How does the weight of airplanes affect atmospheric pressure?

[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?

 

[jedishrfu]

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.

 

[phenom01]

http://www.youtube.com/watch?v=u0k8vGHxYQE&feature=related

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.

 

[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.

 

[AlephZero]

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.

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.

 

[canadave]

http://www.youtube.com/watch?v=u0k8vGHxYQE&feature=related

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.

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.

 

[D H]

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.

 

[Naty1]

And DH wins the 'prize' :

A fixed wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed and the shape of the wings...

http://en.wikipedia.org/wiki/Fixed-wing_aircraft#Wing_geometry

and note the discussion here:

Wing Geometry
http://en.wikipedia.org/wiki/Fixed-wing_aircraft#Wing_geometry

You might also find previous discussions on this topic...I know there have been some...finding them is always the issue.

 

[cosmik debris]

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.

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.

 

[DaveC426913]

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.

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.

 

[CWatters]

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?

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.

That leads neatly on to...

Also, how can airplanes fly vertical?

Google "Helicopter".

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.

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

 

[russ_watters]

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.

 

[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.

 

[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.

 

[azizlwl]

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

 

[russ_watters]

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.

What does the thrust have to do with the lift?

 

[russ_watters]

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

Read my first post.

 

[phenom01]

What does the thrust have to do with the lift?

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

 

[russ_watters]

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

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

 

[phenom01]

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

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

 

[DaveC426913]

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

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.

 

[boneh3ad]

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.

 

[russ_watters]

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.

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.

 

[russ_watters]

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

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

 

[boneh3ad]

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.

That's fair enough, but the distinction I hope to make is that Bernoulli's equation is a useful tool but in no way explains where lift comes from. Newton's laws can, even if they don't explain necessarily how that downwash is generated, only that it's generation signifies lift.

 

[rcgldr]

Given a velocity distribution over a wing, you can use Bernoulli's equation to deduce the pressures on the wing and hence the lift.

I'm not sure how accurate this would be. Part of the lift is due to a non-Bernoulli interaction between air and a wing that increases the mechanical energy of the air (wrt ambient air).

 

[russ_watters]

I'm not sure how accurate this would be. Part of the lift is due to a non-Bernoulli interaction between air and a wing that increases the mechanical energy of the air (wrt ambient air).

What interaction is that?

 

[boneh3ad]

I'm not sure how accurate this would be. Part of the lift is due to a non-Bernoulli interaction between air and a wing that increases the mechanical energy of the air (wrt ambient air).

Explain that because either I just am not understanding what you mean or else I completely disagree. The lift is exacty the opposite reaction to the deflection of the air downward. That isn't a practical calculation, but the pressure distribution, which in many cases gives the same answer, is practical.

The wing only adds energy to the air through the action of viscosity since the wing therefore drags some air along with it. That creates drag, though, not lift.

 

[russ_watters]

That's fair enough, but the distinction I hope to make is that Bernoulli's equation is a useful tool but in no way explains where lift comes from. Newton's laws can, even if they don't explain necessarily how that downwash is generated, only that it's generation signifies lift. [emphasis added]

So again, same limitation, isn't it? You can find the lift by using the velocity profile to find pressure or momentum change, but neither tell you what that velocity profile will look like.

 

[CWatters]

I'm afraid this subject (is it Bernoulli or Newton) comes up all the time on forums around the world. People argue strongly. For every person who favours one theory there is another that has the opposite view.

It's a false dichotomy. Neither is right or wrong.

http://www.pprune.org/professional-...und-studies/468765-lift-bernoulli-Newton.html

Last I time I passed a degree in aeronautics, Bernoulli's equation was derived starting with Newton's laws. It is utter cobblers to separate the two.

 

[DrewD]

http://www.grc.nasa.gov/WWW/k-12/airplane/bernnew.html

This site was posted on PF and I read it a while back and felt that it was informative. It has a simple explanation on the first page and some deeper stuff on the second. My recollection is that it says that either a freebody force explanation or bernoulli fluid laws can work. But it helps to clarify the common mistakes that are made. I haven't the time to reread it now, so I hope that it is as I remember (if its not let's pretend that they changed it since I read it)

 

[boneh3ad]

So again, same limitation, isn't it? You can find the lift by using the velocity profile to find pressure or momentum change, but neither tell you what that velocity profile will look like.

Yes. For that you can apply the Kutta condition and get your answer for the case of no separation. Otherwise you need the full Navier-Stokes equations.

 

[rcgldr]

I'm not sure how accurate this would be. Part of the lift is due to a non-Bernoulli interaction between air and a wing that increases the mechanical energy of the air (wrt ambient air).

What interaction is that?

Explain that

After a wing passes through a volume of air, the affected air ends up with a non-zero exit velocity (the velocity of the affected air when it's pressure returns to ambient). Using the unaffected air as a frame of reference, the affected air's mechanical energy is increased by a wing, which means work is done, which violates the rule for Bernoulli.

A similar process occurs with a propeller or rotor, except that the pressure differential is greater than a typical wing, and there is a greater amount of induced flow (the inwash ahead of the propeller or rotor). From a NASA article:

... at the exit, the velocity is greater than free stream because the propeller does work on the airflow. We can apply Bernoulli's equation to the air in front of the propeller and to the air behind the propeller. But we cannot apply Bernoulli's equation across the propeller disk because the work performed ...

propanl.htm

 

[boneh3ad]

After a wing passes through a volume of air, the affected air ends up with a non-zero exit velocity (the velocity of the affected air when it's pressure returns to ambient). Using the unaffected air as a frame of reference, the affected air's mechanical energy is increased by a wing, which means work is done, which violates the rule for Bernoulli.

A similar process occurs with a propeller or rotor, except that the pressure differential is greater than a typical wing, and there is a greater amount of induced flow (the inwash ahead of the propeller or rotor). From a NASA article:

... at the exit, the velocity is greater than free stream because the propeller does work on the airflow. We can apply Bernoulli's equation to the air in front of the propeller and to the air behind the propeller. But we cannot apply Bernoulli's equation across the propeller disk because the work performed ...

propanl.htm

Yes, you refer to the wake. However, this really relates to drag, not so much to lift.

 

[rcgldr]

Yes, you refer to the wake. However, this really relates to drag, not so much to lift.

The direction of the "wake" is mostly downwards (lift), and somewhat forwards (drag). For a propeller, the "wake" is mostly aftwards (thrust), and somewhat circular (drag).

The force exerted by gravity onto the aircraft is in turn exerted by the aircraft onto the air, and eventually, that force is exerted by the air onto the Earth's suface (forces do not vanish), via a continuous impulse (downward shifting of air) that originated at the aircraft.

The other law of physics involved with an aircraft (in level flight) is that the impulse (force x time) exterted by the aircraft onto a volume of air over some period of time results in a corresponding change in momentum of that air, and in the real world, during the interaction between aircraft and air, the downwards component of velocity is not offset by a reduction of pressure, so there's a net "exit velocity", representing the increase in mechanical energy of the air affected by an aircraft.

This NASA article includes an image of the downwash effect on a cloud:

downwash.htm