Bernoulli's principle

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37 minutes ago, swansont said:

Probably. I'm saying as long as the equations are applicable, you can often pick different approaches to solving/explaining a problem.

As an analogy, you can discuss a falling ball as speeding up owing to a force on it, or due to exchanging potential energy for kinetic energy. Both are correct. It's not one or the other.

As you say the same result can be obtained by different routes.

Here is a very simple derivation of the lift force from Bernoulli for @jfoldbar to aim at.

It shows the angle of attack, the Bernoulli equation, the bunching of the flowlines I mentioned and explains the circumstances and the limitations of the model.

The next thing to then do is to discuss the application of that lift force to the aircraft and the effect upon its trim.

Edited by studiot
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1 hour ago, John Cuthber said:

Stunt plane wings are, I'm told, "thick" because they have to be strong, and more or less symmetrical.
They are pretty nearly "fins".

If you take a flat fin and  streamline it a bit you get the sort of wing you see on aerobatic planes
https://www.amaflightschool.org/getstarted/how-do-i-know-difference-between-basic-trainers-aerobatic-trainers
And if you then tweak it to reduce the drag caused by turbulence behind/ above it, you get a conventional aerofoil.

Nobody cares about the fuel efficiency of stunt planes, but they have wings that look like dolphin fins in cross section.

This has nothing to do with the discussion, so far as I can tell.

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"A symmetrical airfoil like the Avistar’s develops lift using only Newton’s Third Law. But a flat-bottom wing like the NexSTAR’s obtains lift using both Newton’s Third Law and Bernoulli’s Theorem. Therefore, for a given wing area, a flat-bottom wing produces more lift at a given airspeed and angle of attack than would a symmetrical wing with the same conditions. This is why all basic trainers utilize a flat-bottom wing. Aerobatic trainers use more symmetrical wings because these airfoils develop the same amount of lift whether inverted or upright. This is good for aerobatic flight, but not usually required in a basic trainer.
Semi-symmetrical airfoils have some airfoil shape on the bottom but a lot less than on the top. They use a little Bernoulli and a lot of Newton to develop lift. These airfoils develop more lift than symmetrical ones but a lot less than flat-bottomed airfoils. They also sacrifice some aerobatic performance."

It seems to indicate a component produced by Bernoulli, which might be lacking in aerobatic aircraft because of the need to fly upside down.

Wings are very complicated, and trade-offs are made to tailor the plane for a specific flight envelope.
We won't even consider effects like recirculation, vortex flows, and methods, like camber, of keeping flow as laminar as possible over the wing surface before going to turbulent at high alpha.

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On 9/8/2022 at 8:19 PM, jfoldbar said:

so, as i understand, Bernoulli's principle states that an increase in a fluids speed lowers its pressure.
the thing i cant find on any YT vid is the 'why'?
what is happening on an microscopic scale?

Firstly, we can restate this as 'an increase in a fluid's kinetic energy is matched by a decrease in its internal energy'.

The internal energy of an ideal gas is purely a function of its temperature. (High altitude air is to all intents and purposes an ideal gas).

Therefore in order to create kinetic energy, this must be matched by a corresponding drop in temperature.

For 'fast' processes such as the movement of an aircraft wing through a body of air, there is insufficient time for any significant thermal diffusion and therefore the process is very close to one of constant entropy.

Under such conditions

P = P0 (T/T0)^(1-1/k)     where k ~ 1.4 for air

So when gas velocity increases therefore temperature decreases therefore pressure decreases. It's an indirect route so not intuitively obvious.

Having read the posts above, I suggest you consider:

1) The primary direction of motion of the air over a wing is vertical, not horizontal. The air is accelerated up and down.

2) Pressure is a force, not a form of energy. You can safely disregard any post that tells you otherwise (it's a very common misconception).

3) Eugene Khutoryansky's Youtube videos on thermodynamics are very good. The challenge with this one is in visualising how flow in a pipe can be relevant to your area of interest. It is there but you need to have the right mental image in place. If not, then it probably won't help.

Edited by sethoflagos
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On 9/9/2022 at 1:25 AM, studiot said:

but it is only  a first step

yep, i know. but i gotta start somewhere

On 9/9/2022 at 1:25 AM, studiot said:

but it is not necessary.

yes, i have learnt his from YT

On 9/9/2022 at 8:30 AM, studiot said:

According to the video if I stopped the flow by blocking off both ends of the pipe the wall pressure along the length of the pipe would vary with the diameter of the pipe.

this is a very good point.

On 9/9/2022 at 2:38 AM, studiot said:

is a student

im not a student. im just a regular dumb joe that sometimes likes learning all kinds of science stuff thats way beyond me.

On 9/9/2022 at 8:30 AM, studiot said:

A further problem is that the video describes incompressible flow in pipes.

this crossed my mind too, and i wondered the difference (if any) between liquid and gas

On 9/9/2022 at 8:30 AM, studiot said:

There are no pipe walls in the atmousphere

this is one thing i wonder too. how does what happens in a pipe, also relate to what happens over a wing

On 9/9/2022 at 2:38 AM, studiot said:

OP question was about Bernoulli

yep. at the moment i want to keep it to bernoulli as much as possible. i realise because my end goal is flight, the discussion will spill over a bit.

but try to keep to one subject at a time if possible, as im all ready getting lost

thanks

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56 minutes ago, jfoldbar said:

yep, i know. but i gotta start somewhere

yes, i have learnt his from YT

this is a very good point.

im not a student. im just a regular dumb joe that sometimes likes learning all kinds of science stuff thats way beyond me.

this crossed my mind too, and i wondered the difference (if any) between liquid and gas

this is one thing i wonder too. how does what happens in a pipe, also relate to what happens over a wing

yep. at the moment i want to keep it to bernoulli as much as possible. i realise because my end goal is flight, the discussion will spill over a bit.

but try to keep to one subject at a time if possible, as im all ready getting lost

thanks

Thanks for replying I was just trying to row back a little on the discussion, which was getting rather esoterical.

OK so in lower high school (GCSE) enough Physics is introduced for someone to understand but not fully appreciate the mechanics of flight.

Anyone with upper high school ( A level) physics should be able to appreciate fully.

For some reason nearly all treatments of flight mechanics launch straight into the issue of what keeps an aircraft up.

In my opinion this is the hard way to approach it and also leads to a failure to take note of all the forces acting  - something the early pioneers failed to do with as result in so many crashes, despite have enormous (by moderns tandards) wing areas.

When a physicist talks about "The Four Forces"  she means the four fundamental forces of nature and not what we need here.

When an aircraft engineer talks about " The Four Forces" he means the four forces that control the mechanics of flight between them.

These four forces are  Weight, Lift, Drag and Thrust and all are required. (Before you ask, a glider with zero thrust needs a start velocity and eventually fall out of the sky, hopefully in a controlled manner)

In my opinion it is best to start with these, just accept they exist, and postpone how they occur until you a happy with knowing how they interact to control flight.

This is the approach taken by flight instructors to din these interactions into pilots, so they can 'instinctively' perform the necessary balancing act between the four.

I was planning to offer this in more detail when you had responded, but I am about to go out for the day so I will look again this evening.

Edited by studiot
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On 9/10/2022 at 11:13 AM, jfoldbar said:

im not a student. im just a regular dumb joe that sometimes likes learning all kinds of science stuff thats way beyond me.

... at the moment i want to keep it to bernoulli as much as possible. i realise because my end goal is flight, the discussion will spill over a bit.

but try to keep to one subject at a time if possible, as im all ready getting lost

thanks

Let's spill over a bit.

Commercial airliners typically cruise at a tad less than 600 mph. Their wings typically take 1/100 second to pass through the air and in that time a slab of air equal to the wing thickness must separate by about 10" and rejoin.

We'll leave aside what happens at the blunt end of the leading edge (complicated) and focus on the point where the wing surface is at 45 degrees to the oncoming air. At this point the airstream close to this surface must physically be travelling upwards at the same speed the aircraft is travelling forwards. ie -600 mph.

Like any other matter, air at 600 mph will continue travelling at that speed in a straight line unless an external force (and in this case, a substantial one) acts on it.

Therefore, one millisecond later the air 'would want' to be about 75 feet above the thickest part of the wing creating a void. In order to stay in contact with the wing surface the air must expand very rapidly, substantially reducing its temperature and pressure.

The bulk of the airstream now has 'normal' pressure above it and a partial vacuum below. This provides the large driving force necessary to reverse the direction of the airstream and keep it in contact with the wing surface.

On 9/10/2022 at 11:13 AM, jfoldbar said:

this crossed my mind too, and i wondered the difference (if any) between liquid and gas

The Bernouilli equation was developed to describe the flow behaviour of water, not gases.

If you puncture the base of a water tank, the kinetic energy of the jet is directly proportional to the pressure difference.

If you puncture a compressed air vessel, the kinetic energy of the jet is directly proportional to the temperature difference.

In the gas case there is also a drop in pressure (giving some sort of apparent validity to the 'Bernouilli Principle') but it is very far from a proportional relationship, and can be highly misleading if Bernouilli is treated as Gospel. Which some of the lay community are prone to do.

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1 hour ago, sethoflagos said:

Therefore, one millisecond later the air 'would want' to be about 75 feet above the thickest part of the wing creating a void.

That's about 10", of course!

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To continue with my four forces theme here is a diagram to show how they affect an aricraft in flight in the three basic different situations.

Do you wish to discuss and understand these before worrying about how they are generated ?

Edited by studiot
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On 9/9/2022 at 1:11 AM, StringJunky said:

If you start at the front where the air splits into two halves, the top flow close to  the wing surface, whch is humped and lthus onger compared to the straighter bottom surface, must arrive at the same time at the rear of the wing where they join back up. To satisfy that condition the top flow has to travel faster. Bringing in Bernoulli, the faster air at the top puts less pressure on the top half than the bottom half, giving us lift.

Generally speaking, it doesn't. That old explanation has long been debunked.

(seems others have already pointed this out over a week ago)

Bernouilli's principle is somewhat straightforward, but complex with regard to wings as it is incomplete. Shear forces (especially affecting boundary layers and turbulence) are not taken into account.

On 9/9/2022 at 6:10 AM, John Cuthber said:

Why?

Why can't it arrive a bit later?
The air can sort itself out later by swirling about.
It's not like having two queues of people going through customs barriers where the couples need to meet up afterwards.

The "it goes faster over the curved surface so.... Bernoulli... it generates lift" explanation is clearly wrong. You can fly stunt planes upside down indefinitely.

At the very least, you need to consider this as well.
https://en.wikipedia.org/wiki/Coandă_effect

+1

You can also fly an asymmetric wing plane upside down, though less efficiently. The Bernoulli effect is still there despite the curvatures, as it is with the flat plate.

The air still gets accelerated on the upper side.

Ultimately it comes down to Newton explanation, with air deflected downward and/or ground effects.

(Not the explanation of Newton himself...he thought the lift was from the air hitting the underside of a wing where it's primarily from the reduced pressure on the top when the wing is not in stall)

On 9/9/2022 at 12:06 PM, MigL said:

Sure John, a flat fin at an angle of incidence would work, and probably produce more lift than a classic streamlined 'D' shape ( like a Clark Y ), but I can't imagine it would do any good for your lift/drag ratio, and you would have trouble keeping it in the air as a result.

I realize there are many factors which contribute to lift, but I thought we were explaining specifically how Bernoulli makes its contribution.
According to Bernoulli, the fluid needs to move faster, creating a localized area of relatively lower pressure, resulting in 'lift'.

You might have trouble getting it in the air. Once at speed a flat plate can be quite low drag, relative to a typical airfoil assuming the same planform. (but as John pointed out...not the best structural  profile.

Edited by J.C.MacSwell
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On 9/9/2022 at 12:30 PM, studiot said:

According to the video if I stopped the flow by blocking off both ends of the pipe the wall pressure along the length of the pipe would vary with the diameter of the pipe.

Where does the video make that claim? As far as I can tell it assumes a constant flow in the pipe, and with it a difference in pressure due to the different velocities in different diameters.

In practice this is true except for a loss of pressure downstream due to friction, which Bernoulli's principle doesn't account for.

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On 9/20/2022 at 1:39 AM, J.C.MacSwell said:

Where does the video make that claim? As far as I can tell it assumes a constant flow in the pipe, and with it a difference in pressure due to the different velocities in different diameters.

In practice this is true except for a loss of pressure downstream due to friction, which Bernoulli's principle doesn't account for.

That's the whole point, the video doesn't make this claim or even address that situation.

Yet it claims to explain Bernoulli.

I'm sorry if my method of pointing this out was a bit dramatic.

The standard simple version of Bernoulli contains three terms.

Two of these are independent of velocity and solely determine what happens when the fluid is not moving (ie v = 0)

@sethoflagos introduced one of these, though he perhaps didn't explain it very clearly.

So the video information is pretty deficient as it attributes everything to flow.

I also note that @jfoldbar seems to have lost interest in the subject.

Edited by studiot
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1 hour ago, studiot said:

That's the whole point, the video doesn't make this claim or even address that situation.

Yet it claims to explain Bernoulli.

No. It claims to explain the Bernoulli Principle in qualitative layman's terms at the molecular level: an interpretation that wasn't available to Bernoulli who published Hydrodynamica about 150 years before atomic theory became widely established. In my view the video did a perfectly reasonable job of that.

Naturally, this explanation only describes the real world cases where Bernoulli's Principle applies.

The video certainly made no claims regarding the validity and scope of applicability of the Bernoulli Equation. To suggest so would be a bit strawmanish.

2 hours ago, studiot said:

The standard simple version of Bernoulli contains three terms.

Two of these are independent of velocity and solely determine what happens when the fluid is not moving (ie v = 0)

@sethoflagos introduced one of these, though he perhaps didn't explain it very clearly.

So the video information is pretty deficient as it attributes everything to flow.

The Bernoulli equation is a rabbit hole that offers very little insight into the mechanics of flight. The changes to gravitational potential energy of air are three orders of magnitude less than the dominant forces and a little more significantly, frictional dissipation is neglected as pointed about by @J.C.MacSwell.

Most crucially from my perspective the Bernoulli Equation neglects the contribution due to thermodynamic work. Particularly at the leading edge of a wing where air is both pressurised and rapidly accelerated simultaneously. This is the absolute reverse of the expections given by the Bernoulli Principle and a clear indication that too much is missing from the Bernoulli picture for it to be usefully employed here.

2 hours ago, studiot said:

I also note that @jfoldbar seems to have lost interest in the subject.

I noticed that too. Sometimes having to go back to square 1 is a bit dispiriting.

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