Suggested by my CFI, retired Navy Test Pilot, NAWS China Lake.
A good guess for single-engine light (sic...for a twin you have more issues) aircraft is Vx, the best rate of climb speed. But for a better answer:
Flying in calm air, with the engine at idle.
Record your airspeed as and vertical speed vs for at least 3 points - near stall, Vy, Va perhaps. For this purpose, you can just take the ratio of vertical speed to air speed to be L/D.
... Technically you'd need the horizontal component of the airspeed vector, but with realistic errors airspeed will be ok. If you insist, you can correct your airspeed by the factor sqrt( 1- (vs/as)^2 ) assuming of course vs<=as. For as/vs = 2, ( you are descending at 1/2 the airspeed! ) this factor is 0.86 so you can see it's not a big deal.
Given vs in ft/min, as in miles/hour
( mph * 88 = ft/min )
I'll make up some numbers:
VS AS L/D = 88·AS/VS
500 45 7.92
750 90 10.56
1000 110 9.68
If that's still not good enough.. There is a simple formula to create the parabolic approximation f(x) = -ax^2 + bx + c for 3 points... but if you are lazy as I am and/or have more than three points you can use this handy online polynomial regression calculator:
http://www.xuru.org/rt/PR.asp#CopyPaste
Putting in the above numbers, I get f(x) = -a·x2 + b·x + c = -1.273504274·10-3 x2 + 2.097008547·10-1 x + 2.042307692
The best L/D, can be determined by differentiating the above and solving for f'(x) = 0. In general form, I get b/2a, or 82 MPH.
Cool beans.
Any one have a POH for the He-111?