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Old Oct 18, 2004, 12:12 AM
HeliAP'er!
Hogster's Avatar
Surrey, UK
Joined Aug 2003
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How to calculate thrust, given RPM, prop pitch and prop diameter?

It's a question I've often needed the answer to. Without having to build a mechanism around a weighing scale to measure thrust from a prop, is there any way to calculate thrust by using just the prop diameter, pitch and RPM?

Thanks
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Old Oct 18, 2004, 12:48 AM
Nickel what?
Phreakish's Avatar
Prescott, Arizona, United States
Joined Jan 2003
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nope... its going to depend on air density, prop profile, prop blade area, the actual pitch of the prop (everyone has got different ways, or different pitch curves over the length of the prop).

You COULD calculate it, but you're going to need prop constants that are specific to THAT prop, which you'll need to get from real world data... So you're back to making a thrust rig...

Easy way: Buy a 2x4, a scale, and some wire. Put the scale on the end of the 2x4 (or any skinny chunk of wood, at least skinnier than the scale). wrap the wire over the scale, and wrap it around the tail of your plane, and let it dangle from the scale. Then go full throttle and measure the change in the reading on the scale, boom, thrust. Or, if you have a fancy scale, you can zero it while the plane is hanging, and it'll just read thrust...
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Old Oct 18, 2004, 12:51 AM
Good Better Best quest.
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Australia, VIC, Cranbourne East
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mmm

No use even asking the prop' manufacturers they dont consider we need to know,they wont even aknowledge emails on the subject
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Old Oct 18, 2004, 10:33 AM
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Chattanooga, Tennessee, United States
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http://www.lcrcc.net/thrust_calc.htm

or read the the thread on "How do YOU measure thrust"

http://www.rcgroups.com/forums/showthread.php?t=286867
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Old Oct 18, 2004, 12:48 PM
HeliAP'er!
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Surrey, UK
Joined Aug 2003
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Hmm looks like I'll have to make something then - no problem!

Thanks for the link Dr. Kiwi ... sadly GWS props weren't around in 1986 so I can't select it from the pop up menu!

Thanks everyone
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Old Oct 18, 2004, 01:00 PM
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For props with Pitch/Diameter ratios less than 0.6 the formula for static thrust is

Thrust = P X D^3 X RPM^2 X 10^-10 oz.

The constant 10^-10 will have to be changed dependent on the prop and manufacturer.

GWS slowfly props would multiply the above by a factor of 1.0

For GWS HD props the factor would be approximately 0.75.

For props which have P/D ratios greater than 0.6 the equation is not valid for static thrust.

For these props the pitch used in the equation would be approximately the pitch for a P/D ratio of 0.6 and not the stated pitch.

Edited below to correct for propeller constant variations:

For the example shown above, at an RPM of 3660 the equation gives a thrust of 15.3 oz. for a 12 X 6 SF using a propeller constant of 1.1.

A propeller constant of 1.0 gives a thrust of 13.9 Oz.
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Old Oct 18, 2004, 01:20 PM
HeliAP'er!
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Wow what an ingenious formula! I'll try that the next time I get some data .... Thanks Martin!

Edit: Umm .... I don't get 15.3oz of thrust with the example above!

(6) x (12^3) x (3600^2) x (10^-10) = 13.88 oz ....
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Old Oct 18, 2004, 03:07 PM
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Toronto Canada
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OOPS !

The 15.3 Oz. resulted from using a prop constant of 1.1 for the GWS SF props.

I decided that a constant of 1.0 was closer to reality.

It would be worthwhile if someone can post some real numbers for GWS SF props to confirm the propeller constant.

Thanks for bringing this to my attention.
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Old Apr 13, 2014, 08:53 PM
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Quote:
Originally Posted by Hogster View Post
It's a question I've often needed the answer to. Without having to build a mechanism around a weighing scale to measure thrust from a prop, is there any way to calculate thrust by using just the prop diameter, pitch and RPM?

Thanks
Hogster, I know this thread is old as dirt , but since it comes up as one of the top Google searches for "propeller thrust equation" I thought I'd respond, since many users may come across this thread and have your same question. I also had your same question, so I came up with an equation for it. I compared my equation to the (Thrust = P X D^3 X RPM^2 X 10^-10 oz) equation, and I found mine to perform a bit better overall. Additionally, the equation I came up with accounts for static *and* dynamic thrust, so you can see how the thrust will decrease as your airspeed increases.

Here is the simplified version of my equation:

F = 4.3924e-8*RPM*d^3.5/sqrt(pitch)*(4.23333e-4*RPM*pitch – Vac).

F is thrust in Newtons, RPM is rotations per minute, d is prop. diameter in inches, pitch is prop. pitch in inches, Vac is aircraft airspeed in m/s.
If you want thrust in other units: to convert newtons to grams, multiply newtons by 1000/9.81. To then convert grams to ounces, multiply grams by 0.035274. To convert ounces to pounds, divide ounces by 16.

The generics of the equation can be found here, including an Excel spreadsheet to make your life easier, and the equation above is shown here in a more visually appealing form: http://electricrcaircraftguy.blogspo...-equation.html

The full derivation of the equation begins with Newton’s laws, and is shown here: http://electricrcaircraftguy.blogspo...ackground.html

I hope this is useful to many people, as it is very useful to me.
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