Design of an Aerodynamic Measurement System for Unmanned Aerial Vehicle Airfoils

This paper presents the design and validation of a measurement system for aerodynamic characteristics of unmanned aerial vehicles. An aerodynamic balance was designed in order to measure the lift, drag forces and pitching moment for different airfoils. During the design process, several aspects were analyzed in order to produce an efficient design, for instance the range of changes of the angle of attack with and a small increment and the versatility of being adapted to different type of airfoils, since it is a wire balance it was aligned and calibrated as well. Wind tunnel tests of a two dimensional NACA four digits family airfoil and four different modifications of this airfoil were performed to validate the aerodynamic measurement system. The modification of this airfoil was made in order to create a blowing outlet with the shape of a step on the suction surface. Therefore, four different locations along the cord line for this blowing outlet were analyzed. This analysis involved the aerodynamic performance which meant obtaining lift, drag and pitching moment coefficients curves as a function of the angle of attack experimentally for the situation where the engine of the aerial vehicle is turned off, called the no blowing condition, by means of wind tunnel tests. The experiments were performed in a closed circuit wind tunnel with an open test section. Finally, results of the wind tunnel tests were compared with numerical results obtained by means of computational fluid dynamics as well as with other experimental references and found to be in good agreement. INTRODUCTION According to the evolution of unmanned aerial vehicles, commonly referred to as UAVs, several investments have been increasing every year, especially in the field of aerodynamic characteristics which can be obtained through wind tunnel tests. In the Laboratory of Fluid Mechanics at the Czech Technical University in Prague, particularly in the branch of aerodynamics, a measurement system to get aerodynamic data is needed. Based in the methodology of A. Suhariyomo et al. and J. Barlow et al., this paper presents the design of an aerodynamic characteristic measurement system for UAVs which consists mainly in the design of an aerodynamic wire balance. The balance and the wind tunnel complete the data acquisition system which is verified to assess whether the measurement system is suitable for measuring the aerodynamic performance of different airfoils. The verification was performed with a two dimensional NACA 2415 airfoil, comparing it with reference results in [1,6] and [7], measuring lift, drag forces and pitching moment. In order to go further in the validation of the measuring system, four modified models of this base airfoil were made and tested in the balance; the modification consisted in a step located in the suction surface of each airfoil at 30, 40, 50 and 60 percent of the cord. Lift, drag and pitching moment coefficients versus the angle of attack were obtained for all the models tested and were compared to numerical results obtained in [7]. The whole process is described in the following sections. DESIGN OF THE BALANCE This aerodynamic balance was designed for testing the performance of different airfoils for unmanned aerial vehicles measuring lift, drag forces and pitching moment, special attention was paid in this stage because since UAVs fly at low speeds, the magnitude of the forces will be very small, therefore important accuracy is required [5]. Description The structure of the balance was designed according to the dimensions of the open wind tunnel test section using commercial CAD software; the selected parts for the frame were aluminum profiles due its weight and easy connectivity between the other parts as seen in Figure 1. The balance is composed of six high precision digital hanging scales with a capacity of 50N and a resolution of 0,02N located according to Figure 2. Figure 1. Structure of the aerodynamic balance. Six forces are measured in scales A, B, C, D, E and F. The wires attached to A and B are parallel to the incoming velocity vector and define a plane which is taken as a reference plane for the balance (x-y plane), these wires point in the x direction. Figure 2. Location of scales in the aerodynamic balance. The wires attached to C and D are in a plane that is perpendicular to the x-y plane, which is designate the y–z plane. Wires containing A and C are attached to a common point on the left side of the wing. Wires SYSTEMICS, CYBERNETICS AND INFORMATICS VOLUME 10 NUMBER 5 YEAR 2012 39 ISSN: 1690-4524 containing B and D are attached to a common point on the right side of the wing. Finally wires attached to E and F are parallel to those ones attached to C and D. The airfoil is attached to two endplates of transparent polycarbonate as shown in Figure 3, these endplates have the main function of reducing considerably the strength of the tip vortices and the induced drag by blocking the leakage around the wing tips [2]. Figure 3. Endplate detail The balance can produce angle of attack (AOA) changes from 0 to 16 degrees manually with the smallest increment of 2 degrees. MEASUREMENT SYSTEM APPARATUS The measurement system includes three components, a low speed wind tunnel system, the aerodynamic balance and data processing software. Wind Tunnel System The closed-circuit wind tunnel has an open test section of 750 x 550 mm cross section. It is assembled from straight parts of a closed-return passage with rectangular cross section, elbows with corner-vanes, a rectangular settling chamber and a nozzle. The honeycomb and two screens are placed in a closed-return passage as shown in Figure 4. The maximum air velocity of 17 m/s can be obtained in the test section. A 55 kW three-phase induction motor and a frequency changer are coupled with fan. The wind tunnel has a turbulence intensity at a velocity of 7,5m/s of 1,3% [11]. The flow velocity was measured directly with an anemometer vane type. Figure 4. Low speed wind tunnel. Data acquisition and processing The procedure for the acquisition and processing the data is in agreement to the procedure for wire balances, attaching the model in an inverted position (upside down) so that aerodynamic lift added to the weight to prevent unloading the wires as the resulting tension can never be allowed to diminish to zero as reviewed in [6]. Since the horizontal wires A and B cannot transmit bending, the vertical force perpendicular to the flow velocity vector, the lift, is obtained from the sum of the forces in the vertical wires: