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How to calculate thrust?

Old 12-29-2006, 10:38 PM
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MODEL MFG
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Default How to calculate thrust?

Hi There,
I am Manish Borikar from india using 2 o.s. fp 0.46 c.i. 2 stroke engines building one 20' blimp.
i want to know how can i measure thrust of this engine with 11/6 propeller at idling-2500rpm, at 10,000rpm & at 17000rpm.
Please help me in this calculations for thrust.
Manish
Old 12-31-2006, 08:27 AM
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Howard
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Default RE: How to calculate thrust?


ORIGINAL: glueit

Hi There,
I am Manish Borikar from india using 2 o.s. fp 0.46 c.i. 2 stroke engines building one 20' blimp.
i want to know how can i measure thrust of this engine with 11/6 propeller at idling-2500rpm, at 10,000rpm & at 17000rpm.
Please help me in this calculations for thrust.
Manish
Manish,

Your project looks very interesting. It is quite difficult to accurately calculate/predict the thrust from a model airplane engine driven propeller. Propeller manufacturers do not fabricate xx/yy propellers to the same dimensions (which gives us a good variety to choose from) which makes each manufacturer's propeller react differently even though it is sold as an xx/yy. This of course has an impact on propeller efficiency which is where you are trying to measure how much rubber is going to meet the road. Additionally, the propeller angle of attack varies from the hub to the outer edge and as the engine begins to move through the air the angle of attack on the propeller changes so that even if you maintain a constant engine rpm the thrust generated will vary as a function of airspeed (because the angle of attack is changing) When I was in college (many years ago) I built a frictionless sliding platform and bolted an engine on the front end and strain guages on the back end - to measure thrust. I used different sized propellers running at specific rpm (had to build my own tach) and tried to see if I could develop a relationship between thrust and propeller pitch/diameter at specific engine speeds. My conclusions were that you could but it was pretty loose and only useful for limited applications.

What you might want to try is to take the engine you are going to use and mount it on a platform that can move horizontally but not vertically and use a spring guage to measure the load (in whatever units you prefer) generated by the engine at the engine speeds that you are interested in. These numbers will change when your blimp is moving but they can provide a pretty good frame of reference. There are established empiracal and theoretical ways to calculate the thrust - but it is going to be pretty loose for the size engine you are using and may not be of real value to you.

Hopefully, someone else reading your post has a better idea or suggestion than mine. Enjoyed chatting with you and Happy New Year.

Howard
Old 01-02-2007, 11:51 AM
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Bax
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Default RE: How to calculate thrust?

For making calculations, you're best chance of an answer is in the general Glow engines forum. There are people who have posted formulas over and over again.

Please realize that calculations are very, very approximate at best, and you'll likely never come close to the performance levels your results may give you. Also, please be advised that an O.S. Max .46 LA (there was no .46 FP), will not turn an 11 x 7 propeller at 17,000 RPM. The engine just does not have that kind of power. The extremes of the RPM range stated by O.S. were achieved with some propeller, but not the same. If you prop the engine for a reliable idle at 2,500 RPM, you will not get the 17,000 RPM top end. If you prop the engine to turn 17,000 RPM at full throttle, you won't get a very low idle that is reliable, and the transition from low throttle to high will be erratic.

With an 11 x 7 propeller, you'll likely get a maximum RPM in the mid-10,000's or so.

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