Originally Posted by
Mad (Flt) Scientist
I don't think you're understanding my point.
The actual thrust available on a given plane is not relevant to the calculation of VMC.
What matters is the thrust levels which were defined for certification - which can indeed be a function of altitude (and temperature too) - but those may be either above OR BELOW the thrust you have on any given day, on any given aircraft.
This is quite interesting, so I'll add my 2 cents:
I think everyone agrees that
actual VMC is based on certain variables. Given that, (and this might be applicable to only small aircraft with props) isn't
published VMC calculated with the following criteria?
1) Sea level
2) Standard pressure
3) Max Gross
4) Aft CG (less rudder arm)
5) Banked 5 degrees into the operative engine (or perhaps wings level?)
6) Operative engine at takeoff power
7) Critical engine inoperative, propeller windmilling
8) Out of ground effect
9) Gear down & flaps at Takeoff position
Essentially, this is simulating a V2 cut, where you'd most likely encounter the dynamics of flight at or below VMC. 3 factors tend to help, or bring VMC down: gear down (stabilizing effect, lower CG), gross weight (banking effect greater with high weight), and 5 degrees into the operating engine. All the other factors tend to represented a "worst case" scenario, or tend to raise VMC. Changes in the other factors make VMC go down, such as altitude (less engine power), failed engine not windmilling (feathered; less drag) and operating engine not at max power.
The above is what I recall from my ME training. With larger aircraft, I tend to doubt the supposed stabilizing effect that gear down may have. Also, we've discussed the possibility of gross weight (high AoA) having on rudder effectiveness, possibly offsetting it in some models. And lastly, I've not seen instructors enforce the 5 degrees into the operating engine in the jet realm when training. Am I off-base here?