Exactly. The mass and mass distibution of the control surface plays a huge role. This explains why sealing the gap could cause flutter. There are many factors involved in the causes of flutter. The flag explanation is typical of this. Sealing the gaps is not the cure-all for flutter, it just helps with control authority when all other negative factors are eliminated. All scientific fact is a mere approximation under ideal conditions.

Safe Flying!

Here's why these things get spread
note the last line

http://www.modelaircraft.org/newsletters/0304_8.asp

All the r/c sites I've seen that discusses flutter recommends sealing the gap, for one reason or another. Not saying that they're right or wrong, just a note. Do I seal my gaps? Simple answer, yes. Why I do it? That's another topic. I would like to hear more

You guys finally made me to shake the dust off my copy of Martin Simons book, Model Aircraft Aerodynamics, 4th Ed. Here's some excerpts from it, with due credit given:

(p 180) 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. Secondly the control rods and cables, and all their linkages, should be free from play and again as stiff as possible. Finally, the control surfaces themselves should be as lightly built and stiff as possible, and where possible, mass balanced

(p 63) Serious losses occur if there are gaps through the wing at any point. Through such gaps the air flows from the high to the low pressure side of the wing, creating turbulence and reducing lift. Control gaps have similar effects and all such leakages should be carefully sealed

For what's its worth. Well at least, I have made good use of my book

There is an excellent section in one book that covers the formulas needed to predict the flutter speed of a wing structure. Along it's tedious and winding way, it considers the pitching moment of the airfoil. Cutting to the bottom line, different airfoils have different pitching moments, and the results of the computations will be different for different airfoils. Two ideas stand out from that simple fact.



Change the airfoil and the flutter speed changes. (An airfoil with no aileron gap is a "different airfoil" from the same airfoil with a gap.)

Change the airflow over a wing/aileron system, and you've realistically got a different airfoil.

Following those two simple ideas.............

One of the factors is the force the airflow can exert on the wing structure. The wing structure can include the shape and strength of the aileron or it can not include the shape and strength of the aileron. If the aileron is loosely attached, it would not contribute strength would it. It would also not transfer the aero forces it's experiencing into the wing structure would it.

If the aileron were experiencing forces and transferred them into the wing, the sum of forces the wing is experiencing would increase would they not. So if the aileron is more firmly attached to a wing it loosely attached to, you would expect that wing to feel increased forces. Seal the aileron from tip to tip to the TE of the wing with more hinges or gap covering and you've more firmly attached it, haven't you.

And in effect, you've just done something else to that wing, haven't you. You've just changed it's chord. You've gone from a wing with say 8" chord, to a wing with a 10" chord (for example).

It's well understood that increasing the lever arm of a twisting force gives it more "twisting force".

We've got a wing that was strong enough not to flutter when it was 8" wide. The aerodynamic forces weren't strong enough to flutter that TE when it was back at 8". But now that TE is back at 10" and the aero forces have 20% more leverage, don't they. Not only that, but they've had an extra distance to build up even more force.

Seal the gap between an aileron and it's wing and you've just changed two systems into one. The flutter speed of two systems is going to be different than the flutter speed of one system. Which way will the speed change. Danged if many humans or computers can figure it out much less predict it off the top of their heads. And the strength systems have changed for the wing. And the force systems have changed for the aero forces.

And ain't nothing changed in nature about flutter. It's gonna do what it's gonna do and when it wants to do it. But now that you've sealed the aileron to the wing more solidly, and given the aero forces more leverage to exert on that wing, I'd hazard a guess that it's maybe more probable that the flutter can occur at a slower speed.

.... but, then again.........

CA hinges are not the problem. I think the biggest reason you see so many instances of flutter nowadays is 3-D flying.

Since the advent of 3-D, Control Surface size has gone through the roof. This gives the control surface a huge mecanical advantage over the linkage.

So in the past, when the linkage had the mechanical advantage, now the control surface does.

Very true. The other problem in 3D planes is that they are designed for slow flight but have large engines that are often capable of level flight at speeds beyond the flutter speed of the huge, lightly built control surfaces. So if the pilot doesn't practice good throttle control the plane is a flutter time bomb waiting to happen.

Dave

Good point Dave

Yeah, and make sure you put enough rubberbands to hold your wing on.

LOL


Tom

I sealed the hinge line on the elevators of my F-20. Nowhere else. I am happy to report that after the maiden, and two subsequent flights yesterday there was no flutter in the elevator or anywhere else.

As an aside, here is an interesting true story:

I went to A&P school in '78-'79. One of the instructors there was an ex Lockheed engineer. During class one day, discussing flutter on full size airplanes (don't ask, I don't remember past a couple weeks anymore ) he showed us a movie he had taken during wind tunnel testing of the Lockheed L-1011 during it's development. This was inside the famous, huge wind tunnel that Lockheed had in Burbank. It was a large, solid wood scale model of the L-1011 and it was mounted on some sort of stand. The wind was blowing but of course we could not see it. As we watched this model apparently just bolted down and totally stationary, it suddenly and completely exploded into nothingness. Vaporised! Well, as it turns out there were two cameras filming. One was a super high speed camera so the movie could be played back in ultra slow motion. What I saw then was mind numbing. The model in the wind tunnel started to move. The wings started flapping up and down. The engines were simultaneously moving left and right. The empenage was twisting right and left. All this was happening simultaneously. Then the model disintegrated. The instructor said this due to flutter. And this is what can happen to a real, full size airplane. After this, the engineers did some more work and solved the problem. Obviously, as the L-1011 was a successful airliner.

Let me guess... they sealed the hinges

For those who missed the outrageously funny joke just posted, allow me to explain it in a round about way.

Let me guess...... they DIDN'T seal the hinges.

What I meant was... that was my guess as to how they corrected the problem.

Whatever they did, they used pencil and paper to figure it out. Did they have computers back then?

Jeez, and I was thinking you was being funny..... ain't that a kick in the ear.

BTW, I don't think there is a commercial heavy that has anything even remotely similar to hinge gap sealing. Jeez, could you imagine the cost of enough Ultracote for just one big jet????????

And where'd they get a covering iron that big????

Actually, yes they did have computers back then. How do I know that for sure? hehehehe

Actually they even had them in the 1011. The first one delivered to Eastern Airlines was flown block to block from the left coast to Miami without the pilot touching the yoke.

(hmmmmmm is there humor in that last sentence? The thing the pilot flies a big jet with being called a yoke?)

Yes, I was being funny, I just forgot to put the " " after it

I seal mine now because it is easy and it makes the plane fly better and helps prevent adverse yaw by sealing the rudder also. It only take a short time. Why not just do it and then you won't have anything to cry about when you have a disintegrated plane from flutter. Plus if a hinge breaks the seal will hold it together in most cases. It is worth the time. I use 1" Hinge tape from DuBro![8D]

Gibbs

Yes, some big heavy aircraft do have hinge line seals on flight controls.

Thanks for catching that. I really blew what I meant to say. Which was, "I don't think there is a commercial heavy that has anything even remotely similar to our style of hinge gap sealing." You really don't see film ironed across the gaps. You do see lots of surfaces that are inset into the leading component or have fairings that extend past the hinge lines.

Be so kind as to share which ones and how the seals are done?

BTW, it is a somewhat recent change that a lot of the larger commercial aircraft actually don't do aileron turns as often nowadays. They've gone to using spoilers for the turns. To turn, they simply raise a "spoileron" on the inside wing. It increases drag that yaws the airplane in the desired direction and since the wing also loses lift, the airplane will also roll in that direction. Those spoilers (and other surfaces) are often hinged with piano hinging which provides fairly effective sealing.

Well, from my own hands on experience, ailerons on the Lockheed C-141. The seals are made from what appeared to be a heavy plastic coated canvas. They failed 80% of the time. Most of the planes flying would have torn seals, this was not a defect that would ground the aircraft. When the plane would come in for isochronal inspection you could count on changing them out. Lots of hard to reach screws and retaining plates.
More recently I've read that Boeing plans to seal some surfaces on a new model of the 737 designed for short take off and landing. I get to touch a lot of 737s, basically every new one, so when I see it happen, I'll let you know.

Devo, can you post some pics, if available?

Sorry, I'm no longer in the USAF, and Boeing propriety prevents pictures.,