What does the CL versus AOA line represent? Lift?

I understand the relationship is that as AOA increases so does CL.

As a result, the lift production on the wing increases until it gets close to the critical angle of attack where it curves off...

Is this correct? Does the line in that graph represent lift?

Remember your lift formula: Lift = ½psv²Cl

p = is actually the Greek letter rho, for air density
s = wing area
v = true airspeed
Cl = Lift coefficient: this value depends on the actual airfoil (shape) used, angle of attack (you might see the Greek letter alpha), and Reynolds number.

The point where you get maximum Cl is your stall angle. While I won't write a very long text on this topic as there are plenty of references on this elsewhere, the lift formula along with the fact your wing always stalls at a given angle of attack explain why we so often talk about a stall speed, and why it varies.

Hope that helped,

Louis

so does that blue line represent lift?

For example, in a distance/time graph, the line represents velocity, what does the line represent in a CL/AOA graph?

loopa wrote:so does that blue line represent lift?

For example, in a distance/time graph, the line represents velocity, what does the line represent in a CL/AOA graph?

Yes and no. It does not indicate LIFT, because where do you see lift units (ie; how many pounds of lift generated)? It does however measure lift efficiency of a wing at various A0A from ZERO to 1.8 or whatever the graph tops at.

So then what is Lift Coefficient? ... the amount of lift a wing can produce?

Coeficient of Lift is just a number that describes the wing. In Mathmatics a Coeficient is simply a number. if you look at the lift formula posted above:

Remember your lift formula: Lift = ½psv²Cl

p = is actually the Greek letter rho, for air density
s = wing area
v = true airspeed
Cl = Lift coefficient: this value depends on the actual airfoil (shape) used, angle of attack (you might see the Greek letter alpha), and Reynolds number.

You see that most of the formula is fairly generic (½psv²). It does not take into account anything about the shape of the wing. The lift Coeficient does this. This formula, then (including CL) will then give you a value for the lifting force.

It is shown this way to make things simple. A graph showing AOA vs Lift would be impractical, because the lifting force will change based on a number of other things other than AOA (namely air pressure, wing surface area, and wing speed). Making a chart that shows CL vs AOA lets you make the lift calculations for the specific case you are dealing with.

Note the graph is Cl vs. AOA. So the line is Cl (not lift). Your Cl is approximately proportional to the Lift if all other factors remain the same (density, speed, ect.)

I say approximately because there is a difference between Cl and CL (Coefficient of Lift of airfoil versus wing). It is the CL which is in the lift equation and hence proportional to lift.

So would this definition be right?

Cl is the amount of lift a wing produces ?

loopa wrote:So would this definition be right?

Cl is the amount of lift a wing produces ?

No.

The amount of lift a wing produces is equal to Cl x 1/2pv^2S.

As an earlier poster said, if the Cl increases and all other factors remain the same, the lift will increase. But this isn't necessarily related to angle of attack all of the time.

For example, when you extend the flaps, you increase Cl. Many people say that flaps increase lift but actually for all practical purposes it remains the same (nearly equal to the aircraft's weight). After the aircraft has settled down at a higher Cl, we find that we can fly at a lower speed (because the 1/2pV^2 part of the equation doesn't have to work as hard).


For the original question: If you increase angle of attack, Cl will increase until the stall.

So then is there really a definition for Coefficient of lift? cause I sure as hell am either missing something or I need a beer !

Try reading this:

http://en.wikipedia.org/wiki/Lift_Coefficient

loopa wrote:So then is there really a definition for Coefficient of lift? cause I sure as hell am either missing something or I need a beer !

The Coefficient of Lift is simply a multiplier for Lift based on AOA and the design of the wing multiplying the other factors in the equation posted above. Speed, density, and area of the wing also factor in to get the result.

That makes sense !

I've been reading through kershners and ftgu, wikipedia and everything but flapless.. great ! That definition relates to me !


Thanks Tango !

The coefficient of lift is not lift. It's more like angle of attack. If your angle of attack increases, your coefficient of lift increases. The coefficient will also increase with the application of flaps. And it also changes when you change the shape of the wing.. but if you want to make sense of the equation you can just think of it like it's your 172's angle of attack.

Increasing the coefficient doesn't mean you'll climb any better though, as you increase AOA your airspeed (the v in the equation, which is squared!) will drop. Pitching up is just trading off airspeed for AOA, but you need them both for lift. Once you go slower than Vy the tradeoff's a bad deal and you'll be climbing progressively slower and slower as you increase AOA until you stall.

To be factual though, lift is almost always the same. Lift = weight in the climb, in cruise and in the descent. It only changes when you're actually pitching the aircraft nose-up or down.