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Excitations for Rapidly Estimating Flight-Control

Parameters Parameters are estimated, in nearly real time, from responses to these excitations.

A flight test on an F-15 airplane was performed to evaluate the utility of prescribed simultaneous independent surface excitations (PreSISE) for real-time estimation of flight-control parameters, including stability and control derivatives. The ability to extract these derivatives in nearly real time is needed to support flight demonstration of intelligent flight-control system (IFCS) concepts under development at NASA, in academia, and in industry. Traditionally, flight maneuvers have been designed and executed to obtain estimates of stability and control derivatives by use of a post-flight analysis technique. For an IFCS, it is required to be able to modify control laws in real time for an aircraft that has been damaged in flight (because of combat, weather, or a system failure).

The flight test included PreSISE maneuvers, during which all desired control surfaces are excited simultaneously, but at different frequencies, resulting in aircraft motions about all coordinate axes. The objectives of the test were to obtain data for post-flight analysis and to perform
the analysis to determine:

  • The accuracy of derivatives estimated by use of PreSISE,
  • The required durations of PreSISE inputs, and
  • The minimum required magnitudes of PreSISE inputs.

ImageThe PreSISE inputs in the flight test consisted of stacked sine-wave excitations at various frequencies, including symmetric and differential excitations of canard and stabilator control surfaces and excitations of aileron and rudder control surfaces of a highly modified F-15 airplane. Small, medium, and large excitations were tested in 15-second maneuvers at subsonic, transonic, and supersonic speeds. Typical excitations are shown in Figure 1. Flight test data were analyzed by use of pEst, which is an industry standard output-error technique developed by Dryden Flight Research Center. Data were also analyzed by use of Fourier-transform regression (FTR), which was developed for onboard, real-time estimation of the derivatives.

ImageFigure 2 shows results, for one of the derivatives, from 9 PreSISE maneuvers at Mach 0.75. At this Mach number, the airplane is statically unstable. The first set of data represents results from the use of small PreSISE inputs, the second set from medium inputs, and the third set from large inputs. For the derivative in question, the estimate was the same, independent of input size or analysis technique. Typically, the longitudinal derivatives were estimated with acceptably high accuracy and, using FTR, converged to final values after about 5 seconds of inputs. Some lateral-directional derivatives were not estimated as accurately, because signal-to-noise ratios were low. Efforts to optimize the inputs for increased accuracy in estimation of the derivatives are underway.

This work was done by Tim Moes and Mark Smith of Dryden Flight Research Center and Gene Morelli of Langley Research Center. For further information, contact Mr. Moes at (661) 276-3054, Mr. Smith at (661) 276-3177, or Mr. Morelli at (757) 864-4078. DRC-03-06