Slats deployment near critical AoA with flaps set to 0
Hello,
I am an aerospace engineer, aerodynamicist and glider pilot. We all know what the slats are there for right? For increasing the AoA (angle of attack) at which the wing meets airflow separation, or for short to increase the stall AoA. Of course that every benefit has it's own deficiency! Never in this world you'll find something that does only what you need. The slats produce two more unwanted effects which are profile drag and a higher nose down pitching moment which will require a higher pitch trim that also slightly increases drag. Another inconvenient would be that of a structural load limitation on the slat itself during in flight operations, limitation which may not allow for the extension of the slats above a certain dynamic pressure that may damage it.
The question is: Why do all airliners, both old and new designs, only allow the activation of the autoslats when the flaps setting is different than 0 (at least set to 1 unit). The "autoslats" is a term for the deployment of the slats to full extension once the DFCS (digital flight control system) senses that the alpha (AoA) has gone past a certain limit (which is still below critical anyway). This system is designed to not allow the autoslats to operate, no matter the AoA, if the flaps have not already been set to a unit different than retracted.
I am not personally convinced why this happens! Fighter aircraft are using autodroops (similar to slats but are simply a plane leading edge flap and don't travel forward like a fowler (which is a slat)) whenever the angle of attack has passed above a certain value at whatever airspeed, which leads me to think that the structural stress on the droops isn't of concern even if the fighter might be flying supersonic (Mach > 1.2). The slats indeed extend forward (being a fowler type high lift leading edge device) and downward in order to give better aerodynamic performance than a simple leading edge droop, and yes, because of this feature it's structural links and driving mechanism might not withstand the aerodynamic loads (structurally) as easy as a droop, but is this difference really that high in order not to allow the slats to be deployed above a certain IAS?
Maybe I've answered the question myself, but I'm not sure until someone who really knows it, can actually confirm.
For example, an airliner flying at FL 370 at Mach 0.85, by calculation has an IAS of around 300 knots. At 300 IAS, the dynamic pressure is around 51156.61 N/m^2 or 7.4196 lbf/in^2. The structural force suffered by the slat would be this value multiplied by a reference surface. Can that force be so great that could lead to a structural failure in the linkage between the slat and the wing's structure, thus not being allowed to deploy in such conditions, or is there another reason for it?
Let's remember that there were many cases when the aircraft found themselves stalled (by pilot error, system error or both) at more than 270 knots IAS because the pilots pulled the alpha way beyond stall, where an extended slat could've very nicely increase the controllability margin of the airplane beyond stall or simply keep the wing slightly away from an aerodynamic stall.
Thank you!