Experimental investigation into the aerodynamics of fully and partially undulated airfoils at moderate Reynolds numbers

Inflatable wings offer advantages in portability and mass efficiency for novel aerospace platforms, however, their aerodynamic performance is compromised by surface undulations and fabrication-induced trailing-edge truncations. This study provides a systematic experimental investigation into the aerodynamic effects of fully and regionally undulated surfaces, as well as fabrication-induced trailing-edge loss, on the NACA 4318 airfoil at moderate Reynolds numbers ( Re in 3 × 10⁵ ∼ 7 × 10⁵), representative of the operating regime of low-speed inflatable-winged unmanned aerial vehicles. Wind-tunnel tests were conducted on five airfoil models with different surface undulations, ranging from smooth to fully undulated cases, with and without the trailing-edge loss. This experimental design enables the decoupling of region-specific undulation effects that have not been comprehensively examined in previous studies. The results reveal a clear trade-off between aerodynamic efficiency and stall characteristics: full undulation adversely reduces lift and increases drag in the linear regime but substantially smooths and stabilizes post-stall behavior. In contrast, selective undulated configurations demonstrate the potential for performance improvement, with the upper leading-edge region identified as the aerodynamically critical zone governing lift, drag, and stall behavior. Undulations confined to the lower surface, particularly near the trailing edge, demonstrate superior lift generation with only moderate drag penalties compared to the smooth baseline. Additionally, the trailing-edge loss is shown to generally enhance the surface pressure difference for most configurations, except the fully undulated one, where the influence is minimal. The results provide new experimental evidence and practical guidance for the aerodynamic enhancement of inflatable airfoils with similar thickness and camber and suggest that strategic partial undulation can be leveraged as a passive flow control mechanism for conventional airfoils operating under moderate- Re conditions.