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Old Jul 12, 2002, 04:41 PM
Curtis Suter is online now
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Aerodynamics Question

I'm building a G.W. E3D and here wrote the following in the instructions:

Round the leading edges of all the tail surfaces using a sanding block. LEAVE THE TRAILING EDGES SQUARE! DO NOT ROUND THE TRAILING EDGES! Rounded trailing edges tend to make an aircraft "hunt" and not trim properly, and they also exacerbate any tendancy to flutter.
End Quote.

Could someone please explain this to me? I'm a glider thermal duration guy by trade and thin trailing edges are the way we've always done to reduce drag. Also, flutter in models is normally due to linkages, IMHO and experience.

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Old Jul 12, 2002, 04:45 PM
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Someone did some wind tunnel tests and found control surfaces were more effective and less prone to flutter if they were either thin or square rather than rounded.
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Old Jul 12, 2002, 05:05 PM
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If control surface trailing edges are rounded the air will try and follow the curve, but will then break away unevenly, depenent upon the rounded TE profile, the control surface deflection, tiny protruberances earlier in the boundary layer, etc. In other words the flow will be a turbulent/separated mess. Also if the control surface is tapered the thick turbulent or separated boundary layer will mask small movements of the control surface, giving less accurate control near the centre.

OTOH if the TE is square and the control surface is flat the air will stay attached to the TE. The dead space behind the TE will cause some drag, but the streamline will converge with that from the other side better than if there was extra turbulence caused by a rounded TE. Also the surface will cause more linear and more predictable forces when it is deflected slightly.

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Old Jul 12, 2002, 06:29 PM
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The folks that recommend blunt trailing edges have never tried tapered trailing edges.
Flutter is due to many things, the first of which is sloppy or poor geometry on the linkage.
Using the hole in the horn that is closest to the surface to get the maximum amount of travel is the first culprit.
Too few hinges.
Too much weight in the surface aft of the hinges.
Too thin or whippy a pushrod.
Strip ailerons are very prone to flutter, as the sole anti-flutter support is at the root. Moving the horn or servo about 1/3rd the semi-span out works well.
I built up some representation control surfaces to be tested in a wind tunnel for flutter.. square edged, rounded edged, and tapered. Unfortunately the guy with the wind tunnel never accomplished the testing.
I build my airplanes with all types of trailing edges.. and don't get flutter because the linkages are up to the task.. It's NOT the shape that matters.
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