The goal of system identification is to supply a model for flight dynamics simulation. Such a simulation is very useful for design of flight control and flight guidance systems as well as for performance evaluation amongst others.
The way such a flight simulation is built up typically, is by characterizing the forces and moments acting upon the aircraft with respect to airspeed, height, angle-of-attack etc. but also in respect to flap and control surface deflections.
It is important to note, that especially the change of these parameters in respect to the aforementioned parameters determines how the aircraft behaves dynamically. Therefore for example the exact frequency of the dutch roll or other dynamic effects of the aircraft. You can find more information on this under the search term stability or control derivatives.
Several ways exists to determine these forces and moments:
- Wind tunnel tests: This is the most accurate and widespread way to determine these forces and moments without having to build the aircraft (Building the whole aircraft might not only be costly, but in some cases you cannot fly the aircraft without having first designed a suitable flight control computer, because your aircraft might be unstable). A scale model of the aircraft is built, and then tested with the help of a force balance in order to directly measure the forces and moments under various conditions.
- Computer fluid dynamics: Instead of physically building the aircraft for real-life testing or wind-tunnel testing, you model the aircraft for testing inside a virtual wind tunnel. Note that you typically do not let the aircraft fly "free" but rather you try to simulate the aircraft under various conditions again in order to determine the forces and moments given the aforementioned parameters in order to find out the stability derivatives. This minimized the computation time. This approach of course is much cheaper then wind tunnel testing, but because computer fluid dynamics is not 100% accurate, there always remains some uncertainty.
- In-flight system identification: For older aircraft designs or in order to validate the findings which you determined via the aforementioned system identification methods, you can determine the forces and moments/stability derivatives in flight. Again it is not always possible to fly your vehicle, either because you still want to itterate the design or because you just cannot because the aircraft lacks a flight control computer. However, this is of course the most accurate way to determine the forces and moments, as flight tests are the "real deal" containing all "dirt" and real-life effects without any assumptions.
- First principles: For a first evaluation, you can try to roughly determine these derivatives from first principles. While not very accurate it might give you a good enough idea for a first guess/simulation/design itteration.
Once you obtained a set of forces and moments/stability derivates over your parameter space (airspeed, height, angle-of-attack, sideslip angle and control deflections) then it it possible to simulate the aircraft with the help of the equations of motions of an aircraft in order to subsequently either design a flight control system, or calculate performance metrics etc. etc.