<p>The vertical take-off and landing capacity of a helicopter made it a prolific and essential means of transport in a lot of areas including the military operation, medical emergency, firefighting, and transportation of personnel and equipment. The capacity to hover for an extended amount of time, low-speed maneuverability, and the ability to take-off and land in any congested area are a few major benefits of helicopters over fixed-wing aircraft. The circulatory motion of the helicopter rotor produces lift-force in opposite to the weight of the helicopter to hover and maneuver. Rotor harmonic airloads are generated from the rapid variation of flow around the rotor blade due to vortex wake. Vortices are present in many engineering applications and are systematically generated by lifting surfaces. The vortex characteristics and the wake surrounding a helicopter rotor blade play an important role because they affect the flow physics surrounding the rotor blade. Therefore, an advanced mathematical and computational model of rotor wake and blade vortex gives a better understanding on the helicopter rotor dynamics. The strength of the vortex depends on the blade geometry, loading, and the aircraft’s operational state. A concentrated tip vortex line, an inboard trailing vortex sheet, and a shed vortex are accountable for aerodynamic airload generation. Among these, tip vortices have the maximum contribution and are formed by the rolling up of trailing vortices near the tip of the wing. A rapid drop in the circulation near the blade tip causes tip vortices which are the reason for a maximum lift at the tip of the blade. Consequently, tip vortices become the primary source of harmonic airloads. In this study, a mathematical model of the wake is developed consisting of two types of wake geometries: the fundamental wake geometry comprises all three types of the vortex and the distortion. Both the Vasistas and the Lamb-Oseen models for the tip vortex are considered. Multiple numbers of equidistant trailing vortex lines are considered along the blade span to apply the lifting line theory near the portions of inboard trailing vortex lines. There are rapid spanwise variations of the bound circulations for the close blade-vortex interactions of a tip-vortex line, which cause the shedding of some trailing wakes. The near wake due to the close interactions of the blade and the tip vortex has significant lifting-surface effects on the airloads calculations and therefore, are treated by using the lifting-surface and lifting-line theories, respectively. Along with the trailing vortex and tip vortex, the shed vortex lines are included which is caused by the azimuthal variations in the bound circulation. After modeling the wake, the bound circulation at each spanwise station is computed. A Bo 105 composite, hingeless helicopter rotor blade is considered for the computational analysis. A fluid-structure interaction model is developed by coupling the finite element model of the rotor blade with a computational fluid dynamics model of the surrounding air to analyze the helicopter rotor blade dynamic response and to investigate vortex formation due to the blade movement. The swirl velocity is minimum, and the axial velocity is maximum at the vortex center. The axial velocity decreases and swirl velocity increases with increasing the distance from the vortex center to the core radius. Overall, this study will help to understand the effect of rotor wake and predict a few features of helicopter aerodynamics.</p>

An Investigation of the Wake and Vortex Formation of a Helicopter Rotor Blade

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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