**2. Object and methods**

Analytic methods are not applicable for such tasks because for low-wing rotation angles, there is a high sweep angle and, therefore, a low aspect ratio (**Figure 2**). For such a type of geometry, flat section hypothesis is not correct, and more complicated methods must be used.

In view of the experimental research expensiveness, it is reasonable to determinate aerodynamic characteristics with the help of computational fluid dynamics (CFD) methods. Ansys 16.0 software was used that allows proper determining of pressure distribution throughout

**Figure 2.** UAV model geometry for wings' rotation angles equal to 30° (sweep angles equal 60°).

UAV surface and, therefore, appropriate lift and moment coefficients (**Figure 3**). Unstructured meshes were built, and the results independence of cells number was proved for every case. One wing chord equals 110 mm and maximal size of mesh elements are following;


Specificity of such UAVs is that immediately after launch, they have a flight path in which wings are turned from the position along the fuselage to flight position with sweep angles

The **goal** of this work is the determination of the aerodynamic characteristics of tube launched

Analytic methods are not applicable for such tasks because for low-wing rotation angles, there is a high sweep angle and, therefore, a low aspect ratio (**Figure 2**). For such a type of geometry, flat section hypothesis is not correct, and more complicated methods must be used. In view of the experimental research expensiveness, it is reasonable to determinate aerodynamic characteristics with the help of computational fluid dynamics (CFD) methods. Ansys 16.0 software was used that allows proper determining of pressure distribution throughout

tandem-scheme UAV after its start, at the time of wings' unfolding.

**Figure 1.** Tube launched UAVs: "Switchblade," "Sokil-2," "Trident" (in folded state).

**Figure 2.** UAV model geometry for wings' rotation angles equal to 30° (sweep angles equal 60°).

equal to about zero [4].

74 Flight Physics - Models, Techniques and Technologies

**2. Object and methods**


Total height of prism layer is 2 mm which approximately corresponds to maximal boundary layer thickness at wings' trailing edges. There was only one layer of prismatic elements, i.e., mesh was not sufficient to present shear stress and friction drag values in the boundary layer, but as lift and moment coefficients were the priority of this research, this approach that saves a great amount of time is appropriate.

Domain size equals to about 18 spans of rear wing as a larger model dimension. Mesh created in ICEM CFD software was converted to a polyhedral type that decreases calculation time greatly. Final mesh consists of 1.0–1.5 millions of polyhedral cells depending on the wings' rotation angles value.

Use of Menter's turbulence model (k-ω SST) is recommended for aerodynamic characteristics' determination at low and high angles of attack when separation occurs [5, 6]. Only a steady case was considered. A pressure-based solver was used with a coupled scheme and a second-order upwind for all available parameters. Computation model includes

**Figure 3.** UAV surface mesh with wings' rotation angles equal to 60°.

energy equations as well. Air density was defined as ideal gas, and viscosity was set according to the Sutherland model. Pseudo-transient factors and all unmentioned parameters were set on default. All UAV surfaces had roughness equal to 0.05 mm according to wind tunnel model demands. Gauge pressure of 101,325 Pa, velocity of 25 m/s, and temperature of 288 K were set. Calculation process was finished after all residuals had reached 10−4. For high angles of attack, this threshold was not always achievable, so iteration process was stopped after residuals had become minimal and force coefficients had become about constant.

The object has typical geometric parameters of tube launched UAV with equal chords of all wings and the rear wing span slightly bigger than the forward wing span. In the unfolded state, distance between leading edges of forward and rear wings (stagger) was equal to about 4 chords of one wing. Vertical gap between forward and rear wings from one side is about 70% of wing chord. Gap between left and right wings equals 10% of wing chord, i.e., slightly bigger than airfoil thickness. Wings' rotation angles of 15, 30, 45, 60, 75, and 90° were considered (equal for forward and rear wings). So wings' rotation angle of 15° means forward wing swept angle of 75° and rear wing swept angle of −75°. Reynolds number corresponded to one wing chord and is 110,000. Forces and longitudinal moment coefficients are calculated relatively to the total area of all wings, and moment coefficient is calculated relatively to sum of forward wing and rear wing chords. Moment characteristics were defined for conditional center of gravity (CG) that is placed between axes of wings rotation at the distance of 40% from forward wing axis.
