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Thrust versus power

  1. May 28, 2013 #1

    rollingstein

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    Why is it that aircraft engines are always (mostly?) rated in thrust whereas most other heavy equipment I can think of (e.g. ships, earthmovers, etc. ) is mostly spoken about in horsepower.

    First, am I right in my observation. If so, is this merely convention? Or does it make more sense to speak of lb of thrust for a plane and hP for a ship?

    I've heard of bollard pull for tugs or tractive force for locomotives but never the converse i.e. no one mentions the hP for a jet engine.
     
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  3. May 28, 2013 #2

    SteamKing

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    Thrust and horsepower can be converted to one another. IMO, the use governs which number to use.

    Bollard pull is a handy number to have because it allows for a direct comparison of the pulling capability of several different tugs without trying to convert HP. The layout of the tug's propulsion equipment (open propeller v. propeller in a nozzle) makes a difference in how much pull 1 HP is converted to. (For two tugs with the same HP, the boat with nozzles will generally produce greater bollard pull than the boat with an open propeller).

    A similar situation occurs with aircraft. Reciprocating engines were customarily rated by HP. A propeller is driven by the reciprocating engine in order to convert the torque of the engine into thrust. The thrust generated depended on a number of factors: The size and number of the propeller blades, the shape of the blade, pitch angle, etc., so much so that it is impractical to quote a thrust number, especially since propeller thrust also depends on speed of advance of the propeller.

    A jet engine is different, because it is designed to produce thrust without spinning a propeller. If a jet engine is bolted down on a test stand and put to max. throttle, it generates a lot of thrust but no work, because the jet is not moving. Once the jet starts to move, then thrust is being converted to work. If one wishes to compare the performance of several different jet engines, then thrust output is a handy measure.
     
  4. May 28, 2013 #3

    Danger

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    It's extremely complicated, and I don't understand it myself despite being a former pilot and having studied jets enough to design on paper a fighter craft for a novel. Basically, the horsepower-to-thrust ratio changes according to both altitude and speed. A jet bolted to a test stand, running balls-out, produces no horsepower. That same engine, doing 3 Mach at 50,000 ft altitude is probably close to 1,000,000 horsepower.

    edit: I see that SteamKing once again beat me to the punch. :redface:
     
  5. May 28, 2013 #4

    rollingstein

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    Interesting. Why does it depend on altitude? Isn't hp = thrust x speed? Assuming level flight.
     
  6. Jun 2, 2013 #5
    Because the properties of air vary with altitude.
     
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