Originally Posted by
OKDECS
I'd like to see if anyone can give me some clarification on some items that impact Vmc...1) When flaps are extended, Vmc drops. This is usually explained in some simplistic terms like "the flaps have a stabilizing effect" which to me is a little too simplistic. My assumption is that there are two things going on. First, the increased lift from the flaps at a given airspeed is contributing to the horizontal component of lift, thus freeing up some more rudder authority. Second, the accelerated slipstream from the operating engine is creating more asymmetric drag and thus countering the yaw towards the dead engine. One problem I see with the latter part is that the accelerated airflow also increases asymmetric lift on the operating engine side, which is increasing the tendency to bank towards the dead engine...so can anyone clear this up for me?
Flaps serve to degrade the slipstream
quality behind the operating engine, thus and reducing its efficiency and thrust. This in turn allows a lower VMC number. If the air behind an engine is clean, it generates more thrust. Both "keel effect" and slipstream
quality are factors in VMC performance. Yes there is some additional lift on the operating engine side from flap deployment, but obviously the other factors outweigh and when you think about it that additional, asymmetrical lift on the operating engine side is also there with the flaps up. In addition, higher slipstream speeds means more induced drag on the flap behind the operating engine, so there are a bunch of things to consider.
2) When the gear is extended, Vmc drops (usually). There are two explanations - one is pretty straight forward (CG). I understand that if the CG is farther forward with the gear down (mostly depends on which direction the nosegear extends) there's a longer arm from the CG to the rudder, therefore increasing effectiveness. However, the whole "keel effect" explanation confuses me...again because of the "stabilizing effect" simplification. I again default back to the accelerated slipstream in explaining this; if the mains are behind the engine, then the accelerated airflow from the operating engine increases drag on that side, which will aid in directional control. Any insight on whether this is right/wrong?
All that is correct but you are missing the reduction of thrust explained above. The gear hangs down in the slipstream and this reduces the
quality of the air there in terms of using it to generate thrust. It is more turbulent. In turn, less thrust means a lower VMC. I will try and come up with a common sense analogy for this and for the keel effect, but I have to run out for a bit.