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Aviation History
1947
1947 - 1046.PDF
604 FLIGHT TUNE 26TH, 1947 TOWARDS HAPPIER LANDINGS corrective control which is proportional to both the dis- placement and the rate of displacement, then in effect this infinitely variable gear ratio is obtained. The Sperry A. 12 Gyropilot is a " displacement plus rate " autopilot, and is thus suited to automatic approach and, indeed, to controlling an aircraft at any operating speed without adjustment in flight, since correctly banked turns are made at all airspeeds within the operating limits. To solve the first problem of establishing the required ground track, it will be apparent that if the aircraft can be positioned along it, then the signal which will be re- ceived to indicate any deviation can be used to instigate a suitable corrective turn. The received signal and the subsequent turn will be approximately proportional to the angular displacement of the aircraft from the track, and a pure displacement signal will probably suffice to main- tain the ground track, but only afier it has been estab- Fig. 1. is taken to consist of one overshoot of the beam centre to about half-way into the other side and a smooth turn back to the beam centre (Fig. 4), the nature of the turn signals required to accomplish this is shown in Fig. 5. This is, in fact, what is accomplished by the Sperry Auto- matic Approach Control. In operation, it means that the human pilot can fwitch to "Localizer" on the auto / approach Fig- 3- LEFT EDGE / OF BEAM RATE OF /CENTRECROSSING BEAM / TURNLEFT SIGNAL DISPLACEMENT FROM BEAM CENTRE switch ("Gy- ropilot," " Localizer " and ''Ap- proach '' posi- tions) as soon as the localizer needle leaves its stop on lished, i.e., at a small angle of intersection. An alternative system is to introduce a directional reference, but this may involve a more complicated procedure in that the aircraft must be flown parallel to the localizer beam for a certain distance ©r, at least, the runway Q.D.M. must be set on the compass. Consider now the case of an aircraft intersecting the localizer beam at a positive angle of intersection from one side (Fig. 1). The localizer needle will indicate "Turn Right," and an appropriate signal will be fed to the auto- pilot, the subsequent course of the aircraft will be any- thing but that required and severe hunting about the beam centre will take place. Thus, by using a turn signal which is proportional to displacement from beam centre, the limitation is imposed that the angle of intersection of the beam must be very small. Now, if the rate of change of the displacement signal is measured and a rate '' signal'' produced, this can be suitably polarized (or phased) and combined with the displacement signal in such a way that appropriate turns can be introduced to establish the ground LEFT EDGEOf BEAM BEAMCENTRE •RIGHT EDGE1 OF BEAM DISTANCEFROM BEAM CENTRE () DISTANCE FROM BEAM CENTRE Fig. 2. either side, at any intersection on the beam. Jk§L Successful brackets have been made at angles of inter- * section up to 179 deg, but as this depends on the aircraft staying within the confines of the beam during the resul- tant 179 deg turn as explained above, angles of intersection greater than 90 deg are not recommended under operation conditions. From the pilot's point of view, the procedure is to position his aircraft at a range of 10 to 12 miles on the downwind side of the airport at a height of about 1,500ft, following a suitable B/A procedure and using the A. 12 Controller with the Automatic Approach Control (A.A.C.) switched to '' Gyropilot.'' The height selected is maintained within close limits by the A. 12 altitude con- trol. At this stage the localizer needle will be against the stop to left or right and the Glide Path needle against the upper stop, showing that the aircraft is positioned below the • Glide Path to one side or other of the beam. The aircraft is then flown towards the beam on such a course that the angle of intersection is 90 deg or less. The A.A.C. is switched to '' Localizer'' as soon as the localizer needle leaves its stop, whereupon the turning of the aircraft will be undf- auto- matic control and the correct ground track will be es- tablished. While this is going on the wheels can be lowered, the power reduced, the airscrew pitch changed, the flaps lowered and an approximate 2j-deg nose-down attitude introduced by the pitch control and taken out by the A. 12 altitude control. When the Glide Path needle moves to the centre, the A.A.C. is switched man- ually to " Approach." The altitude control is automatically switched off and the aircraft assumes the nose-down atti- tude previously selected, being held in the localizer and Glide Path beams automatically. Any elevator trim which may be necessary is also introduced by the trim tab servo- of the A. 12 so that, after passing over the three marker beacons (by which time the pilot should be able to see the runway) the Gyropilot '' Release'' button on the control column is pressed and the landing of a correctly trimmed Fig. 4. track along the localizer beam centre irrespective of the angle of intersection. This is subject only to the limitation that the aircraft must be within the confines of the beam at some time during the bracketing turns, otherwise, of course, the rate of cutting the beam will be zero. As an illustration of this principle, take the case of the aircraft in Fig. 1, being flown manually across the beam along the track a-b. The displacement signal-and the rate of crossing the beam are shown graphically in Fig. 2. If now the displacement and rate components are translated into turn-producing signals which are allowed to turn the aircraft, these signals will be of the order of those shown graphically in Fig. 3. If an ideal "bracket" of the beam TURNLEFT SIGNAL TURN RIGHTSIGNAL f RESULTANT DISPLACEMENT ^T APPROX. HALFWAY INTO OPPOSrTE SIDE OF BEAM \ BEAU CENTRE CROSSED TIME BACK TO BEAM CENTRE ON CORRECT HEADINGFig. 5-
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