Patent GB-815137-A (GB)
Title: Flight path angle computers
IPC: G05D1/06
Filed: 1957-03-13
Abstract: 815,137. Aircraft indicators. SPERRY RAND CORPORATION. March. 13, 1957 [March 13, 1956], No. 8317/57. Class 4. [Also in Groups XXXV and XXXVIII] The angle of flight of an aircraft with respect to the horizontal is computed accurately from two figures for the angle, one of which follows rapid changes but is liable to contain longterm errors, and the other of which responds to slow variations in the angle ; the computing apparatus contains frequency-sensitive means to combine the accurate features of both figures. The first signal can be obtained from a vertical gyro in combination with a device for sensing the angle of attack, the latter being either a wind vane or else a computer fed with signals representing wing loading and load factor, dynamic pressure, lift coefficient and deflection of landing flaps. The second signal may represent the rate of climb or dive, obtained by pressure-sensitive devices which give signals representing airspeed and altitude. For best accuracy such signal is passed through an arc sine computer to convert h /v into sin<SP>-1</SP> h /v, but this is not essential. The combined signals are passed through an integrating network to remove high-frequency components and the difference between the low frequency components of the two signals is added to the first signal as a correction factor (see Fig. 2). In a modification the first signal is passed through a circuit which causes it to decay with time, e.g. an RC differentiating circuit of the same time constant as the CR integrating circuit used to eliminate the high-frequency components from the combined signal. In Fig. 5 the vane 20 moves the slider of the potentiometer so that a signal appears across AB representative of the angle of attack. The vertical gyro 71 provides a signal across CD depending on the pitch angle of the aircraft, and these two signals are joined in series opposition so that the voltage between D and earth represents the instantaneous flight path angle. An altitude sensor operates a selsyn 77 which drives the follow-up motor 79 and rate generator 80 with a conventional feed-back loop, to give a signal at 82 indicative of the rate of change of altitude. This signal is applied (in parallel with the voltage at D) to the potentiometer 84 whose slider is moved by the airspeed sensor 76 so that the output signal at 85 is proportional to h/v (the arc sine computer is omitted). The electromechanical low-pass filter 90 rejects any rapid fluctuations (due to the gyro faithfully responding to sudden alterations of pitch) and the slow changes affect the " repeat back " signal generator 105 to inject negative feedback into the whole loop. Thus the voltage at D passed to the utilisation devices contains only the rapid changes which the autopilot is to correct. The motor 93 of the filter is driven in accordance with the magnitude and phase.sense of the input signal, and itself drives a rate generator 95 whose output supplies adjustable negative feedback to the input at 91. The slider 99 is driven from the motor via a high step-down gear train to poduce a voltage at 107 depending in magnitude and phase-sense on the low frequency components of the input signal. In an alternative system, Fig. 6, the angle of attack is computed according to a mathematical formula from signals representing the slope of the lift curve #CL/#α, the wing loading factor W/S and a sensitivity factor K to match the output with the other signals in the system, further combined with a signal representing the deflection of the wing landing flaps. This circuit replaces the assembly 46 in Fig. 5. In a further modification, Fig. 8 (not shown), the output from the vertical gyro is corrected to allow for the fact that the angle of attack changes during banking. In another modification, Fig. 7 (not shown) the vertical gyro is replaced by a rate gyro and integrating circuit, the gyro signal being demodulated before integration and then remodulated. Utilization. The signal developed across potentiometer 48, Figs 4 and 5, can be used to control the elevator channel of an auto pilot 55 or a flight director (e.g. as in Specification 690,985), in the latter case being combined with signals from an altitude control 60 (for level flight) or a glide slope receiver 58 (for instrument landing). In another form an indicator, Figs 4 and 9, the signal operates a pointer 117 moving over a dial calibrated in angles of flight path. A further pointer 118 is set manually by the knob 120 which also moves the slider of the potentiometer 121. The pilot adjusts the knob for a desired flight path and a command signal is thereby supplied to the autopilot which will be reduced to zero when the actual and selected paths agree.