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February, 1942 AIRCRAFT ENGINEERING 49 A Letter from Mr. Cleaver, with the Author's Reply a t the design rate of advance. Now, for take To the Editor the rated take-off r.p.m. can be maintained by off, the whole blade is rotated to a finer pitch DEAR SIR, decreasing the pitch. Torque reaction might still be serious, even a t the rated take-off r.p.m., and thus the twist is no longer helical but is With reference to Mr. Andrews's interesting and this might still justify throttling. How such that the blade incidence at the tip is a article in the December issue of AIRCRAFT minimum and at the root a maximum. Thus ever, the main likelihood of throttling being ENGINEERING, there appears to have been some it is seen that only one intermediate section worth while, in the case of a constant speed error in the constant used in equation (18). The works at the correct incidence. This throws airscrew, would be to increase the thrust by text gives a value of 60,000, but the correct into high relief the value of blade root cuffs unstalling the blades at the low power. The for take-off. point which it is desired to emphasize is that value would appear to be 83,100 there is no merit whatever in unstalling an air In the second point raised by Mr. Cleaver, he screw blade unless more thrust is so obtained; on With this alteration, the diameters chosen in says that with a constant speed airscrew the p . 343 it appears to be suggested that throttling Table I appear much more reasonable. For rated take-off r.p.m. can be maintained by might be worth while even if the take-off example, for a four-blader of solidity = ·15, a varying the pitch. As stated in the article this run were lengthened. This could only be be diameter of 8·49 ft. would be undesirably small is not true in certain circumstances. Perhaps for a design condition of 1,750 b.h.p./tip speed cause of advantages associated with reduction th e following will elucidate this point. = ·9a/10,000 ft./400 m.p.h. The revised con of torque reaction, which arc not directly Let the rated take-off power be the maximum stant referred to above would give a diameter associated with blade stalling at all. The same permissible power input to the airscrew. In of 10·0 ft., which compares with about 10·5 ft. direct torque reaction is obtained from a given this condition the engine delivers a certain rated take-off power an d airscrew r.p.m. whether selected by the standard de Havilland Airscrews crankshaft torque which is increased at the air th e airscrew used to satisfy these conditions is method for a wide four-blader at the tip speed screw hub by the reduction gearing. Now, at stalled or not. It is true that slip-stream quoted. constant rotational speed and rate of advance effects on the aircraft structure might compli On the general question of the desirability of (considered to be zero), the airscrew can only cate this issue, but Mr. Andrews docs not make two-speed gearing, it is interesting to note that absorb a certain amount of torque depending on it clear that he is considering these. the modern trend towards very high operating the blade setting. If this blade setting (which Yours faithfully, altitudes has largely removed the need for such governs the blade incidence) be increased the a device. This is because the large diameter, torque absorbed will also increase—this being A. V. Cleaver, high solidity airscrews chosen for such installa the principle of the constant speed airscrew. Technical Assistant (Airscrews), tions give good take-off thrust, and could not Now, when the blades are a t the stall the thrust Engineering Dept. benefit very largely from increased take-off coefficient is a maximum, and if the torque The de Havilland Aircraft Co. Ltd., r.p.m. It is agreed, however, tha t the develop absorbed by the blades is less than the hub Edgware. ment of very fast, low altitude aircraft of the torque the blade setting will increase, thus in December 30, 1941. type envisaged by Mr. Andrews, would make creasing the blade torque but stalling the blades two-speed gearing highly desirable. One won and reducing the thrust coefficient. To the Editor ders whether aircraft development will soon DISAR SIR, When considering the small diameter air show this trend, since at the moment much Many thanks for forwarding me Mr. Cleaver's screws necessary for high speeds the case can more emphasis seems to be placed on high letter. As he rightly points out, an incorrect arise in which whatever the blade setting the speeds at high altitudes. blade torque is always less tha n the hub torque. value for the constant of equation (18) has been In connexion with Mr. Andrews's particular I n these conditions the engine and airscrew used. examination of the two-speed gear problem, it will race, since torque balance is a physical I am most interested to learn that the di is desired to make two points. impossibility. Therefore, to remedy this state ameters obtained from this method compare (1) Any examination of this question which of affairs, the hub torque must be reduced. favourably with those obtained from the stan includes no estimate of compressibility losses is This end ma y be achieved in either of two ways dard de Havilland method. As pointed out in in serious danger of giving misleading results, the Summary, the article does not yet allow —(a) by reducing the power input, or (b) by because a t take-off, when the blades are operat for tip speed effects on induced velocity. The increasing the reduction gearing. In the former case we have stalled blades and reduced input, ing at large incidences (whether actually stalled investigation of thi s aspect is now in a n advanced and in the latter case we have unstalled blades or not), such losses occur a t quite low values of state and has already shown the desirability of tip speed. and maximum input. It is inherent that this a slight increase in diameter over that obtained latte r case will result in the maximum thrust from the charts. This indicates that for the (2) In any case, unless values of thrust and it was on this score that the calculated obtained arc quoted at the end of the calcula design case of 1,750 b.h.p./tip speed = ·9a/ thrus t figures were omitted from the text. tions concerned, it is practically impossible to 10,000 ft./400 m.p.h. the resulting diameter will be in even closer agreement with the D.H . figure form an estimate of the true value of two-speed Mr. Cleaver's statement that the true value of 10·5 ft. gearing. In this connexion, Mr. Andrews's of the two-speed gear is impossible to assess on observations on p. 343 seem rather obscure. With regard to the first point raised by Mr. this basis is therefore not justified when con On the old Schneider Trophy seaplanes, the Cleaver, whilst appreciating the importance of sidered from this aspect. badly stalled fixed pitch airscrews held down including compressibility effects, it should be Yours faithfully, the r.p.m. to such a low figure (well below rated borne in mind that during take-off the blade E. John Andrews, take-off r.p.m.) that torque reaction was dan tips arc not operating at such large incidences Senior Technical Assistant, gerously high and the engine must have been as Mr. Cleaver would suggest. This arises from Aerodynamic Dept. near detonation point. The engine was there th e helical twist given to the airscrew blades in General Aircraft, Ltd., fore throttled back to reduce these tendencies. order that the blade sections work at approxi Feltham. On a modern constant speed airscrew, however, mately constant incidence when moving forward January 9, 1942. 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Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Feb 1, 1942
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