Best option for electric self starting glider

Dne sobota, 11. april 2020 12.02.19 UTC+2 je oseba jld napisala:
The propulsion efficiency has much more direct effect than pylon drag.
In fact, for Takeoff and climb performance, the overall glider drag has secondary effect compared to aircraft weight and available thrust.
Without fuselage interference and opportunities for larger props, the pylon mounted motor provides a significant efficiency advantage. This has a direct effect on climb performance in terms of climb rate and altitude gain.
Running large battery packs, beside running cooler, also provides better energy efficiency and installing large batteries in the wing reduces overall structure weight and provides better flying handling characteristics due to CG.

FES is a nice solution, especially for retrofit of existing gliders.
For a brand new design, there are opportunities for better optimizations.
Is GP at the optimum, probably not (yet:-), but they have certainly made good design decisions for both teh glider and the propulsion.

Actually efficiency of glider in powered mode is quite important:
1.Efficiency of FES powered glider in powered mode is the same as in glide mode.

FES powered glider, L/D 50 and 500kg T.O.W, minimum sink 0,5m/s:
-required thrust for level flight; 500kg/50=10kg which is 100N
-another 100N is required for climb rate 0,5m/s; 200N
-another 100N is required for climb rate 1,0m/s; 300N
-another 100N is required for climb rate 1,5m/s; 400N
-another 100N is required for climb rate 2,0m/s; 500N
-another 100N is required for climb rate 2,5m/s; 600N
-another 100N is required for climb rate 3,0m/s; 700N

2.Efficiency of most powered gliders with retractable systems is about half (or even worse) of the efficiency as glider. Note that pylon drag with its big pulley or motor on top is still there even if propeller is rotating .. Like flying with half opened airbrakes.

Pylon powered glider, with pylon extended, L/D 25 and 500kg T.O.W, minimum sink 1,0m/s:
-required thrust for level flight; 500/25=20kg which is 200N
-another 200N for climb rate 1m/s; 400N
-another 200N for climb rate 2m/s; 600N
-another 200N for climb rate 3m/s; 800N

Efficiency of propeller not depends just on its diameter but also how much thrust such propeller should create. Even with smaller propeller diameter is possible to achieve good efficiency when required thrust is smaller, for instance during level flight or more shallow climb rates and this is typical use of FES. As you can see above, with FES required thrust for climb rate 1m/s is 300N, while with pylon for 1m/s is required 400N. This seems to be possible to offset with higher efficiency of bigger propeller, but bigger propeller requires higher torque. With direct drive electric motors higher torque can be achieved only with bigger motor diameter, but this means even more drag. With more drag it would be required also more thrust from propeller, which means higher power, higher energy consumption, bigger battery packs, more weight. Another possibility is to install motor in the fuselage and transfer power with belts to pylon, but there are quite high loses in efficiency, more that most engineers imagine and so not in equation. Diameter of upper pulley is usually bigger than lower pulley, due to reduction ratio, which is not helpful from drag viewpoint. And number of propeller blades do not affect much to its efficiency. It is only helpful if prop loading it very high but is also possible to increase chord of propeller blades to some extent.

So 700N for FES with 1m prop diameter or 800N with 1,4m (or bigger) retractable system to climb 3m/s, seems to be in advantage of pylon due to better prop efficiency. However, when you put all factors into equation, result is almost the same.
However, in the range of lower power settings FES is much more efficient (100N vs 200N).
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.
I believe that in next months there will be more information about LAK17C FES.