Originally Posted by rickair7777
As far as Vmca certification, higher weight actually improves (lowers) Vmca. This is because in proper (banked) SE flight, all of the total lift generated by the wings is NOT directed vertically...some of it goes horizontally in the direction of bank (towards the good engine). This horizontal lift component helps to offset some of the yaw tendency caused by the good engine...therefore requiring less available rudder, since some of the rudder's work is already done by the horizontal lift component. Since the horizontal and vertical lift components are both part of the total lift, and a heavier airplane requires more total lift to generate the required vertical lift, the horizontal lift component will also be greater for a more haevily loaded airplane.
Everything rickair wrote is correct. Since the manufacturers are always trying to get the lowest possible Vmc, FARs need to specify a weight and max bank angle to use.
HOWEVER, from an operational point of view, a better way to look at it is that a heavier aircraft will require LESS bank angle to achieve zero sideslip. Since you should aways shoot for zero sideslip when single engine, just consider that the required bank angle changes with weight, but Vmc does not.
The Vmc - Weight - bank angle relationship involves 3 variables. Therefore it just depends on which one you want to keep constant to analyze the relationship of the other two. For a given bank angle, a higher weight will lower your Vmc. However, we should not keep the same bank angle at different weights. Instead, we should REDUCE the bank angle at higher weight, keeping Vmc unchanged. This is the only way to keep flying in zero sideslip at different weights.
Heavy airplane - zero sideslip - 2 deg bank into live engine - Vmc=60kt
Light airplane - zero sideslip - 3 deg bank into live engine - Vmc=60kt
(obviously I made these numbers up)
I hope I didn't confuse you. I just thought I'd add my $.02 since I hear too many students saying that a higher weight is "good" for Vmc. Thats not exactly correct. Vmc should not change if flying in zero sideslip (constant heading, 2-3deg of bank into the live engine) at various weights. However, there would be a difference in Vmc only when executing a turn at a given bank angle. Note i said "turn", not "bank", because you are always banked when flying at a constant heading in zero sideslip.