Mig-21-Flight Model - Check six

Cz limit = Stab
efficiency
AoA Max < 33
Cz limit = Stall
AoA_Stall > 33
Mig-21-FM-Identification-Rev. 14
I’ve chose to follow the values extracted from the fig.8 related to “big arm” curve to build the Cz laws
of the flight model.
There is no evidence that the α UUA is true incidence angle, or just incidence index as suggested in
“MAKING THE BEST OF MiG-21” article, but in the rest of this document I do suppose the values
are true incidence angle between the speed vector and the x plane axis (itself equal to the thrust vector)
The analysis of the fig.8 also demonstrates that maximum available lift cannot be modeled by a unique
incidence limit.
We need here to distinguish two separates mach number ranges: under M0.95, maximum lift is
reached for incidence values greater than 33 degrees, over M0.95 maximum lift is reached for
incidence values lower than 33 degrees due to lack of pitch command efficiency.
Comment : I do not have any evidence that the fig.8 value do or do not take into account the
“swimming flap” device introduced in late Mig-21PFM, but this figures seems to be related to a MIG-
21bis/iz.75 that does have it. This device, when activated, allows “automatic” flaps deployment and
retract, between 0 and 25 degrees at low speed, the flaps angle is limited by dynamic pressure when
speed increase, in a quite simple but pragmatic way. But this assumption seems reasonable when we
see Cl value growing up to 1.3 for low mach. (We are not speaking here of the “landing” flaps system,
that use air flow extracted from engine to blow flaps, identified as SPS system).
Even if I’ve no figure describing Mig-21 supersonic lift curve, I keep for mach number over 1.0, a
C z (AoA,Mach) law defined between 0 and 33 degrees with a maximum at 33 and a linear portion
between 0 and 15 degrees.
For mach number under 0.9, I have built law on the basis of the values read from the figure for
AoA=28 and AoA=33, and a maximum C z value, but for an unknown incidence.
This table gives a possible set of values satisfying these
criteria and the figure bellow displays graphically C z main
values extracted from the L-17 manual (big arm curve):
1.4000
1.2000
1.0000
0.8000
0.6000
0.4000
0.2000
0.0000
Max CL (Mach)
0.00 0.50 1.00 1.50 2.00 2.50
Figure 9
Then we can go to the second step: identification of the incidence limitation along Mach number.
In first approximation the (Ng,Mach) reachable domain is limited by
Cz Max
Cz(AoA=33)
Cz(AoA=28)
Mach Max Cz
AoA for
CzMax
0.2 1.302 57.00
0.3 1.207 57.00
0.4 1.093 52.00
0.5 1.018 47.00
0.6 0.964 44.00
0.7 0.931 39.00
0.8 0.911 38.00
0.9 0.899 35.00
1.0 0.894 33.00
1.1 0.841 31.14
1.2 0.650 21.96
1.3 0.524 17.66
1.4 0.440 14.86
1.5 0.393 13.43
1.6 0.365 12.62
1.7 0.352 12.17
1.8 0.344 11.90
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