Gust load alleviation wind tunnel tests of a folding wing tip configuration

Increasing the aspect ratio is one way to improve aircraft aerodynamic efficiency. This reduces the induced drag term but, at the same time, produces an increment of the wing loads, hence an increase of the structural weight. This paper reports the results obtained during a dedicated experimental campaign inside the large wind tunnel at Politecnico di Milano exploring the gust load alleviation capability of a folding wing tip device. This activity has been done under the umbrella of CS2-U-HARWARD project. The high aspect ratio wing equipped with this device was mounted on a half aircraft lying on its side. The model could freely rotate around its pitching axis and a sledge allowed the entire model to plunge. An electromagnetic actuator allowed the application of a dummy weight force to the aircraft counteracting the lifting force, hence permitting the trim of the aircraft. The gusts were produced by deflecting six vanes in front of the model, with different gust lengths produced to excite different frequencies of the wing. A pneumatic actuator was used to keep the wing tip in its standard configuration and release the mechanism allowing it to fold at will. Several delays between the triggering of the gust and the release of the hinge mechanism were tested, to assess the coupling between the dynamics induced by the gust and the one induced by the release of the mechanism. Three strain gauge bridges were installed on the spar of the wing to measure the bending moment at three different sections corresponding to the root, the engine and as near as possible to the hinge. In addition, a camera tracking system composed of six infrared cameras allowed the reconstruction of the 3D motion of the wing hit by the gust and the complex dynamics of the folding wingtip. The measured wing root bending moment shows that the amount of load alleviation depends on the time when the gust hits the folding wing tip. The wing tip needs to be free to float when the gust hits it for the alleviation to be effective, but the transient of the unlocking of the hinge seems to be beneficial to the goal.