On Saturday, September 21, 2019 at 6:06:18 AM UTC-7, wrote:
The main reason motorgliders like the Arcus see such a large decrease in performance with the mast up is that the engine bay doors remain open, and they are about six feet long. That's a huge amount of drag. When we installed jet engines in the Tst-14 and four Arcuses, we had the main doors close over the engine bay and two small "sub-doors" open around the engine mount. With the engine extended, we measured the L/D of the Arcus J (jet) at 38:1.. The Arcus M gets 13:1 with engine extended.

The principal source of drag is the prop; the engine bay doors are aligned with the slipstream, and doesn't matter how long they are, the frontal area remains the same (very small). One ASH26e pilot reported a glide of 17:1 with prop out, and the 26e has fairly long engine bay doors. I have landed the 26e with the prop out and it was uneventful. I have also landed the DG400 uneventfully with the prop out; the one thing to remember is that you don't have the same glide that you do with the engine stowed, so don't make big patterns. I suspect that is what happened to the two cases that were cited. I have also made a simulated engine failure landing in a C152, and its glide was significantly worse (had to go straight for the runway after a 135 degree turn).

Tom