The reason why helicopter accidents occur during takeoff and landing, Ground Effects

The influence exerted on a helicopter during takeoff and landing due to changes in airflow is referred to as ground effects. Ground effects involve changes in the rotor's induced angle and thrust due to variations in downwash and rotor wake.

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Helicopter's Ground Effects

Rotor Wake

Unlike fixed-wing aircraft, which generally experience relatively constant lift across the entire wing, helicopters generate varying lift along different sections of the rotor. Air pressure is primarily concentrated at the rotor's tips, where a downward-flowing vortex phenomenon occurs, similar to a whirlwind. This phenomenon is referred to as "downwash" or "inflow velocities" and affects the rotor's performance by influencing the induced angle.

helicopter-flight

The vortex phenomenon at the rotor's tips not only affects the rotor but also influences the helicopter's body and tail fins. This impact alters the helicopter's dynamic performance, creates unstable airflow, and leads to noise and vibrations.

Ground Effects

When a helicopter approaches the ground or a wall while in flight, the airflow encounters the barrier, causing changes in flow patterns. When the surrounding airflow around the helicopter is disrupted, the sequence of vortices is altered, resulting in instability. This phenomenon is known as ground effects.

helicopter-ground effects

Increase in lift

As the helicopter gets closer to the ground, the airflow changes, leading to a decrease in vortex size. With reduced vortex size near the ground, the lift increases.

Distance from the ground

So, how far should a helicopter be from the ground to avoid ground effect? When considering the effect of helicopter-ground distance, it is known that if the helicopter remains at a distance equal to the rotor radius, it experiences approximately a 7% impact on power at the same power setting. However, beyond a distance more than three times the rotor radius, the ground effect becomes almost negligible.

 

To summarize, helicopters have the characteristic of generating rotor wake at the blade tips, and they are greatly influenced by the unstable airflow caused by proximity to the ground. Helicopters experience an increase in lift due to ground effects, which means they can achieve the same performance with less power. Pilots must take this into consideration when planning speed and landing to prevent accidents.