Buffet envelope prediction of transport aircraft during the conceptual design phase

Abstract

During conceptual design a problem arises when predicting the buffet onset boundary. Due to the pressure on payload-range and cruise altitude capability, improvement on the buffet onset boundary is often of great importance. It is one of the primary constraints in establishing the low and transonic speed performance capabilities of transport aircraft. Buffeting, a high-frequency instability caused by airflow separation or shock wave oscillation, can be seen as a random forced vibration. Depending on the angle of attack and freestream velocity, the separations in the flow can result in an aerodynamic excitation. The main motivation for this thesis research is to create a more advanced but fast transonic buffet onset prediction tool to permit greater design freedom during the conceptual design phase. This implies this tool should be faster than conventional tools, it should be reliable and able to deal with unconventional configurations. In addition, it should be built in a modular way so it is easy to use, alter and replace parts of the tool. The transonic buffet prediction tool was demonstrated using the Fokker 100 wing-fuselage combination test case. A modular transonic buffet onset prediction tool was successfully developed by combing a Vortex-Lattice method, 2-dimensional Euler code, a critical pressure rise separation criterion and the Matrix-V code program. It is approximately 90% faster with respect to the use of only the Matrix-V code and it is reliable in the region left of the coffin corner at high CL and low Mach number combinations. The expected error in the regime which can be correctly predicted by this tool is in the order of 0.05 With respect to further use of this tool, it is most interesting to see how this transonic buffet prediction tool behaves when less conventional wing geometries such as a flying wing, blended wing body, or Prandtl plane are tested.