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Aviation History
1958
1958 - 0666.PDF
682 FLIGHT, 16 May 1958 FLIGHT INSTRUMENTS... perature can be fed into the computing elements. Manual setting,fixed probes and moving vanes drive transducers, from which plug-in units (now in production) calculate height, ram ratio,Mach number, angle of attack, angle of yaw, T.A.S., air density, rocket jump angle, missile jump angle and dynamic pressure.Fire-, camera-, engine-, navigation- and flight-control can be provided. Standard A.C. and D.C. power supplies are needed. The newly formed Electronic Controls Division of GeneralControls Company has been awarded a U.S.A.F. contract for the design of an advanced air data computer for the century-seriesfighters. The only air data system so far announced specifically for civilairliners is the Kollsman Integrated Flight Instrument System— in many ways a misleading designation. It is designed principallyto correct normally sensed air data by electrically cross-checking the various quantities before they reach the panel instruments.In case of a failure, direct indications from the sensing elements are automatically restored. Dial instruments show height, speed,Mach number, outside air temperature and true airspeed, the latter on a digital counter. The system operates over the 0 to5O,OOOft and 0 to 450 kt ranges and has been accepted by Douglas for the DC-8. Wing Sensing Devices. For years pilots have relied on theirA.S.I, for guidance on the behaviour of their aircraft. Reliable A.S.I.s were developed relatively soon after the Wright brothersbegan flying and, by adding a number of knots to the approach speed ("five for a wife and two for each child" has come to beregarded as a good insurance in difficult conditions) a pilot might compensate effectively for rough air, or high landing weight. Butthe stall is related to angle of attack and a knowledge of this quantity offers far better guidance on the performance of the wingat any moment. Various sensing devices have been produced to show angle of attack, almost all requiring some form of vanepivoted by airflow relative to wing or fuselage. Because the pivots were extremely difficult to seal efficiently against weather, andbecause it was hard to find an area of undistorted flow on the airframe, little progress was made until relatively recently. Thepilots, too, found it unnatural to fly by angle of attack rather than airspeed. Mechanical difficulties have now been largely overcome andmany stall-warning and lift-sensing devices are on the market for both light and heavy aircraft. Landings on aircraft carriers havealways required the slowest possible approach and precise control of speed with little or no time for the pilot to study his A.S.I,closely. Aural indication of I.A.S., fed into the earphones as an intermittent or constant tone, was therefore developed by KelvinHughes and is standard in Fleet Air Arm fighters. In addition, this company has produced the stall-warning detector Type C,which consists of a spring-loaded flag mounted above the wing in an area where flow breaks away near the stall. When this occursthe airflow ceases to hold the flag parallel to the line of flow and allows it to turn normal to it. A hermetically sealed magneticswitch is then closed to activate the warning system, whether this be an audio tone, warning light or a stick-shaker. An extension of this system is the Kelvin Hughes Audio Inci-dence System, which compares pressure measured at a point under the wing with total head pressure in the normal pitot head.This ratio is a function of the angle of attack; and audio signals proportional to it are fed to the earphones as either intermittentor continuous tones, representing respectively angles of attack below or above an optimum value. A visual indication can also beprovided if required. Pressure senser and sound generator are mounted in a single sealed case and weigh 4.5 lb. In order to make the transition from traditional reliance on anA.S.I, to use of an angle-of-attack indicator more natural for the pilot, Kollsman Instrument Corporation has produced a combined Left, the new Sperry Rotorace gyro, in which the gimbal bearings are rotated con- tinuously to reduce drastically random drift caused by minute bearing imperfections. Above, Safe Flight Instrument Corporation's Autopower with two throttle servos. instrument on which speed is indicated in the normal way whilean angle-of-attack scale moves round the edge of the dial so that the ruling value lies opposite the A.S.I, needle. The moving scaleis marked simply with A, B and C, respectively indicating the angles of attack for best approach, best cruise and stall. Stall-warning indicators have become very popular for lightand business aircraft in the U.S.A. A small vane protruding through the lower part of the leading edge of the wing is generallyset so that near the stall it flips over and closes a switch. Horns or lights are used as indicators. Two further devices, both made by the Safe Flight InstrumentCorporation, have gained wide acceptance by both airline and private operators. The first of these is called Speed Control and,in effect, shows the pilot on a small instrument whether he is obtaining the best lift from his wing. A small vane protrudesthrough the skin under the leading edge and operates a transducer, the electrical signals from which are passed to a computer. Herethe signals are corrected according to a mechanical input from a shaft recording flap position and also according to a longitudinalaccelerometer signal directly related to the power-drag condition of the aircraft. The integrated signal is displayed as a needledeflection against a scale showing "slow," "fast" or "best" approach speed. Speed Control can be used during the approach, on take-offwhen the best climb is required, or during slow-speed flying. It gives a true indication of wing performance in all aircraft con-figurations and it is free from the lag suffered by conventional A.S.I.S. Over 40,000 Speed Controls have been sold. The second Safe Flight device is called Autopower. Signalsfrom the Speed Control are passed through a lift coupler to a servo amplifier and thence to servos in the throttle control cables, allow-ing the lift signals to be directly applied to the engine power setting, so causing the aircraft to maintain its best speed forapproach or slow flying. Autopower may be fitted in conjunction with (but quite separately from) a standard autopilot; and thiswas in fact done in a DC-3 when making automatic landings with the Bell Aircraft automatic landing system. It was found that thehuman pilot could not control the throttles accurately enough to avoid touching down too fast or slowly when under automaticglide-path guidance. Speed Control and Autopower placed the aircraft at the touch-down position at exactly the right speed fora successful landing. Autopower may be set to maintain an air- craft at specified percentages of its Vsl. During the Bell trialsthe initial value set was 1.6 Vsl, and reduced to 1.4 Vsl shortly before touch-down. At about 23,000 lb weight, the DC-3 firstapproached at 95 kt and then at 84 kt. When full flap was applied just before touch-down, the speed decreased to 80 kt. Groundeffect was automatically compensated for by Autopower. The big jets will probably require some form of instrument toindicate early during the take-off run whether sufficient accelera- tion is being developed for unsticking in the expected distance.The Services have tackled this problem by painting a white marker a given distance from the threshold of each runway. The crewscalculate, for their particular loading, what speed they should have reached when passing this line. Civil operators have pro-posed an instrument which would indicate acceleration against some fiducial mark and show instantly how the take-off wasprogressing. Canadian Marconi exhibited such a system, called the Take-off Monitor, three years ago. It continually indicatesthe relation between airspeed and acceleration throughout the take-off run, showing clearly whether take-off speed will bereached in the normal distance. C. M. L. The Sate Flight Speed Control indicator for mounting on the cockpit coaming. It is 2%in wide.
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