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Flutter - How to find the cause??

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Flutter - How to find the cause??

Old 01-14-2015, 09:02 PM
  #1  
A. J. Clark
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Default Flutter - How to find the cause??

I have a plane that has a flutter at high speed. I have never been able to find the cause
or what might be fluttering. It's a 40 size with 60" wing span. It's probably coming from one
of the control surfaces. Need a way to test each surface. Would appreciate any ideas
anyone would have.

Thanks
Old 01-15-2015, 05:50 AM
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JohnBuckner
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First AJ more info is needed. Question one is what exactly type of airplane is involved, forty size does not cut it. is this a modern type of airplane with relatively large control surfaces or is this an older type and are the ailerons strip airerons?

Also a better description of a flutter episode that you may have survived is needed. Was this just a low frequency flutter that remained throughout the flight or was it something that sounded more like a machine gun that came on suddenly at speed and was stopped when the power was reduced and the nose raised ? After an episode was there any damage or loosening of any of the control surface hinges?

First I wish to state I am not any kind of engineer and only consider myself a simple minded survivor. Actually flutter analysis is an engineering discipline of it own and careers are based upon this discipline. There are no simple answers but fortunately there is a simple method to eliminate the problem of persistant cases when all else fails. It does work and has worked for full scale aviation since the twentys.

This solution is called 'mass balance' of the control surface. I have used a very simple method when an airplane demonstrates a surface flutter and has resisted all the other normal recommended cures (more expensive servo, linkages etc.) . It is very simple method that can be used as a test device. It does require a small hole for a four forty rod end in the surface.

I have used these many times and have posted pictures of how this is done in these forums in the past but also taken major heat over this simple mass balance in the forums. If you are interested I will take the time post the photos this after noon but not otherwise?

John
Old 01-15-2015, 06:29 AM
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AJ... Like John says, more data needed and his questions are very valid. In addition are you using torque rods for the ailerons or a dual servo set up? Flutter needs play or an energy storage unit, and the torque rods are perfect for twisting back and forth and getting into an oscillation. Elevators can flutter if you have a long run of rod or if there is play anywhere. I've had flutter on a few planes and getting rid of torque rods by using dual servos has worked. I just built a large gasser and used aerodynamic balancing and placed a mass balance ahead of the hinge line as the elevator surface was rather large. Make sure you find the source of flutter quickly as it will take your plane down at some point if you don't address it. I almost had a Super Tiger 2500 engine hit me once when a fellow flyer lost a large P-38 due to aileron flutter.
Old 01-15-2015, 07:12 AM
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Buckner is right- flutter is always caused by the control surface having a too rearward CG which lowers its harmonic frequency. If the harmonic frequency is low enough, turbulence can match it and cause flutter. It's the same thing that happens when a loudmouthed soprano breaks a crystal wine glass with her voice. In low speed terms, it's also how a kid on a swing can get some much speed going that they can get even with the top bar even though they aren't nearly strong enough to jump that high. So you have two choices: you can let flutter happen and try to control it using stronger linkages, hinges, and servos, or you can prevent it by raising the resonant frequency of the control surface by mass balancing it. I'm a fan of prevention personally, but others have done just fine by beefing everything up.
Old 01-15-2015, 07:17 AM
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Lone Star Charles
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I experienced some elevator flutter on a Great Planes Escapade 40. The plane made a very loud, low frequency buzzing sound when it got too fast. It was real easy to identify after I landed. The control rod had nearly wallowed out the leading edge of the elevator. I tightened all the connections and removed all the 'slop' in the system. That worked for a while but it seemed to only raise the speed for the onset of flutter. I tried John's method of mass balance. Flutter eliminated.

Both posts above offer good advice. Also, it would help if you could tell us what airplane, engine, and exactly what maneuvers and speeds seem to induce the flutter.
Old 01-15-2015, 08:49 AM
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Keep everything stiff as has been said like the other things. Also make the horn long and the servo hole as close to the middle as you can. Lots of people put in high and low rates, but the mechanical advantage goes away like that.
Old 01-15-2015, 08:53 AM
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are sure it's flutter or perhaps loose monokote. Flutter will generally lead to the destruction of the airplane unless it's caught early enough. While a loose covering will buzz sometimes but that is not flutter.
just a thought
Old 01-15-2015, 11:19 AM
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AMA 74894
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My Late Brother and I wrote an article right here on RCU a couple of years ago... IMHO it's still a very good read and a decent explanation.
http://www.rcuniverse.com/magazine/a...rticle_id=1289
Old 01-15-2015, 10:07 PM
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A. J. Clark
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The plane is a Tauri. I built it sometime mid to late 80's. I scaled it up to a 40 size from a Top Flite plans that came with a
smaller kit version. It has strip ailerons and single servo in the center. The hinges on all the moveable surfaces are monokote.
I made the horizontal stab. with a piece of balsa running length wise on each side and top and bottom to simulate
a built up air foil. Maybe that's causing some bad air current over the horizontal stab.

I'll try to get some photos up this weekend.

The flutter happens when going full throttle then diving a little where it picks up a little speed. Then it almost always starts the flutter.
Slowing down and pulling up always stops the flutter. I put an extra strip of monokote on the aileron hinge line to stiffen it up.
Didn't seem to help any. Also put a rubber band on the aileron linkage as a test to take out the slop . Didn't seem to help any.

I really like flying this plane and would like to get the problem fixed before it destroys
itself.
Old 01-16-2015, 05:29 AM
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thailazer
 
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Originally Posted by A. J. Clark View Post
The plane is a Tauri. I built it sometime mid to late 80's. I scaled it up to a 40 size from a Top Flite plans that came with a
smaller kit version. It has strip ailerons and single servo in the center. The hinges on all the moveable surfaces are monokote.
I made the horizontal stab. with a piece of balsa running length wise on each side and top and bottom to simulate
a built up air foil. Maybe that's causing some bad air current over the horizontal stab.

I'll try to get some photos up this weekend.

The flutter happens when going full throttle then diving a little where it picks up a little speed. Then it almost always starts the flutter.
Slowing down and pulling up always stops the flutter. I put an extra strip of monokote on the aileron hinge line to stiffen it up.
Didn't seem to help any. Also put a rubber band on the aileron linkage as a test to take out the slop . Didn't seem to help any.

I really like flying this plane and would like to get the problem fixed before it destroys
itself.
Could you put a separate servo in each wing so each aileron has its own short linkage to a dedicated servo? Likely that torque rod is the source of your flutter. Had to do this on my Tiger 2 a few years ago and it completely resolved the problem.
Old 01-16-2015, 05:41 AM
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JohnBuckner
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sorry double post

Last edited by JohnBuckner; 01-16-2015 at 05:51 AM.
Old 01-16-2015, 05:44 AM
  #12  
JohnBuckner
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Excellent AJ First in my opinion your airplane may well be at risk for 'aileron' flutter. Strip ailerons are/were quite susceptible to flutter for several reasons one being loose, poorly fitted or otherwise rocking of the torque rod in the trailing edge cavity. That's the obvious reason and the next that may not be so obvious is excessive flexibility of the aileron stock used. This may have been made even worse in your case since your airplane is an enlargement.

How to test for this and be sure hopefully before you loose a control surface in flight and loss of airplane is that small hole I spoke of in the ailerons for the very simple mass balance that you can make up in half an hour. If there is no more flutter then of course that's the answer and I think your airplane is highly suspect for ailerons.

All that is needed for these balances are two 4/40 threaded rod end wires. Two 4/40 hex nuts, four washers and two lead fishing weights with the hole in the middle and something around a half ounce each. The exact weight is not critical and what mass balance does is to move the airspeed that which the onset of flutter occurs to a speed that our airplane cannot reach.

Mass balance will stop control surface flutter of our models at the speeds we fly them at and it works well. It is a last resort when all else fails. Actually I like when building or assembling an airplane that has had a known history that may have aerodynamic balance area forward of the hingeline I will go ahead of insert some lead in this structure that is forward of the hingeline.

OK its early am now and I am getting ready to head to the flying field and will post some pictures that illustrates this simple fix this afternoon.


John
Old 01-16-2015, 08:03 AM
  #13  
A. J. Clark
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jetmech05
are sure it's flutter or perhaps loose monokote.
I'm pretty confident that it's not loose monokote making noise.
Old 01-16-2015, 01:25 PM
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dbacque
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Several years ago I had a very fast Hots that had serious flutter problems. Every time I would fix a problem and go faster, something else would flutter. Early fixes were stiffer pushrods, that fixed some of the elevator flutter. Then one side of the elevator was solid but flex in the elevator joiner let the other side flutter. Dual pushrods driving both sides of the elevator fixed that one. A torque rod broke loose allowing aileron flutter, fixed that torque rod with a carbon fiber wrap. Still had aileron flutter, higher torque servo helped that. Now everything was very solid and speeds were higher than ever. But I'd still get flutter in a dive. Finally on one dive the sun was glinting off the MonoKote and I could see that an aileron was fluttering. The linkage was completely solid but as John pointed out, the aileron was flexing. This allowed the root end to be held fast but the tip to flutter. I added a counterweight and the flutter went away. Flew it a few times and finally it fluttered again. I added a bigger weight and flew the plane at ballistic speeds for years. Unfortunately, it buried itself a few months ago. RIP.

Dave
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Old 01-16-2015, 01:29 PM
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Double post

Last edited by dbacque; 01-16-2015 at 07:38 PM.
Old 01-16-2015, 02:52 PM
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JohnBuckner
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Yup

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Old 01-16-2015, 02:55 PM
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JohnBuckner
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Old 01-16-2015, 07:11 PM
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52larry52
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John, what means do you use to secure the lead weight to end of the 4-40 rod?
Old 01-16-2015, 07:46 PM
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dbacque
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Mine were soldered on with Stay Brite silver solder. A little tricky as too much heat will melt the lead fishing weight but quick work was the answer.

Dave
Old 01-16-2015, 08:24 PM
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JohnBuckner
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Hi Larry. I just drilled the line hole in the sinker for a slip fit and a few drips of thin CA secured them nicely never had one come off. in some cases such at that blue wing it was done in the pits for a buddy at an event very crudely and just folded, hammered and CA dripped worked like a charm and saved the day.

For a while in the old Scale Warbird Racing Association (now defunct) most of the airplanes being raced were typically popular arf's of the day and engines roughly twice the size of the intended power. These things were always popping off like a machine gun. This simple mass balance works so well and could be installed in just a few minutes at the races that I started making up bunches of them and handed them out like candy at the races.

Back when I was messing with what then were called fun fly airplanes (airplanes that morphed into todays 3D) One type was an arrow shaft design (for the fuselage) with absolutely absurdly humongous control surfaces. mostly powered with various .25's, .32 heck even did one with a OS .30 Wankel . These arrow shaft fuselages were so flexable that you could only use full power in the vertical up line. Any thing over that at any other time the flutter that would be induce was frightening. Have had complete control surface separation many time, Heck any of the controls.


After that on the ships I called the Richochet I installed balances at the tips of all the squared off tips and balanced full 100%. remove a pushrod and that surface float just like it was a cork in the sea. It worked beautifully those airplanes could be thrown around with great abandon and power settings. The 100% balance is overkill of course and for a situation like the OP's its only necessary to use a small percentage, getting the flutter threshold into a speed he cannot reach.

John
Old 01-16-2015, 09:19 PM
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52larry52
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Thanks John and dbacque, I thought about soldering but as dbacque points out you can melt the weight. That's what would happen to me! I would melt the weight . Looks like almost anything will work, solder, epoxie, CA, or whatever. Most of my planes it seems are slow flying "old man" planes but I do have a few hot rods in the hanger too. I will give these a try when needed. Thanks.
Old 01-16-2015, 09:44 PM
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A. J. Clark
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Could you put a separate servo in each wing so each aileron has its own short linkage to a dedicated servo?
Yes I could probably put a servo in each wing panel. That would be a big project for me. At the present time
I am not sure which surface the flutter is coming from. I have ask the guys I fly with if they could see it or tell
what was fluttering. They never can spot it either. I wouldn't want to do that much work and not have it
solve the problem. Thats why I would first like to do some test to track down the source of the flutter.

I had one other plane that had a terrible flutter. You could see it in the wing it would twist and vibrate violently. Would
almost stop in mid air when it happened. I peeled back the covering at the trailing edge and added some more sheeting to
stiffen the wing up. That didn't change anything the wing still fluttered as before. Then one day the wood dowel
between the two elevator halves broke. I replace it with a 3/32" music wire. Nothing on the plane ever fluttered again.
So I would like to be more positive about where the flutter originates before I put a lot of work into it.
Old 01-17-2015, 03:31 AM
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This is maybe not what you want to hear, But maybe just fly slower,, not all planes are designed to go super fast,, there is a reason Q500 planes have very small control surfaces and not strip ailerons

good luck
Old 01-17-2015, 03:36 AM
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dbacque Excellent comments in your post #14 above. What many do not know is that the stiffness of the structure is very important in preventing or redusing flutter. Every thing will flutter if given the proper stimulus including iron bars or concrete slabs. The secret to preventing flutter is to have the resonant frequency (by high stiffness and low mass) above the frequency of any stimulus that initiates the action.
Old 01-17-2015, 05:35 AM
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All of the above is good advice, but there may be a simple cure. Seal the aileron to wing gap with clear tape. This will prevent air from passing thru the gap and can DELAY the onset of flutter. I had a similar problem with an old Q-500 racer called a Scat Cat. It had strip ailerons and when we started using tuned pipes, flutter surfaced. Sealing the ailerons stopped it , at least at speeds below 125 mph.

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