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Aviation History
1953
1953 - 0923.PDF
17 July 1953 'OL AND P FRAME Fig. 1. (Left) A freely supported gyro. The gyro wheel is attached to a frame by a pair of gimbal rings, and the gyro axis remains fixed whatever the movement of the frame. Fig. 2. (Right) The spring-restrained gyro, attached to the frame (or aircraft) by a single gimbal ring controlled by a centering spring. This type of gyro is the basis of instruments indicating rates of roll, turn or pitch. It will be noted that the magnitude of the moment is propor tional to the product of the r.p.m. of the engine or airscrew and the moment of inertia of its rotating parts (see preceding table). The early combination of a rotary engine (almost the whole engine rotating) plus an airscrew rotating in the same direction gave a large value of I £ . Pilots who flew Camels in the First World War remember that the fall of the nose during a tight, right-hand turn was so powerful that almost full control deflections were needed. The rotary engine eventually disappeared and the moment of inertia of the rotating parts of the later piston engines remained small. That of airscrews got much larger, and the inertia and speed of revolution of present-day airscrews are of the same order as those of the old rotary engines. The inertia of the aeroplane has, however, also increased and the gyroscopic effect on the whole aeroplane, even of several piston engine-airscrew combinations, is small. It is further reduced if contra-rotating airscrews are fitted. The change to turbine engines has led to high rotational speeds with moments of inertia which are less than those of an airscrew but are, nevertheless, considerable. The product of the two indicates that gyroscopic torque from the pitching or yawing of a turbojet may be several times as big as that from a modern airscrew. The turboprop combination could thus give high gyroscopic forces but, in practice, the directions of rotation of the turbine and airscrew are arranged to be opposite. The gyroscopic couple also depends on the rate of yawing or pitching to which the aeroplane is being subjected. This probably reaches its greatest value during the spin when it may reach 5 radians per second. Similar values are likely during aerobatics (e.g., a stalled turn) both in yawing and pitching, but the greatest rate of pitching (q) occurring in a pull-out is unlikely to exceed 1 radian per second. Since the engine bearings are designed partly by the gyroscopic load it follows that an engine designed for an aerobatic aeroplane is unnecessarily strong, heavy and expensive for a large, civil aeroplane on which aerobatics and spinning are prohibited. Accidents to Comets in which a nose-up attitude was reached during take-off are unlikely to have been connected with these gyroscopic forces. The four Ghost engines rotate in the same direction and each has an I CI of about 6,ooo slug-ft2/sec. At a steady rate of yaw of 0.2 radians per second (equivalent to motion in a circle of only 500ft radius at iooft/sec) the nose-up pitching moment is: M = 4.1£2.r = 4 >: 6,000 < 0.2 = 4,800 lb ft On an aeroplane as large as the Comet this moment would be developed at iooft/sec by only one or two degrees of down elevator movement. In any case, the occurrence of a prolonged turn during the take-off is very improbable. A series of swings of the nose from side to side would result in a series of pitching moments up and down which we have shown to be of a size unlikely to worry the pilot. . Nevertheless, the interconnection of longitudinal and lateral controls on any vehicle makes the pilot's task complicated. There are some aerodynamic couplings between the two (e.g., rudder application sometimes leads to a nose-down moment) and these can usually be avoided, but the gyroscopic coupling is avoided only if the engines are handed; that is, if two engines rotate clock wise and two anti-clockwise. In this case there is no resultant gyroscopic moment on the aeroplane, although the bearings still have to carry gyroscopic loads. Helicopters.—Some interesting gyroscopic problems arise in the helicopter rotor. A typical helicopter rotor has a value of I O of 40,000 slug-ft2/sec, so that any attempt to tilt the shaft will result in a very small precession; alternatively, very large external moments will be required to obtain the rotor tilt necessary for helicopter control. In fact, the blades are usually universally jointed to the hub and the disc can be tilted by the use of an aerodynamic servo—that is, the feathering of each blade so that the aerodynamic force generated does the necessary work. The control problems are thus not so formidable as they appear. A recent paper by Fitzwilliams§ suggested that, for a large helicopter, the rotor drive might be supplied by turbojet engines mounted with the axes across the blade tips. Each turbojet would then be yawing at the angular velocity of the rotor, say 10 radians per second, so that very large gyroscopic couples would be applied to the blades, tending to twist them. In addition to the gyroscopic load on the turbine bearings the centrifugal load would be equal to that during a pull-out at about 200 g. If the turbojet were placed so that its axis lay along the blade (the entry duct and jet pipe turning through 90 deg) the gyroscopic moment would apply a bending moment to the blade which could be arranged to relieve the bending moment due to the blade aerodynamic loads. The centrifugal load on the bearings would then be taken on thrust bearings. The existence of the gyroscopic properties of a rotor are made use of in the "Bell bar." The Bell bar consists of a set of weights rotating with the rotor and forming a gyro. Any change of attitude of tne helicopter fuselage does not immediately affect the plane of this gyro, which tends to remain fixed in space, since the fuselage is universally jointed to the rotor. The tilt of the fuselage relative to the disc is measured by a lever system attached to the rotating Bell bar, and this applies appropriate feathering to the rotor blades which cause a disc tilt until the chosen attitude of the disc to fuselage is regained. The rotor is thus made the gyro of an automatic pilot. "Flying Saucers."—An account which appeared in The Times of April 22nd, 1953, described a projected "flying saucer" aircraft in which "use is made of the gyroscopic effect of a rotating power plant to acquire stability." If the power plant rotates in one direction only about a vertical axis which is rigidly locked in the airframe, then a pitching disturbance induces a rolffng disturbance. Similarly, a rolling disturbance causes a pitching motion. If, however, the power plant had two elements rotating in opposite directions (having equal I Q) the precession effect would be zero and the interconnection between rolling and pitching eliminated. The gyroscopic effect would then be to stabilize the oscillations of the aircraft. This effect of a gyroscope rigidly connected to an oscillating body has been studied in connection with mono-rail cars** and it has been shown that a car having its centre of gravity above the rail will be stable if a vertical axis gyro has sufficient angular momentum. The "flying saucer" might well have little aerodynamic static stability and the contribution from the gyroscope could then be of importance. § "Journal of the Helicopter Association," Vol. 5, No. 4 (1952). ** See, for example, Ferry: "Applied Gyrodynamics," page 259. John Wiley, 1932.
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