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Building Tips and technical articles. => Building techniques => Topic started by: Joseph Patterson on March 19, 2012, 02:52:36 PM
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Maybe some of you guys can help with some U-KEY( the Joe Just design) trimming problems. In reality, the guys solved the slightly O.B. wing high problem with a trim tab after first checking and adding some tip wt. That brought the O.B. WING INTO TRIM. The wing was checked carefully for any twist/warps/incidents and was dead-on and stab was straight, etc. The one thing we did notice was the lead-outs at the tip were not at the same height as the bell crank lead-outs-they were lower at the tip over (3/8").
We had another homemade stunter w/ exposed bellcrank similar to the U-KEY installation except the tip lead-outs were over 3/4 " below the bellcrank height. The O.B.wing flew very high level upright flight and very low level inverted flight. Tip wt. not the problem again. We did not put a tab on the O.B. WING.
We were wondering if the problem could be corrected by raising the lead-out guide to the same height as the bellcrank, if this is really causing the problem. I have looked at other U-kEYS ON THE Web and the leadouts look to be even w/the bellcrank. The wing on this plane is foam and it is straight and true. Both airplanes had aluminum landing gear ( 2 wheels). My question is does the lead-out exiting well below the centerline of the B.C. on an external bellcrank that is above the C.O.GR on top of the airfoil cause this problem, if you have a wing that is true in all regards and no misalignment problems, or is it some other problem such as the wt. of the landing gear below the the C.O.G. IN COMBINATION W/ THE TOP MOUNTED B.C???? We did not have the time or parts on hand at the field to resolve the problem at the field yesterday.
I have looked throughout this forum to see if others had similar problems with their U-KEYS or similar designs and have found no one mentioning these problems.
Thanks,
Doug
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Where the leadouts are positioned with respect to the bellcrank is immaterial to the roll trim (although it does make a difference to friction).
Where the leadouts are positioned with respect to the vertical center of gravity is what's important. Hang the plane by the leadouts, and drop a plumb bob next to the wing -- looking from the nose, the wing should hang vertically (if it has dihedral, the wing should hang vertical overall -- i.e. the tips should be the same distance from the plumb line).
If the wing isn't vertical when you do this, then it's going to get pulled in the same direction in flight -- if the vertical CG is below the leadout point, then the thing will be rolled in while right side up, and out when upside down. If the vertical CG is above the leadouts, it'll do the reverse.
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Thanks Tim, you said what I was thinking, but not expressing very well. We'll check it and see what we got.
Doug
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You might want to check this out I do hope it's not too late.