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Aviation History
1963
1963 - 0317.PDF
FLIGHT International, 28 February 1963 AUTO CONTROL . . . autopilot operation can also be obtained in this mode by engaging lock A and replacing mechanical with electrical feed-back, as described in relation to Fig 10(a). Lock A is designed to yield at some predetermined artificial-feel load to provide torque limita tion at a level determined by safety require- Fig 14 Elliott Force and Moment Computer, which continuously receives thrust and deflector angle signals from each lift unit and computes the necessary corrections for the primary controls. The computer has integral redundancy ments. A force pick-off on the pilot's control column can also be engaged to operate the controls, the power follow-up of the mechnical runs, operated by lever B pivoting around lock A, giving a low feel force. The VTOL controls on nozzle and engines follow the powered control output from the connection rod C, and can also obtain series inputs from the force and moment actuator D on the main powered control assembly. The engine group has an associated Croup Thrust Compensator. Between the auto- stabilizer actuator and Force and Moment actuator sections of the powered control is a hydraulic switch arrangement which has been called a Control Fault VETO. This empowers the autostabilizer actuator to freeze the output of the Force and Moment actuator if the latter makes a demand not in agreement with autostabilizer action—a normal relationship between the two systems determines the operating characteristics of the VETO. Other aspects of the system are explained by the diagram. The overall safety philosophy can be better understood by considering the effects of various failures in the hover, as follows:— Null Failure of Pilot's Control Force Pick-off or its Connections: Pilot's demands are not satisfied with small stick forces, but increased pressure causes lock A to release and direct mechanical operation of the powered control is obtained. The auto stabilizer system will still operate. A utostabilizer Failure: If the autostabilizer has complete integral redundancy, the first internal failure should at most cause only slight performance deterioration. For the most stringent safety requirements if, for example, lateral stabilization is critical during the transition, duplicated auto- stabilizers each with integral redundancy would avoid the remote possibility of a single system being put out of action by physical damage. A carefully designed duplicated system of this kind will have catastrophic failure probability lower than that of the aircraft structure. Nozzle or Ducting Failure: Any type of nozzle or ducting failure could be counter acted, and full control maintained by thrust modulation of the lift units. A Single Lift Unit Failure: ,This would be fully counteracted by the Group Thrust Compensator and high authority Force and Moment Control, and, to a lesser extent, by the autostabilizer system. Failure of the Force and Moment Control System: Integral redundancy would ensure that the first internal failure would cause at most only slight performance deterioration. Any subsequent failure involving a signi ficant demand will activate the auto stabilizer VETO and the autostabilizer itself will then balance out any small dis turbance which does occur. If a lift unit failure occurs after failure of the Force and Moment Control, the effect will be counter acted by the appropriate Group Thrust Compensator plus the autostabilizer. De pending on the type of failure, some minor assistance may be required from the pilot. 301 Group Thrust Compensator Failure: The GTC is very simple and has only a passive failure characteristic. Any lift unit failure subsequent to a GTC failure would be counteracted by the Force and Moment Control. The hypothetical system of Fig IS is an example of the use of dissimilar redundancy for achieving safety in automatic control, and shows the high degree of survivability which can be achieved without using full multiple redundancy. Such systems could certainly be applied to most combat aircraft, where mechanical control runs are relatively short, provided the short-period movements on the pilot's control column in the fully engaged mode were not disturbing to the pilot. For large transport aircraft it would probably be desirable to employ only electrical connections, especially in complex roll-control systems as described in Fig 12. This could be effected in the system in Fig 15 by removing rod E and connecting pivot F to "earth." The electrical link shown would have to be upgraded to achieve the required integrity with the mechanical link removed. Throttle run G could be replaced by electrical signalling connections. There are other alternatives. If the direct mechanical connections are satisfactory, the stick-force sensor and powered control engage lock can be removed, and short- period autostabilizer demands will not appear at the pilot's controls. Complete aircraft systems based on the principles outlined could achieve failure probabilities less than 1 in 108 per hour. This article has broadly covered some of the considerations involved in the use of automatic controls in jet- and fan-lift VTOL aircraft. Many other possibilities, such as rapid automatic starting of multiple lift unit installations and the various associ ated fuel metering problems have not been covered. It is clear that autocontrol technology must continue to advance to meet the growing requirements of new aircraft, and that the associated reliability and safety aspects of autocontrol design must be as carefully considered and as economically solved as is the basic design of the aircraft itself. Fig IS A hypothetical VTOL control arrangement THRUST %^ SENSORS DUPLICATED EFFLUX PRESSURE CONNECTIONS
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