I wanted to straighten out where I have stood the entire time. Nobody has picked a bone with me in a while and that's probably not a bad thing.
I have always referenced in this discussion a multiengine airplane with 2 operating engines (Forget the single engined aerodynamics, that's a can of worms)
I have always said that torque does not magically disappear on a twin, in fact it doubles when compared to the comparable single (for instance, a Cherokee-160 and a PA-23-160 Apache)
The amount of torque applied to the airframe is doubled.
Now, I have said always and still stand, that the effects of torque are not pronounced and also not a factor when flying a twin. There are several reasons, but the big one is that with the torque creators located outboard of the fuselage and thus not aligned with any axis of the Center of Gravity they simply do not have the leverage required to roll the airplane. They still create their force, however they can't roll the airplane. (Not the case in a vaccuum, but we're not in outer space)
This is a different story in the Cessna 337 Skymaster or Adams A-500 with engine out. They are Centerline Thrust airplanes (Engines mounted on the centerline...on the longitudnal axis of the CG) and without the engines to counter one another they will act just as a single would...with engine torque as a factor because torque now has the leverage it needs to rotate the airplane about the long axis of the CG.
I'm quoting page 3-8 of the Pilots Handbook of Aeronautical Knowledge (FAA-H-8083-25) because its handy and was on the desk.
Whenever an airplane changes its flight attitude or position in flight, it rotates about one or more of three axes, which are imaginary lines that pass through the airpalne's center of gravity.
We all learned day 1 in our private pilot ground schools that everything the airplane does rotates around its center of gravity and the effects of torque and what it would cause the airplane to do are no exception. Newton was right, Equal and Opposite reaction. However, Newton wasn't an aeronautical engineer or pilot. His principal applies and is valid in this case, but because the airplane has no axis for the engine torque to have an effect...the airplane simply won't rotate.
We've gotten so far off track in talking about engine torque now we are talking about engine out aerodynamics in twins. There are world class experts out there who don't know everything there is to know about this subject and I'm tempted not to touch that with a ten and a half foot pole.
But I will...
So many things happen so quickly when you lose an engine. The most pronounced is a yaw toward the dead engine and a roll, which is caused by a lack of airflow over the wing of the failed engine (no power thus no slipstream). To reduce Vmc its vital to bank into the good engine which uses the airplanes weight as leverage against the yaw tendency (and it is a yaw tendency) of that good engine so it won't throw you out of control.
There are ways to determine Vmc, and they are a worst case scenario. With much of the emphasis being put on producing maximimum possible power at max weight, and rudder control.
The FAA Requires that the airplane have the least favorable CG. This is typically Aft, which provides the greatest arm between the CG and the downward moving propeller blade of either engine (more leverage, thus making it easier for that engine to Yaw the airplane) and also reduces the arm between the CG and rudder. This will reduce effectiveness of the rudder to the bare minimum. There is also a stipulation that no more than 150lbs of pressure may be applied to the rudder.
With all this emphasis on rudder and creating the worst case scenario in terms of yaw and controlling it...I think it's pretty safe to say that the FAA is concerned about yaw more than roll as a certification issue. This didn't just come out of thin air, lots of airplanes have crashed and people died giving us the rules, guidelines and standards we have today.
