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Aviation History
1963
1963 - 0312.PDF
296 FLIGHT Intematio r.cl, 28 February 1% Fig 4 Ministab (on near corner of table) under test with a Dowty electro-hydraulic power actuator AUTO CONTROL... instruments and automatic aids, and it is surprising that some current VTOL aircraft designs envisage only a bare minimum of automatic controls, to the extent that full potential performance will probably not be attained. An example is the startling reluctance of some designers to specify automatic engine failure-compensation in multiple-engine types. In some cases even automatic stabilization and rate demand- shaping are being avoided. This attitude is understandable if one considers, first, the relatively poor re liability record of automatic control equip ment in the past and, secondly, the effect on an aircraft designer of a suggested solu tion involving an excess of multiplexed equipment! Fortunately the problem can now be solved and reliability comparable with that of the basic aircraft achieved with economic arrangements of automatic sys tems. This allows some dependence on autocontrol, and consequent realization of the various possibilities. In particular, the complex interconnec tions between pilot's controls and combina tions of aerodynamic surfaces, lift engine throttles and deflectors can be more simply handled by electrical signalling allowing considerable weight reduction, more accu rate and repeatable control and low stick and pedal forces. Gear ratio changes can be more simply provided either in the ini tial design or during development. Accu rate cross-synchronization between controls, engines and/or fans on each side of the aircraft becomes relatively simple. Basic aircraft engine and lift unit positioning can be determined entirely by lift geo metry, hot gas recirculation problems, payload storage and so on, without the major limitation of cumbersome mechani cal control system layout. Autocontrol elements may also be advantageous for failure compensation and variable perform ance trimming of lift units in multiple engine installations. Angular-rate gyro and acceleration-feed back signals allow aircraft response to be set for the particular characteristics desired by the pilot for manual control, both in the three translational degrees of freedom and in the conventional rotational ones. As stated before, the techniques for achieving the required performance and reliability in autocontrol devices for aircraft are now well understood, and there may be more experience and relatively fewer problems in the new application than in the develop ment of the basic VTOL aircraft themselves Future automatic control systems are expected to use multiplexed sensors and actuators and microminiaturized electro nic amplifiers with additional internal, or "integral" redundancy, so that the equip ment can survive not only one failure, but many. Internal failures will result at worst in a deterioration of performance. Multi plexing is expected to be kept to the bare minimum because of the severe weigh i penalties and, where safety requirements are highest, "dissimilar" system redundancy will make common failure probability acceptably remote. Basic design techniques in autocontrol elements will therefore tend to follow those used in aircraft structures and engines—and no lesser overall re liability and safety standards will be con sidered. Rate Demand Control and Autostabiliza- tion It is well known that pilot's control de mands yield different results in power- borne flight from wing-borne flight, be cause of the absence of aerodynamic damping. For example, a rolling moment applied in wing-borne flight by sideways stick force will approximately produce a roll rate and, if the stick force is removed, this is converted to some steady roll angle In the power-borne hover, the sideways stick force produces a roll acceleration, and on removal, this may stabilize into a roll velocity. In power-borne flight, therefore, control of roll displacement involves one more integral than in wing- borne flight and can impose severe dif ficulties on the pilot. This applies to all controls in power-borne flight, but in practice is more apparent in the angular controls than the translational ones. The aircraft must therefore be made to Fig 5 Failure survival by triplication and integral redundancy (a) TRIPLICATED SYSTEM MAJORITY VOTE COMPARATOR ->• OUTPUT 1 IOOO , - 1 IOOO 1 IOOO -1 , - 1 IOOO k. 1 IOOO 4 1 1 IOOO 1 •* 1 IOOO 1 1 IOOO (b) SYSTEM USING INTEGRAL REDUNDANCY •+ OUTPUT CONSOLIDATION POINTS
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