How Aeroplanes Fly and Propellers Pull
Have a look at this short discussion to add a little fuel to the fire:
http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html
Best,
Westhawk
http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html
Best,
Westhawk
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Maybe this question is best answered on a more fundamental, but less concrete level?:
Aerodynamic forces are pressure- and shear-forces acting on the surface of a body. Nothing else. Total aerodynamic force is obtained by summing the pressure- and shear-forces over the entire body. The airflow that gives rise to these forces is a result of, or solution to, the Navier-Stokes equations.
Aerodynamic forces are pressure- and shear-forces acting on the surface of a body. Nothing else. Total aerodynamic force is obtained by summing the pressure- and shear-forces over the entire body. The airflow that gives rise to these forces is a result of, or solution to, the Navier-Stokes equations.
Bernoulli works: it describes the pattern of pressures over a shape, and by integrating those over the entire shape, you can accurately predict the total forces and moments acting upon that shape.
Newton works: it describes that for every action there is an equal and opposite reaction. If you are able to measure or calculate the mass flow of air due to the passage forwards of the aeroplane, you'll find it corresponds exactly to the lift.
Now listen carefully: if you know the pressure distribution over the shape travelling through the air, and use this to calculate how much mass of air should be displaced downwards (using, as it happens, Newton's second law). Guess what, you get the right answer, which Newton's laws will then allow you to calculate total forces with.
If we really want to get complicated, we can come to the Navier-Stokes equations, which are in fact derived from a form of Bernoulli's equation (there are several - the incompressible, compressible, and unsteady forms - in ascending order of difficulty to understand). The N-S equations (together with some clever maths called "Cauchy's integral" and a general principle called "vorticity" are basically a way of taking the shape of something and (guess what) predicting the pressure distribution over a shape. From that, Bernoulli's equation(s) allow us to predict forces, and Newtons laws allow us to predict mass flow effects.
They really are all interlinked and inextricable ways of describing the same thing. It is totally incorrect to say that any portion of an aerodynamic force is due to any one, and not any other.
It is of-course true to say that any of these theories can be described also as simplistic - but rarely so simplistic as to be unuseable. Last week I was busy working out the equations for an obscure type of airspeed indicator - using the simplest incompressible form of Bernoulli's equation, and with accepable accuracy. (I'd rather have been flying, but that's another story!).
G
Newton works: it describes that for every action there is an equal and opposite reaction. If you are able to measure or calculate the mass flow of air due to the passage forwards of the aeroplane, you'll find it corresponds exactly to the lift.
Now listen carefully: if you know the pressure distribution over the shape travelling through the air, and use this to calculate how much mass of air should be displaced downwards (using, as it happens, Newton's second law). Guess what, you get the right answer, which Newton's laws will then allow you to calculate total forces with.
If we really want to get complicated, we can come to the Navier-Stokes equations, which are in fact derived from a form of Bernoulli's equation (there are several - the incompressible, compressible, and unsteady forms - in ascending order of difficulty to understand). The N-S equations (together with some clever maths called "Cauchy's integral" and a general principle called "vorticity" are basically a way of taking the shape of something and (guess what) predicting the pressure distribution over a shape. From that, Bernoulli's equation(s) allow us to predict forces, and Newtons laws allow us to predict mass flow effects.
They really are all interlinked and inextricable ways of describing the same thing. It is totally incorrect to say that any portion of an aerodynamic force is due to any one, and not any other.
It is of-course true to say that any of these theories can be described also as simplistic - but rarely so simplistic as to be unuseable. Last week I was busy working out the equations for an obscure type of airspeed indicator - using the simplest incompressible form of Bernoulli's equation, and with accepable accuracy. (I'd rather have been flying, but that's another story!).
G
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Navier-Stokes equations
Now that is a very unfair thing to do bringing them into play.
We used to do a exercise in the lab in the wind tunnel.
Various cross sections and angles etc.
And yes you could get a plank of wood to fly but you got lots of votex shedding and it was pretty crap. And virtually impossible to get any trailing edge controls to work in the turbulent flow. But you needed relatively little mass flow to get a reasonable up force.
Now a pure areofoil with zero angle didn't produce much lift but had very good drag characteristics the flow was laminar and trailing edge controls worked a treat.
As with most things looking at nature has given the optimised shape for the most energy effective design.
Personally I think newton does the bulk of the job. The fancy shape lets you control and lower the drag into a cost effective machine.
MJ
Now that is a very unfair thing to do bringing them into play.
We used to do a exercise in the lab in the wind tunnel.
Various cross sections and angles etc.
And yes you could get a plank of wood to fly but you got lots of votex shedding and it was pretty crap. And virtually impossible to get any trailing edge controls to work in the turbulent flow. But you needed relatively little mass flow to get a reasonable up force.
Now a pure areofoil with zero angle didn't produce much lift but had very good drag characteristics the flow was laminar and trailing edge controls worked a treat.
As with most things looking at nature has given the optimised shape for the most energy effective design.
Personally I think newton does the bulk of the job. The fancy shape lets you control and lower the drag into a cost effective machine.
MJ
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I've been reading Stick & Rudder lately, and by god it makes more sense then piling Bernoulli on top of Newton and having Navier-Stokes jumping on them, and oooh, don't forget Kutta and... hang on, why do I get the feeling that none of these guys have ever flown an airplane? 
G is right - they all work. Who cares why, or how? When you're in the air, that is. Have you ever seen a test pilot in a spinning aircraft muttering to himself along the lines of 'that tosser Bernoulli, getting me into all this trouble. Now, how do I calculate my way out of a flat spin?'
I'll be building a Ban-Bi next year, and the most important instrument is going to be a tribute to the most practical pilot ever - Langweische. The instrument? An AoA indexer.
Problem Solved!
G is right - they all work. Who cares why, or how? When you're in the air, that is. Have you ever seen a test pilot in a spinning aircraft muttering to himself along the lines of 'that tosser Bernoulli, getting me into all this trouble. Now, how do I calculate my way out of a flat spin?'
I'll be building a Ban-Bi next year, and the most important instrument is going to be a tribute to the most practical pilot ever - Langweische. The instrument? An AoA indexer.
Problem Solved!
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An alternate explanation of lift: "ATTACK ANGLE"
As air flows over a wing, the flow adheres to the surfaces of the wing.
This is called the "Coanda effect." Because the wing is tilted, the air
is deflected downwards as it moves over the wing's surfaces. Air which
flows below the wing is pushed downwards by the wing surface, and because
the wing pushes down on the air, the air must push upwards on the wing,
creating a lifting force. Air which flows over the upper surface of the
wing is adhering to the surface also. The wing "pulls downwards" on the
air as it flows over the tilted wing, and so the air pulls upwards on the
wing, creating more lifting force. (Actually the air follows the wing
because of reduced pressure, the "pull" is not really an attraction.) The
lifting force is created by Newton's Third Law and by conservation of
momentum, as the flowing air which has mass is deflected downward as the
wing moves forward. Because of Coanda Effect, the upper surface of the
wing actually deflects more air than does the lower surface.
Beats the hell out of me....
As air flows over a wing, the flow adheres to the surfaces of the wing.
This is called the "Coanda effect." Because the wing is tilted, the air
is deflected downwards as it moves over the wing's surfaces. Air which
flows below the wing is pushed downwards by the wing surface, and because
the wing pushes down on the air, the air must push upwards on the wing,
creating a lifting force. Air which flows over the upper surface of the
wing is adhering to the surface also. The wing "pulls downwards" on the
air as it flows over the tilted wing, and so the air pulls upwards on the
wing, creating more lifting force. (Actually the air follows the wing
because of reduced pressure, the "pull" is not really an attraction.) The
lifting force is created by Newton's Third Law and by conservation of
momentum, as the flowing air which has mass is deflected downward as the
wing moves forward. Because of Coanda Effect, the upper surface of the
wing actually deflects more air than does the lower surface.
Beats the hell out of me....
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I found this interesting link explaining how boat sails work.
http://www.sailtheory.com/sail.html
There's good basic lift theory in the explanations and it's apparent that boat sails and wings work essentially the same way. Both Bernoulli and N3 are explained, and the relationship between them is explained in a fairly straight forward way. Coanda effect is also included and how it relates.
I found this explanation helpful enough to clear up the issue for me as to how all 3 "effects" contribute to lift, and how they relate to each other.
The sailboat guys seem to have a pretty good handle on this.
http://www.sailtheory.com/sail.html
There's good basic lift theory in the explanations and it's apparent that boat sails and wings work essentially the same way. Both Bernoulli and N3 are explained, and the relationship between them is explained in a fairly straight forward way. Coanda effect is also included and how it relates.
I found this explanation helpful enough to clear up the issue for me as to how all 3 "effects" contribute to lift, and how they relate to each other.
The sailboat guys seem to have a pretty good handle on this.
Last edited by Flight Safety; 24th Jan 2006 at 18:07.
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The way I understand the process is that Bernoulli describes the manifestation of Newton 3G in the scenario of an aerofoil passing through the air (or the air passing over an aerofoil). In the case of an aircraft in unaccelerated flight, Lift=Weight and Thrust=Drag. The powerplants create the necessary thrust to propell the machine through the air (and therefore the air over the wing) this thrust is balanced (N3G) by the drag created by the aeroplane in a direction parallel to the thrust line. However, because the wing is curved, the air passing over the top creates a lower pressure than that below resulting in lift (Bernoulli). Because the aeroplane is travelling forwards, the inevitable counter balance to the lift created (3G) is felt at some distance behind the aircraft in the form of wake turbulence. I.E a weight of air equal to that of the air being displaced, rather than being directed vertically downwards is actually deflected downwards and backwards. This process is most easily seen in the form of wingtip vortices at take off and landing when induced drag is at its highest.
Then again, I only fly the things so what do I know? The smelly pilot theory has yet to be disproved...
Then again, I only fly the things so what do I know? The smelly pilot theory has yet to be disproved...
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> Is it Bernoulli or Newton's Third Law?
Both if applied to the whole system correctly. The argument/debate only arises because both camps over-simplify the problem..
http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html
Both if applied to the whole system correctly. The argument/debate only arises because both camps over-simplify the problem..
http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html
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I'm with Genghis on this.
But I must add that he has been very careful to keep it simple and not to confuse the issue by invoking the fundamental effect of the triple ganged reverse flow thronomister arrays at the stagnation point. The Navier-Stokes equations of course follow directly from there.
But I must add that he has been very careful to keep it simple and not to confuse the issue by invoking the fundamental effect of the triple ganged reverse flow thronomister arrays at the stagnation point. The Navier-Stokes equations of course follow directly from there.
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You've all missed the really fundamental point - they fly because you believe they'll fly. If you stop believeing it won't work any more, just like Santa and the Tooth Fairy
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Will power, nothing but will power.
That's why flight deck doors are locked now. Pilots have enough to worry about, without passengers breaking their meditation, willing the aircraft to fly.
That's why flight deck doors are locked now. Pilots have enough to worry about, without passengers breaking their meditation, willing the aircraft to fly.
Originally Posted by BizJetJock
It was Navier-Stokes in my days at RAE, but maybe they've "modernised" them.
Which, I'm delighted to say is somebody else's problem, so far I've managed to spend most of my time worrying about whole aeroplanes, not the ability of computers to predict fiddly little bits of airflow - important though it is, it fascinates me not at-all.
G