With great difficulty.
If at all.
The classic engineering way is a dynamometer, to measure output torque:
that times the rpm with appropriate constant is the output power.
It tells you nothing about propellor efficiency though, and must
perforce include ESC losses and possibly battery losses as well as motor
If you want a a really quick and dirty approximation, though, which for
quite subtle reasons works fairly well in MOST cases, just take the prop
off, measure the RPM, and then put the prop back on. The efficeincy is
very close to the ratio of the on load RPM to the off load RPM, if the
battery doesn't sag too much.
The better approach is to do some fairly tedious mathematics involving
measuring the current drawn under zero load conditions as well, and that
way you can get a better figure.
That will tend to give you the motor + ESC efficiency if you have a
volt/ammeter (whattmeter) between the battery and teh ESC.
You have to do more maths to separate out the controller losses..
ACTUAL power OUT of the motor AND prop is a function of the amount of
air being moved by the prop, and the speed its accelerated to, and is a
fiendish thing to measure.
and almost useless when you do, since it changes according to the models
You best bet is to gear the motor, and use a big coarse pitch prop. The
quieter it is, the more efficient is is.
If you are used to glow engines, forget everything you thought you knew:
very little of it applies with electrics ;-)
Ah the speed and volume of air moved, good! What about
simply hooking the prop/motor to a scale to measure how many
ounces it pulls? I am guessing that this will only measure
thrust, and not Watts because it does not take into account
the speed of the air.
Maybe my measure above combined with some way to measure the
speed of the air (little paddles turning a wheel hooked to a
gear) would derive a reasonable approximation?
Ahh, but what factors or formulas or constants would you use to turn this
thrust into meaningful watts output?
As a way to measure relative performance, it is good, but even better is to
measure the thrust created with the prop-motor combination in a wind tunnel,
so you are not measuring static thrust.
Unless you are wanting to know how well it accelerates from a standing
start, of course.
Absolutely. Watts are somewhat meaningless because of all the of the factors
that can vary it widely and depending on the efficiency and quality of the
motor, a lot of the watts go to heat rather than power. I gave up on watts
a long time ago and use a digital fish scale hooked to the tail of the
airplane and anchored to the ground to get actual static thrust. I at least
know it will fly the airplane! Much more meaningful information.
Here is data on four props I tested years ago. All four props were
diameter trimmed so they absorbed the same power at 11,000 rpm.
Prop 1 static thrust was 1.1 kg. It blew 100 m**3 of air/unit time
Prop 2 static thrust was 1.1 kg. It blew 90 m**3 of air/unit time
Prop 3 static thrust was 1.1 kg. It blew 85 m**3 of air/unit time
Prop 4 static thrust was 1.1 kg. It blew 20 m**3 of air/unit time
All of these props were tested for performance on the same plane
turning at 11,000 rpm. Here are the results:
One of these props flew the plane quite nicely.
Two of these props taxied the same plane ok. One would barely get it
in the air the other would not quite get to lift off speed.
One of these props would hardly even taxi the plane.
Static thrust plus some other stuff tells you some information.
Static thrust by itself is meaningless. From the given data you
should be able to pick out the total dog easy enough. But you will
not be able to tell which of the other three are which.
You have it exactly right. The lowest tech way to get at watts of
output power is to determine the watts of drag at the air speed the
motor/prop will fly at. Output power = drag. Drag can be estimated
well for full scale but it may be hard to get at figures for a model
short of doing wind tunnel stuff. If you try to calculate drag do not
forget to include prop drag as part of total drag.
There are high tech calorimetry ways to get at the output power too.
For instance tie the motor into a calorimeter with a prop in the
calorimeter that absorbs the same amount of power as the airplane prop
at the same motor rpm and measure temp change vs time. Hard to do
outside a lab but not impossible if you are clever.
Or put the motor in the calorimeter with the prop outside and measure
total watts to the engine vs watts of heat gained by the calorimeter.
Again hard to do outside a lab but not impossible.
At any rate your thinking is correct. Static thrust is just about
meaningless by itself. You know how you want the plane to fly, for
instance lots of second gear or lots of speed. You know something
about how fast you want to fly when level and full throttle. You have
all kinds of guidelines about what props others find good on a
particular power plant. So you only have to test three or four props
and find out which one flys the plane the way you like to fly. And
all of that tells you zero about motor efficiency. About the only use
of static thrust is for bragging rights and getting movement started
on a bumpy grass strip.
At one time the level flight prop driven speed record was held by a
sea plane. The reason was simple. To get fast the plane needed a
very high pitch prop. Static thrust was pathetic with such a high
pitch because the prop was stalling. It blew lots of air but did not
give much static thrust. The only level runway long enough to allow
takeoff was water. Just an example of miserable static thrust going
real fast when it finally got underway.
Yeah, I guess that _would_ work on an outrunner, pretty easily. It would be
a bit more difficult on a regular brushed motor, I would think.
I guess you could put a small rotor on a regular motor, and it would not
change the RPMs very much.
I just figured out how to measure the airspeed, something
like measuring the minute electricity generated by a brushed
ducted fan motor that is placed in the airflow, once this
has been calibrated to airspeed. There would probably
already be devices that operate on a similar principle.
Measuring air volume is another matter maybe you could put
the motor/prop in a duct and measure the airspeed at the end
of the duct, by knowing the diameter of the duct you could
figure air volume moved per unit of time. This kind of
measure would be an ultimate measure inherently factoring in
the total efficiency of the motor, prop, gearbox (if any)
and anything else.
I am not sure whether or not I erred in any of my analysis,
I am a software engineer, not an aeronautical engineer.
Several problems, there. I'm not going to take them in any particular
If you use a calibrated airspeed by generating electricity, the airflow is
going to be non-turbulent while you are calibrating it, (before it hits the
measuring prop) and straight flowing against the prop.
If you used this rig to measure airspeed coming off of a prop, the air is
going to be cork-screwing, which will have an undesired effect of the
measuring prop. To be accurate (more so, at least) you would need to use
straightening vanes to straighten out the airflow. Ooops, now we have added
drag which will slow the airflow coming off of the powered prop.
If you put the powered prop in a duct, you still have the corkscrew problem,
and increased drag from the sides of the duct. That will be quite
significant. Then, you might need to factor in increased powered prop
efficiency, _if_ you maintain very, very small clearances between the power
prop tips and the duct walls, as you would with a good ducted fan unit. You
also have engine mount and duct support (possibly not the second) drag to
consider throwing off your measurements.
You have too many unknown variables, and unlike units that need to be
solved, and as everyone knows, you have a big problem trying to solve more
than one unknown variable at a time.
Long to short? Use the ballpark figures for efficiency of the props at the
observed RPMs, and with the ballpark efficiency for the different types of
motors at the amps and speed you are going to be running. Subtract out the
loss for the type of gearbox (if any) you will be using.
Then there is the very best way to figure it all out.
It seems overly simple, but the answer is.....
Put something on a plane, and go fly the sucker ! ! ! ! ! ! <GGG>
Plus, it is more fun than bending your brain like the above would require.
(at least it is for me-I hate math, especially ugly math)
I want to optimize my purchase of brushless motors. In the
ideal case the companies that make these motors would
provide these figures in advance. How exactly can the
efficiency of a motor/prop combo be precisely measured,
disregarding the cost of this measurement?
I am guessing that the expensive part of this may have
already been done and that all that I need to know to
determine how much power that any given motor is generating
is to know the RPMs that the motor is providing to a
specific prop, and then look up the Watts per RPM in a
precompiled table for this specific prop. I already know how
to determine how much power the motor is drawing.
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