Experimental Effect of Liquid Sloshing on the Dynamic Behaviour of Flared Folding Wingtips

A wind tunnel experiment is presented that studies the static and dynamic aeroelastic behaviour of a "floating wingtip fuel tank". This device consists of a 'freely floating' wingtip with an additional mass attached, in the form of a liquid-filled fuel tank, to ensure an optimal lift distribution during cruise, whilst also potentially acting as a passive load alleviation device. The static aeroelastic results show that by altering the fuel tank filling level, the wingtip can float at an optimal angle for aerodynamic efficiency across a range of angles of attack. Furthermore, by altering the position of the fuel tank it is shown that the maximum fuel load can be increased whilst maintaining the optimal equilibrium angle of the wingtip. Regarding gust load alleviation, it is shown that with careful selection of the centre of mass of the wingtip, load alleviation can be achieved that is comparable to that of a wingtip without a fuel tank. Furthermore, the effect of fluid motion is shown to have a negligible effect on the response to one-minus-cosine encounters. However, reductions of up to 10% in RMS root bending moment are seen during random turbulence encounters. Such results are also confirmed by a numerical model incorporating a simple reduced order 'fluid sloshing' model. Overall, the floating wingtip fuel tank is shown to be a viable concept, which could enable longer wingspans whilst increasing the fuel capacity of an aircraft.