As title says most 3rd and 4th gen fighters have overperforming flight models, and most of them are related to reference system error…
first of all there are 2 exceptions out of the aircraft I’ve tested that just straight up over perform for no apparent reason apart from being modelled wrong:
MiG-23ML is overperforming by about 13% percent in a sustained turn: (Ny, which is the normal load factor, is ~5G instead of 4.5G
F-4E is overperforming by a whopping 26% at very low speed (11.85 deg/sec instead of ~9.4 deg/sec). It is also sustaining a quite abnormal 26 degree of AoA which is a really high value (got no source to prove that it couldn’t tought).
Interestingly the higher the speed the less the over performance is for the F-4E. At mach 0.6 it starts to follow the E-M decently well.
Low speed over performance is also proven by this other chart:
Now the fun stuff: there are a lot of aircraft that are currently over performing because charts using what is called “load factor” (usually indicated with the symbol n_z in the west and n_y in Russian manuals) got used to calibrate the aircraft “normal load factor” (usually indicated with the symbol N_z in the west and N_y in Russian manuals and local host).
Difference between the two load factors
The two load factors are different because they belong to two different frames of reference:
n_z takes all the forces perpendicular to the airflow (which also mean all the forces perpendicular to the velocity vector) and divides them by the aircraft weight.
This means that n_z = \dfrac{Lift + Thrust \cdot \sin(AoA)}{Weight}
Since n_z is perependicular to velocity, it can be directly used to calculate a sustained turn (a turn with constant speed and altitude), with the turn rate in deg/sec being:
ω = \dfrac{180}{π} \dfrac{g \cdot \sqrt{n_z^2-1}}{V_{tas}} (g = 9.81 m/s^2, V_{tas} = TAS speed in m/s).
N_z instead takes all the forces perpendicular to the aircraft and divides them by weight.
This means that N_z = \dfrac{Lift \cdot \cos(AoA) + Drag \cdot \sin(AoA)}{Weight} which is clearly different from n_z.
Relation between n_z and N_z in a sustained turn:
Since in a sustained turn speed is not changing (aka velocity vector module does not change) there’s no acceleration parallel to the velocity vector, which means that Drag = Thrust \cdot \cos(AoA) .
By replacing drag in the N_z definition with Thrust \cdot \cos(AoA) and doing some quick algebra, we get that \displaystyle N_z =\Big (\dfrac{Lift + Thrust \cdot \sin(AoA)}{Weight}\Big ) \cos(AoA) which can be rewritten as N_z = n_z \cos(AoA), which means that n_z = \dfrac{N_z}{\cos(AoA)} in a sustained turn.
Why does all of this matter?
Well, unless a lot of aircraft manuals have completely wrong charts, there are are a lot of planes that have charts for n_z that gaijin used to adjust the N_y indicator on the local host.
Since the N_y is smaller than n_z in a sustained turn (the cosine of an angle that is not 0 is always smaller than 1), if one sets the in game N_y values with what in real life are n_z values then the in game n_z will be bigger, resulting in an higher turn rate than real life.
Here are a few examples:
F-15A:
(mach 0.4 test), (mach 0.6 test)
As can be seen with the videos and the chart below, the F-15 seems to be (for example in the mach 0.4 test) matching fine because the N_y value (~4.47G test average, 4.4G chart) is very close to the chart… except that the turn rate is completely wrong (19.1 deg/sec instead of ~18deg/sec).
If we plug 4.4G for n_z in the formula above we get the ~18 deg/sec the chart indicates.
Instead if we take 4.4G as N_z (this is what the devs did), divide it by cos(AoA) (~22 degrees) to get in game n_z and plug it into the formula we get exactly 19.1 deg/sec.
This works with a lot of other planes… to name a few:
F-16A: 200kt test, 16.5 deg/sec instead of 15.4.
If we take N_y value of the local host and use it as n_z we will get chart turn rate.
Instead if we take N_y value (around 3.2G), divide it by cos(22 degrees) and plug it as n_z we get 16.6 deg/sec (within test margin of error, I climbed a bit in the F-16 test).
F-4J: 300kt, 15.31 deg/sec instead of ~13 deg/sec.
(can do again the same thing as before, same conclusion).
F-14B: 300kt, 16.15 deg/sec, instead of 15.3deg/sec.
(again, taking localhost N_y as n_z will give 15.3deg/sec in the formula, dividing the localhost value by cos(18 degrees) and plugging it into the turn rate formula we get 16.2deg/sec.
And I probably could go on if I tested other aircraft.
Why does this sustained turn difference matter?
Because sustained turn rate is directly related to what energy retention of a plane will be in the game… especially a few decimal Gs of difference in low speed sustained turn rate (which is the part where the planes affected by this error with overloads are overperforming the most since the lower the speed the higher the AoA is) can have very big repercussion on energy retention whenever relatively high angles of attack are pulled (even at higher speeds).
Thanks to @Grimtax for helping me making this post far more readable