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First Biplane - Options for More Stability?

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Old 02-05-2014, 10:43 AM
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T-ster
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Default First Biplane - Options for More Stability?

Been flying a long time, but recently assembled a 1.20 size biplane ARF with a YS 1.40L (TWM Ultimate and my first biplane). It flies well, has gobs of power and goes where you point it. The issue is when upright it is almost like a heli, it doesn't want to be stable. It would be happier flying in a 10 degree bank than upright. It is balanced properly and the trim is good, but just feels like the a marble on a hill (right to left). It is short coupled so it is somewhat touchy on the elevator, but that is expected and not a problem. The left/right banking is the issue.

It has one servo for the right aileron and one servo for the left aileron (connects to the bottom wing ailerons with a pushrod to connect the top ailerons to the bottom).

Is this typical of biplanes or is there something with the ailerons (reflex them up or droop them down), that might help?
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Old 02-05-2014, 11:56 AM
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Rodney
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Check the relative incidence of the wings. I'll bet that you have the upper wing at a more positive angle than the lower wing. On some dozen or more bipes I've had, all flew best when the upper wing was set at about 1.5 degrees LESS angle of attack than the lower wing was. One of my bipes was exceptionally difficult when the upper wing was more positive than the lower wing, would not fly level without porpoising . Changed the angle of attack of the upper wing and it tamed down nicely. Of course a bit of change in elevator trim was required as well. Give it a try and if it works out let us know the results. Good luck.
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Old 02-05-2014, 12:53 PM
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rlipsett
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also check the engine mount for left thrust.
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Old 02-05-2014, 03:26 PM
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Try about 40 expo pos on jr neg on futaba, or you could move CG slightly forward
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Old 02-05-2014, 07:02 PM
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Originally Posted by T-ster View Post
............... but just feels like the a marble on a hill (right to left).
What about the roll stability for inverted flight?
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Old 02-05-2014, 11:08 PM
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Interesting question about the inverted flight. I will check that out once this cold weather gets out of town......
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Old 02-07-2014, 09:20 PM
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I did a quick check and the top wing is very slightly positive to the lower wing (0-0.5%). Will try to adjust and hopefully make it out over the weekend.
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Old 02-22-2014, 12:09 AM
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I was thinking back as to how I set the wing incidence and remembered the ARF came with two wood jigs that slide over the ends of the wings to set the incidence. Decided I wasn't going to mess with that because they were spot on with the jigs. Then in a moment of extreme clarity, I vaguely remembered adding lead to the tail and didn't get a chance to fly the plane again for several months. Low and behold I removed the 3 ounces of lead from the tail, added some aileron differential, reduced aileron travel and reduced the aileron exponential. MUCH better flying plane! Not quite as stable as my other planes, but that may be typical of an Ultimate.
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Old 02-22-2014, 07:17 AM
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To answer your question, no, this is not normal for biplanes. I've never flown a biplane that was unstable. The biggest difference I've noticed between a typical biplane and a typical monoplane is that when you pull back the throttle, the biplane slows down a lot faster.

When you reset the upper wing incidence, don't forget to recenter your upper ailerons. If you raise the trailing edge to lower the incidence the upper ailerons will be slightly down from center assuming the servos are in the lower wing. If the servos are in the upper wing then you need to reset the lower ailerons as they will move slightly up.
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