coupar

Once again, Engine offsets for a twin.

I understand the recommendations that say 3-4 degrees of outward offset for twin engines. If one engine fails the offset will help pull the dead engine wing forward and compensate somewhat for the dead engine.

But there is also a recommendation that says the left engine (looking from the rear) should have slightly more offset outwards (say 1 degree) to compensate for Torque. That does'nt make sense to me. On single engine planes torque offset is to the right so should'nt there be more outward offset to the right instead of the left?

What am I missing here?

evan-RCU

I think you are on the right track. You can over think this though. On a decent high po set up I say just point them both straight and try to keep the motors as reliable as possible. If you want to analyze it think about the normal right thrust built in to offset left torque, so the left motor can stay stright and the righ motor shold be to the right, correct? This can make your head hurt.

coupar

My head also hurts. We seem to have concluded the same thing. The Right engine should have a greater outward offset to compensate for torque but that's not what I've read in other threads.

There has to be something I'm missing.

maxpower1954

Guys, it's just my opinion but I've tried it both ways, with out-thrust and without. Conclusion: it doesn't make any difference, at least not a measurable one. I don't bother with it any more, I just set them up at zero.

If using out-thrust was a viable idea, then it would be a full-scale practice, which it isn't. It was tried before World War II, and the Japanese Betty Bomber had a few degrees out-thrust. Same with the counter-rotating prop fixation some guys have. Very few full-size airplanes ever had them.

I think the level of understanding regarding multi-engine flying in the R/C community is about the same as it was in full-size aviation before World War II, when multi-engine airplanes were uncommon and exotic (like in R/C today.) The Army Air Corp flight manual for the B-25 addresses the misconception that turning into the dead engine is dangerous, which most R/C pilots still believe today. Russ Farris

larrysogla

The WWII P-38 twin engine fighter plane had counter rotating props but not in the popular concept that each prop at the top of the rotation has to swing towards the fuselage producing propeller torque to counteract the dead engine drag, dead engine wing drop and dead engine yaw. Instead the P-38 counter rotate the props with each prop at the top of the rotation swinging AWAY FROM the fuselage offering NO COUNTERACTION to the dead engine drag, wing drop and yaw. Actual flight tests indicated that with both engines running and with this NON-TRADITIONAL counter rotation the fighter plane tracked straight and true offering a stable and accurate gun platform for it's machineguns. The emphasis was NOT ON SINGLE ENGINE HANDLING but on GUNNERY ACCURACY. The priority was survival in a dogfight, not on ease of single engine flight. With our RC planes, the emphasis is in assisting the RC pilot in keeping the RC plane flying to a landing. Every little bit helps in a crisis situation. Most twin RC planes have a built in prop offset for that. It does not hurt to have it.
larry

jrf

My Feedback: (551)

Putting the same offset in both engines of a twin is the same as putting a single engine straight ahead. If you need 2 degrees of right thrust in the single to offset torque, you are going to need at least 2 degrees (net total) in a twin to offset torque. You can get that by using left / right offsets of 0 and 2, or 3 and 5, or 8 and 10. Personally, I use 0 and 2 to control torque and keep the nacelles as close together as possible plus enlarging the vertical fin and rudder to control un-symmetrical thrust.

Jim