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.
Sizing the motor on cruise power. The trainer this sheet defaults to cruises on 56 W and wants 324 W installed. Climb sets the motor, cruise sets the battery, and the two answers differ by a factor of six.
Sizing anything on static thrust. A propeller standing still is a stalled propeller, and what it pulls on the bench says very little about the power it needs at speed. Sheet 08 quotes static on a different basis for exactly this reason — 1,391 g against the 1,887 g the flight model would have claimed for the same propeller.
Reading the endurance as flight time. It is eighty per cent of the pack at a steady cruise. The climb to height, the circuit, the go-around and the reserve all come out of the same pack and none of them are in that number.
Trusting the left-hand end of the power curve. The parabolic polar stops describing a wing as it approaches the stall, so the minimum of the curve is optimistic and the region left of it is not to be flown by.
Using one propeller efficiency for climb and cruise. This sheet does, and it is the largest simplification on it. On the default trainer the same 10×6 returns 70.2% at cruise and 56.1% in the climb — fourteen points. Take the two figures off Sheet 08 and put them back in here.
Reading the power curve
Why the curve turns up at both ends
Fly fast and parasite drag dominates — it grows with the square of speed, and the power to overcome it with the cube. Fly slow and the wing must work at a high lift coefficient, so induced drag climbs steeply.
The bottom of the bucket is the speed where total power is least. Slower than that, you need more power to fly slower. That is the back side of the power curve, and it is where a low, slow, dragged-in approach goes wrong.
Climb sets the motor, not cruise
Level flight is cheap. Almost all the power you install exists to climb, accelerate and give you something in hand when an approach goes wrong.
Every extra metre per second of climb costs roughly W × V watts of propulsive
power — so on a 2.4 kg model, about 24 W per m/s, or nearly 50 W at the battery once
propeller and motor losses are counted.
The battery is part of the airframe
A bigger pack buys endurance and then spends it carrying itself. Hobby lithium packs run about 140 Wh per kilogram once case and wiring are counted, so doubling capacity on a light model can add a fifth of its all-up weight.
If the pack mass warning appears below, put the new weight back into the mass field and run the sheet again. Endurance and weight chase each other until they settle.