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Vmc

Asked by: 1704 views Aerodynamics, FAA Regulations

Regarding Vmc, why was a bank angle of 5 degrees determined to be used during flight tests certification to determine Vmc  of which is found in CFR Part 23?  For example, why was 10 degrees or 15 degrees not utilized? My understanding is if the bank angle is less than 5 degrees or greater than 5 degrees that Vmc increases.My thought is that rudder travel and the horizontal lift vector are factors to offset asymmetrical thrust  and sideslip.  Aerodynamically speaking, can anyone elaborate? Thanks.      

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4 Answers

  1. Best Answer


    John D Collins on Nov 15, 2015

    The 5 degrees is a compromise between performance and control. The greater the bank angle the more control you have, but it comes with a performance penalty. It is a specification and not a magic number. VMC is demonstrated by the manufacturer by using a 5 degree bank. One could achieve better VMC values by increasing the bank angle, but by limiting the bank angle to 5 degrees, keeps the manufacturer honest.

    We used to do real VMC demonstrations up to the point of control loss. Recovery was to reduce power on the good engine and lower the nose to gain flying speed. It proved to be too dangerous on many aircraft types as many twins are not capable of recovery if allowed to go too far. It is now frowned on and the effect is demonstrated by the instructor blocking the full use of the rudder.

    In real multiengine operations, the 5 degrees bank does not always provide optimum performance for climb under one engine out conditions. For best performance, I teach using zero side slip. This is different for each aircraft and is often demonstrated by taping an 18 inch section of yarn onto the center of the nose. In flight, with one engine shut down, you increase the bank angle until the yarn is aligned with the axis of the aircraft longitudinal centerline and keep the ball about about a half a ball width outside of center. On a Baron, the best angle to achieve zero side slip is closer to 2 to 3 degrees.

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  2. Dan Chitty on Nov 15, 2015

    Thanks John.

    In the first paragraph you mention the “greater the bank angle the more control you have, but it comes with a performance penalty.”

    To make sure I understand, what performance penalty is experienced?

    Also in first paragraph you mention, “one could achieve better VMC values by increasing the bank angle”.

    In theory and generally speaking, Vmc would decrease with increased bank angle. Is this correct?

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  3. John D Collins on Nov 15, 2015

    Dan,

    I really think you should be able to answer your many of your own questions with a little more diligent research. By performance I mean the ability to climb, maintain altitude, or lose the least altitude. The steeper the bank, the more lift that is diverted from the vertical, the less that is available for the above performance.

    VMC is lower with a greater bank angle because the horizontal component of lift is greater to counteract the turning tendency in the opposite direction because of the asymmetrical thrust.

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  4. Dan Chitty on Nov 15, 2015

    Thank you for the feedback. Much appreciated.

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