Experimental characterization of underwing air injection on lift and drag during water-to-air transition

The wings of waterborne ground effect vehicles experience transition from submerged operation during takeoff to airborne flight near the free surface. This experimental study investigates a hybrid lifting surface using underwing air injection to augment lift across the early stage of this water-to-air transition. A reverse-delta wing model was tested in a free-surface water tunnel over angles of attack from −2° to 8°, vertical positions from 16% chord depth below the surface to 8% chord height above it, air injection rates corresponding to exit Mach numbers from 0 to 0.41, and Froude numbers from 0 to 0.7. Results show that air injection produces distinct force-generation regimes. At zero speed, lift augmentation is dominated by cavity-induced pressure loading beneath the wing. Increasing Froude number shifts cavity venting toward the trailing edge, reducing cavity depth and modifying lift and drag behavior. At sufficiently high injection rates and forward speeds, direct jet thrust contributions can offset cavity-induced drag, resulting in reduced net drag, or forward thrust. Flow visualization reveals unsteady cavity growth, collapse, and venting underpinning these force trends. Results demonstrate that underwing air injection can provide submerged lift augmentation while reducing hydrodynamic resistance during near-surface transition, offering a mechanism for improved beginning takeoff performance of waterborne vehicles.