The Forces on an Airplane How do airplanes fly? It's not magic. Learn about the forces that help (and hinder) airplane flight.
The Forces on an Airplane
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- The Forces on an Airplane
- So this is an airplane here. Ok.
- You probably already knew that.
- If you've flown in one or maybe you just seen them fly.
- But, even if you've seen them or been in one, do you know how they work?
- Is it magic? Wingavium Leviosa.
- Are there invisible fairies that hold the plane aloft?
- Alright men, we've got a busy morning and lots of flights to carry.
- Or is it science?
- Well, you guess that the answer is indeed science.
- That's ridiculous!
- [Hand slap]
- 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 are going to focus on airplnes 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
- its the weight force which points down
- towards the center of earth
- Weight is equal to the mass of the airplane - (M) right here.
- Times the acceleration due to gravity (g)
- Here on earth, (g) is equal to 9.81 meters per second squared.
- Now that is only for earth.
- The acceleration due to gravity really depends on the mass of the planet that you're on.
- The larger the planet, the higher the gravity.
- So, 9.81 meters per second squared here on earth
- The moon however, is smaller than earth.
- So the acceleration due to gravity is only one sixth that on earth.
- So one point six 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 the only two forces then our aircraft only be able to go up and down
- but it won't go anywhere
- So, we're going 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 thurst?
- I'm sorry, we can at least roll down the hill.
- On an aircraft this thurst is produced by engines
- There are two main types of engines
- there are 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
- So we 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 is an equal and opposite reaction
- And that's the thurst force pushing the aircraft forward
- This is really the same thing that happens when you blow up a balloon and let it go
- The air comes out the back and the balloon moves forward
- We have a force that opposes the thrust, its call 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 areodynamic as possible.
- This means that it has smooth lines
- and the air flows nice and cleanly over the front here
- You can feel the pressure drag when you stick your hand out of the window of a moving car
- Ah, honey, honey, your hand, your hand please.
- When your hand is horizontal its areodynamic, and you really don't feel alot of drag.
- But if slowly turn your hand vertical,
- you can really feel the drag increasing.
- So these are our four forces on the airplane
- but perhaps your're thinking
- So this really cool and everything, but how do we increase and decrease the airplane's lift
- to move up and down?
- That's a great question
- Let's look at the equation for the magnitude lift per unit wing area
- we'll call that (L).
- L equals one half times rau times CL times V squared.
- It's that simple.
- Ok, ok. I'll tell you what each of these things mean.
- So rau, its not a P is the greek letter rau.
- Rau 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 if you go higher up the air is thinner,
- so the density is lower.
- So if you want to simplify things we generally use standard density,
- which is one point two seven five four kilograms per meters cubed.
- (V) here is the speed of the aircraft or
- fast its traveling. And (C)(L) is something called the coeffecient of lift.
- Its a number that gives us some information about the shape of the aircraft's wings
- These things right here.
- The coeffecieint of lift changes with the angel of attack.
- Angle of what?
- An aircraft can pitch up and down.
- And even if they pitch up, they are still traveling in a horizontal direction, like that.
- Now, the angle formed here by the horizontal direction from 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 the coeffecient 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.
- So as the aircraft starts to pitch up, the coeffecient 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 its
- not a good thing.
- So we generally try not to pitch up this much.
- There is a similar equation for the drag per unit wing area (D).
- D equals one half rau, not CL, that wouldn't make any sense, but CD.
- As you can guess, its the coeffeiceint of drag times the velocity squared.
- The coeffeiceint of drag is another number that tells us something about the wings.
- And its 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 somehting that we talked about earlier.
- 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 to know
- about how an aircraft flies.
- So the next time you're on an airplane or you've just seen one
- you really know exactly what is that's keeping it up in the air.
- That's not it either.
- Ah...there you go.
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