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Old Oct 13, 2014, 07:01 PM
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blunt trailing edge=vortex shedding->flutter?

Can a blunt trailing edge of a Flat/rectangular control surface cause flutter?

I know with proper reinforcement/no slop linkages, flutter usually is "killed" but would making the trailing edge sharp make the flutter go away?
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Old Oct 13, 2014, 09:21 PM
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Are you referring to the control surface oscillating or the entire wing oscillating?
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Old Oct 13, 2014, 09:55 PM
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Old Oct 13, 2014, 11:12 PM
B for Bruce
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Ideally the trailing edge SHOULD be sharp. That is the proper way and the way that produces the least drag.

I've also seen more flat non-tapered control surfaces with extreme blunt edges flutter than I have with sharp TE's on control surfaces.
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Old Oct 14, 2014, 12:04 AM
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just i had guessed, i asked because my F-22 and F/A-18 both had blunt trailing edges and both had a bad case of flutter up to the point one of the fins flew off
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Old Oct 14, 2014, 10:14 AM
B for Bruce
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With larger blunt edges you're going to get the same sort of alternating vortex shedding that causes flags to flap in the wind with a back and forth flutter. But a smooth airfoil will float with no flutter just fine.

If you're working with Depron foam then the foam itself is generally not stiff enough to work all that well at resisting the twisting. Add on the blunt TE issue and you have a good recipe for moderate to high speed flutter. Maybe try shaving a set of controls down to a triangle shape with a sharp TE then tape or glue on some tissue over all the sides of the control to stiffen it up with a rigid skin.
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Old Oct 14, 2014, 02:28 PM
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also, blunt trailing edges are heavy near the trailing edge wich promotes flutter . the center of mass of the surface nearer to the hinge line is better, as occurs with sharp TE
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Old Oct 15, 2014, 07:03 AM
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Originally Posted by m4rc3l View Post
also, blunt trailing edges are heavy near the trailing edge wich promotes flutter . the center of mass of the surface nearer to the hinge line is better, as occurs with sharp TE
Yes--
blunt trailing edges are good on some designs- notably aerobatic types - the full scale EXTRAS are examples
On our models, the cg of the control surfaces is important in stopping flutter On our large designs, we kept balance point on ailerons an elevators at 20% or less. the rudder - Blunt-- some even use a plate along te. this strange looking device stops tail wagging
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Old Oct 15, 2014, 08:36 AM
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I remember reading somewhere that a rounded trailing edge is actually worse than a square cut one.

Does that hold true ?
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Old Oct 15, 2014, 10:02 AM
B for Bruce
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Yes, rounded is still worse. As I recall the pictures in the article in a magazine from years ago the round TE tends to form that side to side "flag flutter" more strongly with alternating vortices being shed that makes the surface want to flutter.

But the proportionally large and strongly square "blunt" TE was only a little better.

Let's keep something in mind. When you folks call something a "blunt" TE I'm seeing and discussing a flat totally untapered 1/4" flat strip aileron on a 10 to 12 inch chord wing section. Not a tapered section with a 1/32" fat TE done for some small amount of durability.
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Old Oct 15, 2014, 10:45 AM
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Quote:
Originally Posted by BMatthews View Post
Yes, rounded is still worse. As I recall the pictures in the article in a magazine from years ago the round TE tends to form that side to side "flag flutter" more strongly with alternating vortices being shed that makes the surface want to flutter.

But the proportionally large and strongly square "blunt" TE was only a little better.

Let's keep something in mind. When you folks call something a "blunt" TE I'm seeing and discussing a flat totally untapered 1/4" flat strip aileron on a 10 to 12 inch chord wing section. Not a tapered section with a 1/32" fat TE done for some small amount of durability.
We are not all on same page - The surfaces I am noting, are typically 3-5 " chord with the TE 1/4" thick -part of a sym airfoil -say 10% to 15% thick
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Old Oct 15, 2014, 12:01 PM
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Here is an interesting document related to the shape of trailing edges and its influence on lift and drag. This is not related directly to the OP question as it doesn't speak of the influence of the trailing edge's shape on potential flutter. The links in this document are very interesting and you will see some familiar names as well...

http://secretofflight.wordpress.com/trailing-edge/

One thing that is pretty well accepted is that it is better to have a square (we speak of % of chord value, if so much) than a rounded one (or even worse elongated like elliptic, and "pointy") for a control surface as it will give a crisper response. Most of the aerobatic full scale planes have such trailing edges.

Flutter on control surfaces can be linked to a lot of conditions/circumstances. I personally don't think that the shape of the trailing edge only can be a generator. But combined with other variables as balancing (aerodynamically and mechanically), quality and type of hinge setup, gap sealing (or not), control loop tightness (or not) and others, it may exacerbate a potential problem.
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Old Oct 15, 2014, 02:23 PM
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Yes Richard, but if you try to fly the flat foamies at higher speeds, if they were propped up for it, you'd find that those wide surfaces would flutter really well.

Or if you're talking about an outdoor faster model then I'm going to assume that such surfaces are made or otherwise chosen to be stiffer as required.

Quote:
Flutter on control surfaces can be linked to a lot of conditions/circumstances. I personally don't think that the shape of the trailing edge only can be a generator. But combined with other variables as balancing (aerodynamically and mechanically), quality and type of hinge setup, gap sealing (or not), control loop tightness (or not) and others, it may exacerbate a potential problem.
Well, flutter is a mechanical resonance. It's the same effect that causes a tone when we whistle, blow over a bottle opening or play a flute, try to build a symmetrical stayed bridge over Tacoma Narrows or any number of other things. So yes, it all has to come together just right. Which would explain why some blunt surfaces don't want to flutter while others flutter easily.
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Old Oct 15, 2014, 08:13 PM
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I designed, built and flew stuf up to 45 pounds -160 cc engines etc- foamies are typically slow flyers by design.
I LOVE the new EPP molded and cut foam designs The 10% airfoils were common on the bigger models 12% on some but I liked the thinner stuff best .
flutter was more a result of surface rigidity and balance - (surface balance) we had to be careful. I have seen stabs ripped off in a split second ditto for ailerons
Resonance is a field I know quite well - from musical instruments to large commercial machinery . The fix for resonance is funny - you start by changing ---something.
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Old Oct 16, 2014, 04:06 AM
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Or by tuning it out, once you know what the precise harmonic is. The Stockbridge damper is a good example of that approach ( http://en.wikipedia.org/wiki/Stockbridge_damper ). I think that the leading edge counterweights for control surfaces also contribute to dampen out flutter by tuning out resonance.
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