Accurate measurements of airflow angles are among the most expensive and difficult to obtain in flight testing because of the complexity of the airflow near the aircraft and the consequent need to carefully mount and calibrate the sensors. A novel technique was developed for determining aerodynamic stability and control parameters from flight data in real time, without airflow angle measurements (airflow angle and sideslip angle).
The technique combines kinematic reconstruction of airflow angle data and real-time parameter estimation in the frequency domain in a new way to produce accurate characterization of aircraft stability and control in real time, with no requirement for direct measurement of airflow angles. Kinematic reconstruction is used to supply the missing angle of attack and sideslip angle data using data from other sensors, namely an inertial measurement unit (IMU), a pitot tube for static and dynamic pressure, and ambient temperature.
Reconstructed angle of attack and sideslip angle data exhibit unknown bias and drift, making the reconstructed data useless for typical modeling approaches. However, the real-time frequency-domain modeling approach can be made insensitive to unknown bias and drift errors in the data, and therefore can be used effectively with reconstructed airflow angle data.
The approach is simple and accurate, and produces high-quality real-time modeling results for flight testing without airflow angle instrumentation. The method could be implemented in any aircraft with a flight computer and data from an IMU, along with ambient temperature and pressure, dynamic pressure, and measured control surface deflections. Many modern commercial aircraft have this data available at sample rates of 25 Hz, meaning that stability and control monitoring could be provided to the pilot in real time with only a software change to non-flight-critical software in the flight computer, or by adding hardware that accesses flight data already available from the onboard data bus. The technique can also be used for rapid or low-budget flight testing, rotorcraft flight testing in hover, or aircraft accident investigations, because aerodynamic stability and control characterization can be done in real time without direct airflow angle measurements.
This work was done by Eugene A. Morelli of Langley Research Center. LAR-17868-1