High-lift airfoil flap design in XFLR5

Project · Project 21

Design and optimization of a high-lift configuration for a NACA 23015 airfoil using XFLR5, targeting a ≥ 55% increase in maximum lift coefficient under realistic aerodynamic constraints: Re = 3×10⁶, flap deflection ≤ 40 degrees, flap chord ≤ 25%.

Method

The numerical setup was first validated by comparing lift, drag and pitching moment coefficients against experimental data from Theory of Wing Sections, achieving strong agreement across a wide angle-of-attack range. Validating before optimizing is what makes the subsequent numbers worth anything.

A parametric study of trailing-edge flap deflection was then performed, sweeping 0 to 40 degrees.

Results

The trailing-edge sweep demonstrated increased lift and earlier stall onset, reaching a maximum lift coefficient of c_l = 2.25 (+32%).

To improve further, an alternative dual-flap configuration was developed — leading-edge deflection at 27 degrees combined with trailing-edge deflection at 40 degrees — achieving c_l = 2.55, about a 50% increase and approaching the design target.

The study included detailed analysis of the aerodynamic trade-offs: increased drag and more negative pitching moments with higher flap deflection, as well as pressure coefficient distributions showing strong gradients and discontinuities at the flap hinge locations. Results were further validated using thin airfoil theory, linking the camber modification back to lift generation.