Concept feasibility was verified by URANS sliding mesh CFD simulations using the commercial software, Fluent. Flow patterns around the proposed airfoil concept, along with global performance data, are shown in Fig. 51 at 0° angle-of-attack and advance ratio of J=0.46 (φ=0.23). Observe the large difference in flow patterns between the cruise and STOL geometrical configurations. The cruise configuration generates much larger thrust (or negative drag coefficient) than the STOL configuration at the expense of lift.
The standard measure for comparing aerodynamic efficiency of 2D airfoils is the ratio of lift to drag. However, for the propulsive airfoil design presented here, the conventional lift-to-drag ratio is not meaningful because it varies greatly depending on the fan power input. Casparie and Dang [22] defined the concept of an equivalent lift-to-drag ratio (L/Deq) commonly used by the powered-lift community [67] and showed that (L/Deq) on the order of 10 is achievable in the cruise condition, while (L/Deq) on the order of 20 can be reached in the STOL configuration. In their definition, the equivalent drag Deq is the sum of the physical drag force on the propulsive wing, along with a drag force defined as (QΔPt)/U∞, the ideal power required by the cross-flow fan to raise the total pressure of the air stream by the amount of ΔPt at a flow rate Q, normalized to the flight speed U∞.
A unique concept that has received recent publicity is the “FanWing” patented by Peebles [16]. Several generations of model airplanes based on the FanWing concept have flown successfully. A recent version is shown in Fig. 52, and general information about the concept is available on the company web site (www.fanwing.com). In the FanWing concept shown in Fig. 53, the aerodynamic lift generating device consists of a large “unconventional” cross-flow fan positioned in place of the leading-edge of a thick airfoil, forming a unique configuration. In particular, while the lower portion of the cross-flow fan is shrouded, the upper portion (adjacent to the airfoil suction side) is open to the surrounding ambient air. In other words, referring to Fig. 10, while the vortex wall is retained, the rear wall is removed. The rotational direction is such that the exposed blades move aft toward the trailing edge of the airfoil (in the counter-clockwise direction as shown in Fig. 53). One feature of the device is that the lower shrouded housing terminates with a moveable lip or flap, which partially controls the eccentric vortex. The ability of the lip to control vortex size and location affects the local sectional lift and thrust. Therefore, the lip can be used as a movable flap to control both lift and thrust in a 3D configuration, allowing for yaw and roll controls.