Originally Posted by
mattpilot
Vmca is lower with higher weight because (1) heavier aircraft are more stable, higher momentum resisting yaw/rolling,
Not relevant. VMC is a static manoeuvre, mainly, and VMCA is almost always determined through a static balance of forces and moments. Inertial effects are therefore usually irrelevant.
and (2) heavier aircraft produce more lift on the wings, which means when you bank into the 'good' engine you have a greater component of lift acting against the yawing/rolling force, which reduces the requirement of having to use rudder.
This is closer to the truth, combined with HN39s point.
To summarize in one place:
VMCA must be demonstrated/determined within a constraint of not using more than 5 degrees of bank. (It invariable turns out that banking into the live engine is the direction which is most favourable).
When banked, there is a sideways component of the aircraft weight, when resolved into the (banked) aircraft axes. In order to balance this weight component, an aerodynamic sideforce is required. This means that there must be some sideslip on the aircraft. The heavier the aircraft, the more the component of weight in absolute terms, the larger the sideforce and, all other things being equal, the larger the sideslip.
Sideslip generates, in addition to sideforce, both rolling and yawing moments. By banking into the live engine, the yawing moment generated is invariably helpful, in that it assists the rudder in generating the yawing moment required to counter the engine-out moment.
So at a given speed, a heavier aircraft gets more rudder "help" by banking, and so needs less rudder to trim. Alternatively, the heavier aircraft can go to a slower speed before running out of rudder.
This means that a heavier aircraft can maintain a heading within the constraints of the VMC requirements to a lower speed. hence heavier aircraft "have a lower VMCA".