Factors of Vmc

For an airplane with non-supercharged engines, V_mc will Decrease as Density Altitude Increases. So directional control can be maintained at a lower airspeed than it can at sea level. As power decreases with altitude, the thrust moment of the operating engine lessens, thereby reducing the need for the rudder and it’s countering yaw. So at Sea Level the V_mc is the highest. The higher the density altitude, the lower the V_mc.

V_mc is not affected by weight in straight and level flight, only by the weight of the airplane in a bank. When it is banked, a component of the aircraft weight acts along with the horizontal component of lift to create a more effective sideslip towards the operating engine. So for a given bank, the heavier the airplane, the lower the aircraft’s V_mc. The lighter the airplane, the higher the aircraft’s V_mc.

V_mc is greatest when the CG is at the most aft position. An aft CG shortens the arm to the center of the rudder’s horizontal lift, which means a higher airspeed would be required to counteract the engine-out yaw. So the aft CG has the highest V_mc and a forward CG would decrease V_mc.

The change in CG when the landing gear is put down may make the rudder more effective due to a longer moment arm, which would result in a slighter lower V_mc. The extended gear may also have a tendency to align with the oncoming relative wind, which can be directionally stabilizing. So the V_mc for this factor is at its highest when the gear is up, and decreases when the gear is extended.

Extended flaps will increase both drag and lift. The increased drag from the extended flap behind the operating engine may tend to oppose the yawing motion of that engine, requiring less rudder to counteract that yaw. So V_mc will be lower with flaps extended, and higher with flaps retracted (the take-off position for this aircraft).

Cowl flaps in the open position will increase drag on the aircraft, and may also provide a very slight weathervaning/stabilizing effect on the yawing motion. Cowl flaps closed will increase V_mc, cowl flaps open will decrease V_mc.

A windmilling propeller will generate a significant amount of un-balanced drag. This will result in less directional control and a higher V_mc, and is one of the most significant of the factors.

An aircraft in ground effect is “cushioned”, with less drag and greater lift. So in ground effect, V_mc decreases. As the aircraft lifts out of ground effect, it loses that lift and increases its drag, thereby increasing the V_mc.

During engine-out flight, keeping the ball centered and the wings level will actually cause a side-slip situation because of the one failed engine and the resulting drag and loss of lift. A sideslip would make stall characteristics worse, would decrease climb and acceleration capability, and would increase V_mc. Turning up to 5 degrees towards the good (operating) engine will prevent the airplane from being in a sideslip condition, will improve climb capabilities, improve stall capabilities, improve performance and will decrease V_mc.