Lift on aircraft wing
Discussion
dr_gn said:
Been reading this last week - I thought it was pretty interesting:
https://www.scientificamerican.com/article/no-one-...
People often talk about low pressure and downwash etc, but I’m more interested in how the lift force actually acts on the wing surfaces. I can understand the AoA effect (like when you stick your hand out of a car window), and I can understand that if there’s low pressure above a wing, the high pressure pushes the wing upwards.
I don’t get…for example how tip vortices cause drag. I know how they’re formed, but not how anything other than low pressure behind the wing could directly cause drag. I assume the vortices are an effect of whatever causes the drag. I’ve asked several aeronautical engineering graduates that question, and none of them could explain it in terms of physical loads on the wing.
Anyway…
When thinking about anything to do with the complicated subject of aerodynamics, I find it helps if you remember just how dense air is, we think of it as just 'thin air' because we're used to it but 14.7 psi at sea level equates to over 10 tonnes per square metre, (it's still over 3 tonnes at 30000 ft), if you visualise that an aircraft parked on the runway has ten tonnes acting on every square metre of both the top and bottom of the https://www.scientificamerican.com/article/no-one-...
People often talk about low pressure and downwash etc, but I’m more interested in how the lift force actually acts on the wing surfaces. I can understand the AoA effect (like when you stick your hand out of a car window), and I can understand that if there’s low pressure above a wing, the high pressure pushes the wing upwards.
I don’t get…for example how tip vortices cause drag. I know how they’re formed, but not how anything other than low pressure behind the wing could directly cause drag. I assume the vortices are an effect of whatever causes the drag. I’ve asked several aeronautical engineering graduates that question, and none of them could explain it in terms of physical loads on the wing.
Anyway…
wing, you can see how only a small percentage difference in pressure between the two can create considerable lift.
With regard to the induced drag from wingtip vortices, as I understand it, it's just caused by low pressure behind the aircraft (not just the wingtips) basically 'sucking' the aircraft backwards in much the same way as a car with a back like a van has more drag than one tapered at the back.
The vortices have a very low pressure core that soon expands due to rotational forces to cause a large low pressure area, this has a much greater effect at lower speeds.
eta, I've just realised that on the picture I've posted, the aircraft has small winglets, they're obviously not working very well
.
Edited by oakdale on Saturday 25th March 14:41
oakdale said:
dr_gn said:
Been reading this last week - I thought it was pretty interesting:
https://www.scientificamerican.com/article/no-one-...
People often talk about low pressure and downwash etc, but I’m more interested in how the lift force actually acts on the wing surfaces. I can understand the AoA effect (like when you stick your hand out of a car window), and I can understand that if there’s low pressure above a wing, the high pressure pushes the wing upwards.
I don’t get…for example how tip vortices cause drag. I know how they’re formed, but not how anything other than low pressure behind the wing could directly cause drag. I assume the vortices are an effect of whatever causes the drag. I’ve asked several aeronautical engineering graduates that question, and none of them could explain it in terms of physical loads on the wing.
Anyway…
When thinking about anything to do with the complicated subject of aerodynamics, I find it helps if you remember just how dense air is, we think of it as just 'thin air' because we're used to it but 14.7 psi at sea level equates to over 10 tonnes per square metre, (it's still over 3 tonnes at 30000 ft), if you visualise that an aircraft parked on the runway has ten tonnes acting on every square metre of both the top and bottom of the https://www.scientificamerican.com/article/no-one-...
People often talk about low pressure and downwash etc, but I’m more interested in how the lift force actually acts on the wing surfaces. I can understand the AoA effect (like when you stick your hand out of a car window), and I can understand that if there’s low pressure above a wing, the high pressure pushes the wing upwards.
I don’t get…for example how tip vortices cause drag. I know how they’re formed, but not how anything other than low pressure behind the wing could directly cause drag. I assume the vortices are an effect of whatever causes the drag. I’ve asked several aeronautical engineering graduates that question, and none of them could explain it in terms of physical loads on the wing.
Anyway…
wing, you can see how only a small percentage difference in pressure between the two can create considerable lift.
With regard to the induced drag from wingtip vortices, as I understand it, it's just caused by low pressure behind the aircraft (not just the wingtips) basically 'sucking' the aircraft backwards in much the same way as a car with a back like a van has more drag than one tapered at the back.
The vortices have a very low pressure core that soon expands due to rotational forces to cause a large low pressure area, this has a much greater effect at lower speeds.
I can appreciate that a fair bit of energy is imparted to the air as it flows along the wing and wraps around the tips - anyone whose seen the Hawker Tempest displays at Duxford with smoke generators on the tips can see that; they can last for minutes, and still have a significant amount of energy).
I can see how the spanwise flow would reduce the low pressure on top of the wing, and how this would require more thrust to compensate…maybe that’s it - it’s ‘more thrust’ rather than drag? I dunno?
I can see how the spanwise flow would reduce the low pressure on top of the wing, and how this would require more thrust to compensate…maybe that’s it - it’s ‘more thrust’ rather than drag? I dunno?
The wing tip vortices are because the air on top of the wing is at lower pressure than the air under it, and some air "escapes" and goes over the wing at the edge to balance the difference in pressure. This doesn't just create drag from the vortices, but also makes the wings less efficient as the last 10% of the wing is not producing lift. By adding the wing tips it prevents the air moving from under the wing to over it and thus makes the wings more efficient, leading to a saving in fuel.
Edited by Condi on Saturday 25th March 16:35
Condi said:
The wing tip vortices are because the air on top of the wing is at lower pressure than the air under it, and some air "escapes" and goes over the wing at the edge to balance the difference in pressure. This doesn't just create drag from the vortices, but also makes the wings less efficient as the last 10% of the wing is not producing lift. By adding the wing tips it prevents the air moving from under the wing to over it and thus makes the wings more efficient, leading to a saving in fuel.
You’ve just illustrated the problem very nicely: As I said, I know how they’re generated, but nobody can explain how they physically interact with the wing to cause drag.Edited by Condi on Saturday 25th March 16:35
You said “cause drag from the vortices”…how?, why?
dr_gn said:
You’ve just illustrated the problem very nicely: As I said, I know how they’re generated, but nobody can explain how they physically interact with the wing to cause drag.
You said “cause drag from the vortices”…how?, why?
If the air behind the plane is swirling about in vortices, it has kinetic energy.You said “cause drag from the vortices”…how?, why?
That energy comes from the plane.
That must act on the plane as a force, i.e. drag.
But the forces on the plane are basically pressure, integrated over the surface, and frictional drag?
dr_gn said:
You’ve just illustrated the problem very nicely: As I said, I know how they’re generated, but nobody can explain how they physically interact with the wing to cause drag.
You said “cause drag from the vortices”…how?, why?
Any energy put into the air (ie creating the vortexes) is energy which is not going into forward movement. If you create a smaller vortex then you move less air and more of the fuel burn is used efficiently. You said “cause drag from the vortices”…how?, why?
Which was my theory outlined in a previous post. Still no explanation on what the physical interaction is with the aircraft though.
Sure, the forward movement of the aircraft imparts energy to the surrounding air, which results in the air moving around the airframe in all directions - even influencing the air in front, before the airframe itself has got to it.
However my question isn’t about that, its about how that movement of air molecules interact with the airframe surfaces to give lift and drag.
Sure, the forward movement of the aircraft imparts energy to the surrounding air, which results in the air moving around the airframe in all directions - even influencing the air in front, before the airframe itself has got to it.
However my question isn’t about that, its about how that movement of air molecules interact with the airframe surfaces to give lift and drag.
If lift is generated, ie. work is done, then the cost is drag; specifically, induced drag. You don't get something for nothing. The ratio between the work done (lift) and cost (drag) is the lift to drag ratio, and as an example, the best sailplanes have a lift to drag ratio of around 70:1 at the point where the sum of the induced drag and the profile drag are lowest.
For a wing of a constant aerofoil section, the lift distribution is elliptical, with maximum lift at the wing root furthest from the tip, and reducing to nothing at the tip.
A sailplane with a high aspect ratio (long span, narrow chord) wing does a little work on a large length (tip to tip) of air, or in other words disturbs a lot of air very slightly, whereas a jet fighter does a lot of work on a small length of air, or disturbs a small amount of air by a lot.
The vortices seen at the wingtips are simply the visible parts of the work done all the way along the wing.
This picture shows how the vortices off the outboard ends of the flaps are much larger than those off the wing tips, as lowering the flaps put that part of the wing at a high angle of attack so it is doing a lot of work, whilst the tips at a lower angle of attack are doing very little.
For a wing of a constant aerofoil section, the lift distribution is elliptical, with maximum lift at the wing root furthest from the tip, and reducing to nothing at the tip.
A sailplane with a high aspect ratio (long span, narrow chord) wing does a little work on a large length (tip to tip) of air, or in other words disturbs a lot of air very slightly, whereas a jet fighter does a lot of work on a small length of air, or disturbs a small amount of air by a lot.
The vortices seen at the wingtips are simply the visible parts of the work done all the way along the wing.
This picture shows how the vortices off the outboard ends of the flaps are much larger than those off the wing tips, as lowering the flaps put that part of the wing at a high angle of attack so it is doing a lot of work, whilst the tips at a lower angle of attack are doing very little.
dr_gn said:
48k said:
It's a bit like why you have to stand behind the yellow line on train platforms because after the express train thunders through at 125mph you can get sucked off by the negative pressure behind it.
TGCOTF-dewey said:
dr_gn said:
48k said:
It's a bit like why you have to stand behind the yellow line on train platforms because after the express train thunders through at 125mph you can get sucked off by the negative pressure behind it.
Edited by Flying Phil on Sunday 26th March 17:27
OutInTheShed said:
GliderRider said:
If lift is generated, ie. work is done,......]
Lift is a force.Work is force times distance.
Lift is not work.
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.
Why should it be explainable completely in simple terms?If you read some of the better books on sailing, then you will find that separation of flow from the low pressure side is well observed, if not utterly understood.
If you consider the issue with sailing, where not only is 'lift' observed, but also forward drive with an upwind component, no it's not simple.
Not everything can usefully be reduced to on line explanations.
OutInTheShed 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.
Why should it be explainable completely in simple terms?If you read some of the better books on sailing, then you will find that separation of flow from the low pressure side is well observed, if not utterly understood.
If you consider the issue with sailing, where not only is 'lift' observed, but also forward drive with an upwind component, no it's not simple.
Not everything can usefully be reduced to on line explanations.
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”
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