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

What I know (AFAIK?) is that MiG-21 has "speed instability" characteristic
in the transonic area. This was a quote from "pilots manuals", and as I
understand it, it's a tuck mode, where two phugoid poles become real
poles only, and one becomes unstable (crosses onto right side of the
imaginary axis). In any way, if you take a look at the "energy
characteristic", it is clear (because of performances) that these 
regions of speeds are allways avoided.


BTW, I see other inherent problems; take a look at the _huge_ vertical
tail, and very massive fuselage, with allmost "no mass" in the wings. It
suggests problems with inertia coupling, especially when taken into
account that max allowed roll rate is 90 deg per second.


Another one is very high max angle of attack. It turned out that during
final approach, pilots were unable to see clearly, because the nose of
the ac was "above horizon" (very highly positioned compared to position
of pilot's eyes). It was partially solved by introducing artificially
blown upper surface of the flaps.


 > 1) vortex bursting

As I know, it is a "way" delta wing stalls, it is not a particular 
problem of MiG21, but characteristic of delta wnigs in general. BTW, LE 
vortexes are greatly responsible for so high max AoA (cca 35 deg for 
MiG21), as well as nonlinear characteristic of delta_CL_over_delta_alpha.

 > 3) falling-leaf mode" of asymmetric deep stall??

I suppose you mean spin, not stall?

Configuration of this particular AC makes it quite hard to enter into 
(fully developed) spin. I've heard of no cases that someone succedded in 
spinning MiG-21 (at least in Croatia, where I live :). I heard the story 
that someone in former YU spinned MiG21, and after several thousand 
feets of height loose, ejectid himself. I state it is a story only, and 
I can't confim it nor deny.

I also heard that Russians _did_ successfull spin entering/recovery, and 
MiG behaved "according to theory";

(bit about stall warnings)

It is true that there are no mans of "stick shake warinings", but there 
are two "red lights" that will start blinking when AoA approaches 
dangerous values. It also receives signal of "speed of Max AoA approach" 
(i.e. alpha_dot), and if the alpha_dot is greater, lights start blink 
eariler, starting from AoA == 15 degs.


PS
I am not a pilot, just a aeronautical engineer, and this is my 
_personal_ view of MiG-21 A/C :)


ciao,

Gordan