On the Effect of Sideslip Angle on the Behavior of Flared Folding Wingtips

The flared folding wingtip (FFWT) is an emerging concept that offers potential benefits in reducing airframe loads during maneuvers and gust encounters, while also enhancing aircraft handling qualities. FFWT performance is strongly influenced by the orientation of the hinge line relative to the incoming flow, making the system particularly sensitive to nonzero sideslip angles, such as those encountered during crosswind landings. This paper investigates the static and dynamic behavior of FFWTs under sideslip conditions using a custom-designed wind tunnel model capable of large wingtip deflections, alongside a geometrically nonlinear numerical model. Results demonstrate the existence of stable equilibrium configurations up to, and beyond, a 90 deg fold angle, even when the effective flare angle becomes zero or reverses sign. Assuming that wingtip contact with the inner wing bounds the safe flight envelope, the maximum achievable sideslip angle is shown to be approximately equal to the flare angle but is also dependent on angle of attack and wingtip twist (or equivalent aerodynamic camber). These effects introduce additional unstable equilibrium branches at sideslip angles up to 5 deg below the flare angle, narrowing the viable operational envelope. Finally, it is shown that accurate prediction of frequency variations with sideslip requires accounting for additional geometric effects, such as changes in wingtip sweep, which reduce flutter speed and may further constrain the flight envelope.