Small Signal Compensation of Magnetic Fields Resulting from Aircraft Maneuvers

Tolles and Lawson identified three permanent, five induced, and eight eddy-current fields as sources of magnetic interference associated with airframe maneuvers. Small signal approximations are used here to separate the eddy-current terms and thus decouple the sixteen equations into two sets of eight equations. It was found that a singularity exists in the small signal equations for the permanent and induced terms. This causes an ambiguity amoung three of the coefficients which can be resolved mathematically by resorting to large maneuvers. Flight test data exhibit a large amount of magnetic hysteresis and the magnetic anomality detector (MAD) equipped aircraft will not remain compensated from one flight to the next. This complicates the problem of resolving the dip angle ambiguity. Furthermore, significant differences exist in the compensation terms as determined from manual pilot maneuvers and the terms when the maneuvers are performed by the automatic pilot. This has been attributed to differences in altitude stability and the frequency content of the maneuvers. It was found that both optimal frequency filtering and altitude compensation can be used to improve the figure of merit (FOM) resulting from pilot maneuvers.