Advice on motor glider wanted - FES - Jet - Engine

On Saturday, January 7, 2017 at 9:31:46 PM UTC+3, Paul Ruskin wrote:
On Wednesday, January 4, 2017 at 3:48:53 PM UTC, Dan Marotta wrote:
Jets are cool, no doubt, and highly reliable, but they're very noisy
and, at the speeds gliders fly, very inefficient. I don't know how they
cost compared with a Wankel or piston engine, but I wouldn't have one.
The novelty will wear off.


Noise can certainly be an issue, and you wouldn't want to use them much near your home site. But for us glider pilots, efficiency is not a big issue (we don't use it enough to worry). However, they have a very distinct advantage over a piston or wankel, and that's the lack of drag when you extend them.

That means that your commit height is much lower - people I know with conventional engines have to start them above 1000 ft AGL (some use considerably higher) and be downwind on a suitable landing area in case they don't start. With a jet, there's effectively no extra drag, so waiting until 500 ft AGL is perfectly feasible - or you can be further away from the field. That's the difference between completing a flight and not doing so, surprisingly often.

For me, at the moment it's a toss up between jet and FES - though I suspect within a few years FES will win out as battery technology gets better.

Isn't FES already better?

According to reviews, 30 kg of batteries and 30 kg of jet fuel both seem to provide about 90 miles or 1 hour of range. FES provides more thrust/climb rate, is instant starting, non-smelly/mess, much quieter, the motor is lighter. Liquid fuels take minutes to refuel rather than hours, but that's probably not a limiting factor for a recreational glider. LIPO batteries may be a bit more likely to immolate themselves than liquid fuels but the risk is pretty low with both and you do have insurance and a parachute I hope :-)

I side with Bruce. It's pretty obvious to me that FES is the better choice.. I believe Bruce left off the most important advantage of FES...

That being the higher inherent reliability of operation. I can't prove it, but my own feeling is that FES would be an order of magnitude more reliable in starting up. And you'll know much more quickly whether it did or did not. The only real downside to FES is the drag that the folded props will introduce. I'm thinking that the tactical benefit of having a reliable motor escape will prove to be well worth the small added drag over the course of a multi-day event.