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Propeller problem

  1. Jun 14, 2007 #1
    "Pitch is defined as the theoretical forward movement of a propeller during one revolution — assuming there is no "slippage" between the propeller blade and the water"
    i dont, kinda, really get this definition of the pitch. i searched it at so many sites. they all say the same thing, like pitch of an airscrew(prop) is analogous to the pitch of a thread on a bolt. but the profile of the thread is same everywhere in the threaded shank whereas the profile of an airscrew (prop) changes.
    can anyone clarify, where i am getting it wrong?

    also what does one mean by the stalling of an airscrew, i mean stalling of airfoil means the sudden lost of lift, so that way, is it the loss of thrust in case of an airscrew
     
  2. jcsd
  3. Jun 14, 2007 #2

    FredGarvin

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    Science Advisor

    You are thinking correctly in the bolt analogy. I think the part that is getting you is that when you measure the pitch on the bolt, you move from thread to thread, down the shank. Here there is no shank. Think of it this way: If you were holding on to the prop and it was spinning, you and the prop would move forward a certain distance with every rotation of the prop. That distance you move is the pitch.

    Here is a good picture of what it is.
     
    Last edited: Dec 20, 2007
  4. Jun 14, 2007 #3
    but when is this distance constant??
    i mean if the airspeed is 0, the prop is not moving at all. or if the airspeed is high, prop covers a lot of distance in one rotation.
    actually the problem is that i am not able to look at it from the correct frame of reference.
    first i tried to think it like it was moving through a solid and leaving its precise geometry in the solid(this is the most weird thing that i have thought) and the distance between two consecutive imprints is the pitch. but i am most probably wrong in thinking it that way?
    could you please clarify a bit more on this distance thing?:confused:
     
  5. Jun 14, 2007 #4

    russ_watters

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    Staff: Mentor

    The definition of pitch makes no mention of speed whatsoever, only distance. For every revolution, the propeller would move a specific distance forward. So if you want to relate that to speed, use RPM.

    I was a power nerd in High School. I once used a model airplane engine to drive a wind tunnel. The propeller I used had a pitch of 11 inches per revolution and the motor spun it at 16,000 rpm. So can you calculate for us what the speed of the air was...?

    And yes, stalling is stalling - same issue.
     
  6. Jun 15, 2007 #5
    74.4 m/sec???
    so the pitch actually determines the airspeed???
     
  7. Jun 15, 2007 #6
    but when the prop is already moving, then what??
    i mean is this the total relative airspeed, ie speed of air relative to prop.
    is this the reason why thrust decreases with the increase in airspeed??
     
  8. Jun 15, 2007 #7

    russ_watters

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    You got it (everything you said was pretty much correct) - though with turboprop airplanes, they use variable pitch propellers to keep the engine horsepower roughly constant in changing speeds.
     
  9. Jun 15, 2007 #8
    so that varying pitch is called feathering?
    equivalently we are changing the angle of attack, right??

    so they vary the pitch for varying airspeed instead of varying the engine speed:surprised kewllll
     
  10. Jun 15, 2007 #9

    russ_watters

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    Sorry, I said that wrong - they keep horsepower constant by changing pitch. But I think you knew that...

    Feathering is changing the pitch (or releasing it from control) to completely eliminate the thrust. You may do this, for example, if you have an engine failure, to minimze drag.
     
  11. Jun 21, 2007 #10
    interesting idea for a prop moving through a solid and aside from the fact that is completely impossible the theory there is correct.
    i think i see what is confusing you and if i am correct i beleive i can answer your question. see, the definition is the forward movement of the propeller through air per revoloution but it doesnt mean that aslong as your prop is spinning you are moving because the thrust still needs to overcome drag aswell as weight.
    perhaps a better explanation(for when the propeller is moving but the vehicle is static) would be the distance air is pushed backwards by the propeller
     
  12. Jun 23, 2007 #11
    i used a programme that calculates the thrust, power output, power absorbed and efficiency with airspeed, rpm, no. of blade, prop dia and pitch as input.
    i should get max thrust at 0 airspeed, that i m getting!!
    but the programme says "prop is definety stalled"!!!!

    what does this mean??
    wont the system produce any thrust at this stage??(this is what stall means, i guess)
    but how is this possible, every system starts from 0 initial airspeed, what is wrong here??
     
  13. Jun 24, 2007 #12
    anyone?????
    plzzzzzzzz answer!!!!!!!!!
     
  14. Jun 25, 2007 #13
    The system WILL produce thrust even at stall. Getting max thrust at zero is true, you could even get higher if your airspeed is negative or you're using 'reverse thrust' while moving forward.

    Stall is (as wikipedia says it) - the fairly sudden loss of effectiveness of an aerodynamic surface(like propeller blades). It doesn't say 'total loss' as you might be thinking.. Or perhaps you're thinking of engine stall?

    Stall on airfoils or prop blades happen when there's too much angle of attack(like inadequate speed for a heavily loaded wing). Props gain speed too, as you accelerate from zero airspeed. But firmly held at zero airspeed while spun, not only is not it's having much airspeed, the propeller blades are also heavily loaded so it stalls..
     
  15. Jun 25, 2007 #14
    nop.. i m not cofusing it with the engine stall

    so it is like airfoil, where at large angle of attacks, it stalls.
    but even at small pitch(pitch determines the AoA, i guess) it shows the same message as with larger pitches.
    Is it wrong??
     
  16. Jun 25, 2007 #15
    Yep, pitch can determine AoA, if I'm right, that is at the propeller tip only..

    I think a propeller shouldn't stall up to 10 degrees AoA with the craft held at zero airspeed. If it stalls even at 1 degree, you should also see if the propeller has varying incidence-most propellers have more angle of incidence at the root than at the tip.

    If it is a variable incidence, and the angle of incidence at the root is greater than 10 degrees while/even if tip incidence or pitch angle is zero, it might stall(but at the root only). So stalls are possible even at very low pitch angles for varying incidence propellers.

    However for constant incident or with very little variance in incidence(root angle of incidence is equal or nearly equal to tip incidence), if it stalls with very low pitch angle, there's something wrong with it, it's no longer realistic.
     
    Last edited: Jun 25, 2007
  17. Jun 26, 2007 #16
    oh i get it now. it is almost similar to the airfoil case. actually the results from that software were driving me nuts, so i was confused.
    i think i understand now where i was thinking wrongs.
    thanks gaming addict(well i cant believe how dumb i m)
     
  18. Jun 26, 2007 #17
    Hey, don't mention it. I learned much about propellers by tweaking them in the X-Plane flight simulator, but I can't trust it all the time too, it gives very wrong efficiency, and engine power readings in certain situations.
     
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