I can see your thinking, but let me explain it a bit different, if I may.
With the landing gear extended Vmca will be lowered. Drag rises as the square of airspeed; the increase in airflow from the operating propeller will increase drag on the landing gear that is hanging in the propeller slipstream. This will be greater then the drag from the other landing gear which is in the free stream velocity. Hence, the drag vector will be displaced more toward the operating engine then with your gear up. This drag vector positioned on the same side as the thrust vector reduces the yawing moment about the CG. Additionally, lowering the gear will increase stability and lower the CG, in some cases increasing the moment arm from the rudder.
Vmca is determined with maximum available takeoff power or thrust, it is a constant. There is no requirement in part 25.149 that at Vmca one maintains level flight, (performance is covered in other places, such as 25.121) hence there is no need or even ability to add extra power. With the landing gear out you reduce your Vmca and you reduce your performance. Two different aminals.
One can think about Vmca as controllability only, whereas Vyse/ Vxse addresses performance
Last edited by '%MAC'; 31st May 2002 at 17:04.