Aeroelastic Roll Control for High Aspect Ratio Wings

Abstract

Modern aircraft tend to be light and slender. The high aspect ratio reduces the induced drag, increase fuel efficiency, and provide enough lift a low speeds. Therefore such wing types are used for long endurance unmanned aerial vehicles (UAV). A drawback of these wings is that they are more vulnerable for aeroelastic phenomena like divergence, aileron reversal, and flutter. To increase the dynamic pressure, at which one of these phenomena occurs, the stiffness of the wing needs to be increased leading to an increase of the wing weight. Instead of increasing the wing weight, the flexibility of the wing can be used to change the shape of the wing to optimize flight performance or to control the rolling motion of the aircraft. This research area of Active Aeroelastic Wings has grown in the recent years. With this concept, very little control surface motion is used to employ the energy of the airstream to achieve the desirable wing twist. In this thesis work, an Active Aeroelastic Wing is designed, built and tested in a low-speed wind tunnel. After testing, the possibility to predict the performance accurately with a state-of-the-art modeling tool is verified. First, a qualitative background overview is given about the three most important aeroelastic phenomena. These phenomena are torsional divergence, control reversal, and classical flutter and are described to understand the effects of the different wing parameters on the deformations. Secondly, an overview is given of the techniques that have been used to change the shape of the wing. From these concepts, the sweepable spar method has been chosen. This method uses the ability to change the position of the elastic axis, so that the twist deformations can be increased or decreased. Based on this method, a test article has been built that has similar planform shape as the Global Hawk. Instead of ailerons for roll control, the test article is equipped with an aeroelastic outer wing segment that can change its elastic axis. With this aeroelastic outer wing part, the wing is able to provide a rolling moment. Afterwards, its performance has been tested in the Open Jet Facility of the TU Delft. The results of the bench tests showed that the moveable spar is able to change the position of the elastic center at the wing tip. With the main spar at different positions, a change in the torsional stiffness of the wing has been observed. During the wind tunnel test the influence of the sweep angle on the rolling moment has been analyzed. The results did not clearly indicate a beneficial effect of the sweep angle on the rolling moment coefficient. Therefore the wing configuration is set to sweep angle of zero degrees to continue the testing. For this configuration, the main spar moved aft on the left and forward on the right wing, the maximum achievable roll helix angle is similar to that of a Boeing 747-200 during cruise. However, the roll performance of the Global Hawk is about five times larger than the test article. Note that aeroelastic effects have been neglected for the Global Hawk, which reduces the aileron control effectiveness. Finally, the results of the wind tunnel experiments are compared with the simulation results of Proteus. Proteus is an analysis and design framework that uses a non-linear beam model combined with lifting line theory to determine the aeroelastic effects. To analyze the test article some structural and aerodynamic modifications have been made in Proteus, to integrate the effects of a sweepable elastic axis and cambered airfoils. With these modifications the predicted rolling moment coefficient was about 40% smaller compared to the wind tunnel results. Therefore it is concluded that Proteus is currently not able to predict the results accurately.