# Why Bernoulli? (latest - to turbines, from hitting me in the face?)

I made this small table for an older topic to illustrate for the 787, trade-off between AOA, weight, speed, and altitude:

Can I understand such behavior of AoA beyond just saying Bernoulli, but by somehow better grasping first principles of Bernoulli?

A wing makes lift by curving the air’s path.

AOA adjusts the amount of turn the wing’s curved shape gives to the air.

The wing’s curvature turning the air, drops pressure above the wing which pulls air down from above.

Forcing mass down to overcome gravity ties together the physics of rockets, helicopters and fixed wing aircraft.

Bernoulli is what we label as the phenomena that drops pressure from curving the air.

But how does Bernoulli work?

A dance between static and dynamic pressure:

The relative wind has to maintain continuous flow passing over the wing: If air were to continue to “pile-up” anywhere along the curved path, the accompanying static pressure at that location acts like a balloon’s pressure accelerating air out it’s small opening.

The “ballooning” static pressure accelerates to faster dynamic-pressure at the exit.

So, the tendency to “pile-up” becomes self-limiting. More accurately, maintaining the continuous mass flow of air becomes self-moderating.

The pressure drop over the top leading edge of the wing results from the continuous flow’s “pile-up” self-moderation: The mass density (and therefore the pressure density) profile along the wing’s camber will self-distribute to cause just enough pressure drop to force the flow acceleration around the contour to maintain a continuous flow rate.

If it’s not maintained continuous, the distribution self-adjusts (to relieve pressure imbalance) until it regains continuity of mass flow.

So your nose pitch-up has meaning: it regulates how much air curvature you need for enough pull down on air from above to keep you up.

And the Bernoulli part of that is: you set the pressure change by how much curved path disruption you need to impose on a continuous mass flow of air, so that it’s pressure change (including drop below atmospheric static pressure) is sufficient to maintain its horizontal flow rate.

Simpler ways to see Bernoulli?

edit: velocity distribution:

Hotter the color, the faster the speed. Air accelerated to clear the leading edge is moving so much faster than its starting speed (relative wind speed), so it’s static pressure has dropped pulling the higher static pressure air down from above.

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epic science stuff 👌

idk I didn’t read it but it looks cool ig 😂

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Summary:

this is how rockets, helicopters and airplanes make lift against gravity.

this is how air wants to bunch up at the wing’s curve :

here’s how the bunching up keeps the flow going by accelerating it around the curve (incoming DYNAMIC pressure logjams, increasing STATIC pressure which keeps the logjam limited by accelerating DYNAMIC pressure along the curve - Bernoulli’s pressure drop paired with speed increase):

the depleted static pressure from the “balloon deflating” pulls down on denser air from above:

roughly speaking…

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Another summary occurred to me. Bernoulli is about two competing demands:

1)Continuity of flow (conservation of mass)
2)Dynamic pressure vs static pressure seesaw (conservation of energy)

Airfoil design is about putting stress on 1), to get a result for 2) that pulls down on enough air in the most optimal way to balance the particular aircraft’s performance goals.

Btw, there are about 10 times more Google hits for “equal transit time theory airfoil” than for “equal transit time FALLACY airfoil.”

“Equal transit time” is an attempt to simplify 1) above. But it apparently oversimplifies as shown by wind tunnel videos.

Continuity of flow is intuitively a more fundamental principal: What comes in has to go out at the same rate (the wing can’t be forever stockpiling more mass of air).

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I forgot to tie the commentary back to the 787 table:

AoA is fundamental to generating lift.

So as the table shows, AoA has to increase to make more lift for a heavier weight or from thinner air (higher altitude).

Mach speed is TAS but the air may be more or less dense for the same TAS. IAS is the “air density speed” which directly affects how much more AoA you need (how much more you need to curve the air flow’s path to get a bigger pressure drop above the wing).

For the lift equation:

AoA is, unfortunately, inside of

I say unfortunately because it’s just a bit harder to directly see its relevance from the equation.

But

is the kinetic energy density of the relative wind (it even looks like the basic physics formula for kinetic energy), which is directly related to the need to fly by reference to IAS (the v is relative wind, TAS; the density term relates it to IAS).

Finally, a one-liner summary: Bernoulli explains how the 787 uses AoA to deflate static pressure from an airstream so that its fixed wing can do what a rocket and helicopter do, pull air down to stay up.

edit:

Bernoulli explanations usually seem to be accompanied by: “it’s counter intuitive but pressure drops when speed increases.” And so most of us (me) end up memorizing the fact, plus the label Bernoulli to slap on the memorized fact.

So…Devil’s advocate to my commentary: But how does the bunching up of air relate directly to the fall in pressure BELOW the surrounding air rather than just accelerating it from increased pressure where it hit the leading edge?

Answer: Deprivation. The logjam builds until downstream deprivation of air density becomes however low it needs to be to ramp up the acceleration enough to clear the torrent of incoming airflow (the surrounding static air pressure is no floor to pressure drop).

@alexNine99 About your wind tunnel considerations, it occurred to me wind blown off the top of a house from strong wind causing drop in pressure is analogous to the stationary airfoil in a wind tunnel.

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I don’t know who might eventually wander into the IFC to read the above, given such publications as the New York Times, and Scientific American have said flight can’t be explained:

zo67Kyvc7BmVqZkzE3oja2IJx.jpeg)

Given the possibility, I wanted to restate the following with, hopefully, a somewhat clearer example:

How does pressure drop relate to the same mass flow rate of air? Clearly the larger the pressure drop, the bigger the acceleration (you’d fall faster into a planet with stronger gravity).

But why is the same amount mass being transported if the density is less?:

“Bernoulli”: Because in 10 seconds you can pass the same number of race cars over a finish line, PACKED LESS DENSLY, if the speed increases.

Same number of cars passed in 10 seconds is the same mass flow rate maintained, at higher speed, but with lower density (less packed air molecules can keep up the same mass flow rate).

That is, the airfoil’s “deprivation detour” (curved path) causes the air flow to drop pressure to accomplish (acceleration to) higher speed, all for the purpose of maintaining unbroken mass flow rate.

So lift is (edit: “one turn deserves another:” airfoil’s turn of the airflow drops pressure which turns air above downward):

1)Turn the air in a different direction, which pushes the object doing the turning in the opposite direction.
2)The air is turned by the drop in pressure imposed by the object not being symmetrical (air is detoured more on the top than bottom).
3)The drop in pressure is the “unpacking” of the air density to achieve the increase in speed necessary to keep the mass flow continuous over the detour. (the race cars example makes the case this is an intuitive result)
4)The pressure drop is “regulated” by instability of any air mass accumulating excessively from imposing the curved path on the passing air.

(As far as keeping the air glued to the trailing upper surface of the wing, it’s the pressure drop from the “unpacking” that moves higher pressure air down from above.)

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Man got the 100% in basic aerodynamics 👏👏

GG mate

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Thanks! Now if only someone would explain flight to the New York times and Scientific American, and more than a few websites…:)

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Why would I (or someone) not be able to come up with a realistic explanation of lift that ticks the key boxes?? Is it not even possible?

The following examples are fascinating and not uncommon:

It’s not just the New York Times and Scientific American that seem to have trouble with explanations for lift (mentioned further above):

1)University of Cambridge, Department of Engineering, academic Holger Babinsky in 2003:
“The explanation of lift taught by the Royal Air Force to cadets, I’m claiming is completely wrong
How do wings work - Common misconception on lift - YouTube 2:05 sec
Lift - Prof. Holger Babinsky - YouTube(edit: this is the better version - presentation diagrams included)

2)University of Michigan Engineering, aerospace professor Krzysztof Fidkowski:
“After I finished undergrad, I couldn’t explain how airplanes fly
Krzysztof Fidkowski | How Planes Fly - YouTube 0:20 sec

3)Retired Boeing Technical Fellow, Dr. Doug McLean (coded CFD for transonic wing design, etc.):
The Coanda effect is not needed” (and other misconceptions) 21:16
Doug McLean | Common Misconceptions in Aerodynamics - YouTube

I believe it’s possible. It’s not magic is it?

This simply must have an intuitive explanation, but that doesn’t violate the key elements of reality or use “plug” explanation (replacing one mystery with another). And I believe I’ve finally made that case.

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translation plz :)

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Yall preparing me for science class already?

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Was literally about to say the same thing… I leaned this in 10th grade this year and only remember a bit of what they are talking about lol

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I’m 2 years behind you in school

It should still be explainable to everyone I would think. Why the experts can’t agree is what is so interesting. It shouldn’t be “rocket science”(?)

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Previously, here I made quick notes that my editing made too chaotic to be understandable.

I’ll just replace it with words from an earlier post, changed a bit:

Lift results from two competing demands 1) and 2) below, causing air to be accelerated down 3):

1)Continuity of flow (conservation of mass)
2)Dynamic pressure vs static pressure seesaw, dropping the pressure (conservation of energy)
3)Low pressure pulling air down

A wing puts stress on 1), to get a low-pressure result for 2) that pulls down on enough air 3) to hold up the aircraft’s weight.

1 and 2 are what the Navier-Stokes equations describe in detail (used universally in aircraft design). Navier-Stokes equations are Newton’s 2nd law applied to fluids and gases. This is Bernoulli.

3 is Newton’s 3rd law (action-reaction).

A basic description of lift tied together as above (that 3 results from 1 and 2) sticks to 1st principles of physics (it’s all Newton), and so stops a lot of the cognitive errors baked into the dogma dominated history of explaining the basics of lift.

I believe adopting some form of the above would improve teaching and understanding.

(advanced note: the camber+AoA restriction in passage presumably lets pass out only molecules with sufficient velocity to keep up the mass flow rate continuity (through the restriction’s detour), so the Bernoulli speed increase comes from restricting speeds to the right of a vertical line moving to the right (as the restriction is encountered) through the following diagram - so average speed goes up → achieving the same mass flow rate, but at lower density):

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That does make more sense now, thanks!

Do you have a career in some field of science or something? You seem pretty smart lol

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Your welcome. And thank you. But I tend to be skeptical about me being smart. Which I consider an asset (because if I suspect I might be not so smart, then I don’t mind looking dumb so much, which keeps me thinking, which can give a better outcome).

My field was aerospace electronics hardware (engineer).

Trying not to get technical but in a gas such as air, the molecules have an average speed, but a huge range around the average. The hurdle, that is the airfoil’s departure from a straight path
it confronts the air with, basically chops this curve somewhere toward the higher speeds to the right.

Those moving in the direction of the airfoil’s path supply the higher speed flow (we see as increased dynamic pressure); and the rest being blocked is the drop in static pressure, that becomes the pressure drop that holds the wing up.

Basically throwing a bunch of ping pong balls into a room, which bounce back and forth forever (lots of different speeds and directions - you filter fast enough ones, traveling in the direction you want to go - like air rushing out of a balloon opening).

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Youtube channel Veritasium (14 million subscribers) has a short video “How Does A Wing Actually Work?” that makes almost no improvement on the problem of explaining lift. How Does A Wing Actually Work? - YouTube

Because he doesn’t actually tie together the two concepts he is trying to reconcile (pressure drop and downward air movement), he’s left with the common “plug explanations” of Coanda effect and air deflected down from the bottom surface.

This lack of cause and effect discipline between the two explanations leaves no hint at how to deal with the “unequal transit times,” or how to decode the Coanda effect intuitively in terms of Newton’s laws.

I just think this is a good example of why I never felt I honestly understood lift.

Again, my test is, can I explain to myself what’s going on here with AoA’s dance in relation to camber in keeping me flying:

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Interesting, but doesn’t tbe Coânda effect thing have something to do with buoyancy and not lift? I could be wrong, not sure tho

Wikipedia’s first line is: The Coandă effect is the tendency of a fluid jet to stay attached to a convex surface.

So it’s used to describe things we observe.

And there are various observations given as examples.

But seeing how it’s being used so frequently as an explanation for how various things work, where is the clear and consistent explanation for what it is in terms of Newtonian physics?

Without that, it can’t be considered a first principles explanation.

Wikipedia itself has the disclaimer, apparently permanently attached:

It seems it’s not necessary to insert “the Coanda effect” as the missing link in understanding lift. (Also, the Boeing scientist mentioned earlier said the Coanda effect is not needed in the explanation).

But you’ll find a massive amount of content online claiming this is necessary.

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