Session: 01-02-03: Adaptive and Multifunctional Structures

Paper Number: 137940

137940 - Experimental Evaluation of a Low-Speed Aeronautical Ducted Propeller 

An aerodynamic performance evaluation of a low-speed shrouded aeronautical propeller physical model made of PLA (Polylactic Acid), is presented in this study using an experimental methodology. The model evaluated is a 20.32 cm diameter, three-bladed constant chord Clark Y airfoil propeller, with a geometric twist, and a Chen-IL airfoil for its shroud, which is not a common selection based on literature review, making this work an alternative technological approach. The technological interest of this configuration comes from aircraft propellers linear momentum theory, where the jump between the upstream and downstream velocity field magnitude of the propeller is inverse to its thrust generation, and by placing a Chen-IL airfoil duct shrouding the propeller both flows can be modified to approximately balance the stream velocity magnitudes, thus increasing its efficiency. 

A proof of concept is developed starting with the estimation of the free-propeller and shrouded-propeller geometric configuration. The shroud is based on an axisymetric annular fuselage, generated by a Chen-IL airfoil, with the purpose of increasing thrust through the enlargement of the exit area and duct effects. Also, a low-cost test bench was built with a scalable philosophy, where a propeller performance characteristic is obtained by testing it in a wind tunnel setup for 60k < Re < 180k. The measured variables were the Thrust force (Tth), force due to the electric motor torque (Ttq) and angular velocity of the propeller shaft (). 

The experimental setup was placed in the wind tunnel section of 1.80m width, 1.20m heigth and 4.0m length where 6 experiments are carried out, three of them with the free-propeller configuration and the rest with the shrouded-propeller configuration. For all tests, the angular velocity of the propellers are set within the range of 2000 - 4000 RPM with 100 RPM incremental steps. Each configuration is tested in three different free-stream velocities (V) : 0m/s, 5m/s and 8.5 m/s. With the data collected of Tth,  Ttq and , two characteristic curves of thrust and efficiency  are plotted against the break power to make a comparison between the free-propeller and shrouded-propeller performance.

The curves obtained for thrust follows a logarithmic trend. The comparison of the mean values indicates that the thrust generated by the shrouded propeler increased in 24 % against the free case, while the comparison with the theoretical approach highligths an increase about 32 %. 

Since static conditions, efficiency value is computed from the ratio between the mechanical shaft power and the electrical power supplied. The point of best efficiency differs in both configurations: For free case, the maximum efficiency is 48 % ± 4% between the shaft power range of 8.78W ±1W and 11.83W ±1W. In shrouded case, the maximum efficiency is given for a shaft power of 7.5W ±1W taking a value of 53 % ± 7.6%. 

At free-stream conditions(V=5 m/s and V= 8.5 m/s), the aerodynamic effects of the airfoil become more relevant and according to the literature, a shroud made of an airfoil basis decreases the pressure on the shroud airfoil upper surface located heading to the inner duct improving the intake flow for the propeller. This effect was observed during the experimental testing of both cases where a runaway condition was registered at 8.5 m/s as a free stream condition for free case at 2645 RPM while, for shroud case that runaway situation was recorded for free stream conditions at 5 m/s and 8.5 m/s with 2488 RPM and 3251 RPM respectively, caused by the aerodynamic performance explanation given previously in the shrouded propeller. The runaway condition is explainded by the unbalance betwen the propeller torque and the electric motor torque leading a rotational speed increase.

Presenting Author: Juan Pablo Velasco Cataño IMPETUS INDOMITUS - Universidad del Valle

Presenting Author Biography: Senior mechanical engineering student with experience in design and fabrication of turbomachinery for various purposes. Project experience includes transducer and control hardware related to software applications.