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Horsepower to Thrust Conversion?

Discussion in 'CFI Corner' started by meritflyer, Apr 24, 2007.

  1. meritflyer

    meritflyer Well-Known Member

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    Does anyone know how to convert horsepower to thrust and visa versa?

    For instance, the Pilatus PC 12 has 1200 HP. Is there any way to convert that to lbs. of thrust? The CRJ 200 has 8700 lbs of thrust. Can you convert that to HP?
  2. OldTownPilot

    OldTownPilot Well-Known Member

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    Hp is a unit of work, Thrust is a force.

    There is no straight conversion, between the two.
  3. meritflyer

    meritflyer Well-Known Member

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    Gimmie a ballpark here..
  4. tgrayson

    tgrayson New Member

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    Power = Thrust * Velocity.

    When velocity = 0, power is zero by definition.

    Now, if you want to calculate power using units of horsepower, then you need the proper conversion factors.

    Power (in hp) = Thrust (in lbs) * Velocity (in knots) * 6076/(3600*550)

    So, the CRJ, at say 300 knots, developes

    8700 * 300 * 6076/(3600 * 550) = 8009 HP.
  5. E6BAV8R

    E6BAV8R New Member

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    For a turboprop you can use:

    Fn = SHPx375xProp Efficiency / Speed in MPH

    Fn = Total Net Thrust in pounds.

    For an estimate, roughly 80% on average for prop. efficiency. Prop Efficiency is also put into the equation in hundredths; 100% efficiency = 1.0; 80% efficiency = .80.

    Example with a turboprop rated at 1200 SHP with 80% efficiency at 350 MPH:

    FN = 1200x375x.80 / 350

    =1028.57 lb. of thrust
  6. amorris311

    amorris311 Well-Known Member

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    justin is correct! this is physics 101. horsepower was first developed to unify a measure of work output. thrust, as justin stated, is a force which is not work. im sure there is something out there that can give you somewhat of a ballpark but that would mean me calling my mom to have her mail me out my old ap physics book.
















































    NOT GOING TO HAPPEN!!!!!!!!!!!! :):):):):):):):)
  7. tgrayson

    tgrayson New Member

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    No, force is not work, but force * distance is. And airplanes move a distance and therefore do work.

    See if your Mom can Fedex that physics book.:)
  8. fish314

    fish314 Well-Known Member

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    One of the aircraft design books I have (Introduction to Aeronautics: A Design Perspective, by Brandt, Stiles, Bertin and Whitford) uses almost exactly this same formula. The only difference between the two seems to be that this one also has an adjustment for altitude.

    It uses:

    TA=SHP*density ratio*Prop efficiency/Velocity

    Where the parts are:

    TA: Thrust available at the given condition (IN POUNDS).

    SHP: Shaft Horsepower at SEA LEVEL given in ft-lbs/sec. If you have SHP at sea level in horsepower, multiply by 550 to get it into ft lbs/sec.

    density ratio: Gotten from a standard atmosphere table. Or density at the given conditions divided by density of sea level standard day

    Prop efficiency: .8 or .9 is a pretty good guess for most props, but you might be able to find a better number if you know exactly what type of prop you have. May be in your POH or performance manual, or you may be able to find it online.

    V: True Airspeed of the given condition in ft/sec. If you have your true airspeed in MPH you need to multiply by 1.4666 to get ft/sec. If you have it in knots, then you need to multiply by 1.687777 to get ft/sec.

    So for the same conditions at sea level standard day (density ratio =1), I come up with

    V=350 MPH= (350*1.4666)ft/sec= 513.33333 ft/sec
    Prop efficiency= .8
    SHP= 1200 Horsepower= (1200*550) ft lbs/sec=66000 ft lbs/sec

    so TA=(66000*0.8)/(513.33333)=1028.57 lbs. Or the same answer as E6BAV8R found.

    But if you wanted to know the thrust available at 10000 ft, standard day, the density ratio would be .73875, so the thrust available would become:

    TA=(66000*.8*.73875)/513.3333=759.86 lbs.

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