Prevention of retreating blade stall by asymmetrically generated lift: Free-flight investigations with a fully autonomous helicopter testbed

Abstract

Helicopters in forward flight experience highly asymmetrical flow conditions. While transonic effects on the advancing side of the rotor are responsible for a high generation of drag and noise, the retreating side operates in low-speed flow at high angles of attack, close to dynamic stall. This inherent aerodynamic dissymmetry of the main rotor could be counteracted with the intentional generation of asymmetrical lift on the fuselage. To gather more information about the effects of asymmetrical lift and their consequences for the rotor, a UAV helicopter testbed was developed and equipped with a four-axis autopilot system, which enables it to perform fully autonomous flights using GPS waypoints. As a result, the measurements can be conducted in precisely defined flight conditions which can be maintained over long periods of time. Consequently, the averaged data reach a very high level of accuracy that could not be achieved with conventional, manually controlled systems. Successful tuning of the PID loops and other controller parameters for low and high-speed flight (up to 125 km/h fully autonomous cruise flight) was conducted. A remotely adjustable horizontal stabilizer was used to investigate different situations of asymmetric lift. Due to gyroscopic effects of the rotor, a phase shift has to be taken into account, which means that the lift should not be produced directly on the retreating side. The results prove that asymmetrically generated lift can indeed counteract and even overcompensate for the natural rotor asymmetry of a helicopter in forward flight. The retreating blade operates at significantly lower angles of attack, leaving a greater margin to dynamic stall, which allows for lower rotor speeds and therefore resolves the transonic problems of manned helicopters on the advancing side. As a result, either higher flight speeds or lower fuel consumption and noise emissions (eliminated HSI noise) can be achieved.