Lift Video

[Big Pistons Forever]

I am looking for a short video that adequately explains lift in a practical way and in a way that provides information that a pilot should know and has actual relevance to them when they are flying.

Any recommendations ?

[Bede]

That's going to be a difficult find. What we are taught in aviation on how lift is created is nowhere near how a physicist (who knows something about fluid mechanics) would explain it. Start with Kutta-Joukowski theorem and start learning (not you specifically BPF, pilots in general). The problem is that you'll likely get those aerodynamic questions wrong on the PPL written because the exam writers at TC (and most pilots) don't understand the phenomenon or the math.

I'm not claiming I fully understand the concept, but I do know enough to realize how little I know.

I think I'll let photofly chime in here. It would be nice if Col. Sanders was still here too.

[photofly]

Wing pushes air down. Air pushes wing up.

[rookiepilot]

Wing pushes air down. Air pushes wing up.

Cows get bigger. Cows get smaller.

[photofly]

The problem is that it's difficult to give a simple explanation that's not incorrect. The next not-wrong step up from "wing pushes air down, air pushes wing up" is more sophisticated than a pilot needs while flying. At least, I can't think of anything else that's needed in the air.

[Bede]

At least, I can't think of anything else that's needed in the air.

It's not that it's needed per se, but I believe that it's a good idea to teach from first principles. I guess you can give a five minute explanation of lift, because it's all that's needed as a pilot, but I'm a bigger fan of understanding the concepts.

It's the same reason why engineers need to calculate second moments of inertia using an integral while in university, yet once practicing, they look at charts.

The usual explanation of lift- air over top has farther to go, therefore goes faster, therefore less pressure, therefore lift- is just flat out wrong.

[photofly]

I agree on the basis of general pilot knowledge, But BPF asked specifically for what’s relevant to a pilot *while flying*, which isn’t very much.

[C.W.E.]

It would be nice if Col. Sanders was still here too

He is over on the other forum almost every day.

[North Shore]

I always thought that lift was generated by equal, and substantial, amounts of paperwork and money...

[Big Pistons Forever]

I agree on the basis of general pilot knowledge, But BPF asked specifically for what’s relevant to a pilot *while flying*, which isn’t very much.

I would suggest downwash, ground effect, wing tip vortices, and the relationship between AOA and lift are effects that have meaningful application in practical piloting.

[photofly]

Downwash:wing pushes air down behind wing

Wing tip vortices: air pushed down behind wing meets air not pushed down behind the wing. They say “hello”, hit it off and do big dance

Ground effect: wing pushes air down, air pushes wing up up up

Angle of Attack: twist wing more, wing pushes more air down, more air pushes wing more up


Maybe you should be asking for a simple explanation of drag. That’s much harder to explain.

[Zaibatsu]

Drag is easy. Increases by frontal area, shape of the object (Cd), and the square of the velocity.

Parasite drag increases because of velocity.

Induced drag increases because the frontal area and drag coefficient are increasing at a greater rate than velocity is decreasing.

Topping out a car analogy for parasite drag. Wakeboard/water ski analogy for induced drag.

Some quick illustrations and a family tree on what form, friction, and interference drag are.

Done.

[photofly]

Induced drag increases because the frontal area and drag coefficient are increasing at a greater rate than velocity is decreasing.

In your explanation you're using two different versions of the drag coefficient. One is related to frontal area and is applicable to cars, that's the one you see in the auto ads. But for wings and airplanes, the relevant area is wing area: induced drag isn't related to frontal area.

Saying the drag coefficient increases faster than the velocity decreases isn't actually correct. The overall drag is a product of (among other things) the square of the airspeed and the drag coefficient, so to be correct you should be saying the drag coefficient increases faster than the square of the velocity decreases. Then you should also point out that presupposes you increase the angle of attack to maintain lift as the velocity decreases.

And even then, what your explanation says in fancy words is that induced drag increases when you maintain lift at a slower airspeed. It doesn't tell you anything about why that should be so.

I don't think drag is that simple at all. Although, it may also be true that the pilot doesn't need to know very much about it.

[Zaibatsu]

There’s only one drag formula. Only frontal surface area is in it. Like with lift, we have a dimensionless coefficient to balance out the things we can actually measure (specifically, air density, air velocity, frontal area, and the force produced by the drag as measured in a wind tunnel).

I already explained why induced drag increases. The frontal area and Cd are increasing faster than the [square] of the velocity is decreasing.

[photofly]

Ok, let's look up the drag equation in an aerodynamics text (pick any one you like!) and note that the "area" term is the area of the wing. It must be the same area as in the lift equation, otherwise the lift to drag ratio (Cl/Cd) would have no useful meaning: it would depend on both the "frontal area" of the airplane and the wing area.

I'll kick things off. My favourite book is Von Mises' "Theory of Flight" (Dover, 1948) who defines (Chapter 8, equation 1)

Cl = 2L/ρV^2S

and

Cd = 2D/ρV^2S

where S is the area of the wing.

Oh, screw it, just read the wikipedia entry on "drag coefficient", if you need more. Section 4. I'm very happy to talk about lift and drag, but we do have to agree on the basics first. Maybe you're helping me make the point that drag is more complicated than you think!

[Conflicting Traffic]

Any recommendations ?

Here are a couple:

Why an Airplane Flies Part 1: www.piloteffect.com/bernoullis-principle.html
Why an Airplane Flies Part 2: http://www.piloteffect.com/newton.html
The Lift Equations: www.piloteffect.com/the-lift-equation.html

** EDITED to correct the second link.

[photofly]

Any time you see this, you know the person who created it has no idea what they’re talking about. A wing is quite emphatically not half of a Venturi restriction.

FCE80815-37AE-4091-94CB-30F5778889FE.jpeg (207.03 KiB) Viewed 1839 times

Please don’t use these videos.

Oh, and his exposition of Bernoulli’s principle is horribly wrong, too. Bernoulli’s theorem derives from the fact that the enthalpy of a parcel of air is constant, not its energy. The energy of a parcel of air isn't constant at all. For more on this latter point, please see https://www.av8n.com/physics/bernoulli.htm

[Conflicting Traffic]

Any time you see this, you know the person who created it has no idea what they’re talking about.

The person who wrote the script for these videos has a masters degree in aerospace engineering, so he probably has some idea what he's talking about:). (I know many people on this site are down on degrees, but I bring this up because it's relevant to the topic).

A wing is quite emphatically not half of a Venturi restriction.

A venturi is a tube with a constriction in it. The streamtube of airflow over an airfoil does in fact have a constriction in it. So, yes, the flow pattern over a wings is quite emphatically geometrically equivalent to a venturi. You can see an illustration of this here: https://en.wikipedia.org/wiki/Streamlin ... treamlines.

Sure, you can be pedantic and insist that a venturi must have solid boundaries to exist. But all that does is change your terminology: instead of a "venturi", we have a "variable cross-section streamtube". I'm not sure how that adds value to the explanation.

Oh, and his exposition of Bernoulli’s principle is horribly wrong, too. Bernoulli’s theorem derives from the fact that the enthalpy of a parcel of air is constant, not its energy. The energy of a parcel of air isn't constant at all. For more on this latter point, please see https://www.av8n.com/physics/bernoulli.htm

The distinction between enthalpy and energy only matters in compressible flow. Bernoulli is not valid in compressible flow.
Further, how many student pilots know what enthalpy is? And for the many who don't, how much time are you planning to take to explain it to them? Finally, you can in fact derive Bernoulli's equation from first principles without referencing enthalpy. Energy shows up indirectly as an integral of force (pressure) applied over distance.

As for Denker's site, that's a great resource. But I'm not sure how much time you want to spend with student pilots teaching them calculus-based derivations of Bernoulli. They need an intuitive understanding of the physical mechanisms of lift with minimal calculations. The above videos are near ideal for this objective.

Note that I corrected a typo in the second video link, so it's no longer a duplicate.

[Bede]

Photofly,

I knew that this thread would eventually come to this. Thanks for finally chiming in

I agree those videos are terrible and underscore my point that lift is far more complex than most pilots realize because they've been fed a steady diet of 1/2 venturi's during their PPL.

I don't care if the guy has an MSc in engineering, his explanation is, at best, vastly oversimplified.

[photofly]

Well, we crossed posts. Here are my thoughts on those videos.

Overall, It doesn't really matter what degrees the person who wrote the video has, it's still wrong.

There are three broad categories why it's wrong to compare a wing to half a venturi.

Firstly, it doesn't match the physical situation. The point of representing a wing as half a venturi is to say that the air flowing over the curved top of the wing is restricted from moving upwards and out of the way by the air close above it, as though there were a rigid wall there. Unfortunately for a real wing, the air extending at least a wing-span or more above the wing is deflected upwards and out of the way. So if you say that somewhere above the wing is an undeflected streamline that "represents" the top of the venturi tube, your "venturi" tube has a width of many tens of feet and a restriction of only a few inches. That wouldn't generate any flow acceleration, or any lift.

Secondly it doesn't explain any of the interesting features of a wing:
- it doesn't offer you way to calculate how fast the air moves over the wing,
- it doesn't help you determine how far above the wing the air is disturbed
- it doesn't help you explain why angle of attack is important to lift
- it doesn't help you explain the vortices generated by a wing
- it doesn't explain why the sharp trailing edge of a wing is necessary to generate lift
- it doesn't explain why there's upwash in front of the wing and downwash behind the wing; and
- it doesn't help you explain why lift changes when the wing is close to the ground

There are probably a thousand other interesting features of a wing that pretending it's like half a venturi don't help you to understand.

Thirdly it leads you to some wrong conclusions: if a wing generated lift because it represented a flow narrowing, then for many orientations of a wing with positive angle of attack, the "channel" underneath the wing would narrow too, generating low pressure under the wing. This is not what happens. Happily for the video, the wing shown as half a venturi has a flat lower surface parallel to the airflow, so this awkward fact is ignored.

You can use a venturi tube as a demonstration of Bernoulli's principle, if you find it helpful. But you can't pretend a wing has anything to do with a venturi restriction.

Bernoulli is not valid in compressible flow.

Bernoulli's equation when derived correctly absolutely is valid in compressible flow; which is lucky, because all flow is compressible. Air is terrifically compressible. Happily, the simplified form of Bernoulli's theorem presented in the video and used in teaching pilots is correct for compressible flow to first order, and so is applicable to subsonic flow around a wing. Pretending the flow is incompressible is wrong, and in almost all cases will give you the wrong answer. Deriving a (correct) result from a wrong foundation is either ignorant or crooked. I'll be charitable and say the first.

Yes, you absolutely can derive Bernoulli's equation from first principles without using enthalpy. But you can't do it by pretending that the energy of a parcel of air doesn't change; because it does.



No, students of flying don't need to be exposed to enthalpy, or even energy, or many other complicated things. What students need is honesty, and pretending a wing is like a venturi restriction, and pretending that the energy of a parcel of air doesn't change in order to "derive" a simplified form of Bernoulli's equation are both fundamentally and unequivocally dishonest. At best they're fairy tales used to give students a warm fuzzy feeling that they "understand" something when they don't. Like saying babies are brought by storks. Or that Easter Bunny is real. At best. At worst, they simply reveal that the instructor doesn't know what he's talking about.

if you can't think of a simple way to explain lift that's actually honest and correct, then don't provide an explanation at all. It's better than lying to people.


I will write something about the second video, tomorrow.