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

So you’re saying that lift is produced by the pressure difference created by the Coanda effect, where air sticks to the wing’s surface and results in increased pressure above the wing, causing it to lift upwards?

Honestly, I lost understanding of all of this when you started solving the wind tunnel dilemma with kinetic energy and whatever. While I understand how the energy is conserved and whatnot, it isn’t an intuitive explanation. “The air curves because [insert explanation with static pressure and dynamic pressure and energy stuff]” is just as mysterious as “the coanda effect”

Not to be rude or anything, but an explanation is never intuitive if it’s abstract in nature.

In short, “the air curved because it not curving would violate conservation of energy. [explains the energy]” isn’t an intuitive explanation for why/how the air curves. It’s only an explanation for why it curving doesn’t violate conservation of energy.

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I definitely agree with you, if I don’t understand something, then I can’t be satisfied trying to tell myself I understand it. Which is what drives me to question.

As far as the first part, static pressure turning the air, would you say it’s not intuitive

Like how the low is bending the direction of wind around it?:
image

I want to be clear I’m not trying to be sarcastic or flippant here. I’m trying to understand what explanations don’t work (or are otherwise flawed).

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I’m actually saying the opposite - it’s best not used in the explanation of lift:

(my poor wording choice)

Air is forced down from above as a result of the low pressure above the wing (like happens in the weather image in the post above).

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Analogy 1. A window blows out on an aircraft at cruise, the air rushes out the window. It illustrates why air above the low static pressure area is being curved down (replaces Coanda effect):

Analogy 2. Once the aircraft’s static pressure has equalized with the outside by blowing off enough pressure through the window, the dynamic pressure of the air gushing out, stops. It illustrates Bernoulli conservation of energy: speed increase (air rushing out the window) is accompanied by pressure dropping: the speed increase comes at the expense of static pressure.

Analogy 3. The window frame is an air passage restriction, as is the curved contour of an airfoil. These restrictions in passage are what cause the static pressure to be converted into faster rushing dynamic pressure.

Analogy 4. Previously mentioned further above: in 10 seconds you can pass the same number of race cars over a finish line, PACKED LESS DENSLY, if the speed of all the cars increases.

Keeping the same number of cars passing per unit of time is the analogy for the need to keep the rate of mass flow moving out exactly the same as its speed moving in.

It shows you can do it (to compensate for the detoured path of the airfoil curve), by tipping the seesaw balance between static pressure and dynamic pressure.

Static pressure is how dense the race cars are spaced together. Dynamic pressure is how fast they are moving.

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It’s like when the news organizations thought the reverse thrust being opened on a 737 meant the engines had a “gaping hole.”

When I first read your post, I thought what!? Second thought was it sounds too crazy, it must be true. So, I read the story.

Here’s another one. Along with the New York Times, Scientific American and youtube channel Veritasium getting lift wrong, here is Neil De Grasse Tyson making several significant errors in his explanation of lift:

How Do Airplanes Fly? | Neil deGrasse Tyson Explains… - YouTube

1)sheet of paper lifting by blowing on it does not demonstrate Bernoulli principal (the energy from his lungs is used to accelerate the air, violation: the air acceleration is not created by the act of going over the curve)
2)ascent from the runway occurs mostly from air ramming under the wing when the aircraft pitches up (error: no, the increased AoA increases the pressure difference causing sufficient air to be forced down to propel the aircraft up, even giving allowance for ground effect)
3)he claims that it’s not just Bernoulli doing the work, it’s also AoA, as a separate thing, which explains why planes can fly upside down etc. (absolutely not: AoA is Bernoulli at work as much as camber is Bernoulli; it is not some separate phenomena)

He is a public figure science educator with a doctorate level in physics and I wish he could answer this.

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@alexNine99 Have you seen this one?:
How Do Airplanes Fly? - YouTube from (minutephysics, 5.7m subs)

Interesting he says it’s sponsored by Airbus (they hosted him to observe A350 being made).

At 1:07 he brings up the trailing void, even though as mentioned before, the professor at Michigan says it’s a false explanation Krzysztof Fidkowski | How Planes Fly - YouTube).

The first link above is wonderfully short and has a lot of content given quickly (fan blades as mini wings etc.). It has to be a new favorite of mine.

But I think he still gets the core issue wrong: To get a proper insight, you have to tie together:
Bernoulli pressure drop, action-reaction from air being forced down, and no interruption in the rate of mass going out.

One can’t separate them, simply saying “it’s either/or.” Or a “little bit of this and a little bit of that.”

If watched, you’ll notice he throws a few different ideas together about how to get to the Bernoulli pressure difference, and then uses the disclaimer “whatever the reason.”

The vaguely plausible “whatever’s” he mentions are not “food and water”-like essentials. No interruption in the mass flow out, is such an essential. It’s the essential that directly forces the Bernoulli pressure drop.

@Generic_Flyer Your rather funny “='s” joke in Playing infinite flight benefits may be tied to me trying to tie the above together?:)

Oh my gosh! The physics explanation of the following sounded so good until he got it so wrong: “Newton’s 3rd law downwash is an additional lifting force” (adding to Bernoulli he apparently means). From the Archives: The Physics of Flight - STEMCON 2021 - YouTube (an example of why the lack of “='s” is maybe the Achilles heel to all these explanations)

(Listed credentials of the above presenter: FAA Certificated Commercial Pilot and FAA Certificated Flight Instructor and Advanced Ground Instructor. Long standing member of the Aircraft Owners and Pilots Association, the Experimental Aircraft Association, the National Association of Flight Instructors, as well as the Society of Aviation and Flight Educators)

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I agree with this statement yes

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This is a bookmark to myself to ideally maybe make some simplifying illustrations in the future, that makes this more digestible.

I tried to do a better rough draft outline, that I burned out on before.

I put it here just in case anyone wants to try to tear it apart (always appreciated) in the meantime.

Maybe key theme here is that “='s” signs matter :) (there are equations behind the words)

The rough draft (little punctuation etc.)

energy of the relative wind cannot be destroyed - but any dissipation will go to either parasitic drag (energy loss as turbulent flow, and heat)
or pure lift generation.

energy for pure lift generation is experienced also as drag, which engine power or descent must be engaged to keep speed from reducing (just like parasitic drag).
the energy of pure lift is simple: kinetic energy of the air mass moved down to accomplish Newton’s 3rd law (reaction for every action) (1/2)mv^2

edit: MATH ERROR WAS IN THIS LOCATION!

relative wind energy not lost as parasitic drag (parasitic is that which leaves the system without helping lift the plane, and slows its forward motion) edit: as well as that lost going into producing pure lift,
has to leave the vicinity of the wing with the mass flow rate unchanged (it can’t sustainably pile up)

so the detour of camber + AoA, cannot slow the local relative wind’s progress,
yet it tries to
the pressure drop has to be there to accelerate air around the detour so that it leaves at the same rate it comes in
the speed increase caused by the Bernoulli pressure drop has to maintain the horizontal mass flow rate
it’s caused by deprivation at the detour bottleneck, that drops the pressure until the speed acceleration is high enough
(more advanced: air static pressure at a given temp represents air molecules at a wide range of speeds; the bottle-neck/pressure drop represents a filter for molecules with the necessary higher speeds, that average to a higher dynamic pressure)

The pressure drop pulls down the air from above, which causes the Newtonian action-reaction lift
but accelerating the air above down takes energy (the kinetic energy as before)
because the air moving down into the low pressure area attempts to raise the pressure
this is experience as “pure lift drag” to keep up the pressure drop (sufficient to maintain the mass flow rate)
which burns engine fuel
edit: simpler way to say to illustrate this: contour lines being pulled down by low pressure area reduce relative wind horizontal speed to increase vertical speed. Apply 1/2mv^2 to both of these: The vertical v is energy for lift

because there is clearly mixing of horizontal contour flow wind, and air descending from above
this process gets complicated in reality

but presumably this doesn’t affect the fundamental soundness of the principles tied together here

Sorry for the obsession

Shoot, I got an edit: I wanted to cram in that in wind tunnel or simulation diagrams, the Newton’s 3rd law air forced down will be become less evident illustrated as sinking flow lines, the higher the air density and TAS. Because if the air is "thick, the squared v term in kinetic energy means you need much less air forced down to make lift. So, higher density altitude, for example, means you have to generate disproportionally more down speed (steeper flow lines).

This is what makes airliners fuel efficient for long haul’s etc., with high L/D ratios (and why high bypass makes engines more fuel efficient)…all to minimize the squaring of the downward air speed needed. More air mass is much cheaper than airspeed (mass is not squared).

edit: Yes, the above is still too wordy. Engaging in the IFC for the past couple or three years has made me keep returning to the fact I never honestly felt like lift had been explained to in a way I could understand.

But this is leading to, I believe, a much more concise way to put it…

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bro can we stop with the science. it’s giving me terrible flashbacks

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Yeah, but it’s leading to this:

You’re flying from, say, Tokyo to LA. Can your aircraft wings keep saving up more and more air mass? Clearly not. What comes in has to go out.

So when the wind in confronts the “kink” of the camber combined with AoA,

the supply of what’s going out gets choked off…for some moments. But the choking off drops the pressure downstream from the kink because downstream from the kink is not getting fed.

Pressure drop from being choked? Yeah! That’s Bernoulli: “feed me air, I gotta make the exit quota!”

It’s increased speed of air around the contour caused by deprivation leading to a downstream pressure drop. (the cause is the fix, and the fix is pressure drop, and the secondary result is mass forced down, like a rocket or helicopter)

You can’t accumulate air on the wing like a savings account; only a bit of air savings accumulates before the choking off of air drops the pressure enough, to accelerate the air around the disruptive kink to fast enough higher speed to make up for the kink’s detour.

So, lift: a kink inserted into an airflow drops the pressures as it tries to adjust for the kink’s disruption.

The pull of the pressure drop from the kink disruption bends all other passing air in the vicinity down.

The math I put in above is showing how engine power is related to sustaining the bending of air down (from the pressure drop caused by the kink).

It’s not Bernoulli or Newton’s 3rd law it’s both.

It’s kink removal.

or, TLDR version candidate: lift is when a kink placed in an airflow, causes the airflow to pull back harder with lower pressure (to get itself through the kink), which just so happens to pull air down from above (action-reaction, like a rocket’s thrust)

Is this likely true or not? Does it connect the different pieces of lift, which are usually left as either/or explanations?

Is a similar such explanation anywhere else to be found? (I’ve failed to find it)

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Bro y’all just using chatgpt to sound smart

That’s a fair concern. I have tried ChatGpt. It absolutely fails to recognize the core ideas of the above logic. You can certainly test it out for yourself. But make sure you know what you’re asking for so you have a chance to filter out the gibberish.

Not trying to sound smart. I’m trying to answer my own personal burning question and I’m looking for my ideas to be rationally destroyed as a way of stress testing.

Plus it is related to IF.

And looking like I’m trying to sound smart is the price I pay for trying to clear up what I have honestly felt I’ve not understood about a major issue to do with why airplanes stay in the sky.

And I’ve never seen the key ideas tied together as I outlined, in any explanation from any source (if you can find such a source, please let me know).

(edit: I think you may have been joking:)

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Another quick way to word the changing AoA below, directly in terms of the physical mechanism of lift:

image

Changing AoA is the change in obstruction to airflow over the wing.

AoA going from 3 to 4, for example, is increased airflow obstruction.

The airflow reacts to this increase in flow obstruction by dropping pressure (less airmass is let in though the narrowed “window” for mass flow), which forces the mass flow rate to be maintained at the original level by faster flow speed caused by the lowered pressure.

(CONTRADICTION TRANSLATION: THE OBSTRUCTION, of AoA and asymmetrical camber, TO TAS MASS FLOW RATE, DEPLETES PRESSURE UNTIL IT’S LOW ENOUGH THAT THE LOW PRESSURE INDUCED FLOW ACCELERATION MAINTAINS THE ORIGINAL TAS FLOW RATE).

(the pressure will continue to drop until it levels off at a low enough value to draw in the flow at the ORIGINAL MASS FLOW RATE, which corresponds to a HIGHER MASS FLOW SPEED, at LOWER DENSITY).

And secondly, more air is pulled down from above from this increased drop in pressure (initiated from this increase in obstruction). Thus, more Newton’s third law of action-reaction against gravity.

The 787 therefore continues to fly at level altitude, at the increased MTOW compared to the lower MLW, from the extra lift produced at the higher AoA.

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The “secret” to why tons of steel float to the clouds. (What better wonder than the soon to be reworked A380?)

Bernoulli pulling air down is only natural (or why even barn doors can fly):


Above, incoming relative wind: 6 air molecules go by per second, in a given length of space.


When mass started to pile up at the path restriction, pressure started dropping due to the deprivation of passage.

The pressure drop accelerates speed (Newton’s second law).

This process continues until the speed is fast enough to prevent any more pile up.

So, the pile up is prevented when, again, 6 air molecules can go by per second.

In the 2nd image above, the speed had to double to maintain the rate of 6 molecules per second passing by.

Mass won’t pile up, because the mass flow rate has been moderated to remain unchanged. The higher particle speed maintains the same mass flow rate because twice the amount of occupiable space goes by per second.

So, the spacing density is cut in half, which is the pressure drop.

This Bernoulli induced lower density (with same mass flow rate), is exposed to air flow above at the original density (because it’s flow is free of path restriction).

Air at the original density is pulled down toward the lower density air. Newton’s 3rd law (action-reaction force of lift).

That’s why, in addition to the A380 and B787, barn doors can also fly.

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I try to limit the comments to new progress on the topic of AoA behavior we see in IF as related to Bernoulli’s principle.

How can the change in density caused by AoA not lead one think of what lifts balloons?:


The mass above is less dense than the mass below, caused by the wing.

For a symmetrical wing, only AoA is used to make the pressure difference (an asymmetrical wing adds to the pressure difference).

But what’s really causing a balloon or other lighter-than-air vehicle to lift into the air?

Note below (substitute water with air; buoyancy is the same in either case), the balloon is similar to a wing in that a difference in gas density pushes air down, which results in Newton’s 3rd law of action-reaction.

In both cases difference in pressure has to be tied at the hip to Newton’s third law. One explanation vs the other is not an explanation that really explains the lifting force.

“net pressure difference pushes up on the object at the more submerged point(s) thereby supporting the weight of the object. As such, objects appear to have less weight in water than air. The buoyant force can also be thought of as derived from Newton’s third law (i.e. equal/opposite forces). The force of gravity of a mass pushes down, displacing a volume of water equal to the volume of your mass. However, the displaced water (or other substance) will exert an EQUAL and OPPOSITE force, thereby pushing the object up.”

source: In air, gravity causes a gradient of higher/lower densities of gas molecules which causes pressure and buoyancy through more/less collisions. What is the basis for the force of pressure (and therefore buoyancy) in water? : r/AskScienceDiscussion (reddit.com)

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I tried to rework the statement below many times (trying for a better TLDR condensation of all the above commentary). I believe this explanation is true, and a more comprehensive and original form of fundamental explanation for lift (please, anyone let me know if it’s implausible for any reason, and I’d be grateful being directed to anything similar, previously written, if known or discovered):

Lift:

A path disturbance placed in an airflow depletes static pressure until causing sufficient flow acceleration to maintain mass flow continuity through the disturbed path. The depletion must progress until outflow is no longer deprived of sufficient inflow. The resulting terminal pressure drop (to the final equilibrium level) bends the direction of surrounding flow, imparting a force on the object of disturbance.

This ties together the 787’s specific AoA behavior in IF shown in prior tables, why a beast like the reworked A380 can float the weight of 140 adult elephants, why roofs blow off houses in a strong windstorm, and why barn doors can fly.

The lift on a balloon is a physics “cousin” of lift from the above airflow path disturbance. A cousin in the sense a pressure difference accelerates air down to force the balloon to move up until the pressure difference force equals its weight (weight of balloon’s solid materials). It is a pressure difference tied to Newton’s action-reaction law (though not caused by flow disturbance).

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Now we’re talkin!:


Bernoulli and Newton | Glenn Research Center | NASA

“There are many explanations for the generation of lift found in encyclopedias, in basic physics textbooks, and on Web sites. Unfortunately, many of the explanations are misleading and incorrect. Theories on the generation of lift have become a source of great controversy and a topic for heated arguments for many years.”

Notice that we place the names in quotation marks because neither Newton nor Bernoulli ever attempted to explain the aerodynamic lift of an object . The names of these scientists are just labels for two camps.”

So both “Bernoulli” and “Newton” are correct . Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object. We can use equations developed by each of them to determine the magnitude and direction of the aerodynamic force.”

“Arguments arise because people mis-apply Bernoulli and Newton’s equations and because they over-simplify the description of the problem of aerodynamic lift.”

This link’s particular list of errors: equal transit time fallacy, venturi effect(?), Newton’s 3rd law applied to the bottom of the wing (like rock skipping - Sorry Neil Degrass Tyson, please re-consider your explanation:)

“Newton’s laws of motion are statements concerning the conservation of momentum. Bernoulli’s equation is derived by considering conservation of energy. So both of these equations are satisfied in the generation of lift; both are correct.”

“For a gas, we have to simultaneously conserve the mass, momentum, and energy in the flow. Newton’s laws of motion are statements concerning the conservation of momentum. Bernoulli’s equation is derived by considering conservation of energy.”

One more time from me, in relation to the last statement above:
1)mass flow in has to equal mass flow out (no mass accumulation)
2)horizontal flow momentum cannot be erased (it must be accounted for)
3)increased dynamic pressure to hurry through the detour, deflates static pressure (conservation of energy)
4)a bit of a twist (no pun intended): the overall course of air flow is turned downwards, as some of the horizontal momentum is sacrificed for vertical momentum (1,2 and 3 above affect adjacent airflow bending it down. And the complex reality is that 1,2,3 and 4 interact with each other.)

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In addition to Neil Degrass Tyson, sorry Derek Muller of Veritasium. Besides deflating the “skipping stones” emphasis, again no need to mention “Coanda effect” in relation to lift in the above link.