Tip vortex trajectory in proximity to the heliport

The last decade has brought a significant increase in the number of heliports built in Poland mostly for medical purposes to enable helicopter medical transport directly to the hospital. Some hospitals, however, could not build ground-level heliports due to limited space in densely populated areas. The solution to this problem can be elevated rooftop heliports. The main drawback of elevated heliports can be vibrations of the whole building, caused primarily by unsteady rotor slipstream which impinges on the heliport surface during take-off and landing. It is especially awkward in case of medical heliports due to strict vibration requirements. The frequency of these vibrations is known to equal a blade passing frequency of about 25 Hz for the Eurocopter EC-135 that is commonly used by, e.g. Polish medical service. However, the pressure measurements made by the authors showed that a low-frequency component of pressure oscillations of a significant amplitude is recorded in the outer part of the rotor disc. To explain the source of this component, we performed a PIV measurement of the velocity field in rotor slipstream. An object of investigation was the remotely controlled T-REX 450 Pro helicopter with a rotor diameter of 0.7 m. A key conclusion is that tip vortices from blade tips are located roughly along the Archimedean spiral, but vortices distanced from the blade tip shift up and down (or towards the blade tip and rotor axis) in a periodical manner.