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Formula for sink rate for aircraft landing?

  1. Aug 30, 2012 #1


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    my first post here.

    I'm an avid flight sim enthusiast. I wanted to ask what is the formula for calculation of sink rate for aircrafts?

    I know 4 forces act on the airframe:


    sink rate is the amount of distance/time the aircraft sinks from final position to reach touchdown on the runway threshold. usually measured in feet/second.

    Another thing I wanted, is how drag is represented. I mean what is drag? how to calculate drag on an aircraft grossing 31,000 lbs. what factors must be taken into account?

    thanks in advance.
  2. jcsd
  3. Aug 30, 2012 #2


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    Sink/climb rate is determined by aircraft weight, thrust, air speed, and glide ratio at current air speed. As long as angle of descent/climb is relatively small, it can be estimated as speed*(thrust-weight/glide)/weight. Positive value for climb, negative for descent.

    You can estimate drag on the aircraft by dividing weight of the aircraft by glide ratio.
  4. Aug 30, 2012 #3


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    I had the impression the equations below applied, although maybe they turn out to be the same as what K2 posted.

    Assume aircraft is at constant velocity, and no wind (else use air as frame of reference).

    let θ = angle of aircraft path versus horizontal (zero = horizontal, + is climb, - is sink)

    climb or sink glide ratio = tan(θ) (+ for climb, - for sink)

    climb or sink rate = speed x sin(θ)

    drag = thrust - weight x sin(θ)
  5. Aug 30, 2012 #4
    The "sink rate" in the last stages of a flare, which is what the latter part of the landing process is called, is mostly a process of drag vs kinetic energy.

    The process is to increase the drag by raising the nose of the aircraft, therefore increasing the angle-of-attack of the wing. The variables involved in the flare would be the lift/drag characteristics of the wing,including of course the flaps, the drag of the fuselage, the "entry" speed (how much kinetic energy) and the amount of power you use to modify the level of that kinetic energy.

    Gravity is a constant unless you get heavy-handed, which does happen. Chuckle.

    As for the formulas, look into aerodynamics online. You might want to take some aspirin first.
    Last edited: Aug 30, 2012
  6. Aug 30, 2012 #5
    No - the sink rate during flare is not significantly affected by drag.

    What raising the nose does is increase the angle of attack, which incidentally does increase drag - but the main effect is to increase lift. Since the lift becomes larger than weight - it used to be equal to weight during approach - the plane starts to accelerate upwards, and the sink rate therefore starts to decrease.
  7. Sep 1, 2012 #6


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    thanks for all suggestions.
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