Hi L
I was thinking of any heavier than air fixed wing device that had no thrust
I should also have said in my first post that the best gliding speed that I was trying to work out was the one that gave you the most range for the height you had (usually the pilots problem) as opposed to the one that gave you the longest time before you got to the ground.
I fully accept that if you have thrust available from a donk then the best "range" speed will depend on both the airframe and the engine characteristics and as such is a much more complex issue than the simple gliding for range thing.
JF