Real-Time Estimation of Bare-Airframe Frequency Responses from Closed-Loop Data and Multisine Inputs

Area of Science:

• Aerospace Engineering
• Control Systems Theory
• System Identification

Background:

• Estimating bare-airframe dynamics from flight test data is challenging due to feedback control and control effector mixing.
• Existing methods may struggle to accurately isolate the airframe's inherent frequency responses in the presence of these complexities.

Purpose of the Study:

• To develop and validate a novel method for accurately estimating multiple-input multiple-output (MIMO) bare-airframe frequency responses.
• To address the confounding effects of feedback control and control mixing in flight test data analysis.

Main Methods:

• Utilizing orthogonal phase-optimized multisines to simultaneously excite each input with unique harmonic frequencies.
• Computing frequency responses as ratios of output-to-input Fourier transform data at excitation frequencies.
• Resolving confounding effects of feedback and mixing through frequency response interpolation.

Main Results:

• The method successfully estimated frequency responses from simulated and real flight test data.
• Effectiveness verified on a T-2 generic transport airplane simulation with rate feedback.
• Demonstrated on the X-56A MUTT aeroelastic airplane with both feedback control and mixing.

Conclusions:

• The presented method provides an effective means for estimating MIMO bare-airframe frequency responses from flight data.
• The technique can be applied in both post-flight batch analysis and real-time flight analysis.
• Accurate estimation is achievable despite the presence of feedback control and control effector mixing.