What Is Rotor Wash From A Helicopter & Is It More Dangerous Than It Looks?

Helicopters are marvelous pieces of engineering, providing a live demonstration of the laws of physics as they carry us into the skies. With metal or composite blades angled just right and spinning at hundreds of rpm, there is enough downward thrust to lift helicopters weighing thousands of pounds along with gear and passengers. Of course, the incredible winds generated by those whirling blades presents their fair share of risks to property and human safety. When all that air is displaced downward to get the 'copter in the air, it doesn't just dissipate when it clears the rotor blades. 

The air flies outward in a massive, powerful cone around the helicopter's perimeter at wind speeds more than strong enough to knock an unprepared adult flat on their back. This is a phenomenon known as rotor wash, and it's a big reason passengers and onlookers need to be careful when there's a helicopter flying low overhead. Rotor wash is so named because the air produced by the spinning rotors of a helicopter "washes" downward and outward like falling water. It's kind of like holding the back of a spoon underneath a running faucet; the water washes away in a similar fanned pattern.

 Air and water tend to follow the laws of fluid dynamics — like Bernoulli's Principle — in much the same way. A helicopter's rotors and blades constantly produce downward thrust by drawing in air and pushing it downward to gain and maintain altitude. As a helicopter approaches the ground, though, that displaced air starts to deflect against the surface and fan outward. Downwash is the vertical gust produced right below the helicopter, and outwash is the air that fans out and creates turbulence as it hits the ground. It's less powerful the further you are away from the descending helicopter.

Because a helicopter's rotors need to produce a strong enough lift to move the vehicle, its contents, and fuel, those wash gusts flying under and outward from it are equally powerful. If a person, animal, or object were right under a helicopter as it lands, the wind force could easily be enough to cause serious injury or damage. Depending on the size of the helicopter and the construction of its rotors, these gusts can exceed 25 mph. That's easily enough to knock down a healthy adult and turn bits of debris and larger objects into potentially dangerous projectiles.This is why helicopter pilots can't land just anywhere, and part of why you see actors in movies and on TV crouching low to the ground as they approach a landing helicopter. Moving with a lower center of gravity helps keep them stable in the wind and to avoid getting whacked by the spinning blades, which flex and sag as the aircraft dips lower.