What is Vmc and how is it effected?

Answer 1

Vmc is defined as the minimum controllable airspeed of a multi engine aircraft operating on a single engine.

There are 12 basic factors that affect Vmc.

David Carson Bought a L O W F A T C O W

1. Density Altitude

2. Critical Engine Inoperative

3. Bank - up to 5 degrees into operative engine

4. Landing gear position

5. Operative engine at max power

6. Weight at max

7. Forward CG

8. Aft CG

9. Trim

10 Cowl Flaps

11 Out of ground effect

12 Windmilling Prop

Density Altitude

If density is high, then Vmc is decreased, which is a good thing. So if an aircraft's published Vmc is 65kts, a high density altitude may actually cause the aircraft to lose directional control at an airspeed lower than 65kts and vice versa for a low density altitude.

Critical Engine Inoperative

This is pretty self explanatory, with the critical engine inop, the amount of airflow affecting the aircraft's control surfaces is dramatically reduced and thus Vmc is increase. i.e positive control is lost at a higher airspeed.

Bank (Up to 5 degrees)

Studies have shown that as much as 3-5 degrees of bank into the operative engine can significantly reduce Vmc giving the pilots a larger margin to play with.

Landing Gear Position

Of course with the landing gear extended..drag is increased but it also provides a stablizing effect and Vmc is decreased.

Operative Engine at Max Power

With the operative engine at max power...aircraft thrust is increased..and with and increase in thrust comes an increase in drag and so Vmc is increased.

Weight at Max

Simply put, a heavy aircraft is a stable aircraft and so the heavier the aircraft the better it is for Vmc and so Vmc is reduced.

Forward CG

A forward CG reduces Vmc.

Aft CG

An aft CG increases Vmc.

Trim

Not really a significant effect on Vmc but depending on its position it will still affect Vmc.

Cowl Flaps

With cowl flaps open..increrase in drag(not significant but still an increase in drag) and so Vmc is increased.

Out of Ground Effect

Again, not a significant impact of Vmc but still, as an aircraft accelerates out of ground effect the pilot needs a greater angle of attack and this increases Vmc.

Windmilling Prop

With the inoperative engine's prop windmilling drag is SIGNIFICANTLY increased and so is Vmc.