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The Truth About Lift!  
User currently offlineQFA011 From United Kingdom, joined Aug 2012, 2 posts, RR: 0
Posted (1 year 9 months 4 days 17 hours ago) and read 3655 times:

Hi all.
I know this was hot and much discussed topic in the past (last one was 10 years ago if I'm correct) but that fact doesn't help me.
I am deep into the flight training now, yet one of the most basic things is not crystal clear to me (thanks to instructors? Books?) as it should be - lift.
There are several ideas on how is lift created and I often see people disagree with each other (including this forum, pprune and others), and all have some pretty solid references behind them.
The best learning is learning from multiple sources, everyone say. That's what I do.
Yet when I am being asked "what is lift and how is it created?" I still can't answer with confidence.
I will present a few short cuts from training material. Notice the emphasis on just one idea or mixing the several.


Derivatives following are the property of their respective owners. No copyright infringement intended. Made available under Fair Use Doctrine for purposes of commentary and criticism.


[from Jeppesen's "Principles of Flight"]

Quote:
As air flows around an airfoil the pressure differential set up over the upper and lower surfaces
produces a force. This force acts perpendicular to the relative airflow and is known as lift.

[from Oxford's "Principles of Flight"]

Quote:
The aerodynamic force of lift results from the pressure differential between the top and bottom
surfaces of the wing.

[from Burke's "Principles of Flight"]

Quote:
Since for every action there is an equal and opposite reaction (Newton’s Third Law of Motion), an additional upward force is generated as the lower surface of the wing deflects the air downward. Thus both the development of low pressure above the wing and reaction to the force and direction of air as it is deflected from the wing’s lower surface contribute to the total lift generated.

[from FAA's "Pilot's Handbook Of Aeronautical Knowledge]

Quote:
Lift opposes the downward force of weight, is produced by the dynamic effect of the air acting on the airfoil and acts perpendicular to the flightpath through the center of lift.

[from Davies' "Handling The Big Jets"]

Quote:
A wing produces lift by accelerating the air which passes over the top surface to a higher speed than that which passes under the bottom surface. The greater the difference between these two speeds the higher the difference in pressure, hence the larger the lift vector.

[from Wolke's "What Einstein Told His Barber"]

Quote:
... We can give Isaac Newton still more credit for lifting airplanes, because the lift doesn't all come from downwash (with a slight assist by Mr. Bernoulli). Some of it comes from yet another application of Newton's Third Law. Airplane wings are not parallel to the ground; they are made to be tilted slightly upward in front—usually about 4 degrees when the plane is in level flight. That makes more pressure on the bottom surface than on the top, thereby pushing the wing upward and contributing to the lift. The pilot can increase the angle of attack to get even more lift from this effect. Sir Isaac's Third comes in because as the plane moves, the wing is pushing the air down in front of it, so the air responds by pushing the wings up...

[from Preston's "Aerodynamics For Professional Pilots"]

Quote:

1. A solid wing passing through the air accelerates air over its top surface in accordance with the continuity equation
2. In accordance with Bernoulli‟s equation: increased velocity causes reduced static pressure above the wing
3. Static pressure below the wing remains unchanged and is greater than pressure above the wing
4. The difference in pressure between the top and bottom of the wing results in lift
5. The total lift is the net pressure pushing on the entire surface area of the wing (S.)
6. In accordance with rule 3 the wing provides an equal but opposite force to the airflow, deflecting the passing air downward.

So it is unclear to me, is Mr. Bernoulli right or is it Mr. Newton, or both of them?
Math, physics and aerodynamics experts fight over the basic principle of flight and refuse to agree that there might be more than one reason for lift involved.
I don't know if you would believe, but I got two different explanations from two different CFI's.
Some also say it depends on shape of the wing (symmetrical or cambered).
Other say it doesn't matter because "equal transition time" Bernoulli's principle works on is wrong!
Lastly, is there something that we may say "this is 100% correct", not just speculate?
How we new pilots should learn what's correct? I will be very grateful if anyone could clarify things and maybe provide a source.

[Edited 2012-12-01 16:07:44]

[Edited 2012-12-01 16:53:14]

11 replies: All unread, jump to last
 
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 1, posted (1 year 9 months 4 days 15 hours ago) and read 3577 times:

Quoting QFA011 (Thread starter):
There are several ideas on how is lift created and I often see people disagree with each other (including this forum, pprune and others), and all have some pretty solid references behind them.

Actual aerodynamicists don't disagree on how lift is created. The squabble comes about primarily outside the aero field, and from two general sources: some persistant bad (i.e. truly incorrect) theories, and a huge disagreement about how to *teach* lift (as opposed to how it actually happens).

Quoting QFA011 (Thread starter):
As air flows around an airfoil the pressure differential set up over the upper and lower surfaces
produces a force. This force acts perpendicular to the relative airflow and is known as lift.

This is a terrible explanation and someone at Jepp should be shot. The force does *not* act perpendicular to the relative airflow. If it did, this would imply an L/D ratio of infinity. The force due to pressure differential (generally) act upwards and backwards. We decompose that force into two perpendicular axis (and call the up one lift and the back one drag) but that's just a conceptual convenience, not a physical thing.

Quoting QFA011 (Thread starter):
So it is unclear to me, is Mr. Bernoulli right or is it Mr. Newton, or both of them?

Both are correct. They're exactly equivalent descriptions of the same physics and flow.

Quoting QFA011 (Thread starter):
Math, physics and aerodynamics experts fight over the basic principle of flight and refuse to agree that there might be more than one reason for lift involved.

Nobody who is a physics or aerodynamics expert fights over the basic principle of flight or believes that there is more than one reason for lift...the fight is over which explanation is pedagologically better. If there are people actually fighting over how lift happens then, essentially by definition, they're not experts.

Quoting QFA011 (Thread starter):
Some also say it depends on shape of the wing (symmetrical or cambered).

The amount of lift depends on the shape of the wing. The explanation for lift does not.

Quoting QFA011 (Thread starter):
Other say it doesn't matter because "equal transition time" Bernoulli's principle works on is wrong!

Equal transit time is complete and total bullshit. However, that's not what Bernoulli's principle works on so Bernoulli's still OK. Air really does go faster over the top of a lift-generating wing, and the pressure really does drop due to Bernoulli's principle. Air does not, however, go faster enough to make equal transit time a reality.

Quoting QFA011 (Thread starter):
Lastly, is there something that we may say "this is 100% correct", not just speculate?

Yes. Lift producing bodies deflect air downwards. Lift is the equal and opposite reaction on the body due to air deflecting downwards. Anything that deflects air downwards generates lift. Generating lift is easy...it's generating lift with minimum drag that's hard. As a true aerodynamics expert once told me, "Newton reigns in aerodynamics." If you shove air down, you get shoved up. That's all lift is.

*All* of the other stuff around lift is either bad science (e.g. equal transit time) or fights over which explanation you want to use (momentum or pressure). All the true explanations are 100% physically equivalent so it's not a fight about how lift happens, it's a fight about the best way to explain in.

Quoting QFA011 (Thread starter):
How we new pilots should learn what's correct?

If you really want to understand it, I would recommend getting a good aerodynamics textbook (NOT a pilot's handbook or anything from a regulator) and reviewing that, with help if needed, until it makes sense. Fundamentals of Aerodynamics by John Anderson is my favorite but there are other good ones. From a pilot's standpoint, I think you're fine to just recognize that wings deflect air downwards and get pushed upwards as a result.


User currently offlinekalvado From United States of America, joined Feb 2006, 491 posts, RR: 0
Reply 2, posted (1 year 9 months 4 days 15 hours ago) and read 3577 times:

I would say these statements describe same thing from different perspective. Asking which one is correct is like asking if apple is green or round? Both!
There will be a downward flow of air from the wing, just from basic momentum conservation principle. Force exerted by gas on solid is called pressure, so lift requires pressure differential.
As for professional description.. I bet A and B do it mostly in terms of finite element analysis, or some similar simulation method, without bothering too much about great names of the past.


User currently offlineairmagnac From Germany, joined Apr 2012, 307 posts, RR: 44
Reply 3, posted (1 year 9 months 4 days 15 hours ago) and read 3569 times:

Quoting QFA011 (Thread starter):
I know this was hot and much discussed topic in the past (last one was 10 years ago if I'm correct)

Here's a good thread from about 7 months ago :
All Wing Lift = Net Downward Airflow? (by faro May 4 2012 in Tech Ops)


Quoting QFA011 (Thread starter):
is Mr. Bernoulli right or is it Mr. Newton, or both of them?

See the thread for details, but it's both. Those are just 2 different ways to look at the same thing. Same as for a jet engine : you can see it as the engine shoving air out its back and being pushed forward as a reaction, or you can see it as the integral effect of the pressure field around and inside the engine.
And actually, the Bernouilli equation is "simply" the application of Newton's Second Law F=ma to an inviscous, incompressible, irrotational flow

Quoting QFA011 (Thread starter):
Math, physics and aerodynamics experts

Each has his own purpose, and will model reality in different ways to suit those purposes. But as all the models relate to the same real phenomena, there are always links between them (at least, there should be, if the models are properly done).
And I'll be pedantic   : you mention the math experts, but not the mathematical model of lift based on circulation and vortices. That's another explanation for lift !   

Quoting QFA011 (Thread starter):
Other say it doesn't matter because "equal transition time" Bernoulli's principle works on is wrong!

The Bernouilli principle does not "work on that" ; as mentioned above, it is an expression of F=ma, and it only establishes a relation between pressure, density and speed.
Usually, the explanation of lift is given to people in form of pressure difference, itself linked through Bernouilli to a speed difference between upper and lower sides of the wing. So far, so good...The problem then is to explain WHY there is such a speed differential, and this is where the "equal transition time" nonsense pops up. I guess it's a popular explanation because it's quick and easy to visualize...


Quoting QFA011 (Thread starter):
I will be very grateful if anyone could [...] maybe provide a source.

I'll highly recommend John D. Anderson, Jr's "Fundementals of Aerodynamics" and "Fundamentals of Flight" (and I'll add "History of Aerodynamics" too, it gives some useful background on how the knowledge evolved).
All very readable and even enjoyable books, so don't be terrified by how thick they are !   
Edit: Seems I share my opinion about Anderson with Tdscanuck !

[Edited 2012-12-01 18:57:02]


One "oh shit" can erase a thousand "attaboys".
User currently offlineairmagnac From Germany, joined Apr 2012, 307 posts, RR: 44
Reply 4, posted (1 year 9 months 4 days 14 hours ago) and read 3557 times:

Quoting kalvado (Reply 2):
I bet A and B do it mostly in terms of finite element analysis, or some similar simulation method, without bothering too much about great names of the past.


Those are not models of the flows, but only a way to solve the flow equations. First you have to establish the equations in questions. That means modeling the phenomena, and therefore invoking all the basic principles and the established knowledge. You don't want to re-invent the wheel all the time, so engineers will usually use this knowledge indirectly through tools that contain it, but sometimes you just have to go back to basics.
So Newton, Euler, Bernouilli, Navier-Stokes, Prandtl and many other great names are still very much bothering people at Airbus & Boeing !

[Edited 2012-12-01 19:10:08]


One "oh shit" can erase a thousand "attaboys".
User currently offlineQFA011 From United Kingdom, joined Aug 2012, 2 posts, RR: 0
Reply 5, posted (1 year 9 months 4 days 14 hours ago) and read 3538 times:

@airmagnac I searched, but wrong keywords I guess. I'll read that thread surely.

And thank you everyone for replies and book recommendations, interesting answers. Finally it begins to make sense! If anyone else have something to add I'll be glad to read.


User currently offlinestarlionblue From Greenland, joined Feb 2004, 17035 posts, RR: 67
Reply 6, posted (1 year 9 months 4 days 14 hours ago) and read 3526 times:

I have found that the problem most people have is that they start at the wrong end. They start by talking Bernouilli, equal transition time and so forth. Instead, you need to start with pressure differential/downwash, since that is, after all, what produces lift, and work backwards towards the reasons for it. It makes the explanation much more manageable for the layman.

Quoting QFA011 (Thread starter):
I don't know if you would believe, but I got two different explanations from two different CFI's.

This does not surprise me. CFIs are not, in general, aerodynamicists. As noted above, the question is not "how is lift generated?" but rather "how to explain lift?"

Quoting tdscanuck (Reply 1):
Quoting QFA011 (Thread starter):
As air flows around an airfoil the pressure differential set up over the upper and lower surfaces
produces a force. This force acts perpendicular to the relative airflow and is known as lift.

This is a terrible explanation and someone at Jepp should be shot. The force does *not* act perpendicular to the relative airflow.

There's a question on the FAA Commercial Pilot License exam which more or less has the correct answer "lift is a force acting perpendicular to relative airflow"...

Quoting tdscanuck (Reply 1):
Lift producing bodies deflect air downwards. Lift is the equal and opposite reaction on the body due to air deflecting downwards. Anything that deflects air downwards generates lift. Generating lift is easy...it's generating lift with minimum drag that's hard.

As an example, a paper airplane produces lift just fine, and that is with a completely flat wing and no camber. It just doesn't produce lift very efficiently in relation to drag.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineMir From United States of America, joined Jan 2004, 21571 posts, RR: 55
Reply 7, posted (1 year 9 months 4 days 13 hours ago) and read 3506 times:

Quoting tdscanuck (Reply 1):
Air does not, however, go faster enough to make equal transit time a reality.

If I'm not mistaken, shouldn't this be "air goes too fast to make equal transit time a reality"? I've always learned that the air going over the top of the wing will go sufficiently faster than the air going under the bottom of the wing that it will get to the trailing edge first.

http://www.youtube.com/watch?v=UqBmdZ-BNig

-Mir



7 billion, one nation, imagination...it's a beautiful day
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 8, posted (1 year 9 months 4 days 13 hours ago) and read 3496 times:

Quoting Mir (Reply 7):
If I'm not mistaken, shouldn't this be "air goes too fast to make equal transit time a reality"?

You're not mistaken. There are situations where you could (by happenstance) get equal transit time, or slower on top, but the general situation is that the upper surface air beats the lower surface air. I was sloppy in my phrasing.


User currently offlineairmagnac From Germany, joined Apr 2012, 307 posts, RR: 44
Reply 9, posted (1 year 9 months 4 days 7 hours ago) and read 3373 times:

Quoting QFA011 (Reply 5):
@airmagnac I searched, but wrong keywords I guess

No surprise, I don't really understand how the site's search engine works. Unless you have the exact wording of the title of a thread, you'll never find anything specific.
So I usually just use Google and add 'site:airliners.net' at the end of my string of keywords.

BTW if you want more info, here's another older thread on the topic :
The Ultimative "Why Do Aircraft Fly?" Question... (by Superstring Aug 8 2007 in Tech Ops)



One "oh shit" can erase a thousand "attaboys".
User currently offlinejetmech From Australia, joined Mar 2006, 2695 posts, RR: 53
Reply 10, posted (1 year 9 months 2 days 13 hours ago) and read 3061 times:

Quoting starlionblue (Reply 6):
Instead, you need to start with pressure differential/downwash, since that is, after all, what produces lift, and work backwards towards the reasons for it. It makes the explanation much more manageable for the layman.

I think that's a great strategy, for the layman and scientist alike! In fact, J.D. Anderson - whose book "Fundamentals of Aerodynamics" has been given as a reference by others - echoes this sentiment in the opening section of his book, where he states;

" However, in these and all cases, the aerodynamic forces and moments on the body are due to only two basic sources;

1. Pressure distribution over the body surface,
2. Shear stress distribution over the body surface.

No matter how complex the body shape may be, the aerodynamic forces and moments on the body are due entirely to the above basic two sources."


As with many scientific endeavors, finding answers for why the pressure and shear stress varies over the body is where complexity arises, and often raises additional questions that are more fundamental yet again.

Regards, JetMech



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlinefaro From Egypt, joined Aug 2007, 1543 posts, RR: 0
Reply 11, posted (1 year 9 months 2 days 8 hours ago) and read 2986 times:

Quoting airmagnac (Reply 3):
Here's a good thread from about 7 months ago :
All Wing Lift = Net Downward Airflow? (by faro May 4 2012 in Tech Ops)

From that thread, this sentence from reply 42, which has since been seared into my mind:

"Airfoils are not required for flight; they are only required for efficient flight."

This perspective changed quite a lot my understanding of what constitutes lift.


Faro

[Edited 2012-12-04 01:28:13]


The chalice not my son
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