Inverted Flight
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USMCmech
Well-Known Member
Most areobatic airplanes that can fly inverted have symetrical airfoils.
When flying upside down, all the normal rules for lift still apply, except the built in camber of the wing is working against you, not helping.
They still must have an "positive" AOA, which causes the air flowing over the "top" of the wing to flow faster creating lift.
Rember, the plane dosen't care what you see out the window, it only care about the forces acting on it.
When flying upside down, all the normal rules for lift still apply, except the built in camber of the wing is working against you, not helping.
They still must have an "positive" AOA, which causes the air flowing over the "top" of the wing to flow faster creating lift.
Rember, the plane dosen't care what you see out the window, it only care about the forces acting on it.
Grabo172
Well-Known Member
It really doesn't matter too much about Bernoulli's principle, as long as the airplane has a + AOA like said above. Bernoulli only produces about 10 % of the airplanes lift. The airplane just needs to be pushing down enough air to support its weight (ie Newton's 2nd Law of Motion.)
The fact that aerobatic airplanes have the symmetric wings is to fly upsidedown efficiently. You can fly on a flat board if you want, as long as you can push down enough air, but Bernoulli's priciple adds that extra bit of lift and a WHOLE lot of efficiency.
(yes I've been reading Stick and Rudder again)
The fact that aerobatic airplanes have the symmetric wings is to fly upsidedown efficiently. You can fly on a flat board if you want, as long as you can push down enough air, but Bernoulli's priciple adds that extra bit of lift and a WHOLE lot of efficiency.
(yes I've been reading Stick and Rudder again)
seagull
Well-Known Member
AAARGH!
The lift is ALL Bernoulli, where do people pick up this stuff? The positive AoA leads to the rotational flow which leads to higher velocity over the top of the wing, which leads to lower pressure over the top, which lifts the wing. The downward flow aft of the wing is a RESULT of lift, not the cause of it. If it was the cause, how would lift happen in ground effect?
Camber just makes the wing more efficient in normal flight, it does not preclude inverted flight, just makes it less efficient.
The lift is ALL Bernoulli, where do people pick up this stuff? The positive AoA leads to the rotational flow which leads to higher velocity over the top of the wing, which leads to lower pressure over the top, which lifts the wing. The downward flow aft of the wing is a RESULT of lift, not the cause of it. If it was the cause, how would lift happen in ground effect?
Camber just makes the wing more efficient in normal flight, it does not preclude inverted flight, just makes it less efficient.
Grabo172
Well-Known Member
[ QUOTE ]
AAARGH!
If it was the cause, how would lift happen in ground effect?
[/ QUOTE ]
The fact that the wing forces the air down... there is still 3 feet of air below a piper wing and 5 feet of air below a cessna wing... in fact I can go further and say that the air after being pushed down and hits the ground acts to add more lift as it has a surface to act upon thus giving the wing more lift at a slower speed than can sustain flight outside of ground effect...
Here's an excerpt from NASA...
[ QUOTE ]
What causes lift in an airplane? There are two current interpretations to explain lift
of an airplane. These two interpretations have been debated for many years, thus
they are both presented to you.
The first explanation is the pro-Newton, which
states that wings are forced upward because they are tilted and deflect air
downwards. Both the upper and lower surfaces of the wing act to deflect the air.
The upper surface deflects air downwards because the airflow "sticks" to the wing
surface and follows the tilted wing. This interpretation is called the "Coanda effect."
The second explanation is the pro-Bernoulli or Airfoil-Shape, which states that
wings do not deflect air; instead they are pushed upwards. Because air is a
continuum, when it is divided at the leading edge of a wing, it must rejoin at the
trailing edge. The curvature of the “airfoil” shape causes the air to flow faster over
the upper surface than over the lower surface, which creates a lower pressure
above the wing than below the wing (figure 2).
[/ QUOTE ]
We can fight about this all day but both methods are widely accepted by many people. You like your way I like my way...
AAARGH!
If it was the cause, how would lift happen in ground effect?
[/ QUOTE ]
The fact that the wing forces the air down... there is still 3 feet of air below a piper wing and 5 feet of air below a cessna wing... in fact I can go further and say that the air after being pushed down and hits the ground acts to add more lift as it has a surface to act upon thus giving the wing more lift at a slower speed than can sustain flight outside of ground effect...
Here's an excerpt from NASA...
[ QUOTE ]
What causes lift in an airplane? There are two current interpretations to explain lift
of an airplane. These two interpretations have been debated for many years, thus
they are both presented to you.
The first explanation is the pro-Newton, which
states that wings are forced upward because they are tilted and deflect air
downwards. Both the upper and lower surfaces of the wing act to deflect the air.
The upper surface deflects air downwards because the airflow "sticks" to the wing
surface and follows the tilted wing. This interpretation is called the "Coanda effect."
The second explanation is the pro-Bernoulli or Airfoil-Shape, which states that
wings do not deflect air; instead they are pushed upwards. Because air is a
continuum, when it is divided at the leading edge of a wing, it must rejoin at the
trailing edge. The curvature of the “airfoil” shape causes the air to flow faster over
the upper surface than over the lower surface, which creates a lower pressure
above the wing than below the wing (figure 2).
[/ QUOTE ]
We can fight about this all day but both methods are widely accepted by many people. You like your way I like my way...
Grabo172
Well-Known Member
If you want to see the effects of camber and AOA and such you can play with...
Nasa Foil 1.4
And see that BOTH have an effect on lift and you can even play with inverted flight charicteristics.
Nasa Foil 1.4
And see that BOTH have an effect on lift and you can even play with inverted flight charicteristics.
seagull
Well-Known Member
FIrst off, notice that nowhere in NASA does it say it's "part Newton and part Bernoulli". Note that. There is something called impact lift, which does play a major factor at very high altitude hypersonic speeds, but is virtually negligible below that.
Now, the issue is between what you found on the NASA site. That is a nice engineering model, but it begs the question of "why". Nothing ever moves because it is "pulled", it is always pushed, this is basic Newtonian physics (which, of course, is just a rough model because it doesn't include relativisitic effects).
The air does stick to the surface, but what is actually causing the force itself. It is not Newton's third law. If you look at Newton's laws, they are all very intuitive, but often misapplied.
Bernoulli could be considered an adaption of Newton's laws also, for that matter. The Newton model you described is a nice model, but does not tell you "why". It is like the description of how swept wings reduce the effective mach number by dividing up the flow into vectors, one along the leading edge and the other perpendicular. Nice, you can make some calc's, but does not tell you WHY. Don't confuse models with how something really works.
Reading your excerpt again, I find it hard to believe that NASA would put that. The notion that the air above and below the wing have to "join" at the trailing edge is flat out wrong. In fact, the air across the top gets there well ahead of the air on the bottom.
As to your "the air hits the ground and pushes". Pushes against what? Do you think a balloon that you untie and let go flys around becase the air is "pushing" against the air it hits as it leaves the open end? Is that how rockets work, they "push" the air the exhaust hits (oops, they work in a vacuum too).
I do not mean this to sound harsh, but think about the actual physical process that's involved here, not a esoteric model that is nice to use to make these things opaque.
Now, the issue is between what you found on the NASA site. That is a nice engineering model, but it begs the question of "why". Nothing ever moves because it is "pulled", it is always pushed, this is basic Newtonian physics (which, of course, is just a rough model because it doesn't include relativisitic effects).
The air does stick to the surface, but what is actually causing the force itself. It is not Newton's third law. If you look at Newton's laws, they are all very intuitive, but often misapplied.
Bernoulli could be considered an adaption of Newton's laws also, for that matter. The Newton model you described is a nice model, but does not tell you "why". It is like the description of how swept wings reduce the effective mach number by dividing up the flow into vectors, one along the leading edge and the other perpendicular. Nice, you can make some calc's, but does not tell you WHY. Don't confuse models with how something really works.
Reading your excerpt again, I find it hard to believe that NASA would put that. The notion that the air above and below the wing have to "join" at the trailing edge is flat out wrong. In fact, the air across the top gets there well ahead of the air on the bottom.
As to your "the air hits the ground and pushes". Pushes against what? Do you think a balloon that you untie and let go flys around becase the air is "pushing" against the air it hits as it leaves the open end? Is that how rockets work, they "push" the air the exhaust hits (oops, they work in a vacuum too).
I do not mean this to sound harsh, but think about the actual physical process that's involved here, not a esoteric model that is nice to use to make these things opaque.
Grabo172
Well-Known Member
[ QUOTE ]
Do you think a balloon that you untie and let go flys around becase the air is "pushing" against the air it hits as it leaves the open end?
[/ QUOTE ]
Nope, it's being propelled by the gasses escaping the nozzel of the balloon. Therefore as the air is forced out of the balloon, the air acts on the ballon pushing it forward. Hence Newton's 3rd LAW (yes it's a LAW) of Motion. "For every action there is an equal and opposite reaction"
[ QUOTE ]
Is that how rockets work, they "push" the air the exhaust hits (oops, they work in a vacuum too).
[/ QUOTE ]
nope, see above
[ QUOTE ]
I do not mean this to sound harsh, but think about the actual physical process that's involved here, not a esoteric model that is nice to use to make these things opaque.
[/ QUOTE ]
You do sound harsh, and there's no reason for it. I fully understand the physics involved here. Your arguments sound like they are directed at a Jr. High student, very condescending. I work with Bernoulli's Principle every day in my day job, either teaching it to my students or using it, working in the propulsion plant.
The fact is that the high pressure below the wing is produced by BOTH Bernoulli's Principle AND Newton's Laws. Bernoulli's is a fluid flow high pressure and Newton's is an impact high pressure. They are BOTH present. They BOTH contribute to lift.
Do you think a balloon that you untie and let go flys around becase the air is "pushing" against the air it hits as it leaves the open end?
[/ QUOTE ]
Nope, it's being propelled by the gasses escaping the nozzel of the balloon. Therefore as the air is forced out of the balloon, the air acts on the ballon pushing it forward. Hence Newton's 3rd LAW (yes it's a LAW) of Motion. "For every action there is an equal and opposite reaction"
[ QUOTE ]
Is that how rockets work, they "push" the air the exhaust hits (oops, they work in a vacuum too).
[/ QUOTE ]
nope, see above
[ QUOTE ]
I do not mean this to sound harsh, but think about the actual physical process that's involved here, not a esoteric model that is nice to use to make these things opaque.
[/ QUOTE ]
You do sound harsh, and there's no reason for it. I fully understand the physics involved here. Your arguments sound like they are directed at a Jr. High student, very condescending. I work with Bernoulli's Principle every day in my day job, either teaching it to my students or using it, working in the propulsion plant.
The fact is that the high pressure below the wing is produced by BOTH Bernoulli's Principle AND Newton's Laws. Bernoulli's is a fluid flow high pressure and Newton's is an impact high pressure. They are BOTH present. They BOTH contribute to lift.
Grabo172
Well-Known Member
[ QUOTE ]
How does an aircraft fly upside down? I don't see how Bernoulli's Principle can generate lift on an upside down wing.
[/ QUOTE ]
Back to the orignal question...
If Bernoulli's Principle were "ALL" the lift an airfoil gets, then how does it fly upside down? It seems the air would be flowing faster under the wing now and slower on top, then the resulting High Pressure would be on top and Low pressure on bottom, generating a lift DOWN.
Even with a symmetical wing, the air would be at the same velocity top and bottom (same camber same distance to travel) then there would be no difference in pressure and no lift, so the airplane would just fall.
We obviously know this is not the case, an asymmetic wing only cambered on one end can fly inverted, and a symmetrical wing can fly inverted, it's all been done before and done well.
A designer can make a flate plate wing fly. A designer can make a purely cambered wing fly, but the fact is that they work together to create the charicteristics of a contollable, reliable wing we know today.
How does an aircraft fly upside down? I don't see how Bernoulli's Principle can generate lift on an upside down wing.
[/ QUOTE ]
Back to the orignal question...
If Bernoulli's Principle were "ALL" the lift an airfoil gets, then how does it fly upside down? It seems the air would be flowing faster under the wing now and slower on top, then the resulting High Pressure would be on top and Low pressure on bottom, generating a lift DOWN.
Even with a symmetical wing, the air would be at the same velocity top and bottom (same camber same distance to travel) then there would be no difference in pressure and no lift, so the airplane would just fall.
We obviously know this is not the case, an asymmetic wing only cambered on one end can fly inverted, and a symmetrical wing can fly inverted, it's all been done before and done well.
A designer can make a flate plate wing fly. A designer can make a purely cambered wing fly, but the fact is that they work together to create the charicteristics of a contollable, reliable wing we know today.
seagull
Well-Known Member
Your posts do lead me to believe you do not quite understand what Bernoulli actually said. It has nothing to do with camber, per se. Camber is just a way to create rotational flow. A high AoA will also do the same thing, but if a cambered wing is inverted it will just take more AoA. It is fundamentally incorrect to think that the camber is all Bernoulli is talking about. While a venturi tube is one description of Bernoulli principal, it is NOT the only one!
It is flat out incorrect to state that lift is partly Bernoulli and partly Newton. For a basic read, go to the Illustrated Aerodynamics (by prof of Aero at Univ of Indiana, I think) book to understand this. For a nice understanding, read Flightwise by Chris Carpenter (head of Aerodynamics at the Royal Air Force Academy), or, for more in depth, John Anderson's books.
As to your balloon. No, that's not how they work either. The air pressure is pushing outward in a balloon in all directions. In every part, the balloon walls push back, except that one spot where it is open. The side opposite that is getting pushed, so there is not a balance, so it moves. Simple as that.
It is flat out incorrect to state that lift is partly Bernoulli and partly Newton. For a basic read, go to the Illustrated Aerodynamics (by prof of Aero at Univ of Indiana, I think) book to understand this. For a nice understanding, read Flightwise by Chris Carpenter (head of Aerodynamics at the Royal Air Force Academy), or, for more in depth, John Anderson's books.
As to your balloon. No, that's not how they work either. The air pressure is pushing outward in a balloon in all directions. In every part, the balloon walls push back, except that one spot where it is open. The side opposite that is getting pushed, so there is not a balance, so it moves. Simple as that.
MDPilot
Well-Known Member
Try this explanation:
Engineers like the Bernoulli model because this model allows derivation of equations that provide engineering (calculas based) methods of determining airfoil lift and drag in the theoretical world, thus they can design actual wings that act like their theoretical ones.
Physicsists (sp) and flight instructors like the Newton model because it allows them to explain concepts of flight in a verbal (not mathematical) manner, so that it makes sense to their listeners.
Neither model is necessarily "right" or "wrong", they are both methods of explaining "flight." Both models well serve the intended purpose of their users.
Engineers like the Bernoulli model because this model allows derivation of equations that provide engineering (calculas based) methods of determining airfoil lift and drag in the theoretical world, thus they can design actual wings that act like their theoretical ones.
Physicsists (sp) and flight instructors like the Newton model because it allows them to explain concepts of flight in a verbal (not mathematical) manner, so that it makes sense to their listeners.
Neither model is necessarily "right" or "wrong", they are both methods of explaining "flight." Both models well serve the intended purpose of their users.
seagull
Well-Known Member
MD
CFI's may like the Newton method of explaining things, but that is usually because they learned it that way, but it is really not a very good way to understand it.
Physicists, or at least those that actually study Aero, so usually referred to as Aerodynamicists, do not like that Newton method of explaining things, not sure where you got that.
No math is needed to explain Bernoulli, incidentally.
CFI's may like the Newton method of explaining things, but that is usually because they learned it that way, but it is really not a very good way to understand it.
Physicists, or at least those that actually study Aero, so usually referred to as Aerodynamicists, do not like that Newton method of explaining things, not sure where you got that.
No math is needed to explain Bernoulli, incidentally.
MDPilot
Well-Known Member
Didn't say math was needed to explain Bernoulli, I said Bernoulli was particularly well suited to allowing derivative equations to be created in order to resolve lift and drag for infinite wing sections. Newton is definitely NOT.
Neither the classic Newton or Bernoulli methods explain the complete "mystery" of flight. Both have drawbacks, depending on the engineering assumptions that are made in the beginning. Newton is easier for a "aerodynamic layman" to understand, Bernoulli actually allows aerodynamicists and engineers to accurately calculate lift and drag results for both theoretical and real wings, and allow computer flow modeling to achieve accurate results.
Don't know why you are arguing with me seagull, I'm on your side.
Neither the classic Newton or Bernoulli methods explain the complete "mystery" of flight. Both have drawbacks, depending on the engineering assumptions that are made in the beginning. Newton is easier for a "aerodynamic layman" to understand, Bernoulli actually allows aerodynamicists and engineers to accurately calculate lift and drag results for both theoretical and real wings, and allow computer flow modeling to achieve accurate results.
Don't know why you are arguing with me seagull, I'm on your side.
USMCmech
Well-Known Member
[ QUOTE ]
Even with a symmetical wing, the air would be at the same velocity top and bottom (same camber same distance to travel) then there would be no difference in pressure and no lift, so the airplane would just fall.
[/ QUOTE ]
This would only be true at 0 deg AOA.
"normal" wings have camber to increase efficency, but the camber does not creat lift on its own. It adds to the efficency (acro planes are redicusly slow).
Cambered wings also require a positive AOA. They can fly inverted, but they require a much higher AOA when inverted as opposed to flying upright.
Even with a symmetical wing, the air would be at the same velocity top and bottom (same camber same distance to travel) then there would be no difference in pressure and no lift, so the airplane would just fall.
[/ QUOTE ]
This would only be true at 0 deg AOA.
"normal" wings have camber to increase efficency, but the camber does not creat lift on its own. It adds to the efficency (acro planes are redicusly slow).
Cambered wings also require a positive AOA. They can fly inverted, but they require a much higher AOA when inverted as opposed to flying upright.
seagull
Well-Known Member
As was said, a cambered wing flies inverted just fine, just not as efficiently as one that is not.
Something about the post talking about "how far" the air has to travel gnaws at me, as it leads me to suspect that the writer believs the air over the top and bottom of the wing reach the trailing edge at the same time. Certainly implies that.
Incidentally, if you tried to fly a symetrical wing at zero AoA, you would rapidly have a positive AoA as the flight path changed as it started down....
Something about the post talking about "how far" the air has to travel gnaws at me, as it leads me to suspect that the writer believs the air over the top and bottom of the wing reach the trailing edge at the same time. Certainly implies that.
Incidentally, if you tried to fly a symetrical wing at zero AoA, you would rapidly have a positive AoA as the flight path changed as it started down....