Why can’t we use helicopters to rescue climbers on top of Mount Everest?
In: Physics
Here’s the Explain Like I’m 5 version:
Imagine you are crossing a giant lake by walking across stepping stones. At one side of the lake the stepping stones are close together and you easily step from one to the other. As you go farther and farther the stones get farther apart and after a while you are forced to literally jump from one to the other. At a certain point you are unable to reach the next stone and you’ve hit the point that is the farthest you can get.
All aircraft work the same way. The airfoil, the specific shape of the wing, creates lift by moving through a mass of air. At sea level the air mass has more air molecules in it in a given volume, similar to how the stepping stones are closer together at the start. At higher altitudes that same volume of air has fewer and fewer molecules of air in it, stones farther apart. Both the airfoil and you will have greater and greater difficulty using a fewer and fewer number of molecules or stepping stones to continue climbing.
There are things you can do to mitigate this. You can change the shape of the airfoil to be more efficient at higher altitudes, like having springy shoes that help you jump further. Or by going faster. At faster speeds the airfoil is able to hit more air molecules in a given time even though they’re farther apart kind of like how I’d you started running you can better jump from one stepping stone to the other.
It’s harder for helicopters to take advantage of those two things though.
An airfoil that works well at a high altitude doesn’t work that well at a low altitude. Airplanes are able to change the shape of their airfoils through the use of things like flaps that change the shape of the airfoil enabling them to work efficiently at a greater range of altitudes. Helicopters cannot change the shape of their airfoils as the mechanism that would be required to change the shape of the rotor blade, the airfoil in a helicopter, would be astronomically complex.
It’s also easier for airplanes to just go faster. There are two reasons that it’s really difficult for helicopters to “Just go faster”.
The first is the speed of the rotor blade through the air. As the rotors spin a rotor blade on one side of the helicopter is going forward and the one on the other side is going backwards. Relative to the air mass that the helicopter is flying through the blade going backwards is going slower than the one going forward. Just like when Mythbusters shot a bowling ball at 50mph backwards out of a truck going forwards at 50mph and the ball dropped straight to the ground at a certain speed the blade going backwards has zero airspeed relative to the mass of air it’s moving through. An airfoil without airspeed creates no lift. To make a comparison to an airplane this would be like if at high airspeeds the wing on one side of the plane gets chopped off and an airplane can’t fly with only one wing.
The second reason it’s hard for a helicopter to “just go faster” is that the speed of the rotor blade though the air mass isn’t constant for the whole length of the rotor blade. The tip of the blade travels faster than the root of the blade. In effect, the tip of the rotor blade could be traveling at 100mph but the part closest to the center would only be traveling at 25mph. This means that the most efficient section of the rotor blade for any combination of altitude and airspeed changes as the blade spins.
Those two things together create retreating blade stall. At slower speeds the airfoil has to be shaped differently in order to be effective than at high airspeeds, the blade of a helicopter has a different shape at the tip of the blade compared to the root of the blade to counteract this. But at a certain point it doesn’t matter. An airplane with the most complicated flap system in the world still can’t fly with zero speed relative to the air, it’s wings are no longer creating lift and it lands … harshly. As a helicopter flies faster and faster more and more of the rotor blade on the retreating side isn’t creating lift. After a while it’s that airplane that only has one wing.
It’s really hard to fly a helicopter at high altitudes. They are made specially to fly at low altitudes and slow. It could be possible to make a helicopter that could fly comfortably and efficiently at super high altitudes but it would probably have to have rotor blades as long as a football field in order to be able to grab a much air as possible while countering things like retreating blade stall and airfoil shape and it would probably be very awkward to fly at low altitudes too.
Latest Answers