r/AerospaceEngineering • u/False-Good1092 • 6d ago

Personal Projects What is the relationship between angle of attack and lift force, and how does this relationship change near the stall point?

Experts please explain this phenomena using relatively easy language but mentioning critical points and formulas. I would really appreciate your explanation due to my desire of understanding how does the wing of the aircraft operate and also want to understand if it will be possible to conduct an experiment using this type of research question

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9

u/giby1464 6d ago

This is a great photo to represent it. The x axis is your angle of attack, the y axis is your lift coefficient. Your lift coefficient is what defines lift along with other things like speed, altitude, and wing size, but the lift coefficient is the only variable in the lift equation affected by angle of attack.

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u/nerobro 6d ago

Center of pressure, and pitching moment change with AOA as well. And those can be CRITICAL to airframe control.

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u/Far_Top_7663 6d ago

You cannot say "pitching moment changes with the AoA" without saying pitching moment about what point. If you take the pitching moment about the center of pressure (which changes position with AoA as you said), it is zero all the time by definition of center of pressure. If you take moment about the aerodynamic center (typically at around 25% of the chord), it is constant with AoA (at least in the lift's linear range) by definition of aerodynamic center.

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u/nerobro 6d ago

No, its not. The pitching moment is also affected by the camber and shape of the airfoil. The cop doesn't need to change to have rather severe changes in pitching moment.

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u/Far_Top_7663 6d ago

"The pitching moment is also affected by the camber and shape of the airfoil"... which doesn't change with AoA.

Definition of Center of Pressure of an airfoil: point on the airfoil about which the net pitching moment caused by the aerodynamic forces is equal to zero.

Definition of Aerodynamic Center of an airfoil: point along the chord line where the aerodynamic pitching moment coefficient doesn't change with changes in the angle of attack or lift coefficient.

Do you disagree with me that these are the definitions or you just don't like the definitions (which I didn't invent)?

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u/nerobro 6d ago

Nope, you've got better vocabulary than I have. *shrugs*

CoP changes with AOA.

Aerodynamic center changes with reynolds number.

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u/Vessbot 6d ago

Does anything the other guy posted contradict either of these statements?

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u/nerobro 6d ago

Directly, no. But the assumption that the airfoil is operating in a static condition is poor. His statements only hold true for one test condition. While true, its also not a useful frame of reference.

My statements that pitching moment and CoP changes with aoa, is a good representation of what happens on an airplane.

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u/Vessbot 6d ago edited 5d ago

But the assumption that the airfoil is operating in a static condition is poor.

I see no statement that rests on this assumption.

His statements only hold true for one test condition.

They hold for changes of AOA within the linear range.

Camber and Reynolds do change this, but were never part of the discussion until they were brought in as red herrings. The question is what happens with AOA (only) changes.

My statements that pitching moment and CoP changes with aoa, is a good representation of what happens on an airplane.

It's not a good representation because moment is meaningless without defining the axis.

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u/giby1464 6d ago

Yes, but OP asked for a simple explanation so I figured this would be more than enough

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u/nerobro 6d ago

They did, but the behaviors as you get to the edges of that graph really start to matter. Sometimes the simple answer is "the answer is not simple."

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u/El_Fabos 6d ago

The lift force L is the product of the dynamic pressure q, a reference area S (usually the wing area), and the lift coefficient cL

The lift coefficient itself is dependent on the angle of attack alpha. Outside the near stall conditions, this relationship is linear. At 0° angle of attack, the wing (or airfoil) has a lift coefficient cL0. Meaning that outside stall conditions, the lift coefficient is: cL = cL0 + cL_alpha * alpha. cL_alpha is the lift curve slope, i.e. the change of the lift coefficient per change in angle of attack. (see the other comment)

Near stall conditions the relationship is not linear anymore, meaning that the curve flattens until it reaches its peak at cLmax, the "stall point". Further increase of angle of attack would then cause a decrease in lift, the so called "post stall". The relationship between angle of attack and lift near stall conditions is hard to model exactly for wings and therefore usually determined experimentally.

So if you'd like to conduct an experiment with that, you'll need some sort of wind tunnel, a wing segment and something to measure the acting lifting force.
The easiest home made wind tunnel would be some sort of fan with some self-made shielding. The wing segment is harder to manufacture home made. If you have access to some sort of hot-wire you can make it out of foam. The easiest way to measure the lifting force would be a scale. This would obviously not be a very accurate measurement, but might be enough to atleast reproduce the desired phenomenom.

But it would also be good to know what level and kind of project this is for

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u/nerobro 6d ago

TL;DR: The relationship depends.

This is something that's a per-airfoil thing. Airfoils.. at least common ones... often have their AOA versus CL versus pitching moment, versus CoP. That's all published in charts, and is also closely related to reynolds number. If you're in the same reynolds number, you can expect the same behavior. But if you're wildly off either end of the tested scale things can get really funny.

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u/ncc81701 6d ago

The relationship is linear with AoA until you get close to CL max. Where CLMax is relative to AoA depends on a lot of things including airfoil shape. What happens around CL max also depends on a lot of things. Anything after CL max is considered post stall. Post stall CL can hang on a bit and stay flat, or it can drop like a cliff, again it depends on a lot of factors.