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Aviation History
1936
1936 - 0528.PDF
FEBRUARY 27, 1936 11 THE AIRCRAFT ENGINEER SUPPLEMENT 10 FLIGHT 228c creases the static thrust of the 1.5 pitch 275 per cent. A 0.fD pitch airscrew would, generally speaking, be fitted to aircraft having a top speed of no m.p.h. The 1.5D pitch to an aircraft of 250 m.p.h. Herein lies not only the reasons and justifications for variable pitch, but also the explanation for its late arrival. In the days of aircraft having airscrews with pitches less than their diameter, which, roughly speaking, means speeds of less than 150 m.p.h., variable pitch was not worth while. Not until aircraft of greater speed came into production was there sufficient incentive to attract the necessary capital for the development of V.P. airscrews. I think you will agree that an increase in static thrust of 13 per cent, would not have justified the increased weight, cost and complications. When speeds increased over this " no man's land" lying in the region of 150 m.p.h. the situa tion completely altered, and the conditions necessary to justify V.P. airscrews arose. As I have already shown, and as I shall emphasise more fully as I proceed, the con sideration which so completely altered the outlook was blade stalling. The conventional idea that the object of variable pitch variable gear is to maintain constant engine r.p.m. is not sound. The real vital object is to prevent blade stalling. Any device which does not do this fails to justify itself. blades are fitted with counterweights, and when the oil pressure is released the centrifugal turning moment on those counterweights is such that it overcomes the natural turn ing moment on the blades and forces them into high pitch setting. This method of operation has the advantage that oil pressure, with the possible troubles due to leakage, is only used for the relatively shorter period of flight in the low pitch setting. The second case is illustrated by the Curtiss Wright. In principle, this is simple yet effective. The blades are rotated to any required angle, even negative angles, by a 36,000 to 1 gearing driven by a I h.p. electric motor. In both the above types the pitch is set at the will of the pilot —in the case of the Hamilton to one of two pitches, and in the case of the Curtiss to any desired position. For both types automatic constant speed devices are being developed. If I do not show or even briefly describe the Hele-Shaw Beacham variable pitch airscrew, it is only because it has already been fully described in a paper to this Society by Dr. Hele-Shaw in 1928. It is of the constant speed type, yet controllable in that the pilot can set it to run at any predetermined constant rate of rotation. Like the Hamilton, it operates between a minimum and a maximum pitch, whilst the Curtiss Wright can be set at any angle in the 360 deg. range, a feature with certain obvious advantages. & ^\\ FIG. 8 1-5 oi* 'o* sme-»o->4o urn 1 "\. •40 v/v I have indicated an ideal airscrew. I want now to examine a number of more or less practical devices and to see how closely they approach this ideal. I hope to show that with some of them the ideal can be nearly realised. (i) Two pitches—fixed gear. (ii) Variable pitch—fixed gear. (iii) Fixed pitch—variable gear. (iv) Fixed pitch—two-speed gear. (v) Two pitches—two-speed gear. (vi) Variable pitch—two-speed gear. For each combination I propose to confine myself to three airscrews with primary pitches 0.7, 1.5 and 2.5 dia meters. These in practice correspond roughly to speeds o no, 250 and 450 m.p.h. respectively, and two types of engine are considered : — (a) Non-supercharged. (b) Supercharged to maintain constant power up to, say, 20,000 ft. (a) Non-supercharged Engine W Two Pitches—Fixed Gear, and ii) Speed and (ii) Constant Variable Pitch and Fixed Gear. Ine first case is well exemplified in the Hamilton two- P«cn controllable airscrew. This device is a hydraulically- operated mechanism. Oil pressure plus the natural twisting moment moves the blade into its low pitch position. The Having briefly referred to the actual mechanisms, let us examine the effect of the change of pitch in terms of thrust and aircraft speed. For purpose of illustration, I am assuming an airscrew having a primary pitch of 1.5 diameters corresponding roughly to a speed of 250 to 280 m.p.h. Note on Fig. 4 how the airscrews are working. A represents the designed conditions. The fixed pitch screw works along a torque curve from A to E. At low speeds it is working in the stalled region. The two-pitch works in high pitch along a torque curve from A to C. At C, which corresponds to a climbing speed between 0.55 and 0.6 of the level speed, the pitch is changed and the air screw continues to work from climbing speed to static condi tions along a torque curve shown by BD. ' You will note that in the static condition the change of pitch has not brought the screw out of the stalled region. The constant speed screw works along a constant torque curve ABF but just fails to avoid stalling. The ideal airscrew works along the torque curve shown as AGK completely clear of stalling. Fig. 6 shows the resulting thrusts, both the two-pitch and the constant speed airscrewrs fall short of the ideal, but are marked improvements on the fixed pitch. Note the fall of thrust with the two-pitch, due to entering the stalled region near static conditions. The static thrusts are 1.29 for the fixed pitch, 2.18 for the two-pitch, 2.47 for the constant pitch, compared with 3.54 for the ideal. There is evidence and also experience which leads one to think that the curves used are not fully representative
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