I've got a 12 v drill motor. When I hook on a propeller off of a slo-v and power it up I get lots of thrust - this is direct drive. Can anybody guess whether this is a terribly inefficient setup? I know drill motors have been used for large scale airplanes using a belt drive, but I can't find anybody who has tried direct drive.
You're pretty much guaranteed that it's an inefficient setup. Way too many RPMs; you're whipping the air into a froth, not slicing through it...
The problem is you don't know how badly you're overloading the motor to get "lots of thrust." It may seem impressive, but you've only run it for a few seconds. If you let it go for a minute or two, you're likely to discover that the motor and battery get VERY hot. It may even shut down of its own accord.
Stupid comment...
He said motor and prop, not egg-whisk... Does my 5x4.5 at 19,000 revs whip the air into a froth or push my plane thro' the air at 70mph?
A sensible way to tell whether a motor is running at max. efficiency or power is to do the following:
Using a tacho, measure the no-load speed of the motor.
The motor is most efficient at about 75% of this speed, a point on the graph commonly used when running as brushless motor.
The motor is most powerful at about 50% of it's no-load speed. Ferrite can motors are commonly used like this as their power to weigh ratio is fairly low.
These %s are rough rules of thumb and don't take into account prop efficiencies etc.
Plot the curves in Motocalc and you'll find that these rules of thumb work regardless of motor, gearbox etc...
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