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Aviation History
1961
1961 - 0244.PDF
246 FLIGHT, 24 February 1961 SYSTEM Ferranti Flight Director WORK on a flight director for helicopters was begun by theaircraft equipment department of Ferranti Ltd as a private venture during 1958, but MoA endorsed their ideas andbegan to support the project during the following year. A flight director, or an autopilot, was evidently needed to extend thecapability of the helicopter in poor weather; and a director would also become a requirement for monitoring the more capableautopilots. It was realized that there was a fundamental difference betweendirectors for fixed-wing aircraft, which made attitude demands based on the presumption of a safe airspeed, and a helicopterdirector, where no consistent "background motion" of the aircraft could be assumed. The helicopter director would therefore hiveto provide demands for control position, rather than attitude; and four axes—roll, pitch, yaw and vertical—were involved. A fifthfunction, engine power-control, was necessary if the powerplant/ rotor combination was not automatically governed to run at con-stant speed or power. A prototype of the attitude director was first exhibited at theSBAC display at Farnborough last year. Considerable test flying with an experimental director has been completed in a Whirlwindby RAE, which showed that even without autostabilization a pilot is able to fly the machine with considerable precision for morethan an hour. The eff ctiveness of the system was substantially proved. A prototype of the navigation display which will formpart of the director system is now almost complete and flight tests should soon begin. The complete system includes the attitude and navigation dis-plays, a vertical gyro from which attitude and pitch and roll rate are derived, an azimuth gyro giving heading and rate of change ofheading and a control computer. An air-data computer giving height, speed and rate of change of these quantities is required. The computer f jeds the displays, but could also control auto-pilot servos. In addition, Ferranti are designing a coupler for radio aids such as ILS (or presumably, a helicopter equivalent),Tacan and Doppler in order to allow long-term flight-path control. An approximate block diagram is shown in Fig 1. The attitude director, illustrated here, occupies a 5in X 5in panelcut-out. It has been made as large as possible in order to provide the maximum sensitivity without having to resort to unrealistic ornon-linear indication. The aircraft silhouette traditional in arti- ficial horizons has been replaced by a fixed datum ring in thecentre and two pitch scales placed at the sides in order to leave the central area as free as possible. Attitude is represented by therelationship of the pitch scale and ring with the horizon bar. The scales cover a pitch range of ± 5" to allow for attitude variationsin steady flight. An adjustable datum for this purpose was con- sidered undesirable, because of parallax errors and possible mis-taken setting. A traditional bank-angle scale is at the bottom of the dial. The director function is provided by a moving ring index sup-ported on cross-wires and controlled in such a way that when the control column position is correct the ring index is centred andthe helicopter is on or properly approaching the attitude required to produce the wanted flight-path. The linear index across the top edge of the dial provides turndemands in the same sense as the slip-ball of the turn-and-slip indicator, but it controls long-term heading, the short-term cor-rections being fed into the director ring for correction by cyclic- stick movement. At speeds below about 20kt, all turns are madeflat by yaw alone, and all turn demands are then fed into the yaw index alone. Obviously, the vertical linear index in the left edge of the dialcontrols collective-pitch demand and is used during controlled descents on a glide-slope or for height holding. During an instru-ment approach, the speed demands would be fed in through the fore-and-aft cyclic demand and only height terms would appearin the vertical scale. Some collective-pitch demand would also appear during sustained turns, when the helicopter, like manyfixed-wing aircraft, tends to lose height. A single attitude and director combination, such as a horizonand Zero Reader, can keep a pilot fully occupied; and the addition of yaw and collective-pitch directors seem to risk making thedisplay too "noisy" at times. The number and diversity of informa- tion channels requiring control reaction from the pilot are fairlyhigh, so simplicity (which is the main purpose of a director) looks like being lost. Practical trials, however, seem to disprove this;and Ferranti point out that the attitude director will probably be used mainly to monitor autopilot operation, in which case the pilot VERTICAL , ATTITUDE GYRO iPITCH AND ROLL) RATE AZIMUTH GYRO .HEADING AND YAW . ; POSITION AND RATE AIR DATA COMPUTER HEIGHT AND RATE OF CHANGE RADIO ILS TACAN DOPPLER COUPLER FLIGHT L)|RE COMPUTER |C^ j CONTROL " SERVOS CONTROL PC TRANSDUt Fig 1, above, shows the schematic layout of the Ferranti director system. Top left, the attitude director instrument has only to observe that all three director elements are centred.Hand-flying with the director in cruising flight would also be simplified, because then only the cyclic stick (and therefore thedirector ring) would be in constant use. As far as taking over manually following an autopilot failure is concerned, it is possiblethat only one channel or axis would fail, thus making it necessary to take over only pzrt of the complete control task. The first trials at RAE with the four-axis director equipment,in a Whirlwind without autostabilization, showed that the pilot could fly blind for more thant an hour without fatigue and withgreat accuracy. Pitch attitude was in fact held to within ± 1 ° and the results were directly comparable with those achieved by anautopilot. The main difference was that the autopilot's control movements were sharper than those of the human pilot using thedirector. The first engineered version of the director attitude display, complete with the first navigation display and computer,should be test-flown during the next few months. While the attitude director tells the pilot where to move hiscontrols, it can tell him little if anything about the flight path his aircraft is following. This is the function of the navigation display(Fig 2) which is a situation indicator and not a director. It also provides controls with which the long-term flight-path parameters Fig 2. The navigation display with its rotating compass card, height deviation scale and control knobs. The four different roller-blind displays which can appear inside the compass ring are shown overleaf TRACK POINTER' - MOVII CAR COM Pi HEIGHT DATUM 3O HEIGHT SELECTOR
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