Leishman uses the Figure of Merit to define the aerodynamic efficiency of a hovering rotor. It compares the ideal power required to hover (from momentum theory) to the actual power consumed (which includes profile drag losses):
To simplify the physics, the helicopter rotor is initially modeled as an infinitely thin "actuator disk" that induces a uniform pressure jump across its surface. This model assumes: Inviscid, incompressible, and steady flow. No swirl or rotation imparted to the wake. Uniform induced velocity (inflow) across the disk. Hover Performance Leishman uses the Figure of Merit to define
When a helicopter hovers close to the ground (typically within one rotor diameter), the physical barrier alters the airflow pattern. The ground restricts the downward deflection of the wake, reducing the induced velocity and tip vortex strength. This allows the helicopter to maintain a hover with significantly less power than it requires at higher altitudes. 4. Key Takeaways for Helicopter Design No swirl or rotation imparted to the wake
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In certain flight regimes, such as descending flight, a rotor blade passes directly through the wake vortex shed by the preceding blade. This interaction causes rapid changes in local aerodynamic loading, resulting in the distinct, high-amplitude "impulsive noise" (often called blade slap) and structural vibration. Ground Effect The ground restricts the downward deflection of the
): The volume of air passing through the rotor disk per unit of time.