How to simply determine the L/D of your glider

On Jan 11, 9:12*pm, Andreas Maurer wrote:
On Tue, 11 Jan 2011 13:56:32 -0800 (PST), Gary Evans

wrote:
This flight computer I used required 3 pieces of speed/sink rate data
to represent the polar. Best L/D, speed at best L/d and V2.

This what all flight computers do - they convert these three points
into an approximated polar curve.

But how do you get these three points by the method described by Dave?

Andreas

Well, I'll bite. Here's what I do for the planes I fly:

First, I've been fortunate enough to fly planes that have Johnson
reports
available. I know there are probably Akaflieg reports as well, but I
haven't
seen those. I carefully extract the data points from Dick's polar
charts and
correct them for my flying weight (unfortunately always considerably
higher
than Dick!). I input the adjusted values into my PDA (which just
wants
the sink rate at 3 airspeeds, rather than the numbers listed above).
Finally,
I set the Polar Potential via experiment. Typically I'll set it to
90% and then
see how well my final glides work out. If I have a bunch of altitude
left over
on a glide then I'm doing better, and I'll increase the potential. If
I tend to fall
below glideslope a lot then I'll decrease the potential. For the most
part I've
wound up with values around 90% or 92% (which probably means I need to
work harder at tuning up the planes I fly).

Essentially this is a refinement of the beginner approach to glide
slopes:
take the published value and divide by 2 as a safety factor. I divide
by something
closer to 1.1 and usually make it home just fine. The times I've had
to
break off have been because I was below glideslope to begin with.

-- Matt