I'm just getting back into RC after a 16 year absence. I flew a trainer for the lact 10 months to get the feel back.

I just bought a used low wing, don't know the name of the plane. It wants to go to the right. In order to get it to fly straight I have to put in left rudder and left ailerons.

The engine is mounted right of the fuselage center point, and has a lot of right thrust (by my figuring about 5 deg.). I measured from the tip of the prop to a fixed point on the tail and my triangle dimentions are 44.5", 44" and 11". I rotated the prop 180 deg in my measurements to preclude any prop warp.

I've never known much about aerodynamics, so any help you can give me to fix the right roll situation would be appreciated. I remember now my old rule about buying used planes .... you're buying sombody elses problems. Lol

Thanks

The first thing I'd look for is a twisted wing.

If you don't have one, find a buddy with an incidence meter. Block up the plane and check the incidence of both wing panels at the center and at the wing tips. If you find a twist and the wing is built up and not fully sheeted, it can be worked out by twisting the other direction and reshrinking the covering with a heat gun.

Dave

The fuselage may be warped. The vertical fin may be built in at an angle. The wing and or horizontal stab may be attached at the wrong relationship with the fuse or each other.

You can visually check and/or measure all these to see if they are close. To check for warped wings you can hold it up and sight from the trailing edge toward the leading edge while varying the cordwise angle and noting the amount of wing surface showing during this movement. Sighting both L/R sides can be done individually or together to see the differences/similarities.

Terry in LP

have you trimmed the model out?

yes, I trimmed it out. It required both left rudder trim <u>and</u> left aileron trim to get straight and level flight. I'm just wondering if the amount of right thrust and / or the offset of the engine from center point could be effecting things.

thanks,

Scott

If the engine is expected to have a lot of left thrust you mount it slightly to the right, but it has right thrust! Well there's your problem!

Check out this thread: [link=http://www.rcuniverse.com/forum/m_8843896/anchors_8843969/mpage_1/key_/anchor/tm.htm#8843969]Click me![/link]

You need to start with the basics of checking the wing for warps. Then check the fuselage for lengthwise bowing (bannanna shape). Then check for lateral wing balance. Finally ensure that the ailerons are both sitting identically when centered and do not jut above or below the main panels. A bad hinge line where the control surfaces are not "fair" to within better than 1/32 inch can generate a yaw producing drag even when the aileron is supposedly "centered" and also act like it is adding roll trim.

Only when you KNOW from testing with long straight edges that the fuselage and fin are all straight and centered can you mount the wing and check the wing alignment to ensure it is sitting square to the center line and that the semi spans on each side are identical to within 1/8 inch. Any diagonal misseating should be held to within 1/16 inch as measured from the wing tips to the fin centerline. These diagonals can only be checked once the wing is confirmed to be seated so the span on each side is identical to within about 1/32 inch. Otherwise set up measurement points that are that accurate to check the diagonals to the fin. The wing only needs to be skewed a couple of degrees to genenerate that much of a trim issue.

5 degrees of right thrust does seem like a lot. You could very likely reduce that to around 2 to 3 and that much would do all that is required. I'd suggest start with setting it to 2 degrees and check for warps and wing alignment as mentioned above. Wing warps of as little as 1 degree difference from panel to panel can produce what you're getting. Any warping much be dealt with unless it's an identical amount and direction on both panels such as when adding in washout to aid in avoiding tip stalls. The obvious message here is that both sides of the wing much be identical in every way. Even some differences in mis-shaping of the airfoil from side to side could cause this issue if the mis-shaping is pronounced enough.

Take nothing for granted, Check everything looking for anything that is non symetrical.

What did that have to do with my post about the plane having right trim when it should have left trim?

Personally, I thought it added value to the OP's original question and gave him/her food for thought in terms of what to do next with their plane. Your post wasn't exactly an object of clarity! Now, if you'd said mount left for right thrust or vice versa so that the end point of the crank was somewhere in the centre of the airframe we'd have been getting somewhere...

While that is true, it is more the fact that the engine is not mounted correctly in the first place. I would take out all of the right thrust to begin with then if it needs left thrust move the engine to the left side of center and add left thrust.

Within the confines of the size of the firewall shifting the thrustline right or left will have minimal or no noticable effect and isn't really worth the effort. However having too much right thrust could definetly be an issue. But I've never seen 5 degrees produce as strong an effect as mentioned by the OP. That's why I'm more concerned with a proper check for warps and overall model alignment.

Another point to check would be the horizontal stabilizer for twist or warp-especialy if it is a simple slab type assembly. On a few of my older planes they seem to gradually develop a twist over the courese of the flying season and need to be hit with the heat gun and reshaped.