Expert: Mark Janus - 4/3/2009

Question
QUESTION: I know its not impossible for a modern plane to stall as speeds around or in excess of Mach 2, but I have been arguing that its NEARLY impossible... so my question is this, what conditions are needed to cause an airplane to stall at speeds in at or in excess of Mach 2
I have contended that it would take a catastrophic event to cause it, the person I have ben arguing with maintains it can be done simply by pulling back to hard of the stick from level flight. Just so you know we have been discussing this event regarding modern jet fighter aircraft.

ANSWER: Actually I'm wondering what your definition of a "stall" is ...   If you are considering stall to be the loss of lift and accompanying increase in drag to the point where weight overcomes lift and drag overcomes thrust (i.e. the aircraft slows and drops) then I'd have to say it would be hard to "stall" at Mach 2.  If on the other hand you are referring to the flow separating prematurely on the surfaces of the wing for example, that is entirely possible at Mach 2.  
Lift is equal to 1/2 rho v^2 S CL ... so when v is large, you can get away with relatively small CL and still have enough lift to stay up....  You've probably heard that you can even make a brick fly....  basically this is true, if it's traveling fast enough....   Fighter aircraft have so much excess power that they can overcome tremendous drag forces...  also, fighter wings benefit from vortex lift.... (many are designed for the flow to separate and form a stable vortex above the wing) not the most efficient L/D but certainly can produce large CL values and hence A LOT of lift....  
I would say at Mach 2 you would possible overstress the wings (to where they yield (bend or break) before you would experience a "stall" at that speed (by pulling back hard on the stick).  Anyway, by the time a maneuver like that creates a stall you would have probably done a couple loops and would have decelerated to much less than Mach 2 in the process....

mj

---------- FOLLOW-UP ----------

QUESTION: In your answer you say "I would say at Mach 2 you would possible overstress the wings (to where they yield (bend or break) before you would experience a "stall" at that speed (by pulling back hard on the stick)"
wouldn't bending or breaking the wings BE a catastrophic event as I suggested in my original question?

ANSWER: Yes....  I'd say bending or snapping your wings off would be catastrophic.... but what follows that event, I doubt would be classified as a stall... :)
 
At very high speed, even if the flow separates from the wing surface (which is the classic characteristic of a "stall" at low speed), the velocity of the aircraft more than makes up for the drop in CL due to flow separation.  
So again, if you define "stall" as a massive flow separation over the wing, drop in CL, increase in CD then that may occur at any speed... it is primarily a function of angle of attack.... but that said, the aircraft would still be able to produce much more lift than the weight of the aircraft and thus would not drop as in a low speed incident....

Hope this helps a bit,
mj

---------- FOLLOW-UP ----------

QUESTION: I want you to know I do apriciate the info you've given me here. And I believe I understand what you are saying but just to clarify, and please correct me if I am misunderstanding, but what I think you are saying is that while technically, on paper the conditions of a stall can be met, but in a real world situation, at mach 2, there would be no discernable effect on the plane?
And if I may, I'd like to ask if you can think of or describe any event or situation where at mach 2 the classic definition of a "stall" where there is some degree of loss of control or the plane drops might happen?

Answer
yes - other than your "no discernable effect" statement you are correct...  I would say you would notice the reduction in lift and increase in drag but the aircraft would/could still climb...it would just climb slower and decelerate faster...which I'm sure would be very noticeable...

I would say under any violent maneuver such as trying to stall a Mach 2 aircraft you run the risk of loss of control...  it depends on the aircraft, its control surfaces, and the maneuver

I believe that the supersonic capability of a modern fighter is primarily to get there fast and get out fast...  not for use in a "dog fight"....although it may happen that you end up supersonic in a dog fight at some point...

mj