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Aviation History
1956
1956 - 0243.PDF
ter and appearance have been essentially unaltered, despite astonishing increases in the powers it has had to harness and the control functions it has been called upon to perform. For many applications it will remain much the same for as far ahead as can be foreseen: but for high-speed flight a new formula is required. The reason is simply that, in order to absorb the very high take-off and cruising powers offered by the new turbines, and to achieve high flight speeds without noisy tip speeds, very wide blades are needed. A blade of conventional width trying to handle such large take-off powers finds sections of itself working at incidences above the stall, and stalled flutter can result, causing high torsional stresses. Wide chords effectively reduce incidence, and torsional stiff- ness can be preserved without too heavy a weight penalty by the use of hollow steel blades. But the larger the blade, the greater the centrifugal and aerodynamic twisting moments acting on it. In normal flight, with power on, centrifugal and aerodynamic twisting moments act in opposing senses, and are always tending to cancel each other out; but, if an engine sud- denly loses power (or is throttled back) and the airscrew starts to windmill, the aerodynamic twisting moment changes sign. Unless opposed by the pitch-change mechanism the blades fine-off rapidly, causing engine overspeed, sudden asymmetric drag, and perhaps loss of control. Clearjy, the higher the flight speed and the bigger the blades the greater the power required by the pitch-change mechanism to prevent them from fining- off in such circumstances. More pitch-changing power requires either a larger mechanism, or—with a hydraulic system—a higher-pressure oil supply. A bigger mechanism is ruled out on the score Another impression of the new "500 m.p.h." DM. airscrew, first details of which are given in the text. It shows the blades feathered: in the illustration on p. 237 they are in the take-off position. RPM SELECTION CONTJ3NTROL SHAFT OUTPUT TO ROTATING SECTION SERVO PINION HELICAL PITCH CHANGE MOTOR DISTRIBUTOR VALVE PRESSURISED OIL SUPPLY Fig. 1. of weight, engine-shaft bending moments, and bulk, only answer lies in the use of higher oil pressures. The conventional airscrew system in which oil is supplied from a controller mounted on the engine via a transfer bearing on the engine shaft has proved a convenient and efficient arrangement where operating pressures can be kept below about 800 lb/sq in. If airscrew operating pressure is increased beyond this value, however, an unacceptable leakage rate from the transfer bearing is experienced. The self-contained type of hydraulic airscrew, introduced some time ago by Hamilton Standard of America, eliminates the transfer bearing by including the entire oil system and control mechanism within the airscrew itself, so permitting the use of the higher work- ing pressures demanded by the larger airscrews. This system has been adopted by de Havilland to exploit to the full the potentialities of the new British turbine engines. Prominent among these is the Bristol BE.25, which in its developed form will power the 1960 family of 500 m.p.n. SHAFT ORIVE FROM SERVO PINI- Fig. 2. The Bristol 187 airliners. It is for this engine that the new airscrew is intended. The initial version of the BE.25 of about 4,000 sJi.p., as ordered by B.O.A.C. for its Britannia fleet, will be fitted with airscrews which are substantially similar to the existing D.H. units for the Proteus. The illustration on this page (another view is on page 237) shows an impression of die new airscrew as it will look installed on the BE.25 engine. The very wide blades are of the hollow steel variety having approximately constant chord, and have N.A.C.A. series 16 sections of very low thickness/ chord ratio and camber. The airscrew is designed specific- ally for the cruise condition, and the blade sections are chosen to give satisfactory L/D ratios at the transonic relative speeds at which they must work in this con- dition. The illustration, which shows the airscrew blades in the feathered position, emphasizes the blade-root and spinner fairings necessary to pre- serve low thickness-chord ratio right down to the spinner surfaces; the fixed spinner fairings are designed to coincide with the blade in the design cruise condition to give maxi- mum engine air intake efficiency. The circular intake at the spinner nose admits air to cool the closed hydraulic system of the propeller, in which considerable heat is continu- ously generated. It should be noted that although the airscrew is designed for cruising conditions, its take-off performance is entirely satisfactory owing to the great blade width neces- sary to absorb the cruising power of the BE.25 engine at altitude. Sufficient pitch range is provided to permit full power-on reversing for landing-brake purposes. The control system will incorporate synchrophazing; and provision is made in the design for precise manual control of blade angle and therefore of thrust, whilst the engine is controlled by fuel flow at con- stant r.p.m., to facilitate manoeuvring on die ground. The hub consists of a rotating section carrying the blades and pitch-change mechanism and, to the rear, a section held stationary relative to the engine which houses the controller and other ancillary equipment. Each of the two sections has its own oil system, and control signals from the stationary controller are transmitted mechanically to the rotating pitch change mechanism by means of a differential gear system. Fig. 1 is a much simplified diagram of die control servo system in the stationary section of the hub; a small flyweight governor-valve meters oil at comparatively low pressure to a servo rack which consists of a cylinder sliding about a fixed piston. The position of the servo rack is determined by the
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