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Old Sep 10, 2012, 03:12 AM
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Congratulations on your years of experience. It is possible your experience with turbomachinery may not have offered significant insight into how wings generate lift at low Mach numbers. Although the air is compressible at all Mach numbers, it can be shown that as long as local Mach numbers in the vicinity of a wing remain below about half the speed of sound, the wing will affect the density of the surrounding air by only a fraction of a percent. In this Mach number regime you predict the same values for the aerodynamic forces whether you include or neglect the air's compressibility.

Your observation that an airplane flies due to buoyancy is an interesting one. I'll have to check the numbers on that.
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Old Sep 10, 2012, 07:26 AM
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Originally Posted by Truth Squad View Post
I am sorry to hear that it’s getting worse. I hear they are making great progress to a cure.

Who’s Frankly?
You really don't understand how air interacts with solid objects, do you ?
Sorry I even tried to be civil .
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Old Sep 10, 2012, 08:50 AM
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Originally Posted by c131frdave View Post
They displace an equal mass of air as the aircraft per unit of time.
Do you have anything to back up this claim?
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Old Sep 10, 2012, 09:08 AM
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Originally Posted by c131frdave View Post
The "downwash" is the displacement of air behind the wing as it passes through a body of air and immediatly and violently rises back up, which is a phenominon known as wake turbulence (and several other forms of turbulence). Yes, it is not uniform or even remotely organized (though it is far better today than it was 50 years ago)
I see where you're coming from... not even remotely organized.

It is a relief to learn the downwash has made progress toward getting itself organized. Perhaps recent aerotheological advances will spur even more progress in the next 50 years. Turbulence and violent uprisings do nobody any good.
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Old Sep 10, 2012, 09:17 AM
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and I thought Dilbert was funny
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Old Sep 10, 2012, 09:18 AM
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Quote:
Originally Posted by c131frdave View Post
First, air is VERY compressable. Extremely compressable, especially at altitude.
Actually, it can be shown using isentropic gas relations that as long as the flow remains below about Mach 0.3, the density change is less than 5%. Altitude doesn't have anything to do with it. Of course, this assumes there is no heat added to or removed from the flow. You might not encounter such conditions very often in turbomachinery.

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Second, air has no momentum.
Momentum is relative to the reference frame, as it depends on velocity. That's why momentum itself is not that important, but its time rate of change is (see Newton's 2nd Law).

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But again, AIR IS EXTREMELY COMPRESSABLE, and the effects on the planet is zero.
That is due to viscosity, not compressibility.
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Old Sep 10, 2012, 09:33 AM View Post
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A moderator felt this post violated the following rule: Personal Attack. Show it to me anyway.
Old Sep 10, 2012, 10:09 AM
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You donít know how to be civil. Thatís one of the reasons so many of us dislike you.
.
Go away.
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Old Sep 10, 2012, 10:41 AM View Post
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Old Sep 10, 2012, 10:55 AM
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If you enjoy lurking - continue - If you have something to add or discuss without being unpleasant - join in.
Simply disagreeing or making derogatory remarks because you don't have a relevant answer, is not helpful to anyone..
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Old Sep 11, 2012, 09:52 AM
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Old Sep 11, 2012, 12:23 PM View Post
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Old Sep 12, 2012, 06:19 AM
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With the sideshows of late, any focus this thread might have had has eroded. I'm going to make an effort to get back to what I think is the fundamental question here:

Do wings in steady flight generate lift purely by transferring momentum to the air, or are there circumstances where other mechanisms are involved?

It has been suggested that: "The wing develops lift by transferring momentum to the air. Momentum is mass times velocity. In straight-and-level-flight the momentum is transferred toward the earth. This momentum eventually strikes the earth. If an airplane were to fly over a very large scale, the scale would weigh the airplane. This should not be confused with the (wrong) concept that the earth somehow supports the airplane. It does not. Lift on a wing is very much like shooting a bullet at a tree. Lift is like the recoil that the shooter feels, whether the bullet hits the tree or not. If the bullet hits the tree, the tree experiences the event, but has nothing to do with the recoil of the gun."*

Consider how the gun/bullet/tree analogy applies to a rocket that has just lifted off from the moon. Consistent with the above discussion, we imagine the rocket's propellant as a stream of bullets. As the bullets hit the moon, they are absorbed inelastically (brought to rest in the moon's surface rather than bouncing back up). If the bullets are spaced such that there is no physical contact between them in flight, it should be apparent that the impact of a bullet with the moon's surface in no way influences following bullets. As long as it operates in a vacuum and its propellant collides inelastically with any nearby surface, the rocket's performance is independent of altitude. In this scenario, the gun/bullet/tree analogy provides an accurate description of the underlying physics.

If the gun/bullet/tree analogy accurately describes a lifting wing, then a wing's performance should also be independent of its altitude. It is well documented that a wing's performance is significantly affected by proximity to the ground. Unlike a disconnected bullet that flies the same path to the ground regardless of its predecessor's fate, air influenced by a wing is very much connected to the surrounding air. The path that air takes when influenced by a wing is different depending on how close the surrounding air is to the ground. Wings are not rockets, and it should be apparent the gun/bullet/tree analogy applied to a wing provides a very misleading description of how a wing develops lift.

*"Understanding Flight", David F. Anderson and Scott Eberhardt, McGraw-Hill, 2001
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Old Sep 12, 2012, 08:43 AM
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[QUOTE=ShoeDLG;22711145]With the sideshows of late, any focus this thread might have had has eroded. I'm going to make an effort to get back to what I think is the fundamental question here:

Do wings in steady flight generate lift purely by transferring momentum to the air, or are there circumstances where other mechanisms are involved?


I take" the most simple answer is probably the right answer" approach.
Pressure difference generated, explains it for me
Which side is the most influencial ?
Trick question
it does not matter.
How one chooses to explain the diference, flow or measure efficiencies-is another matter- That subject is efficiency.

Lift is just pressure difference.
No/yes?
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Old Sep 12, 2012, 09:25 AM
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Quote:
Originally Posted by richard hanson View Post
I take" the most simple answer is probably the right answer" approach.
Pressure difference generated, explains it for me
Which side is the most influencial ?
Trick question
it does not matter.
How one chooses to explain the diference, flow or measure efficiencies-is another matter- That subject is efficiency.

Lift is just pressure difference.
No/yes?
What causes the pressure difference?
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