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Session: Poster session & drinks
Session starts: Tuesday 18 June, 18:00
Presentation starts: 18:07
Room: Room 1.1

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Nichoas Stathopoulos (L3Harris)
Viresh Wickramasinghe (NRC)
Devon Downes (National Research Council Canada)

Abstract:
The horizontal stabilator (H-Stab) of a fighter aircraft develops significant freeplay as the control surface mechanism ages. Modal tests on a simplified model of a H-Stab structure have been performed to investigate the nonlinear dynamic behavior of the spindle and bushing interface related to varied levels of wear. Extensive testing on this simplified model successfully revealed the critical changes in its dynamic properties. The H-Stab structure was represented by a rectangular aluminum block (labeled exciter plate), attached to a steel rod as a spindle fixed to a mast. A set of bushings with varying diameters was used at the inner edge of the plate to simulate different levels of freeplay, which was controlled in the order of 0.034° to be comparable with MIL-A-8870 standard. Steel bushings were fabricated that represent three freeplay cases, the tight fit, nominal tolerance, and double tolerance. The modal parameters of the first three modes of the structure were identified in three test configurations, and each configuration involved testing using two excitation loadings, namely burst random and swept sinusoid input. Each load type included several input levels. Comparing modal frequencies and damping ratios revealed that the spindle and bushing interface demonstrated nonlinear dynamic behavior. Relative to the tight fitting bushing, the nominal tolerance case experienced a frequency reduction of between 4 to 17 % in the first three modes, while the damping ratio, moved from 0.99% to 1.56% for the first mode. The double tolerance case exhibited frequency reductions of between 1 to 17%, while the damping ratio increased from the original value of 0.99% to 1.72%. In summary, with the spindle freeplay within the range of MIL-A-8870 standard, the modal frequencies of the H-Stab major modes decreased with increased spindle freeplay and increased input load levels, while the damping ratio increased with increased spindle freeplay and increased input load levels. These results indicate that tracking the modal parameters of the H-Stab during the aircraft’s service may be a promising approach in determining when the structure has undergone excessive wear.