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Date: Wed, 21 Mar 2007 18:17:47 -0400
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Subject: Proud control surfaces
From: "bob mackey" <n103md@yahoo.com>
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Darrl Bretag asks...

>  when the manual calls for 0.050 gap between top skin and aileron and
> 0.050 gap between bottom skin and aileron and 0.050 gap between top skin
> and top of flap and ( the factory are know saying the bottom skin can be
> extended straight back to the bottom of the flap with a 0.050 gap as well)
> HOW much leakage through these gaps is going to
>
> - slow the aircraft down ?  ----   %   ---   or loss in air speed,

Well, I'm not going to do the calculation (or the experiment) for
you, but I'll offer some hints on how you can figure it out
for yourself.

First, how much power is consumed by air flowing through the gaps?
From the airfoil design, determine the pressure distribution
and the pressure difference between upper and lower gaps.
Given the viscosity of air and the size of the gaps, what
volume flow rate passes through the gaps at that pressure difference?
Energy = Pressure * Volume
Power = Pressure * Volume/time
That's how much power is lost to the leakage itself.


Second, as the flow exits on the top surface, how much drag does
that add? The flow exits the gap with approximately zero velocity
towards the rear of the wing. It collides with the air flowing over
the top surface. The slow air gets accelerated. The fast air gets
decelerated. When you mix fast and slow, momentum is conserved
exactly, but some of the kinetic energy is lost to turbulence
or entropy. The boundary layer is thickened, meaning that the
airfoil is now dragging more air along with it. More of the
airplane's energy is left behind in the disturbed flow field.
So... how much energy is that? The calculation to figure out
how much energy is lost when mixing fast and slow is not difficult.
The calculation to determine how much the boundary layer thickens
is much harder. As a first guess, I'd take the first calculation
and double it. Oh, and then double it again because the air that
flowed through the gap came from the fast stream on the bottom of
the wing.

Each gram of air in the gap had to be decelerated, leaked through the
gap, then re-accelerated, losing energy at each step.

The drag reduction associated with gap seals can also be
measured experimentally with the use of a drag rake
as pioneered by Dick Johnson on sailplanes.
For more on that, Google "drag rake dick johnson".

> what improvement would a 0.025 give us ?  -- %
Consider that part of your homework assignment :-)

> what improvement would a 0.005 give us ?  --  %
Consider that another part of your homework assignment :-)

> If we had a 0.005 gap all-round and had the 0.040 proud aileron
> leading edge what do you think the efficiency gain would be ??????
? If any  And is it worth all the effort.

How much does that 0.005" gap change when the wing bends under a 4G
load? What would happen if your ailerons jammed during a steep turn?
I think a 0.050" gap is perfectly reasonable given the construction
techniques in a Lancair, the chances of frost, bug guts, and rag fibers
getting in the gap. It's also easy to sand a uniform 0.050" gap
with a popsickle stick :-)

>  And should we be looking at the flap leading edge too ,
> ( standing proud above top skin ).

An air molecule can't tell the difference between a flap
and an aileron. Both want to keep the flow attached at
extreme deflections, so both should probably be well-rounded
and slightly proud of the forward surface.