Jump to content

# Why an Airplane Flies (Bernoulli's Principle vs. Newton's Third Law)

## Recommended Posts

An airfoil will produce upwards lift regardless if it is upside down or right side up, base on its angle of attack. Even a flat board can produce upwards lift if the angle attack is such that it will allow the airflow to flow around the board, and the airflow around the upper camber has a longer distance to travel than on bottom. (upper camber being defined as the side furthest away from the ground.) See here at Nasa. You will need Java to run the interactive demonstration.

The longer camber forces the airflow to speed up. As the airflow comes off the trailing edge, because of the angle of the airfoil, forces the air down, aka Downwash. This gives rise to Newton.

The faster airflow on top creates a low pressure (Bernoulli) when compared to the pressure under the airfoil. This differential air pressure and the downwash helps create the lift to keep the plane flying.

All wings are attached to aircraft with a positive angle of incident. This will allow the airfoil to have a positive angle of attack, and producing lift, while leaving the fuselage to be level. Now an aerobatic airplane, with symmetrical airfoils, will have a small angle of incidence. This allows the plane to fly upside down and appear level. Normal aircraft have asymmetrical airfoils, which makes the wing much more efficient in normal (non upside down) flight. An asymmetrical airfoil can fly upside down, unfortunately the airplane will need to have an extreme pitch up attitude to accomplish this.

• Replies 206
• Created
• Last Reply

#### Posted Images

An airfoil will produce upwards lift regardless if it is upside down or right side up, base on its angle of attack. Even a flat board can produce upwards lift if the angle attack is such that it will allow the airflow to flow around the board, and the airflow around the upper camber has a longer distance to travel than on bottom. (upper camber being defined as the side furthest away from the ground.) See here at Nasa. You will need Java to run the interactive demonstration.

The longer camber forces the airflow to speed up. As the airflow comes off the trailing edge, because of the angle of the airfoil, forces the air down, aka Downwash. This gives rise to Newton.

The faster airflow on top creates a low pressure (Bernoulli) when compared to the pressure under the airfoil. This differential air pressure and the downwash helps create the lift to keep the plane flying.

All wings are attached to aircraft with a positive angle of incident. This will allow the airfoil to have a positive angle of attack, and producing lift, while leaving the fuselage to be level. Now an aerobatic airplane, with symmetrical airfoils, will have a small angle of incidence. This allows the plane to fly upside down and appear level. Normal aircraft have asymmetrical airfoils, which makes the wing much more efficient in normal (non upside down) flight. An asymmetrical airfoil can fly upside down, unfortunately the airplane will need to have an extreme pitch up attitude to accomplish this.

I don't disagree with any of this, but would like to add some extra material.

Not all aircraft can fly inverted especially helicopters. This is despite the recent movie from Hollywood.

Those that can are not as nimble (could any actually take off or land if they had the gear?). As noted above they have to adopt an odd attitude to counter the inverted tailplane contribution and any assymetries in the main airfoils.

It is good to see that someone is observing that there is more than one physical law obeyed. That is it is not Newton v Bernoulli, since both are obeyed. Of course Kelvin and Jukowski are also obeyed.

As regards the pressure and velocity, there is not a single upper and lower pressure and velocity. Both are subject to a large variation along the airfoil.

Further not only is there a relative difference, but in general the air above is less than average stream pressure and the air below greater than average stream pressure.

The attached sketch shows typical detail (the airfoil has a valid angle of attack and will not be horizontal)

##### Share on other sites

So, just to be clear, we all agree that an asymmetrical airfoil of the Clark y type and zero angle of attack will produce lift due to the Bernoulli effect (reduced average pressure above the airfoil due to higher fluid speed above the airfoil ).

Whether we invoke Jukowski's circulation or not, Bernoulli is very easy to explain. If we consider a streamline to be a tube, then, in the absence of sinks or sources, continuity dictates that the amount of fluid entering is equal to the amount exiting. If we now introduce a constriction in the pipe, that same continuity condition dictates that the fluid must speed up through the constriction so as to pass the same amount through a smaller area. There is only one force in the tube that can spped up the fluid as it enters the constriction and slow it down again as it exits the constriction, and that is pressure. The pressure in the pipe ahead of the constriction must be higher than the pressure in the constriction to speed up the fluid, and again, after the constriction it must be higher to slow the fluid down again. Through various 'slight of hand' arguments, we can consider a square tube for the streamline, move the opposing side of the tube out to infinity and even get rid of the tube altogether leaving just a D ( on its back ) constrictive element to represent the wing.

That is my verbal explanation for Bernoulli lift.

If, on the other hand we consider wings, flat plates or even hands sticking out a car window at speed, we note that to produce lift an angle of incidence MUST be used to produce a very inefficient lift with lots of induced drag. That is how planes can fly upside down, and it doesn't involve Bernoulli.

And that is the crux of my original discussion with John.

##### Share on other sites

Studiot- I hope this picture is what you are looking for. I found a really neat App on the iphone that allows me to have a windtunnel at any time.

The picture is on an airfoil and the blue indicates low pressure and the red/yellow high pressure. This is Bernoulli's. Notice the downwash off the trailing edge, that is Newton.

##### Share on other sites

That's a nice pic, ccweb.

It shows all four laws in action at once, though you have a slight misapprehension about what they refer to.

Newton's laws apply throughout and can be used to derive Bernoulli's equation (though there are other ways). Bernoulli's equation is a direct consequence of Newton's laws.

Newton's laws, however, are not directly responsible for what you call the downwash (which is not a force but a movement of air). That is due to the clockwise Jukowski circulation around the airfoil.

This is important because it explains the velocity difference. The circulation flow is up in front of the airfoil. left to right across the top, down at the back and right to left underneath.

As a result the circulation flow assists the air stream over the top and opposes it underneath.

So the velocity is higher over the top and lower underneath.

So to the fourth theorem, that of Kelvin. To the right behind the airfoil you can see the wake of trailing countervortices. They are revolving in an anticlockwise direction. This reduces the overall rotation or vorticity back to zero along the streamlines, in accordance with Kelvin's theorem.

And talking of streamlines, MigL, Bernouilli applies along any streamline or streamtube, not just those in pipes.

But remember it applies only to parcels of fluid along the same streamline, but at different locations or times and only in steady flow.

The pic also shows how the positive angle of attack leads to a clockwise rotation for the circulation. This is because the first part of the airfoil to encounter the flow is the leading edge. This solid object wants to deflect the flow up and down but comparing the two paths it can be seen that above the airfoil the air is clear, but below the deflected air is 'obstructed' by air gathered under the airfoil by the next part to encounter the flow. This obstruction = higher pressure.

I am sorry that last was not well explained, I have been meaning to draw a series of sketches showing how the circulation develops as a consequence of the attack and this gathering process.

##### Share on other sites

• 1 year later...

Airplanes can and do fly in stationary air. This means air cannot flow across the wings. The airplane moves...not the air. So bernoulli has nothing to do with lift. A wind tunnel uses moving air across a stationary wing, and those with tunnel vision assume that Bernoulli is proven. This is on a par with Ptolemy who said the heavens go round the Earth. We all know that Ptolemy was wrong... and pretty soon we'll all know that the use of Bernoulli's theorem is wrong in explaining lift.

Incidentally, the physics that explain the lift of an airplane are the same as the physics that explain the lift of a helium balloon. Put simply: air must descend in a downwash in order to produce lift.

##### Share on other sites

Airplanes can and do fly in stationary air. This means air cannot flow across the wings. The airplane moves...not the air. So bernoulli has nothing to do with lift. A wind tunnel uses moving air across a stationary wing, and those with tunnel vision assume that Bernoulli is proven. This is on a par with Ptolemy who said the heavens go round the Earth. We all know that Ptolemy was wrong... and pretty soon we'll all know that the use of Bernoulli's theorem is wrong in explaining lift.

Incidentally, the physics that explain the lift of an airplane are the same as the physics that explain the lift of a helium balloon. Put simply: air must descend in a downwash in order to produce lift.

1) Have you never heard of relative velocity?

2) We are talking about 'heavier than air' craft which, unlike a helium balloon do not benefit from positive bouyancy forces.

Why is it so difficult for people to accept that no laws are broken and that more than one physical law is in action in developing the lift force?

There is a fluid, so whether it is standing or moving in a laminar manner you will always be able to apply Bernoulli's theorem to it.

Whether or not this theorem supplies the desired result is another matter.

Equally Newton's laws apply to this motion, or lack of it, since this motion is non relativistic.

In structural engineering it is commonly realised that one may calculate directly witht he stresses and strains or convet these to forces and displacements or use energy methods, including the calculus of variations, or even some combination of these.

Sometimes one method is easier than another, but all yield basically the same results.

That is because they are all satified - they all apply.

There are fewer options in aeronautical engineering so why is the same realisation so difficult?

Both are extensions of the same continuum mechanics.

##### Share on other sites

Airplanes can and do fly in stationary air. This means air cannot flow across the wings. The airplane moves...not the air. So bernoulli has nothing to do with lift. A wind tunnel uses moving air across a stationary wing, and those with tunnel vision assume that Bernoulli is proven. This is on a par with Ptolemy who said the heavens go round the Earth. We all know that Ptolemy was wrong... and pretty soon we'll all know that the use of Bernoulli's theorem is wrong in explaining lift.

Incidentally, the physics that explain the lift of an airplane are the same as the physics that explain the lift of a helium balloon. Put simply: air must descend in a downwash in order to produce lift.

Well, I think you are right about this " air must descend in a downwash in order to produce lift."

But the rest is mainly wrong.

"

##### Share on other sites

Well, I think you are right about this " air must descend in a downwash in order to produce lift."

Yes, but it may help to think about the question

"Where does the descending air come from ?"

##### Share on other sites

Studiot, take a look at my website, talosperdix, downwash page. You'll see the downwash descend behind an airplane 's wings. It is truly frightening.

##### Share on other sites

Post#54 in this thread showed a lovely picture of the downwash, as already noted.

I think it would be entirely appropriate to provide a proper link to the website you mention, as it is not advertising.

As regards this thread, you have not answered my question,

Where does the descending air come from?

##### Share on other sites

Cuthber,

When a helium balloon rises up it gains potential energy. The higher it goes the more potential energy it acquires. Where does this energy come from? How does the displacement theory explain where the energy comes from? What is it that loses potential energy in order for the balloon to gain it.

Likewise, an airplane gains potential energy when it goes up. What is it that loses potential energy in order for the airplane to gain it?

##### Share on other sites

Likewise, an airplane gains potential energy when it goes up. What is it that loses potential energy in order for the airplane to gain it?

Pity you didn't answer my courteous reply to your post.

I would have thought the answer to your question here fairly obvious, but often forgotten.

Fuel is expended and work done to fill the balloon or drive the aeroplane.

##### Share on other sites

Incidentally, the physics that explain the lift of an airplane are the same as the physics that explain the lift of a helium balloon. Put simply: air must descend in a downwash in order to produce lift.

Archimedes is rolling in his grave.

##### Share on other sites

Cuthber,

When a helium balloon rises up it gains potential energy.

How does the displacement theory explain where the energy comes from?

"When a helium balloon rises up it gains potential energy."

No it doesn't.

If I have a big He balloon at ground level, I can tie a string to it and wrap that sting round the shaft of a generator. If I let go of the balloon I can get electricity from the generator.

If I start with the balloon further up I can't get as much electricity before the lift from the balloon is too small to turn the shaft.

So, as the balloon rises it looses potential energy.

"How does the displacement theory explain where the energy comes from?"

You tell me- it's not my theory.

However the answer to the question 'Where does the energy come from to make the electricity' is simple. As the balloon rises an equal volume of air falls.

In that regard, it's true to say that there is some downward flow of air that you can stretch a definition, and call a "downwash".

##### Share on other sites

Airplanes can and do fly in stationary air. This means air cannot flow across the wings.

Incidentally, the physics that explain the lift of an airplane are the same as the physics that explain the lift of a helium balloon.

Ah, that explains why they have to tie airplanes down after they have landed.

##### Share on other sites

Ha-Ha

I like that Strange.

##### Share on other sites

Airplanes can and do fly in stationary air. This means air cannot flow across the wings. The airplane moves...not the air. So bernoulli has nothing to do with lift. A wind tunnel uses moving air across a stationary wing, and those with tunnel vision assume that Bernoulli is proven. This is on a par with Ptolemy who said the heavens go round the Earth. We all know that Ptolemy was wrong... and pretty soon we'll all know that the use of Bernoulli's theorem is wrong in explaining lift.

Incidentally, the physics that explain the lift of an airplane are the same as the physics that explain the lift of a helium balloon. Put simply: air must descend in a downwash in order to produce lift.

The extensive history of rigid airships show that an aircraft with or without neutral buoyancy can attain lift from an airfoil regardless of whether it is the crafts airfoil or the air itself that is moving.

##### Share on other sites

Further, rotation or downwash from a wing produce induced drag, as energy is expended to alter the direction of the airflow downwards. This induced drag increases with pressure ( lo altitude, higher density ), slowing the plane.

A helium balloon, on the other hand, rises faster in higher pressure/density air implying that a different mechanism is at work.

##### Share on other sites

Further, rotation or downwash from a wing produce induced drag, as energy is expended to alter the direction of the airflow downwards. This induced drag increases with pressure ( lo altitude, higher density ), slowing the plane.

A helium balloon, on the other hand, rises faster in higher pressure/density air implying that a different mechanism is at work.

And you can measure lift with no airflow (downwash) whatsoever.

##### Share on other sites

Barn doors” or flat plates can fly.

Symmetrical cross section wings fly very well. (Same distance over top and bottom).

If there truly is symmetry here, how do you get lift in a particular direction? How does nature "know" which way is up?

Electrons are said to possess spin, contributing vorticity to atoms and molecules. These in turn form part of larger vortices such as ocean and atmospheric currents.

While the sentiment is true, it is exceedingly hard to measure, and the net intrinsic angular momentum of atoms is negligible on a macroscopic scale.

———

As someone else has pointed out, the Bernoulli equation is merely a restatement of Newton's laws in terms of energy. As long as the assumptions of the equation hold, to say that the equation doesn't apply (or explain lift) is sort of like saying energy isn't conserved.

##### Share on other sites

Well, Talos, you finally did post a link.

This one is marginally better than the website under reconstruction you referred to before in that there is one statement I can agree with.

Air molecules separated at the wings leading edge do not neatly re-join at the
tailing edge.

Further the author seems to be under some misapprehension about vorticity in fluid dynamics.

The air in front of the aircraft has zero vorticity, in general.

Therefore, by Kelvin's theorem, the air some distance to the rear of the flying craft must also have zero vorticity.

The trick is to create not one but two opposing vortices that cancel each other out over the region around and immediately behind the plane.

For a craft flying from right to left across the page the surrounding vortex is clockwise and gives rise to the circulation in Jukowski's theorem and the downwash you have already described.

The counter vortex is found some way behind the craft in the trailing wake. It is usually in the form of shed vortices from the wing tips.

Concentrating solely upon the vortex surrounding the craft does not conform to Kelvins theorem.

Edited by studiot
##### Share on other sites

Thanks for the reminder. Our other website is:

www.talosperdix.com

You may find some more apparently contradictory views.

You could also take a look and listen to this David F. Anderson TED talk on understanding flight: It lasts about 50 minutes.

http://www.youtube.com/watch?v=hQ99JkaOwEk

##### Share on other sites

Talos, do you just wish to preach or are you interested in discussion, which involves acknowledging the contributions of others present in the discussion?

##### Share on other sites

This topic is now closed to further replies.

×

• #### Activity

• Leaderboard
×
• Create New...

## Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.