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Electric SSDR Motor Glider

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hole in the ground

Active Member
Joined
May 27, 2018
Messages
32
Location
UK
The following is the rather delayed action of my inspiration derived from Topaz' Conceptual Design of an "Inexpenseive" Single-Seat Motorglider. Like that thread this is probably only a conceptual design but is more of a feasibility study than anything else: Is it possible using today's commercial technology to build a viable light aircraft powered by battery electric propulsion?

I think that the answer is yes and to me it seems that electric propulsion will require a very efficient aircraft in order to achieve a decent endurance. Endurance not range because pilots measure their experience in hours not miles and frankly you wont be able to go very quickly under electric steam (so to speak). It think that electric propulsion and motor gliders seem like a natural combination. Electric propulsion provides a very reliable restart system so that glider pilots can get lower looking for lift, confident that the donkey will restart when requested. Gliders with their long wings and clean fuselages are aerodynamically efficient requiring little power to keep them airborne. Additionally, turning the motor off is a very good way of conserving battery power and increasing flight duration.

So far I've just got to the point where I have done an initial sizing so the following will just be a quick skim over what I've done and found, with some eye-candy at the end. I've mainly followed Raymer's simplified but with some deviation for an electric build and SSDR requirements.

My reference aircraft for this are the Fournier RF4, Schleicher ASK14, and the AMF Chevvron 2-32. The first two because they are beautiful single seat motor gliders and the Chevvron because it fits the (now earlier) UK microlight regulations and has the lowest empty weight despite being a two seater (to be fair it is 20 years younger than the other 2 though).

Mission requirements are pretty modest:
Max mass = 300kg (SSDR limit - I will design up to it, using aspect ratio and/or battery mass to get maximum performance)
Stall speed = 35kts (SSDR limit - I expect that with glider sized wings that I can achieve this without flaps)
Cruise speed (power) = 60kts (I did a quick trade between 60 and 90 kts and it required nearly 1/2 of the aircraft to be battery for 90kts)
Endurance (power) = 1 hour (I havent yet fully worked out what the mission profile of my 1 hour power will look like - so this may change, at the moment I have estimated just 1 hour at cruise conditions. This will be one of the next things I need to spend some time firming up)
Climb rate = 500 fpm (this isnt a hard limit, more something to hang some figures on at the moment. I will play around with this along with the mission profile to see if I can find a knee in the curve somewhere)
Payload = 95kg (me plus some baggage, threshold would be 85kg - me plus some water and a light sandwich (maybe some cucumber in a pita))
Empty mass = 205kg (not so much a requirement as a fall out of the maximum mass limit)

For reference the RF4 is 270kg empty, the ASK14 is 245kg and the Chevvron is 175kg.

Take-off length - conspicuous by its absence (something I need to work out but I'm not planning on visiting any postage stamps).

In order to get to a reasonable CLmax the wing area popped out as 10.7m (I may just round this to 11m). I havent settled on a wing planform yet, I've got two parallel sets of calculations going on at the moment with a planform similar to the ASK14 and one similar to the RF4. This leads to one design with a 13.2m wingspan and one with a mere 11m, the trade-off I expect will be mass (though whether or not they are sufficiently different in AR to have much effect I will wait and see).

Using the other aircraft as references for power loading suggests a motor of ~ 22kW would be suitable. Coincidentally such a motor for gliders already exists. Front Electric Sustainer/Selflauncher - Future of gliding now. However this swings a 1m diameter prop at 4500rpm which is probably not too great for cruise but is clearly good enough for take-off and climb. I've used their battery mass/power to estimate how much weight of battery I would be lugging around too.

Initial L/D estimation and cruise duration of an hour spat out a battery mass of 51kg for the 11m span and 42kg for the 13.2m

One thing that I am hoping to achieve is having the wingtips further aft with respect to the pilots eyeline than on the RF4 and ASK14. Hopefully without an ICE at the front, and with the batteries mounted somewhere amidships, I'll be able to shift the pilot forward a bit.

I've done an initial layout in fusion 360 of the 13.2m wing using the data I've calculated/guessed, in order to get better numbers for Swet etc. (Tail is way too high an aspect ratio)

DrroGnw.jpg


E2Tziix.jpg


w2GaHvw.png


Thoughts, comments, criticisms?
 
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