Stability Analysis of a Segmented Free-wing Concept for UAS Gust Alleviation in Adverse Environments

Abstract

High altitude, long endurance (HALE) aircraft feature large wing spans and have very low wing loadings resulting in sensitivity to turbulence. While turbulence is usually quite low in the stratosphere where HALE aircraft typically operate, even high altitude aircraft must transition through the lower atmosphere during takeo and landing operations. Sensitivity to turbulence may restrict the weather conditions under which HALE aircraft can be launched or retrieved. A compounding consideration for HALE aircraft is that because of their large wing spans, their wings may be longer than the length scale of the turbulence they encounter. This means that di erent portions of the aircraft's wings will see di erent aerodynamic conditions and will result in signi cant additional structural loads on the wing structure. Alleviating the aircraft's response to time-varying gust elds as well as spatially-varying gust elds is thus important for HALE aircraft. One promising technology for gust alleviation is the \free wing". A free-wing design allows the wing to adjust itself in pitch about a spanwise axis in response to aerodynamic loads rather than being rigidly attached to the aircraft fuselage. Free wings historically have shown the ability to reduce an airplane's response to turbulence. An extension of the concept proposed here is called a \segmented free wing". A segmented free wing di ers from the conventional free wing by sectioning the wings into multiple, independent segments. This design provides a greater reduction in turbulence response than both the standard free wing and the xed wing as demonstrated in initial wind tunnel tests. A conceptual design of such a planform along with a study of its stability characteristics was examined. Initial results from a wind tunnel model showed a reduced rolling moment coe cient when compared to a traditional free-wing design. Experimental tests of the larger model showed a divergent oscillatory mode that appears with increasing velocity. An analytical model of the experimental test was developed and successfully predicts the instability. Comparison of the analytical model versus the experimental results shows an over-prediction of the stability of the system by the analytical model and causes for the over-estimation were investigated. The e ects of unsteady aerodynamics, apparent mass terms, and wake e ects on the analytical model were studied and all were determined to signi cant in the aerodynamic model. The analytical model was used to predict the crossover velocity of a wind tunnel model but the wind tunnel model failed to become unstable due to the stabilizing friction force in the bearing surfaces.