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Quad Tilt Rotor Simulations in Helicopter Mode using Computational Fluid Dynamics
Baeder, James D.
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The flow field around a simplified Quad Tilt Rotor (QTR) vehicle is simulated using computational fluid dynamics (CFD) for various low speed flight conditions in helicopter mode. A time-averaged rotor model is utilized, where the velocity field computed by CFD is coupled to blade element theory and a trim model to provide an equivalent time-averaged body force term in the compressible Navier-Stokes equations, instead of moving overset meshes; reducing the computational time while capturing the essential physics. Overset meshes are used to model the complicated geometry of the simplified aircraft fuselage and wings in order to ensure good resolution of viscous effects. The solution of the compressible Navier-Stokes equations are suitably modified using low Mach number preconditioning to properly scale the dissipation and enhance convergence. This approach is validated for the current work by comparison with experimental data for the downwash velocity underneath an isolated tilt rotor system as well as for the pressure distribution resulting on the surface of a single wing placed underneath such a tilt rotor system. A total of 8 grids with approximately 5.2 million grid points is then employed to simulate half of a simplified QTR geometry for a range of flight conditions. A high download (9% of thrust) is obtained in hover, as expected, when the QTR operates Out of Ground Effect (OGE), primarily from a strong download on the front and rear wings. A detailed analysis of the calculated flow field, along with chordwise pressure distributions and spanwise loadings on the wings, is performed to explain the observed decay in download on the vehicle with an increase in the forward flight speed. The high download obtained OGE in hover, becomes a strong upload (9% of thrust) when the vehicle operates In Ground Effect (IGE) with the wheels placed on the ground; primarily from a strong upload on the fuselage and inner portion of the rear wing. Upload observed IGE in hover gradually fades away with an increases in forward flight. An increase in forward flight speed eventually results in the flow along the ground unable to travel far upstream; the simulation shows the expected horseshoe shape of the wake near the ground. The simulations suggest that the uploads obtained IGE persist for high enough forward flight speed such that a significant increase in payload should be feasible for rolling takeoffs.