Aerodynamic Study in a Wind Tunnel of a BiCP-VTOL UAV for SAR Missions

This article presents wind tunnel experiments conducted on a Bi-rotor Convertible Vertical Take Off and Landing (BiCP-VTOL) Unmanned Aerial Vehicle (UAV) in order to characterize the entire full aerodynamic flight envelope. The motivation behind these experiments come from the need of having precise aerodynamic models that will serve as benchmark where to test the innovative design presented within the EMERGENTIa (acronym for the full title: DevElopment or fan unManned convERtible aircraft for rapid and efficient deployment in emerGENcy situationTIons) project.

In this paper, two different VTOL-UAV models are studied, being the only difference the forward sweep, supressed for the second model. The elements have also been tested separately, resulting in six different systems to study: wing 1 (forward sweep wing), wing 2 (straight wing), fuselage, v-tail, aircraft 1 (wing 1) and aircraft 2 (wing 2). The airfoils used for wing and v-tail are the Bell A821201 and a NACA 0012 respectively. The original full-size model had to be scaled in order to conduct the wind tunnel experiments following two criteria to select the scaling factor of 1:4: the wind tunnel blockage factor and the size limitations of the 3D-printer. Dimensions of the scaled model are fuselage length of 42.3 cm, wingspan of 62.73 cm, wing surface of 310 cm², wing dihedral of 5.27º and v-tail dihedral of 29.3º.

The wind tunnel is an open circuit one of 23.52 meters length, divided into three sections: contraction chamber, test chamber and diffuser, with a length of 5 meters and a rectangular section 1.4 meters wide and 1.8 meters high for the test chamber, and a maximum design speed of 30 m/s. Speeds used for the experiments range of 5.69 -19.44 m/s from which a (10.69 m/s) speed is defined as “characteristic” for being the one with a highest value possible that avoids including oscillations into the measures.

The number of experiments and systems to study have been increased with respect to previous studies, doing experiments of angle of attack and sideslip in the range of [-90º, 90º]. In addition, CFD analyses have been conducted in order to validate the results obtained by the wind tunnel experiments.

A smoothing technique is applied to the results, which captures the presumed underlying trend in noisy data acquired experimentally and allows the extraction of interpolation polynomials of the different parameters for all systems tested. Examples of the process of obtaining these interpolation polynomials will be shown in the final article. A key aspect is the study of all the different elements that constitute the aircraft separately (wing, fuselage, and V-tail) that reduces the number and cost of experiments. Then, the superposition principle was applied in order to compare the results obtained with the numerical ones (CFD).

All wind tunnel experiments have already been carried out for each of the six systems: wing 1, wing 2, fuselage, v-tail, aircraft 1 and aircraft 2. Results for the forces and moments have been obtained and will be detailed in future versions of this paper.