Ground-based simulation of complex maneuvers of a delta-wing aircraft

Abstract

In the process of aircraft development numerical approaches are gaining more and more importance. Not only steady flight conditions need to be modelled but also dynamic derivatives and last but not least realistic flight maneuvers. In particular in the case of delta-wing aircrafts a small change in the flight conditions can strongly influence the vortex dominated flow field about the wings and thus result in large changes of the aerodynamic loads. Numerical tools that are developed for predicting such behaviours need to be validated by experimental data. In order to obtain a data base for validation ground-based simulations of complex maneuvers of a model of the X-31 aircraft have been performed in the low-speed wind tunnel NWB of the German-Dutch Wind Tunnels DNW. In the wind tunnel tests a newly installed novel test rig with six degree of freedoms (DOF) was used for the first time for moving the model. Furthermore, the model was equipped with eight remotely controlled moving flaps. In this manner realistic flight maneuvers could be reproduced in a ground-based facility. Both, the specific technical equipment of the model and the novel six DOF test rig will be reviewed. Thereafter, experimental results obtained will be discussed and compared with numerical results. The X-31 is a single-engine, single-place cockpit, delta-wing aircraft. For control the aircraft had a small, forward-mounted canard; single vertical tail with conventional rudder, wing leading flaps and trailing-edge flaps (elevons). A fully equipped wind tunnel model of the X-31, the so-called X-31 remote-control model, was developed and built to a scale of about 1/7.25 at the German Aerospace Center DLR (cf. Fig. 1). The model is made from steel and carbon fiber reinforced plastic. Its control surfaces can be moved via a remote control system. The main part of the X-31 model is a wing-fuselage section including eight servo motors for changing the angles of canard, leading-edge inner and outer flaps, trailing-edge flaps and rudder. Dynamic surface pressures are measured by miniature piezo-resistive pressure sensors located at 60% and 70% chord length on the upper surface of the delta wing and on the leading edge flaps. Forces and moments are obtained by a 6-component strain gauge also included in the main part of the model. Data are transferred back and force between the model and the external data acquisition system by a 64-channel telemetric system. The transfer rate of the telemetric system is about 3 kHz. The novel configuration of the six DOF dynamic test rig used in the present tests for simulating real flight maneuvers was developed at DNW for its low-speed wind tunnel NWB located in Braunschweig and is called a “Model Positioning Mechanism” (MPM) hereafter. The MPM is based on the concept of a Stewart platform. This platform is linked to the wind tunnel fixed base by six constant-length struts that are connected to six carriages which can move along two parallel guiding rails so that the position and orientation of the platform is adjusted (cf. Fig. 1). The six carriages run independently of each other on the guiding rails thus allowing a displacement within all six degrees of freedom. Because each guiding rail is shared by three carriages, the design is simplified and has fewer components than conventional systems. The six linear motors used for moving the carriages allow accelerations up to 2.5 g. The workspace spans 1100 mm in the flow direction, 300 mm in the lateral direction and 500 mm in the heave direction. The range of pitching or rolling motions can be enlarged by an additional actuator on the MPM and a corresponding joint between the ventral sting and the internal balance. The accuracy of the system, for example, in pivoting angles is better than 0.005°. At the top of the sting the first eigenfrequency is above 20 Hz. The MPM allows for a payload of up to 5000 N. The complex three-dimensional motion of the model is controlled by an optical position tracking system. All measurements were performed in the open test section of the 2.85 X 3.20 m2 low speed atmospheric wind-tunnel NWB of DNW. The X-31 remote-control model was connected to the MPM by a belly sting (cf. Fig. 1). Already during the commissioning phase of the MPM complex maneuvers were successfully simulated. An example based on a real flight maneuver corresponding with steady-heading sideslip test points is shown in Fig. 2. On the left side, the variation in time of the angles of pitch, yaw and roll that were performed by the MPM and the corresponding motions of the flaps that were realized by the remotely controlled servo motors in the model, are shown. As an example, the coefficients of the lateral force and the rolling moment resulting from this maneuver are shown on the right side of Fig. 2. In the same manner many more scientific data have been gathered that are to be used for validating a numerical simulation framework that is under development at DLR for calculating a freely flying maneuvering combat aircraft. In the numerical approach a maneuver is realized by a time-accurate coupling of aerodynamics, structural mechanics and flight mechanics. Results of first numerical simulations will be compared with experimental data obtained under unsteady flow conditions. Thus, a ground-based simulation method has been successfully developed and tested that provides the possibility of simulating complex maneuvers in a subsonic facility.