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Aviation History
1984
1984 - 0405.PDF
AVIONICS ETPS to get variable stability Hawk CRANFIELD Trainee test pilots at the Empire Test Pilots School, Boscombe Down, are to receive a variable stability BAe Hawk developed by Cranfield Institute of Tech nology. The Hawk will be delivered in January 1986, and student training will begin in the following July. Variable stability training at ETPS is currently done on a Beagle Basset converted by Cranfield in the early Seventies. The piston twin has proved a versatile training tool, but its analogue electron ics have limited the scope of features that can be simulated to simple instabilities and to changes in the airframe such as bigger or smaller control surfaces, or wing dihedral. The Hawk's variable stability system will be fully digital, and capable of gener ating a much wider range of handling characteristics than that of the Basset. There will also be a reserve of computer power to enable extra facili ties to be added without major hardware modifications. Rejecting the idea of using ground-based flight simu lators for training test pilots at lower cost, David Williams, Cranfield's instrumentation chief, says that while such simulators enable trainee test pilots to experience un desirable handling character istics without risk, the fidelity of such machines still leaves much to be desired. "This is particularly true in marginal situations where the pilot may require accurate visual and motional cues to maintain control of the simulated aircraft." The Hawk, says Williams, will be essentially an airborne simulator, designed as a pure test pilot training vehicle. Indeed, its specifications have been written by the variable stability Hawk project manager at ETPS, Sqn Ldr James Giles. Early candidates for a new variable stability aircraft were the BAe Jetstream and 125, but these were rejected, says Williams, because they offered only a relatively restricted manoeuvre envel ope. Eventually ETPS was offered a brand new Hawk for the project. The aircraft ^FLIGHT International, 10 March 1984 Key 1 VSS noseboom 2 Static inverters. Tacan 3 Artificial feel actuators 4 Control rod actuator pack 5 Computers (flight control, safety monitor, simulation and HUD) 6 Safety pilot's VOU 7 Test pilot's sidestick 8 Control-rod load transducers 9 TaHplane trim transducer 10 Flight data recorder 11 Aileron trim transducer 12 Programming keyboard 13 eg. pack 14 Fin load transducers 1 5 Aileron position transducer 16 Tailplane position transducer 17 Rudder position transducer 18 Rudder trim position transducer arrived at the end of January. Principal features of the variable-stability Hawk will be a sophisticated artificial feel system located in the front cockpit, where the trainee test pilot sits, a digital simulation computer, a system controller, a flight safety monitor, and primary flight control actuators. The instructor, or safety pilot, sits in the rear seat. His controls are physically linked to the primary surfaces at all times, whereas the trainee controls the Hawk indirectly via the artificial feel system and computer. At the same time the natural flying qual ities of the host aircraft are electronically modified to simulate the instructor's requirements. The instructor can therefore programme any linear or non-linear handling characteristics within the Hawk's flight envelope into the system. The effects of varying the wing sweep angle on a Tornado can, for exam ple, be demonstrated. To avoid complication and excessive cost, the Hawk's variable stability software is single-channel, or simplex, unlike that of the multiplexed system used on fly-by-wire aircraft such as the F-16. Dave Williams says that the two systems should not be compared, that of the F-16 being designed to make a destabilised aircraft flyable, while the Hawk's is aimed at modifying the handling char acteristics of a naturally stable aircraft, including providing a completely un stable situation. The Hawk system will also be able to simulate practically any other aircraft with known stability derivatives. Test pilots would be able, for example, to experience RAF TriStar char acteristics one day, and those of a Tornado the next. The key to simplex system performance is a flight safety monitor, developed in analogue form for the Basset and greatly refined for the Hawk. Cranfield says that it is breaking new ground in the use of an "intelligent" computer which will scru tinise handling limits, provid ing a variable chop-out according to the rate at which a limit is being approached. The system will, for example, ensure an early warning, or drop-out, if the Hawk is rapidly nearing its g limit, leaving the warning until later if the rate-of-change is slower. The safety monitor is also responsible for system health monitoring. It is charged with ensuring that actuators do exactly what the computer wants, and checking that transducers are intra-con- sistent, which means, for example, comparing pitch rate and vertical acceleration to ensure that they are compatible. The safety pilot "talks" to the system via a microcom puter which flashes up appro priate messages on a multi function CRT located in the centre instrument console. Aircraft parameters can be changed in flight, and a range of other alterations, such as stick feel, can be instituted. The variable stability Hawk will be equipped in the trainee station with both side and centre sticks, the latter being able to be conditioned to reproduce any force gradient. The side stick, says Dave Williams, will probably have a measure of compliance, although the exact amount has yet to be decided, and may be adjustable. The lesson input keyed in by the instructor will be fed to the student via his instru ments and the individual multifunction display. Six ordinary needle-type instru ments will be provided, indi cating any previously decided parameter, and there will be a target tracking facility on the Jaguar type hea-up display. 627
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