Over-estimating the power - RCCanada - Canada Radio Controlled Hobby Forum
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Old 10-24-2005, 09:42 AM   #1
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Over-estimating the power

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I was breaking in my OS 25fx yesterday. By the book, 10s lean, 10s rich.

5% synthetic, Master Airscrew 9x6

Second tank I measured 13 800 RPM. This calculates to 0.698 HP (maybe my air density was a bit higher than my calculation?)

OS states 0.86 HP at 18 000 RPM.

This is one up from the smallest propeller recommended, why doesn't it spin like OS suggests it can?

More break in? thinner air? more nitromethane? ether? different planet?
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Old 10-24-2005, 10:43 AM   #2
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All engine manufactures state hp at high rpm's ...looks great on paper to the buyer .Don't worry about it ...just find the best prop for your plane/engine combo . I bet if you put a small enough prop on your engine , it will get those rpm's ...but then it will prob be no use for the plane setup you have .

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Old 10-24-2005, 11:19 AM   #3
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You would need about a three inch propellor to reach 18000 rpm, and the noise would drive your neighbours batty! You will never get there, so don't worry about it..it is just advertising numbers....

Just prop it so you get the kind of thrust yoiu need.....

The sooner you fall behind, the more time you will have to catch up.
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Old 10-24-2005, 12:57 PM   #4
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OS 25 Prop

Hi Erg,

I noticed the same with my OS .25 and so I asked the combat guys when they were in Stratford what prop they use - they seem to prefer the OS. They stated a 9X4 works best and showed me an example of a black Master Airscrew with 15% Cool Power fuel.

I haven't tried it yet. You may want to check with your local combat guys.

Hope this helps.


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Old 10-24-2005, 04:23 PM   #5
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I run a well worn OS25FX on 15% fuel with the stock muffler (baffle removed) I get 16500 RPM with an APC 8x6. This works out to .789 horses. Not too bad and I bet with a pipe it could be above the rated numbers. I have to say that a 9x6 was never one of my favorite props on that eninge. I would stick with a 9x4 or a 8x6. Master airscrews take more of a beating than the APC, but the APC seem to have a performance edge. Just my 2 bits.
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Old 10-25-2005, 09:51 AM   #6
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Peak RPM and power readings or calculations mean little,especially as claimed by some manufacturers,both gas and glow.
Same thing with some weight claims for gassers.
A good tach,and performance in the air are the best tools to judge by and input by your local motorhead or SIG expert make the job easier.IMO.
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Old 10-25-2005, 07:21 PM   #7
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Ya the os 25 like the MA 9x4 with cool power 15% I get around 16,500 -17k[/quote]
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Old 10-26-2005, 06:52 AM   #8
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An engine's power increases with RPM until it reaches its peak power. Then the power falls off as the RPM increases. Presumabley, OS are quoting the engines maximum power, and the speed at which it is attained. I'd be surprised if OS were not quoting a factual number, however, it is not a terribly useful number - as others have stated.

A similar relationship exists between the engines torque and RPM. However, the peak torque occurs at a lower RPM than power. One could argue that is best to prop the engine so that it runs near its torque peak. But that is a matter of preference... and depends on your application. Further, nobody quotes that number!

In the end, experience (yours and other's!) is your best guide when it comes to choosing a prop.

Look at this page: http://www.glensmodels.com/Nuzg45sl.htm

There is a graph with two curves. One is power versus speed. The other is torque versus speed. Notice that they both peak, then fall off, and that the torque curve peaks before the power curve. That will always be the case since it is the fall off in torque that leads to the fall off in power.
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Old 10-26-2005, 09:35 AM   #9
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My guess is that you would be best to prop for peak torque at static speeds, and when the aircraft flys and unloads the prop a bit you could obtain peak horsepower.

This would give you oodles of acceleration at launch.

Anyone out there have a wind tunnel? A calibrated wind tunnel?

Second best would be a calibrated brake dynomometer.
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