Question on Lift.

Sandesh

Sandesh

Well-Known Member
Dear Fellow Aviators,
I was taught in school that lift is created because of the fact that the upper camber has more area than the lower camber and since air when it parts wants to meet together at the trailing edge of the wing, the air on the upper camber increases its velocity to catch up to its counterpart in the lower camber and this creates a lower pressure on top and lift is created. but I am reading myself now and I noticed that it says that the air velocity increases because when we increase the angle of attack, the effective upper cross-sectional are of the airfoil is increased and the effective lower area is decreased, this reduces the area of the airflow over the top surface and with that in mind putting law of continuity AreaxVelocity=constant, when the air area of flow reduces, its velocity must increase to keep a constant, and when the happens, Bernoulli principles states that when velocity increases pressure decreases causing a lower pressure on top and thus creating lift.
So I am confused on which one is right? 2nd one makes alot of sense but I was taught different, or is it the same thing both explanation. Just wanted to clear stuff.

Greatly appreciate it,
Sandesh
 
Simple answer: A difference in pressure creates lift.

Beyond that, your question or display of that question is utterly confusing. Can you rephrase in a more organized fashion? Give the two separate definitions in two separate paragraphs and reread it for spelling and readability.
 
Sandesh said:
effective upper cross-sectional are of the airfoil
Click to expand...

You must be reading "The Illustrated Guide to Aerodynamics" by Skip Smith.

Skip's book is great, but his explanation of lift is not complete, which he acknowledges. But the first explanation you posted is wrong; the camber doesn't produce lift, since non-cambered airfoils work just fine. In fact, a flat board will generate some lift.

The fundamental theory of subsonic lift production is called "Circulation" theory. Here's a brief summary of how it works:
 
Sorry,
What I was taught- Lift is created because the upper camber has more area than lower camber, when air parts in the stagnation point, the air going over the top has to increase velocity to cover the larger area on the top than botttom, this creates lower pressure on top than bottom and lift is created.

What I just looked up- Air accelerates because the area on the top reduces the area of airflow more than the bottom, this causes this acceleration proven by Law of Continuity Area X Velocity= Constant. Then Bernoulli comes into play, velocity increase pressure decrease= difference in pressure which results in lift.

So, I am basically confused about Which explanation is right?, or Are they the same?

Greatly appreciate it,
Sandesh
 
Wow,
Thanks Tgrayson, Thats a new explanation, I have never read this before, it makes sense. The diagram really helps grasp the concept.
 
Sandesh said:
Thats a new explanation, I have never read this before, it makes sense. The diagram really helps grasp the concept.
Click to expand...

Thank you. The theory actually is over 100 years old, first advanced in a qualitative way by Frederick Lanchester in 1890, but further developed mathematically by Wilhelm Kutta and Nikolai Joukowski a decade or so later. If you're interested, here's a book that goes into the subject in great depth: Flightwise.
 
Just wait if you ever get into rotary-wing aero, with such items as reverse flow, negative lift, negative stall, etc.
 
MikeD said:
Just wait if you ever get into rotary-wing aero, with such items as reverse flow, negative lift, negative stall, etc.
Click to expand...

No one's mentioned the flux capacitor variable. That's when it gets just downright ridiculous. :D
 
deek said:
and 1.21 gigawatts!
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IIRC, didn't Doc Brown pronounce it "Jigawatts"? I suppose you could still spell it with the "G". In 1985 there was nothing operating in the "Giga" range, so you could probably say it however you want.
 
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I got these from www.av8n.com and they pretty much explain how a wing creates lift. It's a great site that explains the nitty-gritty behind the aerodynamics and physics of flight pretty well. Check it out!

The airfoil circulates the surrounding air thats rushing by thus causing the air on "top" to speed up and the air on the "bottom" to slow down which, because of Bernoulli's principle, causes a pressure differential. Slower air on the "bottom" exerts more upwards force than the faster air on "top" is exerting downards. And voila, up you go! You'll realize I put top and bottom in quotation marks because the wing doesn't care if its right side up or upside down. It's the angle of attack that's important. So, a positive angle of attack, as we see in the above pictures, creates lift towards the top of the picture. If the airfoils were angled in the opposite direction, then the lift force would be towards the bottom of the picture.
 
http://firstflight.open.ac.uk has some good stuff also including this video:


[YT]vLY7HTazNCw[/YT]. <--- I posted the youtube version, because I'm not sure how to embed video from anywhere else.

By the way, here's a couple of MATLAB models of starting vortices: Unfortunately, the view screen follows the starting vortex itself as the airfoil (a flat plate) moves off the screen to the left. So you can't really see the flow around the airfoil well to see the resulting circulation.

[YT]V3Fz6QajeIw[/YT] [YT]Qfbb0g3-Q-k[/YT]

Hey, Tgrayson. Here's a question for you. Once the aircraft is moving, if AOA increases circulation should increase right? Or in other words, as AOA increases the speed difference between the top and bottom of the wings gets larger. That part I know is correct, there are several simulations and some good smoke chamber videos to show it.

So my question:

Does that mean that downstream of the wing the magnitude of the starting vortex increases also? I'm using the term "magnitude" unscientifically, BTW. I would imagine the angular velocity (rotation rate) of the starting vortex increases. Any idea what happens in terms of flow with AOA increase?
 
fish314 said:
Does that mean that downstream of the wing the magnitude of the starting vortex increases also?
Click to expand...

No, you'd just get another starting vortex, just strong enough to satisfy the Kutta condition, moving the stagnation point to the rear of the airfoil. Real world, you probably have a constant flow of starting vortices downstream, as circulation varies due to minute AoA changes. (Haven't seen that last stated anywhere, but it stands to reason.)
 
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