Altimeters, static pressure, and lift

CarlosSi · Fri Dec 24, 2021 5:14 am

I had an interesting thought earlier today.

We know that when the air is accelerated, the pressure decreases, which is why the airfoil causes the airplane to lift up (in combination with pressure under the airfoil being relatively changed/high).

We also know that altimeters in-flight only read the static pressure since they are roughly perpendicular to the "dynamic pressure" which we get through movement.

But then why is it that, if we were to be flying a speed other than zero, a nonzero airspeed doesn't make the pressure around the static air source decrease, and thus read a higher altitude?

Clearly the altimeter works as intended and it isn't calibrated for this for a fact. I think there's one misunderstanding here.

What is it though?

GalaxyFlyer · Fri Dec 24, 2021 12:32 pm

First, lift is caused by the differential pressure above and below the wing, the faster flow is on the upper surface, not the lower. Second static ports are placed in aerodynamically calm areas on the skin-there’s no lift, typically very flat surfaces.

Dalmd88 · Fri Dec 24, 2021 2:43 pm

When the Reduced Vertical Separation Minimum limits came into being about 15 years ago there were a lot of new inspections and restrictions put in place for static ports and the surrounding area. For a plane to be RVSM legal it has to pass a very detailed flatness inspection in the area around the static ports.

When it was first implemented many static ports on our fleet had to be changed and any structural skin patches in the area replaced. They were good enough for the old altimeter tolerances, but came up short of the RVSM tolerances. There is zero tolerance for anything but a flat surface around the ports now. In the past a small dent could be within the limits of the Structural Repair Manual, or could be repaired with a surface patch. Now it means reskinning the entire area in the RVSM critical section.

CarlosSi · Fri Dec 24, 2021 5:18 pm

GalaxyFlyer wrote:
First, lift is caused by the differential pressure above and below the wing, the faster flow is on the upper surface, not the lower. Second static ports are placed in aerodynamically calm areas on the skin-there’s no lift, typically very flat surfaces.

Oh yes.

Or I guess, maybe I should ask is the pressure of air lower at 400 knots than it is at 200 knots? (forgetting about the wing for a second). We know the static pressure has to be the same, or the altimeter would start to read higher the faster we go, not because of any "lifting", but because faster speed = lower pressure.

Again forgetting about the wing for a second.

Separate question, /which/ pressure is dropping when the airflow accelerates over the top? Static pressure? Total pressure?

GalaxyFlyer · Fri Dec 24, 2021 8:06 pm

Total pressure is lower on the upper surface, but higher on the lower surface.

https://web.mit.edu/2.972/www/reports/a ... rfoil.html

kalvado · Sat Dec 25, 2021 1:50 am

As far as I understand, this is not about high airflow velocity per se, but about change if velocity.
Total airflow energy has several components - related to pressure, velocity, and temperature (Bernoulli equation). When you accelerate airflow, you move energy from one into the other, changing all those components
Incoming flow is fast, but as long as you don't change velocity, pressure (as in temperature and density) remain the same

Horstroad · Sat Dec 25, 2021 9:46 am

An interesting question and I had to think about it for a few minutes.
The static pressure component of the total air pressure changes only when you compare pressure values before and after you have accelerated the air, e.g. infront of and right on top of the wing. This drop in static pressure in combination with more or less constant static pressure below the wing is what creates lift.
At the place where you measure static pressure for altitude indication the air is not accelerated. It's stationary with the aircraft moving through it at high speed.

zeke · Sat Dec 25, 2021 11:21 am

GalaxyFlyer wrote:
First, lift is caused by the differential pressure above and below the wing, the faster flow is on the upper surface, not the lower. Second static ports are placed in aerodynamically calm areas on the skin-there’s no lift, typically very flat surfaces.

Whilst this is one of the popular reasons at flight schools why lift is generated, it is false.

Lift is generated when a fluid (liquid or gas) is turned by a solid.

The idea of lift being generated by a differential of pressure can simply be disproved by looking at a rudder on a boat. Pressure in water increases with depth. At the same depth water has the same pressure.

kalvado · Sat Dec 25, 2021 3:04 pm

zeke wrote:
GalaxyFlyer wrote:
First, lift is caused by the differential pressure above and below the wing, the faster flow is on the upper surface, not the lower. Second static ports are placed in aerodynamically calm areas on the skin-there’s no lift, typically very flat surfaces.

Whilst this is one of the popular reasons at flight schools why lift is generated, it is false.

Lift is generated when a fluid (liquid or gas) is turned by a solid.

The idea of lift being generated by a differential of pressure can simply be disproved by looking at a rudder on a boat. Pressure in water increases with depth. At the same depth water has the same pressure.

Pressure differential explanation is perfectly correct. After all, pressure is the force of air interacting with the surface, and there has to be such force in order for the wing to generate lift.
Now higher pressure air under the wing is trying to escape down, and air above the wing moves down as it tries to fill the void above the wing - that is the downward flow.

It is the nature of velocity difference on top and bottom surface that makes it complicated. Common expanation of equal travel times is outright wrong. You have to talk about airfoil bound vortex, start vortex - and most of the audience would be lost by this point, until you're talking to aerodynamics engineering students.

"lift" generated by static pressure differential - such as on the still boat - does exist. It is called "Archimedes force" and is a slightly different story.

zeke · Sat Dec 25, 2021 11:22 pm

kalvado wrote:
Pressure differential explanation is perfectly correct. After all, pressure is the force of air interacting with the surface, and there has to be such force in order for the wing to generate lift.
Now higher pressure air under the wing is trying to escape down, and air above the wing moves down as it tries to fill the void above the wing - that is the downward flow.

It is the nature of velocity difference on top and bottom surface that makes it complicated. Common expanation of equal travel times is outright wrong. You have to talk about airfoil bound vortex, start vortex - and most of the audience would be lost by this point, until you're talking to aerodynamics engineering students.

"lift" generated by static pressure differential - such as on the still boat - does exist. It is called "Archimedes force" and is a slightly different story.

Ummmm no.

The real details of how an object generates lift are very complex and do not lend themselves to simplification. For a gas, we have to simultaneously conserve the mass, momentum, and energy in the flow.

The simultaneous conservation of mass, momentum, and energy of a fluid (while neglecting the effects of air viscosity) are called the Euler Equations.

If we include the effects of viscosity, we have the Navier-Stokes Equations. To truly understand the details of the generation of lift, one has to have a good working knowledge of the Euler Equations, the simplistic difference in pressure is wrong as I explained if you look at the rudder on a boat, pressure is the same on both sides of a rudder as it turns, yet it still generates lift.

In aerodynamics we consider and substance that does not resist shear as a fluid. Gas and liquids in aerodynamics are considered as fluids.

GalaxyFlyer · Sat Dec 25, 2021 11:39 pm

Nice to know, but keeping it simple is good enough. Pilots don’t need a PhD in Aero to be pilots.

kalvado · Sun Dec 26, 2021 12:04 am

zeke wrote:
kalvado wrote:
Pressure differential explanation is perfectly correct. After all, pressure is the force of air interacting with the surface, and there has to be such force in order for the wing to generate lift.
Now higher pressure air under the wing is trying to escape down, and air above the wing moves down as it tries to fill the void above the wing - that is the downward flow.

It is the nature of velocity difference on top and bottom surface that makes it complicated. Common expanation of equal travel times is outright wrong. You have to talk about airfoil bound vortex, start vortex - and most of the audience would be lost by this point, until you're talking to aerodynamics engineering students.

"lift" generated by static pressure differential - such as on the still boat - does exist. It is called "Archimedes force" and is a slightly different story.

Ummmm no.

The real details of how an object generates lift are very complex and do not lend themselves to simplification. For a gas, we have to simultaneously conserve the mass, momentum, and energy in the flow.

The simultaneous conservation of mass, momentum, and energy of a fluid (while neglecting the effects of air viscosity) are called the Euler Equations.

If we include the effects of viscosity, we have the Navier-Stokes Equations. To truly understand the details of the generation of lift, one has to have a good working knowledge of the Euler Equations, the simplistic difference in pressure is wrong as I explained if you look at the rudder on a boat, pressure is the same on both sides of a rudder as it turns, yet it still generates lift.

In aerodynamics we consider and substance that does not resist shear as a fluid. Gas and liquids in aerodynamics are considered as fluids.

We're jumping across many different approaches here.
On a grand scheme of things, momentum conservation dictates that forces acting on the wing (drag and lift) are equal and opposite to forces acting on something else - and there is nothing but air to fulfill that role. Force acting on air will make airflow deflect, moving it - in case of level flying airplane - down and forward. Can you say that movement causes lift? I am not comfortable with that word, it is more of two results of the same interaction for me.

Force of fluid (gas or liquid) acting on a solid surface is called "pressure". If there is a net force on a wing due to interaction with air, it means that there is difference in pressures that creates that force. And that force (or counterforce), in turn, causes air to deflect.

Now if you want to describe how that that pressure is created due to airfoil-air interaction, then you do need Navier-Stokes and a lot of non-trivial explanations which are still all incomplete. As they say, easiest explanation for pilot is "it's magic, and here is what you need to know about that magic".

And for the boat rudder, there is absolutely a (relatively small compared to hydrostatic) pressure differential which creates lift.
Two images I could find:
https://keyassets.timeincuk.net/inspire ... 00x400.jpg
https://i.ytimg.com/vi/G98QpfaHIhc/maxresdefault.jpg
You can clearly see water level being higher on one side than on the other. This is a result of pressure differential, and is similar to wingtip flow where winglet/fence can be used to reduce that flow - and here rudder extending above water does the same thing.

zeke · Sun Dec 26, 2021 6:56 am

kalvado wrote:
Can you say that movement causes lift? I am not comfortable with that word, it is more of two results of the same interaction for me.

You put an aircraft in a hanger to weigh it as if you to weigh it outdoors, the movement of air over the aircraft will generate lift.

If you have a boat moored at the dock on a lake, rotating the rudder will not steer the boat. If you moor a boat on a dock on a river, the movement of the water over the rudder as the rudder is rotated will steer the boat even when the boat is stationary. It is the movement of the fluid over the solid that generates the force.

kalvado wrote:
Force of fluid (gas or liquid) acting on a solid surface is called "pressure".

No, the units for force and pressure are not the same.

kalvado wrote:
Now if you want to describe how that that pressure is created due to airfoil-air interaction, then you do need Navier-Stokes and a lot of non-trivial explanations which are still all incomplete.

The NS equations describe the physics beautifully, the problem with the NS equations is we cannot absolutely solve the non linear equations, we can only solve approximations of them. Even the largest computers on earth cannot calculate the forces in a coffee cup being stirred by a spoon.

In order to solve NS equations on a digital computer we need to make approximations, it is these approximations which make the results incomplete. When I started my engineering work we used analog computers, we could do integration and transfer functions in hardware without approximations, they worked beautifully for non linear control systems, digital computers were cheaper, more reliable, and produced results which were good enough. We are going full circle again back to analog computers again with quantum computers.

kalvado wrote:
And for the boat rudder, there is absolutely a (relatively small compared to hydrostatic) pressure differential which creates lift.

Hogwash, hydrostatic pressure is the change of pressure in a fluid with depth due to gravity.

CarlosSi · Wed Dec 29, 2021 5:24 am

zeke wrote:
kalvado wrote:
Pressure differential explanation is perfectly correct. After all, pressure is the force of air interacting with the surface, and there has to be such force in order for the wing to generate lift.
Now higher pressure air under the wing is trying to escape down, and air above the wing moves down as it tries to fill the void above the wing - that is the downward flow.

It is the nature of velocity difference on top and bottom surface that makes it complicated. Common expanation of equal travel times is outright wrong. You have to talk about airfoil bound vortex, start vortex - and most of the audience would be lost by this point, until you're talking to aerodynamics engineering students.

"lift" generated by static pressure differential - such as on the still boat - does exist. It is called "Archimedes force" and is a slightly different story.
.

If we include the effects of viscosity, we have the Navier-Stokes Equations. To truly understand the details of the generation of lift, one has to have a good working knowledge of the Euler Equations, the simplistic difference in pressure is wrong as I explained if you look at the rudder on a boat, pressure is the same on both sides of a rudder as it turns, yet it still generates lift.

At high humidity, isn't the reason the water vapor above a wing condenses into visible vapor because of the pressure reduction (paired with a temperature reduction causing supersaturation)? Also venturi effect and carburetor ice?

You clearly know what you're talking about though. I've been exposed to all those equations before but haven't worked with them for so long that I couldn't relate.

Though that still begs the question, if airplane A flows through a body of air faster than airplane B, is the pressure of the air the same or is it "lower" to the faster airplane? The answer is /static/ pressure is the same (while dynamic pressure/ram pressure changes, driving our ASI). The altimeter reads this same static pressure and outputs our altitude.

But then shouldn't that mean that the static pressure as the airflow goes over the wing must stay the same too and not "drop"? What exactly is changing? Is it merely just action/reaction?

The idea that the wings are "surfing" over the air by "turning fluid" as you said is pretty agreeable. This other concept appears to be more controversial.

vikkyvik · Wed Dec 29, 2021 3:46 pm

zeke wrote:
No, the units for force and pressure are not the same.

A force applied over an area is by definition a pressure. Newton (force) per meter squared (area).

zeke wrote:
The idea of lift being generated by a differential of pressure can simply be disproved by looking at a rudder on a boat. Pressure in water increases with depth. At the same depth water has the same pressure.

Wait....are you saying that marine propellers, rudders, etc., do not induce a pressure differential?

Then how do you explain cavitation?

dennypayne · Wed Dec 29, 2021 6:35 pm

I think what is being missed in this discussion is that the pressure differential is not ultimately what causes lift - I would say it is a part of the mechanism by which an airfoil causes an air mass to be pushed downwards. And it is Newton's third law of motion which thereby explains the phenomenon we call lift - that is, the downwards "push" generated by the airfoil causes an equal and opposite reaction upwards against the airfoil. This also explains why changing the shape of the airfoil (via flaps, etc.) changes the lifting characteristics at a given speed.

So to get back to OP's original question, it seems that since static ports are placed in the part of the airframe that is not displacing air in any meaningful way (i.e. not an airfoil), any pressure differential caused by air movement around it is likely negligible.

kalvado · Wed Dec 29, 2021 8:16 pm

dennypayne wrote:
I think what is being missed in this discussion is that the pressure differential is not ultimately what causes lift - I would say it is a part of the mechanism by which an airfoil causes an air mass to be pushed downwards. And it is Newton's third law of motion which thereby explains the phenomenon we call lift - that is, the downwards "push" generated by the airfoil causes an equal and opposite reaction upwards against the airfoil. This also explains why changing the shape of the airfoil (via flaps, etc.) changes the lifting characteristics at a given speed.

So to get back to OP's original question, it seems that since static ports are placed in the part of the airframe that is not displacing air in any meaningful way (i.e. not an airfoil), any pressure differential caused by air movement around it is likely negligible.

What causes lift is sort of a tricky question in terms of giving a good answer. In general, an answer to such question should have some predictive power in terms of how to design the airfoil.
Pressure differential is an absolutely correct answer, but a follow up question question is "what causes pressure differential? how to increase it?
Airflow deflectionis another absolutely correct answer, but a follow up question question is the same - "what causes flow deflection? how to increase it?

vikkyvik · Wed Dec 29, 2021 9:20 pm

kalvado wrote:
dennypayne wrote:
I think what is being missed in this discussion is that the pressure differential is not ultimately what causes lift - I would say it is a part of the mechanism by which an airfoil causes an air mass to be pushed downwards. And it is Newton's third law of motion which thereby explains the phenomenon we call lift - that is, the downwards "push" generated by the airfoil causes an equal and opposite reaction upwards against the airfoil. This also explains why changing the shape of the airfoil (via flaps, etc.) changes the lifting characteristics at a given speed.

So to get back to OP's original question, it seems that since static ports are placed in the part of the airframe that is not displacing air in any meaningful way (i.e. not an airfoil), any pressure differential caused by air movement around it is likely negligible.

What causes lift is sort of a tricky question in terms of giving a good answer. In general, an answer to such question should have some predictive power in terms of how to design the airfoil.
Pressure differential is an absolutely correct answer, but a follow up question question is "what causes pressure differential? how to increase it?
Airflow deflectionis another absolutely correct answer, but a follow up question question is the same - "what causes flow deflection? how to increase it?

Long story short, one doesn't happen without the other.

Airfoil bends the flow downwards. How is the equal-and-opposite force applied to the airfoil? Well, there's only one way - through a pressure differential.

Or.....airfoil causes pressure differential, causing wing to feel upward force. How is an equal-and-opposite reaction applied? By bending the airflow downwards.

dennypayne wrote:
This also explains why changing the shape of the airfoil (via flaps, etc.) changes the lifting characteristics at a given speed.

Changing the shape also changes the pressure field. You can explain it either way.

kalvado · Wed Dec 29, 2021 9:38 pm

vikkyvik wrote:
kalvado wrote:
dennypayne wrote:
I think what is being missed in this discussion is that the pressure differential is not ultimately what causes lift - I would say it is a part of the mechanism by which an airfoil causes an air mass to be pushed downwards. And it is Newton's third law of motion which thereby explains the phenomenon we call lift - that is, the downwards "push" generated by the airfoil causes an equal and opposite reaction upwards against the airfoil. This also explains why changing the shape of the airfoil (via flaps, etc.) changes the lifting characteristics at a given speed.

So to get back to OP's original question, it seems that since static ports are placed in the part of the airframe that is not displacing air in any meaningful way (i.e. not an airfoil), any pressure differential caused by air movement around it is likely negligible.

What causes lift is sort of a tricky question in terms of giving a good answer. In general, an answer to such question should have some predictive power in terms of how to design the airfoil.
Pressure differential is an absolutely correct answer, but a follow up question question is "what causes pressure differential? how to increase it?
Airflow deflectionis another absolutely correct answer, but a follow up question question is the same - "what causes flow deflection? how to increase it?

Long story short, one doesn't happen without the other.

Airfoil bends the flow downwards. How is the equal-and-opposite force applied to the airfoil? Well, there's only one way - through a pressure differential.

Or.....airfoil causes pressure differential, causing wing to feel upward force. How is an equal-and-opposite reaction applied? By bending the airflow downwards.

dennypayne wrote:
This also explains why changing the shape of the airfoil (via flaps, etc.) changes the lifting characteristics at a given speed.

Changing the shape also changes the pressure field. You can explain it either way.

Thats all absolutely correct. But it still doesn't address basic questions of how to design better wing, or why one side of the wing is more sensitive to ice accumulation.
So "black magic" is still the best answer in the field. Magicians from design office may try explaining, but usually they aren't too successful.

zeke · Wed Dec 29, 2021 9:39 pm

vikkyvik wrote:
Wait....are you saying that marine propellers, rudders, etc., do not induce a pressure differential?

Then how do you explain cavitation?

Cavitation is caused by a phase change, the rate of phase change depends upon local flow conditions (pressure, velocities, turbulence) as well as fluid properties (saturation pressure, densities, and surface tension). It is not a result of one factor.

Cavitation is used all around us, like fuel injectors in engines and gear driven pumps.

Cavitation can be modeled in CFD with NS equations.

DH106 · Thu Dec 30, 2021 12:15 am

zeke wrote:
Cavitation is caused by a phase change, the rate of phase change depends upon local flow conditions (pressure, velocities, turbulence) as well as fluid properties (saturation pressure, densities, and surface tension). It is not a result of one factor.

Cavitation is used all around us, like fuel injectors in engines and gear driven pumps.

Cavitation can be modeled in CFD with NS equations.

By 'phase change' do you mean liquid turning to gas?
My understanding of cavitation is that it's when the pressure over a boat/ship's propeller surface drops to the point where gas forms (it effectively 'boils'). Prop blades nearer the surface are more prone to cavitation due to the lower pressure at that reduced depth.

Is this understanding wrong then?

zeke · Thu Dec 30, 2021 8:01 am

DH106 wrote:
By 'phase change' do you mean liquid turning to gas?
My understanding of cavitation is that it's when the pressure over a boat/ship's propeller surface drops to the point where gas forms (it effectively 'boils'). Prop blades nearer the surface are more prone to cavitation due to the lower pressure at that reduced depth.

Is this understanding wrong then?

Cavitation involves highly complex physical processes with highly non-linear multi-phase flows, I don’t understand it completely. It is a highly specialised field.

It’s a function of the vapour pressure of the fluid, however in air and natural water bodies there are cavitation nuclei like microscopic particles, pollutants, and dissolved gas (in natural water bodies) facilitates cavitation inception.

It is not a simple as people make out, it can be located on the tip, root, leading edge or trailing edge, suction side (back), face, etc. the form it can take include sheet, cloud, bubble, vortex. It can be stationary, instationary, or migrating.

hitower3 · Thu Dec 30, 2021 10:01 am

Dear all,

This thread is a highly interesting read for me, thanks to all contributors.
I have the impression that the debate over the origin of lift generation through pressure differential or flow deflection is somehow similar to the debate over the the particle or wave nature of light. It's a duality where both phenomena exist, although in some observations one phenomenon is more apparent than in others.

Best regards!
Hendric

vikkyvik · Thu Dec 30, 2021 3:08 pm

zeke wrote:
Cavitation is caused by a phase change, the rate of phase change depends upon local flow conditions (pressure, velocities, turbulence) as well as fluid properties (saturation pressure, densities, and surface tension). It is not a result of one factor.

I see the word "pressure" in there.

So I'll ask again:

vikkyvik wrote:
are you saying that marine propellers, rudders, etc., do not induce a pressure differential?

masi1157 · Thu Dec 30, 2021 7:51 pm

zeke wrote:
It’s a function of the vapour pressure of the fluid...

It is primarily a function of the static pressure of the fluid dropping below the vapour pressure of the fluid. If and where that happens, you can (or will) have cavitation. And why does the static pressure drop in certain areas? Because the dynamic pressure (i.e. the local velocity) rises, while the total pressure (static + dynamic) remains unchanged.

Gruß, masi1157

zeke · Thu Dec 30, 2021 9:38 pm

masi1157 wrote:

It is primarily a function of the static pressure of the fluid dropping below the vapour pressure of the fluid. If and where that happens, you can (or will) have cavitation. And why does the static pressure drop in certain areas? Because the dynamic pressure (i.e. the local velocity) rises, while the total pressure (static + dynamic) remains unchanged.

Gruß, masi1157

Vapor pressure is a thermodynamic property of a fluid, the phase change is a function of temperature.

masi1157 · Thu Dec 30, 2021 9:43 pm

zeke wrote:
Vapor pressure is a thermodynamic property of a fluid, the phase change is a function of temperature.

I didn't say that the vapour pressure changes. I said the the static pressure drops below the vapour pressure, and that causes cavitation.

Gruß, masi1157

zeke · Fri Dec 31, 2021 2:52 am

masi1157 wrote:

I didn't say that the vapour pressure changes. I said the the static pressure drops below the vapour pressure, and that causes cavitation.

It is not an isothermal process, hence the reason advanced weapons systems use active temperature control to reduce noise and vibration. Some weapons systems can travel in excess of 200 kts underwater using cavitation.

masi1157 · Fri Dec 31, 2021 9:46 am

zeke wrote:
It is not an isothermal process..

It doesn't matter! You argued earlier, that there is no difference in (static) pressure between the sides of a rudder of a boat. And that is not true. While you won't see cavitation on the rudder, there is cavitation on propellers and even on non-moving parts in e.g. hydraulic pumps. And the reason for that is the static pressure of the fluid dropping below the quasi constant vapour pressure, in places where the local velocity rises and causes higher dynamic pressure.

What then happens in the cavitation bubbles is a totally different story, but that is irrelevant here. Even the occurrence of cavitation is nothing but an example in this discussion. We are talking only about local pressure differences in fluids.

Gruß, masi1157

kalvado · Fri Dec 31, 2021 12:57 pm

masi1157 wrote:
zeke wrote:
It is not an isothermal process..

It doesn't matter! You argued earlier, that there is no difference in (static) pressure between the sides of a rudder of a boat. And that is not true. While you won't see cavitation on the rudder, there is cavitation on propellers and even on non-moving parts in e.g. hydraulic pumps. And the reason for that is the static pressure of the fluid dropping below the quasi constant vapour pressure, in places where the local velocity rises and causes higher dynamic pressure.

What then happens in the cavitation bubbles is a totally different story, but that is irrelevant here. Even the occurrence of cavitation is nothing but an example in this discussion. We are talking only about local pressure differences in fluids.

Gruß, masi1157

Just bring over some airfoil simulations, there are plenty of those.

LH707330 · Fri Dec 31, 2021 4:55 pm

Anybody who thinks there are no pressure differentials on boat rudders needs to go sail a Laser dinghy. Get on a broad reach in 20 knots of breeze, let the boat heel over 30 degrees, and then pull the tiller to try and maintain course. Spoiler alert: you'll hear a "khkhkh" sound as the upwind side starts to ventilate and suck air bubbles down from the surface, then the rudder will stall, and unless you're quick hiking out and un-stalling the rudder, you'll round up and flip the boat.

kalvado · Fri Dec 31, 2021 5:08 pm

LH707330 wrote:
Anybody who thinks there are no pressure differentials on boat rudders needs to go sail a Laser dinghy. Get on a broad reach in 20 knots of breeze, let the boat heel over 30 degrees, and then pull the tiller to try and maintain course. Spoiler alert: you'll hear a "khkhkh" sound as the upwind side starts to ventilate and suck air bubbles down from the surface, then the rudder will stall, and unless you're quick hiking out and un-stalling the rudder, you'll round up and flip the boat.

I was pretty impressed when someone explained me that rudder and sail follow same aerodynamic as plane wing, generate lift, and can stall the same way as a wing

zeke · Fri Dec 31, 2021 10:02 pm

LH707330 wrote:
Anybody who thinks there are no pressure differentials on boat rudders needs to go sail a Laser dinghy. Get on a broad reach in 20 knots of breeze, let the boat heel over 30 degrees, and then pull the tiller to try and maintain course. Spoiler alert: you'll hear a "khkhkh" sound as the upwind side starts to ventilate and suck air bubbles down from the surface, then the rudder will stall, and unless you're quick hiking out and un-stalling the rudder, you'll round up and flip the boat.

Most people associate pressure as being a force per unit area, in SI units N/m^2. Pressure is also the amount of energy in a volume, N.m/m^3=N/m^2. Conservation of energy is the first law of thermodynamics, it is neither created or destroyed, for a given volume energy is constant, i.e pressure is constant. By saying you have a "pressure differentials" means you are saying there are "energy differentials", i.e, energy has been created or destroyed. This is incorrect, the amount of energy is constant.

The common approximation used to explain this is the Bernoulli’ equation, where the total pressure, i.e. the sum of dynamic, static, and hydrostatic pressure is a constant. This is only an approximation, to fully understand what happens inside a fluid we need to also consider the principles of continuity of mass, momentum, and energy, that is the Navier–Stokes equations which can explain the stall.

kalvado · Fri Dec 31, 2021 10:51 pm

zeke wrote:
LH707330 wrote:
Anybody who thinks there are no pressure differentials on boat rudders needs to go sail a Laser dinghy. Get on a broad reach in 20 knots of breeze, let the boat heel over 30 degrees, and then pull the tiller to try and maintain course. Spoiler alert: you'll hear a "khkhkh" sound as the upwind side starts to ventilate and suck air bubbles down from the surface, then the rudder will stall, and unless you're quick hiking out and un-stalling the rudder, you'll round up and flip the boat.

Most people associate pressure as being a force per unit area, in SI units N/m^2. Pressure is also the amount of energy in a volume, N.m/m^3=N/m^2. Conservation of energy is the first law of thermodynamics, it is neither created or destroyed, for a given volume energy is constant, i.e pressure is constant. By saying you have a "pressure differentials" means you are saying there are "energy differentials", i.e, energy has been created or destroyed. This is incorrect, the amount of energy is constant.

The common approximation used to explain this is the Bernoulli’ equation, where the total pressure, i.e. the sum of dynamic, static, and hydrostatic pressure is a constant. This is only an approximation, to fully understand what happens inside a fluid we need to also consider the principles of continuity of mass, momentum, and energy, that is the Navier–Stokes equations which can explain the stall.

I see where you're coming from. Yes, if you talk about pV=nk[sub]B[/sub]T, you can get that dependence. But no, since volume is not constant, your interpretation is incorrect.
Same with downdraft - looks like you're coming from some distorted interpretation of Zhukovsky-Kutta theorem, where pressures are not involved....

masi1157 · Fri Dec 31, 2021 10:56 pm

zeke wrote:
The common approximation used to explain this is the Bernoulli’ equation, where the total pressure, i.e. the sum of dynamic, static, and hydrostatic pressure is a constant. This is only an approximation, to fully understand what happens inside a fluid we need to also consider the principles of continuity of mass, momentum, and energy, that is the Navier–Stokes equations which can explain the stall.

Are you trying to tell us we need to apply Navier-Stokes to understand the basics about static and dynamic pressure around an airfoil, a boat rudder and all that? No, sorry, Bernoulli is more than sufficient.

Grüßchen, masi1157

kalvado · Fri Dec 31, 2021 11:07 pm

masi1157 wrote:
zeke wrote:
The common approximation used to explain this is the Bernoulli’ equation, where the total pressure, i.e. the sum of dynamic, static, and hydrostatic pressure is a constant. This is only an approximation, to fully understand what happens inside a fluid we need to also consider the principles of continuity of mass, momentum, and energy, that is the Navier–Stokes equations which can explain the stall.

Are you trying to tell us we need to apply Navier-Stokes to understand the basics about static and dynamic pressure around an airfoil, a boat rudder and all that? No, sorry, Bernoulli is more than sufficient.

Grüßchen, masi1157

Problem of Navier-Stokes is that those are local equations, and they have no global analytical solutions - while Kutta condition describes airfoil as a whole, for example.
Similar situation exists in classic mechanics, where Newton laws description is mathematically equivalent to Lagrange and Hamilton model, but the proof is much less than trivial; or in electronics where going from Maxwell equations (which are much simpler mathematically) to, for example, Ohm's law is not obvious.
Appealing to Navier-Stokes hints me there is too much reliance on finite element analysis instead of more generalized descriptions. Which is, actually, a bread and butter of industry these days,

zeke · Fri Dec 31, 2021 11:11 pm

kalvado wrote:
since volume is not constant, your interpretation is incorrect.

Natural water bodies are normally considered incompressible.

kalvado wrote:
Same with downdraft - looks like you're coming from some distorted interpretation of Zhukovsky-Kutta theorem, where pressures are not involved....

Not at all, any explanation for lift (a component of the total force) should equally also explain the various forms of drag.

The various simplifications poorly explain boundary layer effects and turbulence, the main reason for skin friction drag, the dominant drag in cruise flight.

kalvado · Fri Dec 31, 2021 11:23 pm

zeke wrote:
kalvado wrote:
since volume is not constant, your interpretation is incorrect.

Natural water bodies are normally considered incompressible.

kalvado wrote:
Same with downdraft - looks like you're coming from some distorted interpretation of Zhukovsky-Kutta theorem, where pressures are not involved....

Not at all, any explanation for lift (a component of the total force) should equally also explain the various forms of drag.

The various simplifications poorly explain boundary layer effects and turbulence, the main reason for skin friction drag, the dominant drag in cruise flight.

I thought we're talking water? Then pV=nkT simply doesn't apply. You do have difference is surface levels on different sides in leu of that. I showed some pictures before.

And yes, you're right in terms of what needs to be described - but you're making huge leaps over very basic stuff.
If you have to explain lift, you must start with Bernoulli. However, next step is explaining a flow pattern which corresponds to lift creation- which is where things become interesting. That includes introducing Kutta condition, talking about bound and starting vortex with honorable mention to Zhukovsky-Kutta and Kelvin theorems, go to the bathroom to play with cardboard airfoils, mention the role of viscosity in moving of stagnation point, apply Bernoulli to airflow with a bound vortex - and only now you can talk about downdraft as obvious coming from Kutta stagnation point.
And then - and only then - you may talk about all the effects at the airfoil surface.

zeke · Fri Dec 31, 2021 11:40 pm

masi1157 wrote:
Are you trying to tell us we need to apply Navier-Stokes to understand the basics about static and dynamic pressure around an airfoil, a boat rudder and all that? No, sorry, Bernoulli is more than sufficient.

To calculate the effects over a rudder in real applications, for sure, that is exactly how the engineering is done on large ships and performance boats. In most cases a rudder has disturbed flow from the hull and normally placed behind the propeller on powered craft. Bernoulli will not explain why a rudder will stall, it will not explain ways to mitigate a stall like increasing surface roughness, vortex generators etc. It does not explain drag, it does not explain the interaction between the propeller flow and the rudder.

One would also not use Bernoulli to locate static ports on an aircraft like the OP was asking about.

zeke · Fri Dec 31, 2021 11:45 pm

kalvado wrote:

If you have to explain lift, you must start with Bernoulli.

Really ?, better tell NASA https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html

kalvado · Fri Dec 31, 2021 11:50 pm

zeke wrote:
kalvado wrote:

If you have to explain lift, you must start with Bernoulli.

Really ?, better tell NASA https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html

The wrong part in that explanation - which is called "equal transit theory", if you noticed - is

In order to meet up at the trailing edge, the molecules going over the top of the wing must travel faster than the molecules moving under the wing.

Of course, next you have d'Alembert paradox and all that. And equal transit idea is nice, but actually wrong.
And that's where bound vortex comes into play.

PS. I wish Einstein airfoil was brought up in connection with that more often. While totally useless from aerodynamic perspective, it is nice to show that scientific mistakes can happen with the smartest people - and (unlike, say, criminal conviction) don't affect past achievements. That is just a part of normal process of doing science

masi1157 · Sat Jan 01, 2022 9:09 pm

zeke wrote:
To calculate the effects over a rudder in real applications...

I asked..

masi1157 wrote:
to understand the basics...

To understand the basic principle Bernoulli is just fine. But of course you need a lot more for engineering work. However, I think it is always a good idea to start from the simple end and then make it only as complicated as is absolutely necessary.

Gruß, masi1157

LH707330 · Sun Jan 02, 2022 3:57 am

zeke wrote:
LH707330 wrote:
Anybody who thinks there are no pressure differentials on boat rudders needs to go sail a Laser dinghy. Get on a broad reach in 20 knots of breeze, let the boat heel over 30 degrees, and then pull the tiller to try and maintain course. Spoiler alert: you'll hear a "khkhkh" sound as the upwind side starts to ventilate and suck air bubbles down from the surface, then the rudder will stall, and unless you're quick hiking out and un-stalling the rudder, you'll round up and flip the boat.

Most people associate pressure as being a force per unit area, in SI units N/m^2. Pressure is also the amount of energy in a volume, N.m/m^3=N/m^2. Conservation of energy is the first law of thermodynamics, it is neither created or destroyed, for a given volume energy is constant, i.e pressure is constant. By saying you have a "pressure differentials" means you are saying there are "energy differentials", i.e, energy has been created or destroyed. This is incorrect, the amount of energy is constant.

The common approximation used to explain this is the Bernoulli’ equation, where the total pressure, i.e. the sum of dynamic, static, and hydrostatic pressure is a constant. This is only an approximation, to fully understand what happens inside a fluid we need to also consider the principles of continuity of mass, momentum, and energy, that is the Navier–Stokes equations which can explain the stall.

With the boat rudder example, one way to accommodate the higher pressure is the difference in the water column heights: in my example, the leeward side of the rudder will have water higher than the normal waterline, while the windward side will have a much lower level, which points to a lower pressure in the water column for a given depth next to the rudder. As far as where the energy goes, much of it gets wasted as turbulence, which is why your coach yells at you to keep the forces balanced so you're not holding weather helm and going slow because of unnecessary induced drag.

CanukinUSA · Sun Jan 02, 2022 10:36 am

I think that everyone involved in this discussion needs to read the book entitled “Understanding Aerodynamics. Arguing from the Real Physics” by Doug McLean. Published by John Wiley & Sons in 2013. ISBN 978-1-119-96791-4.
It has about 580 pages covering in a very detailed fashion what you are debating. Unfortunately what an design engineer and a flight crew member need to know to do their jobs is probably quite different and a lot of very questionable theories have come into use that are incorrect but work in the world of the particular user. Doug is a retired Engineer/Scientist and Technical fellow from Boeing back when new aircraft were designed and built not just modified using Amended Type Certificates like what seems to happen now.

CanukinUSA · Sun Jan 02, 2022 5:44 pm

I think what has been missed here is that incorrect theories can be used in some cases because the assumptions that they were based onthat are used work. When things go wrong is when aircraft manufacturers, airlines and operators use the incorrect ideas in new and unexpected ways that cause the theories to break down without being properly thought out and realistic testing.

masi1157 · Sun Jan 02, 2022 6:50 pm

CanukinUSA wrote:
I think what has been missed here is that incorrect theories can be used in some cases....

You should never use an "incorrect" theory. But you should try to start with a simplified theory and only move to a more complicated theory if you approach the boundary of its applicability. Once the theory in use sufficiently covers your problem, it's fine. There is no need to start with an over-complicated, complete and "correct" theory. Especially not if that requires so many conditions and inputs, that you don't know or don't understand. You only need to be aware of the limitations of the simplified model you are using. And that seems to be a big obstacle for many.

I remember so many younger colleagues that tried to use complicated FEM tools to cover even the smallest mechanical problem. They made mistakes with their inputs and didn't see, that their results can't be correct, because they didn't even dare to take a simplified view on it.

Gruß, masi1157

CanukinUSA · Sun Jan 02, 2022 7:32 pm

Keep in mind that a lot of the theories were once believed to be correct particularly in fields such as Engineering where the background behind the theory has not been taught. Many occupations such as Engineers have only been taught how to calculate using rote memorized mathematical formulas and have no idea of what was assumed to get there.
I think back to some of the ideas I was taught in Engineering and realize now after a lifetime in the Aircraft industry both as an Engineer and Pilot that a lot of what I learned and was taught was incorrect but believed to be correct. It always amazes me how some people will continue to hang on to previous ideas without taking a new look at where they came from.
I was a pilot and Flight Instructor before I went back to do my Engineering background and realize now that a lot of what I taught student pilots about aerodynamics was totally incorrect but worked In getting them to learn how to fly the aircraft safely.
In addition I can’t say that many of my Engineering professors had any idea of how their mathematical theories related to the practical world because they had limited experience outside of the academic world and of course were only interested in teaching their version of engineering or teaching anything.

Jungleneer · Mon Jan 03, 2022 1:04 am

CarlosSi wrote:
I had an interesting thought earlier today.

We know that when the air is accelerated, the pressure decreases, which is why the airfoil causes the airplane to lift up (in combination with pressure under the airfoil being relatively changed/high).

We also know that altimeters in-flight only read the static pressure since they are roughly perpendicular to the "dynamic pressure" which we get through movement.

But then why is it that, if we were to be flying a speed other than zero, a nonzero airspeed doesn't make the pressure around the static air source decrease, and thus read a higher altitude?

Clearly the altimeter works as intended and it isn't calibrated for this for a fact. I think there's one misunderstanding here.

What is it though?

Modern air data systems currently are very integrated, with the dynamic, differential and static ports on a single pitot. They have very complex calibration using sometimes neural networks to provide very accurate AOA, airspeed and altitude readings.

CanukinUSA · Mon Jan 03, 2022 2:16 am

Don't forget that there are Air Data Computers that will correct for any known errors in pressure readings that were determined during Flight Testing with accurate instrumentation for altitude, airspeed, etc. are all altitudes and speeds within the operational limits of the aircraft.

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