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Old May 20, 2012, 02:18 AM
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Question
Aileron flutter

I've got a 150 size Sukhoi 26.
It has only been flown about 6 times and each time I moved the CG back to make it more neutral and on the last occasion it got a realy bad aileron flutter, enough to strip the gears on one servo and brack the glue joint of the other servo mount.

Question is was it the change in CG that started the flutter or is it just my imagination?
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Old May 20, 2012, 05:00 AM
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I would think that it would be wear on a bad servo, linkage, clevis etc. unless you were flying it harder each time you moved it back
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Old May 20, 2012, 05:02 AM
Zor
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Originally Posted by GeorgeM View Post
I've got a 150 size Sukhoi 26.
It has only been flown about 6 times and each time I moved the CG back to make it more neutral and on the last occasion it got a realy bad aileron flutter, enough to strip the gears on one servo and brack the glue joint of the other servo mount.

Question is was it the change in CG that started the flutter or is it just my imagination?
It is just your imagination.

Flutter is due to resonance taking place when there is looseness in the installation.

Zor
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Old May 20, 2012, 06:32 AM
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Thanks guys.
I've got some better Hitech servos to put in it but the only thing I could find anything wrong with is the control horns have a bit of flex in them. It is a cheep ARF that came with poor hardware but I used heavy duty linkages as the kit stuff was rubbish.
Still seems odd to me that when it was nose heavy it didn't flutter.
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Old May 20, 2012, 07:34 AM
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Every thing will flutter if given the right stimulus. The objective of not having flutter in any portion of our planes is to design them so that the resonant frequency (the cause of flutter) is out of the range of any stimulus imparted by variations in flight loads or outside inputs. This is usually done by making sure that the resonant frequency (where flutter will take place) is higher than any stimulus. This is done by insuring that the structure is very stiff and that no slop occures in any connections. Check your push rods for stiffness or bending under loads, make sure the aileron stucture is stiff (will not deform under pressure), make sure no slop inconnections at servo arm or control horn, rigid control horn, all hinges with no play or slop, etc.
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Old May 20, 2012, 10:22 AM
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I've read that flutter can be caused by too large a gap between the control surface and the wing (or stab or fin). Anyone know if there's any truth in that?
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Old May 20, 2012, 03:10 PM
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I've read that flutter can be caused by too large a gap between the control surface and the wing (or stab or fin). Anyone know if there's any truth in that?
Generally, no this is not true. However, I would not say "never" as it is possible for a gap to set up some frequency of air flow that could start an oscillation. A gap will usually be less sensitive than a gapless installation but rarely if ever contributes to flutter. Now, some have found that sealing a gap on ailerons that did flutter cured the problem but that is probably because, in adding the material to close the gap, they also stiffened up the structure by adding a layer or two of tape or other material which raised the resonant frequency of the structure.
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Old May 21, 2012, 03:05 AM
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Thanks for clarifying that Rodney.
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Old May 21, 2012, 02:04 PM
Zor
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Originally Posted by DeeBee1 View Post
I've read that flutter can be caused by too large a gap between the control surface and the wing (or stab or fin). Anyone know if there's any truth in that?
One thing you may consider is that if the hinges are so called CA hinges (or similar) and the gap is wide enough then it becomes possible to move the control surface up and down without rotation.

Such an installation can easily initiate some flutter.

In a similar case if the hinges are not solidly cemented to their hinge axis and the hinge tabs can flex due to their groves being too wide then again it is an invitation to flutter.

This is not easy to explain in text and assure that the reader undestand clearly what I am trying to say.

I will try to make a sketch and post it as soon as I can find time.

Zor

Edited to add a sketch
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Old May 21, 2012, 06:15 PM
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Zor. Thanks for the explaination.

I've checked and check again the hinges and there quite solid with only a minimal gap but I did find as I said earlier that I had some flex in the control horns and last night as I was changing them I found the wood for and aft under the horn was badly bruised. The horns only had 2 screws attaching them which let them rock back and forth with enough force applied. The new horns use four bolts and are much stronger.
Only a another flight will tell if that is the fix. I should be able a give her a try this weekend weather permitting.
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Old May 21, 2012, 06:22 PM
TigreJohn
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I have a sneaky hunch that changing the CG location may increase/decrease the probability of flutter. Effectively moving the CG changes the distance between the CG and the Center Of Lift of the wing. As an example of this effect, pick up a weight (CG) with your hand (COL). Now try picking up the same weight at the end of a ruler or yardstick with your hand at the other end. Feels a lot heavier. Now, your wing is going to need a greater Angle Of Attack to generate the lift needed for the perceived additional weight in the latter situation. In this case, this is going to add wind pressure to one side of your surface which would minimize the slop in your linkage. I may be wrong, but why does flutter disappear when you are maneuvering versus flying straight and level.
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Old May 21, 2012, 11:14 PM
Zor
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You are answering your own question.

When maneuvering the air pressure on the control surfaces picks up all the backlash wherever it is and the flutter stops.

Consider also that if the CG location is such that a lot of trim is needed for the elavator then the horizontal tail is creating some lift either negative or positive and that takes out the backlash.

It is not possible to predict the event of flutter.
The thing to do is avoid any flop between the servo shaft and the control surface. That includes any backlash at the servo shaft.

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Old May 22, 2012, 12:07 AM
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Originally Posted by jjkupinski View Post
I have a sneaky hunch that changing the CG location may increase/decrease the probability of flutter. Effectively moving the CG changes the distance between the CG and the Center Of Lift of the wing. As an example of this effect, pick up a weight (CG) with your hand (COL). Now try picking up the same weight at the end of a ruler or yardstick with your hand at the other end. Feels a lot heavier. Now, your wing is going to need a greater Angle Of Attack to generate the lift needed for the perceived additional weight in the latter situation. In this case, this is going to add wind pressure to one side of your surface which would minimize the slop in your linkage. I may be wrong, but why does flutter disappear when you are maneuvering versus flying straight and level.
Interesting theory, I'm feeling that you could be onto what I expierencing. With the change in CG I'm sure the air flow over and under the wing must change to some degree, which in turn changes the wing angle of attack and aileron loadings.
I think we need a wind tunnel to do some testing.
To give you an idea of the CG change, I moved the 5 AA cell rx battery back about 3" and a the same time went from a heavy MA 18x6 prop and spinner to an APC 18x6 without spinner.

I may move the RX battery back forward just to be on the safe side for the next test flight as the severaty of the flutter scars me. I feel that I'm lucky to have even got it back on the ground in one piece.
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Old May 22, 2012, 02:51 AM
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Thanks for the sketch Zor.
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Old May 23, 2012, 09:00 PM
Zor
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Thanks for the sketch Zor.
You are welcome.
A picture is worth 1000 words.

Zor
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