As I understand it, altitude should not matter when doing these experiments to determine glide ratio. So for example if we are at 10,000 feet, gliding at 90Kt IAS, of course our true airspeed is say 20% higher, but then our true rate of descent is also 20% higher than indicated on our VSI. These two factors cancel each other out, so our glide distance is the same whether we are 5000 feet over sea level terrain, or at 10,000 ft MSL over terrain that is at 5000 MSL. Do I have that right?
The other interesting thing is that glide distance does not depend on weight, as shown on the glide distance chart. Heavier weights require higher glide speeds, but end up at the same distance. I always thought that this was a fascinating and very counter intuitive fact.
Next chance I get, I am going to try the glide with the prop control all the way out, to see how much better I can do with my 3 blade prop.