Unwanted Yaw In Inverted Flight

[jdmilla]

50cc Yak 54. CG and thrust seem OK. Gentle downward arc on 45 deg inverted and generally straight vertical climbs. Here's the problem. Full throttle hands-off straight and level flight into the wind. Pull into half loop and fly inverted in the opposite direction and I get a left yaw (left to the ground observer). The only difference I can deduce between upright flight and inverted flight is the angle of attack on the wings and stab. Is this an aileron differential/alignment problem, or maybe elevator alignment? Thanks for any insights.

[bodywerks]

Is it just not in the same heading as it was when upright or do you actually see an arc in the flight path to the left?

[Danny Baker]

Danny: Does the yaw happen during the 1/2 loop. I.E. Does it exit the 1/2 loop heading to the left? Or does it exit the 1/2 loop on the proper heading, and then starts to yaw left once on that inverted line?

[jdmilla]

It arcs while inverted. If I start level and upright and half roll to inverted it will start heading off course. I can correct with rudder. I can't tell exactly where it starts to deviate in the half loop, but I'm thinking its when the angle of attack goes from the bottom of the wing to the top of the wing. If I pull to vertical from level flight it tracks straight through the first 500', so I'm thinking its not a thrust issue. I have 2.3 deg right thrust. I didn't want to start adding washers until I was sure it was a thrust problem. All the surfaces seem to be centered, but I do have slightly more gear slop in the ailerons and elevators than I would like. I'm currently using PowerHD 9150's but am in the process of switching out to Hitec 7955TG's and aluminum arms. Rudder servo already has the Hitec. I'll try to check wing incidence along the wing span to see if there is a warp. I'll keep plugging away at this and all suggestions are welcome.

[airbike]

Check your engine side thrust again. Center the rudder and do many verticals and see if the engine is pulling the plane to the right. Could have too much right rudder trim also.

When inverted, the right thrust becomes left thrust. You will see most inverted harriers flown with lots of rudder input, especially when popping up to a hover with application of power.

[Danny Baker]

Are you holding a lot of down elevator to keep it level when inverted? If so, then one elevator has more throw in it than another in the down elevator command position. Causing drag and yaw to one side. So far it sounds like you need to reduce the throw (down elevator only) of the right side elevator by a few % points on the ATV (end point). Try 3-4% less on that side. Let me know.

Hope this helps,

[nate428]

i would check the throws like danny says... also to check thrust of motor vs control surfaces you can fly full speed and trim it for straight flight then kill the throttle, if it continues to track straight the thrust of the motor should be correct. try that upright and inverted. do you find yourself favoring or holding left or right rudder in a hover?
inverted does it yaw on rudder or is it a wing dropping needing aileron correction? have you balanced your wings (lateral cg)?

[Wojcigitty]

I agree with Nate, check your lateral CG.

[storklappe]

I had this same problem with a plane a long time ago. When people told me that the wings were not balanced I thought they were nuts because it did not make sence to me that the plane would fly fine upright and only had the yaw issue inverted. Finally out of desperation, I started applying weight to the wing tip that was the high side in the yaw. It fixed the problem and it flew great!

[Wild Thang]

	Quote: Originally Posted by jdmilla
	50cc Yak 54. CG and thrust seem OK. Gentle downward arc on 45 deg inverted and generally straight vertical climbs. Here's the problem. Full throttle hands-off straight and level flight into the wind. Pull into half loop and fly inverted in the opposite direction and I get a left yaw (left to the ground observer). The only difference I can deduce between upright flight and inverted flight is the angle of attack on the wings and stab. Is this an aileron differential/alignment problem, or maybe elevator alignment? Thanks for any insights.

I'm from Lakeland as well. Where do you fly at? Maybe i'll see you at one of the fields and we'll try to get her flying straight.

Matt

[jdmilla]

Thanks for all the suggestions! I read Peter Goldsmith's trimming guide, so I'll check lateral cg by his method and see if it helps. I don't need to keep much down elevator for inverted flight but the amount of gear slop I have is worrisome. If one of the elevators or ailerons is getting more blowback than the other then I could get a difference in centering when the angle of attack changes. I'm going to dial back my throws as well. I was trying to find the compromise between IMAC and 3D but I can't do either one of them well at this point. I'll set up for IMAC then progress to 3D. Looks like this weekend will be pure trim flights.

[jdmilla]

	Quote: Originally Posted by Wild Thang
	I'm from Lakeland as well. Where do you fly at? Maybe i'll see you at one of the fields and we'll try to get her flying straight. Matt

Hi Matt. I think we met before when I was flying at Propbusters. I didn't have a gasser yet. I switched to Imperial since it's a lot closer to my house. Let me know if you'd like to come down to Imperial one weekend.