Originally Posted by gums
- The stabilators, or horizontal stabilizers/elevators, moved +/- 21 degrees.
- They had a symmetrical airfoil, and...
- They were interchangeable from one side to the other
- They moved independently in order to help with roll, especially when supersonic
gums, many thanks for the data.
I am aware that the Viper has a delta wing and strakes that generate powerful vortices at high AoA. The stabilators also form a delta-shaped plan (but without the strakes). Such wings tend to “stall” rather smoothly
AFAIK, meaning there is no sharp drop in lift as AoA increases past Clmax AoA, thanks in part to vortex lift.
I tried to understand why the stabilators efficiency (measured as the distance between the neutral and full deflections curves of pitch moment vs AoA) starts decreasing at a lower AoA for ND than for NU deflection. I surmised this had to do with the downwash of the wing and since the stab airfoils are symmetric it is possible to evaluate the downwash and stabilators local Clmax AoA. This gives 21 deg and 15 deg respectively. I was puzzled by a 21 deg downwash angle until I remembered that the FCS adapts the wing curvature at high AoA, drooping slats and flaperons.
Of course, we miss pitch moment curves for the A330 to understand what ND or NU inputs could do. Owain Glyndwr posted a very interesting explanation but I am not convinced the THS was not stalled at 60 deg AoA. We know that the elevator was still working from BEA’s note but perhaps this was due to the elevator presenting a larger frontal area to the airflow in ND position than in NU. The effect of a stalled THS would be a positive slope of the Cm curve in the stalled area, meaning a decreasing downward moment with increasing AoA.
Another point where I doubt Owain’s conclusions is the estimation that, at high AoA, the mean aerodynamic center will move longitudinally to the wing center of area, hence behind the CG. Looking at the Viper curves that move apparently occurs at about 60 deg AoA when the curves turn south. Owain considered the wing only whilst he should consider the wing/body assembly. Adding the fuselage will generally move the mean aerodynamic center forward so I think it is unclear whether the wing/body adds up to or opposes the ND moment of the tail at high AoA.
The stall was very probably recoverable but imho it would take much more time and obstinate ND input to recover than the crew expected.