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simulation T he Saab Skeldar V200 is a rota- ry-winged unmanned aerial vehicle (UAV) in a market dominated by fixed-wing aircraft. The Skeldar does not re- quire runways and can hover in one position. Designed for land- and sea-based patrol, light transport, electronic warfare, and surveillance applications, the 4.0m x 1.3m x 1.2m UAV flies at speeds up to 130km/h with a range of 150km. Challenge Early in the Skeldar development process, problems arose in the prototype’s stability. The prototype had been developed with the aid of mathematical models, but the prototype behavior demonstrated that these models did not fully capture the UAV’s flight behavior. Accurate simulation of the flight behavior of a rotary-wing aircraft requires the ability to capture the interactions between the lifting forces on the rotor blades and downwash, which is the air deflected by the blade in producing lift. To the best of MSC’s and Saab’s knowledge, no simulation had ever managed to address this issue. Validation, solution Dr. Per Persson, technical fellow, structural dynamics for Saab, decided to use MSC Software’s Adams simulation to model Skel- dar’s flight behavior. Dr. Per Weinerfelt, also MSC’s Adams Simulation software model of the Saab Skeldar V200 rotor mechanism. X Saab Skeldar V200 Airframe Length: incl. rotor: 5.2m (17.1ft) Height: 1.3m (4.3ft) Max. take-off weight: 235kg (518 lb) Flight performance Payload: 40kg (88 lb) Service ceiling: >3,500m (11,500ft) Max. speed: 140km/h (75kts) Endurance: 6 hours Mission radius (D/L): >100km (62 miles) TOL area: 10m (33ft) diameter Propulsion system • Heavy fuel, 2-cylinder, in-line, 625cc, 2-stroke, liquid-cooled internal combustion engine • 6,100rpm • Electronic fuel injection/ignition system • 55hp a technical fellow at Saab, provided support regarding aerodynamics and inflow model- ling. Persson imported a structural model of the helicopter into Adams. Researchers modeled the two rotor blades as eight flexible bodies in MSC Nastran and incorporated their modal representations into the Adams model. Dividing the rotor blades into smaller segments makes it possible for the rigid body motion of the outer part of the rotor blade to apply forces to the inner part of the blade to more accurately model the deformation of the blade during flight. Each segment con- tains about 25 beam elements with varying characteristics. The flight study was focused on the rotor system, so the main helicopter frame is represented simply as a rigid body. The aerodynamic forces and moments acting on the UAV are calculated by a model that is implemented as a user-defined func- tion (UDF) in Adams. The actuator motion of the rotor blades and helicopter frame provide input to the aerodynamic model. A U G / S E P T 2 0 1 5 | O N L I N E A M D. C O M | 7 1