Here we'll break down VMC - what it is, how it works, and what can affect the minimum controllable airspeed.
Vmc is the speed at which the rudder no longer has the authority to overcome the yaw caused by the critical engine being inoperative, under specific criteria mandated by the FAA.
The lower Vmc is, the safer the aircraft is. It makes sense that the slower an aircraft can go while still maintaining control with an engine failed, the better.
Vmc strictly deals with maintaining directional control, irrespective of climb performance.
Say, for example, an operating engine at max thrust produces 1,000 lbs of yaw (T x X) toward the dead engine. This force will remain consistent regardless of airspeed. Now, assume that at 150 kts the rudder can produce 2,000 lbs of force (R x Y). As airspeed is reduced, so is the rudder’s force. At 100 kts, this rudder can generate 1,400 lbs of force, more than enough to control the 1,000 lbs from the failed engine. But, as airspeed continues to decrease, the rudder will reach a point, say 70 knots, at which it can produce exactly 1,000 lbs (the same as the yaw from the engine). This is the minimum controllable airspeed. As soon as the aircraft decelerates below 70 kts the rudder’s force drops below 1,000 lbs. At this point, the rudder is unable to overcome the yaw caused by the engine being inoperative and the aircraft begins an uncontrollable yaw toward the dead engine. If the airspeed is increased, or thrust is reduced on the operating engine, the rudder can regain control.
The FAA sets forth criteria for aircraft manufacturers to follow in order to establish Vmc for an aircraft. The criteria is as follows:
MAXIMUM TAKEOFF POWER - BAD for Vmc
The more power on the operating engine, the greater the force pulling toward the dead engine. The greater the force, the earlier the rudder will lose control. The minimum controllable airspeed will be higher with greater power.
CRITICAL ENGINE INOPERATIVE - BAD for Vmc
The critical engine is the engine that has the most adverse affect on control of the plane. By failing this engine, the rudder has more force to overcome than if the R-engine was failed, therefore Vmc will be higher.
INOPERATIVE ENGINE WINDMILLING - BAD for Vmc
A windmilling prop creates more drag than a feathered prop. Increased drag on the inoperative engine will create a stronger yaw toward the dead engine. Therefore, the rudder has to overcome more force, raising Vmc.
SEA LEVEL CONDITIONS - BAD for Vmc
At sea level the dense air allows the operating engine and prop to produce maximum thrust. Since there is more thrust, there is a greater force toward the dead engine for the rudder to overcome, therefore Vmc is higher.
MOST UNFAVORABLE LEGAL WEIGHT (LIGHTEST WEIGHT) - BAD for Vmc
Vmc increases as weight is reduced so the lightest legal weight is most unfavorable. The lightest weight provides the aircraft the least momentum. The heavier the aircraft, the more likely its inertia will carry it forward and help prevent the yaw and roll associated with a failed engine.
MOST UNFAVORABLE LEGAL CENTER OF GRAVITY (AFT CG) - BAD for VMC
Vmc increases as the C of G is moved aft. The further aft the C of G, the shorter the rudder’s arm is. The shorter the arm, the less effective the rudder. Vmc will be higher since the rudder produces less force at any speed than if the C of G was forward.
OUT OF GROUND EFFECT - BAD for Vmc
Vmc decreases in ground effect. As the aircraft yaws and rolls toward the dead engine the dead engine’s wing would dip further into ground effect, reducing its drag as it became more efficient, thus reducing the yaw toward the dead engine.
GEAR RETRACTED - BAD for Vmc
When the gear is down it acts as a keel (like on a boat) which aids in directional stability and decreases Vmc. With the gear up the keel effect is removed and it cannot help keep the aircraft straight.
COWL FLAPS OPEN - GOOD for Vmc
With the cowl flaps open the operating engine’s prop will push air into the cowl flaps resulting in increased drag. Increased drag on the operating engine decreases Vmc since it assists in counteracting the yaw toward the dead engine.
5 DEGREES OF BANK INTO THE OPERATING ENGINE - GOOD for Vmc
The horizontal component of lift generated by bank assists the rudder in counteracting the yaw from the inoperative engine. Vmc is reduced considerably with bank angle so the FAA limits the bank during testing to 5 degrees.
FLAPS IN THE TAKEOFF POSITION - Could go either way
Most twins takeoff without flaps, therefore there will be no effect. But, with many different flap sizes, types, and settings, having the flaps down could help or hurt Vmc. Having the flaps down could produce more drag on the operating engine
(reducing the yaw), it could also create more lift on the operating engine’s wing (increasing roll toward the dead engine).