PanAm747
Topic Author
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How Does A Wing Generate Lift?

Tue Mar 19, 2002 11:25 am

Wow, I'm a little confused on this.

Every groundschool and every pilot book I have taught me that this was the CORRECT theory:

http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html

So is Jeppesen wrong?
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PPGMD
Posts: 2398
Joined: Sun Sep 30, 2001 5:39 am

RE: How Does A Wing Generate Lift?

Tue Mar 19, 2002 11:41 am

Yes, its is wrong. Its a little too complicated to explain in text with out some sort of drawings. Maybe someone will dig up a page to describe it more in depth for you.
At worst, you screw up and die.
 
IMissPiedmont
Posts: 6200
Joined: Wed May 23, 2001 12:58 pm

RE: How Does A Wing Generate Lift?

Tue Mar 19, 2002 12:05 pm

It's half wrong. The molecules do not travel faster in order to "meet" the molecules they were with at the leading edge.

Think of the top of the wing as one half of a constricted pipe, the top half being formed by the air pressure from above. The air must travel faster to get an equal amount through a smaller area. This is the heart of the Bernoulli effect. An equal amount of air passing through a smaller space must travel faster, hence lower pressure. Nothing magical, just elementary physics.
The day you stop learning is the day you should die.
 
SkyGuy11
Posts: 532
Joined: Fri Oct 26, 2001 7:09 am

RE: How Does A Wing Generate Lift?

Tue Mar 19, 2002 12:42 pm

^^ Is there such a thing as 'elementary' physics!?

That is a good website BTW! Yeah, the reason most private pilot textbooks explain lift in the incorrect mannar is that it is simpler and easier to understand. The real theory of lift involves physics and difficult to explain theories that takes a lot of study to really understand. But it makes complete sense when you think about it. Think of an inverted wing. There is less distance on top of the inverted wing than there is on the bottom. Does this mean that the wing is creating lift in the 'wrong' direction? Absolutely not! Lift is a product of the downforce created by the wing forcing the air down, which is directly related to angle of attack and airspeed.
.
 
ThirtyEcho
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Joined: Tue Jan 01, 2002 1:21 am

RE: How Does A Wing Generate Lift?

Tue Mar 19, 2002 1:06 pm

Leave us not forget the Coanda Effect, which describes why the airflow above the surface of the wing doesn't just keep going in a straight line away from the wing's curvature but, instead, conforms to it.
Its the same reason that, if you pour coffee out of a cup at a 45-degree angle, the coffee runs down the side of the cup instead of falling straight down. If you increase the angle of the coffee cup, its "angle of attack," the coffee "separates" from the cup and falls straight down. The Coanda Effect is why angle of attack, and not speed, defines the stall break.
 
Guest

RE: How Does A Wing Generate Lift?

Tue Mar 19, 2002 3:07 pm

Just for the record, some commercial pilot texts describe it incorrectly, as well as private pilot texts. But as a pilot, I think I'll let the engineers and physicists worry about the exact mechanics. When I'm shooting a partial panel approach to minimums, I'm not wondering, "Is this newtonian lift or Bernoulian?" Although it is good for a pilot to grasp the general concept.
 
FredT
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Joined: Thu Feb 07, 2002 9:51 pm

RE: How Does A Wing Generate Lift?

Tue Mar 19, 2002 6:28 pm

IMissPiedmont,
that's another incorrect theory. Bernoullis principle is valid but not really relevant. The pressure decrease comes from the air accelerating downward as it follows the curvature of the wings upper surface. There is no pressure gradient in a fluid without acceleration and vice versa. SkyGuy and ThirtyEcho are spot on.

I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
SailorOrion
Posts: 1959
Joined: Tue Feb 06, 2001 5:56 pm

RE: How Does A Wing Generate Lift?

Tue Mar 19, 2002 6:54 pm

Kutta-Joukowski theorum óf Fluid Mechanics. the lift, L, of an airfoil is given by L = pTVl, where p is the density, T is the circulation, V is the velocity, and l is the length of the airfoil. Basically, the wing adds a circulation to the airflow. (use simple vector analysis on the flow to compute it)

SailorOrion
 
Guest

RE: How Does A Wing Generate Lift?

Wed Mar 20, 2002 5:52 am

Dang guys...
Now you've all got me really confused. I thought all along it was the "Lifties" that pushed up on the wing when you were flying.
Jetguy
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: How Does A Wing Generate Lift?

Wed Mar 20, 2002 6:12 pm

Jetguy,
these are the official reasons, of course. The real reason aircraft fly is of course....

...PFIL, or Passenger Fear Induced Lift. The lift comes from all the pax grabbing onto their armrests and pulling. It is easy to see. The engines and take off roll are simply there to put enough fear into the pax. On takeoff, as the noise and vibration picks up, they all grab hold of their armrests and off you go.

On approach, since they're all relaxed after inflight meals and a drink or two, you need flaps and perhaps even spoilers to make sure you get enough lift. If you find yourself below the glideslope, you might even have to add a burst of power to frighten them enough!

Yes, cargo ops is dangerous. It really takes quite a pilot to scare a cabin full of mail!

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
Klaus
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Joined: Wed Jul 11, 2001 7:41 am

FredT

Thu Mar 21, 2002 11:45 am

Wow! Now I´ve got it!!!  Big thumbs up  Big thumbs up  Big thumbs up

(Next time I´ll remember not to let my fellow passengers do all the work for me!  Wink/being sarcastic)
 
ThirtyEcho
Posts: 1409
Joined: Tue Jan 01, 2002 1:21 am

RE: How Does A Wing Generate Lift?

Thu Mar 21, 2002 12:45 pm

FredT, you forgot the effect of flaps and slats. Both devices are primarily noisemakers; as the aircraft slows for landing, flaps are deployed and they make a lot of noise, causing the pax to pull up on the armrests again. Slats serve the same function except that they make a kind of groaning noise that causes the pax to say "What's that?" and pull harder. During a go-around, the engines are set for max noise and the flaps are retracted at the same time to cause max terror; this is accompanied by a recording played through the PA system of people screaming, crying and saying "Oh my God!" that causes the greatest amount of armrest pulling.
 
airplay
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RE: How Does A Wing Generate Lift?

Thu Mar 21, 2002 1:10 pm

There are many instances in our lives when scientific concepts are just too complex to expect everyone (including pilots) to understand the principles responsible for certain phenomena as intimately as someone who has devoted a great deal of time studying.

In the effort to effectively teach a lay-person the basics, we sometimes take liberties with the accepted theories in order to simplify them to a level that gets the point across well enough for the context it's presented. It may not be exactly correct, but it's easy to accept.

The theory of lift with respect to aircraft is one such subject and this debate can go on ad-nauseum as everyone adds their 2 cents....


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Guest

RE: How Does A Wing Generate Lift?

Fri Mar 22, 2002 12:27 am

Now, now. We all know that it's money that makes airplanes fly.
 
PPGMD
Posts: 2398
Joined: Sun Sep 30, 2001 5:39 am

RE: How Does A Wing Generate Lift?

Fri Mar 22, 2002 1:11 am

Yep money it is, espically after that last check I wrote.
At worst, you screw up and die.

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