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Aviation History
1936
1936 - 0529.PDF
SUPPLEMENT TO FLIGHT 22Sd . 12 THE AIRCRAFT ENGINEER FEBRUARY 27, 1936 as regards static torque of full-scale conditions. But in order to arrive at any relative results it has been found essential to use some set of curves based on tests of similar blades. If, as I admit is possible, the stalling point is somewhat more delayed and the static thrust higher under full-scale conditions, the various speeds I am quoting will alter, but the relative results will remain substantially the same. (iii) Fixed Pitch and Variable Gear, (iv) Fixed Pitch— * Two-Speed Gear. There has from time to time been discussion as to whether a two-speed gear would constitute an alternative to variable pitch. The answer is definitely " No," because it does not prevent the airscrew blades stalling. The purpose of the two-speed gear is to allow the engine to maintain maxi mum r.p.m in spite of the airscrew running slower. The increase in power will slightly increase the airscrew speed and will result in an increased thrust, but this increase in the static rate of rotation causes the airscrew to commence stalling at an even higher speed than before. Fig. 7 shows the improvement in thrust due to the use of a two-speed gear for an airscrew of 1.5D pitch (aircraft speed 250-280 m.p.h.). To make a fair comparison it is again assumed that the second gear will come into opera tion on climb at a speed between 0.55 and 0.6 of the top of stall for all conditions of flight. An even better result would be obtained by using a higher gear and dropping to a still lower pitch, but there are obvious limits to such a procedure. The lower the second pitch the higher the airscrew revolutions, and either very high tip speeds would be encountered at low speeds (a condition in which high tip speed is most detrimental), or alternatively the primary rate of rotation would have to be correspondingly lower with consequently larger and heavier airscrews. In this particular case if the tip speed in the second gear is goo feet/sec, then the tip speed in the designed condition will be 730 feet/sec. Fig. 8 shows how closely these combinations approach the ideal. The static thrust of the fixed pitch, two-pitch two-speed, and ideal are 1.29, 3.06, and 3.54 respectively. To complete the story, note the thrust curve for the con stant speed variable pitch with two speed gear with its static value of 3.3, an increase of 256 per cent, over the ordinary fixed pitch and within 7 per cent, of the ideal. The two-speed gear, together with variable pitch, has a further advantage. The airscrew runs in its top gear only for take-off and climb, but most of its flying is done in the low gear with a reduction of 20 per cent, in tip speed. Apart from improved efficiency, the airscrew will be less noisy, a point of importance for both military and civil aircraft. 2-5 ^*Kv s. '"•Vsj. VT > , , FIG. II. ^7 ^STA^ TOP SPEED CONSTANT ALTITUDt * ' ltO-120 M.P.H. - • -I -2 -2 4 speed. The result compared with the two-pitch shown in Fig. 6 is scarcely worth the expenditure and weight involved. It forms no alternative to a variable pitch or a two-pitch airscrew, and the more difficult and complicated suggestion of a variable gear automatically changing to keep the engine revolutions constant is little better. The static thrust in the case chosen is 1.29 for the fixed pitch, 1.48 for the two-speed gear, 1.56 for a variable gear, compared with 3.54 for the ideal (v) Two Pitches—Two-Speed Gear, (vi) Variable Pitch Two-Speed Gear. As an alternative to variable pitch there is no field for the two-speed gear, but it has a field, and for high-speed aircraft a very valuable field, as an addition for use with variable pitch. Indeed, I hope to show that by a combina tion of these two devices the ideal thrust curve can be closely approximated to. In referring to the performance of the two-pitch airscrew, it was shown that for speeds of 250 m.p.h. or more, even in the second pitch, the blade would stall on the ground. In order to prevent stalling it is necessary to drop down to a pitch of 1.0D or less. To do this with the fixed gear engine would mean over-revving the engine on the ground. This could be prevented by introducing a second gear which would allow the airscrew to run faster to the necessary extent, but with the engine kept to its permissible r.p.m. For the case under consideration I have used a gear of 1.235, and thus have been able to drop down from a high pitch of 1.5Z) to a low pitch of 0.8D, a pitch which is well out Fig. 9 shows the thrust curves of all the various com binations superimposed. Fig. 10 shows similar thrust curves for the same combina tions, but for an airscrew with a pitch of 2.5D corresponding to a speed of 450 to 500 m.p.h. The general order of merit remains the same, but their relative values have altered. All combinations except one are now stalling badly. The exception is the constant speed airscrew on the two-speed engine, which follows closely the ideal curve. Fig. n shows the opposite end of the range illustrated by thrust curves for an airscrew with a pitch of 0.7D, cor responding to no to 120 m.p.h. In this case all the curves lie so closely together (and are not all shown), and so near the fixed pitch airscrew that, as previously stated, the gain to be obtained by variable pitch or variable speed is not worth the price to be paid. The question of immediate interest is at what speed does variable pitch become worth while and at what speed may we expect the further addition of a two-speed gear to be come desirable. This can best be answered by reference to Fig. 12, which shows the static thrust plotted against pitch. To give a more readily appreciated meaning, a scale of approximately corresponding speeds has I e added. The fixed pitch airscrew designed for a top speed of 140 m.p.h. gives a static thrust equal to nearly 80 per cent, of the ideal. Whatever gain is possible can be ob tained equally well with variable pitch or two-speed ge • Fig. 12 further shows that variable pitch, either in tne two-pitch or constant speed form, only meets the lu
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