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Aviation History
1941
1941 - 2415.PDF
OCTOBER I6TH, 19 JI. FLIGHT TORQUE REACTION FIG 2. FI6.3 FORWARD AIRSCREW P 2p REACTION AT X at the centre of each of these wheels D. These reactions 2P at points X are equivalent to a torque about the thrust line • = 2PXD' = 2XJ^.XD' = 2T,. Now if the wheels D are supported from the crankcase of the engine, this torque reaction of 2T, from the gear box is taken back to the crankcase, where it acts opposite to the engine torque of 2T. We now introduce the condition that T, = T2, i.e., each airscrew absorbs an equal amount of torque Total engine torque 2T = T, + T2 = 2T.,. That is, the torque reaction of 2T., = 2T from the gear box balances the engine torque of 2T. Or simply the system is in equilibrium with no reaction to the engine mounting. Friction losses in the drive to the counter-rotating com- ponent of the airscrew, which have been neglected up to this stage, will now be considered in conjunction with Fig. 4- The three main points of friction loss are: — (1) At the teeth of the gears C and D ; (2) at the teeth of the gears D and E, and (3) at the bearings of the gears D. (1) This loss requires a torque T, in the crankshaft, and a balancing torque T, in the crankcase. (2) This loss is of a similar magnitude to (1), but is transmitted through the gears D. By analogy with the considerations of torque to the airscrew, this requires 'a torque T, in the crankshaft, a balancing torque T, in the crankcase, and a torque 2T, in the gear box. (3) This requires a torque T., in the crankshaft, and a balancing torque T., in the crankcase. This torque gives a force SP at the teeth of the gears C and D, and is reacted at point X by a friction torque in the bearings of gears 1> and a force &P, which puts a torque TL, in the.gear box. Thus the total friction torque reaction from the gear box= 2T[ 4. T,, and balances the torque from the crank- shaft necessary to overcome friction. This discussion could be carried further, but would always show that during conditions of steady running, both components of the airscrew absorbing equal torques, there is no torque reaction to be applied to the engine bearers. Inertia Torque During conditions of unsteady running, i.e., accelerating and decelerating, there will be a resultant inertia torque to be 'taken off. This is the summation of the clockwise and counter-clockwise inertia torques of all rotating parts. We may conclude then from the above that there is no torque reaction to the engine mounting when the airscrew gear box is anchored directly to the engine. If, instead, the gear box is mounted on the airframe, the torque is taken through those parts of the airframe structure which lie between the engine mounting and the gear box mount- ing. This point would only be important in the case of a machine with a layout similar to that of the "Bell Aira- cobra'' with contra-airscrews situated at the end of a long extension drive shaft, and the gear box mounted at the end of the shaft adjacent to the airscrews. The torque loads would then have to be taken through the interme- diate structure. With reference to gyroscope effect, the following points are worthy of note. If the two components of the. airscrew" are of equal Moment of Inertia, and they both rev at the same speed, then the pitching and yawing couples will be equal and opposite for the two components, and there- fore cancel out. This applies for any manoeuvre of the aircraft. These pitching and yawing couples, in cancel- ling out, must, however, be taken in bending of that por- tion of the shaft between the two components of the air- pcrew ; this gives no loads to the engine mounting. In the case of the tandem engine arrangement they must be taken through the structure of the nacelle. In either case this is purely a consideration of internal forces, and there it no external reaction on the airframe.
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