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Old Sep 20, 2013, 03:23 PM
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gquiring's Avatar
United States, SC, Summerville
Joined Jun 2001
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Need help with sloppy flap

I am building an Elegant 700 sailplane and the flap has a huge amount of slop in it when I drop it 90 degrees. The supplied horns are only 5mm off the flap surface which I am thinking may be the issue. Here is a video.

Sailplane Flap is Sloppy (1 min 10 sec)
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Old Sep 20, 2013, 08:10 PM
No bounce, No play.
davidmc36's Avatar
Canada, ON, Ottawa
Joined Oct 2010
3,422 Posts
No matter what you do once you get to 90 deg it will be the same. The only thing that a taller arm on the flap will do is change the length the rod needs to be and how much the servo needs to move to get the travel you want.

I would ignore the slop the you see when you are pushing the flap towards the servo. Just put a bit of load on the flap that would simulate the air load and see if the flap gets to the position you want it. Then increase the servo travel as needed to get it there.
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Old Sep 20, 2013, 09:30 PM
C-c-c-custom!
United States, NM, Albuquerque
Joined Nov 2011
330 Posts
Well, you can see in the video that the linkage geometry is quite bad when the flap is fully deflected - as you rotate the flap the servo is hardly resisting because you aren't putting much force on the pushrod. Moving the attachment point further from the hinge will help with the deployed stiffness (this increases the "lever arm" in the deployed configuration), but will place higher loads on the hinge itself.

A taller horn will help with the stiffness, mostly in the flaps up configuration, but won't do a whole lot to the deployed deflection scenario.
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Old Sep 20, 2013, 10:02 PM
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lesterpk's Avatar
Australia, WA, Perth
Joined Oct 2003
877 Posts
4 opinions:
1. Your servo isn't anchored well, its moving when you push on the linkage. Brace the servo in the box with a piece of balsa or carbon rod or even ice block stick across the servo and it needs to also be touching both sides of the servo box. A drop of CA at end end as well as on the servo will help immensely in stiffening up the mount.
2. Offset the servo arm further so at full flap its pointing more forward. At the moment your servo is near 90 degrees so is at the worst possible mechanical disadvantage trying to hold the flap.
3. The flap horn needs it hole further back from the hinge line. Move it another 5mm back toward the TE so when your flap comes to 90 degrees it will be 10mm away from he hinge, that will give it more leverage. Having a taller horn on its own wont change the issue when the flap goes to 90 degrees, it will still be too close to the surface to have enough leverage. I'd make the horn a bit longer, say 8mm and move it behind the hinge line 8-10mm.
4. Hard to say from the vid but also looks like your pushrod is flexing, maybe go up a wire size or two.
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Last edited by lesterpk; Sep 20, 2013 at 10:03 PM. Reason: spelling
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Old Sep 22, 2013, 08:10 AM
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gquiring's Avatar
United States, SC, Summerville
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The sloppy flap is fixed. I pulled the tiny horn out and copied a horn from a Top Model Prelude. The horn on the Prelude is 9mm tall vs 5mm from the Elegant. I also placed the horn back from the hinge line which solved the slop problem.

Sloppy Flap Fixed (1 min 11 sec)
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Old Sep 22, 2013, 06:25 PM
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Australia, WA, Perth
Joined Oct 2003
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Great to hear you got it sorted.
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Old Sep 24, 2013, 08:16 PM
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320pilot's Avatar
Joined Mar 2009
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good fix..I was off the Interweb for a week, or could have offered my 2c worth..

I was going to suggest, reversing the way the flap horn sticks up (closer to TE of flap) and get it higher, but you did all that.
You may notice you don't get the full 90 degrees travel, but IMHO you don't need it. 80 degrees or so is fine.
Remember to retract the flaps on landing!
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Last edited by 320pilot; Sep 24, 2013 at 08:47 PM.
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