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Old Feb 03, 2005, 11:00 PM
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Gainesville, Florida, United States
Joined Sep 2001
1,456 Posts
Control surface deflections and drag

On my 38" WS DLG, I'm trying to reduce drag as much as possible. This led to a question - to reduce drag on the control surfaces and maintain authority, is it more beneficial to use smaller surfaces with larger deflections, or larger surfaces with smaller deflections? My instinct says the latter is better from an efficiency standpoint - it may be slightly heavier, though. Your input?
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Old Feb 09, 2005, 06:31 PM
can ya do that??...
electroboy's Avatar
Joined Jan 2004
531 Posts
I dunno for certain, but particularly in the realm of soaring flight, weight and surface area(drag) are concerns.
Just offa the top o my head I'd suggest keep wt.+drag to a minimum with smaller surface.

The goal for me when soaring is to make as few movements as possible.
so when a surface has to move, it will only be for as long as necessary to adjust attitude. Then back to most efficient flight profile- CLEAN

There's all kindsa stuff going on in flight and tradeoffs galore.

I hope some others look at your question, I'm curious too.
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Old Feb 09, 2005, 07:17 PM
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gouch's Avatar
Lake Macquarie, Aust.
Joined Dec 2002
14,330 Posts
Great question. Looking forward to the replies.

I THINK Ollie once replied that the latter was more efficient, but I'm not 100% on that. Would it really need to be havier to have a large surface-less throw setup? Heavier for slope soaring for example can be an advantage as well, at least around here.

The goal for me when soaring is to make as few movements as possible
I still consider myself a relative newbie at flying ~2 years, and even though i can fly, doing so efficiently seems to be the real test I try to build my planes as drag efficient as I can, only to waste it all when I fly them !!

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Old Feb 10, 2005, 02:12 AM
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vintage1's Avatar
East Anglia, UK
Joined Sep 2002
29,707 Posts
Something tells me that the lowest drag would be an all moving wing...twisted very very little. And whose twist changed along its axis,like a propellor.

So whatever is closest to that. Large outboard ailerons, and little movement 'd say.
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Old Feb 10, 2005, 07:12 AM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
3,693 Posts

A reasonable size surface with minimal deflection is best. But, rigging is more important than control surface size.

The purpose of the stabilizer is to set the angle of attack of the wing. The stabilizer should fly at zero angle of attack. Ideally in level flight the wing is rigged so that the angle of incidence is the same as the angle of attack required for level flight. If the rigging is correct then there will be no control deflection at all.

If control trim is required for level flight then you have a high drag situation. It means that you are changing the angle of attack for the entire fuselage. If the fuselage is angled it creates lift. Since it has an aspect ratio less than one the induced drag is very high.

You will get much greater drag reduction by rigging the wing incidence for zero trim deflection than you will by varying control surface size.
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Old Feb 10, 2005, 08:00 AM
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Punta Gorda, FL
Joined Apr 2002
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For small angles (<3 to 5 degrees) of control deflection, the drag doesn't increase much. Above large angles of control deflection, the drag goes several times more because of the turblunce increases. You can make the controls work better for small angles. You can make the work tail area with tail long fuselage behind the wing. You can you can reduce the tail drag with small tail area but at reduced controls. You can increase drag by making the CG aft up to a point. Etc, etc.

The designer can ballance those conflecting goals. There are other goals about designing tails. With many goals, the choices are the designer's science and art.
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Last edited by Ollie; Feb 10, 2005 at 08:03 AM.
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Old Feb 10, 2005, 11:58 AM
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Palmdale, CA
Joined Oct 2000
13,551 Posts
I prefer as small as practical, especially chord-wise, with the span altered to suit. And low deflections for non-aerobats.
On some planes I have no ailerons at all, to maintain the wing as clean as possible.
With a flying horizontal, it's the best of all worlds...
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