Dynamic stalling of delta wing a/c (wrt MiG 21)

Don suggested:

<This creates a handling problem as pilot may not realize he is on back
side of lift curve (also sometimes called back side of power curve, or
region of reversed control).

If the aircraft is in this region (past point of max CL)  continued aft
stick causes lift to decrease, craft to sink more rapidly.  Only way to
get out of it is to LOWER nose, counter-intuitive.>

I don't think it's proceeding beyond Cl(max) that's the issue here - 
surely it's a Cl/Cd issue. Glide angle is simply the vector sum of the 
ratio of net lift (lift plus any component of thrust) to net drag (drag 
minus component of thrust parallel to velocity vector). Once past 
(Cl/Cd)(max) further increases in alpha might boost Cl but the 
corresponding increase in Cd is greater, so the glide angle (and hence the 
sink rate) increases. Hence low-aspect ratio deltoids require phnomenal 
amounts of power to fly at the very low speeds that the high-alpha 
capability of the configuration permits. 

All aircraft display this characteristic to some extent although it's far 
more pronounced on low aspect-ratio configurations; its the resultant of 
different forms of drag that vary differently with airspeed. Form drag 
increases with airspeed and induced drag decreases with airspeed, so there 
is a minimum total drag airspeed (at which in principle form drag equals 
induced drag). This airspeed is the speed for maximum rate of climb, being 
the condition in which the minimum amount of power is required to oppose 
drag and thus the maximum amount is available to oppose gravity, and in 
simple cases it usually occurs at around root-two times the stall-speed for 
the configuration. It should theoretically occur at EXACTLY root-two time 
Vs, but factors like the vertical component of thrust when the fuselage is 
rotated to achieve the required alpha confuse the issue in any real-world 
case, so it becomes an approximation.

On a recent Discovery programme (take the health warnings as read) on 
privately-owned Migs in the USA an american Mig-21 owner stated that it 
was an "energy management" aeroplane which required afterburner for any 
kind of manoeuvering - even comparatively gentle turns. Even though the 
source was a Discover programme I wouldn't be that surprised if it was 
true.

Peter D Rieden