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Old Jun 15, 2007, 12:23 PM
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Crazyrcer's Avatar
Pleasant Hill, Missouri
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What causes flutter, and how can it be eliminated?

I have recently experienced wing flutter for the first time. I was performing an inverted pull out from a dive when my 48" Unicorn started fluttering severely. The plane was not damaged, but it left me wondering what fluttering is and if it can be eliminated. I would appreciate any thoughts, ideas or suggestions.
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Old Jun 15, 2007, 12:40 PM
Gone Flying.
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Western NY
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Could be wing warp, or just plain ol' turbulence from the wind during your flight, hard to say. Is there anything protruding/sticking out of the fuselage (like a piece of tape) that could be catching wind?
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Old Jun 15, 2007, 12:51 PM
Slave to the Bean
United States, NJ, Montclair
Joined Jan 2005
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this is not a scientific explanation by any means but the flutter you are referring to is most likely more stress than your unicorn is set up for, the epp is naturally floppy and soft so it needs a little extra support if you want it to hold its shape under more radical forces than just scootin around the park.

the unicorn kit i had came with one spar down the center and with a mega 16/15/4 and 1800mah nimh pack would flap like a bird if i pulled out of a long dive, scared the crap out of me the first time it happened, but just like you said, no damage. try adding two thin carbon spars in the leading edges and put a thin strip of carbon fiber tape down the length of the ailerons, top and bottom. these will make the balsa so strong you can hold one in your hand with a can of chunky soup balanced on the other end and no sag whatsoever. good luck, hope this helps.
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Old Jun 15, 2007, 12:52 PM
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United States, WI, Muskego
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crazy - flutter - To vibrate or beat rapidly or erratically. this is mostly seen at high speeds and can be caused by a number of factors.

do you mean wing flutter (wings flapping like a bird) or control surface flutter (medium pitch buzzzz with some control loss during)?

wing flutter - only thing to do here is to stiffen the wing. a good 'A' spar layout works very well using carbon rods.

control surface flutter - get this under control quickly. blown servo gears are a real bummer. many things (or combination there of) can contribute to this - hinge slop, gaps in hingeline, play in control linkages, pushrods flexing or elevon flexing. there are probably more, but this is off the top of my head.

hinge slop - eliminate any play in the joint between where the bevel of the elevon meets the trailing edge of the wing core.

hingeline gaps - seal these up with a strip of tape (Scotch wrapping tape works well). high pressure air passing through the hinge line can do very unpredictable things.

control linkage play - eliminate all play. if you move the elevon the slightest amount and do not see the servo try to move, find the culprit and fix it. most common are the pushrod to control horn connections (both at servo and elevon horns). check for wallowed out holes. the holes should be just big enough to let the pushrod (or ez-link) go through, but not so tight as to put any strain on the servo.

flexing pushrods - either use stiffer pushrod material or support the pushrod with a guide tube.

elevon flexing - if you can flex (twist) your elevons without causing the servo to move, then they need to be stiffened. either replace completely with stiffer (usually heavier) balsa stock, or glass them (on the balsa not the tape) with .75 oz glass and WBPU or epoxy.

hope this helps.
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Old Jun 15, 2007, 04:34 PM
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Pleasant Hill, Missouri
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Quote:
Originally Posted by wingnutt
do you mean wing flutter (wings flapping like a bird) or control surface flutter (medium pitch buzzzz with some control loss during)?

wing flutter - only thing to do here is to stiffen the wing. a good 'A' spar layout works very well using carbon rods.

control surface flutter - get this under control quickly. blown servo gears are a real bummer. many things (or combination there of) can contribute to this - hinge slop, gaps in hingeline, play in control linkages, pushrods flexing or elevon flexing. there are probably more, but this is off the top of my head.

elevon flexing - if you can flex (twist) your elevons without causing the servo to move, then they need to be stiffened. either replace completely with stiffer (usually heavier) balsa stock, or glass them (on the balsa not the tape) with .75 oz glass and WBPU or epoxy.

hope this helps.
The flutter I am seeing during flight is wing flutter, not control surface flutter. I have had problems with the elevons loosing effectiveness during high speed dives, but figured it was just elevon flex due to the lack of a buzzing noise. I am using the stock elevons which flex quite a bit. Cutting and glassing some new elevons is still on my list.

The Unicorn uses three spars, one parallel to each LE and one over the CG. Would I need to add more CF spars or would extra strapping tape suffice? That would be a bummer if I had to beef up the wing as I just completed a nice new covering job.

I can easily fly the wing in such a way that it won't experience major flutter (wing flapping up and down 3+ inches), but flying slow enough to eliminate all the flutter will be hard to do. Will minor wing flutter (i.e just barely noticeable, looks like the elevons moving up and down) eventually destroy the wing?
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Old Jun 15, 2007, 05:13 PM
Mum is the word!
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It is the airfoil causing it whe it hits its max speed wall,.Super lifting foils get separation of flow across the controll surfaces that make them verry inefective. The flow goes rite over them and the elevator has to reach up super high to get them to do anything.

This requires tons of elevator to get it to react.It also seems to start an uneven oscilation over the wing and controll surfaces .The flow then over the foil that gets violent sometimes ending up in total destruction.

A few mods might help it go slightly faster but not much.I know what foils work, that is what I have done over the last 10 years with flying wing only.
I have seen 48 unicorns built even more rigid than I build my wings.Start the flap around 110 mph.
It just depends on what you want in a wing ,the uni is a great wing in its range.
Is this what you are experiancing?
ck
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Last edited by klique; Jun 15, 2007 at 05:32 PM.
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Old Jun 15, 2007, 06:39 PM
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Quote:
Originally Posted by klique
It is the airfoil causing it whe it hits its max speed wall,.Super lifting foils get separation of flow across the controll surfaces that make them verry inefective. The flow goes rite over them and the elevator has to reach up super high to get them to do anything.

This requires tons of elevator to get it to react.It also seems to start an uneven oscilation over the wing and controll surfaces .The flow then over the foil that gets violent sometimes ending up in total destruction.

A few mods might help it go slightly faster but not much.I know what foils work, that is what I have done over the last 10 years with flying wing only.
I have seen 48 unicorns built even more rigid than I build my wings.Start the flap around 110 mph.
It just depends on what you want in a wing ,the uni is a great wing in its range.
Is this what you are experiancing?
ck
That sounds like it could be why the elevons are loosing effectiveness in dives. After really pushing the plane in a dive from 400+ feet, the controls become very mushy and it rolls back and forth. Sounds like what you are suggesting. I plan to get some more balsa and cut some new elevons that will be fiberglassed for extra strength. My biggest problem right now is the wing flutter.
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Old Jun 15, 2007, 07:09 PM
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Quote:
Originally Posted by Crazyrcer
That sounds like it could be why the elevons are loosing effectiveness in dives. After really pushing the plane in a dive from 400+ feet, the controls become very mushy and it rolls back and forth. Sounds like what you are suggesting. I plan to get some more balsa and cut some new elevons that will be fiberglassed for extra strength. My biggest problem right now is the wing flutter.
It is gona keep hapening,no matter what you do,It is just hitn' that wall.
ck
good luck
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Old Jun 15, 2007, 09:55 PM
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Without knowing how the wing was built it is very hard to say. There are a nuber of reasons as mentioned by every poster. If you are not using tapered aileron stock ,get them. I have a member at my local field who flys his nitro plane pretty much wide open the whole time. You can here his control surfaces on the brink of shearing off.
I took a close look and hinted at why he had gaps between all of his control surfaces.
If this is your situation ,I would do as other have recommended and hinge top and bottom to rule it out. This doesnt require any more than clear packing tape.

Airfoil seperation may limit control but it should have little affect on flutter. You have a weak link in the build or control surfaces. In a straight line seperation is NA. The most adverse reation to serperation comes at high angles of attact. Seperation will cause control lose at high AOA not flutter. Please check your elevons. I use tapered aileron stock which is more dense then flat balsa as well as rigid and hinge them top and bottom. Not slop ,no gap, no flutter. If you used strapping tape in your build I would check to see if it has released. If you wing seems a little more flexable than before or youve upgraded the motor for perfomance ,you may have tomod the wing to handle the upgrade.
Jeremy
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Old Jun 15, 2007, 10:15 PM
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On highlift foils the pressure over the top and bottom is extremely unequal,at higher speeds it causes the ocilation to start wich goes through the wing and flutters the wing and controll surfaces .
ck
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Old Jun 15, 2007, 10:34 PM
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just trying to weed out the easy stuff first. The airfoil isnt an issue for me unitl I see the wing and know a little more about it.It very hard to make an educated guess without all the info.
Jeremy
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Old Jun 16, 2007, 12:10 AM
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The reason I am saying that is foils is because I have seen it on the corn alot.
Charger here on the groups,built one a while back with major sparring , stiff elevon stock 3/16 push rods, w ball links at each end and storage tape hinges.It was glassed with henries 183 and 3m 90 it was way over built and over powered.
Also the elevons were 3/8/ 1/1/2 stiff elevon balsa.
He asked me to flying on the highspeed passes because he was afraid of the speed.
It wasnt the speed that was the scarry part, it was the mushy elevator and the wagle while it was tucking at the bottom of the dive.

At the bottom of the dive I was way into the elevator w no avail then as it hit top speed it violently fluttered ,I thought it was all over ,but I was able to recover it befor it went in luckily.

We landed it and did an inspection to see how dammaged it was and you couldnt even tell and all gear was tight and intact.
If it had not had such a soild build it would have ben destroyed and gear all over.
He later realized it wasnt the speed of the uni's he was worried about it was the instability at the top end that he was associating w speed. I have found with wing with the rite design for those speed ranges, It can be a dream to fly at twice the speed.
ck
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Old Jun 16, 2007, 06:22 PM
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Anyone got a used corn they want to sell?
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Old Jun 16, 2007, 06:51 PM
Mum is the word!
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Nope but I have a set of sleghammer cores and a 52'somthin someone sent me recently,That I will never build if you want them back.
ck
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Old Jun 16, 2007, 06:59 PM
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lol,

Flutter is a self-starting vibration that occurs when a lifting surface bends under aerodynamic load. Once the load reduces, the deflection also reduces, restoring the original shape, which restores the original load and starts the cycle again. In extreme cases the elasticity of the structure means that when the load is reduced the structure springs back so far that it overshoots and causes a new aerodynamic load in the opposite direction to the original. Even changing the mass distribution of an aircraft or the stiffness of one component can induce flutter in an apparently unrelated aerodynamic component.

At its mildest this can appear as a "buzz" in the aircraft structure, but at its most violent it can develop uncontrollably with great speed and cause serious damage to or the destruction of the aircraft.
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