A long time ago, I did glide performance testing, with the engine off and the prop windmilling. A week ago I finally did glide performance testing with the engine at idle, to see how much of a difference there was.

Glide Ratio vs IAS - Engine at IDLEBoth sets of testing were done at 1600 lb weight. The best glide speed with the engine at idle was about 96 kt, and the glide ratio was about 10.1:1.

 


Glide Ratio vs IAS - Engine OFFThe best glide speed with engine off was about 104 kt, and the glide ratio was about 8.75:1.

 


Rate of Descent vs IAS - Engine at IDLEThe speed for minimum rate of descent was about 65 kt with engine at idle.

 


Rate of Descent vs IAS - Engine OFFThe speed for minimum rate of descent was about 88 kt with engine off.

Most builders conduct their glide testing with the engine at idle, but my results suggest that this gives a false impression of the speed for best glide and the glide ratio. In my case, the glide ratio is about 15% worse with the engine off, and the best glide speed is about 8 kt faster.