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Aviation History
1935
1935 - 0993.PDF
MAY 2, 1935. FLIGHT. 467 airscrew. During the acceleration prior to take-off the forward speed V is even smaller, and the airscrew is work- V ing at a lower value of -—- , and consequently at even lower efficiency. Actually the engine speed varies slightly during the take-off. In other words, n varies with V, but not to any very great extent, and it is sufficiently near for our present purpose to assume it constant. Mention has been made above of the fact that airscrew efficiency is influenced by several factors. Although the general efficiency curve of all airscrews is similar to those in the adjacent column, it does differ according to certain features. One of these is the ratio of pitch to dia meter, usually expressed as P/D. The left-hand curve represents an airscrew of fairly low P/D ratio. The right- hand curve relates to an airscrew of greater P/D ratio. It is interesting to note that not only is the maximum efficiency of this airscrew greater, but the curve has a flatter top, so that actually its efficiency is better over a greater range V of forward speeds. In fact, it is seen that from -=,= o.ss to «D V -^=0.96 the efficiency is equal to or greater than the maxi mum efficiency of the low-pitch airscrew. Let it be assumed that the propeller, the efficiency of which is shown in the right-hand curve, is used on a much faster aeroplane, in which the cruising speed is 170 m.p.h. and the take-off speed 50 m.p.h. At 170 m.p.h. V (250 ft./sec), and at the same airscrew speed n, -=r = 0.94, and the corresponding efficiency is 0.71, or 71 per cent. At the other end of the speed scale, at the assumed V take-off speed of 50 m.p.h. (74ft./sec.) —=r —0.28, and the corresponding efficiency is seen to be 0.43, or 43 per cent. "Stalling" of Blades Although they give a general idea of the fundamental principles involved, the two curves do not tell the whole story. For example, the coarser-pitch airscrew, the effi ciency of which is shown in the right-hand curve, would probably slow down the engine at low forward speeds. This would have the effect, because, by reducing n the V value of the ratio —=r would be increased, of bringing the wD ° working point up on the efficiency curve, but unfortunately the power would be correspondingly reduced, so that prob ably there would be an overall loss. Taken to extremes, as in a machine with an unusually wide speed range, it means that the airscrew blades are stalled at low forward speeds of the aeroplane, exactly as an aeroplane wing is stalled, with resulting loss in power and thrust. A prac tical example of this occurred with one of the seaplanes in the last Schneider Trophy Contest. One airscrew was tried which would have given better efficiency at maximum speed than those actually used, but the blades were so badly stalled that the machine could not be persuaded off the water! This is, of course, an extreme example, and Two typical curves which show the comparative efficiency of two airscrews of different pitches. This figure should be studied in conjunction with the text rarely does one get a machine with almost enough power to '' helicopter '' straight upwards and yet incapable of getting off, but it serves to show what can happen in actual practice. An examination of the efficiency curves shows at once that if one could find means of always keeping the air screw at the peak of its efficiency curve a great deal of extra thrust horse-power would become available which could either be used to increase the performance, notably the take-off and the climb (the best rate of climb occur ring in most aeroplanes nearer to the stalling speed than to the maximum speed), or it would enable an engine of lower power to be used for the same performance. A few moments of contemplation reveal that there are two obvious ways in which this could be achieved: By varying n, the rotational speed of the airscrew, and thereby V selecting the value of —j-giving best efficiency, or by vary ing the pitch /diameter ratio. The former method would consist in a variable gearing between the engine and the airscrew, and the latter in varying the pitch or the dia meter. So far, no one has succeeded in designing an air screw of variable diameter, but many are in existence in which the pitch is changed. The efficiency curve of a vari able pitch airscrew would form an envelope curve over the two fixed-pitch curves shown. At very low speeds the pitch/diameter ratio would be low, and therefore able to give a high efficiency for take-off and climb, while at high speed the ratio would be high, and giving a still higher efficiency. Most efficient of all would, of course, be a com bination of variable speed and variable pitch, but the mechanical difficulties would be great and the extra weight considerable. The choice between variable gear ratio and variable pitch is influenced by very many considerations, and it would take us too far to go into them here. Suffice it to say Slotted counterweights and pitch-angle adjustment on the Hamilton Standard. On the right is a view of one of the limit stops which permit independent adjust ment for high-pitch and low-pitch positions in dependently. >__. L>- COUNTERWEIGHT
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