Build a pulse-jet for my final year project

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Discussion Overview

The discussion centers around the design and construction of a test cell for measuring thrust in a pulse-jet engine as part of a final year project. Participants explore various methods for thrust measurement, including the use of strain gauges, load cells, and alternative setups involving dead weights.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant suggests using a strain gauge pin to attach the jet to the test cell for thrust measurement.
  • Another participant raises concerns about the complexity of thrust measurement and the importance of aligning the engine and load cell centerlines.
  • A different approach is proposed involving a simpler setup with dead weights and a spring balance, emphasizing the need for safety due to the nature of pulse jets.
  • A participant provides a rough design of the test cell, detailing the attachment methods and the use of a strain gauge for electrical signal output.
  • One suggestion includes using an interferometer for small thrust measurements.
  • Concerns are expressed regarding potential bending moments on the pin affecting thrust readings, with a recommendation for calibration of the load readout.
  • A participant expresses enthusiasm for the dead weight method and requests to see the engine itself.

Areas of Agreement / Disagreement

Participants present multiple competing views on thrust measurement methods, with no consensus reached on the best approach. Various suggestions and concerns are raised without resolution.

Contextual Notes

Participants mention potential issues such as alignment of components, calibration of measurement devices, and safety considerations related to the operation of pulse jets. Specific assumptions about the configuration and setup are not fully detailed.

Who May Find This Useful

Students and hobbyists interested in experimental thrust measurement techniques, pulse-jet engine design, and those seeking practical advice on project implementation in engineering contexts.

Rhino
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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??
 
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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.
 
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.
 
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..
 

Attachments

If the thrust is small, use an interferometer. Thats what I've seen done.
 
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.
 
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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