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Date: Mon, 19 Mar 2007 13:07:50 -0400
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Subject: Proud control surfaces
From: "bob mackey" <n103md@yahoo.com>
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Darryl Bretag asks:

> Could you please tell us more about the 0.040 proud leading edge
> of the aileron !
>   - reason

The forward edge of the aileron is aft of the laminar-turbulent
transition on the LNC2 (observed transition is about 2" ahead of the
aileron hinge on my LNC2). Probably similar on the IV and other
Lancairs also. This is in the "pressure recovery" portion of the
airfoil. The flow is decelerating as it transitions from the lowest
pressures near the thick part of the wing, to ambient pressure near
the trailing edge. This "uphill" flow or "adverse pressure gradient"
is an area where flow separation can be a problem.

If the aileron surface were faired with the wing surface, the airflow
would likely trip at the gap (or where the wind from the lower surface
is blowing through the gap). So... to keep the flow attached at this
junction, the aileron can be made slightly thicker than the wing,
creating a small area where the flow is accelerated. The more
favorable pressure gradient at the step helps keep the flow attached.

>   - extra drag

Nope, the proud surface offers a drag reduction relative to the
faired aileron-wing junction.

>   - speed restrictions

Not that I know of.

>   - wind tunnel tests results.

I don't know of a Lancair in a wind tunnel, but this idea has
been thoroughly tested. It has been the standard design practice
since about the end of WW2. Frequently people try to improve
kitplanes by fairing the gap and end up making their planes
slower and more prone to tip stall when moving the ailerons.

>   - any thing else about this concept we should know about

It applies to all hinged control surfaces. The rudder, elevator,
ailerons, flaps, and trim tabs should all be slightly proud of the
surface upstream. As Paul Lipps mentioned recently, the idea also
applies to the spinner-cowl gap as well.

How high the trailing surface should be depends on the boundary
layer thickness, airspeed, local pressure gradient, etc...
0.040" would be pretty typical for a Lancair wing.

>   Thanking you in advance for your expertise

You're quite welcome. It's good to know the money my folks spent
sending me to a fancy college wasn't entirely wasted :-)
Though I did get there a little too late to actually take
any classes from Theodore von Karman.
http://en.wikipedia.org/wiki/Theodore_von_K%C3%A1rm%C3%A1n

-bob mackey