I've read in several posts where some people attribute the gap in the hinge line as a cause of inducing flutter in a control surface. I just thought I'd attempt to explain why this is in fact a myth. (Please feel free to disagree and present your opinion, as if I have to ask )

First of all the best diffenition I have for flutter is the uncommended or uncontrolable movement of a control surface during flight.

If you agree with this defination then it is logical to conclude that a gap in the hinge line will not cause flutter since the gap allows some of the airflow to pass throw the gap ruducing the effectivness of the control surface.

Gaps in the control surface can be best described as causing the model to become sluggish and less responsive to the control inputs, Where as models that are experiencing flutter become eratic and uncontrolable.

If you are able to easily move a control surface by hand WITHOUT seeing or hearing the servo move then you may experience flutter on the surface and the mechanical linkage and servo should be carefully examined before your next flight.

Stalled servos some people use the term flutter when this happens. A control surfaces linkage that appears mechanically sound appears to flutter (or move under heavy load) In some cases the cause of this is using a servo that is not powerfull enough to hold the required position under the load.
Evey is rated for a specific load or torque usually measured in ft/oz if you exceed that load rating the surface can be forced into another position i.e. pulling up from a high G dive and finding all of a sudden that the elevator doesn't seem to be working what could have happpened is that the servo became stalled and could not overcome the preasure of the airflow across the horizontal stab there for servo was unable to deflect the elevator enough to pull the model out of the dive (we all no whats coming next)

I don't claim to be an expert in these or any other matters I just thought I'd share my thoughts. Since I believe that the first step in preventing your next crash is understanding the last one. Sometime planes crash and we have no idea why thats just part of the sport but if you make every effort to learn why something is happening with your aircraft then at least for me the several hundred dollars worth of toothpicks wasn't a total loss since I learned something new..

The biggest flaw in that argument is that flutter is not uncommanded movement of the whole model but, as you initially wrote but then ignored, just in a control surface. Gaps in the hinge line allow the air to flow in an unintended fashion between top and bottom of the airfoil. This disturbance in the clean flow can easily cause flutter. Flutter of one control can very often not affect the overall control of the model too much (until it gets so bad that the surface parts company).

Servo stalling or blowback are not flutter and I've never heard anyone suggest that they are. However your analysis of other things that can go wrong but which are not flutter seems reasonable to me, if a bit irrelevant to a discussion of the effect of gap sealing on flutter. BTW gap sealing will not improve stalling or blowback either.

Steve

Servo stalling or blowback are not flutter and I've never heard anyone suggest that they are.

I have heard several people over the years attribute what I assumed was a servo stalling as flutter as a matter of fact I think they even called it "High speed" flutter which I believed was incorrect but everybody is intitled to the opinion and since it wasn't my plane that crashed I wasn't the one that needed to justify the cause.


I'm not sure I understand what you mean if the control surface isn't moving to cause the whole model to move then what is causing it?

A gap in the hinge line reduces the efficiency of the control surface. How on a mechanically sound control surface can flutter be induced? If the gap allows air to pass thru the control surface where is the uncommanded movement or "flutter"?

Just curious?

Wait so you are saying that the air passing thru the gap in the hinge line creates turbulance with causes the surface to move or flutter...

I want to get a hold of an old trainer and drop a over powered engine on it and set it up to flutter.

I want to record this on tape so people can understand what flutter looks and sounds like.


here is some in flight footage of a full scale expeirienceing flutter on the stab

http://www.airandspacemagazine.com/a...TCFlutter.html

there are others on that page that show diffrent angles and examples of flutter


edit:

here is a good article on flutter.

http://www.airandspacemagazine.com/A...FM/Hammer.html

Thanks unstable, Sounds like my defination of "flutter" is off

I was only considering flutter in relationship to the control surfaces.

Crashem:

This is extremely simplified, but think of a whistle. The smaller the whistle (gap between the control surface and its mount) the higher the pitch.

By decreasing the gap we get the whistle "High" enough that it does not have any effect at our air speeds. Conversely, with a wide gap (low pitched whistle) we can tear the control surface off at common air speeds.

By sealing the gap we remove the "Whistle" entirely. Sealing the gap works, it should always be done if there is any worry about flutter. It also makes the control surface more effective, and it's no trouble to do.

My twin had gaps, wide. When it came apart in the air I said "I Tried."

Bill.

PS: Leaving the trailing edge of the surface blunt also decreases the tendency to flutter. In other words, don't taper your ailerons. wr

I have never experienced flutter, but many years ago I had a Cub that had terrible aileron control even though the aileron deflection was substantial. One day at the field when I was bemoaning this fact, someone told me to seal the gap. I did. I then had to decrease my aileron throw to less then half of what it was and I still had much more control than before.

Flutter or no flutter, that made a believer out of me. I always seal ALL of my gaps now.

Are the hinge lines in full scale aircraft "sealed" ? Or do they maintain a minimum hinge gap ?

In a FS they are rarely sealed. But a 1/4" gap on a Full Scale plane is so minimal that it's unnoticable.

That will be equivalent to 1/32" hinge gap in r/c ? With this hinge gap size, will it be necessary to seal ? Will it improve control surface response significantly ?

Mike:

Not too long ago I had an airplane on the downwind leg, a strip of something came off, there was a short moan from the plane, then a balsa cloud with several heavy things falling from it. It was an OLD airplane, the observed strip was the seal from an aileron.

Speak true - the above description is something you would love to get on videotape, but it is far more dramatic than the actuality. I did lose an aileron, and the plane did have a hard landing, but it's repaired now with all the sealing strips replaced.

One good result, I now check all the gap seals more closely than I have in the past, found some more that needed replacement. If you don't keep your planes flying for ten or fifteen years you'll probably never have this worry.

Take care of them, they last a long time. My oldest r/c is about 20, it has worn out three engines so far. My oldest twin is seven, never had even a hard landing. Or never hard engough to do any damage, I should say.

Flown with care, twins stay in the air.

Bill.

Rajul:
In scale that gap is closer to 1/3200 inch. If you can maintain your gaps at 1/32" you'll probably not notice any difference with gap seals, unless you're a 3D flyer, and maybe not even then.

But seals are still a good idea, the added weight can't be noticed, and they're extremely easy to add.

Comment, Minn?

Sealed twins fly nice, and the eyelerons stye in plyce. (Sorry)

Bill.

PS: I should sat in scale EFFECT it's closer to 1/3200. wr

I totally agree with you Bill.

Rajul, As Bill said, the gap on an airplane this small will have a much greater effect than it would on a larger plane. Now take into consideration the fact that you may have a tiny difference between the gaps on each aileron. These gaps may have no effect whatsoever on how your plane flies. On the other hand, they might have a major effect. So the way I see it, by simply applying a strip of tape or covering, you are removing this factor. Now, if your plane acts "funny", that's one thing that you don't have to suspect.

Rajul:

Just a quick note. Mike and I have kept saying "Ailerons" but the elevator and rudder should be sealed too; they also can flutter.

Unsealed, a crash from flutter,
. that's the hobby shop's bread and butter.

Bill.

Bill, is your real name Nipsy Russell? LOL

Mike:

I have stomach problems that prevent my watching television, so I had to do a quick search to find out who or what Nipsy Russell is.

I can't believe he would have become the "Poet Laureate" of TV if his doggerel were as bad as mine. Haw.

But I dearly love puns, limericks, and twin engined airplanes. I'll forgo a twins couplet this time, and give you a limerick with a terrible pun:

A young maid named Anheuser,
Swore no man could surprise her.
. Then a fellow named Colt,
. Gave her virtue a jolt,
Now she's sadder Budweiser.

Bill.

PS: The stomach problem that keeps me away from TV? 90% of the programming turns my stomach. And the rest is usually not worth the time it takes to watch. I have a TV, it has literally not been switched on in over a year. wr

Bill, my stomach agrees with yours. Fortunately I have a remote control, and lots of documentaries to watch. LOL

And speaking of bad puns...

A guy walks into a psychiatrists office with a banana sticking out of his nose.

He says "Doc, what's wrong with me?"

and the doctor replies,

"You're not eating properly."

Mike, sent you a PM - wasn't anything about the thread.

A twin with a flutter, is worse than a stutter.

Bill.

Bill and Mike, thanks !

That A-6 wing flutter really makes you want to go out and check to make sure everything is set up right! I like watching how it excites a resonance, amplitude grows, and wing root explodes off the plane. (Tacoma Narrows anybody? )

LoboFlyer, the Tacoma Narrows Bridge was the first thing I thought of after reading this thread. I think everyone needs to understand that every structure has the potential to destroy itself because of vibration/resonance. That bridge collapsed in only 40 MPH or so winds.

You guys went from a very complicated description of flutter to a discussion of gap sealing. To set some of this straight. FLUTTER is caused on any surface by something being loose. It could be linkage, hinges, loose servo in it's mounts or broken gears. It is NOT cause by gaps between the moveable surface and the fixed surface. Gaps on fullscale aircraft of the aerobatic type and competition gliders are sealed for two reasons . One is to make the surface more efficient there by resulting in less required deflection and two causing less drag. If you experience flutter for any other reason it is an over speed condition caused by too much power resulting in speed for which the aircraft was not designed.

I was reading Martin Simon's book on model aircraft aerodynamics. Here's an excerpt on flutter:

"To prevent flutter it is essential for the hinged surfaces, especially on fast models, to be without slop, and for the wings and fuselage to be stiff. Stiffness is not the same thing as stength. A fibre glass rod or arrow shaft is very strong, but is not very stiff, indeed the flexibility of glass combined with strength is its main recommendation to archers who use it for bows as well as arrows. Some sailplane models like their full-sized counterparts, with slender fiber glass fuselages, are inviting tail flutter at high speeds. Second, the control rods and cables, and all their linkages should be free from free play and gain, as stiff as possible. This does not mean that pivots should be hard to move (although this too will help to prevent flutter), but the control rods should not be easily bent of flexed by end loads. Finally, the control surfaces themselves should be as lightly built and as stiff as possible, and where possible mass balanced"

An excerpt on control hinge sealing:

"With modern plastic materials it is quite feasible for a model to have a flexible surface on one side, so that the joint is fully sealed and smoothly curved, rather than sharply kinked (aerodynamically superior). Some contest sailplanes have adopted this method of hinging for both flaps and control surfaces with very good results"

Hope this helps..............Mike

seeling the hinge line is one way of helping stop flutter.

the reason this is done is that a gap invites airflow between the control surface and the main stab/wing this can cause a vibration like a whistle. this vibration can, at a certain speed create a vibration that matches the resonant frequency of the surface, amplify itself until you have uncontroled vibration, or flutter.

anyone into music, esp speaker/instrument design can tell you that if two peices are closed matched in resonent frequency a small input from an outside source will make those two peices feed each other and you will get a louder sound (more vibration)

sealing the gap eliminates the whistle which is a source of vibration that can change its pitch due to speed, and is more likely to match the resonance of the surface.

MinnFlyer,

I have a roll 3M 3/4" multitask tape I like to use to seal hinges. What do you think about the sheets of peel and stick covering sold at the LHS for patching? I bought one that matches the covering on a plane I'm currently working on. I'm thinking of using it to seal my ailerons and elevator.

Do you think sealing control surfaces can cause excessive stress on servos; either wearing them prematurely or causing battery drain? I had a trainer that was sealed with covering material. I could never seem to get the control surfaces to move as freely as I thought they should. The 3M tape seems to do OK though. But still a little stiff. I do deflect the surfaces as I apply the tape, to provide freedom of movement.

At an airshow last year, I saw a full scale Waco biplane with sealed hinges. I'm not sure what was used. But the tape was clear and appeared to be thicker than packing tape. It really surprised me.

(Oops! I just realised I dug this old thread up during a search. Thanks for any replies.)