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Aviation History
1948
1948 - 2001.PDF
NOVEMBER 25TH, 1948 FLIGHT 639 'esy ue IK Ut'ic ipli I 4 \xocMiosi Installation ofAlvis Leonides and trqflsmission in the ( sured low profile power loss at top speed, and the low discloading a low induced power loss when hovering. Low disc loading and low rotor pitch had contributed to theremarkable safety achieved by rotary wing aircraft in the pre- helicopter era. In contrast to most other helicopters, a resultof the low-pitch operation of the Gyrodyne was that it could not be stalled under any condition of flight by a possible misuseof the controls. Another advantage of low pitch was that it effectively minimized vibration. A significant feature of the Gyrodyne was its independentmeans of propulsion, which enabled both fuselage and tip-path plane to remain substantially level under cruising conditions.This removed an important barrier to the propulsion of heli- copters. Periodic lift distribution in autorotative rotors hadresulted in stalling a part of the retreating blade near the root, but in the helicopter with forwardly-inclined rotor,periodic stalling could occur at the blade tip. Operation of the helicopter close to the periodic stall of the retreating blade waslimited by vibration. No matter how much power was avail- able, it could not be utilized for propulsion beyond theperiodic tip-stall barrier. As higher-powered helicopters be- came available, the feature of independent propulsion wouldbecome more and more essential. Many sources of vibration were inherent in rotary-wing aircraft, but the periodic tipstall was one which could be avoided. Continuity of operation in adverse weather could only be ensured by ability to main-tain a high cruising speed. With the change-over from the autorotative sio the poweredrotor,-the simplicity of control had deteriorated. The number of controls had increased from two to four, and the ever-presentnecessity for torque balance and pitch-throttle synchronization required the four controls to be operated simultaneously. Thiswas a further penalty that had to be paid for the ability to hover. In the Gyrodyne an attempt had been made to minimizethis penalty by suppressing one control. The only flight controls were the stick, the throttle and thepedals. Collective pitch was automatic by giving the drag hinge a slight downward and outwardinclination. Thus there was an immediate increase in manifold pressure whenever the throttle wasopened, without any appreciable change in angular speed. An over-riding control had been designedbut not yet fitted. This was mainly for trimming jf. ' at altitude, but could also be used for momen- ' \.tary hovering before making a power-off landing. The device operated additional hinges, which werelocked in normal flight. The Gyrodyne design also eliminated the usualtorsional'bearings. Instead of a swashplate, the rotor head itself formed the swashplate. Thisdid not mean that the hub axis was tilted, but instead, the hub axis (the axis of the main bear-ings) remained fixed, and the rotor head was tilted relative to the hub axis. In forward flight theforward inclination of the head balanced the back- ward inclination of the tip-path plane, which therefore re-mained substantially at right angles to the hub axis. Stick shake was completely eliminated by the use ofhydraulic irreversibility. Steady loads had also been sup- pressed hydraulically, but it was now thought advisable toretain the steady leads, which provided a natural "feel," and to use hydraulic irreversibility only for suppressing stickshake and as a precaution against: misuse of the controls by violent handling. An electrically operated actuator was usedfor controlling the rate of clutch engagement, and Dr. Bennett thought that a torque-limiting device ought to be an essentialairworthiness requirement. In describing the construction of the blades of the Gyrodyne,Dr. Bennett explained that the blades were of accentuated flexibility in bending but rigid in torsion. The steel tubularspar is of circular section at the root, but for most of its length it is of oval section. The profile is maintained bywooden ribs of plywood skin, the ribs being friction-clipped to the spar. As the Gyrodyne was designed for transport at cruisingspeed, a tailplane and two side fins were provided. No attempt had been made at providing dynamic stability at zeioforward speed. Dr. Bennett also pointed out that the stub wings of the Gyrodyne have a stabilizing effect in roll, whererotary-wing aircraft tend to be too sensitive. Designed as it is for high cruising speed, the Gyrodyne, Dr.Bennett said, is best adapted for fairly long non-stop journeys, such as from city centre to city centre, rather than from cityctifctre to airport. THE BRISTOL 171 HELICOPTER CONFINING himself to points raised by W/C. Brie andCapt. Liptrot, Mr. Raoul Hairier explained how thevarious problems had been attacked in the design of the Bristol 171. He dealt with the subjects of vibration, controls,stability, safety, and capital and maintenance costs. Vibration was a fundamental symptom in rotating-wingflight. It could not be eliminated, but it could be reduced to generally acceptable proportions. Because the factors govern-ing blade feathering were complex, the feathering motion could not be expressed mathematically in a simple form, butonly by an infinite Fourier's Series. The conventional rotor control, which provides a simple sinoidal incidence-variationduring rotation, satisfied the theoretical requirements to a fair degree at low tip speeds. At high tip speeds (high transla-tional speeds) blade lift could not be held constant, and vibration arose. Mechanisms could be imagined which wouldproduce complex cyclic movements, but they would be too complicated, and he regarded the limit of rotating-wing flightas the tip-speed ratio at which higher harmonics become notice- able. Calculations showed that a high tip-speed ratio couldonly be obtained when the blade was flying at a low basic lilt coefficient with an aerofoil section capable of producing ahigh maximum lift coefficient. To delay shock stall an aero- foil section of low thickness/chord ratio (below ten per cent)was desirable near the blade tip. The rotor flight envelope was defined in the cockpit by a very wide r.p.m. range forlanding (this applied to speeds below 35 m.p.h.), a smaller normal r.p.m. range up to speeds indicated on a special scaleon the altimeter, and a narrow r.p.m. range applicable up to maximum speed. Blade drag was mainly caused by blade coning. Even in the171, with its exceptionally small coning angle, -it represented more than 50 per cent of the total mean drag. The drag of allblades added-up *^ectorially to the tota.1^ rotor force in the plane of rotation. \Jhe greater the number of blades the The Bristol 171 rotor is characterized by a high moment of inertia in con- 'unction with an unusually wide speed range. The kinetic energy is 680,000 Ib. ft.
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