AoA briefly (tilt angle observable in IF)

AoA (Angle of Attack) in the fewest words? Lift occurs when a moving flow of gas (air) is turned by a solid object. The flow is turned in one direction; lift is generated in the opposite direction (Newton’s third law).

AoA is the aircraft’s tilt angle contribution to increasing the amount of turn of the moving flow of gas.

Zero AoA: only the wing’s shape turns the flow of gas.

Positive AoA: the tilt of the wing adds to the amount of turn of the flow of gas (more change in direction), increasing lift at a given airspeed.

At any particular airspeed, there is only one amount of lift that will ever be created at a given tilt angle. Flying the right AoA is what enables you to match total lift to your weight (stay flying) at any airspeed.

edit note:
(Newton vs Bernoulli debate? Nature isn’t debating: Newton=Bernoulli; conservation of momentum solution is equivalent to pressures exerted from change in flow velocities solution)

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To add to this: you could show the AoA in the bottom bar of IF.

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Thanks for this mini-tutorial!

If I may say so, perhaps you could expand on it a little bit and create a full tutorial on AoA and its effect on flying, in the #ground-school:community-tutorials category? I’m sure it would help the community!

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Yes, thank you. That is important info. Also it’s the same angle you see visually made between the nose mark and the FPV (small circle). And this is of course different than the pitch angle (nose mark above the horizon).

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Thank you. I was thinking about what might be enough info without being too much. I definitely agree with you on the benefit of expanding on this in some way though.

Referring to your last part, technically, at a given airspeed you can have 2 different AoA values giving the same amount of lift. However the second AoA would come from a stalled wing, which by definition is when you have a decrease in the coefficient of lift (directly proportional to the lift force) as AoA increases.

A great idea for a tutorial!

I suggest that in terms of general concepts it may help to expand it to include an explanation of airfoils and AOA together.

For example you have not explaned that the fundamental physics of lift is caused by the pressure difference created by the airfoil (and that pressure difference changes with AOA and speed changes). Its the suction effect upwards caused by low pressure over the top surface of the airfoil (wing) that lifts the plane.

So understanding that firstly and ideally seeing a diagram, will help a lot and lead to easier understanding of the impact that faster speeds and / or higher AOA has…(basically increases the air pressure under the wing and lowers the air pressure above it).

So your explanation only refers to air impacting a “solid object”. Whilst this would impact airflow directions, on its own it does not generate lift. A flat plank-like wing won’t fly. The airfoil has to cause airflows at different speeds over its surfaces to create the pressure differences that generate lift (air passing faster over the top than the bottom).

I think I see what you mean - after the lift curve peaks it drops back down again? You got me on that! Hopefully we won’t be flying there much any time soon. But it does emphasise the point that there is a safe range for AoA beyond which it doesn’t work anymore. A limit that is somewhere up into the low double digits.

Making a tutorial for this will also contribute to the upcoming rework of the F-18, the F-18 is flown by controlling its AoA for approach/landing.

So I think you’re getting into the issue of the debate over whether it’s Bernoulli’s principle vs Newton’s 3rd law for best explaining lift?

Bernoulli is about the pressure differential caused by the increase in speed of the air forced over the top of the wing (due to longer curved path): increase in speed of fluid (air in this case) causes a drop in pressure.

Newton is about conservation of momentum: throwing a cannon ball while standing on a skate board or why a fire hose has to be held with great strength.

Consider the firehose for a moment. This is a thrust scenario similar to a rocket engine. The force on the rocket and the firehose are both caused by the “equal and opposite reaction” of the momentum of the propellent expelled: rocket fuel or high pressure water.

In a sense it’s a physics accounting equation: assets must equal liabilities. Or what you give up is what you get back. It fully explains what curving the air’s path will do in terms of lift. So it is a physically complete explanation of the system of air flowing over a wing.

What it doesn’t do is show where the force is applied within the system. Similar to the firehose - the force is measurable directly from the momentum of water expelled. It explains the force but doesn’t show how forces are pushing within the hose to make this all happen.

So I’m thinking Bernoulli comes in to better show how the forces resulting from Newton’s law are actually acting on the wing. But apparently the numerical modelling of Bernoulli’s principle is very messy. And somewhat less intuitive one might argue.

But for me, a mystery that still remains is how to most intuitively connect Bernoulli and Newton. But I still thought that Newton gives a full and more intuitive explanation, assuming one has to choose between them.

Do they use AoA as a target number for the approach procedure?

That is what makes an aeroplane fly, so in this context here I would stick with that. You’ll lose peoples attention if it becomes a physics debate …

So I’m exposing my ignorance here a bit. I had been reading (don’t know how reliable) that pilots tend to prefer Newton’s 3rd law, and engineers go more with Bernoulli. Which I would think would be opposite. But that the debates continue. It’s not my debate. I’m happy with both theories where they apply. And they both apply. But I believe a fire hose is more intuitive than what’s happening inside a venturi tube (Bernoulli).

Why does pressure drop when speed of a gas increases? We tend to answer by definition because we’ve heard that. But how have we intuitively experienced it?

I disagree.
I might add, you can generate lift on a brick under the right conditions.

“Any physical body moving through a fluid can create lift if it produces a net turning of the flow.” https://www.grc.nasa.gov/www/k-12/airplane/factors.html

Yes, in correlation with the airspeed.

The NATOPS for the legacy hornet uses “AoA” 720 times excluding the index.

An other example where it is used is cruise:

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You might wish to know that the velocity of the air being larger over the upper surface of the wing is not trivially due to “a longer path”. NASA has a short article on this, you may wish to read up on it.

Also, a complete numerical simulation over the entire wing with Newton’s 2nd Law (which is then equal in magnitude to the lift force by N3L) can very well be done, over infinitesimal wing elements if you know enough details about the airflow (velocity at every point, pressure, density etc.) and make some assumptions. Though it would be tedious.

The current consensus is that both points of view are physically, equivalent. Again, I recommend you read the NASA article for more details.

Cheers.

I did read that. Thank you. I think that’s very important. It’s a complex modelling problem. The analysis has to be done for design. But it’s not at all as simple as many popular explanations.

That was a point I was trying to make. It’s a bit like, physics or chemistry, which is right. They’re both right. They’re tools that answer questions at different levels about overlapping phenomena.

By the way, a simple thing you can try is to take an A4 sheet of paper, fold it into 4 along the breadth, and staple (or stick) the 2 far sides together, forming a triangular “tube”. Blow through this tube. You will see the tube shrink, showing that indeed the pressure does drop as velocity increases.

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Thank you! I need to learn more about this.

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I like that idea. Thank you! This is a kind of related experiment blowing air between two sheets of paper (fluids engineering source): Blow Air Between Two Sheets of Paper - YouTube

Unfortunately as their link describes:
“Bernoulli’s theorem describes the energy conservation law in a fluid. Forcing air to flow increases its energy relative to that of the surrounding air. Therefore, Bernoulli’s theorem cannot be used to compare the moving air with the surrounding, stationary air.”

So a wind tunnel is valid for testing Bernoulli pressure changes, but not a forced stream of air within a body of otherwise stationary air.

I have to admit, this wasn’t clear to me at first. So either way, what you said added to my understanding!