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Build a pulse-jet for my final year project

  1. Nov 1, 2005 #1
    Hello,
    I have taken on the task to build a pulse-jet for my final year project and could do with help designing an outside test cell to measure thrust.
    I am considering attaching the jet to the test cell using a strain gauge pin.
    What do you think??
     
  2. jcsd
  3. Nov 1, 2005 #2

    FredGarvin

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    Science Advisor

    Can you be a bit more specific? Thrust measurement can get a bit tricky. How do you plan to mount the engine? What exactly is the configuration going to be for this "pin".

    The way we do it involves, basically, a table with a floating top (thrust bed). The thrust bed is attached to a load cell that is calibrated in place, i.e. the entire thrust bed is cal'd as a single unit. There are a lot of issues with getting the centerline of the engine and the centerline of the load cell lined up.

    Keep asking questions.
     
  4. Nov 6, 2005 #3
    I'd go for a simpler set-up using dead weights. Mount the engine horizontally, so that it can slide forwards a few inches along a rail. Use a spring balance (as used by fishermen) to quickly measure thrust, and run a string over a pulley, to a pan containing weights for more accurate measurements.

    Pulse jets are difficult to get going (fill them with propane gas and ignite), dangerous, and unbelievably noisy. I hope you run your tests far from civilisation, or you will need very understanding neighbours.
     
  5. Nov 11, 2005 #4
    Hello,
    If you have a look at the attachment i have drawn a test cell which i will build,it is a rough drawn without taking into consideration any applied stress and strains.If you look at the cylindrical pin this is where i will attach the jet to the cell (I’ll attach a plate to the jet then use some kind of pylon to attach to the pin).At the back i'll have a pin attached to the back end of the jet to stabilize it..
    The pin will have a strain gauge which will give an electrical signal out..

    Any opinions appreciated.

    Ryan ps sorry about size of bitmap..
     

    Attached Files:

  6. Nov 11, 2005 #5
    If the thrust is small, use an interferometer. Thats what ive seen done.
     
  7. Nov 12, 2005 #6

    FredGarvin

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    Science Advisor

    It's a bit tough to see. It looks good. Again, the only thing I would worry about is inducing a bending moment on that pin and thus getting a thrust value that is not the true reading. I would recommend that you spend a bit of time and do a calibration on the load readout to make sure you can account for this. It would be very easy to do if you had a hand held load cell or a fish scale type. Pull and compare the two readouts. That will develop your cal curve.
     
  8. Dec 5, 2005 #7
    I think that the idea of running the engine on a track and pulling on a fisherman's scale is brilliant.:biggrin:
    ps: I'd like to see the engine itself.:wink:
     
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