Originally Posted by
DC-8
Could you explain me more deeply how AoA affects VMC assuming the bank angle is zero?
Sure.
Simplifying the VMC trim problem, we have to generate enough yawing moment with the rudder to counter the yawing moment generated by one good engine and one dead engine (on a twin)
Suppose the Cn(dr)max is the maximum yawing moment coefficient we can generate from the rudder, then the maximum rudder yawing moment at a given speed is:
Cn-rudder = 1/2 * rho * V^2 * S * c * Cn(dr)max
and if the asymmetric thrust T has a moment arm x, then for exact balance we get:
1/2 * rho * V^2 * S * c * Cn(dr)max = T * x
Now, if you really simplify things, you ignore the fact that T is actually a function of speed, V. You also assume that Cn(dr)max is a fixed, known, value. Which means it's quite simple to solve for speed once you know all the other values, by simple algebra.
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. Usually the weight component effect (for the 5 deg bank case) overpowers this effect, but it may not be insignificant for some aircraft. I think I saw an Airbus paper presented at some conference or another where they went into some detail about the alpha effect, because it was significant for one of their aircraft.
Sorry for asking so much but I find Aerodynamics really interesting!
Stay patient; a cure is expected any day now for this condition.
