On Mon, 18 Sep 2017 09:16:34 -0700 (PDT), 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.
... text deleted

If generated from flight test data, the polar plots from all the
flight test measurement programs I have been involved with include the
reduction of all data (including airspeed) to sea level standard
atmospheric conditions, (e.g., 29.92 mb, 59F).

In addition, if measured data, airspeed is corrected for instrument
erros as well as pitot/static errors. Altimeter data is similarly
corrected for instrument errors.

If the polar is from a theoretical calcuation rather than a flight
measurment, I'd assume they do similar correction to standard
atmospheric conditions.

Anyone interested in the full details of flight preformance
measurements can consult Dick Johnson's comprehensive article in April
1968 SOARING magazine. Also, Dick published a more general update in
May 1989 SOARING.

Bob

On Mon, 18 Sep 2017 09:16:34 -0700 (PDT), 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


---
This email has been checked for viruses by AVG.
http://www.avg.com