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Aviation History
1947
1947 - 1293.PDF
AUGUST 7TH, 1947 FLIGHT Airscrews for Gas Turbines 1 direct-connected type requires an airscrew with provision for a superfine pitch (approximately zero) in order to ensure easy initial starting of the engine, maintain suitable idling conditions, and allow quick acceleration of the engine up to maximum r.p.m. Further, the extra range afforded by allowing the airscrew to constant-speed at the superfine angle prevents the serious situation arising of the engine stalling when coming in to land. Further addition to the airscrew system is required in the provision of a mechanical safety stop at an appro- priate safe positive pitch which will prevent the blades ^.advertently moving into superfine pitch should an air- screw control system failure occur in normal flight. At MAX. PERMISSIBLE OVERSPEED R.P.M. PRER4RATI0N NORMAL ^MAX.R.P.M. FOR DIVE DIVE ENTRY EMERGENCY DIVE EXIT -A.O.L. SETTLING DOWN TO STEADY DIVE R.P.M. -3MY0RMAL MAX.R.P.M. MAX. • PERMISSIBLE OVERSPEED R.P.M. 3 4 SECONDS Fig. A. Direct-connected turbine engine with airscrew as a dive brake. The normal dive exit would take place at a later period than that shown for the emergency condition, which is indicated merely to show that exit must not occur before this point- the same time, flight will be sustained in the new fixed pitch. Without such a stop, the airscrew and engine would violently overspeed beyond permissible limits, with the most serious consequences. In the case of exceptionally high-speed aircraft, consideration will have to be given to providing a further safety stop at some angle coarser than normal take-off fine in order to prevent serious overspeeding if control system failure should occur at this high speed. The .system is designed to enable the safety stops to be overridden where necessary when coming in to land. For airscrews using negative pitch as a land- ing brake it is essential, in order to prevent overloading the engine under low-power run- ^ning conditions, to have the airscrew under c§nstant-speed control right through to nega- tive pitch. This latter feature adds a require- ment to the airscrew system by having to in- corporate automatic change-over valves to re- verse the sense of the pressure-oil supply and permit constant speeding in negative pitch. Additionally, a second mechanical safety stop is provided at the starting superfine pitch to prevent* the airscrew inadvertently moving through this pitch and into reverse in the event of control system failure. Tf. when coming in to land, such a condition occurs while the aircraft is still airborne, the heavy : reverse pitch would stall the engine and un- doubtedly incur serious consequences. Means are again provided deliberately to override this stop when applying reverse pitch at touchdown as a land- ing brake. In the event of this type of engine being used in a dive- bombing aircraft where the airscrew is employed as a dive brake, braking provision has to be made for a blade angle of approximately -3 deg, this angle being the con- dition at which the airscrew develops sufficient windmill- ing torque to rotate the engine (power off) at an r.p.m. which, combined with the forward speed in the dive and blade angle incidence, will produce the required drag. In changing pitch from the fine angle to the dive-brake angle and vice versa, the blades pass through a pitch range in which the airscrew develops a torque sufficiently great to accelerate the engine to high r.p.m. This period of pitch change must, therefore, be moved through with great rapidity and demands a pitch-change rate of about 40 deg/sec. The speed .to which the engine can be accelerated depends on the inertia of the system ; the direct-connected turbine possesses the highest inertia and motoring power of the engine parts mechanically connected to the air- screw and, hence, the least tendency to overspeeding. Emergency Dive Recovery At the point of dive entry (see Fig. 4) the engine r.p.m.will surge instantaneously to a value within predetermined acceptable limits, but will quickly decrease—taking abouttwo seconds—to a steady figure as the dive progresses It is essential to allow this period of two seconds to elapsebefore any attempt at dive exit is made, otherwise the r.p.m. will not have settled down to a sufficiently lowfigure to prevent overspeeding when coming out of brak- ing pitch. In addition, a delay of approximately ihseconds is necessary, even after the steady dive r.p.m. ha9 been obtained, before the power lever is moved out ofbraking pitch and into the normal forward flight power- on position. This i| seconds' pause is to ensure that thefuel feed is not reopened until the airscrew pitch has reached the normal flight condition. To achieve this, itis necessary to include some form of dashpot in the air- screw control system. The foregoing is only concernedwith emergency recovery; in a normal dive the period is usually long enough to allow the r.p.m. to settle downto steady dive conditions before recovery has to take place. Figure 4 also shows the progressive change in blade anglffrom the dive entry and exit. The compounded-compressor type of turbine functions,•*so far as the airscrew is concerned, very much on the lines of a piston engine inasmuch as no superfine pitch is re-quired for starting or idling. Neither is constant-speeding in reverse pitch required, as the engine cannot whollystall under low-pressure running conditions since the com- PREffcRATION FOR DIVE ^NORMAL MAX. R.P.M. -%• NORMAL MAX.R.P.M. — 0 r—•- SETTLING DOWN TO STEADY DIVE R.P.M. SECONDS Fig. 5. brake, for the Compounded-compressor type turbine engine.with airscrew as dive The normal dive exit would take place at a later period than shown emergency dive exit, which condition is merely to show that exit must not occur before this point.
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