QUALITY OF FLIGHT TEST MANEUVERS

It is very important to keep track of the quality of the flight test maneuvers, because the quality of the estimates obtained and any deductions made from using these in analysis programs depend on the quality of maneuvers [1]. The aim should be to maximize the information content in the data. This requires good planning and experiment design. If the maneuvers are conducted to generate the data for parameter estimation, then specific maneuvers are required to be planned. It so happens that the maneuvers conducted for other purposes generally do not provide reliable data for parameter-estimation exercises. For the latter purpose, very careful consideration of many factors is essential: control inputs, signal magnitudes, sampling rates, and signal to noise ratio (SNR). If the information content in the aircraft responses is not adequate, then these data will not provide good estimates of the aerodynamic derivatives. The SNR should be at least 10 for the most important signals required for the analysis. In addition, such data with low SNR are not good to model the plant in general. It is often felt that to obtain good estimates of the control effectiveness derivatives, application of one control input at a time would be a good strategy. Other control inputs can be applied in some sequence. The question of sufficient excitation of the modes is of great importance. As we are interested in studying the dynamic modes of the vehicle, it is important that the modes are excited properly and they become apparent in the time responses. If the modes are not adequately excited and captured in the data, then system identification will be ill-conditioned. One should use a small magnitude for inputs so that the assumption of linearity is maintained for subsequent analysis. This is mainly because aerodynamic derivatives are defined on the assumption of small perturbation theory. The SNR consideration is important for the small amplitude maneuvers. Typical maneuver amplitudes for a fighter class of aircraft would be (peak) alpha = 2°, beta = 1°, roll rate = 30°/s, pitch rate = 10°/s, yaw rate = 5°/s, normal acceleration = 0.25g, and lateral acceleration = 0.1g.

Large amplitude maneuvers will excite nonlinear phenomena/dynamics; how­ever, these might be required for specific reasons, as will be discussed later. Often and almost always repeated maneuvers are needed to obtain consistent estimates of the derivatives. If a few maneuvers are bad then they can be neglected and the overall flight test time is reduced. This would save fuel and test efforts. To that extent the maneuvers should be conducted in calm atmospheric conditions, unless the purpose is to specifically test the aircraft responses in turbulent weather.

The command inputs types, if they are simple, can then be applied manually very easily. If not, then the pilot should practice sufficiently on a real-time flight dynamics simulator or on a computer connected to a joystick. Some input types can be programmed or computerized and applied through a flight test panel. The issue of generating the data for parameter estimation from closed loop responses of an inherently unstable/augmented aircraft is of paramount importance. The generated responses can be often checked on the site, i. e., at the flight test center itself. This will ensure the adequacy of the data for subsequent offline/batch processing analysis. One should also ensure that the data are adequate and of sufficient length (in time) to be suitable for consistency analysis.