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3 Answers

Propeller pitch

Asked by: 4979 views Aerodynamics, Aircraft Systems, General Aviation, Student Pilot

hey everone

i am a bit confused about propeller pitch. propeller pitch is the forward distance which the propeller blades cover in one rotation.now in respect of a piston engine aircraft having constant speed HARTZEL propeller the manufacturer has defined different RPM for take off, cruising flight and landing. My question is why it is recommended to Take off with high Rpm and cruise at lower RPM?

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3 Answers

  1. Jim Foley on Sep 15, 2012

    Propeller pitch is the angle of attack which the airfoil (prop blade) meets the oncoming air; just like a wing. A constant speed/variable pitch prop can be though of similar to the gears on a car. Assuming the same RPM, a lower pitch will provide more power. On takeoff, you want maximum engine power (RPM), you set the prop to the lowest pitch. In cruise, you slow the engine down (reduce RPM) and increase blade pitch. The prop is taking a bigger “bite” out of the air, thus more speed. As I said, you can equate this to shifting to a higher gear in a car. Read http://www.free-online-private-pilot-ground-school.com/propeller-aerodynamics.html for more info.

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  2. Brian on Sep 15, 2012

    Brake horse power is directly proportional to engine RPM. The higher the RPM the greater your BHP. This explains the desire for high RPM on takeoff.

    In flight the propeller is just like your wing, an airfoil. Each airfoil has an L/D max, just like your wing. Recall this corresponds to best glide, or a point of maximum efficiency for the airfoil; occurring at a specific AOA.

    With a propeller, if pitch is kept constant, as forward speed is increased there is a decrease in the propellers AOA. By altering the propellers pitch we keep the propeller’s AOA at or closer to it’s L/D max angle. Thus improving it’s efficiency in both cruise and climb applications.

    I really dislike the ‘takes a bigger bite out of the air’ phrase because in reality the propeller AOA is not changing as this statement implies. Instead we alter the blade pitch in an attempt to keep the AOA relatively constant as forward speed is increased; that is, at L/D max. Think of how the two relative winds, forward speed and propeller speed, interact when trying to grasp this concept.

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  3. H. Odell on Sep 16, 2012

    When transitioning to cruise from low pitch climb position and full power, you reduce the rpm to cruise speed and then increase the pitch to a predetermined manifold pressure as per the manual. At 10 to 14 thousand feet, you will not be able to have too much manifold pressure as the engine, having been leaned out carburator wise to the best mixture willl not be producing as much horsepower as it does at low altitudes.
    These propellers are called constanst speed as the RPM might otherwise increase in a decent, the propeller increases pitch thus keeping the RPM the same.
    Always reduce pitch before reducing rpm to keep from exceeding max. manifold press.
    I have often thought that this would be good for an auto automatic transmission. ie. control the manifold pressure for max power.
    BTW most engine manuf. have a min. manifold pressure, maybe 15 lbs.

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