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Dynamic & Static propeller pressure

  1. Jul 21, 2007 #1

    I have this theory about static and dynamic pressure generated by a propeller, but I might be wrong, so I could use some comments on this.
    Its kinda long and whiney, but well.. here goes;

    Lets say I have a motor with propeller, spinning at a certain rpm.
    -In the first scenario, I put the propeller so that it can spin freely in the atmosphere. The volume flow in this situation is at its maximum (no static backpressure), and the generated pressure is a pure velocity pressure, equal to: ½ . Rho . v^2
    Because no static pressure is generated, the total pressure equals Pvelocity.
    (Ptot=Pvel + Pstat). Assuming Pvel = 2500 Pa => Ptot = 2500 Pa

    -In the second scenario I take a sealed box, cut a hole in it with the same diameter as te propeller, and place the propeller so that it can spin in this hole. The prop will blow air into this box.
    Initially the flow will be large (see scenario 1), but as the static pressure in the box rises, the airflow (think velocity pressure) wil decrease, eventually stalling the airflow (Pvel=0). At this point, the prop is purely maintaining the boxpressure, this being the maximum static pressure the prop can generate, and all kinetic energy has converged into potential energy (Pvel to Pstat). Analog but opposite to scenario 1, Ptot now equals Pstat. since Pvel is zero.

    Now my question:
    Since Ptot is constant, can I say that the static pressure in scenario 2 is equal to the velocity pressure in scenario 1, being 2500 Pa??

    ps: Ignore small losses (This is ofcourse a simplified situation, but I need to get the concept)
  2. jcsd
  3. Jul 21, 2007 #2
    I see this question is related to your other question in some way. I think both are good questions. Wish I had the answers.

    In theory the pressure the propeller will generate will be exceeded by the pressure in the box and force it's way back out. In real life excess pressure will escape out the edges (space between the opening and blade tip).

    We get similar conditions with hovercraft and their lift fans. I only own and operate one, have never done the engineering on one.
  4. Jul 22, 2007 #3
    Yeah spot on, I'm asking these questions with the hovercraft in mind :smile: (Still RC for now)
    I'm trying to work my way through the engineering part, but its hard to find any good info on the internet. It seems like most hovercraft builders are doing this trial and error, or by copying successful setups. I allready dug up my fluiddynamics courses, but we never worked with air as a medium, so I'm a bit lost on some parts.
    There seem to be some very bright people on this board so I thought; piece of cake for them, but replies on this are less than expected..
    Thats pretty cool you own a hovercraft! Have you built it yourself?
  5. Jul 23, 2007 #4


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    Do you want us to assume that there would be no effects due to stall?

    I guess in a perfect world it would have to equal out. In typical industrial fan curves, the static pressure is highest at a 0% efficiency and flow.
    Last edited: Jul 23, 2007
  6. Jul 23, 2007 #5
    Ash, there are lots of pdf downloadable files (techincal/design papers) on several hovercraft club sites. I think you have to be a member for the American one, but you can post as a guest. I'm a HCA member and can start a thread for you if you like.

    You are right, a lot of the work by the self builders is according to plans. This has had an adverse effect on small hovercraft develpment in my opinion. On the otherhand I know how hard it is to get even a slight modification to work, let alone be an improvement.

    Links to have:




    There are of course many other forums and clubs, these are just my stomping grounds and sites which have helped me. I strongly encourage you to become a HCA member.

    My latest thread on my personal hovercraft project:

    You will need this forum for your model work.
    Last edited: Jul 23, 2007
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