On Saturday, December 23, 2017 at 9:00:08 AM UTC-5, Chris Davison wrote:

If olnly we could retrofit FES units to
existing standard class gliders (please)!

problem is weight -- the weight hit on an 18 m (or bigger span) glider matters a lot less than on a 15, and then as they have better L/D you need less total power to sustain them (although don't forget the impact of gross weight)

It's worth going through the numbers just to see what's going on -- first, take the optimistic case:

The power required to fly level is just

P = m * g * v / (L/D)

Taking an 18 m glider at 400 kg gross weight, coming home at 60 kts ≈ 30 m/s and an L/D of 40 yields 2.9 kW (before motor and propeller losses) ... taking 3.2 kW as battery output required would be reasonable.

The very best Lithium-sulfur batteries are now around 400 Wh/kg, but 200 is more typical of common Lithium batteries.

So if you wanted 60 nm return range @ 1 hour cruising time you'd need 16 kg of standard lithium batteries. Motor and propeller for this power will add 4 - 5 ... so up around 20 kg ... sounds good, right?


But if you look, people aren't fitting FES that wimpy. Pilots will want more than 60 nm return range and will want to come home FASTER, also have more margin for sink or headwind en route. The real FES units being put into gliders just about double all the numbers, and that becomes a real load on a 15 m glider ... and L/D at cruise matters, the 18 m ships win there too.

Better batteries are the only thing that can change the equation.