Hi

I have a second hand CM with a 91fx in it - it is pulling left a lot
does any body have an idea of the degree of right and down
thrust needed

Thanks
Tim

nashtm:

General power trim:

If the plane "Pulls left" regardless of throttle settiing the thrust angle wont cure it - something is out aerodynamically.

If the pull varies with the throttle use some washers under the engine mount to shift it to the right. Fly it to test, add more if needed.

Down thrust is adjusted the same way. If your elevator trim has to be changed with the throttle to maintain level flight you can add up or down thrust to compensate.

But.

If you want full aerobatic capabilities, this right and down thrust will work against you when inverted. Trim for full throttle in upright flight, inverted and full throttle (At the same neutral control and trim) will result in a climb to the left.

You pays your money and you takes your choice.

Bill.

I have 1 1/32" thick washer under the left side of the motor mount on mine and it seems about right. You still need to hold some right rudder on and after takeoff til the speed builds up, but it pulls pretty straight in the verticals.
The test for side thrust I was taught was to pull vertical and watch. If it diverges to one side or the other right away, it's aerodynamic; if it goes straight for a while, then pulls to one side or another after some of the speed comes off, it's sidethrust.

Check CG -2 down at most. CG is the main thing ,you can trim most fly 0 the CG is a must for what you want the plane to do.
Great air plane you can make it do what you want it to do by changing balance and control throws.

Thanks for the replies,

I should have been more specific - it ballooned when I chopped the throttle on landing so I removed some of the down thrust which sorted that out.

But in full power verticals it pulls left some much that I need almost full right rudder. It also only has enough rudder to hold it in a knife edge with left rudder.

I have put in a washer for right thrust and will test it.

Is using a mix on the radio an good solution ?

Thanks
Tim

If it pulls that hard it almost seems like it would have to be an aerodynamic problem.........or maybe a heavy wing. Have you balanced it laterally?
Mixing can be very helpful, but it's best to work out the big problems the old fashioned way first!

Nash:
One last test. Full power level flight, chop the throttle. If you have an immediate right yaw, then yes. Crank some right thrust in. Everything else you say points to the need for it.

On a trainer type airplane I don't mind using thrust angles to correct design problems, as they lead to greater upright stablilty. But these same angles detract from an airplane's inverted capability. Keep this in mind.

If you aren't stuck on the plane being scale, you might consider a rebuild of the vertical fin and rudder - keep the same total area but increase the rudder to 60 or 65% of the total area, do the same with the horizontal stab and elevator.

There are two other possibilities. Strat mentioned one - horizontal balance. The other possibility is built in left thrust. Did you build the plane yourself? Are you sure the left and right fuselage sides weren't interchanged?

Do you have a thrust angle gauge? The Robart incidence meter will work. If you don't, find a straight yardstick and drill a hole at the 18" point. Bolt it on the engine and measure the distance to the leading edge at each end. Works very well.

Let us know what you find.

Bill.

PS: Mixes are nice for a lazy pilot, or one trying to get the utmost precision in flight. The very last thing to use in basic trimming. wr.