Numerical Investigation of Thrust Vectoring for a High-Aspect Ratio Nozzle of an Unmanned Combat Air Vehicle

Abstract

When observing the progress in the technology of unmanned combat aerial vehicles(UCAVs) than it can be foreseen that in future the role of manned combat aircraft will be taken over more and more by unmanned systems. In particular, the capability of long endurance flights joined with low observability can make the UCAV preferable in combat and reconnaissance missions. The needed stealth constraints have a severe impact on the design of UCAV configurations, especially on the aerodynamic shape of the UCAV and on the integration of engines and their associated subsystems. Especially the design constraints of engine nozzles and their integration in the UCAV fuselage have been tightened and drastically changed due to tight requirements in regard to infrared and radar signature. The latter is the reason why highly integrated engines, submerged air intakes and at engine nozzles are preferred solutions for UCAVs. From the infrared point of view the engine exhaust plume should possess an overall low temperature level and its characteristic temperature profile should be smeared over in a way that it can't be recognized and assigned to an engine jet by any hostile anti-aircraft missile. Therefore, high-aspect ratio nozzles are used although they show deficits in regard to the thrust level due to additional friction and redirection of flow. The strong demand for low observability leads to another design constrain: due to strong back-scattering of radar the side rudder of modern UCAV configurations has been dispensed. Consequently, this arises the problem of a limited yaw control, which eventually reduces the agility and maneuverability of the vehicle. Therefore, unconventional control effectors like mechanical and fluidic thrust vectoring devices gain more and more interest. Existing solutions for fighter aircrafts can't be used since they are made for rotationally symmetric or nearly quadratic nozzles. The challenge is to develop efficient thrust vectoring devices for high-aspect ratio nozzles under the aspect of low radar and infrared observability. In this work a CFD study is presented which investigates different mechanical and fluidic thrust vectoring approaches. Focal points of this investigation are not only the efficiency of the thrust vectoring devices itself but also their capability to replace classical split or spoiler flaps currently necessary for yaw control. Last is important in regard to minimize the detectability of the vehicle. Based on preceding CFD simulation results of 2D generic nozzle flows with different thrust vectoring concepts, selected approaches are integrated in a UCAV test configuration. The vehicle is the generic MULDICON configuration, which is well known from NATO AVT activities. An evaluation of the thrust vector control efficiency for this considered UCAV is presented under the aspect of control flap replacement for lowering observability.