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Old Apr 18, 2014, 10:58 PM
rookie
wilf370's Avatar
Canada, SK, Saskatoon
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engine power

So im thinking of comparing power on some of my glow engines. my thoughts are if I put a prop on the engine run it wide open and see what rpm it turns. then put the same prop on a different engine and compare rpm. this is on 40 to 46 engines old and new?
Wilf
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Old Apr 19, 2014, 06:19 AM
Complete RC Idiot Savant
The Netherlands
Joined Nov 2009
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Not conclusive:

You can get a decent .45 2 stroke to run 16000 RPM with a 10 x 6 for example, but a 1.20 fourstroke with that same prop will not reach 16000 RPM.

What I am saying is: two engines that are designed to develop their respective peak power at too different RPMs, cannot be effectively compared by this method.

It would be the same as comparing a 100 bhp racing bike engine with a 100 bhp tractor engine, and saying that the tractor engine is much slower, therefore MUST be much weaker....

Brgds, Bert
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Old Apr 19, 2014, 07:36 AM
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I think he meant to compare some .40 and .46 two stroke glow engines to each other. So if he ran the same 9x7, 10x6 and 11x6 propellers on all of the engines and recorded the results, using the same fuel, then he could compare the results to see what happened. It seems fairly reasonable to me.

But you are correct in that one has to be careful as to what they are comparing though.
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Last edited by earlwb; Apr 19, 2014 at 07:36 AM. Reason: typo correction
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Old Apr 19, 2014, 07:51 AM
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United States, NY, Eldred
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Are you doing this just for fun? Or are you trying to pick the "best" engine from several you have for a specific application? If it's the latter, than you must also consider weight.
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Old Apr 19, 2014, 08:24 AM
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Canada, ON, Cottam
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That is a pretty good comparison. As has been mentioned, other things come into play. It is a good guide. I checked a bunch of motors that way and it gives a good guideline. Some motors are happier at higher rpms, and may not give good numbers on a bigger prop. A 10-6 is a good guide. I found OS LA, TT GP .40's to be in the 11,000 range, and the BB Supertigers and AX to be closer to 14, and 15,000. I am happy with say, my AX .46 because the throttle is so linear and faithful and would sacrifice a bit of top end for that. (it is ok there too though.) Some others stumble and don't have a reliable idle which enters into my choices. Also some motors may wind up a bit more while in the air. Especially with a higher pitch prop and/or a pipe.
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Old Apr 19, 2014, 08:47 AM
Complete RC Idiot Savant
The Netherlands
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Quote:
Originally Posted by earlwb View Post
I think he meant to compare some .40 and .46 two stroke glow engines to each other. So if he ran the same 9x7, 10x6 and 11x6 propellers on all of the engines and recorded the results, using the same fuel, then he could compare the results to see what happened. It seems fairly reasonable to me.

But you are correct in that one has to be careful as to what they are comparing though.
Not even then: a Rossi for example was designed for FAI fuel, and a K&B typically for 15% Nitro or more.
Hardly likely, a direct comparision on the same fuel is providing an accurate picture....

Brgds, Bert
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Old Apr 19, 2014, 11:06 AM
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I tach all my engines with different combos and record the findings along with other data like weight in a journal. Very handy when deciding what engine/prop/fuel combo to use on a particular plane.
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Old Apr 19, 2014, 03:03 PM
rookie
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Canada, SK, Saskatoon
Joined Dec 2012
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So i'm building a engine toque dyno. It will measure rpm and thrust. I do not have a clear picture in my head of the linear power band of a propeller. Does anyone here know where I can find some info on that/
Wilf
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