Effect of shockwaves on primary flight controls
I understand that once an aircraft exceeds its critical mach number, shockwaves form on the wings and stabilisers and further increases in speed cause the shockwaves to grow and move towards the trailing edges. For aircraft with conventional flight controls (elevator, ailerons, rudder) this can lead to loss of control, or even control reversal, as the shockwaves and/or separated flow behind the shockwaves can render the controls ineffective. Hence trimmable horizontal stabilisers or all moving tailplanes / tailerons on aircraft designed to approach or exceed Mach 1.
My question is, how come elevons on delta wings still work in these conditions, e.g. Concorde, Eurofighter, Mirage fighters, etc? Is it simply a question of having enough force to move the control surfaces, i.e. through use of PFCUs? What about when the shockwave is sitting just in front of the control surface hinge line? Surely there must be a speed range where the control surfaces become ineffective temporarily?