Effects of strong rotor wake vortex interference on helicopter airspeed measurement

Accurate airspeed measurement is crucial for helicopter flight safety. This article establishes a numerical simulation method for the rotor/fuselage aerodynamic interference flow field based on the unsteady momentum source theory and investigates the influence of rotor wake vortex interference on helicopter airspeed measurement. Using a reference helicopter as an example, we analyze the variation characteristics of airspeed values within the entire speed range for different rotor thrusts and airspeed probe locations. Due to factors such as rotor blade motion and fuselage blockage, there are differences in the static pressure phase and peak values at different airspeed probe locations. The phase difference is influenced by the impact of rotor blades, while the peak value difference is affected by the rotor disc load and fuselage blockage. The study also identifies and explains the “airspeed hysteresis” phenomenon caused by rotor tip vortices passing through the airspeed probe area within specific speed ranges. The variation of airspeed in the airspeed probe area can be divided into three regions: the downwash-dominated region, the hysteresis region, and the incoming-flow-dominated region. For conventional configuration helicopters, the hysteresis region occurs only at medium to low speeds. Further analysis of parameter effects suggests that the closer the airspeed probe is to the front of the fuselage and the smaller the rotor thrust, the further forward the speed range of the hysteresis region. When the airspeed probe is positioned near the nose, no significant hysteresis appears. Finally, the article provides recommendations for the placement of airspeed probes from the perspective of aerodynamical interference.