Nichoas Stathopoulos, Viresh Wickramasinghe, Devon Downes
DOI Number: N/A
Conference number: IFASD-2024-073
The horizontal stabilator (H-Stab) of a fighter aircraft develops freeplay as the control surface mechanism ages. Modal tests on a simplified model of a H-Stab structure were performed to investigate the behavior of frequency and damping ratio of the spindle and bushing interface on different levels of wear on the structural dynamics. Testing on this simplified model successfully revealed critical changes in its dynamic properties. The H-Stab structure was represented by a rectangular aluminum block (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 center of the plate to simulate different levels of freeplay, which was controlled in the order of 0.034° to be comparable with the MIL-A-8870 standard. Steel bushings were fabricated that represent three freeplay cases: tight fit, nominal clearance, and double clearance. The parameters of the first three modes of the structure were identified for each of the three bushing fits, and each configuration involved testing using two excitation loadings, namely a burst random and a sweep sinusoidal input. Comparing modal frequencies and damping ratios revealed that the spindle and bushing interface demonstrated nonlinear dynamic behavior. Horizontal bending mode showed a clear reduction in modal frequency when going from the tight bushing to the nominal bushing in the order of 3% to 4%. Vertical bending showed a 1% reduction in modal frequency resulting from the nominal bushing compared to the tight bushing and with no further change from the loose fit bushing. This suggest that this particular mode is rather insensitive to the freeplay. Torsional mode displayed a large decrease when going from the tight fit to nominal freeplay
bushing, in the order of 15% to 16%. However, going to the loose fit bushing, the reduction of modal frequency is only a further 1%. In summary, with the spindle freeplay within the MIL-A-8870 standard range, the identified modal frequencies of the H-Stab major modes were observed to decrease with increased spindle freeplay and the type of excitation. 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 resulted in freeplay at the hinges.