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6 Answers

Why power for turboprop engine and thrust for jet engine ?

Asked by: 9177 views Aircraft Systems

can  any one please explain me why, when performance of turboprop engine is referred, it is always in terms of power where as engine performance of turbojet is referred to in terms of thrust ? why is the difference ?

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6 Answers

  1. Nathan Parker on May 05, 2011

    It really depends on what aspect of performance is being measured.  Jets produce power and thrust, and so do props.  Rate of Climb is always determined by the quantity of excess power, regardless of whether you’re talking about jets or props.  Angle of climb is always determined by excess thrust, regardless of whether you’re talking about jets or props.
    The main performance difference between the two types of propulsion is how power and thrust vary with airspeed.  Jets produce fairly constant thrust throughout the operating range, but props produce high thrust at low airspeeds, but thrust drops off rapidly with increasing airspeed.  Jets experience increasing power with airspeed, but props have a fairly constant amount of power with airspeed, at least with a constant speed prop.
    Regarding power vs thrust, power being applied to the airframe takes into account the velocity of the airplane, whereas the amount of thrust being applied does not.  If you have two identical airplanes climbing at the same angle, the faster airplane is generating more power, even though the thrust is the same, because Power = Thrust X Velocity.

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  2. Kenneth Wells on May 05, 2011

    In a straight jet engine, fuel is sprayed into the chambers, combusted and expelled out in the back. This means that (for all practical purposes) all fuel is used for thrust production (thrust producers)In a prop, after the fuel is ignited, a lot of the heat energy is wasted throught gears/transmissions etc before it is transferred to the propeller disc (which isn’t realy that efficient either) which in turn produces thrust, hence it is more convenient to talk about power (Watt)!So, it is convenient to relate fuel flow to thrust output in a jet, and power output (internally) to fuel flow in a prop.It is difficult to know exactly how much thrust a propeller generates, but for a jet engine it is quite simple.

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  3. Steve Pomroy on May 10, 2011

    The distinction between thrust and power often causes confusion.  As Nathan said, both props and jets produce both thrust and power.  And as Kenneth said, fuel flow is related to thrust in jets, while it more closely related to power in prop aircraft.
    The thrust v. power conundrum is discussed a little more here with specific regard to climb performance:  http://www.askacfi.com/3551/excess-thurst-versus-excess-power.htm.
    Steve Pomroy

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  4. Scott Stahl on Aug 28, 2011

    Another reason for measuring turboprops in terms of HP, is that HP is effectively a function of torque and RPM (HP=Torque x RPM/5252).  Since a turboprop is applying a torque force to a shaft, and spinning it at a certain RPM, it is easy to measure the HP being put into the prop.  It is also a little more consistent with other prop driven airplanes.

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  5. Mick on Jan 28, 2012

    If I have a P51 with 2000hp engine and top speed is 400mph, and I fitted a jet engine to the same airframe how much thrust would I need to get 400mph.

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  6. Keith Williams on Mar 17, 2013

    1 HP = 33000 ft lbf / min

    2000 HP = 2000 x 33000 = 66 000 000 ft lbf / min

    1 MPH = 1760 yds hour so 400 MPH = 400 x 1760 = 704 000 yds hour

    Multiplying this by 3 ft/yd and dividing by 60 min/hour gives

    400 MPH = 704 000 x 3 / 60 = 35 200 ft / min

    Thrust Power TP = Thrust x TAS

    Rearranging this gives

    Thrust = TP / TAS

    Inserting the above figures gives the following

    Thrust = ( 66 000 000 ft lbf / min ) / ( 35 200 ft / min ) = 1875 lbf.

    So the thrust required = 1875 lbf.

    The above calculations assume a prop efficiency of 100%.

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