Counter rotating propellers
I am in need of tribal wisdom....
I will be involved in testing a twin engined aircraft, upon which counter rotating engines are being installed. I am not aware that it would have flown this way before. I would like to understand better the reason for the choice which will be made (before I can influence it) as to which engine on which side.
I'm expecting to see the left hand engine on the left wing, as this makes the two propellers rotate toward each other at the top. This seems to be the common arrangement, but is it the best?
Here's the logic argument going on in my mind:
We are choosing the added expense and nuisence of the odd engine so as to improve single engined handling on the critical engine. The critical engine is commonly thought of as being the left. This, because at high power, it is creating torque, which must be overcome with flight control input. The torque of a right hand engine on the right wing will cause a rolling force to the left, which is the same direction toward which flight control inputs will be required to maintain straight and level flight on only that engine, so it is good. That same right engine on the left wing would create a torque force which will roll the plane further away from straight and level flight, and thus require maintaining even more control input, so Vmca goes up because of incresased drag and/or need for speed to make more effective those flight controls at increased deflections.
However, With counter-rotation engines arranged so as to have the propellers moving toward each other at the top, the resultant increased angle of attack (and local lift increase) from the up flowing propwash is on the outboard (of the nacelle) area of the wing, not the inboard. Thus the additional lift on the outboard area of the wing will require more control to counteract that lift 'way out there, rather than the lesser effect of the increased upward flow propwash acting on the inboard area of the wing, which would seem to balance everything out.
So, is it a case of the two opposing propeller forces cancelling each outer out somewhat, but the torque still having the greater effect of the two forces? Are the counter rotation engines so arranged only on the basis of the "feel" of Vmca during flight test of a type, or is it possible that both engine arrangements should be tested to see what the actual difference is? Is there tribal wisdon of the engine swapping thing having been done during other twin flight testing, to actually validate this, or is the comparision of the Vmca handling with a right engine on both sides enough to not bother going any farther?
Thanks for your thoughts...
Pilot DAR