The influence of the tip sails shape on the wing aerodynamics in ground effect
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摘要: 翼尖帆片将原型机翼集中的翼尖涡分散成多个小涡，加快翼尖涡的耗散，从而降低机翼诱导阻力。为进一步了解翼尖帆片对机翼在地面效应下流动特性的影响，分别对安装有3片椭圆形和梯形帆片的NACA4412机翼开展了风洞实验研究。测量了2种帆片机翼的气动力和翼尖涡结构，并通过比较流动结构，分析了2种机翼气动力产生差异的原因。机翼的升、阻力用六分量盒式风洞天平测量，翼尖涡速度分布用七孔探针扫描获得，以机翼弦线为特征长度的雷诺数为1.5×105。当远离地面时，梯形帆片与椭圆帆片的升、阻力差别较小，但随着机翼逐渐接近地面，梯形帆片的增升减阻效率逐渐高于椭圆帆片。而机翼升阻力的差异，主要是由于局部气流方向角对各帆片形成的有效迎角有所差别，使得帆片对主翼产生不同的增升和减阻贡献。Abstract: The function of the wing tip sails is to scatter the concentrated tip vortices into several smaller scale vortex structures, and accelerate the dissipation of tip vortices, thereby reducing the induced drag. In order to study the influence of wing tip sails on the flow fields and aerodynamics of a wing in ground effect, wind tunnel experiment is conducted to measure aerodynamics and tip vortex structures of a NACA4412 wing fitted with three elliptic tip sails and three trapezoidal tip sails respectively, and the reason of the differences between the aerodynamic loads on the two wings is analyzed by comparing the flow fields of tip vortices. The lift and drag forces are measured using a 6-component balance, the velocity distribution of tip vortices is scanned by a 7-hole probe. The Reynolds number based on the chord length of the wing is 1.5×105. The experimental results show that the differences of lift and drag forces between the two wings increase as the wings get closer to the ground, and the trapezoidal tip sails is more efficient in lift augmentation-drag reduction than the elliptic tip sails. The local flow direction and local incidence of each sails are different for the two wings, which result in different contributions in increasing the lift and reducing the drag.
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图 1 翼尖帆片和鸟类飞行中张开的初级飞羽
Figure 1. Wing tip sails and the separation of primary feathers of birds
图 5 升力系数随迎角变化曲线图
Figure 5. Variation of the lift coefficient CL with angles of attack α
图 9 翼尖帆片局部气流及升力方向示意图
Figure 9. Schematic diagram of local flow direction at the wing tip sails
图 10 2种机翼翼尖涡静压分布随迎角变化过程及比较(α=2°、6°和12°，间隙比h*=0.15)
Figure 10. Comparison of the static pressure distributions of tip vortices for two wings at different angles of attack
图 11 2种机翼翼尖涡涡量分布随迎角变化过程及比较(α=2°、6°和12°，间隙比h*=0.15)
Figure 11. Comparison of the vorticity distributions of tip vortices for two wings at different angles of attack
表 1 机翼诱导阻力因子随间隙比的变化关系
Table 1. Variation of induced drag factor with the gap ration
(1+δ)/πλ h*=0.15 h*=0.3 h*=1.0 梯形帆片 0.069 0.082 0.113 椭圆帆片 0.080 0.089 0.114 
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表 2 翼尖涡涡通量在各迎角下的比较(ω为涡量，S为面积)
Table 2. Comparison of tip vortex flux at different angles of attack
涡通量/(ω·S) α=2° α=6° α=12° 椭圆形帆片 8.26 15.83 32.12 梯形帆片 9.93 15.38 30.44
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