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Date: Mon, 01 Oct 2007 18:43:46 -0400
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Subject: Drag coefficient
From: "bob mackey" <n103md@yahoo.com>
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Here's another data point...

Lancair 235/O-320
Aymar-DeMuth 64x78 fixed pitch, wood prop
I have measured the glide ratio on several
occasions, and found it be 500 feet per nautical mile
at 100 kias.
That is mixture shutoff, throttle open to reduce pumping losses,
prop spinning.

With a 1500 lb gross weight, the drag at 100 kias is

   (500 ft / nm) * 1500 lbf = 123 lbf

  Or in rational units:
   (152.4 m / 1851 m) * 6672 N = 549 N

The equivalent parasite drag area, (CD=1)
is given by

   Drag = C*A*((rho*v^2)/2),
with the dynamic pressure (1600 N/m^2) in the parenthesis.

I can't separate drag coefficient and area, because I haven't measured
the area, but we don't really need to. The equivalent (total) drag area
is:

   C*A = Drag / ((rho*v^2)/2) = 0.343 m^2 = 3.7 ft^2

Which is quite a bit greater than the ~1.6 that Paul Lipps sees in cruise.
I'll take that to mean that the prop/engine drag is about 2.1 ft^2
equivalent drag area, assuming his airframe and mine are otherwise similar.