Why Do Aircraft fy? Flat Plate Lift Vs Bernoulli?
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cc2180, you can fly a Bellanca Decathlon at a 90 degree bank angle
with only 180 hp. And what is creating lift here? It has to be the
fusselage, the vertical fin, and the engine.
with only 180 hp. And what is creating lift here? It has to be the
fusselage, the vertical fin, and the engine.
Ah.. post #9. Started as witticism but then moved onto some serious stuff. I see, now.
Still sounds a little like a circular argument: viscosity because then we can treat flat plate as if it were another Bernoulli object which requires viscosity.
So:
flat plate + viscosity => asymmetric flow => Bernoulli => lift
I must say that asymmetric flow is not the first think that I would expect from a flat plate but I guess my intuition developed without the help of hours spent in wind tunnel.
Again, a question arises:
flat plate + extremely low viscosity fluid => nearly symmetric flow => nearly no Bernoulli => nearly no lift ?
PS: Thick, I tell you! Thick is my head!
Originally Posted by Dash&Thump
The answer IS Viscosity. Without that, you have a pointy land vehicle with wings.
A flat plate will fly because the viscosity allows it to generate asymetric airflow, the fundemental requirement to allow Mr Benoulli to do his bit.
The viscosity is the property that allows the air to seperate at the trailing edge and not come back up and join the other stream in the opposite place to the separation point at the front.
A flat plate will fly because the viscosity allows it to generate asymetric airflow, the fundemental requirement to allow Mr Benoulli to do his bit.
The viscosity is the property that allows the air to seperate at the trailing edge and not come back up and join the other stream in the opposite place to the separation point at the front.
So:
flat plate + viscosity => asymmetric flow => Bernoulli => lift
I must say that asymmetric flow is not the first think that I would expect from a flat plate but I guess my intuition developed without the help of hours spent in wind tunnel.
Again, a question arises:
flat plate + extremely low viscosity fluid => nearly symmetric flow => nearly no Bernoulli => nearly no lift ?
PS: Thick, I tell you! Thick is my head!
flat plate + extremely low viscosity fluid => nearly symmetric flow => nearly no Bernoulli => nearly no lift ?
If we balance a pencil vertically on its tip, does the direction in which it will fall "depend on gravity"? How can we answer that?
If we took the same pencil, on the same surface, to the moon (where the gravity is lower) would it fall in the same direction? The answer to this second question is, I hope, obviously "yes", as the magnitude of the vertical force makes no difference to the direction of fall. (Of course the rate of fall may depend on the magnitude of the vertical force, but the direction does not -- it depends on things like the mass distribution within the pencil, and the detailed shape of the tip, and the surface.)
If we took the same pencil, on the same surface, into outer space (where the gravity is zero) would it fall in the same direction? Well, with no gravity at all, the pencil would not fall, so the behaviour is quite different. We could legitimately say "no".
Does lift depend on viscosity? Well in the sense that absolutely zero viscosity would completely change the pattern of flow around the wing, yes. But all it takes is a tiny trace of viscosity (very high Reynolds Number) to create the flow pattern, and then the wing lifts as you'd expect. Increasing the viscosity will change the lift a little, but not much, even as the viscosity increases by orders of magnitude, and the lift actually tends to decrease as viscosity increases (Reynolds Number decreases).
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2. "reduce" air density by a factor of 1000.
Lift should decrease 1000 times, except that Reynolds number also decreases 1000 times.
1. "reduce" air viscosity by a factor of... (pregnant pause.. + Dr. Evil cunning facial expression..) 1000.
So, for a flat plate at an angle of attack, lower pressure on top, same higher pressure on the bottom.
No lift, just drag.
No lift, just drag.
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Because if its diverting flow in any direction, there has to be a resultant force as a result of Newtons Laws Surely.
It interests me, as I wonder how CFD software predicts lift ... As someone has mentioned, without viscocity, you cant get assymetric flow, thus without Newton, a flat plate could not produce lift.
It would be interesting to see what effects this would have on a flat plate in superfluid flow (0 viscosity).
It interests me, as I wonder how CFD software predicts lift ... As someone has mentioned, without viscocity, you cant get assymetric flow, thus without Newton, a flat plate could not produce lift.
It would be interesting to see what effects this would have on a flat plate in superfluid flow (0 viscosity).
bookworm:
Thanks!
At first you've lost me on the relevance of the pencil example, but eventually I think I get it. Any amount of viscosity changes things.
The question still remains: is it absolutely necessary?
Is it a question that can be demonstrated in a realistic experiment?
I'll break it down to a short sentence for thick heads like me:
Increasing viscosity descreases lift. Hm.. Do you meant, beyond certain point, or always? Or, is it that you are mentally changing size to change Reynolds, which you equate with viscosity? Something doesn't tally with the need for viscosity.
chornedsnorkack:
I appreciate your replies, but... my questions aim to find how much the two physical properties of air / fluid contribute to the lift.
Your 1st example didn't separate the two.
Your 2nd example promises to decrease viscosity [effects] by
. Hm.. And divide the results by surface area? Any link with results?
Can we perhaps confine our discussion to one flat plate of fixed dimensions, and not introduce this dreaded Raynolds number. To place us on any graphs that you may have at your disposal, let's say that the flat plate is 10m X 1m (not terribly, but close to C-172).
PS: Merry Christmas.
Thanks!
At first you've lost me on the relevance of the pencil example, but eventually I think I get it. Any amount of viscosity changes things.
The question still remains: is it absolutely necessary?
Is it a question that can be demonstrated in a realistic experiment?
Increasing the viscosity will change the lift a little, but not much, even as the viscosity increases by orders of magnitude, and the lift actually tends to decrease as viscosity increases (Reynolds Number decreases).
Increasing viscosity descreases lift. Hm.. Do you meant, beyond certain point, or always? Or, is it that you are mentally changing size to change Reynolds, which you equate with viscosity? Something doesn't tally with the need for viscosity.
chornedsnorkack:
I appreciate your replies, but... my questions aim to find how much the two physical properties of air / fluid contribute to the lift.
Your 1st example didn't separate the two.
Your 2nd example promises to decrease viscosity [effects] by
... increasing the size of the airfoil
Can we perhaps confine our discussion to one flat plate of fixed dimensions, and not introduce this dreaded Raynolds number. To place us on any graphs that you may have at your disposal, let's say that the flat plate is 10m X 1m (not terribly, but close to C-172).
PS: Merry Christmas.
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Because if its diverting flow in any direction, there has to be a resultant force as a result of Newtons Laws Surely.
As someone has mentioned, without viscocity, you cant get assymetric flow,
The airflow below the plate is compressed on meeting shock front, and then again on approaching the surface of the plate. The airflow above the plate meets no shock, but when the neighbouring air below is no longer there, it starts to expand against the upper surface of the plate. But the pressure remains below ambient. There is no way for the high-pressure air below the plate to affect the low-pressure area above in any way, because this would take upstream signals, which are forbidden.
The question still remains: is it absolutely necessary?
Is it a question that can be demonstrated in a realistic experiment?
Is it a question that can be demonstrated in a realistic experiment?
Increasing viscosity descreases lift. Hm.. Do you meant, beyond certain point, or always?
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Well, well, well.
I apparently must repeat my previous post to re-concentrate some minds.
KISS. Newton's third law. To every action is an equal and opposite reaction.
That is the answer in a nutshell.
Aircraft fly because the upward force opposite to their weight is equal.
The way aircraft produce such opposing force is by deflecting air downwards. Now whether it is by aerofoils , bernoulli, coanda, flat plates or pure chance, it makes no odds. It is simply the downward deflection of the air that causes an opposite force upward on the aircraft.
If in doubt, ask the birds. ( OK we cannot do that).
I apparently must repeat my previous post to re-concentrate some minds.
KISS. Newton's third law. To every action is an equal and opposite reaction.
That is the answer in a nutshell.
Aircraft fly because the upward force opposite to their weight is equal.
The way aircraft produce such opposing force is by deflecting air downwards. Now whether it is by aerofoils , bernoulli, coanda, flat plates or pure chance, it makes no odds. It is simply the downward deflection of the air that causes an opposite force upward on the aircraft.
If in doubt, ask the birds. ( OK we cannot do that).
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The word according to NASA
Lift is the force that holds an aircraft in the air. How is lift generated? There are many explanations for the generation of lift found in encyclopedias, in basic physics textbooks, and on Web sites. Unfortunately, many of the explanations are misleading and incorrect. Theories on the generation of lift have become a source of great controversy and a topic for heated arguments for many years.
The proponents of the arguments usually fall into two camps: (1) those who support the "Bernoulli" position that lift is generated by a pressure difference across the wing, and (2) those who support the "Newton" position that lift is the reaction force on a body caused by deflecting a flow of gas. Notice that we place the names in quotation marks because neither Newton nor Bernoulli ever attempted to explain the aerodynamic lift of an object. The names of these scientists are just labels for two camps.
Looking at the lives of Bernoulli and Newton we find more similarities than differences. On the figure at the top of this page we show portraits of Daniel Bernoulli, on the left, and Sir Isaac Newton, on the right. Newton worked in many areas of mathematics and physics. He developed the theories of gravitation in 1666, when he was only 23 years old. Some twenty years later, in 1686, he presented his three laws of motion in the Principia Mathematica Philosophiae Naturalis . He and Gottfried Leibnitz are also credited with the development of the mathematics of Calculus. Bernoulli also worked in many areas of mathematics and physics and had a degree in medicine. In 1724, at age 24, he had published a mathematical work in which he investigated a problem begun by Newton concerning the flow of water from a container and several other problems involving differential equations. In 1738, his work Hydrodynamica was published. In this work, he applied the conservation of energy to fluid mechanics problems.
Which camp is correct? How is lift generated?
When a gas flows over an object, or when an object moves through a gas, the molecules of the gas are free to move about the object; they are not closely bound to one another as in a solid. Because the molecules move, there is a velocity associated with the gas. Within the gas, the velocity can have very different values at different places near the object. Bernoulli's equation, which was named for Daniel Bernoulli, relates the pressure in a gas to the local velocity; so as the velocity changes around the object, the pressure changes as well. Adding up (integrating) the pressure variation times the area around the entire body determines the aerodynamic force on the body. The lift is the component of the aerodynamic force which is perpendicular to the original flow direction of the gas. The drag is the component of the aerodynamic force which is parallel to the original flow direction of the gas. Now adding up the velocity variation around the object instead of the pressure variation also determines the aerodynamic force. The integrated velocity variation around the object produces a net turning of the gas flow. From Newton's third law of motion, a turning action of the flow will result in a re-action (aerodynamic force) on the object. So both "Bernoulli" and "Newton" are correct. Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object. We can use equations developed by each of them to determine the magnitude and direction of the aerodynamic force.
What is the argument?
Arguments arise because people mis-apply Bernoulli and Newton's equations and because they over-simplify the description of the problem of aerodynamic lift. The most popular incorrect theory of lift arises from a mis-application of Bernoulli's equation. The theory is known as the "equal transit time" or "longer path" theory which states that wings are designed with the upper surface longer than the lower surface, to generate higher velocities on the upper surface because the molecules of gas on the upper surface have to reach the trailing edge at the same time as the molecules on the lower surface. The theory then invokes Bernoulli's equation to explain lower pressure on the upper surface and higher pressure on the lower surface resulting in a lift force. The error in this theory involves the specification of the velocity on the upper surface. In reality, the velocity on the upper surface of a lifting wing is much higher than the velocity which produces an equal transit time. If we know the correct velocity distribution, we can use Bernoulli's equation to get the pressure, then use the pressure to determine the force. But the equal transit velocity is not the correct velocity. Another incorrect theory uses a Venturi flow to try to determine the velocity. But this also gives the wrong answer since a wing section isn't really half a Venturi nozzle. There is also an incorrect theory which uses Newton's third law applied to the bottom surface of a wing. This theory equates aerodynamic lift to a stone skipping across the water. It neglects the physical reality that both the lower and upper surface of a wing contribute to the turning of a flow of gas.
The real details of how an object generates lift are very complex and do not lend themselves to simplification. For a gas, we have to simultaneously conserve the mass, momentum, and energy in the flow. Newton's laws of motion are statements concerning the conservation of momentum. Bernoulli's equation is derived by considering conservation of energy. So both of these equations are satisfied in the generation of lift; both are correct. The conservation of mass introduces a lot of complexity into the analysis and understanding of aerodynamic problems. For example, from the conservation of mass, a change in the velocity of a gas in one direction results in a change in the velocity of the gas in a direction perpendicular to the original change. This is very different from the motion of solids, on which we base most of our experiences in physics. The simultaneous conservation of mass, momentum, and energy of a fluid (while neglecting the effects of air viscosity) are called the Euler Equations after Leonard Euler. Euler was a student of Johann Bernoulli, Daniel's father, and for a time had worked with Daniel Bernoulli in St. Petersburg. If we include the effects of viscosity, we have the Navier-Stokes Equations which are named after two independent researchers in France and in England. To truly understand the details of the generation of lift, one has to have a good working knowledge of the Euler Equations.
Lift is the force that holds an aircraft in the air. How is lift generated? There are many explanations for the generation of lift found in encyclopedias, in basic physics textbooks, and on Web sites. Unfortunately, many of the explanations are misleading and incorrect. Theories on the generation of lift have become a source of great controversy and a topic for heated arguments for many years.
The proponents of the arguments usually fall into two camps: (1) those who support the "Bernoulli" position that lift is generated by a pressure difference across the wing, and (2) those who support the "Newton" position that lift is the reaction force on a body caused by deflecting a flow of gas. Notice that we place the names in quotation marks because neither Newton nor Bernoulli ever attempted to explain the aerodynamic lift of an object. The names of these scientists are just labels for two camps.
Looking at the lives of Bernoulli and Newton we find more similarities than differences. On the figure at the top of this page we show portraits of Daniel Bernoulli, on the left, and Sir Isaac Newton, on the right. Newton worked in many areas of mathematics and physics. He developed the theories of gravitation in 1666, when he was only 23 years old. Some twenty years later, in 1686, he presented his three laws of motion in the Principia Mathematica Philosophiae Naturalis . He and Gottfried Leibnitz are also credited with the development of the mathematics of Calculus. Bernoulli also worked in many areas of mathematics and physics and had a degree in medicine. In 1724, at age 24, he had published a mathematical work in which he investigated a problem begun by Newton concerning the flow of water from a container and several other problems involving differential equations. In 1738, his work Hydrodynamica was published. In this work, he applied the conservation of energy to fluid mechanics problems.
Which camp is correct? How is lift generated?
When a gas flows over an object, or when an object moves through a gas, the molecules of the gas are free to move about the object; they are not closely bound to one another as in a solid. Because the molecules move, there is a velocity associated with the gas. Within the gas, the velocity can have very different values at different places near the object. Bernoulli's equation, which was named for Daniel Bernoulli, relates the pressure in a gas to the local velocity; so as the velocity changes around the object, the pressure changes as well. Adding up (integrating) the pressure variation times the area around the entire body determines the aerodynamic force on the body. The lift is the component of the aerodynamic force which is perpendicular to the original flow direction of the gas. The drag is the component of the aerodynamic force which is parallel to the original flow direction of the gas. Now adding up the velocity variation around the object instead of the pressure variation also determines the aerodynamic force. The integrated velocity variation around the object produces a net turning of the gas flow. From Newton's third law of motion, a turning action of the flow will result in a re-action (aerodynamic force) on the object. So both "Bernoulli" and "Newton" are correct. Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object. We can use equations developed by each of them to determine the magnitude and direction of the aerodynamic force.
What is the argument?
Arguments arise because people mis-apply Bernoulli and Newton's equations and because they over-simplify the description of the problem of aerodynamic lift. The most popular incorrect theory of lift arises from a mis-application of Bernoulli's equation. The theory is known as the "equal transit time" or "longer path" theory which states that wings are designed with the upper surface longer than the lower surface, to generate higher velocities on the upper surface because the molecules of gas on the upper surface have to reach the trailing edge at the same time as the molecules on the lower surface. The theory then invokes Bernoulli's equation to explain lower pressure on the upper surface and higher pressure on the lower surface resulting in a lift force. The error in this theory involves the specification of the velocity on the upper surface. In reality, the velocity on the upper surface of a lifting wing is much higher than the velocity which produces an equal transit time. If we know the correct velocity distribution, we can use Bernoulli's equation to get the pressure, then use the pressure to determine the force. But the equal transit velocity is not the correct velocity. Another incorrect theory uses a Venturi flow to try to determine the velocity. But this also gives the wrong answer since a wing section isn't really half a Venturi nozzle. There is also an incorrect theory which uses Newton's third law applied to the bottom surface of a wing. This theory equates aerodynamic lift to a stone skipping across the water. It neglects the physical reality that both the lower and upper surface of a wing contribute to the turning of a flow of gas.
The real details of how an object generates lift are very complex and do not lend themselves to simplification. For a gas, we have to simultaneously conserve the mass, momentum, and energy in the flow. Newton's laws of motion are statements concerning the conservation of momentum. Bernoulli's equation is derived by considering conservation of energy. So both of these equations are satisfied in the generation of lift; both are correct. The conservation of mass introduces a lot of complexity into the analysis and understanding of aerodynamic problems. For example, from the conservation of mass, a change in the velocity of a gas in one direction results in a change in the velocity of the gas in a direction perpendicular to the original change. This is very different from the motion of solids, on which we base most of our experiences in physics. The simultaneous conservation of mass, momentum, and energy of a fluid (while neglecting the effects of air viscosity) are called the Euler Equations after Leonard Euler. Euler was a student of Johann Bernoulli, Daniel's father, and for a time had worked with Daniel Bernoulli in St. Petersburg. If we include the effects of viscosity, we have the Navier-Stokes Equations which are named after two independent researchers in France and in England. To truly understand the details of the generation of lift, one has to have a good working knowledge of the Euler Equations.
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About 75% of the lift is due to upwash/downwash. Look underneath a helicopter. The grass will be blown flat, only possible due to the wind going down underneath the wing/rotor. Action going down is reaction going up of the rotor itself. Same with a paper airplane which has no airfoil, and same with an aircraft flying on its side.
25% rest is Bernouilli.
25% rest is Bernouilli.
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About 75% of the lift is due to upwash/downwash
Hold your hand palm down out of the window of a moving car.Tilt the angle of your hand up ,your whole hand then raises.Tilt your hand down,your entire hand moves down. 
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A helicopter flies by directing the thrust downwards to overcome its weight. In the same way that a harrier jet does on take-off.
A red arrow flies at 90 degree bank by a slightly positive AoA and raw thrust/weight ratio to literally drag it up through the air to keep it level.
It has nothing to do with the aerodynamics of a fixed wing a/c creating lift.
A red arrow flies at 90 degree bank by a slightly positive AoA and raw thrust/weight ratio to literally drag it up through the air to keep it level.
It has nothing to do with the aerodynamics of a fixed wing a/c creating lift.
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Sorry fellas but how you describe the way choppers fly is incorrect.
They are just so ugly the earth repels them, just like the A380!
TWT, thats a good explanation. True or not well, that will be argued about but all i know is that it works!
They are just so ugly the earth repels them, just like the A380!
TWT, thats a good explanation. True or not well, that will be argued about but all i know is that it works!
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beachbumflyer's post #41 and cc180's #57 address "knife-edge" flight.
Sorry, but it IS an aerodynamic exercise; the fuselage and fin, rolled to 90 degrees, does form an (albeit imperfect) airfoil. It creates lift (opposite to gravity) because of its AOA, while the wing is now at a zero-lift AOA.
To be fair, since its AOA is 10-15 deg., the prop axis is at this same angle, and thus it's creating some lift too.
So it's not a brute-force "hanging on the prop" condition, but just a poor airfoil with very low aspect ratio making do as a wing.
Sorry, but it IS an aerodynamic exercise; the fuselage and fin, rolled to 90 degrees, does form an (albeit imperfect) airfoil. It creates lift (opposite to gravity) because of its AOA, while the wing is now at a zero-lift AOA.
To be fair, since its AOA is 10-15 deg., the prop axis is at this same angle, and thus it's creating some lift too.
So it's not a brute-force "hanging on the prop" condition, but just a poor airfoil with very low aspect ratio making do as a wing.
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I agree with one of the paragraphs posted by Brian Abraham below...
Surely Bernouilli's theory is not completely relevant when discussing wings because his theory talks of a venturi.
Is it not wrong as well as inaccurate to assume that all the effects that occur in a venturi will be the same if you were to cut the venturi in half, ie. a wing?
Surely Bernouilli's theory is not completely relevant when discussing wings because his theory talks of a venturi.
Is it not wrong as well as inaccurate to assume that all the effects that occur in a venturi will be the same if you were to cut the venturi in half, ie. a wing?
