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Test rig to demonstrate propulsion

  1. Apr 18, 2005 #1
    hi all,

    i need to construct a test rig to demostrate propulsion,
    the rig need sto be onstructed according to factors that affect thrust

    any ideas how to go abt doing it?

  2. jcsd
  3. Apr 18, 2005 #2


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    I would give you all the info you need on the topic but I don't understand what it is exactly you are asking. Do you simply need a thrust stand that will allow you to measure thrust?
  4. Apr 21, 2005 #3
    hi fred

    sorry 4 the late reply
    bascially i need to construct something that shows how thrust could be demonstrated by serveral factors, such mass flow rate, speed ofair and so on

    something like holding a firehose and varying the hand pressure and varying the amt of thrust generated..but in now in laborotary conditions..


  5. Apr 21, 2005 #4


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    Basically all you need to do is figure out a holding apparatus to hold your object and have that able to slide. Connect the opposite end of the flow to a scale (like a fish scale) or if you want to get a bit more techie, a load cell.

    I guess this will all depend on how involved you want to get.
  6. Apr 25, 2005 #5
    This guy makes his own load cell out of a brake cylinder and a pressure gague..

  7. Apr 26, 2005 #6
    I am just finishing a fluid mechanics course this semester where we talked about modelling things that you are talking about. The first thing you want to do is to determine what parameters are important to you and how they depend on eachother.
    Take thrust for example.
    You want to find the thrust generated by a firehose. Now you must determine what parameters affect the thrust generated by the hose. Pressure would be one, velocity, diameter, legnth, density, and viscosity. Now you have to determine what parameters are going to be the same for the model and the real life situation and what parameters you can control by changing. Well I would assume you are going to using the same type of fluid in your model as would a real firehose so you can get rid of the density and viscosity parameters.
    However, if you wanted to you can test the thrust using a different fluid then what would be used you would simple have to set up a relation between the two.

    By understanding the relations of the parameters you can set up a much more meaingful experiment.
  8. Apr 27, 2005 #7


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    Ahhhh.....similitude and pi terms. Those were the days.
  9. Apr 27, 2005 #8
    Ormund in Santa Fe Springs Ca. Might be able to help you. I'm checking my lit now to be of more help.

  10. Apr 27, 2005 #9
    Thrust stand

    I had this same question when I first started firing a pulse jet. I visited Mr. Ormund's facility where he manufactured load cells for large engines ie.Pratt& Whitney, General Electric and Rolls Royce even the Space Shuttle main engines. Anyway his suggestion was to build a frictionless bearing horizontally mounted holder and then mechanically linking the assembly to a digital scale. Most digital scales are quite accurate because they incorporate a wheatstone bridge measuring technique which can provide up to 1/4 pound resolutions . The frictionless bearing consists of two .001 thick by 1/2 in wide by 6 in long stainless steel 'feeler' gages tightly anchored to the engine holder and the stationary rig.

  11. Apr 27, 2005 #10
    That wasn't too, clear the bearings should be mounted one fore and one aft on the engine holder and on the stand.

  12. Apr 27, 2005 #11
    also the digital scale provides large displays either in English/pounds or Cgs outputs in either case the displays are easily read when video taped.

  13. Apr 27, 2005 #12

    density and viscosity are extremely important as well as temperature. You already know that as the viscosity increases the 'absorbed energy' increases as well which is a parasitic loss. Also with increased viscosity increased drag upon the confining medium increases the total or gross efficiency is severely affected when these parameters are not given the weight they deserve when evualting performance of a system requiring thrust. for example : using a surfactant on the pumped fluid can easily reduce parasitic 'absorbed' energy losses by as much as 15% which in the case of a fire hose can mean increasing the 'reach' by almost a correspoinding amount or depending on the nozzle choice an increase in delivered water to the fire......anyway just an observation.......

  14. Apr 27, 2005 #13


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    Most digital load cells are reduced down to a strain gauge. I can't recall if I have seen a piezoelectric load cell, but I'm sure they exist. For the cost and the accuracy, you really can't do much better without spending a fair amount of money.

    The method Mr. P mentioned is what I was eluding to and is pretty much what we use in our engine thrust measurements. The only difference is that the bed that actually supports the object hangs from relatively thin flexures instead of riding on bearings.
  15. Apr 27, 2005 #14
    Mr. P,
    What I was saying is that if you were to model an engine or a pump or whatever you would have certain things that will remain constant based on the model and your real life situation. These parameters such as viscosity, density, and pressure. Therefore you can cancel them as long as your models parameters are exactly the same as your real life situation.

    When testing something you need to hold certain things constant. However, if I were designing a pump that might be used to pump several different types of fluids I would need to find a relation between how say for example the power of the pump varies based on the different fluid properties. At the same time realizing that when testing certain aspects of the pump such as rotational velocity and its effects I would need to cancel out or hold certain things constant. In that case it would be set up so that the fluid being tested with is the same as the fluid being used in real life. That way I can just find the affects of rotational velocity.

    Perhaps I didn't make these things clear in my previous post. Hopefully you understand what I am saying.
  16. Apr 27, 2005 #15

    You are exactly right! When establishing a test proceedure I generally do as you have said using three test sequences to identify a direction to head or to find mathetical relationships. This result creats a new mostly intuitive begining point in which the process is repeated.

    Mr. Garvin, the Feeler gage material I mentioned above is referred to as a 'frictionless bearing' because ,technically, the displacement or distance that the holding fixture and engine moves is typically measured in tenths or ten thousandths of an inch and for all practical purposes isn't meaningful. Ion thrust measurements however do require more sensitivity. According to Mr. Ormund His equipments' accuracy has to be traceable to the NIS for purposes of contractual obligations and the type of rig listed above is for identifying 'which way to go' when changing things like fuel pressure diameter vs length etc.


    frank MR. P
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