Patent GB-1051161-A (GB)
Abstract: 1,051,161. Radar. BELL AEROSPACE CORPORATION. May 30, 1963, No. 21647/63. Heading H4D. Relates to an automatic landing system for aircraft, Figs. 1, 2, in which a land radar affords the Cartesian co-ordinates XR, YR, ZR of the aircraft relative to the touchdown point and a computer calculates pitch and roll command signals #c, #c conveyed by a data link for the control of the automatic pilot of the aircraft, the signal #c including terms proportional to the altitude error ZE determined by the difference between the aircraft altitude ZR measured by the radar and the correct altitude Zc corresponding to a predetermined glide path, ZE, and fZEdt, and the signal #c including terms proportional to the lateral position error YE of the aircraft, YE and fYEdt. According to the present invention the computer provides also another pitch command output which supplants the above, primary, pitch command during flare-out. Radar.-The radar aerial, of the parabolic reflector type, scans in horizontal and vertical planes and transmits circularly-polarized waves to reduce rain echoes; a corner reflector is fixed on the aircraft. The radar incorporates a gate corresponding to a range of a few miles and when the aircraft enters the gate the radar locates and " locks on " to it to supply the computer with the aircraft's instantaneous co-ordinates. General.-The computer determines the pitch and roll command signals #c, #c taking into account the aerodynamic characteristics of the aircraft. The data link also transmits discrete signals for " wave-off and "de-crab." Pitch command computation.-The general form of the pitch command signal #c is: where: Kc is the static gain coefficient (degrees per foot error) optimum TR is the rate-gain coefficient values (seconds) of the lead chardependent acteristic of the pitch comon type of mand computer. aircraft. Ti is the integrator time constant (seconds). The corresponding apparatus is illustrated in block diagram form, Fig. 3: Zc, from which ZE is derived, is obtained by multiplying a factor dependent on glide slope angle γ by a voltage (XR - 8XR) where XR is radar-measured range; a switch 45, closed only when the aircraft range has decreased to 16,000 ft. controls the insertion of the term. fZEdt for reducing over shoot or hunting. The coefficient Kc is attenuated until the range decreases to 4,000 ft., Fig. 4 (not shown), to avoid excessive commands when the error ZE is likely to be large. Roll command computer.-The roll command signal similar notation to that for #c above being used. Factor K<SP>1</SP>c mK<SP>11</SP>c where K<SP>11</SP>c is the maximum value of K<SP>1</SP>c and 0<m#1, m being a function of range, Fig. 5A (not shown); noise amplitude is then substantially independent of range. The corresponding computer is outlined with reference to Fig. 5 (not shown). To avoid excessive over-shoot the term fYEdt is inserted only when the aircraft is nearer than 7,000 ft. The maximum value attainable by signal #c is limited. Flare-out.-A pitch rate command #<SP>1</SP>c is formulated such that the values ZR and ZR are brought to required magnitudes at the predicted instant of touchdown, at the desired position along the runway. In one system, Fig. 7 (not shown), an expression: is integrated to give the pitch command signal, where: #ZR is the instantaneous ZR minus that required at touchdown, TR is predetermined for a given type of aircraft and takes account of airframeautomatic pilot time lags, and V is aircraft velocity, measured by the radar. In a simpler arrangement the voltage ZE in the pitch command computer, Fig. 3 (not shown) is modified by substituting a voltage Z<SP>1</SP>c for Zc, obtained as shown in Fig. 9 (not shown): a switching arrangement 126, 128 provides, when flare-out is to begin, for the stored value of Zc at that moment to be applied to an exponential potentiometer 118 supplying an adder also fed with a voltage, derived from a further non- linear potentiometer 120, used to introduce an increasing " sink rate " gradually, to ensure actual landing at the designated touch-down point. A servo-mechanism 110, fed with the factor T, actuates the wipers of potentiometers 118, 120. Manual " de-crab."-In practice it is necessary only to provide a visual signal to the pilot who has a visual display of actual azimuth and that of the runway; the signal is applied to the data link by a relay 140, Fig. 9, energized by a switch actuated by servo-mechanism 110 two seconds before touch-down. Indicators. The control signals are fed also to a cross-pointer indicator for monitoring the automatic operation and for use if the automatic pilot fails.