On Wednesday, January 13, 2016 at 3:30:05 PM UTC-8, Dave Walsh wrote:
I think there is little doubt that the FES system has lower drag
(thus better performance) than a pylon mounted electric
motor. Pylons are weight and complication that the FES just
doesn't have. With the pylon stuck out and the engine not
running the drag will be much greater than a failed FES.
Dave Walsh

Dave,

I don't think so. A small prop is not as efficient as a large prop. In terms of thrust, it's better to move a lot of air slowly than a small amount of air faster. The Wright brothers got that part right, big props turning slowly are most efficient. In most glider applications, a pylon allows for a larger prop that does a nose mount. So if available energy limited, a pylon mounted larger prop offers significant advantages. Other examples are the human and solar powered aircraft

With the pylon prop stowed, there is essentially no drag penalty. The same cannot be said for FES, though I don't know what any drag penalty might be. On MKIV customer contacted me because his yaw string did not stream straight back in flight but stayed off to the side. We determined it was due to flow disruption from the stowed FES blade/s. Moving the yaw string further aft on the canopy apparently solved the problem. The point being that the stowed blade was not just tripping laminar flow behind the blade (I think stowed at roughly 3 and 9 o'clock) but was influencing a much larger area, as the string was obviously at 12 o'clock. Strange.

I agree a failed FES should have less drag than a failed and still extended pylon and prop. And the risk of being stuck with an extended but non functional FES should be near zero. Whatever degree of risk, for a stuck extended pylon, one might assign, my understanding is it's fairly rare in the ASH26E community (self launch, but the idea is the same). The engine's been running, so when it's time to get rid of the noise, the battery has adequate capacity to drive the linear actuator to lower the pylon - and the actuator has an easy time of it as it's not fighting air flow as it would be when extending the pylon in flight. I imagine there have been more instances of failure to extend pylons than getting them stowed.