Your vario sink rate is an indicated rate, as is the airspeed indicator,
so your calculated glide ratio, /_in the airmass_/ should be a simple
calculation.Â* Having said that, the air is too dynamic to care what your
instantaneous glide ratio is and, like thermals, and investments, past
performance is no guarantee of future performance.Â* It's all "golly,
gee-whiz" stuff at the end of the day.

Have fun!

On 9/18/2017 10:36 AM, Scott Williams wrote:
On Monday, September 18, 2017 at 11:16:37 AM UTC-5, 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
Hey Jim,
My less than technical use of IAS/sink rate is a moment by moment comparison of airspeed indicator (in Knots) vs vario indication, also in Knots. the result is compared to conditions and how far away from my turnpoint or landing site.
Generally, if I'm worse than polar, I'm in less buoyant air, or outright sink and might entertain a course divergence, if better, I'm happy.
For Me, during soaring flight I find changes in lift/cruise too dynamic to really consider the difference of TAS vs IAS. Of course other than the limits imposed by lots of altitude. (I wish that was my problem more often)

I hope I have not revealed some fundamental ignorance on my part,
Good lift,
Scott, standard cirrus

--
Dan, 5J