Aerofoils.. Suck or Blow..?
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Aerofoils.. Suck or Blow..?
I seem to remember that for PPL training, the aerfoil section was considered to be mostly supported by the vacuum above the wing.
However the effect of batting so many air molecules downwards by the underside of the wing must also account for lots of lift.
Has there ever been any wind-tunnel tests done to determine which supplies the most upward lift..?
My own tests of blowing over a piece of paper seemed to show no difference between the suction side or the blown side.
However the effect of batting so many air molecules downwards by the underside of the wing must also account for lots of lift.
Has there ever been any wind-tunnel tests done to determine which supplies the most upward lift..?
My own tests of blowing over a piece of paper seemed to show no difference between the suction side or the blown side.
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There was a lecture on YouTube given by the retiring aerodynamicist at Boeing that seemed to boil down to 'we don't understand lift'.
The bit on top is Bernoulli and he bit underneath is Newton (3rd Law).
But when you resolve all these forces, there seems to be a bit left over (or under).
I had a look at Kermode 'Mechanics of Flight' , didn't get very far, then found his book 'Flight without Formulae' which helped a bit.
There's a principle called 'Lies to Children'. This is where we give simple, incomplete explanations to children to answer their questions, to the extent that the over-simplification could be called an untruth or a lie, because it misses out essential stuff.
This is certainly the case in our explanations of aerodynamics at the PPL stage and probably beyond that.
TOO
The bit on top is Bernoulli and he bit underneath is Newton (3rd Law).
But when you resolve all these forces, there seems to be a bit left over (or under).
I had a look at Kermode 'Mechanics of Flight' , didn't get very far, then found his book 'Flight without Formulae' which helped a bit.
There's a principle called 'Lies to Children'. This is where we give simple, incomplete explanations to children to answer their questions, to the extent that the over-simplification could be called an untruth or a lie, because it misses out essential stuff.
This is certainly the case in our explanations of aerodynamics at the PPL stage and probably beyond that.
TOO
Last edited by TheOddOne; 7th Jul 2023 at 18:31.
In my aero course we plotted the pressure distribution around the airfoil from ports at intervals on the wing section. The largest pressure variation was pressure reduction over the top of the wing with a mild increase in pressure over the underside and a pressure spike aligned with the leading edge at the stagnation point.
At it's root it's all Newton - the difficulty being that it's Newton applied to statistically significant numbers of individual molecules. Bernoulli is handy because it is far easier to measure the air pressure and velocity than it is to account for a number of particles so large I have no idea what the prefix would be. Bernoulli explains the relationship between pressure and velocity and, by including density, also includes energy. This eliminates the need to consider all the motions of individual molecules which simplifies design, especially for such things as pitot-static sensors.
If you want more amazement look at the source/sink/vortex mathematical model. One places a source of air at one spot on a 2D plane (geometry, not passenger) and a sink and one gets streamlines. Add more sources and sinks until there is a boundary that matches the airfoil and add a vortex component that causes the air to turn. Sometimes called the Circulation theory of lift it allows understanding airfoils without Newton or Bernoulli.
When people say "How does a wing produce lift when it is upside down or how does a flat plate produce lift" the answer is "poorly."
Still, if one gets a little distance from the airfoil one can see that the downward flow from a level-flight airplane has the same momentum as required to offset the weight of the aircraft, no matter what the shape and that is Newton's contribution.
In subsonic flight the most interesting thing is that air at some considerable distance ahead of the airfoil is getting shoved forward. This makes the most sense when such flight is mentioned as "incompressible." The wing is moving so slowly compared to the speed of sound the air can just move out of the way. This is what produces the lower speed on the underside of the wing - the distant air is getting slowed relative to the wing. On the upper side the wing is making a gap into which air flows, increasing the speed of the distant air relative to the wing. It does this without a change in density.What is changing is the dynamic pressure, exchanged with the static pressure.
At it's root it's all Newton - the difficulty being that it's Newton applied to statistically significant numbers of individual molecules. Bernoulli is handy because it is far easier to measure the air pressure and velocity than it is to account for a number of particles so large I have no idea what the prefix would be. Bernoulli explains the relationship between pressure and velocity and, by including density, also includes energy. This eliminates the need to consider all the motions of individual molecules which simplifies design, especially for such things as pitot-static sensors.
If you want more amazement look at the source/sink/vortex mathematical model. One places a source of air at one spot on a 2D plane (geometry, not passenger) and a sink and one gets streamlines. Add more sources and sinks until there is a boundary that matches the airfoil and add a vortex component that causes the air to turn. Sometimes called the Circulation theory of lift it allows understanding airfoils without Newton or Bernoulli.
When people say "How does a wing produce lift when it is upside down or how does a flat plate produce lift" the answer is "poorly."
Still, if one gets a little distance from the airfoil one can see that the downward flow from a level-flight airplane has the same momentum as required to offset the weight of the aircraft, no matter what the shape and that is Newton's contribution.
In subsonic flight the most interesting thing is that air at some considerable distance ahead of the airfoil is getting shoved forward. This makes the most sense when such flight is mentioned as "incompressible." The wing is moving so slowly compared to the speed of sound the air can just move out of the way. This is what produces the lower speed on the underside of the wing - the distant air is getting slowed relative to the wing. On the upper side the wing is making a gap into which air flows, increasing the speed of the distant air relative to the wing. It does this without a change in density.What is changing is the dynamic pressure, exchanged with the static pressure.
Birch and Bramson kept things simple. Whatever the cause the airflow over the wing reduced in pressure (not in density so not a vacuum) and the flow below the wing increased the pressure. Displacement takes place and we call this lift. The effect could be demonstrated with a piece of folded paper or the back of a table spoon in the flow from a tap. It didn't pretend puritan truth but it taught the pilot the importance of angle of attack/lift force and speed of flow/lift force. The wing stall could also be demonstrated and so his method taught all the pilot need know.
The circulation theory whilst absolutely true is complex and difficult to grasp by ordinary folk, like me, who become pilots. The Circulation theory needs to be demonstrated in a wind tunnel. Reducing complexity into four vectors; lift/weight, thrust/drag can easily be drawn as a schematic diagram on the white board or by use of a model.
The circulation theory whilst absolutely true is complex and difficult to grasp by ordinary folk, like me, who become pilots. The Circulation theory needs to be demonstrated in a wind tunnel. Reducing complexity into four vectors; lift/weight, thrust/drag can easily be drawn as a schematic diagram on the white board or by use of a model.
Last edited by Fl1ingfrog; 7th Jul 2023 at 21:44.
In this video one can see the rapid slowing underneath the airfoil:
This video discusses the pressure distribution around an airfoil:
I think the second video has an error as the distribution is of the pressure perpendicular to the surface. The high positive (negative-negative 1) at the front is pointing horizontally. to push the airfoil left to right. For thin airfoils at low angle of attack it's not too big a deal, but thick foils (Clark-Y) at high angles of attack the difference will be substantial. The rest is close enough.
This video discusses the pressure distribution around an airfoil:
I think the second video has an error as the distribution is of the pressure perpendicular to the surface. The high positive (negative-negative 1) at the front is pointing horizontally. to push the airfoil left to right. For thin airfoils at low angle of attack it's not too big a deal, but thick foils (Clark-Y) at high angles of attack the difference will be substantial. The rest is close enough.
Last edited by MechEngr; 8th Jul 2023 at 04:42.
How I describe it as a dumb pilot. Avoid trying to explain why it happens, focus on what happens.
An aerofoil is efficient at deflecting air below it as it moves through it.
An appropriately curved form is able to deflect the air more (and over a wider range of angles) than a simple flat plate (although a flat plate will still work).
The top surface of a curved aerofoil draws the air downwards more than a flat top plate will do it. The bottom surface of both an aerofoil and a flat plate also deflects it downwards if it travels with a slight angle.
If you can push air downwards, you are pushed by the air upwards.
The movement of the air caused by the aerofoil as it moves through it must correspond to a change in pressure. If you flap your hand it moves the air, your hand feels the change in pressure of the air.
You can look at the overall amount of air the wing pushes down and calculate lift. (Newton)
You can also measure the pressure of the air causing it to flow as it does around the aerofoil and calculate lift. (Bernoulli)
Both will give identical answers as they are simply answering the same question but from different perspectives.
Beroulli's thereom is derived from Newtons laws.
That is what happens.
Why does a curved surface deflect the air better than a flat plate? Much more difficult question. Avoid trying to answer it. If the student persists invoke magic.
An aerofoil is efficient at deflecting air below it as it moves through it.
An appropriately curved form is able to deflect the air more (and over a wider range of angles) than a simple flat plate (although a flat plate will still work).
The top surface of a curved aerofoil draws the air downwards more than a flat top plate will do it. The bottom surface of both an aerofoil and a flat plate also deflects it downwards if it travels with a slight angle.
If you can push air downwards, you are pushed by the air upwards.
The movement of the air caused by the aerofoil as it moves through it must correspond to a change in pressure. If you flap your hand it moves the air, your hand feels the change in pressure of the air.
You can look at the overall amount of air the wing pushes down and calculate lift. (Newton)
You can also measure the pressure of the air causing it to flow as it does around the aerofoil and calculate lift. (Bernoulli)
Both will give identical answers as they are simply answering the same question but from different perspectives.
Beroulli's thereom is derived from Newtons laws.
That is what happens.
Why does a curved surface deflect the air better than a flat plate? Much more difficult question. Avoid trying to answer it. If the student persists invoke magic.
Best description I've heard is that lift is obtained by throwing air at the ground. A helicopter rotor or a propeller is nothing but a wing traveling in a circle, stand beneath a hovering helicopter or behind a propeller fixed wing running at high power. The arrows point in the direction of "lift" and its magnitude on a typical airfoil at a particular angle of attack. Good thread on the subject here.
Lift %, Upper/lower wing sections
Lift %, Upper/lower wing sections
bound vortex theory around a foil gives a good solution, and accounts for the start and stop vortex structures, Each version has some constraints, but Bernoulli has the biggest problems with multi element foils.Whatwever floats yer boat, oops, that's Pythagoras, Trump, Archimedes!
thin foil theory works by the deflection.
bound vortex results in a defection
bernouilli keeps the FAA happy, even if it is rong as often as it is Wright.
Kutta-Joukowski theorem within the framework of inviscid potential flow theory is pretty good, needs add ons in more complex conditions.
backspin, slice, topspin...slats..., flaps... etc, Bernoulli falls short, as it does for unvented spoiler effects, (why there is usually a gap at the base of the spoiler or vent holes or slots...)
Zhukovsky's elegant conformal transformation is nice 'n eezy;
morphing circulation theory into lifting line gives good results, still a lot easier than a RANS/URANS/DES/LES numerical solution of lift, which will look a fair bit like any of these, but will function in low speed, and high speed cases, and will model stuff like compressibility, supercritical design fairly well, although it can be said that a lot of pretty simple potential flow code will give a fair approximation of pressure and velocity distributions. Dreese's code, Ilon Kroo's PANDA etc were nice n easy and give good results. Doc Pats Multi surf and visual foil is a bit more advanced, but it would be nice to have more cells, it is a step up from javafoil, which is surprisingly good. Going to Ansys, MSC, Simscale, Siemens, Dassault etc gives pretty good outcomes and other than needing a large pot of cash to use the code and a similar size bucket of loot for the core time, gives answers that apart from frequently having artefacts avoids answering the question of Bernoulli or newtonian reactions at play.
Whatever works and explains what you are playing with, alternatively, whatever the FAA says.
What will normally ness with the mind of the observer is that what happens at the trailing edge of a foil is as important or more to what happens at the front, and it affects the flow at the front of the foil in real time, and is not "spooky action at a distance", but arises due to the fact that one of the things above is not like the other, one of these is just not the same... etc...
I’d like to see this retired Boeing engineer stating “we don’t understand lift” as a 777-9 lifts off the runway in the background. Sort of a Joe Isuzu moment.
Thread Starter
It's just not Cricket.
Being from the UK, I try to think of the situation as the same as a Cricket Bat and Ball; with the bat being the wing and the ball being an air molecule. If you swing correctly and hit the ball a glancing shot, the ball changes direction and heads to the outfield. If you miss and the ball passes behind the bat, it doesn't deflect at all... So maybe the cricket analogy just does not work here.
