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Measuring Actual Thrust

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Old 05-03-2007, 09:28 PM
  #1
KojakDave
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Default Measuring Actual Thrust

FYI: (THIS WAS ALSO POSTED IN THE GLOW ENGINES FORUM, SO IF YOU SAW IT THERE, IT'S THE SAME POST).

I've recently experimented with measuring the actual thrust of an engine/prop combination as it is mounted on the airplane. I would like to run this by all the engine gurus out there to see if this seems like a valid way to do so.

The way I've been doing this is to use a digital fish scale which is hooked to a 10' leather dog leash (it seemed to have less elasticity to it than some of the different types of rope I looked at...plus I know it will hold at least 40 lbs, the weight of my stupid mutt pulling her heart out). The other end of the leash is placed around the tail of the plane that I'm testing and the digital fish scale is attached to a metal spike driven deep into the ground. I start the engine and let it idle, take the slack out of the leash, and zero out the scale. I then throttle up and read the "weight" of the pull generated in lbs and oz.

It seems like it has been a fairly reliable test, especially when comparing different propeller sizes/pitches/brands to determine which one can generate more thrust. Does anyone see any flaws in this?

Thanks,

Dave

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Old 05-04-2007, 12:37 AM
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Default RE: Measuring Actual Thrust

Hi Dave,
I am not an engine "guru" and thus I tried something like that EXTENSIVELY couple years
ago. The results were very inconsistent. The measured force varied in too huge range of
about +/- 5-to-10 % and that is too much. I am sorry to disapoint you but there are lot
of sourses of errors in that set-up, like low-frequency vibrations which are any way transfered
to the scale-metter, of coure the high-frequency vibrations are filtered out by the connection
you make. Mechanical engineers can tell many more. At the end, after burning almost 10
liters of fuel for measurements I found out that all this was in fact quite useless because there
are simple accurate methods, lot more accurate and lot more comfortable(!!!) too than to
measure thrust - just use Measure RPM and use Calculator Program (Excel file) in your
home computer. Among several calculators I have checked, the best so far has been
the one developed by Pe Reivers and available free on
http://www.mvvs.nl/
That calculator in few moments will give you at any time what you would measure and
without any dust, smoke etc and for almost any engine, propeller, fuel, altitude etc.
All my measurements converge pretty well with that calculator.
Cheers,
Nick
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Old 05-04-2007, 07:59 AM
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Default RE: Measuring Actual Thrust

Dave, this is very much the convenient system we all use for static thrust. The main advantage with static thrust is the direct relationship to engine power. How very convenient. Under air speed things change, but here engine power derived from static rpm measurements is a great asset.
Using a digital scale however is not a good idea, because of the sampling the electronics have to do. If you hook up an analog scale, the pointer will swing over a range of about a pound or more. Humans have no problem defining the midrange of the pointer swing. Digital scales however will show errors, unless a damping system is present in the hook-up. (a analog scale will bebefit from that as well)
Defining the thrust with scales has the drawback, that net thrust is measured. The fuselage drag in the propwash counteracts the prop thrust.
Despite the above, I found the way to define the static thrust very adaptable to calculations, which proved a great method for prop size evaluation that takes about 80% out of the guessing.

Test flights will always have the final say, because in flight, plane air resistance, pitch and prop geometry play a large part that cannot be evaluated so easily by simple calculations. Complicated datalogging is needed to gather data, which is way beyond the scope of us simple souls. We have our extremely accurate visual contact for that.
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Old 05-05-2007, 12:03 PM
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Default RE: Measuring Actual Thrust

Me and a friend have used digitale scale with great success. We get quite stable consistent readings and have no difficulty in determining the prop with the most static pull. I find it interesting and usefull information at least for prop choice on a 3-d plane. I know flying is the best way to determine the best prop, but this is something we like to do on a rainy day and we like to experiment and investigate a little. The last time we had a pulling contest, I put my MVVS 49 up against his magnum 91 fs. He managed to pull 4.5 kg with a 14-6 and I managed 4.2 kg on APC 12-4, so I came close (identical airframes). Most importantly, we had fun that afternoon even though it was raining (we did this inside his garage)
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Old 05-05-2007, 02:59 PM
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Default RE: Measuring Actual Thrust

Asmund,
Try an analog device, and you will see the difference. Digital devices give "jumpy" readings, whilst from the analog poimter movements a lot more info is available. The difference is such, that for example, an analog vacuum gauge in an engine can be used to check ignition timing or valve leaking just by the pointer movement, where a digital meter does not give that kind of information at all.
I think the words " not a good idea " were not chosen that well. I should have said that an analog scale is better for the purpose.
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