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3 Answers

RPM and power

Asked by: 11624 views Aircraft Systems

hello everyone

i am a student pilot. i fly a single prop piston engine aircraft. anyone please clear following doubts in my minds....

  in constant speed propeller aircraft if power is increased i mean boost is increased how the speed of the aircraft or its lift is increased, despite the RPM of the engine is same? suppose i was flying at power 20 inch with 2500 RPM at speed 90 knots. i increased the boost from 20 inch to 25 inches and rpm is same(2500) i get speed almost 105. RPM is still the same. it means prop is rotating at the same speed. how do i get more speed or lift?

  i will really appreciate your kind guidance.

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3 Answers

  1. Nathan Parker on May 17, 2012

    Please note that increasing power doesn’t mean you get “more lift”.  Some people use that terminology to mean “climbing”, but a climbing airplane doesn’t have any more lift than an airplane in level flight.  (In fact, it has less.)
    But, to answer your question, when you increase the throttle in an airplane with a constant speed propeller, the pitch of the propeller blades increases so that the RPM stays the same.  The propeller blade rotates at the same speed, but its AoA is larger so it produces more thrust.

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  2. Lucas on May 18, 2012

    Also think of a car: in first gear at 2000 RPM you might be going at 15 MPH, but if you switch to second gear at 2000 RPM you would be going at 30 MPH. Now it doesn’t really work that way, but the concept is similar. Pretend that the propeller lever is your cars accelerator and your throttle the gear shift and there you go: increase the gear and the airplane flies faster without affecting RPM.
    So like Nathan said increasing the throttle increases the angle of the blade of the propeller (putting it, if you want, into a higher gear), increasing thrust and therefore speed.

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  3. Wes Beard on May 18, 2012

    The way I think about it is like this.  If the propeller were to go through a solid gel like object.  The path it would make would be different depending on the blade angle.  If the blade angle is fairly flat compared to the gel object, it is going to take a lot of rotations for the propeller to work it’s way through the gel.  As the blade angle is increased, the number of revolutions required decreases but the amount of energy required to twist the blade will increase.
    It is the exact same thing,  when the propeller moves through the air, the energy required to rotate it through the air is directly related to the blade angle.  This explains why a propeller will remain at a constant RPM even though power is increased in the engine.  The extra energy supplied is consumed by the extra rotational force needed (due to a higher blade angle) to keep the propeller at that RPM.  Since, the propeller takes fewer rotations (due to a higher blade angle) to go a given distance, the airplane will have an increase in speed as well
    Obviously, there are speeder springs and flyweights that are designed to move the blade angle to keep the RPM constant.  These components are the supporting cast for the end product of a constant RPM and not the direct reason why the RPM stays constant.  The amount of energy supplied to the propeller plus the propeller blade angle will equal the rotational rate.

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