How wings generate lift?

  • Thread starter Neon
  • Start date
47
2
Wings generate lift because of the curved shape of the top wing air flow faster over the top and sucked up the plane. This would be bad for a fighter jet flying upside down. Wright brothers plane wings are flat so the wing must be deflected down. So curved wings are just aerodynamic. Soo which is correct?
 

jbriggs444

Science Advisor
Homework Helper
7,815
2,751
Wings generate lift because of the curved shape of the top wing air flow faster over the top and sucked up the plane. This would be bad for a fighter jet flying upside down. Wright brothers plane wings are flat so the wing must be deflected down. So curved wings are just aerodynamic. Soo which is correct?
Which of what is correct? What two contradictory statements do you see?
 
47
2
Curved wings produce lift or air deflection do everything as flaps the deflection of air.
 

jbriggs444

Science Advisor
Homework Helper
7,815
2,751
Curved wings produce lift or air deflection do everything as flaps the deflection of air.
How are those statements contradictory?
 
47
2
If curved wings produce lift, you cant fly upside down. If flat wings are better why so many curved wings?
 

jbriggs444

Science Advisor
Homework Helper
7,815
2,751
If curved wings produce lift, you cant fly upside down.
Curved wings do produce lift and you can fly upside down. What leads you to think differently?
If flat wings are better why so many curved wings?
Because "better" depends on your requirements. What is better for fighter jets may not be better for commercial airliners or for paper planes.
 

russ_watters

Mentor
18,992
5,146
The airfoil of the Wright Flyers was most certainly not flat. Identifying the optimum curvature was one of their major innovations.

As others said, what is optimum depends on the requirements. But it is noteworthy that by increasing the angle of attack you can make a surface that curves up (the underside of a wing) direct air down.
 

Nidum

Science Advisor
Gold Member
2,990
848
Some aircraft designed specifically for aerobatics have symmetrical wing profiles .
 

CWatters

Science Advisor
Homework Helper
Gold Member
10,528
2,293
If curved wings produce lift, you cant fly upside down.
That's not correct. The angle of attract and the shape of the wing both contribute to the production of lift. A flat plate can generate lift if given an angle of attack. Some curved wing sections can generate lift with zero angle of attack or even a negative angle of attack.
 

cjl

Science Advisor
1,760
347
Some aircraft designed specifically for aerobatics have symmetrical wing profiles .
Yes, and even aircraft with asymmetrical airfoils can fly upside down - they just need to use a much larger angle of attack to do so than they need when flying normally.
 
47
2
Ok after some research, I got it. Thanks everyone
 
Good job Neon. It is relaxing to hear just that. I have witnesses some horrific aftermath from aircraft crashes and they are almost always pilot error. Like John Kennedy and hundreds more. They were up and did not fully understand what can happen and how to get out of a lethal situation.
Again great job NEON. If you become an aviator, know every aspect of the aerodynamics and the physics behind it and you will have a great and wonderful and long life enjoying the freedom of the skies.
 
The phenomenon of lift can be explained in its entirety using Isaac Newton's 2nd and 3rd law. The blades/wings have a huge forward momentum and exact force on the air. As a result, the air is deflected in a slightly downward direction. Bringing in the Newton's 3rd law, this deflection 'air stream' caused by the wing will provide a net upward force on the wing, which is termed as lift.
No matter what, the air stream HAS to be deflected downwards. You can induce this deflection through a non-symmetric airfoil with or without an angle of attack, or a symmetric one with an angle of attack.
There are other subsidiary effects which make the airflow to 'stick' to the profile of the wing. One should look up Coanda effect. It explains why a person standing next to a charging train is pushed into the train.
For a particular velocity, a higher angle of attack means higher lift only until a certain angle. After that, the effects like flow separation and turbulence come into picture.
 

boneh3ad

Science Advisor
Insights Author
Gold Member
3,064
727
There are other subsidiary effects which make the airflow to 'stick' to the profile of the wing. One should look up Coanda effect. It explains why a person standing next to a charging train is pushed into the train.
The Coanda effect pertains most correctly to a fluid jet adhering to a surface, not the flow over an object such as a wing like you cite. The air "sticks" to a wing due to viscosity and the simpl fact that if it didn't stay near the surface, there would be a vacuum, which obviously can't happen.

This is also not why a person would tend to get drawn toward a passing train. In fact, in general that doesn't happen near the sides anyway. The real danger of that is right after the train passes and air tends to rush into the low-pressure wake.

For a particular velocity, a higher angle of attack means higher lift only until a certain angle. After that, the effects like flow separation and turbulence come into picture.
Flow separation and therefore stall causes the effect you cite. Turbulence can occur at any angle of attack, however. In fact, in most situations, the majority of the flow over a wing is turbulent.
 

sophiecentaur

Science Advisor
Gold Member
23,768
4,042
No matter what, the air stream HAS to be deflected downwards
Yes, this is something that some Bernouli enthusiasts will argue against, vehemently, on the grounds that Bernouli is 'enough' of an explanation. But there absolutely has to be a vertical force and that can only be caused by a continuous process of changing momentum of some air, possibly quite distant from the wing. To see this downward flow of air, it's no good going into a wind tunnel (the argument commonly used) because a wind tunnel has that vertical force directed at its bottom half and such an experiment only shows the amount of lift force but does not 'explain' it. The wing will effect the region of air in the immediate vicinity and Bernouli works in the near region but you need to look outside this region to get the downdraft effect. I make that statement on the grounds of faith in Newton's laws and not as result of any fluid dynamics argument. But Newton is ultimately in charge of all these situations The downwards force reveals itself impressively in the wake of a large jet aircraft which is a region of lethal downdrafts. .
 

DaveC426913

Gold Member
18,315
1,927
The phenomenon of lift can be explained in its entirety using Isaac Newton's 2nd and 3rd law. The blades/wings have a huge forward momentum and exact force on the air. As a result, the air is deflected in a slightly downward direction. Bringing in the Newton's 3rd law, this deflection 'air stream' caused by the wing will provide a net upward force on the wing, which is termed as lift.
No matter what, the air stream HAS to be deflected downwards. You can induce this deflection through a non-symmetric airfoil with or without an angle of attack, or a symmetric one with an angle of attack.
By this logic then, as long as you provide an angle of attack, a flat slab of a wing with no curve and no asymmetry would get jut as much lift as a modern cambered wing.
Yet such is not the case.
 

boneh3ad

Science Advisor
Insights Author
Gold Member
3,064
727
By this logic then, as long as you provide an angle of attack, a flat slab of a wing with no curve and no asymmetry would get jut as much lift as a modern cambered wing.
Yet such is not the case.
The logic you cite makes no such claims. It only claims that a flat slab will generate lift provided it has nonzero angle of attack. It makes no claims about the amount of lift relative to an actual airfoil. In fact, this is why we have airfoils, as they operate much more efficiently at subsonic speeds.
 

sophiecentaur

Science Advisor
Gold Member
23,768
4,042
By this logic then, as long as you provide an angle of attack, a flat slab of a wing with no curve and no asymmetry would get jut as much lift as a modern cambered wing.
Yet such is not the case.
Is this not ultimately affected by Drag? If you pull a flat plate through the air on a bar, you can get deflection force at all angles. It would just be a lunatic design for a wing to have such a steep attack angle.
 
3,377
942
I think BreakingBaDude meant that you could have a working wing without the necessity of the aerofoil profile.
A completely flat winged plane COULD fly as long as the wing has an angle of attack.
It would be very inefficient and would waste a lot of energy as unnecessary drag, but it could actually fly.
In fact I know it can, A Iong time ago I made catapult launched balsawood models with flat wings (delta wings too!), and yes they worked.
They just didn't stay airborne very long, but they flew alright .. and they looked cool too:cool:.
 
Last edited:

boneh3ad

Science Advisor
Insights Author
Gold Member
3,064
727
i.e., he is claiming there is no other component.
Well in a sense there isn't. 100% of lift can be accounted for by the downdraft generated by a lifting body. It just turns out it's not easy to measure that downdraft, and a cambered airfoil, for example, generates a whole lot more downdraft than does a flat plate at angle of attack relative to drag.

That said, some of these "Newton purists" seem to act like Bernoulli's equation is simply useless. It is not. It's a lot easier to measure pressure or calculate velocity (which can be converted into pressure trough Bernoulli's equation) near the surface than it is to measure or calculate the entirety of the downdraft created by a shape, so Bernoulli's equation is still very useful.

If you want to know how lift is generated, therefore, you can take several approaches. Newton's laws and the downdraft can provide a compact, easy to understand picture of where lift comes from but does not address the multiple ways to generate said downdraft. Lift can also be directly connected to the fluid pressure on all of the surfaces of a lifting body. This naturally lends itself to using Bernoulli's equation, and that's completely justified in many cases. It naturally leads to the question of why the air on the top and bottom travel at different velocities, and that usually leads to a whole host of misunderstandings.

The fun fact is that the nature of the downdraft from an airfoil and the reason the air moves faster over the upper surface are related concepts, and therefore, the two approaches honestly have some of the same "weaknesses". At the end of the day, the how portion of lift lift generated by an airfoil is because it generates a downdraft and because there is a pressure difference on the top and bottom. Those facts cannot be separated. Why an airfoil has a faster velocity and lower pressure on the top and why it generates a lot of downdraft can really only be addressed fluid dynamically, and that's a lot more complicated.
 
  • Like
Reactions: cjl

boneh3ad

Science Advisor
Insights Author
Gold Member
3,064
727
Is this not ultimately affected by Drag? If you pull a flat plate through the air on a bar, you can get deflection force at all angles. It would just be a lunatic design for a wing to have such a steep attack angle.
Interestingly enough, this would be the most efficient possible design for a supersonic wing. It just isn't very practical for many reasons.
 

boneh3ad

Science Advisor
Insights Author
Gold Member
3,064
727
To see this downward flow of air, it's no good going into a wind tunnel (the argument commonly used) because a wind tunnel has that vertical force directed at its bottom half and such an experiment only shows the amount of lift force but does not 'explain' it.
A wind tunnel is a perfectly valid way to look at lift, and s properly-designed experiment will show a large flow deflection as a result of the airfoil. The issue is that you have to have a tunnel test section that is substantially larger than the model. This is why, if you look at pictures of plane models being tested by someone like Boeing or Lockheed or NASA, the mode usually looks tiny relative to the tunnel. Even when the downwash impacts a tunnel wall, it is often possible to correct for that.
 

Vanadium 50

Staff Emeritus
Science Advisor
Education Advisor
23,372
5,756
What is better for fighter jets may not be better for commercial airliners or for paper planes.
Agreed. As someone who has flown 2 million miles on commercial airliners, I am glad they don't regularly fly inverted.

100% of lift can be accounted for by the downdraft generated by a lifting body
That is the key insight that people forget in this argument: air goes down, plane goes up, The rest are details.
 

boneh3ad

Science Advisor
Insights Author
Gold Member
3,064
727
That is the key insight that people forget in this argument: air goes down, plane goes up, The rest are details.
The rest really aren't just details though. Heavier-than-air flight would simply not be possible if that's all we understood about lift. Air goes down, plane goes up is true, but you can't ignore the importance of how much air and how much drag.

EDIT: That was kind of confusing. Let me clarify. You can explain lift pretty much entirely with downwash. The same cannot be said about airfoils (and why they are better than a flat plate) or heavier-than-air flight.
 

Related Threads for: How wings generate lift?

  • Last Post
Replies
11
Views
10K
  • Last Post
5
Replies
124
Views
6K
  • Last Post
Replies
1
Views
643
  • Last Post
Replies
2
Views
573
Replies
1
Views
2K
Replies
23
Views
5K
  • Last Post
Replies
4
Views
5K
Replies
17
Views
2K

Recent Insights

Top