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what causes rudder flutter

Old 10-09-2008, 01:21 PM
  #1  
cliffh
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Default what causes rudder flutter

Okay, this isn't an electric question, but it is an electric plane.

What causes control surface fluttering at speed, specificaly, the rudder?
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Old 10-09-2008, 01:30 PM
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firemanbill
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several things can cause that.

A weak, or too thin, control surface
Air gaps between the rudder and stab that cause turbulence in the joint
Too weak, or thin, of a pushrod
Too small, or weak, servo holding an otherwise strong enough control arm.
Too much speed for the airframe

But basically it is air turbulance that auses the flutter
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Old 10-09-2008, 01:30 PM
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jb48
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Most obvious cause is sloppy control rod linkage to rudder horn and/or rudder servo arm.
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Old 10-09-2008, 01:52 PM
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AEAJR
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Yes, I would look at slop in the linkage first. If you can move the rudder without moving the servo or bending the control linkage, you have slop that will lead to flutter.
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Old 10-09-2008, 02:14 PM
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cliffh
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Thanks for the quick answers.

There is no slop at all in the conrol linkages.

I think the problem is that it is a rather large rudder on a "pattern" type aircraft. It is only made from fan fold foam and the rudder itself is therefore rather flexible.

Maybe I'll try a small carbon rod across the rudder from front to back at the top and see if that helps.
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Old 10-09-2008, 02:58 PM
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Sabrehawk
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Yes if it's too flexible, it will flutter. Just like a flag flapping in the wind, or like when we were kids an put a small leaf in yer mouth and blow making it flutter and make that buzzing sound.
And it'll rip that rudder right off it it gets out of hand. Not to mention the load it puts on the servo, yep it can strip it's gears.

But I've seen control sufaces get ripped from the plane due to this.
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Old 10-10-2008, 12:08 AM
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cyclops2
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Default How fast & how big is the rudder in size ?

If the rudder has a crease in it by accident that weakens the stifness completly. Looks ok, but is not when flying.

No creases in any flying surfaces.
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Old 10-10-2008, 12:42 AM
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cliffh
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Well, there are no creases or damage to the rudder. The carbon fiber helped quite a bit though.

The aircraft is a 44" pattern plane made out of fan fold foam. The rudder is only one layer thick and is quite flexible. I'm going to just cut off the vertical fin and rudder, then remake them using double thickness foam.

I'm pretty sure now that the rudder is just too flexible and thin.

The plane sure flies great if I just keep it at good pattern speeds. That is what I should do, but it is too easy to get a little too fast sometimes
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Old 10-10-2008, 04:15 PM
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eflight-ray
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One other point that I'm pretty sure on, don't radius the trailing edges of control surfaces. Trailing edges should have sharp/square edges whether they are chamfered down or left constant thickness.
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Old 10-10-2008, 08:11 PM
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cyclops2
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Thumbs up Thanks for the update on your R & D!!

It is great when modelers who are push the limits of fan fold post information like you have.

Thanks a lot.

Rich
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Old 10-10-2008, 08:20 PM
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Conveluted condor
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How many hinge points ? How stiff and what material? I had the same problem on a P51 . I split the hinge points in half and added more. The lack of radius does help due to the laminar air flow off the trailing edge with no curves to induce vortices. This was meary my remedy for a glow powered semi scale war bird. I'm new to the e flight experience . I'm only on to my second electric.lol
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Old 10-11-2008, 12:45 AM
  #12  
cyclops2
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Thumbs up Good point Condor.

Flexiable surfaces, fixed and the movable control part attached to it, can cause each other to start the vibration.

It does NOT always have to be the smaller part.

Some foamies parts are just too " springy ".

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Old 10-11-2008, 10:10 AM
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cliffh
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UPDATE: Yesterday I finished remaking the vertical fin and rudder out of double thickness FFF. I also cut a bit off the top of the vertical fin and rudder as shown in the diagram. This completely solved the problem. I believe the single layer of FFF was just a little too flexible for the speeds encountered. I'm using 3 CA hinges by the way. I'll use 4 next time.
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