Originally Posted by
Mad (Flt) Scientist
Where angle-of-attack comes into play is our assumption that Cn(dr)max is fixed - it isn't. If an aircraft is at low angles of attack, in most cases the fin will be in quite undisturbed flow, and the rudder will be aerodynamically effective.
take the same aircraft to an angle of attack near the stall, and the flow coming off the top of the fuselage may be blanking (washing over) much of the fin; this causes a loss in directional stability (which in this case isn't important) and also a loss of rudder effectiveness.
This may mean that a light aircraft can trim single-engine-out at a given speed, but a heavier aircraft at the same speed may not be able to.
Many thanks for the answer! Really interesting! And in addition to the loss of rudder effectiveness at high AoA the heavier aircraft would require more thrust for a given speed, so this would also increase VMC, wouldn't it?
It's really incredible how 5º can change everythig!!!