Aerodynamic optimization of propellers for High Altitude Pseudo-Satellites

Abstract

The propulsion system of High-Altitude Platform Stations or High-Altitude Pseudo-Satellites (HAPS) is commonly based on propellers. The properties of the atmosphere at those high altitudes and the characteristic speed of HAPS entail that the flight is performed at very low Reynolds numbers. Hence, the aerodynamic behavior of the propeller sections changes substantially from the hub to the tip of the blades. Under those circumstances, the ordinary methods to develop optimized propellers are not useful and must be modified. We present a method of propeller design adapted to HAPS features. It combines traditional solutions with modern numerical tools. Specifically, Theodorsen analytical theory is used to minimize induced drag. This process leaves one free parameter that it is fixed optimizing a cost function depending on the Reynolds number with a viscous-potential numerical code. It leads to an optimal determination of the geometrical characteristics of the propeller, i.e., chord and pitch distribution, increasing its total efficiency. The resulting algorithm has low computational requirements what makes it very appropriate for the preliminary design of HAPS missions, when it is necessary to simulate many different cases. That methodology has been applied to a relatively small HAPS airship with a wind speed of 10 m/s and required thrust of 100 N. The propeller is assumed to be made up of NACA4412 airfoils and the cost function to be minimized is given by the ratio of the 2D drag and lift coefficients. With those conditions we perform a parametric analysis where different combinations of diameters, thrust coefficients, and propeller advance ratios are considered. Over a Reynolds number range from 103 to 106, the new method provides a gain about 5% in the propeller efficiency when compared with the ordinary design procedure that employs a constant Reynolds number. That gain is of utmost importance for HAPS operations, since, for example, it allows an increase in the payload of up to 25% for a 90 meters long airship.