How do airplanes fly with heavy weight and air resistance?

[ProgressNation]

I wonder how airplanes fly? I know maybe it's a tough question. Does the airplane fans produce huge force that makes the airplane boosts it's speed?The wings make it fly, and I don't know how it's tail works. But how do the airplanes fly according to it's heavy weight? Plus there is a big air resistance. I'd really love to know how the airplanes work, they're beautifully amazing!
Thank you!

 

[DiracPool]

This is kind of what the PF mentors typically consider an overbroad question. However, I'll interject one interesting tidbit here I just stumbled across because it's interesting and relevant to your question. I've always learned that airplanes fly because the wings create lift through a differential in air pressure created by the shape of the wing. Supposedly, the curved shape of the wing at the top causes air to move more rapidly there, creating a lower air pressure over the top of the wing which causes the plane to lift. However, I just recently watched a documentary that aimed to debunk this as a myth, saying that there is no air pressure difference since the laminar flow of the wind simply wraps around the wing and meets at the other side.

The actual cause of the lift, according to this documentary, was simply the force of the wind hitting the bottom of the wing, and that lift was created because planes fly slightly tilted with their pitch upward into the wind which forces the plane upwards. They debunked the pressure differential idea by saying if that were true, then planes wouldn't be able to fly upside-down, which they obviously could do. I thought that was kid of interesting. Unfortunately, I can't remember the name of the documentary.

 

[ProgressNation]

Thank you for sharing your answer, it's honor for me! But another thing that I want to know is that why does the airplane fans exist? Does the airplane fans exert very huge force which causes the airplane to accelerate very fast where can air resistance go through the wings and make it fly?
Thank you!

 

[D H]

See [thread=618500]this thread[/thread].

 

[CWatters]

Before long you will come across groups of people who argue endlessly over two different theories. One group say that lift is caused by the pressure difference above and below the wing. The other group say it's caused by the wing deflecting the air downwards. The real answer is both are correct...

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

 

[ProgressNation]

Ok does the fan exerts a huge force to accelerate the engine?? If so, how?

 

[CWatters]

I don't know how it's tail works

That's one reason why it took man so long to figure out how to fly.

Put simply.. There are lots of different forces that act on an airplane. An airplane has to be designed so that if one of these forces causes the aircraft to pitch (point) up or down then other forces restore the balance. This is known as stability.

Consider a ball balanced on top of a hill. If you give it a slight knock it will roll off down the hill. The position of the ball is said to be "unstable". If you have a ball at the bottom of a valley and you give it a slight knock the ball may move but it will eventually return to it's original position. In this case the ball is said to be "stable".

The tail on a plane basically makes the plane stable. It also allows the plane to be steered but that's another story.

 

[CWatters]

Ok does the fan exerts a huge force to accelerate the engine?? If so, how?

Start at the back of the engine...

Near the back there is a combustion chamber where fuel is burnt in compressed air. As it burns it expands and rushes out of the back of the engine. A turbine fan in this hot exhaust stream works like a windmill and drives a shaft running down the middle of the engine. This shaft power an air compressor in the centre and the big fan at the front.

The big fan does two jobs. It provides some air for the compressor in the core of the engine but it also sucks in a lot of air that bypasses the core and is sent straight out the back.

Pushing air backwards has the effect of pushing the plane forwards.

 

[D H]

Before long you will come across groups of people who argue endlessly over two different theories. One group say that lift is caused by the pressure difference above and below the wing. The other group say it's caused by the wing deflecting the air downwards.

That's what I call the "Tastes great!" "No! Less filling!" debate over how airplanes fly. The real answer it's not an either/or proposition. Both explanations are correct.

The real answer is both are correct...

Birds of a feather!

 

[Dale]

That's what I call the "Tastes great!" "No! Less filling!" debate

 

[A.T.]

What do you guys make of the "New Theory of Flight" by Claes Johnson?

https://www.youtube.com/watch?v=t7e_6bkUFzE

http://secretofflight.wordpress.com/

Is this really something completely new, or just more detail? Does his "elegant separation" apply to stalled wings only, or in general?

 

[boneh3ad]

However, I just recently watched a documentary that aimed to debunk this as a myth, saying that there is no air pressure difference since the laminar flow of the wind simply wraps around the wing and meets at the other side.

This is incorrect. If there is a lift force that is generated by the air, it must be accompanied by a pressure difference. Further, the concept of laminar flow has essentially nothing directly to do with the generation of lift. It can affect the values of lift and drag, but it is not part of the fundamental reason lift exists.

The actual cause of the lift, according to this documentary, was simply the force of the wind hitting the bottom of the wing, and that lift was created because planes fly slightly tilted with their pitch upward into the wind which forces the plane upwards. They debunked the pressure differential idea by saying if that were true, then planes wouldn't be able to fly upside-down, which they obviously could do. I thought that was kid of interesting. Unfortunately, I can't remember the name of the documentary.

Well, they are mostly right. You can really look at lift two ways: either through the deflection of the air downward or through the pressure difference created on either side of the airfoil. Both of these can accurately describe the lift generated by a wing and both allow for a plane's ability to fly upside down. The thing that makes many of the pressure different explanations wrong is that they don't explain (or incorrectly explain) why that pressure difference exists (hint: it has nothing to do with "equal transit time"). If done correctly, you could solve for the flow around an airfoil and use the pressure differences to calculate lift and drag or draw a control volume around it and look at hte net momentum change due to action of the body and you would get the same answer for lift and drag. Both explanations work equally well. The pressure explanation is often a lot easier to use in practice, however.

Which brings me to...

Before long you will come across groups of people who argue endlessly over two different theories. One group say that lift is caused by the pressure difference above and below the wing. The other group say it's caused by the wing deflecting the air downwards. The real answer is both are correct...

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

As CWatters alludes to, this causes a downright comical debate between two warring factions who, in essence, are both correct anyway. It is sort of a chicken vs. egg debate.

What do you guys make of the "New Theory of Flight" by Claes Johnson?

https://www.youtube.com/watch?v=t7e_6bkUFzE

http://secretofflight.wordpress.com/

Is this really something completely new, or just more detail? Does his "elegant separation" apply to stalled wings only, or in general?

This sounds like a bit of over-promoting their own work. They really didn't present anything new in that presentation, and I would even argue that some if it is wrong. For example, saying Kutta-Joukowski does not describe real physics is true by definition, as that theory makes the assumption that the flow is inviscid and incompressible, which obviously doesn't describe the real world. Further, pretty much everyone already knew that you had three major factors affecting lift and drag: vertical pressure differences, horizontal pressure differences and separation (viscosity would be another major contributor as well). I really see nothing new here. Further, I disagree with the idea that a large rounded-edge airfoil will necessarily produce lift. At low angle of attack with no separation, it would not. It requires that separation to prevent the flow from simply remaining symmetric.

 

[sophiecentaur]

The thing that the 'pressure difference' brigade seem to ignore is the fact that there needs to be a net downwards motion of the surrounding air in order to produce a net lift force on the plane (reaction / rate of momentum change of the air). How that downward flow is achieved, and how efficiently, is what everyone is really arguing about, I think. The downdraught from a hovering helicopter is more noticeably there because all the displaced air is under the same spot in the sky. For a fixed wing craft, the effect is much more subtle because it's spread out over the whole of the flight path and any coherent motion will quickly be damped out and the kinetic energy dissipated.
Why do Scientists always want to say "this is what really happens" instead of "this model works"?

I did like the animations on that youtube sequence,

 

[nst.john]

Well if the air is being split by the winds at two different forces, wouldn't they be split into two component pressures and add together to the original pressure in the end?

 

[boneh3ad]

The thing that the 'pressure difference' brigade seem to ignore is the fact that there needs to be a net downwards motion of the surrounding air in order to produce a net lift force on the plane (reaction / rate of momentum change of the air).

The other side of that coin is that a lot of the downward deflection group like to ignore the fact that there also must exist a pressure difference and that the two are really part of the same phenomenon.

Why do Scientists always want to say "this is what really happens" instead of "this model works"?

Ego.

 

[OmCheeto]

..."this model works"...

This model always worked for me:

There was also a version with a high power elastomer based engine, as I recall.

(google google google)

ah ha!

That model worked too.

 

[sophiecentaur]

Ego can often result in Egg on Face!

Om Cheeto - the wheels came off your idea!

 

[sophiecentaur]

That pressure difference idea really sucks.

 

[boneh3ad]

That pressure difference idea really sucks.

Come now, science never sucks!

It blows.

 

[Filip Larsen]

The actual cause of the lift, according to this documentary, was simply the force of the wind hitting the bottom of the wing, and that lift was created because planes fly slightly tilted with their pitch upward into the wind which forces the plane upwards.

If by "simply" they mean "only", then how did they explain loss of lift when flow separates from the upper surface (e.g. "stall"), or when a wing deploy spoilers. How would they explain that blowing air between two sheets of paper will make the papers move closer to each other rather that away from each other?

In general you have to look at the complete flow to understand and model what is going on, and from that you can often give equivalent explanations using either forces, pressure differences, conservation of momentum, etc. As others also have noted, it may not make much sense to try assign one explanation as being more fundamental. However, what I think you can consider fundamental in this context, is that an air flow give rise to a lower static pressure (as the example with the two sheets of paper illustrate).

 

[DiracPool]

If by "simply" they mean "only", then how did they explain loss of lift when flow separates from the upper surface (e.g. "stall"), or when a wing deploy spoilers. How would they explain that blowing air between two sheets of paper will make the papers move closer to each other rather that away from each other?

I don't think they got into it that deeply, it was a side note to some other main theme of the documentary. And I'm not taking sides, I'm no fluid mechanic, I just remember being a bit stunned when I heard it because I was brought up on the pressure differential model. But it kind of made sense to me having been a surfer for the first 30 years of my life. When you surf or waterski or wakeboard, you maintain lift above the surface through angling your nose up and deflecting the water down. You can slow down your velocity by driving your back foot down if you want to let the barrel catch up to you so you can get "tubed" If you angle the pitch of your nose high enough you actually will "stall" and fall off the wave. If you're a water skier, you start out underwater and "lift" to the surface through angling the pitch of your ski's or wakeboard upwards as the boat begins to pull you from rest. So, to me, this idea of lift being generated through the deflection of air downwards by the airfoil made sense. I had just never thought of it that way before since I had been indoctrinated into the pressure model. But I can see how both may play a combined role.

 

[D H]

The actual cause of the lift, according to this documentary, was simply the force of the wind hitting the bottom of the wing, and that lift was created because planes fly slightly tilted with their pitch upward into the wind which forces the plane upwards.

What documentary are you talking about? A link would be nice. There's a lot of junk science on the internet and on TV.

That explanation is pretty much bogus. It describes why a sheet of plywood gets lift. It does not explain how a well-designed wing gets lift. That sheet of plywood is pushed by the wind. A well designed wing gets most of its lift from the upper surface, not the bottom one.

They debunked the pressure differential idea by saying if that were true, then planes wouldn't be able to fly upside-down, which they obviously could do.

That sounds like a non sequitur.

 

[DiracPool]

What documentary are you talking about? A link would be nice. There's a lot of junk science on the internet and on TV.

I scoured my DVR but nothing rang a bell. I even ran a search online and couldn't find it. It was just a few minute segment of an hour show so I'm not surprised. However, I did come across an article in that search that is essentially the guy's argument verbatim, so here it is:

<crackpot link deleted>[/color]

Again, I'm not qualified to make a scientific assessment on this so it would be interesting to hear some of your reactions to it.

 

[Buckleymanor]

Well if the air is being split by the winds at two different forces, wouldn't they be split into two component pressures and add together to the original pressure in the end?

Yes, after the air had moved across the surface of the wing.
When the flow is split and moveing over and below the wing then the pressure is different because one of the split flows travels a longer distance than the other over the wing, which gives rise to lift.

 

[DiracPool]

Yes, after the air had moved across the surface of the wing.
When the flow is split and moveing over and below the wing then the pressure is different because one of the split flows travels a longer distance than the other over the wing, which gives rise to lift.

Is there a way to quantify the lifting force of that pressure difference taking in, say, the area of wing, angle of incidence to the incoming air, airspeed etc?

 

[Buckleymanor]

Is there a way to quantify the lifting force of that pressure difference taking in, say, the area of wing, angle of incidence to the incoming air, airspeed etc?

Yes, though there would be a lot of variables to consider some of which I would probably miss.
If you were an aircraft designer, probably a piece of angel cake.

 

[DiracPool]

Yes, though there would be a lot of variables to consider some of which I would probably miss.
If you were an aircraft designer, probably a piece of angel cake.

I guess there would have to be. That's what aerospace engineers do. That's why it's so puzzling to me why there is any ambiguity at all in this area. These engineers must know where the forces are coming from and what they are quantitatively and qualitatively, don't they?

 

[D H]

I scoured my DVR but nothing rang a bell. I even ran a search online and couldn't find it. It was just a few minute segment of an hour show so I'm not surprised. However, I did come across an article in that search that is essentially the guy's argument verbatim, so here it is:

<crackpot link deleted>[/color]

Again, I'm not qualified to make a scientific assessment on this so it would be interesting to hear some of your reactions to it.

As you might have noticed, I deleted that link. It has a few things right, but it also has far too many things that are flat out wrong.

 

[DiracPool]

As you might have noticed, I deleted that link. It has a few things right, but it also has far too many things that are flat out wrong.

Fair enough.

 

[cjl]

Yes, after the air had moved across the surface of the wing.
When the flow is split and moveing over and below the wing then the pressure is different because one of the split flows travels a longer distance than the other over the wing, which gives rise to lift.

As was pointed out (I believe by bonehead) earlier in this thread, it isn't because the travel distance is different. The shape of the wing causes the air velocity over and under the wing to be dramatically different, and that is what "causes" lift (although as mentioned before, there are several correct ways to look at it).

 

[cjl]

That explanation is pretty much bogus. It describes why a sheet of plywood gets lift. It does not explain how a well-designed wing gets lift. That sheet of plywood is pushed by the wind. A well designed wing gets most of its lift from the upper surface, not the bottom one.

Interestingly, even a flat plate airfoil still gets a large fraction of its lift from the upper surface (possibly even a majority, but I don't know off the top of my head). There's no shape you can really make (to my knowledge) that will get almost all of its lift from the lower surface at low speed. At very high (high supersonic) speed, this can change, but that's an entirely separate problem

 

[boneh3ad]

I'd like to know how you define a portion of lift as coming from an upper or lower surface. I have never run across that statement except here on this forum. To be honest, I find that statement to be incredibly misleading in every logical way I can see it making sense.

 

[voko]

It is often said that with an engine big enough, you can make a barn door fly.

Note that does not necessarily imply a stable fly.

 

[Buckleymanor]

As was pointed out (I believe by bonehead) earlier in this thread, it isn't because the travel distance is different. The shape of the wing causes the air velocity over and under the wing to be dramatically different, and that is what "causes" lift (although as mentioned before, there are several correct ways to look at it).

bonehead mentions, (hint: it has nothing to do with "equal transit time").
By that I reckon he is hinting that the transit times are not equal nor should be to creat lift.
If the shape of the wing by any design causes the air velocity over and under the wing to be dramatically different then you get lift.
It does not have to be the case that the shape of the wing causes the travel time to be different but when it does you still get lift.
You could apply flaps and creat drag in one direction to creat lift.
The distance can be the same or different but as you say it's the difference between air velocity over and under or vice versa that causes a pressure differential which causes lift?

 

[Oryon]

As a pilot,we are taught that lift is provided by both the pressure difference and the angle of attack of the wing, (angle between the chord line of the wing and the direction of motion) lift and drag both increase as aoa increases, up to the critical aoa, which is when the airflow separates from the top of the wing to cause a stall. Aoa must increase as speed decreases to maintain lift. At some speed the aoa required exceeds the critical aoa, and the plane stalls. To see an extreme example, google blue angels slow flyby, and you will see a pronounced nose up attitude comapred to the forward motion of the airframe. The op question is very broad, and covers a large part of the initial ground school for a private pilot, due to its complexity. I would highly recommend reading a book of some form on the subject, perhaps something aimed at the fledgling student pilot.

Apologies for errors in spelling and typos, am sending this via my tablet.

 

[boneh3ad]

If the shape of the wing by any design causes the air velocity over and under the wing to be dramatically different then you get lift.
It does not have to be the case that the shape of the wing causes the travel time to be different but when it does you still get lift.
You could apply flaps and creat drag in one direction to creat lift.
The distance can be the same or different but as you say it's the difference between air velocity over and under or vice versa that causes a pressure differential which causes lift?

That is reasonably accurate. There are a couple of clarifications or fixes to make to your statement though:

For one, in general, the transit time is not the same for a lift-generating body. You would need a very special case for that to occur, and I am not even really sure that you could design a lifting body to do that. Generally speaking, the air traveling over the wing is much faster than that traveling under it (several times faster), so the top surface would have to be designed to be substantially longer, at which point it would likely stop being a lifting body. So, yes, it does not have to be the case that the shape causes the travel times to be different, but to take that one step further, I cannot imagine a situation where the travel times would be the same.

Second, be careful with correlation and causation. Lowering the flaps to create drag will certainly happen, but it is not the creation of that drag that causes the generation of lift. Lift and drag are two sides of the same coin here. Essentially, in lowering flaps, you are increasing the camber of the airfoil. This allows the airfoil to generate more lift at lower speeds and lower angles of attack (of the plane, anyway), and of course along with lift comes drag. You can't have one without the other. To explain why that is, it is perhaps easiest to think in terms of the net momentum change of the flow around the wing.

Any time you have lift, your airfoil will be necessarily deflecting air downward. If you were to draw a large box around the airfoil and do a control volume analysis, you would be able to integrate the total momentum coming into the control volume, which will be entirely in the horizontal direction, and then integrate the total momentum leaving the control volume, which, due to the deflection, would be partially horizontal and partially vertical. That addition of vertical momentum to the fluid is done by a force coming from the wing whose opposite according to Newton is lift. The flow will also have lost some momentum in the horizontal direction, which occurs due to a force coming from the body whose opposite and equal counterpart is drag. Put simply, with lift and drag, one doesn't causes the other. They both must occur at the same time.

Of course, you could explain this same thing by looking at the pressures over the airfoil and you would get the same answer for an inviscid flow. I simply find it easier to illustrate why and how drag and lift are related using Newton's laws and flow deflection.

 

[rcgldr]

It describes why a sheet of plywood gets lift. It does not explain how a well-designed wing gets lift. That sheet of plywood is pushed by the wind.

Interestingly, even a flat plate airfoil still gets a large fraction of its lift from the upper surface (possibly even a majority, but I don't know off the top of my head). There's no shape you can really make (to my knowledge) that will get almost all of its lift from the lower surface at low speed. At very high (high supersonic) speed, this can change, but that's an entirely separate problem

Even a flat plate airfoil, such as those balsa models posted by Omcheeto or the one in the link below get most of their lift by accelerating air downwards from above the wing. Part of the reason is that the low pressure zone above a wing diverts some of the oncoming flow to over the wing, reducing the flow below the wing.

http://www.4p8.com/eric.brasseur/glider2.html

As a pilot,we are taught that lift is provided by both the pressure difference and the angle of attack of the wing, (angle between the chord line of the wing and the direction of motion) lift and drag both increase as aoa increases, up to the critical aoa, which is when the airflow separates from the top of the wing to cause a stall. Aoa must increase as speed decreases to maintain lift.

Pressure difference and diversion of air are a function of angle of attack, air speed, and the air foil. For a cambered airfoil, effective angle of attack is sometimes used, where effective angle of attack is defined to be zero when the physical angle of attack produces zero lift.

A wing in a stalled state still produces lift, but the lift is reduced, and decreases as angle of attack increases beyond the crictical angle of attack. A stall can occur at high speeds, (assuming the wings can handle a high g force turn), and high speed stalls usually result in snap roll, where excessive elevator input results in a fast roll reaction (one wing "stalls" before the other reducing that wings lift, causing it to roll "downward", the other wing rolling "upwards", reducing the "upwards" moving wing's angle of attack so that it produces more lift than the "downwards" moving wing). Some aerobatic radio control models are deliberatly designed to be able to snap roll.

As far as dispelling the equal transit theory, this pre-shuttle lifting body prototype (m2-f2, the glider version) should do the job (the chase jet is a F104 Starfighter).

 

[Oryon]

You ate correct about high speed stalling, in effect the g load increases the weight of the plane, thus reducing the critical aoa. I'm no aero engineer nor physicist, this id just my understanding based on my flight training, including aerobatic flying. Full scale aerobatic planes can be snap rolled quite well too.. , :-)

 

[OmCheeto]

Perhaps my perception of this problem is skewed by the fact that I'm a boater, and not an airplaner(?). When "planing", I seriously doubt that a negative pressure has anything but the most insignificant effect on my boat flying across the surface of my lovely river.



Same effect, different density medium.

hmmm...

Perhaps I should PM B. Elliott and have him do some underwater experiments.

OMG! He met Einstein!

​

And check out Einstein checking out his neckerchief.

Zis knot reminds me of a zimplification of zee Kaluza-Klein poly-dimenshional nature of zee universe... hmmm...

--------------------------------
Why are people who drive boats called boaters, but people who fly planes called pilots? Though there are river pilots. hmmm... Someone remind me of my ponderings come November. The sun is out, and you know where I should already be.

 

[OmCheeto]

Which begs another question: Why did this cargo airplane, forget how to fly?

No need to watch after 45 seconds into the video.
(The videographer uses "Facebook" language , and then accidentally steps on his dog. )

https://www.youtube.com/watch?v=-MB9JDBe4wA

Angle of attack? Load shift? Old engines? Unexperienced person? People shoot at aeroplanes in Bagram so we need to escape as fast as possible? Photoshop?

 

[A.T.]

As a pilot,we are taught that lift is provided by both the pressure difference and the angle of attack ...

And a car's propulsion is provided by both: the engine and the force at the wheels...

 

[voko]

Load shift?

If I remember correctly, it was transporting a few armored vehicles, and apparently they were not fastened securely. A plane which suddenly becomes tail-heavy at about one thousand feet - let's put it this way, a situation I would rather not be in. Even though one can tell the crew fought till the end.

 

[CWatters]

Which begs another question: Why did this cargo airplane, forget how to fly?

Google is your friend..

http://avherald.com/h?article=46183bb4

On Jun 2nd 2013 accident investigators by the Ministry of Transport and Civil Aviation of Afghanistan reported in a press conference that quickly shifting cargo, consisting of three armored vehicles and two mine sweepers totalling at 80 tons of weight, caused the accident. The cargo slammed so hard at the back of the aircraft, that parts of the aircraft separated and wiring in the back was severed. As result of the shift and loss of aircraft parts the center of gravity moved so far back, that the attitude of the aircraft could no longer be controlled, the nose of the aircraft rose beyond the flying envelope of the aircraft and the aircraft stalled destroying the aircraft and killing all crew in the resulting impact. Parts of the aircraft, that separated as result of the initial load shift, were recovered from the runway. The straps used to tie down the cargo were recovered from the accident site, although charred they provided evidence of having fractured before final impact, it was unclear however, whether the fracture(s) had happened before or after takeoff.

 

[CWatters]

NASA has a series of slides on incorrect theories of how aircraft fly...

The "Longer Path" theory, or the "Equal Transit Time" theory...
http://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html

The skipping stone or planning boat theory...
http://www.grc.nasa.gov/WWW/k-12/airplane/wrong2.html

The Venturi effect..
http://www.grc.nasa.gov/WWW/k-12/airplane/wrong3.html

They conclude there isn't really a simple explanation..
http://www.grc.nasa.gov/WWW/k-12/airplane/bernnew.html

 

[OmCheeto]

Google is your friend..

http://avherald.com/h?article=46183bb4

Thanks! Mystery solved.

NASA has a series of slides on incorrect theories of how aircraft fly...

The "Longer Path" theory, or the "Equal Transit Time" theory...
http://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html

The skipping stone or planning boat theory...
http://www.grc.nasa.gov/WWW/k-12/airplane/wrong2.html

The Venturi effect..
http://www.grc.nasa.gov/WWW/k-12/airplane/wrong3.html

They conclude there isn't really a simple explanation..
http://www.grc.nasa.gov/WWW/k-12/airplane/bernnew.html

Boy those Glenn people are blowhards. And kind of hypocritical too. If one clicks on the "For kids" link, you'll find the following explanation:

How Wings Lift the Plane

Airplane wings are shaped to make air move faster over the top of the wing. When air moves faster, the pressure of the air decreases. So the pressure on the top of the wing is less than the pressure on the bottom of the wing. The difference in pressure creates a force on the wing that lifts the wing up into the air.

Which sounds like the Bernoulli explanation, which they claim is wrong in "Incorrect Theory #1".

Typical government organization. First they lie to you, then they point their finger at you for for spreading their disinformation.



And they don't like my boat analogy... Ok then, explain how a hydrofoil works, Glenns...

 

[boneh3ad]

Did you even read the links? There is nothing inaccurate in there.

 

[OmCheeto]

Did you even read the links? There is nothing inaccurate in there.

No. I skimmed them. They were very long. They struck me as documents written by engineers, edited by management, and then finalized by lawyers.

hmmm...

Were you involved in their creation, bonehead?

 

[boneh3ad]

Boy those Glenn people are blowhards. And kind of hypocritical too. If one clicks on the "For kids" link, you'll find the following explanation:

Which sounds like the Bernoulli explanation, which they claim is wrong in "Incorrect Theory #1".

Only because you didn't read it. Bernoulli absolutely can accurately describe lift. What "Incorrect Theory #1" states is that the oft-stated reason for faster moving flow is due to the idea that two neighboring parcels of air split and moving around opposite sides of the airfoil must meet back up is incorrect. In fact, the parcel of air moving over the top will typically leave the trailing edge of the airfoil long before its underwing neighbor does. The key question there is why the air speeds up over the top. The fact that it does, though, means that you can describe lift with Bernoulli's equation; you just can't explain why the air speeds up over the top directly that way.

And they don't like my boat analogy... Ok then, explain how a hydrofoil works, Glenns...

The difference is that an airplane wing is moving through air. When planing a boat, it rides on the interface between air and water. In that case, the water is so much more dense than the air that its effect on the system is negligible and the real dominant effect for planing is much like "Incorrect Theory #2" in the NASA links. In the boat case, you effectively can ignore the upper surface because of the fact that the momentum of any air turned by the shape is going to be so incredibly less than the momentum of the water deflected by the hull.

No. I skimmed them. They were very long. They struck me as documents written by engineers, edited by management, and then finalized by lawyers.

hmmm...

Were you involved in their creation, bonehead?

So what you are saying is you dismissed an article without actually reading it? Interesting...

 

[OmCheeto]

Only because you didn't read it. Bernoulli absolutely can accurately describe lift. What "Incorrect Theory #1" states is that the oft-stated reason for faster moving flow is due to the idea that two neighboring parcels of air split and moving around opposite sides of the airfoil must meet back up is incorrect. In fact, the parcel of air moving over the top will typically leave the trailing edge of the airfoil long before its underwing neighbor does. The key question there is why the air speeds up over the top. The fact that it does, though, means that you can describe lift with Bernoulli's equation; you just can't explain why the air speeds up over the top directly that way.



The difference is that an airplane wing is moving through air. When planing a boat, it rides on the interface between air and water. In that case, the water is so much more dense than the air that its effect on the system is negligible and the real dominant effect for planing is much like "Incorrect Theory #2" in the NASA links. In the boat case, you effectively can ignore the upper surface because of the fact that the momentum of any air turned by the shape is going to be so incredibly less than the momentum of the water deflected by the hull.

Ok then. How planes fly is complicated. Laminar flows, non-laminar flows, mass flow rates, pressure changes, velocity changes, etc.

[STRIKE]And what about my hydrofoil question? Aren't they simply underwater wings? Seems very similar to me. How do they work?[/STRIKE] (never mind, I'm outa here)

So what you are saying is you dismissed an article without actually reading it? Interesting...

You are absolutely correct.

Incorrect Theory #1

{The upper flow is faster and from Bernoulli's equation the pressure is lower. The difference in pressure across the airfoil produces the lift.} As we have seen in Experiment #1, this part of the theory is correct. In fact, this theory is very appealing because many parts of the theory are correct. In our discussions on pressure-area integration to determine the force on a body immersed in a fluid, we mentioned that if we know the velocity, we can obtain the pressure and determine the force. The problem with the "Equal Transit" theory is that it attempts to provide us with the velocity based on a non-physical assumption as discussed above.

I should have read all the lawyer like talk about the theory being both Incorrect and Correct at the same time. hmmm... Perhaps the final edit was done by a quantum physicist.

Anyways, this thread strikes me as being a bit too pedantic.

Unsubscribe.

 

[sophiecentaur]

I have kept an eye on this thread. It has been interesting to watch two groups of people arguing that their basic idea is not wrong and concluding that this implies the other argument has to be wrong. False dichotomy, I think.
Bernouili seems to give a reason for a lift force but it basically relies on sky hooks to keep the surrounding air up there, even when it must be pushed downwards - so there is a need for a reaction force - which is what the other lot are basing their argument on.
Anyone who says that Bernouili can't apply (in some form or another) is clearly wrong. It has to be operating when air flow is disturbed and the direction of the effect of pressure difference is to give lift with an aerofoil or even just a plane wing, when tilted. The only heavier than air machine that doesn't rely on Bernouili is a rocket. The only machine that doesn't rely on a constant downward flow of air is a balloon.
Kiss and make up, you guys.