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Aviation History
1956
1956 - 1793.PDF
21 December 1956 HELICOPTER AUTOPILOTS 957 J Successful Experimental Work Described to the Helicopter Association AT the third meeting of the Helicopter Association's 1956-57t^ lecture session, held in the library at the Royal Aeronau- J- -m- tical Society on December 7, a paper entitled AutomaticPilots for Helicopters was presented by Mr. H. Collomosse, B.Sc, of Louis Newmark, Ltd., and Mr. M. C. Curries, B.Sc.,A.F.R.Ae.S., of the Royal Aircraft Establishment. Dr. G. S. Hislop was in the chair. The first part of the paper was devoted to an examination ofthe theoretical considerations underlying the design of a helicopter automatic control system. From a detailed analysis of the heli-copter's inherent instability characteristics, a fundamental automatic control law for stabilization had been evolved. Thetheory had then been extended to include autopilot facilities for cruising flight and hovering. The control system under discussionhad three modes of operation: — (1) Stabilizer: In this mode, while the pilot retained overall control, the autopilot introduced artificial stabilization to make the helicopter more easy to handle. (2) Cruising: In this case, the pilot established a desired flight- path in straight and level flight and the autopilot maintained this path automatically on selection. (3) Automatic Hover: To enable the autopilot to maintain a hovering flight condition above any specific point, it was assumed that signals would be available to define the position of the helicopter with relation to the point on the ground over which it was desired to hover.To arrive at the general form of control law required as a basis for the design of the automatic stabilizer, an elementary theoryrelated to stability in the hovering flight condition was first dis- cussed in mathematical terms; it was then shown how the desirablecharacteristics of the automatic stabilizers had been chosen and approximate values of control parameters calculated. These valueswere then shown to be satisfactory for the forward flight con- dition in the "stabilizer" mode of operation. The lecturer thenwent on to demonstrate how an extension of the theory could be made to determine control laws required for the "cruising" and"automatic hover" modes of operation. The investigation had been confined to a study of single rotorhelicopter characteristics. The more complex aspects of the problem had been examined on a simulator and typical responsesfor the Westland Whirlwind helicopter were given (Fig. 1). The second part of the paper contained a description of theflight trials carried out at R.A.E. Farnborough. One interesting aspect of the experimental programme was that the results hadbeen achieved almost entirely from flight tests. At the time when the work was begun, aerodynamic information on the test aircraftwas very limited. While the theoretical form of the control law was known, the optimum values of control parameters could onlybe roughly forecasted. The actual values were obtained in flight. The first step was to add artificial stability to the helicopter bymeans of suitable "black boxes" so that it could be converted into UNCONTROLLED STABILIZE MODE FORWARD FLIGHT (e.g. FORWARD) CRUISE MODE I UNCONTROLLED STABILIZE MODE FORWARD FLIGHT (e.g. AFT) CRUISE MODE 13 2: UJ Q 5O AIR SEC UNCONTROLLED STABILIZE MODE 75 TRUE SEC HOVERING SCALE Fig. 1. Simulator responses in pitch to a forward gust (Whirlwind). Disturbance while under control is about ten times that while uncontrolled. a somewhat more rational machine. The pilot would still retainoverall control. The test aircraft used was a Whirlwind, loaned by the RoyalNavy, and this was fitted with an experimental set of equipment comprising, in the main, "off-the-shelf" components. The rategyros and magnetic amplifiers were components of fixed-wing autostabilizer equipment, while the yaw servomotor was a com-ponent designed for a pilotless target aircraft. Vertical and heading gyros from another fixed-wing autopilot were added later. For the original experimental installation, a three-axis systemwas thought to be sufficient and a diagram of the pitch channel (Fig. 2) showed the typical layout of the system. It was decidedthat the most suitable detecting element would be the rate gyro and the best arrangement for the servomotor installation wouldbe a parallel one—with full authority, since unrestricted manoeuvring was required whilst under automatic control. Thisscheme seemed particularly suitable in the yaw axis, where large changes of rudder-pedal position may be required as a resultof changes in main rotor torque. To provide safeguards for the failure and runaway cases, electrically operated clutches wereincorporated in the system so that instant disengagement or easy manual overpower was available to the pilot in all axes. However, for the pilot to change the flight condition with thestabilizer in operation, it was not desirable that he should merely overpower the system; means had to be provided for him to "flythrough" the system. Although a miniature controller was not considered ideal, it was decided to make use of one in the earlyexperiments in order to get the flight trials started. Later, a means of operating the equipment through the standard flying controls 1 fig. 2. Diagram of first experimental set (pitch channel). RATE GYRO INTEGRATING I CIRCUIT , Pi LOTS ^ CONTROLLER SERVO- MOTOR CLUTCH CYCLIC STICK AIRFRAME FLYING CONTROLS HELICOPTER AERODYNAMICS
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