Wind tunnel investigation of a flexible wing high-lift configuration with a variable camber continuous trailing edge flap design

This paper presents wind tunnel experimental results of a flexible wing high-lift configuration with a variable camber continuous trailing edge flap (VCCTEF) design for drag minimization, tested at the University of Washington Aeronautical Laboratory (UWAL) in July of 2014. The objective of the high-lift test in UWAL is to assess the high-lift performance of the VCCTEF. The wing bending stiffness is tailored to achieve a wing tip deflection of about 10% of the wing semi-span at 1-g flight conditions. The VCCTEF is a multi-segment flap design having three chordwise camber segments and five spanwise flap sections for a total of 15 individual flap elements. The high-lift design includes a Variable Camber Krueger (VCK) leading edge device and an inboard high-lift trailing edge flap with a Fowler motion. Two inboard high-lift flap configurations are tested: a single-element plain flap and a three-segment cambered flap. The outboard VCCTEF is rigged at varying flap deflections of up to 30° formed by a circular arc camber and has no Fowler motion. A premature flow separation associated with the initial configuration of the VCK leading edge device, as indicated by an abrupt stall, was encountered during the initial runs. A final VCK configuration was found experimentally with a varying rigging angle from 65° at the inboard to 50° at the outboard. Wind tunnel test results indicate a CL_max = 2:13 is achieved for a wing-body configuration with the single-element plain flap versus CL_max = 2:09 with the cambered flap. This CL_max is close to the desired CL_max for a typical Boeing 757 landing configuration. The cambered flap achieves a L=D improvement by 6% over the single-element plain flap due to the reduced profile drag with the cambered flap. Sensitivities due to VCCTEF spanwise deflection shapes, combined Reynolds number/aeroelastic effect, and Fowler slot width were studied. Reynolds/aeroelastic and Mach number corrections were performed to estimate the values of CL_max at flight Reynolds and Mach numbers. The corrections result in estimated CL_max = 2:22 for the plain flap and CL_max = 2:19 for the cambered flap, roughly about 4% increase. An aeroelastic analysis was performed to analyze the bending deflection measurements from an optical VICON system and the torsional twist data computed from the VICON measurements. The bending deflection measurements show very good data consistency, but the computed torsional twist data exhibit high degree of scatter. The wind tunnel test confirms that the high-lift design for the VCCTEF is capable of providing high-lift performance for transport aircraft. The test results also confirm the potential drag reduction benefit for the three-segment cambered flap as compared to the plain flap.