Flight testing is expensive. It is therefore important that necessary flight data be collected in the most efficient manner possible. Inputs traditionally used for flight test maneuvers to collect aircraft stability and control data include doublets, impulses (stick raps), multisteps, and frequency sweeps. All of these input types are designed for single-axis response, although often the inputs are applied sequentially to different controls to collect multi-axis data.
Recently, an input design technique has been developed that combines the time efficiency of multi-axis excitation with optimized (minimum) input amplitudes, wideband frequency content, and multiple-input orthogonality in both the time domain and the frequency domain. This development in flight test input design has enabled new approaches to efficient stability and control flight testing that previously were not possible.
Several novel flight test maneuvers for efficient aerodynamic modeling were developed and demonstrated in flight. Orthogonal optimized multisine inputs were applied to aircraft control surfaces to excite aircraft dynamic response in all six degrees of freedom simultaneously, while keeping the aircraft close to chosen reference flight conditions, or while flying the aircraft through a range of flight conditions. Each maneuver was designed for a specific modeling task that cannot be adequately or efficiently accomplished using conventional flight test maneuvers. Real-time and post-flight modeling results were used to show the effectiveness and efficiency of the maneuvers, as well as the quality of the aerodynamic models that can be identified from the resultant flight data.
The general idea is to excite the aircraft using perturbation inputs with wideband frequency content over a range of frequencies that encompasses the expected modal frequencies of the aircraft dynamic response. The excitations are implemented as perturbations to the control surface deflections by summing the designed perturbation inputs with the actuator commands from the pilot and feedback control system.
Multiple excitation inputs are designed to be mutually orthogonal in both the time domain and the frequency domain, and they are optimized for maximum data information content in multiple axes over a short time period, while minimizing excursions from the nominal flight condition or flight path. The mutual orthogonality of the inputs allows simultaneous application of multiple inputs, which helps to minimize excitation time, and provides continuous multi-axis excitation as the aircraft flies through time-varying or precarious flight conditions.
Unique features of the flight test maneuver design method include good data information content in all degrees of freedom for many controls simultaneously, robustness to unknown dynamics, suitability for time-domain or frequencydomain modeling, ease of design, excellent flight test time efficiency, and the capability for effective use in precarious and/or unusual flight conditions, such as stall and departure.