Qwain Glyndwr,
Thank you for constructing the graph, and curves, illustrating, and explaining what appears to be the science behind some of the interpretations and assumptions made based on the existing BEA Report graphs in earlier posts .
Originally Posted by
Owain Glyndwr
"> In fact, for the A330 geometry and using standard aerodynamic calculation methods it turns out that 1 deg of THS is roughly equivalent to 1.5 deg elevator.
Your post is substantial, while also dense, and at a quick reading - I don't have much time now - I can react with some quick questions, but I may have more later....
1. Regarding the Elevator to THS 1:1.5 ratio, what is the ratio between the surface/area of the Elevator versus surface/area of the THS?
2. Is this 1:1.5 ratio the reason (or are there more?) behind qualifying the THS as the "most powerful control surface on the plane" - paraphrasing text from some earlier posts on this thread, related to the THS?
So when we talk of the PF not recognising that he was stalled, one of the classic indications was missing.
The other criterion is buffet. ... a 5 Hz component. .... but it seems to me to be perhaps expecting too much for a pilot to distinguish a change in frequency content of cockpit motion when he has been experiencing turbulence and was expecting more severe turbulence to come shortly. I note that the AI Chief Test Pilot has said that it is very difficult, even for experienced test pilots, to distinguish the transition into stall.
The explanation above seems to emphasize the role of the Stall Warning as being by far the most important element present among the Stall Warning mechanisms at the time, and thus the need for it to be reliable.
...
For virtually the whole of the event, and certainly for the whole time the aircraft was stalled, the THS had a positive AOA so that it was generating upwards lift and a nose down pitching moment despite the fact that it was set at -13.5 degrees!. This is consistent with it being a stable aeroplane as shown by that pitching moment curve.
The THS reached the 13 degree NU when the airplane was already in STALL, so the effect of the max 13 degree deflection can be counted on from that moment, and not earlier - I would think.
3. At what AoA, and Elevators max NU, would the THSmaxNU pitching moment switch into becoming a NU moment?
While at 40 degrees AoA, the THSmaxNU ND momentum (in spite of its NU position) opposed the Elevators NU momentum; at this switching point, it would start helping the Elevators NU pitching moment, or opposing the Elevators ND pitching momentum, if Elevators were moved to be deflected ND.
Of course the net HS lift was negative and the net pitching moment positive (nose up), but this was made up of a very large downwards lift from the elevators partly offloaded by the positive lift from the THS itself. If the elevator had been returned to neutral the THS lift would have given a ND pitch and attitude reduction. Look at the traces - that is exactly what happened!
You seem to have taken in consideration a first important element regarding the THS contribution to the pitching momentum, which is the THS angle in itself.
But there is a second element, which I think may be, or is important, which depends on the question:
Is the Elevator swing action relative to the THS surface, that is, Elevator max NU and max ND angle relative to the THS surface?
If the answer is yes, than in that case, in absolute value relative to the airplane longitudinal axle (absolute - for shortness):
a) Elevators max NU (absolute) angle is at
- maximum at THSmax NU
- minimum at THSmax ND
and
b) the Elevators max ND (absolute) angle is
- minimum at THS max NU,
and
- maximum at THS max ND.
This implies a difference between the absolute Elevators ND angle relative to the a/c longitudinal axle:
- Elevators max ND angle with THS max NU, versus
- Elevator max ND with THS at 3 degree NU (call it Neutral), and further
- Elevator max ND with THS max ND.
4. I would be interested in reading your comments extended to these above elements.
The combination of answers to 3. and 4. would explain further, I think, your conclusion above.
The nose was being held up by the application of elevator.
5. At AoA 40 degrees, are the THS + Elevator, Thrust, (and Cg) the only (force) factors contributing to the pitching momentum?
Of course, the THS setting made the elevator’s job easier, and if the THS had been (sensibly in my view) restricted to 3 deg the eventual AOA would have been lower, How much lower you can get from the first chart – with 3 deg THS and 30 deg elevator you could expect to arrive at 35 deg AOA – big deal! – you are still well stalled and although the descent would have been shallower the end would have been the same unless he had recognised early on that he was in a stall.
I am confused. Is perhaps my first reading. I read "first chart", but I don't see more than one chart, so I must be missing something.
Or are you perhaps referring to first curve? Also you mentioned in your text "traces", or "trace" which I read as "curves" or "curve" on the graph, but I want to make sure my reading was correct.
Overall things are very complex, but it is nice to peal off one by one the layers involved.....
Without having a Normal Acceleration in the picture, is there any indication that in order to being able to quickly and effectively react and have full control of the control surfaces at Stall, would have been better achieved by being in Direct Law?