Bob - watts per pound are found when you use a watt meter to determine how much power you system produces.
The meter goes in between your batter and ESC on your power system and it measures the watts/amps/volts. It is used for ground testing.
If the meter shows 100watts and you have a 1 pound airplane - you have a 100w/lb.
Simple as that.
Watts per pound was what people used to talk about when determining how powerful a motor you needed to fly a plane. Watts was sort of a measure of power, except when prople satrted flyingthier electric planes the brushed motors were pretty inefficient, so 200 watts measured going into the motor was anywhere from 20% to 70% efficient.
People generally quoted 50 watts per pound as "adequate." I think this would be at the 70% efficiency, 35 watts OUTPUT per lb, and this is flying pretty marginally, like a piper cub style plane with a light wingloading, like a powered glider.
For ROG, 70 watts per pound is better. For a 3D plane, you are talking around 150 watts per lb.
If you are building a kit, the manufacturer would likely recommend a motor size.
The manufacturer of motors will list a watt rating for the motor, but prop size and cell count and cell type will all effect the output.
I would use the 50 watts per pound as a minimum. At 100 watts per pound, you will be testing the structure of the plane, and it needs to be as robustly constructed as a power plane at 100 watts per lb. an imbalanced prop ofr bengt shaft wil make it vibrate as much as a nitro engine.
770 watts is about a horsepower. Do not compare this to nitro motors horsepower, because you might have a motor rated over 1 hp, like a .40 pylon motor, at 21,000 rpms, but it only runs at 12,000 rpm as propped, so an electric motor putting out 400 watts would fly about the same.
Go with a recommended power system, ie battery and ESC and motor, until you have some practice with different planes and power setups. And keep in mind that BRUSHED motors are not very efficient, so you might double the power in to get the same watts output. I have a wattmeter. It measures the watts going into the motor. You could get watts output using calculations based on rpm, prop size and prop speed.
I would just go with watts input, and maybe go a little higher on the watts per lb. 75 watts per lb, input for brushless motors should be enough watts per lb. unless you need to go straight up. This should give "spirited performance," like loops from level flight.
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Watts per pound is really determined by your prop choice too, I.E. a 3D plane with 150 watts on the nose, weighing in at 15-16oz with a 10x4.7SF prop on 3 cell will go vertically pretty quickly. But a pylon racer on the same battery, same weight but an inrunner swinging a 6x4 prop may need at least 200 watts per pound to go vertically.
Generally, for your parkflyers swinging 7-12" props, here's a good chart, if you must:
50 watts per pound = slow flight, enough power for cruising.
100 watts per pound = Good flight performance, quick takeoffs.
150+ watts per pound = Unlimited vertical power, will takeoff vertically.
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