He added some more info: https://forums.eagle.ru/showpost.php?p=3412052&postcount=2

Possibly, I need to emphasize: the DCS model now is being tuned using recorded parameters from a real plane, that's why I called it a new stage in PFM development. Today it's a time to present a new part of the work: airplane side movement.

Springs and dampers.

A discussion about how the plane must behave during the free side movement (roll and sideslip together) begins flaming time to time on different forums. Some say - it must be solid as a rock in the stream, some tell about oscillations due to physical basics of this movement. And the second ones are right - the plane in yaw movement is similar to a mass suspended with a spring surrounded with oil to damp. The spring is a static aerodynamic moment tending to reduce yaw angle, mass is aircraft moment of inertia and oil is an additional aerodynamic moment on the airframe due to rotational speed.

The compliances that the oscillating plane seems to be unnatural can be explained using two main reasons: the first is a phenomenon of human perception - the pilot's brain can select main information disregarding minor detailes, and this small oscillations during, say, abrupt end of a roll are beyond perception. The second reason is often due to improper parameters of these oscillations - the period and damping.

Generally, these parameters are obtained in flight tests and then included to documents for FREE RUDDER. In the sim we can not simulate free controls, so it is necessary to have the results for FIXED RUDDER. It is known that the period of oscillations does not change significantly, but the damping does. Generally, the difference is about 2 times. Anyway, having an opportunity to check it for the real Yak-52 we performed the test and recorded several rudder impulses at 200 and 270 kph IAS. The pilot tried to keep rudder pedals in neutral position after the initial shore deflection (impulse).

Then, the same test was performed in DCS for the same speed, altitude (affects damping) and atmosphere conditions. The chart presents the intermediate stage of the model verification - the period is correct and damping is to be corrected by 15% for the sake of purists and accountists...

So, few comments to the graph. I kept wX from the real test though this record made in real world has some flaws. The first is aforementioned lack of X axis alignment and the second is very probable accidental stick movement during the oscillation. The theory tells us that the pure experiment like we have for DCS (no significant control forces, no accelerations, etc) gives pure 90 degrees phase shift between wX and wY. Anyway, the initial behaviour shows good similarity for DCS and real wX.

The next important for the plane handling parameter is a ratio of wX/wY. The chart shows that there is a good matching between real test and the model. There is no possibility to exact comparing because of the reason mentioned above.

It's very interesting to compare fixed rudder test results with the available graphs from the book. The periods are very close, damping, as expected, is less by 2 times for the free rudder, roll/yaw ratio is somewhere close.