How these are worked out — the equations behind this sheet, their sources, and what each one assumes.
Where people go wrong
The errors this method actually invites, and what it does about each. Every one of them is already a warning somewhere — in a note that appears when your own figures trip it, in the scope statement, or in the equations behind the page. Collected here so you can read them before you need them.
Forgetting what the payload draws. A payload is two bills — its mass and its watts — and the second is charged for every second the aeroplane is switched on. On the default trainer 10 W of avionics and sensor is 3.1 Wh, 17% of the mission and twice what the whole climb costs, and it takes 6.9 minutes off the loiter. Nothing about the airframe changed.
Assuming energy sets the payload ceiling. Usually it does not. Stall speed grows as the square root of weight, so at some payload the cruise speed falls inside 1.3 times the stall and flying stops you before the pack does. On the default trainer the pack would pay for 1,756 g and flying stops it at 845 g.
Counting the reserve twice. This sheet shipped doing exactly that — an 85% usable fraction for the chemistry and a 20% reserve on top of it — which put it 15% below Sheet 09 on the same pack. Depth of discharge is the reserve; do not take another bite out of the same energy.
Treating the descent as free. The propeller is off, so nothing goes to it — but the avionics run the whole way down, and the descent covers ground the cruise distance has already been charged for. Both are on the sheet now, the second as a stated over-charge rather than a silent credit.
Expecting the segments to add up to a flight. Each segment is one steady condition held for a time, not an integration. The take-off run, the acceleration to cruise, the turn onto station and every transition between them are uncharged. Worth measuring before worrying: the kinetic energy of the whole speed-up from standing still to cruise is under one per cent of the default mission, and it bounds every one of them together.
| Segment | Flown at | Time | Power | Energy | Of the pack |
|---|
The descent is flown power-off, so it is charged only for the avionics on the way down; a windmilling propeller is a brake, not a generator. The depth of discharge and the derate are compiled, not measured, and are Sheet 09's figures rather than this sheet's.