Originally Posted by PBL
Just in case there is anyone here interested in analysing the accident - that is, what actually happened to the airplane, or might have happened...
My last comment, then I will shut up:
A little physics 101:
Spinning any object requires torque (that is, rotary force). Producing torque on any object involves producing an equal and opposite torque on the producer. The effect of these forces will be proportional to the size and mass of the object and the producer. In case of a prop aircraft engine, the force required to spin the propeller will produce a twisting force on the engine itself around the axis of the engine, opposing the direction of the propeller spin. The torque is low when the propeller is at a constant rpm, as only the air resistance needs to be countered. It will shoot up exponentially when accelerating the prop, and will be at its maximum when applying full power from idle and the engine spools up at maximum fuel burn.
As the engine is bolted to the airframe, the negative torque on the engine will be transferred to the aircraft to the centre of mass (which is very close to the bisection point of the roll and pitch axes, where the fuselage and the wing meet). A clockwise spinning prop (viewed from front) will produce an anticlockwise twist on the engine and airframe. This translates to a starbord engine producing an upward lift on the centre of mass, while a similarly placed port engine will produce an equal downward force on the centre of mass. (Iolar's linked photo clearly shows both props on a Metro spinning clockwise). This is torque balance, engine acceleration will have no effect on the aircraft as the two equal and opposing forces balance out.
In an engine out scenario (or single engine aircraft), the torque imbalance needs to be countered. On fixed wing aircraft this is the job of the ailerons, on helicopters this is done by the tail rotor. If for any reason the torque remains imbalanced, the aircraft will rotate around the axis of the torque, the engine.
In present accident, the roll to the right is perfectly consistent with the port engine producing little or no torque on the airframe, while the starbord one continuing to do so. This scenario will twist the aircraft right around the axis of the starbord engine, and is also consistent with right wingtip strike and the inversion, as well as the relatively undamaged state of the port propellers.
All I'm saying is that what little facts we know are
consistent with a mishandled port engine out at go-around scenario. They are however inconsistent with a low visibility CFIT type accident.