The Longitudinal Static Stability and Control Characteristics of a Flying V Scaled Model

Abstract

Despite widespread research into the possibilities of improving aerodynamic efficiency, a plateau seems to have been reached for the conventional configuration. Hence, the potential of unconventional configurations are being investigated in recent years. Flying V is one such configuration that promises a lift to drag ratio about 24 in cruise conditions, an improvement of 25% with respect to the NASA Common Research Model that was used as conventional configuration benchmark. In addition, the flight dynamic characteristics of such an aircraft must be investigated to ensure flight safety. Sub-scale flight testing (SSFT) allows the characterization of flight dynamics using sub-scaled models. In order to mitigate the risk of loss of control situations, the static stability and control characteristics of the model must be investigated. This research work aims to support SSFT by designing a sub-scale model and assessing its aerodynamic characteristics by wind tunnel testing. The sub-scaled design is representative of a 4.6% geometrically scaled model of full-scale Flying V design based on Froude scaling laws.

To test the aerodynamics of the future flying model, wind tunnel testing have been conducted. Balance measurements of 4.6% scaled half-model have been collected in an open jet wind-tunnel. A total of three control surfaces are installed on the model and the two inboard have been prescribed, during the design phase, to provide pitching moment control authority. The shift in aerodynamic center at higher angles of attack has been registered and large deflections of the control surfaces have been noticed to influence the shift of the aerodynamic center up to 6% of the mean aerodynamic chord.

Aside of the experimental investigations, RANS simulations have been performed using the Spalart-Allmaras one equation turbulence model. Although discrepancies have been identified between the wind tunnel and numerical results, especially in terms of drag and pitching moment coefficient, the CFD results are used to get a better understanding of the influence of vortical flows on the genesis of lift and drag over the scaled model. Based on the performed CFD simulations, it can be concluded that the performed CFD simulations are insufficient to reproduce the pitching moment behavior recorded during wind tunnel testing.

The designed sub-scaled model proved to be able to substain flight loads up to more than 2.5g at MTOM conditions. Based on the performed analyses, the center of gravity is suggested to be located between 1.33 and 1.39 meters behind the nose of the configuration. The deployed control surfaces can trim the aircraft up maximum lift coefficients between 0.6 and 0.7, depending on the location of the center of gravity, with an ultimate static stability margin equal to -4.4%. The results highlight that a reduction in pitching moment control authority would cause a reductions up to 20 % on the maximum lift coefficient achievable in trimmed conditions due to lack of control authority for forward location of the center of gravity. The designed model can therefore be used for future SSFT activities and landing speeds are estimated to be lower than 20 m/s for the proposed range of center of gravity locations at MTOM conditions.