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Aviation History
1941
1941 - 2442.PDF
FLIGHT OCTOBER I6TH, 1941.Correspondence The Editor does not hold himself responsible for the views expressed by correspondents. The names and addresses of the writers, not necessarily for publication, must in all cases accompany letters I CONTRA-ROTATING AIRSCREWS " On the Wrong Track " HOPE I am not too late to enter into this_controversy aboutcontra-rotating airscrews, but the whole answer seems so clear to me that 1 had to write to somebody about it! All the arguments I have so far read in your pages seem tome to be on the wrong track Surely the aerodynamical torque and the structural torque are two entirely different things. Inthe structural torque the airscrew or airscrews do not affect the issue in the slightest. The engine will tend to rotate aroundits crankshaft irrespective ot what happens to that shaft after it leaves the engine. But, then, this torque is surely theproblem of the fuselage and engine-mounting designers, and it probably did not enter the heads of the Rotol designers whenthey were planning contra-rotating screws The aerodynamical torque, however, is a very differentmatter. This is caused by the slipstream tending to rotate the aircraft around its longitudinal axis. This torque is, of course,cancelled by the contra-rotation of the screws, as the slipstream emerges from behind without any twist. I hope you will print these efforts of a hopeful sixteen-year-old, if only to get at the constructive criticism which will surely follow. P. TAYLOR. Arithmetical Exposition .. -; I HAVE noted with amazement the great difference of opinionexpressed in your correspondence columns concerningcounter-rotating airscrews, and shall be most grateful if you will publish this letter belore any more confusion occurs ! It is quite obvious that many people are not clear as to thedifference between the torque reaction in the airframe and that due to piston side thrust. The torque which it is desired toeliminate is the torque reaction on the airframe, as it is this torque which causes " swing " at take-off. The torque reactionon the crankcase due to piston side thrust is an internal concern of the engine, and has no direct effect on the handling of theaeroplane. If one could assume that all the engineers who have arguedover this subject were conversant with Newton's laws of motion, it would be quite unnecessary to have to prove that torquereaction is eliminated by contra-propellers ; the answer would be evident. But, for the benefit of those who must complicatematters by bringing in the torque reaction due to piston side thrust, the following simple reasoning may be of assistance. Consider first a typical geared aero engine such as the" Merlin." Using the dimensions given in Fig. (1) below, where T is the airscrew torque, and clockwise torque reactions asexperienced by the pilot are considered positive, we have T*Torque in crankshaft = -=r- T_ R F1G. I 11 FIG. 2 r| N- Torque reaction on the crankcase due to piston side thrust Tr +)R T The tangential force at the gear teeth is -j^, and this causes normal loads on the reduction gear bearings in the directions shown. The torque reaction on the crankcase due to these bearing loads will be -.,'. .,,,., From (T) and (2), we have the resultant torque reaction on thecrankcase (which is the torque reaction on the airframe) as We have thus proved the obvious fact that the torque reactionon the airlrame is equal and opposite to the torque in the airscrew shaft. Now consider the system shown in Fig. 2, where the airscrewshaft rotation has been reversed by the introduction of an idler gear into the system. The torque reaction due to pistonside thrust is TrJR,, but the reduction gear bearing loads are altered as shown. These bearing loads give no resultant out-of-balance force, but they do give an out-of-balance couple of magnitude T 2T •~ + 2x + rj — — (r, + x)R, :•;.-£; : _ TV .. - - ... ... . ieT-R7 •• •••• •• Combining equations (1) and (4) and writing x for R, we have torque reaction on airframe for (4) and IT r^\+Th vl ~ "R;; + R, _ -IT •••-•••\". = +T .. .,.; ...... .... .. (5) which is equal and opposite to that obtained for Fig. 1.Combining Fig. 1 and Fig. 2, we can have " contra-props," twin crankshafts rotating in the same direction, and zero torquereaction on the airframe. It is easily seen that the twin cranks rotating in the same direction may be replaced by a single shaftgiving twice the torque, and the reaction on the airframe is still zero. The same answer will be obtained for any geared systemdesired, and the torque reaction on the airframe will always be zero so long as the two airscrew torques are equal and opposite;making them so is a job for the airscrew designer to effect by adjusting his diameters, pitches, and chords, but that has nobearing on the argument. .._. ., .. - I am afraid that Mr. Watts was wrong.."...s. •.,.- .. '.'.'. " R.A.F.V.R." •"• ' [M.Sc. (Eng.), A.F.R.Ae.Sv G.I.Mech.E.j. Something More to Explain I HAVE read with great interest the whole correspondencepublished in your issue of September 25th on the subject of counter-rotating airscrews, and 1 have come to the con-clusion that there is something to explain in addition to the statement made by Henry C. Walts in his letter. We read there as follows: " Equally, a counter-rotatingairscrew, one part of which is mounted on the shaft and the other on the engine casing (the latter being free to rotate),would serve the sams puipose." Quite true, but it is necessary to say more exactly, " thelatter being free to rotate together with the engine casing, that is to say, the whole engine must rotate in the bearing?of the engine mounting; one screw is fixed on the shaft and the other on the casing. Two rotating and independent parts(the crankshaft with components and engine casing with accessories) in opposite direction, according to the reactionof a piston on the wall of a cylinder, each equipped with its own screw, will produce neither torque of engine nor torquereaction on the engine mounting of an aircraft. Such a self-counter-babncing engine was designed some
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