stunthanger.com
Engine basics => Engine set up tips => Topic started by: FLOYD CARTER on September 11, 2008, 01:42:53 PM
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I'm looking for some sort of formula to determine static thrust. I'm trying an O&R 29 FRV, which gives 7800 RPM with a 10-5 B-Y&O on 15% nitro with 28% castor oil. I'd like to know how much static thrust I'm getting from that combo. That will determine what size stunter (and how heavy) I should consider.
The motor appears near-new, and probably not yet broken-in.
Floyd in OR
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Basically, mount it on a doll :)y, tie it to a spring balance, and measure it.
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Floyd,
What is tricky is that the static thrust is a function of your prop--it doesn't really depend on the motor at all, just how fast it is being spun around. I know that sounds "obvious", but the issue is that with another prop, lets say an old Top Flite 10-5 or even an APC 10-5 will have different static thrust values, even at the same rpm.
So there would be two ways to get what you want.
1) Put the prop airfoils and info into a propeller calculator and then run several rpms. This isn't trivial unless you can approximate the airfoil with some actual known NACA airfoil. I've played with a couple of propeller calculators and this is always the "rub". Here is a link to one that I have used.
http://www.drivecalc.de/PropCalc/index.html
Here is a link to Martin Hepperle's site.He also has a propeller calculator. http://www.mh-aerotools.de/airfoils/index.htm
2) Build yourself a thrust stand. It isn't too hard to do. The reason I have never bothered is quite frankly I don't believe it is worth the effort to measure static thrust. To me it is only relevant for my Bi-Slob since it is the only CL plane I have that "flies" at zero airspeed on the prop!
Now if I could get access to a wind tunnel,I would make up an electric motor test stand and you could make a lot of nice measurements of how the propellers behave in the real world.
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Floyd,
I just ran your numbers in another prop-power-calculator.
That one indicates 0.14 HP, 1.67 lbs thrust (presumably in-air), and 1.53 lb static thrust.
This calculator runs in MS Excel, and has adjustments for temperature, altitude and barometer, along with a list of modern props and their "prop factors" (?).
Another way to check static thrust is to mount the engine in a model of known weight, turn the model vertical with the engine going, and see if it will lift the model, or you still have to support it. If it lifts the model, tie on some more weight and repeat until thrust just supports the model.
That finds the static thrust directly, for the unusual - and probably not characteristic - attitude the model is in... How well it applies to other conditions is left to find out, but you know what the system is capable of. Model attitude doesn't change the thrust production you can measure...