SSTjumbo
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Confusus On The Concept Of Lift

Mon Feb 24, 2003 2:49 pm

I have a question that I need clarification on. Please help me!!!

I have been told a couple of things about lift: it comes from both Brounelli's principle and newton's third, I think I'm clear on that. Now here's where my question comes in, I have been told by one CFI that lift is proportional to the square of speed, and I have been told by another CFI that lift DECREASES with increased speed. Can someone give me an in-depth of lift to help clear things up here?

Thanx
-Mike
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RE: Confusus On The Concept Of Lift

Mon Feb 24, 2003 3:49 pm

Maybe you are getting lift confused with induced drag, which is a byproduct of lift.

Lift varies directly with the square of the airspeed. Induced drag decreases with an increase in airspeed.

'Speed
 
jhooper
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RE: Confusus On The Concept Of Lift

Mon Feb 24, 2003 4:46 pm

Generally speaking, lift increases with speed (otherwise why would you need to accelerate to get off the ground?). More air molecules are flowing past the wing as it speeds up. At slower speeds, one must increase the angle of attack of the wing in order to generate the necessary lift to maintain level flight.
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Ikarus
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RE: Confusus On The Concept Of Lift

Mon Feb 24, 2003 5:53 pm

Assuming incompressible flow: The easiest way to think of it is via the Lift Coefficient. That is constant for the plane no matter what speed it goes at (until it extends the flaps, which change the coefficient). The formula for that is:

CL = Lift / (0.5 * air density * Speed squared * wing area)

So to get more lift force, you increase the speed...
 
FredT
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RE: Confusus On The Concept Of Lift

Mon Feb 24, 2003 11:23 pm

The lift coefficient (CL) is not constant, as Ikarus appeared to be suggesting even though it probably was not what he meant. However, it does not vary with airspeed (except for slightly, through the changed Reynolds number) but only with angle of attack.

If the angle of attack, and thus CL, is kept constant, lift is indeed proportional to the (total) airspeed squared. However, as you generally want just the right amount of lift to compensate for the weight of the aircraft (in level flight only, if you want to be strict about it), you’ll typically find yourself reducing the angle of attack and thus CL to keep the same amount of lift as speed increases. Thus, the lift coefficient will decrease with increasing airspeed. Perhaps this is what your instructor was talking about?

L = CL * q_a * S = CL * 1/2 * rho * V^2 * S

where L is lift, q_a is the ambient dynamic pressure, S is the wing area, rho is the air density and V is TAS.

Extending flaps does change the CL(AoA) curve. The main purpose of flaps is (typically) to increase the maximum CL, allowing flight (i e lift equals weight) at a lower airspeed.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
Ikarus
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RE: Confusus On The Concept Of Lift

Tue Feb 25, 2003 2:51 am

FredT: Oh, how embarassing. I should never post again after not having slept enough....

Mine was a pretty poor post...


Regards

Ikarus
 
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RE: Confusus On The Concept Of Lift

Tue Feb 25, 2003 10:46 am

I was wondering, if the principle of Bernoulli is used to describe lift, then how does it apply to a symetrical wing (were both sides have equal amounts of camber)?
 
delta-flyer
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RE: Confusus On The Concept Of Lift

Tue Feb 25, 2003 11:51 am

I was wondering, if the principle of Bernoulli is used to describe lift, then how does it apply to a symetrical wing (were both sides have equal amounts of camber)?

You can make a piece of plywood generate lift if you give it an angle of attack. Try it while riding in a car with an open window. All Bernoulli says is that airspeed on top of the plywood will be grater than under, therefore you will have lift. When the board is exactly level, the flow will be symmetircal, and you will not observe any lift.

Pete
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FredT
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RE: Confusus On The Concept Of Lift

Tue Feb 25, 2003 6:50 pm

Ikarus,
I know the feeling. Been there, done that. Many times. Big grin

"I was wondering, if the principle of Bernoulli is used to describe lift, then how does it apply to a symetrical wing (were both sides have equal amounts of camber)?"

Bernoulli’s principle, while valid, is not the best explanation to use. Yes, total pressure is conctant or, in other words, the static pressure decreases as the speed of the air increases. However, you often see (even in pilot’s training manuals, which is something I can’t decide if I should laugh or cry over) the longer-path explanation, where two air molecules split at the leading edge are assumed to magically meet up again at the trailing edge.

They don’t.

A wing deflects air downwards. That is why and how lift is generated. The pressure distribution around a wing is what accelerates the air downward - and accelerates the wing upwards. You can never have acceleration in a fluid without a pressure gradient.

A symmetrical profile will generate zero lift (CL = 0) when the angle of attack is zero. Increase the angle of attack and it starts generating lift.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
Ikarus
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RE: Confusus On The Concept Of Lift

Tue Feb 25, 2003 10:14 pm

FredT: Ah well, are you trying to explain the generation of lift? Risky that... There are too many explanations and too many of them are right...

One of the better explanations I've seen is about the curvature of the streamlines. Assume attached flow over a typical, teardrop shaped airfoil. Above the wing the air is deflected and follows a curved streamline around the wing. Now consider centrifugal and centripetal forces: For the air, the best path to take is of course a straight line. (Think of Newton: A particle without any force on it will move at constant velocity in constant direction, or be stationary). But the air goes through a curved trajectory - therefore, there is a centripetal force on it pulling it inwards (the wing) and due to the equal and opposite reaction, there is an outwards force on the upper surface of the wing (pulling it up). On the bottom surface, the curvature is less prominent, but there is still a curvature of streamlines in the same direction as above the wing. So again, there is a force on the air molecules pulling them down / inwards, and therefore there is an equal and opposite force on the wing, pushing it up.

(For any Germans in here, www.schlechtflieger.de has utilizes three very different very good methods to explain how lift is generated... including the curvature of streamlines method, the downwash / upwards momentum method as explained by FredT, and then something based on Bernoulli, but looking at the airfoil dividing the flow into two different channel flows.... very competent explanations in all...)

Regards

Ikarus
 
FredT
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RE: Confusus On The Concept Of Lift

Tue Feb 25, 2003 10:32 pm

Ikarus,
or in other words, there is acceleration to make the air follow a curved path (streamline). And, as I was saying - no acceleration in a fluid without a pressure gradient.  Big grin

BTW, there will be a lower than ambient static pressure on the lower side of most wings in most flight conditions. In other words, the air is curving upwards below the wing - just less so than the air on top of the wing is curving downwards. By far the main part of the lift is generated on top of the wing - that is why we prefer to have junk hanging below the wings to having junk on top of’em.

Yes, I know the never-ending lift debate is a can of worms. But I believe that each time it is re-iteraded, someone will see the light and drop the ridiculous idea of pairs of eternally mated air particles...  Smile

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
jhooper
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RE: Confusus On The Concept Of Lift

Wed Feb 26, 2003 2:21 am

Just when I think I understand this stuff, some aerospace engineer tell me I'm full of shit!  Pissed
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SSTjumbo
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RE: Confusus On The Concept Of Lift

Wed Feb 26, 2003 2:36 am

Hmm, good to see I nearly started a war  Pissed.

Anyway, the question origionally came up when one CFI told me that lift increases with the square of speed. I get to my stage I oral, and the Assistant Cheif Pilot tells me that I'm wrong, and that lift decreases with speed. This confused the crap out of me, but what you guys stated helped clear things up. Now just what to tell THEM is the question  Big grin.

-Mike
I don't know, so this is my signature.
 
jhooper
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RE: Confusus On The Concept Of Lift

Wed Feb 26, 2003 2:55 am

I'd like to hear your "Assistant Chief Pilot's" explanation.
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FredT
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RE: Confusus On The Concept Of Lift

Wed Feb 26, 2003 6:21 am

JHooper,
nothing wrong with your explanation that I could see? Just did not provide the gritty details that us engineers love to totally get lost in.  Big grin And hey, I'm an AE and people still tell me I'm wrong every now and then. Quite often, actually. And the worst part is - sometimes, the daft bahstids are right! Big grin

SST,
please forward the explanation here. It would be interesting to see where what has been misunderstood!

Cheers,
Fred
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SSTjumbo
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RE: Confusus On The Concept Of Lift

Wed Feb 26, 2003 9:46 am

I don't know if the ACP really intended to mean this point, but to get that part perfect in his oral, I had to say that lift decreases with speed, and he didn't seem to have a problem with that.

Also, another question, does induced drag necessarily have to increase when speed decreases, or is it a biproduct of the extra lift being produced. The reason I ask is what kind of drag causes adverse yaw? Thanks for bearing with my stupidity everyone  Big grin.

Cheers
-Mike
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jhooper
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RE: Confusus On The Concept Of Lift

Wed Feb 26, 2003 11:18 am

Adverse yaw is caused by the induced drag of the raised wing (since it's producing more lift, it's also producing more induced drag). Admittedly, I have a simple understanding of this concept, but that is as I understand.
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SSTjumbo
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RE: Confusus On The Concept Of Lift

Wed Feb 26, 2003 11:45 am

Okay, let's try this question, what's the complete relationship between speed, induced drag, and lift? Maybe that will help me understand things.
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jhooper
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RE: Confusus On The Concept Of Lift

Wed Feb 26, 2003 1:14 pm

I don't have a mathematical answer to your question. However, I do know that as speed increases, induced drag decreases, but parasite drag increases. Therefore, total drag decreases to a point, where you get max L/D. From that point on, induced drag will continue to decrease while parasite drag increases, and thus total drag increases. Look through your textbooks and you should be able to find a simple graph with induced drag, parasite drag, and total drag all related to speed.

[Edited 2003-02-26 05:16:43]
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FredT
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RE: Confusus On The Concept Of Lift

Wed Feb 26, 2003 4:15 pm

Induced drag is proportional to the lift coefficient squared.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
lehpron
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RE: Confusus On The Concept Of Lift

Thu Feb 27, 2003 1:26 pm

"Induced drag is proportional to the lift coefficient squared."

So what is it exactly? Cuz you're saying that if I had a highlift wing a CL twice what it was then the CDI would be 4 times what it was?
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FredT
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RE: Confusus On The Concept Of Lift

Thu Feb 27, 2003 3:44 pm

Lehpron,
if you have a high lift wing with a CL twice of what it was at a lower angle of attack, the induced drag will be four times as large. If you have a high lift wing with a CL twice that of another wing, you don't know. There's a wing specific wing efficiency factor in the formula as well.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
SSTjumbo
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RE: Confusus On The Concept Of Lift

Fri Feb 28, 2003 12:33 am

Hold short everybody, I just thought of something: could the demons have been telling me that the lift of the WING decreases with an increase of speed but the lift of the AIRFOIL increases squarefold with speed? I don't know what prompted me to think of that, but does that make any sense?

Thanks
-Mike
I don't know, so this is my signature.
 
EssentialPowr
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RE: Confusus On The Concept Of Lift

Fri Feb 28, 2003 1:05 am

FredT, Delta-Flyer, Ikarus-

Nice job; this is how a tech forum should go!!

To others, particularly CFIs:

If you can follow this discussion, then you probably "get it". All too often, private pilots through CFIs simply blast through the Gleim, never really learn anything, and then end up confusing their students (as was the case here.)

The lowest, most basic form of learning is "Rote Recall" which is unfortunately heavily relied upon in most pilot education programs and FAA tests. The explanations posted here are largely "math free". Read them, and be able to write a definition of lift of your own that accounts for Newton and Bernoulli. You will have then moved to a higher level of understanding, and will be able to explain it someone else.
REmember-
If you can WRITE your OWN definition, that indicates a higher level of learning. (Doing the math will elevate it to an even higher leve, but that's not nec...)

You'll be a better CFI, b/c anyone can sense a BSer or when someone doesn't "get it". Sounds like the chief pilot and certain CFI need to hit the books, and in my own opinion, both need another oral to retain their quals...

cheers-
 
FredT
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RE: Confusus On The Concept Of Lift

Fri Feb 28, 2003 5:30 pm

SSTJumbo,
no. You have cl for the airfoil, and CL for the entire wing. They work pretty much the same.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.

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