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I am studying in Vmc part, but I have hard time to understand that trim for take off is affected to Vmc. How and why does it make Vmc speed increase?

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  • $\begingroup$ What aircraft? This isn't always the case. $\endgroup$
    – Ralph J
    Commented Mar 3, 2019 at 3:20

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The trim setting doesn't increase or decrease Vmc speed itself. I suspect your question is actually "what is the take-off trim setting's relationship to Vmc".

The term "trim", more or less referring to a state of aerodynamic equilibrium in the balance of pitch related forces, and such equilibrium being achieved at a given speed for a given weight/power/thrust configuration, you trim to a speed. Trim speed is the speed the airplane will seek without physical intervention by the pilot or autopilot, for a given all up weight, CofG location, and power setting (I've always called the trim wheel a "hands free speed setting dial").

Trim settings for takeoff in multi-engine airplanes can normally be expected to be something at or close to single engine climb speed (blue line) in a piston twin, or at or close to V2, or single engine safety speed, in a transport airplane.

The idea is to have the airplane configured so that it will naturally seek a desired speed, or reasonably close to it, without control input, and reduce the workload on the pilot during periods of excitement. To the extent that the trim setting is related to Vmc, it's to achieve this speed margin as determined by the manufacturer.

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Elevator Trim does not change the Minimum Control Speed(Vmc). It is variables such as engine power, weight and balance, and center of gravity that can cause changes to Vmc.

Those variables can also mean changes to the correct elevator trim settings for takeoff.

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