Originally Posted by
Owain Glyndwr
I am usually with you, at least in spirit, in your tussle with Confiture, but I really don't think you need to instruct him on what constitutes a stall
I wasn't (and would never presume to do so), but more on that shortly...
It might not be so bad if it were not for the fact that your illustration isn't actually representative of a modern airliner stall except at low Mach numbers and with flaps and slats deflected.
I did say it was a generic example from basic classroom material! If not being representative of a jet transport is a problem I'll remove it.
What I suspect CONF iture misses is that while the calculation of AoA is not directly affected by thrust and/or airspeed, airspeed (and Mach at high speed) *does* figure in the Coefficient of Lift calculation from which AoA is determined (said calculation is what I'm guessing he means by "performance"). The presence of the graph was simply to show that a correlation exists - not the correlation of a specific type of aircraft.
So to summarise - I certainly wouldn't presume to tell CONF iture (or any other line pilot) of the correlation between AoA and Stall - but the relationship between AoA and Coefficient of Lift (and by extension, the variables involved in the calculation of Cl) shows an indirect relationship between Thrust, Velocity (i.e Airspeed/Mach) and AoA does indeed exist.
Alpha floor was intended to make such a situation less likely - OK, if you want to call it a performance issue I won't disagree, but in my book it is more for maintaining margins than increasing speed and reducing AoA. AoA control is the province of the stick movement
Fair enough, but if the pitch attitude cannot be reduced for whatever reason and the aircraft approaches a low-energy state (precipitating approach to stall), the only way to maintain a sufficient Coefficient of Lift and therefore AoA to keep the aircraft flying is to increase speed, which is done with thrust. Alpha Prot will limit pitch angle and ensure the aircraft won't stall, Alpha Floor will activate to keep the aircraft from bleeding energy until it hits the ground. I've understood it that way for quite some time.
Would it be useful to consider Alpha floor as precautionary and Alpha max as prevention, rather than lumping both in AI's "High AoA protection"?
Personally I'm not bothered by the semantics too greatly, as it is largely subjective. Coming at it from an engineering and systems perspective I see Alpha Floor as a function of autothrust and Alpha Prot as a function of the flight control logic - it just so happens that there are scenarios in which the two will interoperate.
(Which incidentally dovetails with at least some of what CONF iture is saying - he's right in that if you disable autothrust (and by extension Alpha Floor) but remain in Normal Law, the Alpha Prot will prevent the aircraft from stalling, but the aircraft will gradually lose energy until you hit the ground. Funnily enough I've never said that wasn't the case...)
As AZR said, there's a degree of quibbling over semantics going on here that to my mind adds little to the discussion and merely generates unnecessary friction.