No one can explain WHY planes fly...

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jedi

Well-Known Member
But on a wing the lowest pressures happen on the front half of the wing with the smallest radius. Why?
I was saving that till later but since you asked this may be a good time take a swipe at it.

As pointed out in Post #159 the head term of the Bernoulli equation is typically omitted in aerodynamics because of the low density of air and the limited change in pressure with height.

Bernoulli was typically working with straight pipes and the assumption was made that the flow was uniform across the pipe. This is where Bernoulli's equation falls down in aerodynamics. There are other mass related terms to consider. One of those is centrifugal force or centripetal force (I see another conflict brewing) depending on the readers understanding of physics.

Bernoulli does not handle turning airflow. (That was a period!) Bernoulli theorists try to cover that up by pointing out the curvature of the wing and the equal transit time theory, etc. It is close but no joy. The Einstein airfoil discussed in Post #88 was based on that theory and did not work well. The "Vortex Theory of Lift" was invented to compliment, counter and explain that.

Bottom line, the lowest pressure happens on the front half of the wing because that is where the radius is the smallest and the forces needed for the airflow to follow the curvature are the highest. Air molecules going from zero to 150 knots in two inches require one heck of an acceleration. (Stagnation point to max speed over the airfoil versus the nose radius of the airfoil.) If the plane is doing 120 the max airspeed over the airfoil is probably 150 or more.

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Bill-Higdon

Well-Known Member
The esplantaion is JSNM Just Stark Naked Majic in Red Neck terms

Norman

Well-Known Member
But on a wing the lowest pressures happen on the front half of the wing with the smallest radius. Why?
Because that's where acceleration happens ie moving anything around a curved path accelerates it(a change of direction is an acceleration). Aft of the minimum pressure point the flow is decelerating so the pressure is increasing.

poormansairforce

Well-Known Member
I was saving that till later but since you asked this may be a good time take a swipe at it.
Wasn't asking, I was probing. Big difference.

This is where Bernoulli's equation falls down in aerodynamics.
Bingo.

One of those is centrifugal force or centripetal force
Bingo #2.

Bernoulli does not handle turning airflow.
Bingo #3.

Bottom line, the lowest pressure happens on the front half of the wing because that is where the radius is the smallest and the forces needed for the airflow to follow the curvature are the highest.
Bingo #4.

Because that's where acceleration happens
Bingo #5.

Since the air doesn't accelerate simultaneously then you have expansion which is a pressure drop/density reduction. Go back and look at the KF 1 airfoil video I posted. Why does the wing not stabilize until its inverted? Could it be that wings float just like balloons using a density difference? We just put the energy into the system differently and measure the effect as pressure.
(Duck and cover emoji here)

Norman

Well-Known Member
That's true, but even if they did the cars wouldn't get farther apart at the bottleneck, and that's the conundrum.
If you impose a few rules on the cars it may make a bit more sense.

No penetration, the cars all have to stay on the road.
No voids, the cars have to stay on the road and the passengers in the cars can't get too far apart ie they have to maintain approximately the same average spacing at all times.
The speed of individual passengers in the cars is constant regardless of the bulk flow.

Now think of each car as a slug of fluid and the molecules as the passengers. As the car approaches the bottleneck it must stretch out and get thinner in order to maintain the its volume. The passengers are still getting bounced around randomly at the same speed but now their movement is more restricted across the flow but less restricted in the direction of flow and since the no penetration rule doesn't apply to them (they must be in a car but not necessarily the same car) some of them are being thrown from the cars (small scale turbulence).

Now replace some of the passengers with smoke particles in a wind tunnel. When the smoke is pulsed the smoke line shows the length of a section of the stream tube (a car) and the spacing between lines shows the width. As the tube gets stretched it gets narrower and the smoke lines get closer together. Since the length of a given volume of the tube is longer in the bottleneck, and the individual partials must maintain their speed, the drift in the bulk direction is greater than the drift in the cross flow direction.

There... the Walt Disney version of flow acceleration around a curved surface at low Mach numbers.

Norman

Well-Known Member
The esplantaion is JSNM Just Stark Naked Majic in Red Neck terms
That's much more polite than the 3 letter acronym I use, PFM.

BBerson

Well-Known Member
HBA Supporter
Because that's where acceleration happens ie moving anything around a curved path accelerates it(a change of direction is an acceleration). Aft of the minimum pressure point the flow is decelerating so the pressure is increasing.
Then why not put the Venturi probe at the front?

Aesquire

Well-Known Member
The question was answered in the first couple of pages. Wings/rotors/props move the air down, and the reaction is lift. That's an oversimplification AND the net answer after all the math is reconciled.

The car analogy is fun, but real cars ( their drivers anyway ) operate on ego and psychology, which, as "soft sciences" are actually philosophy or religion.

Scienterrific American, has gone seriously downhill since Science was subordinated to Politics/Religion in the Great Dumbing. Some sources claim the dividing line was 1987, but that's off topic.

I was taught Bernoulli in grade school. I was exposed to the "wings deflect air" concept with paper dart airplanes. In the years since I've been exposed to Lippisch, Reynolds numbers, and Prandtl. Every single explanation is incomplete and all of them together get close, but The Science Is Never Settled.

I'm ok with that. I currently use the mental image of overall flow field, since I've actually SEEN it in gliders flying in snow storms. ( we were young, and why not? ) And I've watched the downwash & tip vortices from a B-52 move trees, bushes, dirt, and me as it flew low down a valley.

I take for a given that with a sensitive air pressure instrument I could detect an airplane flying overhead, but haven't tried it.

As to the truck load of canaries? Did that experiment in High school with a cardboard box on a scale and another student's canary. Clear window in the box, and a perch. No weight change.

Alas, never nominated for a Nobel Prize.

jedi

Well-Known Member
In Post # 137 BBerson references the Bernoulli's principle statement from the internet presented in Post #138.

It seems the opposite would be true, like dynamic pressure. Can you explain why pressure would decrease?

Bernoulli's principle
Description
In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid's potential energy. The principle is named after Daniel Bernoulli who published it in his book Hydrodynamica in 1738. Wikipedia

Bernoulli's principle is a statement of conservation of mass and also a conservation of energy. The Mass is conserved by being contained within a pipe or in free flow by being bounded by stream lines. The energy is conserved by a frictionless flow assumption of the constant mass.

In the Bernoulli Equation, p is the static pressure and the term with velocity is the dynamic pressure. The sum of the static pressure and the dynamic pressure is the total pressure, sometimes called the stagnation pressure. For aerodynamics, where the head or density times gravity term is very small and assumed to be zero, the total pressure is constant. In other words, the loss of static pressure is converted to an equal gain in dynamic pressure.

The statement "an increase in speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid's potential energy" can be difficult to picture because it combines both the static pressure change and gravity effect in a single statement. The velocity change is demonstrated with the venturi and is frequently demonstrated.

The velocity change with "potential energy" is more difficult to picture because the velocity change is generally controlled by the pipe size.

Here is a simple experiment to demonstrate the speed change do to the potential energy portion of this statement.

Slowly open the faucet of the kitchen sink until the drip drip becomes a steady flow all the way to the sink basin. The water exits the tap at near zero velocity and a (gauge) static pressure of zero or atmospheric (absolute) pressure. As the water falls it accelerates at 1G. We could measure the distance it falls and calculate velocity and the dynamic pressure or we could simply state that the "total head" is equal to the height measured.

Furthermore, we can see that the diameter of the stream becomes smaller as the velocity increases. If we were to manufacture a venturi with these dimensions and lay it in a horizontal position we would need a static pressure at the large diameter equal to the total pressure measured in the sink to match the flow volume of the sink experiment.

It is the reduction of static pressure that increases the velocity at the throat of the venturi.

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BBerson

Well-Known Member
HBA Supporter
Bernoulli's principle is a statement of conservation of mass and also a conservation of energy. The Mass is conserved by being contained within a pipe or in free flow by being bounded by stream lines. The energy is conserved by a frictionless flow assumption of the constant mass.
I understand conservation of energy in a closed pipe. Air cools in a carburetor Venturi and freezes moisture.
Where did this statement " in free flow by being bounded by stream lines" come from?

Does moisture ever freeze in the "free flow" Venturi on top of a wing? (I have seen it condense into fog above a wing)

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aeromomentum

Well-Known Member
It actually is really simple. You can also derive Bernoulli from Newton. Both are valid when applied correctly.

The issue is many people try to use a scalar version of Bernoulli when with 2D or 3D flow they need a vector version. It is not delta speed that maters but delta vector velocity. The scalar speed could even stay the same but since the vector direction is changing there is a normal force. Of course if the radius is small the velocity vector has a rapid change.

poormansairforce

Well-Known Member
Wings/rotors/props move the air down, and the reaction is lift
Please explain how you deal with the opposite forces generated by the upflow to the wing. Or does that magically get excluded?

Dan Thomas

Well-Known Member
I understand conservation of energy in a closed pipe. Air cools in a carburetor Venturi and freezes moisture.
Where did this statement " in free flow by being bounded by stream lines" come from?

Does moisture ever freeze in the "free flow" Venturi on top of a wing? (I have seen it condense into fog above a wing)
Under the right conditions it will. Flying into very cold rain will result in that water running back from the leading edge and freezing. Clear ice. Rime is flying into supercooled cloud droplets that freeze on contact.

Norman

Well-Known Member
Does moisture ever freeze in the "free flow" Venturi on top of a wing? (I have seen it condense into fog above a wing)
Yes, that was the basis of Harry Ribllet's rant against the NACA 5 digit airfoils. They have their minimum pressure point unusually far forward and can, under severe icing conditions, form a dangerously larg lce dam that acts like a spoiler.

Aesquire

Well-Known Member
Please explain how you deal with the opposite forces generated by the upflow to the wing. Or does that magically get excluded?
After you add up all the motion vectors, if the sum is up, you fly. The motion of the wings through the air move air in multiple directions.

Looked at another way, gliders use gravity to power their flight.
The wings shape the airflows to change the downward force of gravity into forward motion. Adding an engine & it's thrust by propeller or jet exhaust is cheating! That changes the vector sums, by adding a large force at aprox. 90 degrees to gravity. ( in level flight )

Not that I'm against cheating.

The demons explanation is as valid as some others, the sum total of forces. The pulling up on armrest hypothesis, however, isn't IMHO useful. And the cooling fan explanation skips too much math, although the observation that sweat increases when the cooling fan stops is valid, but not useful.

poormansairforce

Well-Known Member
After you add up all the motion vectors, if the sum is up, you fly. The motion of the wings through the air move air in multiple directions.

Looked at another way, gliders use gravity to power their flight.
The wings shape the airflows to change the downward force of gravity into forward motion. Adding an engine & it's thrust by propeller or jet exhaust is cheating! That changes the vector sums, by adding a large force at aprox. 90 degrees to gravity. ( in level flight )

Not that I'm against cheating.

The demons explanation is as valid as some others, the sum total of forces. The pulling up on armrest hypothesis, however, isn't IMHO useful. And the cooling fan explanation skips too much math, although the observation that sweat increases when the cooling fan stops is valid, but not useful.
I have no idea what to do with this!

jedi

Well-Known Member
I understand conservation of energy in a closed pipe.
......
Where did this statement " in free flow by being bounded by stream lines" come from?
........
By definition, a streamline follows the flow, there is no flow across a streamline. If there were flow across it it would not be a streamline. Streamlines require laminar flow. The turbulence of separated flow would cause the streamline to get jumbled up and tied in knots. The streamlines more or less define a pipe wall where the is no friction.

In the faucet example of Post # 170. The surface of the falling water column defines a set of streamlines.

Does that answer the question?

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BBerson

Well-Known Member
HBA Supporter
In the faucet example of Post # 138. The surface of the falling water column defines a set of streamlines.
The faucet example was post #170.
But I can't easily accept that faucet flow is valid for air. The water faucet flow has surface tension.
Nor do I see how confined pipe streamlines is valid for open wings. (what I assume you call "free flow)

We need to find a video course to explain this from the beginning, I think.

jedi

Well-Known Member
The faucet example was post #170.
But I can't easily accept that faucet flow is valid for air. The water faucet flow has surface tension.
Nor do I see how confined pipe streamlines is valid for open wings. (what I assume you call "free flow)

We need to find a video course to explain this from the beginning, I think.
Thanks. #170 is corrected. Yes, free flow is atmospheric where there is no confining "Pipe".

Bernoulli only addresses flow in one direction or one degree of freedom. His equation assumes uniform flow across the pipe diameter or streamlines. His equations do not account for curving flow that would create a pressure gradient across the pipe. This is where Bernoulli falls short but it was the common limit of fluid flow mathematics up to the 20th century and invention of the airplane.

A wind tunnel is a big pipe. A symmetrical wing section at zero angle of attack is the equivalent of a venturi and Bernoulli is a valid equation. As soon as the wing is given camber or an angle of attack and begins to produce lift the flow is no longer uniform and Bernoulli begins to break down.

During and after WWII there was a big push to develop better airfoils. In order to account for the pressure gradients across the upstream flow path and create curved streamlines additional terms and higher math is needed to describe the flow field. Vorticity (tornado type) flow is added to the linear flow to create a two dimensional flow field. Vorticity creates pressure gradients and flow variations across the wind tunnel test section. Wings producing lift introduce vorticity into the flow field. The vortex theory of lift describes this flow field and the creation of lift.

See Post #20 video. I agree we need a better video in English. Search the internet.

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