Beech 18 - left thrust issues

[madmac]

I built the Beech 18 foamie and having had experience with left thrust on single prop jobs when running up for takeoff i fitted it with contra rotating props thinking this would eliminate the problem but it swerves to the left as severely as anything i have flown. Any explanations?

[jrman]

It's caused by gyroscopic precession induced by the rotating mass of the motor. Correct it with right rudder during take off.

[John Privett]

Jrman - if I've understood madmac correctly, he has a twin engined model with the motors rotating in opposite directions. Any gyroscopic effects would cancel each other out, as should any aerodynamic effects.

Maybe the left wheel is binding or the undercarriage is misaligned?

[John Muir]

Your left propeller is on backwards? Or you forgot to use an opposite handed prop on the clockwise rotating motor? Motors are not spinning at the same speed? Your runway has suddenly developed a tilt to one side?

On reflection, maybe not the last one. But being a twin, assymetric thrust for one reason or another would be a possibility.

[Chris Poulton]

i cant talk from experience here, but, loving this plane i've read lots. this swing is a common trait with twin engine taildraggers apparently, and there is not much that can be done about it, other than to hold in 'up' elevator at the very beginning, advance throttle veeerrry slowly keeping the rudder stick employed as necessary, and releasing elevator once you have enough speed for the rudders to be effective. only then advance to takeoff power, again smoothly, and almost let her fly off the deck unaided. i am assuming the plane behaves well once airborne?

[Martin Harris - Moderator]

I'm with John on this one - all things being equal, the model should have all the swing tendencies balanced out of it. Have you got any side thrust built in to either or both engines?

[PatMc]

As John suggests, it could simply be the undercariage. Other possibilites are that in practice - the ESCs don't actually respond identicaly to the Tx (even though they should) or the performance of the motors don't actually match or the performance of the props don't match or an accumulation of small mismatches in the power train. Also worth checking that both ESCs have the same timing settings etc.

You could swap over ESCs to see if this has any effect, if not swap motors but the normal rotation prop should always be on the starboard motor.

[PatMc]

Too late to directly edit the above, I got the prop rotation wrong.

It's the port motor that should always have the "normal" rotation prop.

[madmac]

Thanks for your input guys. I used my limited brain and swapped the motors. It veered to the right. Must be different motor revs. Is there a way of balancing the revs. Obviously they are identical motors. I am operating two esc off one battery.

[Martin Harris - Moderator]

Did your motor swap involve a pure motor swap i.e. leaving the props and ESCs on their respective sides?

Can you confirm that the motors have the same kV ratings? The way to confirm that would be with a tachometer and tests using the same battery, ESC and propeller.

[John Muir]

Madmac, could it be the props? They obviously can't be out of the same mould so there might be a mismatch. Try swapping them? Otherwise I'm not sure what you could do. A tachometer would be handy here I reckon. You couldn't hope to properly set up an i.c. twin without one.

John.

[PatMc]

The full size had same rotation props. I'd try the model with identical props & maybe have both rudders with a degree or two of outward bias.