Influence of Powered Lift Systems on the Aerodynamics of a Turboprop Aircraft

Kurzfassung

The purpose of the present work is to evaluate the influence of active lift augmentation systems, in terms of circulation control and slipstream deflection, on the aerodynamic behavior of a state-of-the-art transport aircraft with short take-off and landing (STOL) capabilities. Both technologies individually affect the aerodynamics of the main wing in order to increase lift generation. Using both technologies together leads to additional integration effects. Moreover, the influence of circulation control and slipstream deflection is not limited to the main wing. Due to the alteration of the main wing's circulation distribution, the main wing wake is affected as well. The flow conditions, and thus the aerodynamic behavior of the tail, are therefore changed. The changes in the aerodynamic behavior of the main wing, and in particular the tail, can have a significant impact on the controllability and stability of the entire aircraft, as observed in experiments by NASA in the 1960s. The present work provides an aerodynamic data base that enables thorough flight dynamic analyses of the investigated aircraft configuration. As the root cause of the change in aircraft behavior mostly remained unknown in past investigations, the work also aims at identifying aerodynamic phenomena that lead to the effects of circulation control and slipstream deflection on the controllability and stability. In order to investigate these effects on the aerodynamic behavior, numerical simulations of the entire aircraft in landing configuration were carried out based on the Reynolds-averaged Navier-Stokes equations. The present investigation indicates a maximum lift coefficient in landing configuration of up to 4.52 for an aircraft utilizing both circulation control and slipstream deflection to enhance low-speed performance. With the propellers off, the maximum lift coefficient is degraded due to adverse engine integration effects. The nacelle thereby induces strong vortices, which interact with the circulation control downstream, causing local wake bursting. The analysis of the longitudinal motion further indicates that the effect of circulation control and the propellers on the static, as well as dynamic stability, is rather small. While circulation control slightly increases the stability, propeller effects tend to decrease the stability. In lateral motion, the influence of circulation control and the propellers has been found to be more severe. Directional stability, as well as the dynamic yaw damping derivatives, are highly non-linear, depending on various parameters such as circulation control mode, thrust, and freestream flow conditions. Both circulation control and the propellers may cause a significant decrease in stability under certain conditions. Propeller effects may even lead to directional instability and reversed yaw damping derivatives. Wake-fuselage interaction has been found to be the origin of this behavior. While the inner flap tip vortices play the dominating role concerning the decrease in stability in the case with circulation control and with propellers off, complex flow interactions of the inner flap tip vortices with the propeller slipstream cause the more severe degradation of stability, if the propellers are on. Circulation control also reduces the lateral static stability (dihedral effect), whereas the effect of the propellers depends on various parameters such as rotational direction of the propellers and sideslip angle. Besides additional influences of circulation control and the propellers on the roll damping and the other lateral dynamic derivatives, the investigation also revealed cross-coupling effects between lateral and longitudinal motion such as the creation of a considerable pitching moment due to crosswind.