Dear friend,
flying multi-engine is fun and rewarding.
to start, controllability and performance are two different and opposite things. what is good for one is bad for the other one.
Part 23 requires manufactureres to test Vmc under certain requirements.
Vmc promises only that if your speed is equal or above it, your airplane will fly straight. there is no guarantee of perfomance what so ever. i.e. Piper Seneca PA34 has a Vmc of 80mph. this promises that if you have an engine failure, while at max wt, good engine full power, bad critical engine windmilling, bank 5 toward good engine......with all this Vmc promises that you will fly straight. you might not be even able to maintain altitude, but you will crash going straight. no loss of directional control. that is the logic behind Vmc.

Now when wt increases Vmc decreases. In the Seneca Vmc is 80mph. this assumes banking not more than 5 toward good engine. when you bank toward the good engine you are using less rudder (rudder is the first line of defense to prevent yawing toward bad engine) by utilizing the horizontal component of lift. when acting at an angle, total lift is divided into components vertical and horizontal. the vertical one has to equal wt to maintain altitude. the heavier you are the more vertical lift you need, however, if your bank is still 5, then this will lead to an increase in horizontal lift as well, same bank angle but because more total lift is produced, then more vertical and horizontal lift you will have. this will lead to less rudder needed, which means you will have a lot of reserve rudder. now revisiting the simplified definition of Vmc. it is simply when the rudder stalls. when the airplane becomes slow enough that full rudder cannot produce enough lateral lift to prevent loss of directional control so airplane continous yawing. if you have full wt then horizontal lift is big enough for you to use partial rudder versus full. this means you need to slow down even more before you have to use full rudder and then slow down on notch more to loose directional control. so more weight means lower Vmc.
Wt is always a penality on performance because the heavier you are the less thrust available you have and climb and performance charactiristics will degrade. so you will fly straight if you hold Vmc or above. if you are unable to maintain altitude with Vyse you will crash but while having directional control. email me with any questions. mfalhout@hotmail.com