Measuring the efficiency of a motor ???

"The Natural Philosopher" wrote

WoW! That kind of power...... Does not even compute!

WoW!

Good post!

"The Natural Philosopher" wrote

You must be one of "those" people that LIKE math. You know, sick people!

I know, my wife are one of thems, toos! ;-)

I know that some people don't mind math, and even enjoy it. I'm just not one of them.

Knock yerself out! :-)

I suppose it is a total waste of time to point out once again that the purpose of a prop is to generate thrust, not to blow air. The two are two different subjects. If all we wanted a prop to do is blow air it would be designed very differently. For instance some practical props generate thrust without blowing any air at all. An example is an auto gyro in level flight where the air flow behind the trailing edge is UP.

What you need to do to calc efficiency is to determine total drag at air speed. That includes not only drag associated with the air frame but also prop drag. Those are calculateable numbers. Knowing total drag at air speed and electrical consumption of the motor at air speed and you can calc motor efficiency. Any static test type of stuff you do is by and large meaningless. At best it will give you some crude general hints on the direction to go.

Unfortunately the only two on this group that have any slight chance of understanding this are TNP and Moleski.

I suppose it is a total waste of time to point out once again that the purpose of a prop is to generate thrust, not to blow air. The two are two different subjects. If all we wanted a prop to do is blow air it would be designed very differently. For instance some practical props generate thrust without blowing any air at all. An example is an auto gyro in level flight where the air flow behind the trailing edge is UP.

What you need to do to calc efficiency is to determine total drag at air speed. That includes not only drag associated with the air frame but also prop drag. Those are calculateable numbers. Knowing total drag at air speed and electrical consumption of the motor at air speed and you can calc motor efficiency. Any static test type of stuff you do is by and large meaningless. At best it will give you some crude general hints on the direction to go.

Unfortunately the only two on this group that have any slight chance of understanding this are TNP and Moleski.

F U mk

"MJKolodziej" wrote

I second the motion.

All in favor, say aye?

With all due respect, that's quite a bold statement lacking any proof, and I strongly disagree.

From the posts I've read from them, TNP and Moleski have way more than a slight chance of understanding that. And I'm sure that there is a lot of more people out there that don't speak too loud, but understand these issues pretty well. And I'm not only talking about those who do this for a living.

Finally, unfortunate comments like that don't help other people to improve their skills in this hobby.

Best regards,

- Angel

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.

Watts of power output is the correct measure that I was trying to obtain. Watts of power output divided by watts of power input yields the efficiency of the whole system. Thrust is not a good measure because the thrust may not provide enough airspeed. Even tons of thrust enabling 10 MPH is useless if stall speed is 15 MPH.

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 =3D 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.

With respect, more fool you then.

Pleny of models with > 1:1 thrust to weight wont fly.

They might hover though ..;-)

1.5:1 and it may hover pretty well, It might even flight straight up, even if not horizontally.