Why airplanes fly: The Truth uncovered...happy reading
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Why airplanes fly: The Truth uncovered...happy reading
Newton Vs Bernoulli
Disregarding for a moment that Bernoulli died over a century before the aeroplane was invented, he was nonetheless right. It is certain instructors unnecessarily over-simplifying the theory, which has been very well understood since the early 1920s, who are lying to you.
It's a nice bit of video isn't it.
G
It's a nice bit of video isn't it.
G
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Here we go again for the umteenth time. Even this article is an incomplete description of lift. For instance even a flat piece of plywood angled to the airflow will generate lift (no curves involved).
And I didn't see any mention of a frictionless conveyor belt .
Time for JB.
And I didn't see any mention of a frictionless conveyor belt .
Time for JB.
Do a Hover - it avoids G
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Crabman
You mention flat plates. I agree with you of course. For my students who cannot understand how a flat plate can ever produce low pressure above its top surface I talk along these lines:
Aero guys call the streamline that defines the boundary between the air which goes over a wing from the air which goes under a wing the ‘stagnation streamline’ and call the point where it hits the aerofoil the stagnation point.
The key thing to realise is that the only time the stagnation point (SP) is located right on the front of the wing leading edge is when the wing is at such an alpha that it is producing zero lift. If the wing, even a flat plate, is at an alpha that allows lift to be produced, the SP will be located on the underneath of the wing a small distance back from the leading edge. Furthermore the SP moves steadily back under the wing as alpha is increased until the stall – and even beyond. This effect can be seen in any simple wind tunnel where smoke is introduced to enable the flow past the wing to be visualised.
If you don't have a wind tunnel and to understand how this can also happen with a flat plate, I offer the following explanation. Imagine it is a windy day and you are carrying a large 8 x 4 ft sheet of chipboard out from the DIY store to your pickup. Being bright you take care to hold it flat and edge on to the wind so naturally the SP is right on the leading edge and all is well. Then out of the corner of your eye you see another chap who is carrying his sheet flat on to the wind.
Here the SP is right in the middle of the board with the air spilling round all four sides. He realises your way is easier and starts to rotate his board towards the flat position you used.
As the alpha is reduced from his 90° to your 0°, the SP has to move forward from the centre to the leading edge. Therefore is it not reasonable to accept that as the alpha of the sheet is reducing through say 10° (below that for the stall but not yet zero), the SP will be well on its way to the leading edge but it will not actually get there until the alpha for zero lift is reached? I apologise for that childishly simple analogy but I find such things useful so perhaps others will as well.
Aero guys call the streamline that defines the boundary between the air which goes over a wing from the air which goes under a wing the ‘stagnation streamline’ and call the point where it hits the aerofoil the stagnation point.
The key thing to realise is that the only time the stagnation point (SP) is located right on the front of the wing leading edge is when the wing is at such an alpha that it is producing zero lift. If the wing, even a flat plate, is at an alpha that allows lift to be produced, the SP will be located on the underneath of the wing a small distance back from the leading edge. Furthermore the SP moves steadily back under the wing as alpha is increased until the stall – and even beyond. This effect can be seen in any simple wind tunnel where smoke is introduced to enable the flow past the wing to be visualised.
If you don't have a wind tunnel and to understand how this can also happen with a flat plate, I offer the following explanation. Imagine it is a windy day and you are carrying a large 8 x 4 ft sheet of chipboard out from the DIY store to your pickup. Being bright you take care to hold it flat and edge on to the wind so naturally the SP is right on the leading edge and all is well. Then out of the corner of your eye you see another chap who is carrying his sheet flat on to the wind.
Here the SP is right in the middle of the board with the air spilling round all four sides. He realises your way is easier and starts to rotate his board towards the flat position you used.
As the alpha is reduced from his 90° to your 0°, the SP has to move forward from the centre to the leading edge. Therefore is it not reasonable to accept that as the alpha of the sheet is reducing through say 10° (below that for the stall but not yet zero), the SP will be well on its way to the leading edge but it will not actually get there until the alpha for zero lift is reached? I apologise for that childishly simple analogy but I find such things useful so perhaps others will as well.
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Interesting post, it inspired me (after having lurked for ages) to register. I am stupid, like the second man with the vertical plywood. But I'm a fast learner, so I gradually reduced my AoA to the right point, until eventually I was just pushing my 4X8 towards the trusty pickup with a couple of fingers (hoping at the same time that the wind didn't pick up and break my nose).
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I posted on Reddit a few days ago talking about the inaccuracies of current lift explanations. I thought I would include the link instead of rehashing everything I said over there! My username is: 4fifty8
I don't believe this is a Newton vs Bernoulli fight. Both of them have contributed to the understanding of lift that scientists have in 2012. It's just that whoever taught you possibly taught you incorrectly that Bernoulli was the one and only reason why an airfoil produced lift and then stated the myth of equal transit times. Those are both wrong, but Bernoulli isn't!
Lift - still being poorly and inaccurately described : flying
I don't believe this is a Newton vs Bernoulli fight. Both of them have contributed to the understanding of lift that scientists have in 2012. It's just that whoever taught you possibly taught you incorrectly that Bernoulli was the one and only reason why an airfoil produced lift and then stated the myth of equal transit times. Those are both wrong, but Bernoulli isn't!
Lift - still being poorly and inaccurately described : flying
Second Law
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OK, head above parapet!
Lift comes primarily from more air molecules hitting the wing below it than above it per unit time.
Hence the behaviour of sheets of 8x4 or upside down flight or flat wings or whatever.
The lower pressure above the wing is largely simply relative to the higher pressure beneath.
Shape helps but is not key.
CW
Lift comes primarily from more air molecules hitting the wing below it than above it per unit time.
Hence the behaviour of sheets of 8x4 or upside down flight or flat wings or whatever.
The lower pressure above the wing is largely simply relative to the higher pressure beneath.
Shape helps but is not key.
CW
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Lift comes primarily from more air molecules hitting the wing below it than above it per unit time.
When you're talking about a piece of plywood or a barn door flying, it would have to be at a VERY low angle of attack to fly by the same principles as an airfoil normally does. This is because the critical angle of attack would be very low due to the lack of a smooth and progressive path for the air to follow as it flows around the top. The change in velocity (direction in this case) would be so high that the surface tension would not be high enough to be able to impart enough force to keep the airflow attached to the upper surface of the plywood. It would break away at the critical angle of attack and it's now stalled. To make the plywood fly beyond this point, then you would be relying a lot on the molecules hitting the underside of the plywood and imparting a force based on Newton's laws that would cause the plywood to "fly". Important distinction is that this is not how airfoils normally fly!
Do a Hover - it avoids G
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chris
Your words sound eminently sensible however they do rather ignore the evidence from those wonderful aids to understanding wings - wind tunnels.
In tunnels it is easy to actually measure the pressures all over the top and bottom surfaces of a wing at different angles of attack. (This is done by making many small holes in the surface of the wing and connecting these 'pressure tapping points' with tubes inside the wing to a bank of manometers or U tubes containing fluid. Naturally the fluid gets 'blown' up at the places where the pressure is increased and 'sucked' up where the pressure is lowered. The height of each column is of course proportional to the local pressure at the tapping point and providing you know the specific gravity of the fluid in the manometer the actual pressure - positive or negative - can easily be calculated.)
You will have to take my word for it that at angles below the stall the suctions are relatively much larger than the positive pressures. If you doubt my word (and why not for goodness sake) you could examine any suitable reference work on such activities.
When it comes to generating forces you are however very right that shape is less important than angle of attack. A traditional 'aerofoil' shape has the edge of course over a flat plate (which is in turn much better than an aerofoil flying inverted) when it comes to the suctions produced on the upper surface. Of course a 'good' shape also wins hands down with that marvellous measure of aerodynamic efficiency - the ratio between the lift generated and the drag generated or the L/D ratio.
In simply recording the facts of this life I do realise that I have done nothing to explain the facts. Such an explanation requires more mental horsepower than I was issued with.
In tunnels it is easy to actually measure the pressures all over the top and bottom surfaces of a wing at different angles of attack. (This is done by making many small holes in the surface of the wing and connecting these 'pressure tapping points' with tubes inside the wing to a bank of manometers or U tubes containing fluid. Naturally the fluid gets 'blown' up at the places where the pressure is increased and 'sucked' up where the pressure is lowered. The height of each column is of course proportional to the local pressure at the tapping point and providing you know the specific gravity of the fluid in the manometer the actual pressure - positive or negative - can easily be calculated.)
You will have to take my word for it that at angles below the stall the suctions are relatively much larger than the positive pressures. If you doubt my word (and why not for goodness sake) you could examine any suitable reference work on such activities.
When it comes to generating forces you are however very right that shape is less important than angle of attack. A traditional 'aerofoil' shape has the edge of course over a flat plate (which is in turn much better than an aerofoil flying inverted) when it comes to the suctions produced on the upper surface. Of course a 'good' shape also wins hands down with that marvellous measure of aerodynamic efficiency - the ratio between the lift generated and the drag generated or the L/D ratio.
In simply recording the facts of this life I do realise that I have done nothing to explain the facts. Such an explanation requires more mental horsepower than I was issued with.
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In tunnels it is easy to actually measure the pressures all over the top and bottom surfaces of a wing at different angles of attack. (This is done by making many small holes in the surface of the wing and connecting these 'pressure tapping points' with tubes inside the wing to a bank of manometers or U tubes containing fluid. Naturally the fluid gets 'blown' up at the places where the pressure is increased and 'sucked' up where the pressure is lowered. The height of each column is of course proportional to the local pressure at the tapping point and providing you know the specific gravity of the fluid in the manometer the actual pressure - positive or negative - can easily be calculated.)
You will have to take my word for it that at angles below the stall the suctions are relatively much larger than the positive pressures. If you doubt my word (and why not for goodness sake) you could examine any suitable reference work on such activities.
Keeping Danny in Sandwiches
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Of course my 6p balsa glider that I used to buy from Woolworths as a kid was nothing more sophisticated than your piece of 8 by 4 and used to fly incredibly well for the money.
i thought cw was saying nothing more than that force is an integral of pressure which in turn is about the macroscopic consequence of all those little billiard balls. don't think he meant overpressure was more significant than overpressure.
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I think we're seeing a clash of the justified equal and opposite reaction to the classical "bullets impinging the wing and bouncing off the underside" flawed explanation of Newtonian lift, and a very similar-sounding explanation of lift through what the air molecules (not bullets) actually do in a strictly physical sense?
Edit: italia458, good example to counter the bullet theory. Adding that one to the repertoire.
Cheers,
Fred
Edit: italia458, good example to counter the bullet theory. Adding that one to the repertoire.
Cheers,
Fred
an explanation of pressure is not the same as trying to explain lift though a transfer of momentum argument. nevertheless conservation of momentum holds (clearly) and the actuality of force, including lift, drag, normal force, axial force is from the physical effects exerted on the wing.