ENHANCING THRUST AND EFFICIENCY: THE SCIENCE BEHIND CONTRA-ROTATING PROPELLER

This study explores the mechanism through which contra-rotating propellers improve the efficiency of stratospheric airships that fly at low Reynolds numbers and low advance ratios under unfavourable flight conditions. Experiments to assess their efficiency using wind tunnels resulted in a finding that, despite constant advance ratios, efficiency can be influenced by varying Reynolds numbers. Compared to traditional single-rotor propellers, the contra-rotating type always demonstrated superior performance, improving efficiency by a few percentage points. This suggests that contra-rotating propellers could be the best solution for improving propulsion in stratospheric airships, especially in low-density atmospheric conditions. From a broader perspective, contra-rotating propellers have undergone tremendous change over the years. Originally designed to achieve increased speed, they were subsequently used as a means of gaining fuel efficiency. Nevertheless, with advancing research, noise turned out to be the biggest challenge, prompting the emphasis on acoustics in the last three decades. These propellers exist in numerous configurations, ranging from co-axial to tandem, open rotor, and prop-fan configurations. Their history can be divided into five key periods: the early pioneering days before 1940, the golden years of 1940–1950, the rise of Western and Soviet-Russian aircraft from the 1950s onward, and the modern developments that began in the 1980s. Experimental planes and test models have further extended the boundaries, most notably in abandoning internal combustion engines in favour of contemporary gas turbines, which brought with them new engineering challenges. While such propellers are more efficient, continued technical challenges still must be overcome for more general acceptance.