On Monday, September 18, 2017 at 9:16:37 AM UTC-7, Jim wrote:
I sure need help here.

As far as I understand polar diagrams the airspeed axis is for indicated airspeed, not true airspeed. This certainly makes sense.

BUT, I have also come to understand that an estimate of L/D for a given airspeed can be obtained by dividing a given airspeed on the polar curve by the associated sink rate for that point on the curve.

BUT AGAIN, if the airspeed on the polar diagram is IAS wouldn't an L/D derived from a polar diagram be reasonably accurage only at sea level in standard conditions in still air?

If a glider pilot is interested in the distance traveled for a given loss of altitude wouldn't TAS be more useful than IAS - especially at higher altitudes? Of course headwind/tailwind would still be a factor in how much distance the glider can cover for a given altitude loss.

I've likely just bunged up this entire thing.

Jim

To the first order anyway, the glider only knows IAS. If you are high, the air is thinner, you are flying faster to generate the same lift (and the same pitot pressure), and also sinking faster (since you are gliding at the same angle). L/D is Lift/Drag ratio, and is numerically equal to the glide ratio. Lift and drag both vary the same with increasing altitude (proportional to air density x velocity ^2). The only thing that changes is your speed over the ground - TAS, if no wind.