Wind Tunnel Tests of the Counter-Rotating Propeller of Stratospheric Airship

In this paper, a design of counter-rotating propeller (CRP) for stratospheric airship in low Reynolds number is tested in experiment for the first time. In consideration of stratosphere environment, a two-bladed counter-rotating propeller is designed for stratospheric airship. With the similarity theory of Reynolds number and advanced ratio, the experiment is conducted in low Reynolds number wind tunnel at Beihang University. The results indicate that counter-rotating propeller were 4%-7% percent more efficient than single-rotating propeller (SRP). This shows that for the counter-rotating propeller: 1) with the same diameter and power, the thrust coefficient is higher; 2) with the same thrust coefficient and power coefficient, the diameter could be reduced; 3) with the same thrust coefficient and diameter, the power coefficient could be reduced. Introduction Stratospheric airship is now received more and more attention. Stratospheric airship need to meet the requirements of long-endurance. So there are many methods are adopted: 1) improve the ability of energy storage; 2) use the lighter materials; 3) improve the airship aerodynamic configuration [1] [2]. As a fact, the propeller for stratospheric airship has low efficiency than the propeller for high speed aircraft. Due to the stratosphere atmospheric density is small, Reynolds number of stratosphere is an order of magnitude smaller than the ground. The propeller propulsion for the stratosphere airship has low efficiency[3] [4]. Some existing methods to raise the capacity of the propeller propulsion: At first, can by changing propeller diameter, but due to the stratosphere critical Mach number is small, blade tip is easy to produces shock resistance, bigger blades bring great difficulty to install; Secondly, increase the number of the propeller can effectively improve the overall thrust, but too much consider the layout and balance the propeller torque, installation also has a lot of difficulties; At last, the counter-rotating propeller (CRP) offers the potential of a higher efficiency, smaller diameter along with a reduction of reaction torque when compared with a single rotation propeller (SRP) for equal input horsepower. This is due to the recovery of swirl energy by the rear rotor and the partial cancellation of moments since they are in opposite directions on the front and the rear rotors although not equal in magnitude[5]. Biermann and Gray took a wind tunnel experiment for counter-rotating propeller with high Reynolds number, verified that counter-rotating propeller can significantly improve efficiency[6] [7]. Biermann and Hartman took an experiments prove that counter-rotating propeller propulsion system for the overall efficiency significantly increase applies to lower advance ratio[8] [9]. Airship propeller propulsion works at a low advance ratio. As the energy increasingly nervous today, counter-rotating propeller propulsion applied to stratosphere airship has a good prospect. Similarity Theory Stratospheric air density is much smaller than the ground. How to simulation the low Re number environment on the ground must be studied first before taking the wind tunnel test. In this experiment, in order to ensure the movement similar and dynamic similar the advance ratio and Reynolds number similar conditions must be satisfied[10]. Advance ratio similarity determines the propeller working state movement is similar. The ratio of wind speed and the blade tip rotation speed is called advanced ratio.