Why Aren't Helicopters Faster? - Popular Mechanics

In the grand contest between air and land, we tend to think flying has a speed advantage over driving. There's less friction, no road to overcome. But physics is a funny thing, and helicopters have some serious disadvantages over good old-fashioned cars in terms of pure speed. Real Engineering breaks down the physics of the race.

Once the lift in a helicopter's blades is greater than the weight of the helicopter, it will begin to rise in the air. Then, when it equals the weight of the helicopter it will begin to hover. To move forward, a pilot will then shift that lift into thrust by getting the helicopter to an angled position.

It's this position, the one the helicopter needs to achieve forward motion in the first place, that dooms it to slower speeds. As the helo moves forward, the left side of its blades move away from the direction of travel and the right hand side moves towards it. Because it is going with the flow, the right side of the helicopter's blades generates more lift than its left.

Of course, helicopters don't spin wildly out of control every time they fly. That's because the blades change what's known as their angle of attack, how they face the win. Sometimes this angle is changed by pilots, other times it occurs automatically through a part of the rotor known as a swash plate.

As a swash plate increases a helicopter's angle of attack, it's lift increases. But that can only take you so far. The helicopter will run into what's known as flow separation boundary where it can no longer compensate for its unequal lift. That point comes at around 400 km/h (216.45 knots, 249.09 mph). With counter-rotating rotors, sometimes helicopters can go faster. But even they can only push against flow separation so much.

Cars have plenty of aerodynamic problems of their own, but no hard limit that hits them as fast as a flow separation boundary. That's why groups like Bloodhound SSC are dreaming of a car that could break 1000 MPH.

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David Grossman is a staff writer for PopularMechanics.com. He's previously written for The Verge, Rolling Stone, The New Republic and several other publications. He's based out of Brooklyn.