TABLE OF CONTENTS The identification of bare-airframe dynamics from flight-test data with feedback regulation active raises some important issues and limitations. The SCAS feed-backs generally suppress the bare-airframe excitation signal, especially at lower frequencies (Fig. 5.6), and introduce cross-control correlation that also reduces the effective input autospectrum for multi-input analyses (Chapter 9).
Perhaps the greatest concern in the identification of the bare-airframe response under closed-loop test conditions is that the SCAS feedback introduces correlation between the output noise and the bare-airframe excitation signal ( v and x, respectively, in Fig. 7.4). This correlation leads to bias errors in the frequency-response estimate. The analytical and numerical simulation results presented herein show that bare-airframe dynamics can be determined without incurring significant bias errors, subject to modest restrictions on the noise-to-signal ratio. These restrictions are consistent with the flight-testing guidelines discussed previously (Sec. 5.7). Although this chapter refers to SCAS feedback, the same considerations also apply to piloted feedback regulation as well.
The following topics are covered in this chapter: situations requiring closed-loop testing, theoretical analysis of bias errors caused by feedback, and numerical study of bias errors in closed-loop testing.
Some situations require that the identification of bare-airframe dynamics be based on test data collected with one or more of the SCAS channels engaged. Possible reasons for these circumstances to arise include the following:
The bare-airframe dynamics might have lightly damped, unstable, or highly coupled modes that make it too difficult for the pilot to execute...
