Dumb Question about Flutter
 

Hi everybody,

I just had a dumb thought (more of a "What if") that I will put out there as a question. I know that when there is a large gap between control surfaces or the plane is flying faster than desgined, flutter occurs and it is more likely than not that the control surface will be torn off. I have seen this happen before and I have heard about it alot. The one common thing that I noticed is that CA hinges were used. I was curious, would flutter have the same destructive effect if Robart hinges were used instead of CA hinges. Now, there is no excuse for gaps in between control surfaces and I am not suggesting to use Robart hinges in the place of good modeling construction. I for one just like using Robarts on planes 60 size and larger. I like to channel the hinges so that there is no gap in my control surfaces. This is just one of those questions that just pop into your head kind of things.

Happy flying and soft landings!

RE: Dumb Question about Flutter
 

Ok, this is a very common misconception that comes up quite often. Flutter has almost nothing to do with the gap between your control surfaces. Flutter occurs when there is slack in the system so that they control surface can move freely. Most often this is caused by slop in your control linkages that allows the control surface to have some free play in it. It can also occur if you control rods can bend and be flexed.

The use of CA hinges or Robart Hinge Points won't affect flutter in any way. You can still get flutter with either system.

Ken

RE: Dumb Question about Flutter
 

Just to help dispel the rumor...

Having a gap has nothing to do with flutter.

Neither does the type of hinge you use.

EVERYTHING in the world has a frequency at which it wants to vibrate. If the turbulant airflow around your control surface reaches the right frequency, the control surface will try to vibrate.

The best cure is good, strong servos and no slop in the linkage.

If flutter continues, you have several options:

You can change the frequency of the turbulence by either sealing an open gap, or opening a sealed gap. You can also add vortex generators to your wing like the commercial airliners do.

OR

You can change the frequency at which the surface wants to vibrate by adding a weight to it.

But keep in mind that either of these cures will still not STOP flutter, they will just allow it to happen at a different speed - the trick is to have it happen at a speed at which you don't fly.

RE: Dumb Question about Flutter
 

As said above, the gap has no bearing on whether or not a surface will flutter. Everything will flutter given the proper stimulus. Those who say they cured a flutter problem by sealing the gap may have eliminated the flutter but it was not because they eliminated the gap. They also changed something else, most probably by making the surface slightly stiffer and eliminating any slop in the hinges which helps prevent the trigger that initiates the flutter.

RE: Dumb Question about Flutter
 

AMEN! to the above posts!

RE: Dumb Question about Flutter
 

And, I might add, flutter may occur when you fly a wing/control surface faster than it was designed to fly. Then, you will quickly find out what that resonant frequency really is. Often, it happens pretty fast. Flutter is pretty destructive. It will tear up a control surface in a heartbeat. Often, if you can hear it, it may be to late.

CGr.

RE: Dumb Question about Flutter
 

Destruction usually isn't limited to the control surface. Many times flutter will destroy the entire airframe. It doesn't take long, usually just a few seconds from the time you hear the flutter to the time that the airframe is destroyed. About the only thing you can do when you hear flutter is chop the throttle and level out the plane. Hopefully that will change the resounance of the flutter enough to stop it. If it does you should immediately get the plane back on the ground. And if you can bring back a plane from flutter then you should immediately head to Vegas and hit the Blackjack tables, because you are one lucky person!!!!Most planes don't survive flutter!!!

Ken

RE: Dumb Question about Flutter
 

Thanks for everybody's replies. I do understand that gap has nothing really to do with flutter. My main question is what is the effect of flutter on Robert hinges as opposed to CA hinges. I did enjoy the detailed descriptions of the causes of flutter. Everybody here is correct. Gap has nothing really to do with flutter directly. I have seen some planes with huge gaps in them but do no flutter. They do not seem to respond very well or not as much as you think they should. As always, I appreciate the input.

Happy flying and soft landings!

RE: Dumb Question about Flutter
 

I think Ken has already answered your question there:

"The use of CA hinges or Robart Hinge Points won't affect flutter in any way. You can still get flutter with either system."

RE: Dumb Question about Flutter
 

I guess I should have amplified my reply, meaning that it starts at the control surface then, in heartbeat... well, you get the idea. I've only seen it once, and once was enough. In that case, it pretty much tore up the horizontal stab of the plane that it happened on. If I recall, it was a 3D'er that was going way to fast for the airframe. The humm started, got louder, then we heard a noise like a bang as the horizontal stab desintregrated and down it came... very quickly. From the time of the hummm sound to the time that it destroyed the stab was perhaps, oh, 10 seconds??? I don't recall, but it was very fast. We were not sure what happened until the club "pro" said that it was classic flutter.

As I said, once was enough.. and it was not my plane.

CGr.

RE: Dumb Question about Flutter
 

Hmmm I am not sure about this statement. If you have a gap say 4 mm and mylar hinges then you have slop. It will allow the control surface to move up and down at the leading edge even if the control linkage is stiff. I have no evidence to support my SWAG on this subject but I would still not pass an aircraft as fit to fly with big gaps.

Flutter can destroy an aircraft in an eyeblink.

RE: Dumb Question about Flutter
 

You are somewhat correct, and this is a place where CA vs Robart hinges WOULD make a difference because Robarts would not flex like CA hinges will.

However, it is still not the "gap" itself that would cause the flutter, but rather the fact that the aileron is not sturdy.

RE: Dumb Question about Flutter
 

Robarts vs. mylar will change the frequency in which the flutter occurs. I feel the Robarts may help, as they are more rigid as a system, moving the frequency for the flutter higher.

The above is a deduction based on my Mechanical Enginering background, not on RC experience and may be flawed.

jack

RE: Dumb Question about Flutter
 

Just adding weight to the surface will lower the frequency, and increase the likelyhood of flutter. To counter weight the surface (place the weight ahead of the hinge line) is a standard cure in full size airplanes, and will work on models as well.

RE: Dumb Question about Flutter
 

Destruction can literally happen within half a second of flutter. It happened to my last Sig Astro Hog when it went into elevator flutter and tore the left hand horizontal stab off in less than 1/2 second. 2 seconds later it impacted the ground (30' pass down the runway) and essentially broke in half. The builder (I picked it up at an estate sale) put one of the control horn screws through the dowel connecting the elevator's (something nobody caught until after the fact).

Hogflyer

RE: Dumb Question about Flutter
 

I have seen flutter only once in person. It was with a really overpowered Great Planes Fundango. It had an E-Flite Park 480 with a 3s battery. This made the plane travel way too fast and it fluttered violently where the pilot chopped throttle and it actually came down in one peice. One of the CA hinges had broken off of the aileron, but that was about the extent of the damage. It was incredible how violent flutter is, especially when you can see it happening in the air!