Dai Q.   Sun C.  

Experimental Investigation on Aerodynamics and flow structures of a Wing with Tip Sails in Ground Effect

Reporter: Dai Q.

The experimental results of a wing with tip sails in ground effect are introduced in the paper.
The experiment is conducted in a low speed wind tunnel, the aerodynamics and tip vortices of a rectangular NACA4412 main wing mounted with 3 tip sails in the vicinity of the ground are measured. In this experiment, the angle of attack α is varying from -4°to 25°, and ground clearance is changing from 10% to 100% of chord length. The Reynolds number based on the chord length of the main wing is 1.3×105. The lift/drag forces and pitching moment are measured by a three-component transducer, and the wing tip vortex structures are measured using a seven-hole probe, which scans 3 cross sections normal to the free stream in the weak flow.
The experimental results compared to a NACA4412 wing show that, for the same ground clearance, the lift force of the wing with tip sails increases significantly and the stalling angle of attack decreases, and the drag force increases slightly at small angle of attack while it decreases obviously at higher angle of attack. Meanwhile the pitching moment increases as well. For the same angle of attack, when the wing is getting closer to the ground, the lift force increases and the drag decreases slightly. In addition, negative lift force is not observed for the wing with tip sails at the minimum angle of attack in present experiment. However, for the prototype NACA4412 wing, it occurs to negative lift when the angle of attack is lower than -3°, it implies that the tip sails decreases the negative lift angle.
Figures 1 and 2 are the vorticity contours of tip vortices of two types of wings. As shown in figure 2, the tip sails break the concentrated tip vortices into several weak vortices, and the intensity of the tip vortices is much lower compared to the prototype NACA4412 wing. Compared figure 1 with figure 2, it shows that the tip sails have observable efficiency in weakening the tip vortex and may reduce the induced drag.

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