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

[quote]ORIGINAL: drela

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My bad!

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

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.

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

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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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.

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)

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.

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/showt...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?

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.

What sort of wing servos have you had good luck with in large wing sailplanes? If the control horn I just put on and my moving in on servo arm hole still results in flutter, the hitecs are going. Anyone else on this forum using the hitecs I have sucessfully?

On average, you'd probably want a horn that's close to 1" tall. I just measured the horns I use on a 3m and the outer hole is 7/8" from the bottom of the horn. The throw I'd get from that 7/8" will be greater by half the thickness of the aileron it's bolted to. So that horn can deliver one inch of lever. It's only a medium duty horn. Heavy duty horns are taller.

It's actually fairly easy to figure out how tall a horn you're going to need. But it takes some measuring of fairly short things that aren't easy to measure.

Look first at the mfg's suggested throw for the aileron. Then measure the aileron cord at the point they give for that throw. Let's say the cord is 3" and they want 1/2" up and 1/4" down. Ready with the tiny little ruler? OK......

Forget the ruler.
Almost any horn close to 1" tall will give 1/2" throw to any surface of over 2" cord easily.

OK, ok...... seriously now..........

Compare the cord of the surface to the height of the horn. That'll give you a ratio. Use the ratio on the recommended throw. That's how far the outer hole in the horn must move.

A 1" horn on a 3" aileron that needs to be moved 1/2" would mean the hole in the horn must move roughly 1/3 the required 1/2" throw.

Remember now, the solution that's important is to use a horn on the control surface that's both appreciably taller than before AND that whatever connector you use fits the hole in the horn with no slop.

And the connector hooked to the servo must also fit with no slop. The entire exercise here actually is focused on first removing the slop, then giving the elevator horn enough lever arm that any slop at that connection is miniscule compared to that lever arm length, and then giving the servo some help by making the connection there slop free and close to the servo.

Good luck but.........
with all this advice, you ain't gonna need luck.
(and you've already shown IMMENSE patience to have read through all this....

I certainly appreciate everyone taking the time to outline something that I am sure must be basic to those with experience. Got to call ZB at Esprit about the horn he sent me as from everything I have read, 1/2 inch just doesn't appear to be long enough. Maybe its me and I have just installed it wrong.
The emphasis on slop elimination still disturbing due to fact not much I can do about the play in the Hitecs. Maybe its a non issue when dynamic forces of actual flight are put into play. Guess there's only one way to find out.

A Banner day for me!
My first launch, by myself. My first fixed wing landings, by myself and no flutter!
I replaced the brass control horns that I hacked down to little nubs and moved the clevis on the servo in one hole toward the servo center. The brass clevis is still only 5/8th of an inch tall, I thought, looking at some other examples on various posts, that I might need to be closer to an inch. The hitec servos, though having a bit of backlash, apparantly were not to blame. It was a nice quiet night with little wind and the only sound was that lovely aerodynamic sound these 3 meter birds make as they pass overhead. No buzzing flutter.
Thanks everyone for your input and lessons in basic control setups. I will post one closing post soon with photos of the changes I made. Hopefully, a future newbie will benefit from my mistakes and your advice.

Glad you nailed it. I got to see a pretty dramatic example of what happens when you dont find the source of flutter. A guy I fly slope with has a homebrew 100" pattern plane thats been converted to a sloper, and it...flew, past tense, pretty well. He was in a mild descent today out at Tick Pt in Marin, when suddenly the elevator fluttered violently, and half the stab blew off. What was left of the elevator became jammed, and the plane bunted from about 200' above the hill to about 100' below the hill, and hit the only rock out there in the shoulder-deep coyote brush. It went in inverted at about 70 mph. This had happened previously with less dramatic effect, so, bottom line, the airplane isnt fooling around when it makes that sound.

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They find those rocks so easily!
There's one on the front side of Vincent that must go down to the center of the earth.
When a plane hits it, the earth rings!
And all that's left of the plane is the tail end!

there's another thread running on here regarding flying alone or with fellow flyers. My experience is a great illustration of the benefits of flying with others, especially when they have experience you don't. Had the fellows that flew with me last week (got the bird in the air and on the ground for me) not told me my bird was fluttering I would have thought "wow, that's a really cool sound". On the other hand, got the solution right here. Guess its good to have a little of both types of help.

as promised, the before and after shots. Photo with short horn and clevis further out on servo arm caused flutter, longer horn and clevis moved in fixed it.