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Why are air plane propellers small?

  1. Jul 9, 2016 #1
    Why are the blades of air plane propellers thin compared to ship propellers? Why shouldnt air planes use the same blade design?
     
  2. jcsd
  3. Jul 9, 2016 #2

    billy_joule

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    Put simply, water is different to air and aircraft are different to boats.
    If you want more specific information you'll need to do some of your own research and ask more specific questions.
     
  4. Jul 9, 2016 #3
    I did. All I found is that the design of the blade has an impact on the power the engine has to output and tension within the blades. Comon design parameters are pitch, size, count. I know that the viscosity of air and water differ. Is that the reason? Thicker, bigger blades to ensure higher stability?
     
  5. Jul 9, 2016 #4

    SteamKing

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    For one thing, ship propellers and aircraft propellers operate at vastly different speeds. If you spun a ship propeller at the same speed as an aircraft propeller, the ship propeller would start to cavitate, which means that bubbles of water vapor would form on the low pressure side of the blade. These bubbles disrupt the flow of the water over the blade, and the propeller cannot absorb the power it is designed to do. These bubbles also collapse quite violently, leaving the blades eroded and damaged where this occurs.


    Picture-59.png

    Ship propellers also tend to be big. Here is a propeller which is 11 m in diameter:


    img_machinery_A24_02_01.jpg

    It would be tough to fit such a beast on a Cessna.
     
  6. Jul 9, 2016 #5

    jack action

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    I'm no expert in either type of propeller designs, but air and water differ in more way than just viscosity. Densities are also different. Air is compressible, so the speed of sound is of concern. Water is incompressible, but cavitation is a problem.

     
  7. Jul 9, 2016 #6

    russ_watters

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    Density is probably the biggest difference.
     
  8. Jul 10, 2016 #7

    Ranger Mike

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    there is a very practical reason aircraft limit propeller size. Look at the Vought F4U Corsair. It was designed from the onset as a carrier-based fighter, not only had the largest propeller of any U.S. fighter, but was also expected to face rough landings aboard a pitching carrier deck. The inverted gull wing allowed the landing gear to be short and strong, and to retract straight back, improving internal wing space. An additional aerodynamic advantage was that the wing/fuselage connection is perpendicular and has inherently lower drag than any other connection. This plane had the largest prop of any plane in the hay day of propeller aircraft at 13 feet 4 inch diameter. You get too big a propeller and the plane will rotate about the air screw in flight!
     
  9. Jul 10, 2016 #8

    NTW

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    In general, it is cheaper to deliver a thrust through a large mass of air launched backwards at a small speed than with a small mass launched at high speed. The momentum of that mass, and the corresponding reaction of the aircraft, may be the same, but remember that the energy has the v term squared... Propellers have a size limited by the tip speed, that should not be too high, lest drag rises too much. But that tip speed is a vector sum of the rotational velocity and the airspeed of the aircraft; thus, that airspeed shouldn't bee too high, either. Besides, there are problems with the undercarriage (that should not be too tall) and with the torque, but this last problem can be solved with contra-rotating props.

    But, even taking into account all the limitations mentioned above, the larger a prop, the better... That's the reason helicopters have very large rotors.
     
  10. Jul 10, 2016 #9

    OCR

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    Maybe not completely limited by tip speed, but it does seem to cause big problems... as in the case of the "Thunderscreech".
    Noise ...
    YouTube ... "Thunderscreech"
     
  11. Jul 11, 2016 #10

    Baluncore

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    Why are air plane propellers small?
    Air plane propellers are not small, they are big compared with a boat propeller for a similar mass vehicle.
    Blades are airfoils. A propeller blade in low density air is travelling relatively fast compared to the speed of sound. If it was not thin it would cavitate.
     
  12. Jul 11, 2016 #11

    NTW

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    Cavitation takes place only in liquids, where a gas bubble may form and then collapse destructively. That never happens in air...
     
  13. Jul 11, 2016 #12

    Baluncore

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    OK, so replace my “cavitation” with “cavity formation”. Cavities can form in air once speeds approach supersonic.

    Cavitation is where a liquid cannot move fast enough to fill the space behind a moving propeller blade. A cavity forms that contains vaporised liquid.

    When air cannot move fast enough to fill the space behind a moving propeller blade, the airflow separates and a partial vacuum is formed. That depression may then contain a mist of condensed water vapour.

    Both situations can be resolved by reducing the angle of attack, or by reducing the thickness of the propeller blade.
     
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