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This thread is privately moderated by bbmaker, who may elect to delete unwanted replies.
Apr 22, 2009, 10:27 PM
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calculating thrust?

hey, im doing a little project for math class. i was wondering how you could calculate the thrust of a propeller? im new, and i dont really know any of the terminology for the parts of a propeller, either. the motor is supposed to run 4400 rpm with no load, but im just going for an aproximate answer. and if you could include the formulas or whatever, that would be nice, too.
Apr 22, 2009, 11:14 PM
Deletedfor proving Nauga wrong
When you are calculating thrust developed by a propeller you are calculating the "lift" of a special type wing.

Because each part of the wing has a different airspeed, and usually a different "chord", each part of the prop develops a different amount of lift (thrust)

The tips move fastest, and if all else was equal would produce the most thrust. But the tips usually have the smallest chord...

Depending on the propeller design it may have the angle of attack at each radius match up to the "pitch" and thus trying to pull the plane forward the same amount per revolution. In most cases that really isn't true though... Especially near the hub the pitch angle rarely matches the listed prop pitch.

And various makers will have different blade designs...but list the same pitch and diameter. I have at least 10 different styles of 10X6 prop and each gives different results an any given rpm. Different airfoils, different materials, different blade shapes... each difference affects how the propeller performs.
A wide blade that is flexible is usually good for low RPM and a blade that is narrow and stiff is usually better for high rpm.
But... I have one 10X6 that is very flexible and the engine I use it on turns it at very high rpm...

If the pitch is high, then it is fairly common to see the prop "stall" at high rpm when doing static thrust tests. There is an rpm for any prop beyond which you will not see an increase in static thrust and might even see static thrust decrease when you increase RPM.

Because of all the variables involved... there is no standard formula for expected thrust. What is more typical is to take a prop of some given design and test it at measured RPM and measure the thrust it develops.

The online thrust calculators actually interpolate empirical data (resuts from tests that have been run by many people using many different propellers).

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