2G wrote on 9/21/2019 1:30 PM:
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).

Was the 13:1 measured with gear down? With the propeller stopped? In the flap
position for best glide?


I agree with Tom. It may be counter-intuitive that the drag of a mast is
relatively small, but low speeds, the wing is generally the biggest drag producer,
not stuff sticking out in the wind.


As I mentioned earlier, 26E owners spend 2 to 5 minutes cooling the engine after
stopping it. The mast is lowered to about a 30 degree angle to the fuselage, which
holds the doors completely open. And yet, we can thermal effectively, and when we
finally stow the mast, we don't notice any improvement in climb rate. We're not
fooling ourselves on this - we have thermalled with other gliders while the mast
is in the cooling position, and still climb with them.

Open doors on 26E make very little drag at 50-55 knots, and I suspect that is also
true of the Arcus M. After all, would SH design a glider with doors that reduce
the climb rate by 300 fpm?

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf