Measurements of lift and ground interference on a lifting-fan wing at zero forward speed

Abstract

Measurements were made at zero forward speed of the forces acting on a lifting-fan wing system of hexagonal planform. The diameter of the wing was reduced in the course of the experiment from five times to twice the diameter of the fan. The wing acted efficiently as a shroud, and for the same power input, the maximum total lift was increased by 60 per cent over that of the fan acting alone. With the wing in position, over 40 per cent of the total was 'induced' lift, i.e., that developed on the wing alone due to a reduction in pressure on the upper surface caused by the inflow to the fan. For wings with diameter four or five times that of the fan, there is a large reduction in induced lift as the ground is approached, the loss being roughly inversely proportional to the square of the ground clearance. For a ground clearance of 0.25 the wing diameter, 25 per cent of the overall lift is lost. The adverse ground effect was reduced when the wing was set at a large angle of inclination to the ground. With smaller wings, the adverse ground effect is less, and for a wing/fan diameter ratio of 2, the loss in induced lift is counterbalanced by the small increase in the fan lift which occurs (for all values of wing/fan diameter ratio) as the ground is approached. The loss in induced lift when in proximity to the ground can be attributed to a reduction in pressure over the lower surface of the wing caused by the inflow which provides air for entrainment in the spreading jet. The deflection of the jet by the ground causes an increase in pressure at the axis of the jet which acts over the hub of the fan and increases the fan lift. Rows of parallel slats (fences) on the ground serve to partly channel the jet effiux, thus reducing the adverse ground effect. For the slats tested, with spacings equal to or larger than the fan hub diameter, the results depended critically on the transverse position of the fan axis relative to the adjacent slats.