On Saturday, April 11, 2020 at 8:06:30 PM UTC-7, Eric Greenwell wrote:
On Saturday, April 11, 2020 at 1:09:56 PM UTC-7, Luka Žnidaršič wrote:
...
In case of FES self-launch, I feel much safer on critical altitudes till 50m,
as in case of motor failure there is still available pure glider efficiency to
perform a turn back if required. With retractable systems you end up flying a
brick, where you can hardly afford turning back without risk of spin entry.
Another big FES advantage is non problematic starting of engine above non
landable terrain. With retractable systems you should never try to start engine
without landing field below, as in case that engine do not start and cannot be
retracted for any reason, you end up flying a brick. Clearly propeller
clearance is advantage of pylon, but with pylon there are more problems related
to take-off in side-wind conditions. With FES there is much better rudder
efficiency. It is hard to compare FES with retractable systems, as they are
very different in also in flying style, and all pilots will prefer one over
another.

One: My ASH26E is hardly a "brick" with the pylon and gear extended, and I can
turn around to land back with just 200' AGL just as well as an unpowered glider
(for which I always used the 200' AGL criteria if the tow failed). I tested
that years ago - it loses very little during a 180 degree turn. A friend found the
same thing for his DG 400.

Two: It is true the FES pilot will have a better L/D after a failed start than a
similar engine/pylon glider (like my ASH26E) with a failed start AND a failed
retract. It is not necessary to be over a field, but only within gliding reach
with the mast extended. It is the same decision process for either propulsion
type. Unlike the ASH26E, the GP15 can risk a start almost as far from the landing
area as a comparable FES glider. Because mast is streamlined like an airfoil, and
the propeller folds in-line with the motor, it has much less drag than the
conventional "engine on a stick".

Three: I am not aware of any crosswind problems for engine/pylon gliders caused by
lack of rudder efficiency. The propeller air goes directly past the rudder from
the nearby propeller on the pylon, increasing the rudder's effectiveness at least
as much as the FES, where the propeller is much further from the rudder.
Regardless, it is not the rudder that is used to keep the glider going straight
down the runway in a crosswind, but the steerable tailwheel (every glider should
have one). On my ASH26E, I hold the tail wheel on the ground until about 25 knots
airspeed, then lift it off by moving from negative flaps to positive flaps (soft
fields may need a different technique).

Four: Off course, pilots will generally prefer one system over the other, but I
think some (many?) will find the GP15 much closer in operation to the FES gliders
than to the gasoline "engine on a pylon" gliders like my ASH26E. From clicking the
"mast up" switch to full power on the GP15 is 5 seconds; retraction is about 3
seconds. Compare this to my ASH26E, with about 20 seconds to initial power, and
about 30 seconds to retract partially for cooling, then another 1 to 4 minutes to
full retraction after cooling.

--
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


I find a huge difference in cross capability in my DG 800 on pavement vs. dirt or grass. You need help from the wheel on the ground to prevent weather vanning. I find the fringe turbulent thrust from the engine is not so effective on the rudder, standing behind the glider you fill most of the prop thrust is up near the elevator anyway. The pylon thrust line lifts the tail and reduces traction on the rear wheel. I have lost track of how many aborted takeoffs I have had in strong cross winds, while gliders being towed were doing just fine off dirt and grass..