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Aviation History
1963
1963 - 0313.PDF
FLIGHT International, 28 February 1963 respond in the hover as it does in wing- borne flight by substituting for the missing natural aerodynamic damping an angular rate gyro feedback in roll, pitch and yaw. Such rate feedback compensation normally also serves for autostabilization of manual and automatic demands in turbulence, both in power- and in wing-borne flight. The automatic system should additionally have some short-term ability to hold a selected displacement, such as a wings level atti tude—normally by adding a rate integral term to the autostabilizer. The term must give adequate short-term positional stabi lity but must not jeopardize the rate- demand characteristic. During the transition from power-borne to wing-borne flight, the changeover from VTOL to aerodynamic controls must be smooth. The amount of rate stabilization at each stage will depend upon the build up, or otherwise, of aerodynamic damping forces, but control authority is normally transferred to the aerodynamic surfaces as quickly as possible in order to reduce re liance on the lift units and/or bleed-air to the absolute minimum. In the more complex systems, self-adaptive gain controls may be used during the transition stages 297 specifications. Limited authority will allow the pilot fine control for small control movements without preventing large, rapid corrections beyond the limited-authority "•compensated" regime when necessary. If the role of the aircraft is limited, and pilot control without automatic assistance is possible, but undesirable, a simple light weight automatic control system will suffice for pilot relief. But if uncompen sated pilot control is difficult or impossible at all times, then some minimum level of failure-survival is needed in the automatic controls. It may not be neccessary in all axes; performance requirements and weight considerations will determine actual de signs. Fig 2 is an example of a simple pitch autostabilizer and rate-demand system. The pilot's control column operates the control unit, and rate-gyro stabilization is added differentially. If the rate loop gain is high enough, the aircraft will be con strained to move at a pitch rate propor tional to control demands. In practice, control run pick-offs are sometimes in cluded, as shown, for special phase-ad vance and demand shaping, but these are not essential to the fundamental operation Fig 6 Rate demand and autostabilizer system using failure survival integral redundancy INTEGRALLY REDUNDANT INTEGRATOR TO PILOT'S CONTROLS TO POWER CONTROLS so that optimum performance can be obtained for all e.g., weight and engine performance conditions. It is worth noting at this point that at least one VTOL aircraft, the Hawker P. 1127, can be flown in good visibility near the ground without any artificial demand compensation or stabilization; adequate damping may here be achieved by other means possibly related to the par ticular pattern of jet intake or efflux. Similarly desirable characteristics may in future be achieved by other VTOL air craft so that rate feedback compensation may not be essential to safety in the hover, but both practical experience and theoretical studies show that rate stabilization is highly desirable to reduce pilot work-load and allow operation in more difficult weather conditions. It is fairly certain that its use will be general in VTOL aircraft. Turning now to other safety aspects, the overall automatic rate demand and stabili zation controls may have limited authority wherever possible, but actual designs will, of course, depend on particular aircraft of the rate demand/stabilization system. A number of miniature stabilizers for VTOL applications have been developed by Elliotts, and two of these are shown in Figs 3 and 4. If reliability is important and single- failure survival is the minimum required, the most straightforward development is to triplicate the autocontrols and add some majority vote system to reject a faulty member. Limited low-speed cross-syn chronization is also required because of tolerance problems. This technique has been widely considered despite the fact that triplication for safety is expensive and heavy and the necessary cross-synchroni- zadon of "independent" channels funda mentally undesirable. In recent years much effort has been devoted to the design of failure-survival systems which do not require complete triplication, cross-synchronization and so on. Some redundancy is clearly necessary, but it can be applied more economically and effectively. A new technique now being used is to give each element of a system Fig 7 Autostabilizer for a VTOL transport. A, e/ectro/uminescent partial failure indicator panel; B, self-test e.l. scale; C, duplex rote gyro; D, internal test buttons; E, screwed connectors failure-survival capability within itself, by means of built-in or integral redundancy. Such elements connected together into a control system allow multiple paths for control signals and it is, therefore, highly probable that numerous internal failures will not put the system out of action, while pure triplication with a majority vote com parator can survive only one fault. Partial failure may cause slight performance deterioration, hut this can normally be tolerated. Fig 5 illustrates the greater survival capability of integral redundancy. Each of the three channels (or lanes) of the tripli cated system in this case includes four elements, each with a failure probability of llXK) °vera" failure probability of this ' / 4 v 1 \ ^ 48 system is therefore 3 /4 x IV V 1,000/ 10" The integral redundancy system shown has only two of each element, but they are arranged in failure-survival pairs. Its over- ( I V 4 —— I = — 1,000/ 10" Although the weight of the system has been reduced by roughly one-third, its failure probability has decreased by a factor of 12. In this example, the triplicated system can survive a maximum of four selected failures, or a maximum of "n" selected failures if there were "n" elemental pairs. Even if the various devices for consolidating the out puts of the individual pairs are heavier than the comparator in the triplicated system— which is doubtful—the integrally redundant system offers a very great survivability advantage. Integral redundancy, now being applied in the more advanced control systems, is well known in its passive-element form to the designers of fail-safe airframes. Fig 6 shows diagrammatically a rate-demand and stabilization system incorporating such principles wherever possible. The rate gyro is duplicated, having two rotor and gimbal assemblies rigidly coupled at their output axes and driving a twin pick-off. These in
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