DEVELOPMENT OF AIRFOIL GEOMETRY FOR IMPROVED PERFORMANCE WITH A PASSIVE VARIABLE-PITCH AIRCRAFT PROPELLER

This paper presents the evaluation of airfoil geometry on a passive variable-pitch propeller aimed at enhancing aerodynamic performance over a wide range of operating conditions. Conventional fixed-pitch propellers suffer from reduced propeller efficiency in off-design conditions due to aerodynamic losses. A passive variable-pitch propeller is one in which the pitch varies based on operating conditions, such that the spanwise blade elements adjust to local flow properties. The objective of this study is to analytically and numerically assess the effects of airfoil geometry on propeller efficiency. The final product is a passive variable-pitch propeller that improves propeller efficiency over a range of operating conditions and removes the need for an operating design point. Conventional aircraft propellers are either selected or designed to closely fit mission requirements, which involves considering trade-offs in different operating conditions. Such trade-offs have significant effects on aerodynamic losses in off-design conditions. This study developed a Blade Element Momentum Theory model, and an airfoil performance model to determine the required airfoil for a given propeller diameter and range of operating conditions. This integrated modeling approach defined the model for an 18-inch diameter passive variable pitch propeller. The propeller and operating conditions were chosen to support further experimental research that will take place in a wind tunnel. The propulsion setup for basing the maximum and minimum operating conditions is based on a subsonic wind tunnel with a free stream velocity range of 15 ft/s to 65 ft/s and an E-Flight 90 motor, rated at a KV of 325, with a peak power output of 1800 Watts and a maximum rotational speed of 6,500 RPM. The corresponding advance ratio range, derived from the specified free-stream velocity and motor rotational speed, spanned from 0.2 to 0.6, facilitating the evaluation of the off-deign performance. Airfoil selection for the passive variable-pitch propeller focused on steady-state operating conditions and the lift-to-drag ratio. The determination of steady-state operating conditions is determined by the aerodynamic characteristics of a given airfoil and a pivot point that the airfoil is allowed to freely rotate about. For an airfoil to have a steady-state condition at a given Reynolds number, the airfoil needs to have a steady-state condition at an angle of attack near the optimal lift-to-drag ratio for that airfoil. This property in airfoils is only found in airfoils with a positive coefficient of moment which a traditional airfoil doesn’t have. This led to the development of a system to define a reflex airfoil based off a NACA 4-digit airfoil series. This method for developing reflex airfoil shapes can adjust camber, camber location, thickness, reflex magnitude, and reflex location. Given the extended parameter space, significant computational resources were required so steps were taken to reduce required resources. To efficiently explore this domain, a rapidly exploring random tree algorithm was employed. The reason for employing this type of algorithm is that traditional optimizers would be unable to reliably generate airfoils. This is because closely related airfoils can have either a steady state or not have steady state making results very jagged. Using the random tree algorithm allows for an unbiased search over more of the testing domain. This is a random tree algorithm that uses Xfoil for each airfoil for a large range of Reynolds numbers based on an 18-inch propeller in the wind tunnel operating conditions. Given the propeller and operating conditions, the maximum Reynolds number was 120,000 and the minimum Reynold number looked at was 20,000. This range of Reynolds numbers drives the needed capabilities of the airfoil. In other words, the random tree algorithm searches for an airfoil that has the largest steady-state operating range with the highest lift-to-drag ratio. After the initial Blade Element Momentum Theory results were found a matching Computational Fluid Dynamics model was made to validate the results. Results indicate that the analytical and numerical results have significantly improved propeller efficiency over a fixed-bladed propeller under off-design conditions while maintaining comparable performance in on-design conditions. This comparison is unique due to passive variable pitch propellers acting as multiple fixed pitch propellers. In the comparison, the one passive variable pitch propeller was compared to 4 different pitch propellers showing how the passive variable pitch propeller has improved operating range. Analytical and numerical results show that the passive variable pitch propeller has a maximum of 5% lower propeller efficiency while having an operating range larger than the 4 fixed pitch propellers combined. The results also show that the airfoil section parameter has a significant effect on the passive variable-pitch propeller performance. The results also show that a reflex airfoil which has lower lift-to-drag characteristics than traditional airfoils makes up for lower lift-to-drag by operating at ideal angles of attack. These findings support the use of passive variable-pitch propellers in aircraft propulsion systems. These insights are particularly relevant for small-scale applications where the cost and mechanical complexity associated with active variable-pitch propellers are not practical.