition. Rapid changes of speed and height, such as are demanded by missilemanoeuvres, induce transient conditions which are particularly severe. The short active life of some missiles may include manyvaried load patterns, resulting from the demands of the guidance system, combined with rapid temperature variation, perhaps fromwell below zero to many hundreds of degrees Centigrade. The task of the test engineer in proving structural integrity hasbeen complicated by this need to include in ground test conditions the factors of heat and time. A complete theoretical analysis of thestresses due to combined aerodynamic loading and thermal stress conditions in a complex structure is not at present feasible. Acomprehensive series of tests must therefore be carried out on a complete airframe structure, combining all the aspects of varyingheat, load and time. Heat is radiated on to the surface of the test structure by a numberof quartz-envelope, infra-red lamps mounted in polished reflectors, shaped to enclose the test structure. By this means up to 20kW/sqft can be delivered into large structures and up to 90kW/sq ft into small structures and specimen panels. Almost double thispower is consumed in the lamp assemblies because of losses. During the heating operation, structural loads resulting fromaerodynamic forces or manoeuvre are applied as point loads and varied to reproduce the flight pattern under study. Generally theenvironment is simulated by closed-loop temperature/time pro- gramming of the skin temperature, while the mechanical load ismanually controlled through orthodox hydraulics. During tests of this kind, which cannot be repeated because of thedeterioration of material properties, a great number of results has to be recorded, sometimes in as little as 30sec. The current techniqueis to record test results (temperature, deflection, strain, etc) upon punched tape for subsequent data processing; or the results can betranscribed immediately upon an automatic typewriter. A more advanced control and recording technique is nowcoming into use at Filton. The control system is intended to pro- vide a more accurate reproduction of the temperature/load/timerelationship through closer simulation of the heat-flow conditions and more precise and rapid control of load variation. It is basedupon synchronized electro-mechanical function generators which prescribe to the heat and load power sources instructions derivedfrom flight parameters programmed on digitally punched tape. The heat control function generator converts the digital informa-tion into analogue form and computes the heat input appropriate to that instant in flight. This information is then compared withthe output of the heating lamps, and the difference signal is used to control the power supply unit. By rapid sequential presentationof data, discrete control can be exercised of the heat input to a number of individually heated areas. The load control functiongenerator operates in a similar manner but utilizes rapid-response hydraulic servos to modulate the jack pressures. The flight load cases to be simulated are programmed to the loadfunction generator as consecutive complete programmes. Two or three cycles of load can be applied within the transient heating time,enabling the progressive deterioration of structure stiffness to be determined. The recording system at present under constructionutilizes telemetry multiplexing and high-speed magnetic recording. Vibration testing has received considerable attention since thewar. The electronics industry has provided much specialized equipment of advanced design, and simulation techniques closelyrepresentative of actuality are being studied. In the recently-equipped vibration section at Filton, structuralresonance investigations are carried out to check the predicted aeroelastic characteristics of Bloodhound through the exactdetermination of the structure resonant frequencies, the identifica- tion and description of the modes of vibration, and the evaluationof structural damping factors. The test structure is suspended upon
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