# How do airplanes really fly

1. Jul 4, 2012

### phenom01

I was taught that due to Bernouli's theory of air pressure the shape of the wing makes air move faster on the top and slower on the bottom thus creating low pressure above and high pressure below the wing. Now, if this was true, then how do airplanes with identical shape wings fly if both the top and bottom are the same shape? Also, how can airplanes fly vertical? Now, i think airplanes fly because of newtons law of motion. When the air craft is moving forward the pilot lowers the aileron and this causes the wind to move downwards thus creating an equal and opposite reaction which lifts the plane up. Anyways i kind of wrote a lot. What do you guys think?

2. Jul 4, 2012

### Staff: Mentor

what planes have identically shaped wings on top and bottom?

flying vertical simply means the planes propulsion is sufficient to counter gravity as in a jet.

with the aeleron down the plane will nose downward. true it deflects air downward but that also induces the plane to tilt downward. planes that are landing use the aeleron to go down and to also slow the plane prior to landing.

3. Jul 4, 2012

### phenom01

This pane has the same shape wings. Also, how do you explain flying upside down? According to Bernouli's theory upside flying would create high pressue above the wing and lower pressure bellow the wing; thus forcing the plane to the ground.

4. Jul 4, 2012

### DrewD

I don't know where the page is, but NASA has an interesting discussion of airplane wings. It is true that Bernoulli's equations are not the reason (or at least not the whole reason), but I don't remember all of it. Try searching for NASA and aeronautical engineering and you might find it.

5. Jul 4, 2012

### AlephZero

Tha's nearly right, except for the bit about "the pilot lowers the aileron".

THe shape of the wings makes the air move downwards. You don't need a fancy shape to make a wing that "works". A flat plate at a small angle to the horizontal will do fine. The only reason why wings are complcated shapes (and have adjustable flaps, etc) is to make them work efficienctly, not to make them work at all.

6. Jul 4, 2012

If planes had no control surfaces (ailerons, elevons, and rudders), then you would be correct, and planes would behave solely according to Bernoulli principles. But planes in real life use those control surfaces to exert pressures in other directions, which can and do counteract the flight behaviour you're describing.

For instance, in upside-down flight, normally, as you say, a plane with just a wing and no control surfaces would not be able to fly upside down; it would tend to "lift" downwards relative to the ground. That's why if you were trying that stunt in a normal plane, you'd use the plane's elevons to deflect the path of the plane "upward" to counteract the "downward" lift of the wing.

Incidentally, that's also why planes that lose control of their control surfaces tend to crash, quickly.

The bottom line is, in answer to your question--airplanes follow a path that is influenced by several other forces besides just the wing shape (thrust from engine, control surface deflection, etc). That's how they fly.

7. Jul 4, 2012

### D H

Staff Emeritus
Ahh. The good old "Tastes great!", "No! Less filling!" debate of why airplanes fly: Do airplanes fly because of Newton's third law or because of Bernoulli's principle?

The question has a hidden implication that this is an either/or kind of proposition. Why must it be one or the other? To me the answer is "yes".

Newton's third law (better: conservation of momentum) dictates that there will be no lift if the airflow is not directed downward somehow. There are a couple of problems here. One is that it doesn't say anything about what that somehow is. Another is that it says nothing about what constitutes a good wing.

Bernoulli's principle starts to answer this latter question of what constitutes a good wing, but Bernoulli's principle alone does not answer the question of what causes lift. Direct the airflow upward from a nicely shaped wing and you will not get any lift.

You need both conservation of momentum and Bernoulli's principle to explain how airplanes fly.

8. Jul 4, 2012

9. Jul 4, 2012

### cosmik debris

Ailerons, as they are on the wings, control roll and usually move in opposite directions. The wing with the aileron down will lift that wing.

10. Jul 4, 2012

### DaveC426913

Only if the angle of attack is set properly. Angle of attack is critical in providing lift. Without an inclined angle of attack, it makes no difference what shape the wing is.

11. Jul 5, 2012

### CWatters

Angle of attack: This also increases the pressure on the lower surface and reduces it on the top surface.

Thrust line: If you watch a plane fly upside down you may notice they usually have the nose higher in the air (pointing upwards more than usual). It's even more pronounced in knife edge flight.

It's neither Newton or a Bernouli but a combination of both.

12. Jul 5, 2012

### Staff: Mentor

Doesn't look to me like anyone answered the OP's main questions until the last two posts(which happens too often in lift discussions), so to reiterate:

1. If the wing has a non-zero angle of attack, the symmetrical airfoil doesn't look symmetrical to the air it is flying through.

2. A plane taveling vertically isn't using its wings to fly, it is using the thrust of the engines, like a rocket.

Last edited: Jul 5, 2012
13. Jul 7, 2012

### phenom01

Bernouli's theory still doesn make sense. If you increase thrust lift does not insrease. So that whole lower vs higher pressure thing is invalid.

14. Jul 7, 2012

### Antiphon

There really is a lower pressure over the top of the wing. Sometimes you can literally see it.

Those of you who fly regularly, next time you are near clouds you may see a haze form over the top of the wing that isn't there ahead or behind the wing. This is caused by the condensation of water out of saturated air due to the lower pressure and thus the temperature drop.

15. Jul 7, 2012

### azizlwl

What about the show airplanes that can do inverted manuever.
Will the plane experience opposite of lift?

16. Jul 7, 2012

### Staff: Mentor

What does the thrust have to do with the lift?

17. Jul 7, 2012

### Staff: Mentor

18. Jul 7, 2012

### phenom01

Because the faster you go, the more air flows between the wing; thus effecting pressure differences. You follow?

19. Jul 7, 2012

### Staff: Mentor

More thrust doesn't necessarily mean more speed. But anyway, if you hold the angle of attack constant, lift DOES increase as a square function of speed, as Bernoulli's equation would predict: http://www.grc.nasa.gov/WWW/k-12/airplane/lifteq.html

Last edited: Jul 8, 2012
20. Jul 7, 2012

### phenom01

constant in a straight line? How does that increase lift?

21. Jul 7, 2012

### DaveC426913

How does increasing speed increase lift??

Just as in a car, wind resistance increases as the square of speed, so, in a plane, does lift.

22. Jul 8, 2012

Holy cow there are some huge misconceptions here.

In a most basic sense, lift can be explained through Newton's laws. The flow is "pushed down" do the plane must be "pushed up". Of course in this sense, it doesn't matter how this downwash is generated or how efficiently, only that it is generated.

Bernoulli's principle is one way to calculate the lift on a wing in certain situations (the wing cannot be separated, for example). Given a velocity distribution over a wing, you can use Bernoulli's equation to deduce the pressures on the wing and hence the lift. It says nothing about how you find sai velocities or the best shape of a wing. Bernoulli's equation is merely a tool; it cannot explain lift completely.

Now, any wing that generates lift must, by definition, deflect the air downward, and the same wing will also have a higher pressure below than above. You can connect the two using what is called the Kutta condition. This states that for an object with a sharp trailing edge, the rear stagnation point must be at that trailing edge rather than the location predicted by inviscid methods, which results in a net circulation around the airfoil and therefore a velocity difference and pressure difference on the surfaces as well as deflected flow coming off the back. This works for any shape with a sharp trailing edge (as all wings have) and does not require any assumptions about how the plane is flying.

A plane with a symmetric airfoil can fly because it has angle I attack and a sharp trailing edge. This allows the airfoil to deflect the flow downward. The same applies for a traditional airfoil flying upside down. In this case, flying inverted is her inefficient, but with enough angle of attack it can be done.

Flying vertically doesn't have lift in the traditional sense. The lift is provided solely by the thrust.

23. Jul 8, 2012

### Staff: Mentor

I'm curious as to why you would say that about Bernoulli's equation but not Newton's laws, since they have the same limitation. Predicting what the flow around a wing looks like is extraordinarily difficult.

24. Jul 8, 2012

### Staff: Mentor

Moving faster means more air is flowing over the wing. More air thrown downward means more force pushing the wing upward.

25. Jul 8, 2012