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2 Answers

Factors of VMC – Bank

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General Aviation

Can sompine please explain why if you have greater than 0 degree side slip with an engine inoperative you will have a lower VMC speed and more control. Also why if you have less than zero degree sideslip your VMC speed will increase.  The way I am thinking of this in my head means that I have it reversed.


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2 Answers

  1. Nathan Parker on May 10, 2012

    A sideslip towards the dead engine reduces the effective AoA on the vertical stabilizer, reducing the pilot’s ability to maintain directional control.  A sideslip towards the good engine increases the effective AoA on the vertical stabilizer, increasing the pilot’s ability to maintain directional control.  With enough bank towards the good engine, you can maintain control with no rudder input at all.
    You can visualize it this way.  On a piece of paper draw the following:
    1) ME aircraft with left engine out.
    2) Rudder deflected to the right.
    3) Virtual chordline from the trailing edge of the rudder to the leading edge of the vertical stabilizer.
    4) Measure the AoA between a straight ahead relative wind and the virtual chordline
    5) Now measure the AoA between the relative wind coming from the left in a left sideslip and the virtual chordline.
    You will see that the AoA in the left sideslip is reduced compared with the relative wind coming from straight ahead of the airplane.  I think you can extrapolate to the right sideslip.

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  2. Bob Watson on May 14, 2012

    Another way to think of it is that Vmc is the speed at which your rudder can’t keep the plane flying straight. If you don’t have any bank, the rudder will be doing all the work of keeping the plane straight, so it will need a certain amount of airflow to make that happen. If you bank into the operating engine, that takes some of the work off the rudder allowing it to keep the plane straight with less airflow (i.e. at a lower speed).
    This is basically what Nathan said, but without thinking about it in terms of AoA (directly).

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