Mark Drella
Appearances
Short Wave
How Does An Airplane Stay In The Air?
Sympathize? I don't get it why it's so mysterious. You know, there's so many examples of phenomena which basically demonstrate lift in a very intuitive way.
Short Wave
How Does An Airplane Stay In The Air?
Well, there's lots and lots of explanations. So, for example, I don't know if you've ever done this in your kid. You can kind of fly your hand, right, as you hold it out the car window.
Short Wave
How Does An Airplane Stay In The Air?
You can let the air support the weight of your hand, right, if you angle it just right.
Short Wave
How Does An Airplane Stay In The Air?
Right, on an air cushion. If you've got to angle it up just a little bit, let the air blow underneath, so to speak, it'll support the weight of your arm, right, of your forearm. That's basically what lift is, intuitively. Air moving past an object pushes on that object. I mean, why is that a mystery, right? And you see it lots of ways. For example... How does a water skier stay up?
Short Wave
How Does An Airplane Stay In The Air?
Well, the water rushing past the water skier pushes up on the skis and keeps the skier above the water, right?
Short Wave
How Does An Airplane Stay In The Air?
The equations of motion for that situation are the same as for an airplane. Based on basic physics, there is really not that much significant difference between water and air. They're both classified as fluids. Big difference is simply the density. You know, water is about 800 times more dense than air. But that's the only difference.
Short Wave
How Does An Airplane Stay In The Air?
So, if you could visualize air going past an object moving through the air, or if you visualize a water moving past an object moving through the water, they literally look the same. So, for example, The Goodyear blimp is almost mathematically identical to a submarine.
Short Wave
How Does An Airplane Stay In The Air?
Okay, so just going back to your hand moving out of the car window, right?
Short Wave
How Does An Airplane Stay In The Air?
The air needs to an area to push against, right? Pressure is the major way how fluids impart force to an object. So, for example, water rushing past a water skier, the water pressure under the skis is larger than the overall surrounding pressure. And that extra pressure on the bottom pushes up on the skier. Just like the air pressure underneath on the bottom surface of a wing pushes up
Short Wave
How Does An Airplane Stay In The Air?
on the wing, and that holds up the airplane. So you need area. In fact, on very simple high school algebra, force is pressure times area.
Short Wave
How Does An Airplane Stay In The Air?
Okay, so again, the simplest explanation is you go back to your hand sticking out the car window. If you hold your hand perfectly horizontally, right, you won't feel a force. To get an upward force, you have to tilt your hand so that you present the bottom of your hand to the oncoming flow, right? That angle is called the angle of attack.
Short Wave
How Does An Airplane Stay In The Air?
That will make the pressure on the bottom of your hand larger than on top. And that resulting pressure difference imbalance times the area of your hand is the lift acting on your hand. Exactly the same thing happens with an airplane.
Short Wave
How Does An Airplane Stay In The Air?
Yeah, yeah. An airplane is far more subtle because the nose is just barely higher up. But it's the same idea, yes.
Short Wave
How Does An Airplane Stay In The Air?
Yeah, so basically the air pushes on the wing, and in the process, the wing also pushes on the air. This is Newton's third law. When two things push against each other, it's never one way. So for example, when the air pushes up on the airplane, the airplane pushes the air down slightly as a result. So what happens is, in practice, if you look at the overall picture...
Short Wave
How Does An Airplane Stay In The Air?
If the airplane wasn't there, the air just would sail through straight. But because there's a wing there, the air curves and that curvature is related to the differences in pressure. So specifically for lifting wing, the pressure under the wing tends to increase and the pressure above the wing tends to decrease just from the angle of attack and the shape of the wing.
Short Wave
How Does An Airplane Stay In The Air?
Yeah. So the thing that holds the wing at the correct angle is the tail. And birds have tail feathers for a very similar reason. Wow. Right. The tail is very much like the fins on a weather vane or a dart, right? It kind of points the airplane into the wind at the correct angle.
Short Wave
How Does An Airplane Stay In The Air?
Well, okay, first of all, the pilot can control the tail. In fact, that's the primary purpose of the pilot is to control the tail. Right, so the pilot, by moving the control stick, moves the tail, and the tail then moves the wing to the right angle. Now, in practice, the airplane kind of passively does the right thing.
Short Wave
How Does An Airplane Stay In The Air?
Because what happens is when you're rolling down the runway, as the pilot then moves the stick, then the tail tilts the entire airplane, including the wing. And then once the angle of attack is such that the resulting pressure difference equals the weight, the airplane lifts off the ground. And then it kind of keeps going.
Short Wave
How Does An Airplane Stay In The Air?
Well, it's basically the atmosphere is not perfectly still. So if the airplane flies into air that's rising or descending or just there's a lot of turbulence mixing air, so to speak, then that will modify the pressures on the wings and the fluctuating pressures then kind of fluctuate the airplane. That's what you're feeling. And airplanes are designed to withstand very, very extreme turbulence.
Short Wave
How Does An Airplane Stay In The Air?
Even the most violent imaginable turbulence that you've ever felt, the airplane can easily withstand loads maybe three times bigger. So there's a very big safety margin on how aircraft are built. In fact, I can't remember hearing of an airplane breaking up in the midair because of turbulence. It just doesn't happen.
Short Wave
How Does An Airplane Stay In The Air?
Well, obviously, you know, if you drive off a cliff, bad things will happen. Or if you drive into a rock face, same thing with an airplane. I mean, that's an operator error. There's nothing wrong with the airplane. It's just it was operated incorrectly. That's why you have two pilots. So we can imagine having... two drivers in your car. If one falls asleep, it's okay. The other one's there.
Short Wave
How Does An Airplane Stay In The Air?
The airplane is designed to almost fly hands-off. Another way to say it is, for example, if you're cruising your car down the highway, and you let go of the steering wheel, nothing will really happen, right? Because the car is passively stable. Passive stability is a good thing. It means that if you don't do anything, it'll just keep doing what it's doing.
Short Wave
How Does An Airplane Stay In The Air?
Yeah, so the pilot could fall asleep for 10 seconds and it'll be fine.