RE: flutter in aileron and flaps  
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All Forums >> RC Airplanes >> RC Gliders, Sailplanes and Slope Soaring >> RE: flutter in aileron and flaps
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RE: flutter in aileron and flaps - 8/14/2006 2:55:39 AM   
Tall Paul



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From: Palmdale, CA, USA
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quote:

ORIGINAL: drela

quote:

ORIGINAL: Tall Paul
THe servo has x amount of torque available.
Moving the pushrod out from center increases the moment arm, which increases the push available to the surface.
Force times distance.

You've got it backwards.

torque = force x distance
force = torque/distance

Doubling the length of both horns will not change the servo to surface motion, but it will halve the pushrod force.
The effect of clevis slop will also be halved.

In contrast, doubling both horns will not change the servo gear slop or servo resolution. To improve these, increase the servo throw as much as possible (to +/-45 degrees or more), by making the surface/servo horn length ratio as large as possible while still maintaining adequate surface travel. In the original photo, the surface horn looks too short, and the servo horn looks too long in comparison.


Doubling the horn length wasn't for slop or resolution, it's for resistance to flutter.
The short horn at the surface, for getting the maximum surface deflection is notorious for flutter initiation.
Making -that- horn longer is usually all that is needed to stop any flutter.
Or doubling up on the servos on the surface.

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RE: flutter in aileron and flaps - 8/14/2006 3:27:09 AM   
Mastertech


 

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From: Dalzell, SC, USA
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quote:

ORIGINAL: Tall Paul

quote:

ORIGINAL: drela

quote:

ORIGINAL: Tall Paul
THe servo has x amount of torque available.
Moving the pushrod out from center increases the moment arm, which increases the push available to the surface.
Force times distance.

You've got it backwards.

torque = force x distance
force = torque/distance

Doubling the length of both horns will not change the servo to surface motion, but it will halve the pushrod force.
The effect of clevis slop will also be halved.

In contrast, doubling both horns will not change the servo gear slop or servo resolution. To improve these, increase the servo throw as much as possible (to +/-45 degrees or more), by making the surface/servo horn length ratio as large as possible while still maintaining adequate surface travel. In the original photo, the surface horn looks too short, and the servo horn looks too long in comparison.


Doubling the horn length wasn't for slop or resolution, it's for resistance to flutter.
The short horn at the surface, for getting the maximum surface deflection is notorious for flutter initiation.
Making -that- horn longer is usually all that is needed to stop any flutter.
Or doubling up on the servos on the surface.



Now that I agree with. But at the servo is what we were talking about. You just had it backwards. No biggy. Night all

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RE: flutter in aileron and flaps - 8/14/2006 3:52:32 AM   
Tall Paul



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quote:

ORIGINAL: drela

quote:


You've got it backwards.
...
torque = force x distance
force = torque/distance

...


.
My bad!

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RE: flutter in aileron and flaps - 8/14/2006 4:11:22 AM   
ArCeeFlyer



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From: York, PA, USA
Status: offline 		I couldn't help but notice that the diagram seems to be in conflict with that quote. If force = torque/distance, shouldn't the 2 inch arm show 48oz/2 = 24oz of force? And that would reverse as well with the torque = force x distance where 24oz of force x 2 inches = 48 oz of torque. It would make sense when lengthening the servo arm, resulting force gets lowered. Does that seem right?

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RE: flutter in aileron and flaps - 8/14/2006 6:31:16 AM   
Tall Paul



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quote:

ORIGINAL: ArCeeFlyer

I couldn't help but notice that the diagram seems to be in conflict with that quote. If force = torque/distance, shouldn't the 2 inch arm show 48oz/2 = 24oz of force? And that would reverse as well with the torque = force x distance where 24oz of force x 2 inches = 48 oz of torque. It would make sense when lengthening the servo arm, resulting force gets lowered. Does that seem right?

.
Yep, I fingered that out.
But the basic premise of moving the pushrod -out- on the surface horn reduces the opportunity for flutter.

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RE: flutter in aileron and flaps - 8/14/2006 12:49:36 PM   
Mastertech


 

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Status: offline 		Out on the Control horn yes, in on the servo yes. There is a point on going out on the control horn that will reverse this but that all depends on the stiffness of the control horn. It's why we use gussets.

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RE: flutter in aileron and flaps - 8/14/2006 1:13:45 PM   
Allie33


 

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From: Palm Bay, FL, USA
Status: offline 		BTW,

With all the discussion. there has been no mention of how the flutter was evidenced. Also, what was the model and how was it powered?

Tell us about it.

AW

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       Post #: 32

RE: flutter in aileron and flaps - 8/14/2006 5:28:58 PM   
slopemeno



Posts: 448
Joined: 5/24/2003
From: San Francisco, CA, USA
Status: offline 		Yes, Sparky...er Tall Pauls diagram reduces the servo torque, but it buys you resistance to flutter. Flutter usually doesnt start because the servo cant hold position, it starts because of linkage slop.

A drop of CA in the threads of the one clevis will eliminate some of the slack. You can also put 2-56 nuts on the clevises. A drop of CA in the horn/clevis pivot point will take out some more slack, just "pop" the bond loose after it hardens to make sure it moves.

If your servos gears are sloppy, that can be a source of slop that can start flutter, so I suggest picking decent servos from the start. I've had better luck long-term with nylon gears, and the JR-341 servo has been a great long term servo for wing applications. I recently put a set of gears in a pair of 341's that had been in a plane that went into a set of trees at warp factor 6, then blew out and lay submerged in water for two weeks. I thought "eh, maybe I can use them in a foamie" so I put new gears in them. They ran like new! It was like finding money on the sidewalk..

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RE: flutter in aileron and flaps - 8/14/2006 6:33:49 PM   
Tall Paul



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quote:

ORIGINAL: slopemeno

Yes, Sparky...er Tall Pauls diagram reduces the servo torque, but it buys you resistance to flutter. Flutter usually doesnt start because the servo cant hold position, it starts because of linkage slop.

...

.
Or too heavy a surface hanging off the hinges.
Or a long thin surface, with the control horn at one end.
The Goldberg Tiger II was notorious for this. Changing from a single servo in the center to two servos for the ailerons, each a couple bays out from the center fixes that problem.

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RE: flutter in aileron and flaps - 8/15/2006 12:07:39 AM   
flaphoto


 

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From: Fenton, MI, USA
Status: offline 		The flutter occured in shallow dive, not high speed, not under propeller power. It appeared that it might have been occuring in both the aileron and the flap. The bird is a Stork E, 3 meter moldie (as seen on Esprit website). Motor is a Hacker B50, but the flutter was not occuring under throttle (that we noticed...could be wrong). The entire trailing edge was trimmed down several degrees, as one might to to provide addtional lift on launch(right or wrong, it was done to eliminate me, newbie, from scrambling to find the right switch if I need to add power to get out of trouble...I am eliminating that setup prior to next flight).
The servos are brand new, Hitec HS5125MG digital wing servos (41.66oz.in torque at 4.8Vand .17sec to 60 degrees)


quote:

ORIGINAL: Allie33

BTW,

With all the discussion. there has been no mention of how the flutter was evidenced. Also, what was the model and how was it powered?

Tell us about it.

AW



< Message edited by flaphoto -- 8/15/2006 12:09:33 AM >

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       Post #: 35

RE: flutter in aileron and flaps - 8/18/2006 3:08:51 AM   
flaphoto


 

Posts: 68
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From: Fenton, MI, USA
Status: offline 		got the replacement brass control horns from esprit models today and fitted them in place of the ones that I originally installed and cut too short. The new ones still only give me 1/2 inch length from the control surface to the center of the hole where the clevis is inserted. Is that going to be long enough for a 3 meter sailplane? Looking through some of the photos of similar sized birds it appears that many have controls horns installed that are greater than 1/2 inch.

Also, the Hitec servos I have (HS5125MG), even when powered up have a tiny amount of play/backlash in the gears...never seen that before. Does anyone else here have those servos? Do you see the same thing? Could I have gotten 4 bad, brand new servos or is this normal? The cheaper sevos I have on the v tail are locked in with no backlash at all.

< Message edited by flaphoto -- 8/18/2006 3:10:25 AM >

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       Post #: 36

RE: flutter in aileron and flaps - 8/18/2006 3:57:34 AM   
flaphoto


 

Posts: 68
Joined: 6/17/2006
From: Fenton, MI, USA
Status: offline 		Did a little research on my servo backlash/slop question, here is a little thread which doesn't speak well of my servos.... http://www.rcgroups.com/forums/showthread.php?t=405902&pp=15
disturbing given the price I paid for these brand new servos. I have a difficult time thinking Esprit would put a package together for me that would include sloppy servos...I hope this isn't causing my flutter problem.
Anyone with experience with these servos share the opinons expressed in the above thread?

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       Post #: 37

RE: flutter in aileron and flaps - 8/18/2006 4:07:46 AM   
Tall Paul



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From: Palmdale, CA, USA
Status: offline 		The servos may not be robust enough to resist the surface flutter which is caused by the short horn.
Reading that link, I'd tend to go that way.


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RE: flutter in aileron and flaps - 8/18/2006 11:15:41 AM   
flaphoto


 

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