stunthanger.com
Engine basics => Engine set up tips => Topic started by: Terry Caron on November 05, 2016, 12:50:51 PM
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What's the process?
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The usual method is to generate torque and RPM points using a number of different "props".
The props usually have no pitch so that there is no airflow to disturb the measurements. Basically paint stir sticks.
Motor is mounted on an axle and the torque determines how much it twists.
Some pictures here:
https://www.rcgroups.com/forums/showpost.php?p=35713764&postcount=669
Lots of details I am sure in getting it all to work properly, but that is the general principle.
There are also picture there of other methods, for example using magnetic loads that you can vary and a load cell to directly measure the torque.
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That link is to some bit of R/C arcana, Pat, but thanks for the overview.
So it's simply the weight of the "props" that loads the engine to specific torque/rpm points?
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I have thrust finder machines that are very good at this, as comparing engines to engines
Randy
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That link is to some bit of R/C arcana, Pat, but thanks for the overview.
So it's simply the weight of the "props" that loads the engine to specific torque/rpm points?
Pretty sure that is the method (or a variation of it) that was used in all the old engine reviews.
They were pictures in many of the old magazines of different setups.
By using a series of props (real or just sticks) you will get different operating points.
It is not the weight that varies, but the diameter. Prop weight has no real effect on HP output.
To Randy's point I would have to disagree a bit. :) Thrust is as much about the prop as the motor.
But if you have "known" props then you can compare motors just by comparing RPM, no need to measure thrust.
But if you want an actual HP curve you need multiple data points, so you need multiple loads and some way to measure the torque absorbed.
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Thanks for the clarification, Pat.
I'm trying to understand about prop selection and this is one of those areas in which more info will probably cause my question to change, but here goes:
Case in point: a Golden Bee for some stuntability, for which a Cox 5x3 prop has been most often recommended, I must assume from experience.
Mine (twin bypass, standard head, 1 gasket) will turn a 5x4 or a 5.5x4 prop on 15% nitro to ~17K static (2-300 more on 25%), and Waring, in his report, says peak power is at 13-15K.
Which means those props peaked would be well past max power in the air (I think Chinn said 1-1.5K unloaded increase). Of course the engine can be richened to bring launch rpm down to 12.5-13K to keep unloaded rpm near peak hp.
Why not choose a larger prop that would unload to the sweet spot but still has "breathing room" in the air?
Seems it would give better mileage, not being richened so much, if nothing else.
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AFAIK, Golden Bees were single bypass, Black Widow was twin bypass.
I seem to recall always running 6x3 on mine, 5x3 on a Tee Dee. That would puts things more in line with the HP curves in the reviews.
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Model Engine News has an article about the different methods sometimes used to measure HP at http://modelenginenews.org/techniques/testing.html and the simplest is the last one on that page by using calibrated props. For general use at home I'd think that a range of closely similar props would give good comparative results for a large range of engines although not quite as accurate as calibrated props. Many years ago I used those graphs to make up a spreadsheet so that, by entering the revs achieved on any one of those props, it would show the HP and torque developed.
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You're correct about the cylinders, Pat - mine has a BW type on it right now.
I'll change it back, not needing the extra power (or noticeably greater fuel consumption).
And in original configuration it turned a 6x3 such that it would unload as I was thinking it should.
Great link, Brian, thanks!
Probably more "mathy" than my noodle can handle but maybe I can glean some understanding from it.
EDIT:
Yep, I was right. ;D
The significant thing I take away is that testing has so many variables that any method is actually most useful as a comparative, much like Randy's thrust measurements.
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To Randy's point I would have to disagree a bit. :) Thrust is as much about the prop as the motor.
But if you have "known" props then you can compare motors just by comparing RPM, no need to measure thrust.
But if you want an actual HP curve you need multiple data points, so you need multiple loads and some way to measure the torque absorbed.
Disagree all you want, but your are wrong ,thrust is not as much about the prop as the motor, "if" you have a more powerful motor turning the exact same prop you will more than likely get more thrust which equals more power, its all about the motor, not the prop as far as this particular test goes
The MOTOR makes the thrust by having power to turn the prop, the single prop does not make more or less thrust, its the motor that does that, as long as you use apples to apples compare
I personally have a set of known props that I use to plot a torque and HP curve, but the thrust finder is, as I said a useful tool.
http://bj-model-engines.com/AmericanHobby/Dyna.html
The company that makes the device I sell also makes a dyna-meter whick measures torque
Regards
Randy
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http://bj-model-engines.com/AmericanHobby/Thrustfinder.html
and the thrustfinder
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I did say that you could use a thrust stand if, like you, you have "known" props. But RPM would tell you the same thing.
But if you don't have any known props and the goal is to measure the HP curve then the dyno is the tool you need.
Or you can use the curves and the RPM.
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Kavan used to make and sell sets of calibrated "propellers", and I believe P.G.F. Chinn used these with some sort of tachometer/strobe to get accurate rpm. Referring to Kavan's chart gave data points for HP & Torque graphs. This may have come from one of George Aldrich's engine articles in Aeromodeller Annuals...around 1970, +/- a couple. D>K Steve
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Like this one, Steve?
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Too kool for skool! I really like the little rubber finger protectors on each side of the kit. I've seen those before, but none of the rest. It'd still hurt like the dickens to get whacked by one of those aluminum propellers!!! ;D Steve
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Should be fairly easy to compare the wattage drawn by an electric motor at a specific rpm for half a dozen or so blank props, like the Kavan test props, if you want to make your own. It's easier to work with using blank props because regular props make a heck of a breeze.
Years ago a friend, Glenn Dye, built several TWA rear exhaust combat motors for me. We tested several of them using Kavan test props he had. They had a very interesting torque curve- it started low, of course, a smooth curve up to about 1.2 hp @18,000 rpm and then turned into a very flat curve that topped out at 23,000 and then dropped back to 1.2hp @ 25,000. Different props had little effect on the speed, so I went with the ones that held speed in maneuvers the best- an 8.5/6 pylon prop if I remember right. 3 minutes of high excitement!