If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

The forces on an airplane

How do airplanes fly? It's not magic. Learn about the forces that help (and hinder) airplane flight. Created by MIT+K12.

Want to join the conversation?

  • male robot hal style avatar for user egrewal1
    What exactly causes lift?
    (12 votes)
    Default Khan Academy avatar avatar for user
    • blobby green style avatar for user Jeffrey Wiedl
      Lift comes from three things:
      *Camber - The net curve of the air foil. The camber creates the downwash that results in an upward force as described in philipyu_sep's answer. Imagine a sail on a sailboat. It has net curve and it makes the air go "down" (sideways for the boat) and that makes lift - a forward force on the boat.

      *Thickness - The thickness gives us Bernoulli effect. Even a symmetric airfoil that has an angle of attack ( in the video) has thickness in the flow. The Bernoulli effect gets a bad rap, but it is real, it just isn't the only thing at play. Think of that sail again. It is very close to zero thickness, yet it produces lift.

      *The kite effect - This is what you feel when you stick your hand out the car window. You can make lift on your hand just by making some angle of attack. Think about that one... You could take a piece of plywood, almost no thickness - no Bernoulli and no camber - so no downwash, but you could make it fly if you moved it through the air fast enough.

      Samuel Langley demonstrated the kite effect by putting a flat brass plate on a rotating arm and showed that it generated lift. This strategy needs a lot of power and a powerful enough engine didn't exist then (1903).
      The Wright brothers, on the other hand, spent a lot of time testing airfoil shapes and came up with a cambered, nearly constant thickness airfoil that was more efficient and could be warped to control the aircraft and ultimately won the race to controlled, sustained flight.
      (10 votes)
  • piceratops ultimate style avatar for user Ben
    Is there lift in a vacuum?
    (3 votes)
    Default Khan Academy avatar avatar for user
  • mr pants teal style avatar for user Aditi A
    What does the v squared represent?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • mr pants teal style avatar for user smacdonald
      v squared aka v^2 is the square of the velocity of the air relative to the wing.
      In the equation it shows that the lift is proportional to the square of the speed of the aircraft. For example, if the speed doubled, the lift would quadruple.
      (3 votes)
  • piceratops ultimate style avatar for user Jason Hironaga
    At when he talks about the angle of attack, he explains what the angle of attack is but he doesn't say why it is called the angle of attack. Could someone please explain?
    (3 votes)
    Default Khan Academy avatar avatar for user
  • leaf red style avatar for user [!] Percy383(Olympian in the first Khan Olympiad)
    Is an airplane capable of travelling straight up? If so how high can it go? If not what do you need to do this? Thanks!
    (2 votes)
    Default Khan Academy avatar avatar for user
  • orange juice squid orange style avatar for user Jude Abishek
    what is the cause of stalling?
    (2 votes)
    Default Khan Academy avatar avatar for user
  • ohnoes default style avatar for user E M
    What is thrust?
    (1 vote)
    Default Khan Academy avatar avatar for user
  • starky ultimate style avatar for user Mortal Kombat Advocate
    When does an airplane actually use drag?
    (2 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user Adam Weintraub
    It seems that you missed the area S in the formulae of lift L and drag D?
    (1 vote)
    Default Khan Academy avatar avatar for user
    • purple pi purple style avatar for user Joost List
      He defines L and D as lift and drag per unit surface area. So the formula is actually correct. However, in aeronautical engineering you would typically include the surface area S and thus L and D are defined as lift and drag on the complete airplane.
      (2 votes)
  • male robot hal style avatar for user chuenyangchen
    Is it true that to decrease the drag of an airplane, decrease the speed of the airplane? Also, if you decrease the speed, won't the lift also decrease? If you are a pilot, how would you control the four forces of flight?
    (1 vote)
    Default Khan Academy avatar avatar for user
    • leafers ultimate style avatar for user The Chosen One
      The drag does increase as speed increases but your speed increase would be more than the drag and you would still go faster. You would just need more energy to increase the speed and more energy to run the plane. Lift does not necessarily need to increase along with speed. I think. Because if you point the nose of the plane downward speed would make the plane go down faster. As a pilot you can control the four forces of flight by changing the tilt of the wind and increasing thrust (propellers) and decreasing the thrust.
      (2 votes)

Video transcript

So this is an airplane here. OK, so you probably already knew that. If you've flown in one, or maybe just seen them fly. But even if you've seen them or been in one, do you know how they work? Is it magic? [CHANTS GIBBERISH] Are there invisible fairies that hold the plane aloft? All right, men. We've got a busy morning and lots of flights to carry. Or is it science? Well, you can guess that the answer is indeed science. That's ridiculous. What? So to discuss how an airplane flies, we first have to talk about the forces on an airplane which push it around in all sorts of different directions. Now we're going to focus on airplanes today because they're awesome, but most of these forces apply to any other vehicle. The first force acts on all these vehicles-- really, it acts on everything. It's the weight force, which points down towards the center of Earth. Weights is equal to the mass of the airplane-- m right here-- times the acceleration due to gravity. Here on Earth, g is equal to 9.81 meters per second squared. Now that's only for Earth. The acceleration due to gravity really depends on the mass of the planet that your are on. The larger the planet, the higher the gravity. So 9.81 meters per second squared here on Earth. The moon, however-- it's smaller than Earth. So the acceleration due to gravity is only 1/6 that on Earth-- 1.6 meters per second squared. This is why astronauts can bounce high on the moon, but not on Earth. This isn't nearly as much fun. Obviously, there has to be another force opposing the weight and pushing the airplane up. This force is called lift. Lift operates perpendicular to the airplane's wings, which are right here in this side view. Now, if these are our only two forces, our aircraft will be able to go up and down, but it won't go anywhere. So we have to have a force that pushes the airplane forward, and this is called thrust. All vehicles have thrust, otherwise they wouldn't go anywhere like our airplane. Why didn't you buy a car with thrust? I'm sorry. We can at least roll down the hill. On an aircraft, this thrust is produced by engines. There are two main types of engines. We have propellers, like this little guy right here. And jet engines, like our first model. Whatever the type of engines, they all work by the same principle. Let's draw a little side view of an engine here. The engines accelerate air out the back this direction. And by Newton's third law, there's an equal and opposite reaction, and that's the thrust force pushing the aircraft forward. This is really the same thing that happens when you blow up a balloon and you let it go. The air come out the back and the balloon moves forward. We have a force that opposes the thrust. It's called drag. It points opposite the direction of flight. The major type of drag is pressure drag, which is the force caused by the air smacking into the airplane. So we try to minimize this type of drag by making the airplane as aerodynamic as possible. That means that it has smooth lines in the air flows nice and cleanly over the front here. You can feel the pressure drag when you stick your hand out the window of a moving car. Uh, honey, honey. Your hand-- your hand, please. When your hand this horizontal, it's aerodynamic and you really don't feel a lot of drag. But if you slowly turn your hand vertical, you really feel the drag increasing. So these are our four forces on the airplane, but perhaps you're thinking-- So this really cool and everything, but how do we increase and decrease the airplanes lift to move up and down? That's a great question. Let's look at the equation for the magnitude of lift per unit wing area. We'll call that L. L equals 1/2 times rho times cl times v squared. That simple. OK, OK, I'll tell you what each of these things mean. So rho-- it's not a P. It's the Greek letter, rho. Rho is the density of the air, which is a measure of the number of air molecules in a certain volume. Density of the air varies with altitude and temperature, so you go higher up. There, the air is thinner, and so the density is lower. If we want to simplify things, we generally use the standard density, which is 1.2754 kilograms per meters cubed. v here is the speed of the aircraft, or how fast it's traveling. And cl is something called the coefficient of lift. It's a number that gives us some information about the shape of the aircraft's wings-- these things right here. The coefficient of lift changes with the angle of attack. Angle of what? Aircraft can pitch up and down, and even if they're pitched up, they're still traveling in a horizontal direction like that. Now the angle formed here by the horizontal direction of travel and the direction of the aircraft's nose is called the angle of attack, and we denote that with the Greek letter alpha. So we can make a little plot here of that. We're going to put coefficient of lift up on the y-axis, and the angle of attack down on the x-axis. So as the airplane starts to pitch up-- if I can get a little hand here-- thank you. As the aircraft starts to pitch up, the coefficient of lift increases. This is a good thing because we have more lift. As we continue to increase, we eventually reach a point where we keep pitching up but the lift starts decreasing. This is something called stall, and it's not a good thing. So we generally avoid try to pitching up this much. There's a similar equation for the drag per unit wing area, D. D equals 1/2 rho. Not cl-- that wouldn't make any sense. cd, as you can guess, is the coefficient of drag times the velocity squared. The coefficient of drag is-- it's another number that tells us something about the wings, and it also varies with the angle of attack. So as the angle of attack increases-- oh, thank you-- the coefficient of drag increases as well. Thank you very much. This is because as the aircraft is pitching up, there is more wing area perpendicular to the flow. Now, this reminds me of something that we talked about earlier. Exactly. This is very similar to whenever you hold your hand out the window of a car. And so, that's pretty much everything you need know about how an aircraft flies. So the next time you're on an airplane or you just see one, you can really know exactly what it is that's keeping it up in the air. Nope. No, it's not them either. Ah, there you are. Now you got it.