Patent GB-1028826-A (GB)
Title: Flight control apparatus
IPC: G01S1/02
Filed: 1964-08-04
Abstract: 1,028,826. Radio-navigation. BENDIX CORPORATION. Aug. 4, 1964 [Aug. 21, 1963], No. 31552/64. Heading H4D. [Also in Division G3] A control system for guiding an aircraft Z, Fig. 2, along a glide slope beam BT, comprises, means generating a signal representing the angular displacement E of the aircraft from the glide slope beam, by means of directional radio properties of the beam, means generating a signal representing the rate of change of said angular displacement, but not using said directional radio properties, means filtering the angular displacement signal and integrating the rate of change signal, means producing a signal being the sum of the filtered signal and the integrated signal, means integrating the angular displacement signal, and means producing a control signal by the addition of the sum signal and the last mentioned integrated signal. It is stated, that with the control signal operating an autopilot, the aircraft can be safely brought down to within 50 ft. of the ground automatically as against the 200 ft. limit obtained when the control signal is comprised of the sum of the angular displacement signal and its integral. The angular displacement signal is produced as a D.C. signal by the glide path receiver 1, Fig. 1, and is converted into an A.C. signal by a modulator 3. The signal then passes through a variable gain amplifier 4, the gain of which is varied such that a given angular displacement represents a smaller and smaller error as the beam transmitter is approached. The amplified angular displacement signal is then integrated at 5, to overcome long-term errors, and is applied via adding circuit 9 as part of the control signal, to autopilot 10. A rate of descent device 21 produces a rate of descent signal h<SP>.</SP>. An accelerometer device 25 produces a vertical acceleration signal h<SP>.</SP>  which is passed through a washout circuit 26, which is a differentiating CR circuit with a very long time constant of 20 seconds, to remove any steady state errors in the signal. The signal is also multiplied by the time constant T of a lag network 24, to become Th<SP>..</SP>. The multiplied signal and the rate of descent signal are combined in circuit 23 to produce a signal:- where S is the time differential coefficient. This signal then passes through an integrating lag network 24 having a transfer coefficient of 1 and a time constant of 4 seconds, to 1+TS produce an output signal h<SP>.</SP>. The resulting rate of descent signal is relatively noise free. The signal h<SP>.</SP> is passed to a wash-out circuit 16 having a transfer function of TS/1+TS and a time constant of 20 seconds. Since by virtue of the autopilot the aircraft seeks to maintain a steady rate of descent h B  along the glide path BT, the workout circuit will only pass variations of the rate of descent from this steady rate. Therefore its output will be ( (TS/1+TS) h<SP>.</SP> = h <SP>.</SP>. h B <SP>.</SP>. This output signal is then amplified at 22 to become K h (h <SP>.</SP>. h B  <SP>.</SP>). As the angles N and N, Fig. 2 (not shown), are small, E = h-h B /x. Therefore assuming an average value of x, the rate of change of E is given by, sE = hÀ-h B À/x. There-     fore if the amplification factor K h  = T/x. the signal from amplifier 22, represents T. sE. This signal is fed, together with the E signal from variable gain device 4 to a lag network 7,   having a transfer function 1/1+TSand a time   constant of 5-10 seconds, to produce a signal E substantially free of noise which is passed to the autopilot. The network 7 this simultaneously acts as the filler and the integrator of the inventor. A signal representing the rate of descent h B <SP>.</SP> may be fed to the input of washout circuit 16, from means 31, to enable the aircraft to initially capture the glide slope quickly. The E signal from network 7 may be fed to a pitch command indicator 27, where the control of the aircraft is by the pilot, together with an opposed signal from a vertical gyro, indicating the pitch of the aircraft, to overcome the tendency to overshoot the glide path.