Power to mass ratio required:

66 – 88 W/kg minimum for hand-start and slow climb

133 – 176 W/kg for runway start, steady climb

220 W/kg for aerobatics

Li-polimer battery power density: 168 W*h/kg

Study Case 1

http://davesrcflight.mysite.wanadoo-members.co.uk/frame_pages/start_frame.htm

Type: glider

Wingspan: 2.1 m

Mass: 1.37 kg AUW (all up weight?)

battery: 8x3000HV NiHd

nominal climb rate: 400m/min (6.6 m/s)

engine run: 7 min.

total flight approx: 1 hour

Lets analyze it and see how much weight we can allocate to batteries:

1.37 kg, we need good climb power approx 150 W/kg:

1.37 kg x 150 W/kg = 205 W needed

7 min engine time = 0.11 hours

total energy needed:

0.11 h x 205W = 23 W*h

minimum battery weight (no losses):

23 W*h / 168 W*h/kg = 0.136 kg minimum must be allocated to battery having no losses at all. So it is only 10% which is not bad at all for theoretical assumption.

Study Case 2: AC Propulsions SoLong

Wingspan: 4.75 m

Wing area: 1.50 m2

Mass: 12.8 kg

Power Source: 120 Sanyo 18650 Li-Ion cells and 76 Sunpower A300 solar cells

Solar panel nom. power: 225 W

Battery mass: 5.6 kg

Max motor power: 800 W

Min electrical power for level flight: 95 W

Stored energy: 1200W*h

Speed range: 43 – 80 km/h

Max climb rate: 2.5 m/s

Control and telemetry range: 8000m

Calculated values:

energy density: 1200W*h / 5.6 kg = 214 W*h/kg (very good, above average)

Power to mass ratio:

Max: 800W / 12.8 kg = 62.5 W/kg (below minimum statistical)

Level flight: 95W / 12.8 kg = 7.42 W/kg (all electrical consumption)

Time needed to fully charge batteries at level flight:

Available panel power 225 W

Consumed by flight: 95W

Remains 120W

Time to charge: 1200W*h / 120W = 10h

Full discharge time at level flight: 1200W*h / 95 W = 12.6h

Sun/Total flight ratio = 10h / (10h+12.6h) = 0.44