Measuring engine torque

Discussion in 'Firewall Forward / Props / Fuel system' started by ArcticDave, Aug 19, 2017.

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  1. Aug 19, 2017 #1

    ArcticDave

    ArcticDave

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    Hello everyone, I have been lurking here for a while as my kinda Legal Eagle project is slowly built. I am currently gathering parts for the half VW power plant. Most of the time I can get the answers I need by using the search function, but I couldn't find much on homemade dyno's.
    I have skimmed over the "wood bar dyno" method used by some of the guys over on the Legal Eagle board some time back, and it looks doable enough, but I don't have MS Excel on my current computer and can't read the xls download any longer.
    I recently ran into an article from the Jan 1982 issue of Sport Aviation entitled "The Whatley torque stand".
    If anyone is familiar with that...is the methodology sound?
    Since I need a test stand anyway for run in and tuning it seems a no brainer to add a lever arm to it and be able to measure the effects of changes to timing, jetting, and whatever else I want to fool with at the time.

    I couldn't figure out how to link a pdf, but a google search of the article title will pull it right up.

    Thank you all for any input,
     
  2. Aug 19, 2017 #2

    clanon

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  3. Aug 19, 2017 #3

    proppastie

    proppastie

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    Yes it is sound, but be very careful, 20 hp or whatever the 1/2 VW puts out is a whole lot of force, make it sturdy.

    The alternative is to wait until you have mounted on your plane, and use a spring scale attached to a tree and tail wheel., or a cheap load cell hanging weight scale from e-bay, you probably do not have to pay even this much if you can get away with a luggage scale.

    Static thrust and RC model RPM gauge should tell you what you need.


    http://www.ebay.com/itm/Portable-50...avy-Duty-Industrial-Hang-Weight-/263049027751
     
  4. Aug 20, 2017 #4

    Dana

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    Static thrust will tell you nothing about HP..

    I once attempted to measure torque by running the engine while the plane was sitting in two scales. It didn't work; the buffetting and shaking me it impossible to get a good reading.

    Dana
     
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  5. Aug 20, 2017 #5

    ArcticDave

    ArcticDave

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    Yep thats the one. Thanks!

    Thanks, it will be built plenty sturdy. I want my engine to running in top form before I even order a prop. A test stand would help I think. If I can give Propman good numbers for my engine...it might save some hassle shipping a prop back and forth to get it just right.
     
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  6. Aug 20, 2017 #6

    proppastie

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    Yes but if you are just trying to tune it or jet it max RPM should be enough.

    sitting on 2 scales is not the same as a fish scale on the tail wheel tied to a tree, but one would be checking out a prop that way.
     
  7. Aug 20, 2017 #7

    proppastie

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    I thought the 1/2 VW was a proven engine....why are you going to this trouble.

    Reading the article you will see a test club prop is attached to take the measurements, you will have to have something to start with.
     
    Last edited: Aug 20, 2017
  8. Aug 20, 2017 #8

    BBerson

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    The torque arm must be at crankshaft vertical axis.
     
  9. Aug 20, 2017 #9

    proppastie

    proppastie

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    Why not horizontal axis?
     
  10. Aug 20, 2017 #10

    BBerson

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    The Whatley torque arm was centered both horizontal and vertical axis.
    Measuring from the wheels on scales can't work.
     
  11. Aug 20, 2017 #11

    Dana

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    Sure it can... in theory; you just have to resolve out all the forces and arms. But in practice, well, it wasn't too practical...

    Dana
     
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  12. Aug 20, 2017 #12

    proppastie

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    The engine is allowed to rotate about the Center of the crank shaft the arm is horizontal to the top of a scale. The arm is not even in the center but is offset to about the location of the carb.
     
  13. Aug 20, 2017 #13

    ArcticDave

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    There is a lot of variation in the VW. There are so many choices in available parts. Stroke, cam, timing of the cam and host of other variables that can have a major impact on available power. A test stand would allow me to see the changes in peak torque as I fiddle with it.
    A few test clubs would easy enough to fashion myself, a prop not so much.
     
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  14. Aug 20, 2017 #14

    wsimpso1

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    A difficult problem that way. Noisy data is definitely a problem in dyno design. Running on a zero wind day and with enough filtering and other processing of the signals, you can get a reading on steady state torque. Trouble is that "the wind always blows". Any torque that wind blowing on your airframe generates gets added on top of your signal and all of that noise.

    Torque meters for turboprop engines used to be strain gages on an engine mount element with spherical joints at both ends of that element. Still takes careful calibration and signal processing to be workable.

    Billski
     
  15. Aug 20, 2017 #15

    ArcticDave

    ArcticDave

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    Thats interesting Bill. What was the purpose of the strain gauges? Were they using the torque measurements to determine load % for a fuel map...or something else?

    Since nobody has come out and said it's an invalid way to determine torque, I think I'll build one. I have a good friend that is very interested in building a little bird like mine. If he does, it will be handy for his engine too.

    Touching briefly on the discussion of the placement of the lever arm, does it really matter? As long as the line of the arm intersects the center of the crankshaft, wouldn't it provide the same reading whether positioned at 9 oclock or 6?

    Thanks everyone!
     
  16. Aug 20, 2017 #16

    proppastie

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    Very valid,

    If you look at the picture you see the torque arm does not intersect the CL of the crank, so probably a little calculation he had to do.

    Because of the power pulses you might have problem with a digital scale like I posted, readings not steady, and or not readable, I would try the $6 luggage scale first before I invested in a larger scale. But with a large lever that may be enough. If it does not work then a spring scale (fish scale) would be what you have to use. I reserve the right to be wrong here because I have never done this,....just trying to help.....

    Post your results.... looks like fun.
     
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  17. Aug 20, 2017 #17

    Autodidact

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    If you draw a line from the end of the torque arm to the crankshaft axis, you get a slightly longer moment arm, but, the force vector is at and angle perpendicular from the arm meaning that it is not pushing perfectly vertically down on the scale but at a small angle, so that the reading on the scale is the force vector divided by the cosine of the angle-from-the-horizontal of the line drawn from the scale to the crank axis. If you do the trigonometry, you will find that the reading on the scale is the same as if the torque arm were extended straight out from the crank axis and a vertical piece attached to the end extending down to the scale, or even if you moved the scale up to meet the arm.

    Don't just take my word, though, spend 5 minutes w/paper&pencil and do the very easy math and find out for yourself if I know what I'm talking about.

    I couldn't resist working it out, and it's a liiitttle difficulter than I made it out to be I guess, so:

    Suppose the horizontal torque bar is 10 ft long and positioned 2 ft below the crank axis, that means that the angle of a line from the scale to the crank axis is 11.537° from the horizontal, and its length is 10.2062 ft. If the engine is producing 100 lb-ft torque, then the force perpendicular to the scale-to-crank-axis radial line is 100/10.2062 = 9.798 lb. The angle of that force vector of magnitude 9.798 lb that is perpendicular to the scale-to-crank-axis radial line is 11.537° from the vertical. So, 9.798/cos(11.537°) = 10 lb, and 10 times the horizontal distance from the crank axis to the scale, which is 10 ft, is 100 lb-ft torque.
     
    Last edited: Aug 20, 2017
  18. Aug 20, 2017 #18

    ArcticDave

    ArcticDave

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    Hahaha...I was thinking a fish scale too!

    It'll be probably be October before I can get to the stand, I'm working on finishing the detail work on the fuselage at the moment. 90% done and almost ready for paint.

    I will certainly post up when the stand gets built. I have a couple of ideas for cleaning up the design and making it easier to use.
     
  19. Aug 20, 2017 #19

    BBerson

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    I think a heavy temporary flywheel would smooth the torque pulse.
     
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  20. Aug 21, 2017 #20

    ArcticDave

    ArcticDave

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    I had to read this 2-3 times to wrap my head around the math. Great info.
    I'm thinking a vertical arm at 6 oclock would be easy to deal with. Less hanging out the side to trip over. The spring could be attached to either support leg depending on the prop rotation of the engine on the stand. I know more force would be applied to a shorter lever, so I'll work through a bit of math this evening and get an idea of the force involved with...say a 4' arm at the level of torque the engine should produce.
    I would like to use a needle on top of the arm and a flat plate with lines marked on it for instant torque measurements. Once it was calibrated to whatever return spring I dig out of the junk pile it should be a simple matter of reading the "gauge" to know the output of that dyno run.
     

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