Parametric Whirl Flutter Testing of Swept-Tip Proprotor Blades

A 4.75-ft (1.45-m) diameter, dynamically-scaled proprotor with swept-tip blades was tested up to very high speeds of 205-kt (380-km/h) including the onset of whirl-flutter. Three important parameters that are difficult to vary at full-scale: hingeless hub, pylon placement, and wing spar, were examined consistent with both straight and swept-tip blades. The stability of all three wing-pylon modes: beam, chord, and torsion were measured. The in-house comprehensive analysis UMARC-II was used judiciously to shed light on the fundamental mechanisms at play and for validation. The key conclusions were: (1) on a gimballed hub, the swept-tip blade has no adverse effect on whirl-flutter, nor does it impede the mechanisms that might eliminate it, such as an aft pylon center of gravity placement or stiffer wing spar, and (2) on a hingeless hub, the swept-tip blade left the beam mode unaffected, but increased the chord>and torsion damping significantly through their interaction with the low-frequency regressive lag mode. Overall, the results demonstrate that swept-tip blades are a beneficial choice for high-speed flight when paired with an aft pylon, hingeless hub, or a combination thereof.