Would somebody explain, on what facts the following happens in the first 12 seconds of the above graph:
Pitch is reduced from +15° to -10° = 25° change
FPA increases from -22 to -34 = 12° change
AOA decreases from +35 to +22 = 13° change
Speed in this timeframe is not specified, but is esentially important for generating lift. Drag in this AOA regime has an influence as well, where can it be found in the graph?
On what ground would the AOA change (13°) be grater than the FPA change (12°) by changing pitch (25°)?
Wasn´t the initial AOA according BEA higher than 40°?
Out of my own expierience with such a high initial AOA and such low speed the pitchdown initially increases the FPA by the same amount, the pitch is decreased,, giving only a minimal change in AOA and an increase in FPA and descent rate.
The expierience in the A330 sim from PJ2 and another Airbus pilot make me believe, that this magic graphs miss some vital points. Also all accepted procedures for combat jets as well as the new updated stall recovery procedures speak of the need to place the stick forward until the stall warning stops, which would not be the case until 45 seconds into the recovery like shown in the graphs.
Franzl,
Good questions that deserve a reasoned reply. Sorry to have got out of sequence, but domestic duties got in the way.
I assume we are not doubting that stuffing the nose down will eventually give increased EAS and reduced AoA, the debate being the mechanism by which one achieves that end state.
I rationalised it this way:
Suppose we start with pitch +15, Gamma -25, AoA +40 and instantaneously pitch down through 25 deg to -10. (this is a thought experiment so we can do that)
AoA remains at +40 for the moment as you suggest, but that means gamma goes to -50. EAS hasn't changed, neither have CL and Cd at constant AoA so drag and lift are unchanged. However we now have a large additional gravity component so the aircraft will accelerate in absolute terms, i.e.TAS will increase, again as suggested in a later post. But in addition the aircraft is now descending more rapidly into denser air so there is a double effect on EAS, and the thrust is also increasing due to altitude effects which also helps a bit although this is not anything like as important as the gravity effect since the maximum thrust one might get at say FL350 is only about 7%W whereas a 25 deg FPA shift is worth 42%W.
If EAS increases at constant AoA the aircraft will develop an upwards acceleration normal to the flight path and thence an upward velocity component which, with the increased airspeed will result in a reduction in FPA. You can see this in HN39's traces. [It will also of course experience an increase in drag which partially offsets the benefits, but I think this only partially offsets them]
A reduction in FPA, WITH PITCH HELD CONSTANT, will give a reduction in AoA.
Cd is very sensitive to AoA above the stall, so the drag reduces more than the effect of increased EAS and the whole response is "speed unstable" as it were, with an increasing acceleration and reduction in AoA as time goes on until the AoA is reduced to below S/W limits.
That is, I think, the basic recovery mechanism, but in the early seconds the exact response depends on a changing balance between the various effects, and these in turn depend on the exact time history of applied pitch, so it is really quite difficult to answer your questions about the first twelve seconds.
Drag (and lift) in this case were calculated by analysis of the traces extracted from the BEA cartoons, the only assumption needed being how thrust varies with altitude and RPM and there are well established scaling rules for that. The justification for staying with these values is that the actual trajectory can be calculated by inputting just pitch and power, the output being speed and altitude. Hence the purple line on my original graph.
I don't think there is any conflict with your point about holding forward stick until the S/W stops, the only qualification is that I assumed the pilot flew constant pitch.
I don't think PJ2 will be upset if I say that in my conversations with him he confirmed that on at least one occasion he executed a recovery from 30 deg AoA using 10 deg ND pitch - it took about 15000 ft.