Effective Date: 15 June 98
Test and Analysis Techniques As mentioned in the introduction, this section will contain test and analysis techniques for some of the more common stability and control tests. The content of these sections is very general and should serve only as a guideline.
Equivalent Model Matching
Some modern aircraft have either marginal or no natural aerodynamic stability. For these aircraft , it is required to have a full time flight control system to provide artificial stability through active control of the control surfaces. The F-16 is one example of an aircraft which employs this type of flight control system.
By constantly monitoring the aircraft state, the flight control system provides the necessary control movements to maintain aircraft stability. In response to inputs from the control stick, the flight control system generates the required deflections in the control surfaces to change the aircraft attitude in pitch, yaw, and roll in addition to maintaining positive aircraft stability.
With a flight control system in the loop between the pilot and the control surfaces, it is impossible to determine aircraft stick free stability. In addition there are typically control surface movements in all three aircraft axes even though the control stick has been moved in only one axis. Aircraft equipped with automatic flight control systems appear to be very highly damped even though they are actually have no natural damping. The apparent damping and natural frequency are actually a function of the flight control laws which are built into the flight control system.
To validate the stability and control characteristics of an aircraft equipped with an active flight control system, it is necessary to employ the use of equivalent model matching. An equivalent model is a computer simulation of the aircraft including effects of the aircraft kinematics, aerodynamics, and flight control system. The math model has very well defined stability characteristics, and is used to taylor the aircraft flight control system so that specification requirements are met.
If the aircraft in free flight behaves the same as the equivalent model, then it is safe to say that the stability of the aircraft satisfies the same specifications as the equivalent model satisfied. Equivalent model matching is performed by using the same control inputs that were used during flight as inputs to the equivalent model. The resulting responses are then compared to the actual aircraft flight test response for the same maneuver.
Almost any flight test maneuver can be used for equivalent model matching, but several maneuvers will produce better results. Maneuvers such as pitch, yaw, or roll pulse or steps and frequency sweeps are typically used because they excite all frequencies. Pulse type maneuvers can be taylored to excite specific frequencies, and can be built in to the flight control system so that they are repeatable. Pulse maneuvers also provide data which can be used for parameter identification techniques. Frequency sweeps can also be used for some limited flutter testing.
Problems can be encountered with equivalent model testing because of atmospheric disturbances and trim effects which can not be duplicated in the equivalent model. It is almost impossible to match exactly the trim required in actual flight with that required in the model. Because both of these problems are extremely long period and sufficiently separated in frequency from the aircraft response, they can usually be ignored. In most cases, it is adequate if the short period responses of the flight test data matches in phase and magnitude with the equivalent model.