Lift on aircraft wing
Discussion
The way I see it, which is enough to operate adequately at sailing, there are at least two modes.
Firstly there is the Bernoulli classical stream line flow, particularly valid in cases like a venturi.
Secondly there is a more chaotic stream of fluid hitting a surface at an angle. Valid in cases like a flat plate rudder at a big angle of attack, or a jet of water hitting a crude turbine blade. That's simpler, the momentum of the fluid hitting the surface exerts a force on a surface.
A real sail or plane wing has elements of both modes. The leading edge will tend to have attached streamlined flow, towards the back of the wing is often turbulent.
The detail of it changes with small variations in conditions, like angle of attack and airspeed. Like a lot of other things, it's a problem that can be resolved with a computer if you slice it finely enough, or you can rely on a century or two of empirical observation,
There is no one 'understanding' that fits every shape of wing under every set of circumstances. Wings and sails are 3 Dimensional, air doesn't just flow parallel to the fore/aft axis, it will follow more complicated paths.
At the lowest level, fluids consist of large numbers of particles, each of which is moving randomly. The way we 'understand' fluid behaviour is just a model which predicts behaviour to a useful standard. Like I know I want a flatter sail in a stronger wind and I know I can cope with the heeling force of a 7sqm sail in a 20knot breeze. It's not always useful to attempt understanding at too low a level.
Also you can't have a one size fits all total understanding of wings or sails, because every one is different, even the same sail will be different in a 6 knot breeze to a 12 knot breeze.
Firstly there is the Bernoulli classical stream line flow, particularly valid in cases like a venturi.
Secondly there is a more chaotic stream of fluid hitting a surface at an angle. Valid in cases like a flat plate rudder at a big angle of attack, or a jet of water hitting a crude turbine blade. That's simpler, the momentum of the fluid hitting the surface exerts a force on a surface.
A real sail or plane wing has elements of both modes. The leading edge will tend to have attached streamlined flow, towards the back of the wing is often turbulent.
The detail of it changes with small variations in conditions, like angle of attack and airspeed. Like a lot of other things, it's a problem that can be resolved with a computer if you slice it finely enough, or you can rely on a century or two of empirical observation,
There is no one 'understanding' that fits every shape of wing under every set of circumstances. Wings and sails are 3 Dimensional, air doesn't just flow parallel to the fore/aft axis, it will follow more complicated paths.
At the lowest level, fluids consist of large numbers of particles, each of which is moving randomly. The way we 'understand' fluid behaviour is just a model which predicts behaviour to a useful standard. Like I know I want a flatter sail in a stronger wind and I know I can cope with the heeling force of a 7sqm sail in a 20knot breeze. It's not always useful to attempt understanding at too low a level.
Also you can't have a one size fits all total understanding of wings or sails, because every one is different, even the same sail will be different in a 6 knot breeze to a 12 knot breeze.
dr_gn said:
I’d be perfectly happy with an explanation in complex terms, or multiple line explanations with or without diagrams - so where are they?
Seems like a lot of aerodynamic theory is based on empirical data rather than first principles. Nothing wrong with that, but it seems odd that a lot of it is just “that’s just the way it is”
Ok, I'll have a go, but yes it's a complicated subject, and it was a long time ago that I used to do any of it Seems like a lot of aerodynamic theory is based on empirical data rather than first principles. Nothing wrong with that, but it seems odd that a lot of it is just “that’s just the way it is”
As already covered, lift is due to higher pressure below the wing than above it, but because the wing has an angle of attack, the lower pressure above the wing is also slightly behind the wing. i.e. your lift vector isn't vertical but tilted backwards slightly, which known as lift induced drag.
If air's flowing along the wing, round the end and back along the top, i.e. a wing top vortex, you're losing some of the lift as it's decreasing the pressure on the bottom and increasing it on the top. In order to generate enough lift you increase the angle of attack slightly, which means more induced drag. So the vortex itself isn't causing drag directly.
coanda said:
This was a fun read.
You're slightly more advanced aerodynamics than the current explanation needs 
Edited by RizzoTheRat on Monday 27th March 10:15
RizzoTheRat said:
[....
Ok, I'll have a go, but yes it's a complicated subject, and it was a long time ago that I used to do any of it
As already covered, lift is due to higher pressure below the wing than above it, but because the wing has an angle of attack, the lower pressure above the wing is also slightly behind the wing. i.e. your lift vector isn't vertical but tilted backwards slightly, which known as lift induced drag.
......]
Nice picture, but imagine your wing is being used as a sail.Ok, I'll have a go, but yes it's a complicated subject, and it was a long time ago that I used to do any of it
As already covered, lift is due to higher pressure below the wing than above it, but because the wing has an angle of attack, the lower pressure above the wing is also slightly behind the wing. i.e. your lift vector isn't vertical but tilted backwards slightly, which known as lift induced drag.
......]The lift force can have a forward component.
Otherwise we wouldn't be able to sail upwind.
As an engineer, I realise I don't understand aerodynamics completely.
I don't need to, I just know enough to use it to a certain level.
The weather on the other hand, is something that's impossible to understand from my point of view.
I'd like to be able to reliably know what the wind will do, in detail, an hour or two ahead.
Will it shift to the left or the right?
Will it be stronger over there. or the other side of the bay?
As for predicting the weekend after next, no chance!
OutInTheShed said:
Nice picture, but imagine your wing is being used as a sail.
The lift force can have a forward component.
Otherwise we wouldn't be able to sail upwind.
As an engineer, I realise I don't understand aerodynamics completely.
I don't need to, I just know enough to use it to a certain level.
The weather on the other hand, is something that's impossible to understand from my point of view.
I'd like to be able to reliably know what the wind will do, in detail, an hour or two ahead.
Will it shift to the left or the right?
Will it be stronger over there. or the other side of the bay?
As for predicting the weekend after next, no chance!
I think it's still the same or very similar for a sail. In that wing diagram, say the induced drag vector is 20 degrees from the lift vector, if you sail at more than 20 degrees to the wind you still have a forward component, with the keel providing the counter to the side component. Of course drag means you need to sail a fair bit further from the wind if you want to actually get anywhereThe lift force can have a forward component.
Otherwise we wouldn't be able to sail upwind.
As an engineer, I realise I don't understand aerodynamics completely.
I don't need to, I just know enough to use it to a certain level.
The weather on the other hand, is something that's impossible to understand from my point of view.
I'd like to be able to reliably know what the wind will do, in detail, an hour or two ahead.
Will it shift to the left or the right?
Will it be stronger over there. or the other side of the bay?
As for predicting the weekend after next, no chance!
I used to do internal aerodynamics, much easier when it's all constrained than this messy external stuff
As for weather, you're in to quantum weather butterfly territory there
Edited by RizzoTheRat on Monday 27th March 11:51
Edited by RizzoTheRat on Monday 27th March 11:53
RizzoTheRat said:
dr_gn said:
I’d be perfectly happy with an explanation in complex terms, or multiple line explanations with or without diagrams - so where are they?
Seems like a lot of aerodynamic theory is based on empirical data rather than first principles. Nothing wrong with that, but it seems odd that a lot of it is just “that’s just the way it is”
Ok, I'll have a go, but yes it's a complicated subject, and it was a long time ago that I used to do any of it Seems like a lot of aerodynamic theory is based on empirical data rather than first principles. Nothing wrong with that, but it seems odd that a lot of it is just “that’s just the way it is”
As already covered, lift is due to higher pressure below the wing than above it, but because the wing has an angle of attack, the lower pressure above the wing is also slightly behind the wing. i.e. your lift vector isn't vertical but tilted backwards slightly, which known as lift induced drag.
If air's flowing along the wing, round the end and back along the top, i.e. a wing top vortex, you're losing some of the lift as it's decreasing the pressure on the bottom and increasing it on the top. In order to generate enough lift you increase the angle of attack slightly, which means more induced drag. So the vortex itself isn't causing drag directly.
coanda said:
This was a fun read.
You're slightly more advanced aerodynamics than the current explanation needs Edited by RizzoTheRat on Monday 27th March 10:15
"I assume the vortices are an effect of whatever causes the drag."
Earlier on Condi said "This doesn't just create drag from the vortices...,"
Can't both be right.
dr_gn said:
This thread shows the difficulty people have in explaining, in simple terms, the physical interaction of air molecules on an aircraft which lead to the generation of lift and drag. I find it fascinating that no matter how the questions are posed, the same inadequate or irrelevant explanations are repeated. It pretty much confirms the findings of the article I linked to earlier.
it's been explained but taken for granted you know what bernoulli's principle is. maybe explained since but i'll have a bash, 20yrs since i graduated and never been in industry, so this is about my level!bernoulli's principle states that the higher the velocity of a fluid (liquid or gas), the lower the pressure it exerts. its due to air molecules generating air pressure by random motion, but if they are accelerated in one particular direction then some of the energy is transferred in that direction, reducing overall pressure as the molecules collide less often.
a wing is shaped so that the distance from front to back is greater on the top surface than the lower. the air splits around the wing and has to travel faster over the top surface for the flow to arrive at the same point at the same time as the air underneath. this means lower pressure above the wing, thus lift.
drag has 2 components, pressure drag and skin friction drag
pressure drag is caused by the air molecules compressing on the forward facing surface and spacing out at the rear. this is caused by turbulent flow - when layers of air seperate from the surface and begin to swirl. the air particles push on the front surfaces more than the rear ones, thus creating a drag force. the aim is to get the airflow to stay more attached to the surface so that the turbulent wake is narrower, making the low pressure zone smaller, thus making the pressure drag force smaller.
skin friction drag is because, as layers of air move over a surface the air molecules collide with that surface, slowing them down and, at the stagnation point, stopping them completely. these molecules then collide with those in layers further out, slowing them down as well, until you get far enough out so that the stream is not affected. the area where the molecules are affected is called the boundary layer, and the aim here is to minimise it by keeping surfaces as smooth as possible.
beyond that its not easily explained without maths, lots and lots of maths. high speed aero [above mach 1] is downright scary maths.
Edited by shirt on Monday 27th March 14:47
shirt said:
dr_gn said:
This thread shows the difficulty people have in explaining, in simple terms, the physical interaction of air molecules on an aircraft which lead to the generation of lift and drag. I find it fascinating that no matter how the questions are posed, the same inadequate or irrelevant explanations are repeated. It pretty much confirms the findings of the article I linked to earlier.
a wing is shaped so that the distance from front to back is greater on the top surface than the lower. the air splits around the wing and has to travel faster over the top surface for the flow to arrive at the same point at the same time as the air underneath. this means lower pressure above the wing, thus lift.https://www.grc.nasa.gov/www/k-12/VirtualAero/Bott...
If interested in the geekery, this is worth a watch:
Doug McLean, retired aerodynamicist from Boeing. His lecture is entitled "Common Misconceptions in Aerodynamics". Vorticity at 28:25 dispelling that even the equations used for computation mislead into suggesting vorticity causes velocity, leading into Induced Drag at 30:35. The whole thing is worth the time though I think.
Doug McLean, retired aerodynamicist from Boeing. His lecture is entitled "Common Misconceptions in Aerodynamics". Vorticity at 28:25 dispelling that even the equations used for computation mislead into suggesting vorticity causes velocity, leading into Induced Drag at 30:35. The whole thing is worth the time though I think.
Bear in mind that the basic case of the wing is that the air is still and the wing is moving.
That means that if 'equal transit time' is not valid, then some air has been accelerated.
Accelerating the air implies force and pressure.
Since the name of the game is to create an upward force on the plane, the nett force on the air must be downwards.
In steady state, the mass off air appearing in front of the plane all has to pass over or under the wing and appear out the back.
Air is conserved. Your plane doesn't move a net amount of air when it flies from A to B.
Conversely, when you consider a sailing boat, it is extracting energy from the wind and using it to drive the hull.
Again, the total mass of air is conserved, but clearly if energy/power is being extracted, then the wind must be losing kinetic energy to the boat.
That is a little more complex, because the boat is moving relative to the sea and the wind is moving relative to both.
It is a mistake to only think about the thin layer of air adjacent to the wing or sail. The air affected by a sail extends a considerable distance from the sail.
Maybe it's useful to think about a propellor, or a hovering helicopter.
The helicopter is balancing its weight by accelerating a cylinder of air under the rotors downwards.
That air spreads out over a big area and returns to above the rotors.
It's all about accelerating air. In laminar flow that means curved surfaces.
That means that if 'equal transit time' is not valid, then some air has been accelerated.
Accelerating the air implies force and pressure.
Since the name of the game is to create an upward force on the plane, the nett force on the air must be downwards.
In steady state, the mass off air appearing in front of the plane all has to pass over or under the wing and appear out the back.
Air is conserved. Your plane doesn't move a net amount of air when it flies from A to B.
Conversely, when you consider a sailing boat, it is extracting energy from the wind and using it to drive the hull.
Again, the total mass of air is conserved, but clearly if energy/power is being extracted, then the wind must be losing kinetic energy to the boat.
That is a little more complex, because the boat is moving relative to the sea and the wind is moving relative to both.
It is a mistake to only think about the thin layer of air adjacent to the wing or sail. The air affected by a sail extends a considerable distance from the sail.
Maybe it's useful to think about a propellor, or a hovering helicopter.
The helicopter is balancing its weight by accelerating a cylinder of air under the rotors downwards.
That air spreads out over a big area and returns to above the rotors.
It's all about accelerating air. In laminar flow that means curved surfaces.
dr_gn said:
So Vortex Induced Drag isn't induced by vortices? That's what I assumed previously:
"I assume the vortices are an effect of whatever causes the drag."
Earlier on Condi said "This doesn't just create drag from the vortices...,"
Can't both be right.
I *think* vortex induced drag is a misnomer. It's induced drag, and the lift and vortices are all part of the air movement that's causing the drag"I assume the vortices are an effect of whatever causes the drag."
Earlier on Condi said "This doesn't just create drag from the vortices...,"
Can't both be right.
OutInTheShed said:
It's all about accelerating air. In laminar flow that means curved surfaces.
I like to think I have a modest understanding of this stuff but it never ceases to amaze me that some aircraft are able to fly upside down.Similarly, I'm intrigued that aircraft power generally pushes against the mass of air behind - like your helicopter example where the air is underneath. And yet a true outer-space rocket has nothing to push against so is compelled to chuck some mass out the back very, very fast indeed!
Mave said:
Panamax said:
Similarly, I'm intrigued that aircraft power generally pushes against the mass of air behind
Generally they don't - jet engines work by accelerating a mass flow of air through the engine and squirting it out the back.
Panamax said:
OutInTheShed said:
It's all about accelerating air. In laminar flow that means curved surfaces.
I like to think I have a modest understanding of this stuff but it never ceases to amaze me that some aircraft are able to fly upside down.Panamax said:
it never ceases to amaze me that some aircraft are able to fly upside down.
It completely fries some people's minds that when you do a barrel roll you maintain positive lift all the way round, ie the passengers wouldn't spill thier gin and tonics. It was done a few times in Concorde, and I wouldn't be surprised other airliners have done it some point as well. Also re. helicopters, the Lynx, and I believe it's successor the Wildcat, are rare in that they can produce negative lift. This means when landing on the deck of a ship it can actually push itself down on the deck to stay in position.
Mave said:
Generally they don't - jet engines work by accelerating a mass flow of air through the engine and squirting it out the back.
And they're most efficient when the speed difference between the incoming and exiting flows are lowest, hence the huge bypass ratio on modern engines, which often put as little as 10% of the total airflow through the core, the rest is just through a huge fan. This also helps massively with noise which related the velocity difference.Edited by RizzoTheRat on Tuesday 28th March 08:47
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