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Stalls and V-Speeds

Asked by: 2840 views Aerodynamics

what is a power on stall? what is a power off stall? how do they occur? V-Speeds: what are they? for a cessna 172 model M VR? VX? VY? VA? VFE? VSO? VSI?

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1 Answers

  1. Wes Beard on Sep 17, 2013

    Hello J-Air23,

    An airplane is said to be in a stalled condition when the wings can no longer support the weight of the aircraft. It does not mean that the engine has stalled or quit running like it would in a car. The defining cause of the wings no longer being able to support the weight of the airplane is when the critical angle of attack is exceeded.

    Imagine driving in a car at highway speeds and you stick out your hand and pretend that it is an airplane. The fingers are closed and facing forward and the elbow is directly behind the hand. The airflow going over the hand is called the relative wind and the angle at which that relative wind strikes your hand is considered the angle of attack. There is a more proper definition for angle of attack but I’m afraid it will just muddle the water. If you were to pitch your hand up sufficiently enough you will find that the air is no longer going over the top of your hand but pushing your hand backwards. When that occurs you have exceeded the critical angle of attack and the hand is no longer producing any meaningful lift.

    A power-off stall occurs when the engine is at idle and the airplane is allowed to pitch up enough so that the airplane exceeds its critical angle of attack. On the otherside, a power-on stall occurs when the engine is producing power and the airplane is again allowed to pitch up sufficiently enough that the airplane will exceed its critical angle of attack.

    I want to be careful and not give you the idea that pitching up is the only way the airplane stalls. It is not. The airplane can exceed its critical angle of attack at anytime depending on the conditions and the flight path. Some factors include whether or not the airplane is turning, the airplane is cross-controlled and wind turbulence. The only constant is that the wings have exceeded their critical angle of attack.

    V-Speeds are defined speeds that allow the pilot to reasonably maximize the performance of the airplane.

    VR is the rotate speed.
    VX is the speed you use if you want to maximize altitude gain over a given distance.
    VY is the speed you use if you want to get to altitude the fastest.
    VA is maneuvering speed. Any speed below this number and the airplane will stall before breaking up.
    VFE is the Flaps Extended speed. It is the speed at which you can put flaps down with damaging them.
    VSO is a speed that the airplane will stall at in straight and level flight with full flaps and gear down.
    VS1 is a speed that the airplane will stall at in straight and level flight with flaps and gear in a specified condition.

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