Influence of Different Wingtip Devices on Aerodynamic Characteristics of UAV

The wingtip device plays an important role in increasing the range of UAVs because the winglets can effectively reduce the induced drag. The purpose of this paper is to explore the influence of wingtip device parameters on the lift-to-drag ratio of UAVs. Using the Fluent analysis system, the drag reduction performance of the winglet-winglet structure based on a UAV was analyzed at different angles of attack. Firstly, from the perspective of aerodynamic performance, the optimal cant and sweep angles of the blended winglets were selected by simulations. Then, the best-blended winglet and the blended split were simulated and analyzed, and the difference between the two on the lift-to-drag ratio and the flow field around the wing were compared. Compared with the traditional wind tunnel test, the CFD technology used greatly shortens the research cost and experimental cycle and provides help for the design of the UAV wingtip work.


Introduction
Wingtip device has been widely recognized and developed as a high-lift and drag-reduction device.Winglets can effectively reduce cruise-induced resistance [1][2] and improve lift-to-drag ratio.The wingtip device has different designs in appearance, there are differences in the effect of different shapes of wingtip devices on the aerodynamic characteristics of the aircraft.At present, for large passenger aircraft, the influence of wingtip devices with different parameters on the aerodynamic characteristics of the aircraft is extensive and in-depth.Li Z K [3] simulated and compared two wingtip devices (blended winglet and wingtip vortex diffuser) which are widely used in civil passenger aircraft at present, and explored the influence of two different devices on the moment characteristics and lift-drag characteristics.Tong J H [4] used numerical calculation and simulation methods to study the effects of four different shapes of wingtip devices on the longitudinal static stability and lift-drag characteristics and of double-swept flying wings.Qiao W [5] used CFD technology to calculate and analyze the aerodynamic effects of the winglet's span length, cant angle, sweep angle, geometric torsion angle and tip-root ratio on wing lift-to-drag ratio and wing root bending moment, which provided a reference for the aerodynamic design of winglet with high aspect ratio of general aircraft.The geometric parameters of winglets include airfoil, span, cant angle and sweep angle [6].Recently, more and more UAVs have begun to install winglets to improve the voyage, endurance and economic efficiency, but there are few studies on the effect of wingtip device parameters on the aerodynamic performance of UAVs.
In this paper, the wing of a UAV is equipped with different blended winglets, and the aerodynamic simulation analysis is carried out and explore the effect of blended winglet parameters on lift-drag characteristics by CFD technology, so as to get a better blended winglet design scheme.And on the basis of the blended winglet, it is transformed into a blended split winglet, the differences between them are analyzed and compared, and the design suggestions of UVA wingtip device are given.

Numerical Model
In this paper, the numerical method is based on Reynolds average N-S(RANS) equation combined with turbulence model, ignoring external heat conduction terms, considering fluid viscosity effect.
Where: V is the velocity vector of the fluid; V is the kinematic viscosity of the fluid; f is the penetrating force vector per unit mass; ρ is the fluid density; P is the pressure where the fluid element is located.
For incompressible flow, the continuous equation of fluid motion can be simplified as ∇ = 0 (2) The turbulence model is SST k-ω turbulence model.The model can provide accurate and reliable turbulence prediction results in the calculation scenario in this paper.

Geometric Model and Mesh Model
In this paper, M6 airfoil is taken as the basic wing, and this wing and the wings with different blended winglet are drawn in CATIA software.The mode of winglet is realized by adding an extension on the wing and then adding a small radius of curvature at the connection location between the wing tip and the blended winglet, which ensures the smooth connection for the winglet-mounted wing.The design of winglet mainly involves two parameters, including sweep angle and cant angle, as shown in Fig. 1.The winglet parameter values are shown in Table 1, and nine kinds of wings with different parameters are established, as shown in Fig. 2.   The wing layout studied in this paper is symmetrical.For the sake of shortening the calculation time, half of the wing is selected for calculation and analysis.The computational domain of the fluid field is constituted of a hemispherical region with a radius of 12m and a semi-cylindrical region with a length of 14m.The computational grid of the wing is generated by polyhedral unstructured grid.To effectively simulate the boundary layer outflow field of the wing body, 15 layers of boundary layer grids are generated around the wing.At the same time, the mesh of the flow field area around the wing is made denser, and the symmetry plane and wing grid are shown in Fig. 3.

Calculation Conditions
The control equation adopts finite volume method, the second-order upwind scheme is selected for each flux discrete scheme, and the pressure-based solver combined with SIMPLE pressure-velocity coupling algorithm is selected to solve the control equation.The wing surface adopts adiabatic non-slip solid wall boundary condition, and the far-field boundary condition adopts velocity inlet and pressure outlet respectively, and the free flow velocity is 50 m/s.

The Influence of Cant angle on Aerodynamic Characteristics
Nine different blended winglets were simulated at 0, 3 o , 6 o , 9 o and 12 o angles of attack (The following is represented as AOA) to explore the influence of winglet cant angles on the lift-resistance characteristics.
Keep sweep angle unchanged, and draw the drag polars of three different cant angles, as shown in Fig. 4, so as to study the effect of winglet cant angle about aerodynamic performance.It can be seen that no matter what kind of sweep angle winglet, when AOA is smaller, cant angle has little effect on the drag polars.When the AOA is larger, for the same Cl(coefficient of lift), the Cd(coefficient of drag) with winglet is obviously smaller than the base wing.With the decrease of cant angle, the drag polars tends to move up.It can be seen from Fig. 6 (Sweep Angle 45°, AOA12°) as the cant angle of winglet decreases, the pressure difference between the lower surface of the wing and the upper surface of the wing increases.This shows that when the cant angle increases, the pressure difference decreases, which a deterioration in the aerodynamic performance and a decrease in the lift coefficient of the wing.From the curve of lift-to-drag ratio versus AOA (Fig. 5), it can also be seen that the smaller cant angle is, and the ratio is greatly improved at the same AOA.

Effect of Sweep Angle on Aerodynamic Characteristics
Nine different blended winglets were simulated at 0, 3°, 6°, 9° and 12°AOA to explore the influence of sweep angles on the lift-drag characteristics.Keep the cant angle unchanged, and draw the drag polars with three different sweep angles, as shown in the Fig. 7, so as to understand the effect of cant angle about aerodynamic characteristics.It can be seen that when the cant angle is fixed, the sweep angle has little different on the drag polaris, but with increase of sweep angle, the intersection of the drag polaris with winglet and the base wing moves to the lower left.In other words, the winglet with larger sweep angle has better aerodynamic performance than the base wing in a larger AOA range.For the winglet with smaller cant angle determined above, when the value of the sweep angle is 60°, installing this winglet can improve the ratio between Cl and Cd the most according to the curve Fig. 8.In addition, the larger sweep angle reduces the weight of the winglet from the structure and reduces the self-load of the UAV.

Comparison of Aerodynamic Characteristics of Two Wing Tip Devices
Based on the blended winglet of 20° cant angle and 60° sweep angle, the blended split winglet was established (Fig. 9).The upper and lower winglet are symmetrical, and the angle between the two is 40°.
The blended split winglet is simulated with the same calculation method and conditions as above, and compared with blended winglet.

Comparison of Lift-resistance Characteristics.
In Fig. 10, the difference of lift-drag characteristics between blended winglet and blended split winglet is shown.It can be seen that when the AOA is small, the ratio of the two wingtip devices is similar, the lift-drag ratio of blended winglet is slightly higher than that of the blended split winglet at an angle of attack of 3°, which is about 0.286% higher.With the increase of AOA, the lift-drag ratio of blended split winglet is gradually greater than that of blended winglet, and the difference of lift-to-drag ratio increases gradually with the increase of AOA, and the difference is about 14.764% at 12° angle of attack.Overall, Blended split winglet is better than Blended winglet in improving lift-drag ratio.
Figure 10.Curve of Lift-to-drag ratio versus AOA.

Comparison of Tip Flow
Characteristics.The wingtip device can dissipate the wingtip vortex.
Taking an angle of attack of 6 as an example, the flow characteristics of the wingtip with two wingtip devices are compared as shown in Fig. 11 both wingtip devices can effectively dissipate the wingtip vortex.For the trend that the airflow in the high-pressure area of the blended winglet flows along the winglet to the lower pressure force area of the upper wing, the airflow at the wingtip is relatively gentle, the streamline distribution at the trailing area of the wing is relatively uniform, and the downstream wake vortex is far downstream.For blended split winglet, the air flow from higher pressure area to lowwer pressure area appears at the upper and lower bifurcated winglets, and there is a trend of tail vortex generation at the downstream of the two bifurcated winglets, while the induced velocity directions of the two vortices at the junction are just opposite, and the interaction between them cancels each other, thus reducing the induced resistance.

Result and Discussion
In this paper, we have studied the effect of various wing with winglet configurations on the lift-drag ratio improvement over different angles of attack using ANSYS Fluent.Nine kinds of blended winglets with different cant angles and sweep angles are simulated, and their effect of them on the aerodynamic characteristics of UAVs is explored, to obtain the best parameter values.Based on the blended winglet, it is transformed into blended split winglet, and the differences between them are investigated, and the following conclusions are drawn: (1) The change of cant angle has an obvious influence on the lift-drag characteristics.The smaller the cant angle is, the more obvious the improvement of the lift-drag ratio.
(2) The change of the sweep angle has less effect on the lift-drag characteristics, but a larger sweep angle can show the advantage of the winglet's improved lift-drag ratio in a larger range of angles of attack, and it is accompanied by less load.
(3) Both blended winglets and blended split winglets can weaken the tip vortex and reduce the induced drag.At a lower AOA, the improvement of the lift-drag ratio between the two is close, and at the larger AOA, the increase of the lift-drag ratio of a blended split winglet is greater than that of a blended winglet.

Conclusion
At present, the application of wingtip device in UAV is rare, by using the different geometrical configuration of winglets it helps to increase the range of UAV.We have discussed the influence of winglet geometric parameters on the aerodynamic characteristics of UAVs and compared the aerodynamic differences between two widely used winglets to UAVs, which will provide some guiding value for the design of UAV winglets.But the design of wingtip device should be studied in more depth and multidisciplinary, for the optimization of winglet parameters of UAVs, we will choose more winglet species such as tip fence and spiroid winglet in future work.Also, more aerodynamic performance will be evaluated including wing root bending moment and the operability of the UAV.

Figure 1 .
Geometric definitions of cant angle and sweep angle

Figure 3 .
Figure 3. Symmetry plane and wing grid and partial enlargement.

Table 1 .
Values of winglet Parameter Variables