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Aviation History
1941
1941 - 2380.PDF
246 HT OCTOBER O,TH, 1941. CORRESPONDENCE CONTRA-ROTATING AIRSCREWS If the Engine Bolts Sheared ? I HAVE a suspicion that your correspondent "Mystified"dropped his query into your letter box with his tongue in his cheek and promised himself a lot of quiet fun when theintellects and amateurs had swallowed the bait. It is ralher like the story of the man who borrowed a poundnoLe and after numerous transactions repaid his loan and found he had "made" a pair of boots for nothing. It was beingtold some years ago and reduced financial and banking experts to a frenzy." Mystified "gives the game away when he asks the crucial question, " How does the engine know what happens to thetorque after it has delivered it to the crankshaft? " It doesn't know or care. All it does know is that no matterhow many gadgets and refinements the wise guys button on to the far end of the crankshaft, the dashed thing is just ashard to turn around and the measure of its effort to keep on turning it around can still be torque reaction, but located beforethe crankshaft enters any gearing. Put another way: if all the holding down bolts were sheared, the engine would make thesame effort to emulate a Catherine-wheel as if no such things as contra-props had been thought of. " R. N." Electrical Analogy I AM not an engineer, but it seems to me that the argumentsof '' Mystified '' are based on the fallacy that the torque of the engine, including the reduction gearing, is necessarily trans-mitted to the rest of the aircraft. The expression " torque " seems to be used in two senses,one being that force produced by the engine (engine torque), and the other that force which tends to rotate an aircraft inflight about its fore-and-aft axis (aircraft torque). The engine torque may be compared to an electric currentwhich "flows" from the crankshaft through the driven parts back to the engine block. In a normal aircraft this current"flows" through the airscrew to the air, thus producing air- craft torque, and from the air to the wings and so back to theengine block. • 'With contra-rotating airscrews the current " flows " out toone airscrew and in through the other to the gearing, which is mounted on the engine block, thus completing the "circuit."• Further proof can be obtained kom model aircraft. If such an aeroplane, with a fully wound motor, be suspended by itspropeller, the plane itself will rotate. If a contra-rotating airscrew model is suspended by bothpropellers-, providing the gearing is an integral part of the model, the plane will not rotate. I read this as proof that thereis no torque applied to the aircraft, as in the previous case. If, however, the gearing is held separate from the fuselage,then the plane will rotate. It would seem, therefore, that the gearing is the key to the situation. I agree with Mr. Watts that engine torque is not eliminatedby contra-rotating airscrews, but it is prevented from affecting the aircraft as a whole. A. N. SMITH. Aerial Differentiation WHEN I first perused Dr. Watts' somewhat inconclusiveremarks about this vexed question in the issue of Flightfor Sept. 25th, I must admit that I was tempted to think that he was rather taking advantage of his exalted position in theaircraft industry in permitting himself to be so enigmatic; much in the same way as Cabinet Ministers write when askedto contribute an article to one of the popular Sunday newspapers. On further reflection, however, it occurs to me that his reticencemay have something to do with a very novel form of transmission, particulars of which may not yet be published. After all, the article in Flight did not inform us as to exactlyhow the two propellers revolve in opposite directions ! Like most of your other correspondents, I rashly assumed that thetorque reaction of the reverse-drive gear box was taken by that of the engine crankcase ; in which case, of course, there wouldbe no mystery at all. Not only would there be no tendency for the aeroplane to bank over but, also, torque reaction on theengine mounting would be eliminated. Fortunately for my professional reputation (if any) my letter was not published,because I now propose to show that it may be possible to secure contra-rotation of the two propellers in a manner which istorsionally independent of crankcase reaction. As Dr. Watts reminds us, the controversy centres round atype of contra-propeller which can be attached to the single shaft of existing engines. From this we may infer that noanchorage to the engine crankcase or mounting is necessary. The accompanying diagram is offered as an explanation as tohow this might possibly be done. E is the engine shaft to which the bevel A is fixed. Both the carrier D of the planet bevels Cand the bevel B can rotate loosely on an extension of shaft E. The outer propeller is attached to B and the inner one to D asshown by the shaded sections, and their air reactions are denoted by the arrows. Now at first sight it would appear that such an arrangementwould simply rotate in one direction en bloc. However, if both propellers rotate in the same direction the magnitudes of theirair reactions will be unequal and one propeller will lag behind. Immediately this occurs the other propeller will be urged inthe opposite direction. For supposing E to rotate in the direc-' tion of the curved arrow, D will be checked by air reaction andB will receive contra-rotation. The scheme constitutes a true differential in the sense that the air torque reactions of the twopropellers must always be equal and opposite, and if both pro- pellers are to rotate at the same speed their air torque reactionsmust be equai at that speed. Otherwise one propeller would rotate faster than the other until the air torque reactions wereequal. If the reference letters A, B and C also represent the pitchdiameters, or numbers of teeth, we can determine how many revolutions A would make in the direction of the arrow if Dand B each made one revolution in opposite directions simul- taneously. Assume B fixed, then D makes one rev (_ and A makes B * 1 + ^-r But during this time B makes one rev ^) which imparts a further — tevs f to A. Hence ,A makes 1 -f- -^- = -—— A ^ A A revs (. A Conversely, if A makes one rev, D and B each make A + 2Brevs in opposite directions. This device, as well as securing contra-rotation, also incor-porates the airscrew reduction gear. For example, if propeller speed is to be half engine speed ———— = £, or A = 2B as- drawn. But any reasonable reduction can be obtained by asmall change in the relative values of A and B. If such an arrangement is feasible it is quite true that, as" Mystified " says, " the engine does not know what is happening to the torque " and torque reaction must therefore be imposedon the fuselage or wings via the engine mounting in the usual manner.We now come to the crux of the problem. If a single engine mounted on the fuselage is fitted with the Rotol contra-prop,will the machine as a whole tend to drop one wing or will it not ? My answer to this is that it definitely will do so. You willnote that the differential gear I have described only divides the torque equally in both directions when the engine shaft rotatesin one particular direction. Hence to the engine, the contra- prop only represents another form of dynamometer ; the airas a whole is the brake and the aeroplane will twist about in it in just the same way as the baseplate and foundations betweenthe engine and dynamometer anchorages are twisted in the test house. . V. MARTEN-GWILLIAM.
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