I know this was hot and much discussed topic in the past (last one was 10 years ago if I'm correct) but that fact doesn't help me.
I am deep into the flight training now, yet one of the most basic things is not crystal clear to me (thanks to instructors? Books?) as it should be - lift.
There are several ideas on how is lift created and I often see people disagree with each other (including this forum, pprune and others), and all have some pretty solid references behind them.
The best learning is learning from multiple sources, everyone say. That's what I do.
Yet when I am being asked "what is lift and how is it created?" I still can't answer with confidence.
I will present a few short cuts from training material. Notice the emphasis on just one idea or mixing the several.
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[from Jeppesen's "Principles of Flight"]
As air flows around an airfoil the pressure differential set up over the upper and lower surfaces
produces a force. This force acts perpendicular to the relative airflow and is known as lift.
[from Oxford's "Principles of Flight"]
The aerodynamic force of lift results from the pressure differential between the top and bottom
surfaces of the wing.
[from Burke's "Principles of Flight"]
Since for every action there is an equal and opposite reaction (Newton’s Third Law of Motion), an additional upward force is generated as the lower surface of the wing deflects the air downward. Thus both the development of low pressure above the wing and reaction to the force and direction of air as it is deflected from the wing’s lower surface contribute to the total lift generated.
[from FAA's "Pilot's Handbook Of Aeronautical Knowledge]
Lift opposes the downward force of weight, is produced by the dynamic effect of the air acting on the airfoil and acts perpendicular to the flightpath through the center of lift.
[from Davies' "Handling The Big Jets"]
A wing produces lift by accelerating the air which passes over the top surface to a higher speed than that which passes under the bottom surface. The greater the difference between these two speeds the higher the difference in pressure, hence the larger the lift vector.
[from Wolke's "What Einstein Told His Barber"]
... We can give Isaac Newton still more credit for lifting airplanes, because the lift doesn't all come from downwash (with a slight assist by Mr. Bernoulli). Some of it comes from yet another application of Newton's Third Law. Airplane wings are not parallel to the ground; they are made to be tilted slightly upward in front—usually about 4 degrees when the plane is in level flight. That makes more pressure on the bottom surface than on the top, thereby pushing the wing upward and contributing to the lift. The pilot can increase the angle of attack to get even more lift from this effect. Sir Isaac's Third comes in because as the plane moves, the wing is pushing the air down in front of it, so the air responds by pushing the wings up...
[from Preston's "Aerodynamics For Professional Pilots"]
1. A solid wing passing through the air accelerates air over its top surface in accordance with the continuity equation
2. In accordance with Bernoulli‟s equation: increased velocity causes reduced static pressure above the wing
3. Static pressure below the wing remains unchanged and is greater than pressure above the wing
4. The difference in pressure between the top and bottom of the wing results in lift
5. The total lift is the net pressure pushing on the entire surface area of the wing (S.)
6. In accordance with rule 3 the wing provides an equal but opposite force to the airflow, deflecting the passing air downward.
So it is unclear to me, is Mr. Bernoulli right or is it Mr. Newton, or both of them?
Math, physics and aerodynamics experts fight over the basic principle of flight and refuse to agree that there might be more than one reason for lift involved.
I don't know if you would believe, but I got two different explanations from two different CFI's.
Some also say it depends on shape of the wing (symmetrical or cambered).
Other say it doesn't matter because "equal transition time" Bernoulli's principle works on is wrong!
Lastly, is there something that we may say "this is 100% correct", not just speculate?
How we new pilots should learn what's correct? I will be very grateful if anyone could clarify things and maybe provide a source.
[Edited 2012-12-01 16:07:44]
[Edited 2012-12-01 16:53:14]