I'll post this in hopes someone might have an answer...
Plane is a ModelTech Giles 202 (don't ask me why I keep fiddling with this thing!)

I've added 2 degrees washout on each wing, which seems to help with it's tip-stalling. The two wing halves are 0.5 degrees different (right side 5 degrees positive to left side - and no, I'm not the builder...), which might induce a left roll... but it doensn't appear to.

Here's the problem:
When trimmed for straight and level upright flight, then rolled inverted, the plane rolls left. Aileron trim is no help, since adding right roll trim only makes it roll right when upright. With me?

I've tried rudder trim while inverted, however all that does is affect yaw - no roll coupling with such small amounts of rudder trim.

I've checked the wing to stab angles, and the wing tips are skewed to the right (as measured from the wing tip TEs to the stab roots) by 3mm. I guess that might give some right roll upright, and left roll inverted? But would 3mm really be noticeable? And is my thinking right?
It also rolls slightly right on inside loops, and hard left on outside loops.

If you've any idea what I could do.... anybody?
Thanks!

Rudder offset is the obvious culprit, that's the only thing that changes response upright or inverted, but usually it does this with lots of dihedral.
Second would be differential elevator since it loops differently.

Have you checked lateral balance?

Yup - right on center

You may think it is laterally balanced but the model is giving one indication that it is not. Imagine the right tip is heavy, so there is a tiny amount of left aileron trim to compensate. The two forces cancel each other out. When inverted the left aileron trim is still left aileron trim. But the heavy tip is now on the left side so the two forces add to one another instead of cancelling out, and the model rolls left.

Weight is not an intrinsic property of matter, mass is. Weight is mass acted on by an acceleration, so the greater the acceleration, the greater the difference in weight between the two tips. You can cause more acceleration, and thus greater weight difference by pulling up into a loop. Since the aileron trim will be constant assuming the speed remains constant, if there is a weight difference between the tips the model will roll off to the heavier side. If it pulls off to the right during a pull up, and goes off to the leftt if you push up from inverted flight, it is a lateral imbalance with the right side being the heavier side.

H

H - thanks![&:]

I balanced the plane using a string wrapped around the prop shaft and one under the rudder counter-balance (at the fin hinge line) -
I'll check it again - I remember it may have been leaning a little.

What's a good way to balance it? Would inverting the plane help?

If there is any offset in the engine, wouldn't that cause the lateral balancing to be off if you use the prop shaft?

I thought about that, too. But the engine is offset on the firewall such that the prop shaft exits the cowl on the centerline of the plane, so I can hang it from there.

I was just coming back to edit my reply to add that it would only affect it if the engine shaft was centered and the right thrust was added after the fact. This is how I had done it before figuring out to center the shaft with the right thrust added.

Don't bother, it's just a waste of time. The rudder counter balance is high up the fin, so you are suspending almost the entire weight of the model below your pivot line. An ounce or so of lateral imbalance is not going to show up against the many pounds of weight of the whole model refusing to be rotated upwards! Unless you can arrange the suspension points to be on the exact longitudinal line of centre of mass, forget it.

Flight behaviour is what matters, so trust what the flight behaviour tells you. If it gives all the indications of lateral imbalance, then deal with that. If it gives all the indications of a warp, then deal with that.

Perhaps there is no warp or imbalance at all. What I often find when asked to check out other people's models is that they only ever use aileron trim to correct a turning tendency which is actually a rudder problem. Consider a model that is yawing left, even with no roll coupling it will gently alter heading to the left. 99.9% of model fliers counter this with right aileron, so the model ends up imperceptibly yawed left, imperceptibly banked right, and seeming to go in a straight line but is actually unbalanced in a knife edge but with 1 degree of bank instead of 90 degrees bank. When you roll inverted the left rudder becomes right rudder from the ground viewpoint so the model yaws right, but the right aileron trim is still right roll and hence you get a good roll and yaw to the right.

It is essential to get the model in proper level balanced flight before making any other tests. I find the best way is to fly directly away from me and directly toward me, in line with the wind, and watch very closely to see if the model is yawing off the original heading and ensure that the wings are truly level. If it yaws off heading then trim the rudder, not the aileron. That probably will start to induce a slight roll and that can be trimmed out with aileron.

Trimming is only correct at one combination of speed and throttle setting. Change speed or change throttle setting and the trim is no longer correct, so do all your trim tests at the speed and throttle at which you normally fly the model. In the full size we are working the rudder all the time in aerobatics to keep the plane from yawing off line, in a plane where the prop rotates the same way as a model's prop it needs left rudder as speed goes above the trimmed speed and right rudder as it falls below trimmed speed, but we have the luxury of the slip ball to guide us.

H

Hi Keefer,
Maybe a little southern advice might create some thoughts?
First the Giles is a straight forward airplane design, should have no quirks
That so, if the airframe is true in all respects then flight should be fine.
What does the above say?
Considering the smallest detail> closely check the entire airframe for
trueness.
Wing and tail is square to center line?
Wing and tail incidences are correct?
Flight surfaces are warp free?
Fuselage is true about the center line?
Fore and aft balance is proper as well as lateral?
Thrust line is paralell to line of flight?
Vertical C.G. is close to thrust line?
As you go through this list more may come to mind, do not hesitate
follow through.
Quite often we can not see the tree because of the forest but going back
to basics carefully often the culprit will be obvious.
Good luck and have fun!

Hal [email protected]

Thanks, Harry and Hal -
I do my best to check the rudder trim, and in this case, when inverted, the rudder doesn't help with roll at all - only yaw.

There may just be something "untrue" in the airframe that I can't measure or detect - so I'll keep plugging along. I may re-check the axial balance again in the air.

Happy New Year!

Keefer