How does an airfoil work?

Answer 1

THE REASONS FOR THE SHAPE

To avoid flow-detachment and "stall," the leading edge of an airfoil must be rounded.

To produce flow-deflection as well as the "circulation" required for lift, the trailing edge of an airfoil must be fairly sharp.

If we need a round leading edge and a sharp trailing edge, then an airfoil must look like a streamlined teardrop shape.

In order to produce a lifting force, an airfoil must induce "circulation" by deflecting air downwards from its trailing edge. (Because of this downwards deflection, air in front of the leading edge will also be deflected upwards at the same time.)

There are two ways to accomplish this deflection of air: 1.Tilt the entire airfoil at a positive angle of attack with respect to oncoming air. 2. Give the airfoil an arch shape, so it's curved upwards in the center. This curve is called "camber." Notice that the trailing edge of a cambered airfoil is tilted downwards. Because of this downwards tilt, the air will flow downwards off the trailing edge. The cambered shape also acts to prevent stall because the leading edge is tilted downwards to intercept the upflowing air.

End result: an airfoil looks like a streamlined teardrop shape which is bent upwards in the center.

BERNOULLI EFFECT AND PRESSURE

Whenever the trailing edge of an airfoil causes air to move downwards, two other things occur. First, the air ahead of the airfoil will move upwards. Second, the air above the airfoil will speed up, and the air below the airfoil will slow down. Each fast-moving parcel of air above the airfoil greatly outraces its counterpart flowing below. Air divided by the airfoil doesn't rejoin again, instead a narrow region of fast flowing air appears above the airfoil, and a wide region of slow air appears below. Because of the "Bernoulli Effect," relatively fast air exerts less pressure than slow air. This difference in pressure above and below the wing will create an upward force: the "lifting force." End result: if we know the velocity of the air above and below the wing, then we can calculate the difference in pressure and discover the value of lifting force.

NEWTON'S TWO LAWS TOO

An airfoil can only deflect air downwards if it applies a downward force to the air. And since parcels of air have mass, any downward acceleration of air must be injecting momentum into the air. If the air gains downward momentum, then the airfoil must gain some upward momentum at the same rate. And if air is forced downwards, then the airfoil must be forced upwards. These are the simple consequences of Newton's 2nd and 3rd laws: F=mA, and each force being paired with an equal and opposite force. End result: if we know the pattern of velocity of air surrounding an airfoil, we can calculate the momentum flow and the lifting force.

So, how does an airfoil REALLY work? Is it because the wing is tilted? YES, the trailing edge of the wing must deflect air downwards. And is it because the airfoil has a special shape? YES, the angle of the trailing edge is caused by airfoil camber and airfoil angle.

AVOID THESE MISCONCEPTIONS

Where airfoils are concerned, some books contain many mistakes. Here are some errors to avoid:

1. THE EQUAL TRANSIT-TIME FALLACY Some authors think that the upper surface of a wing must be longer than the lower surface. They also think that the parcels of air being divided by a wing must rejoin each other after the wing has passed by. Both of these ideas are wrong. First, the parcels of air do not rejoin: wind tunnel smoke-pulse photographs clearly show that no rejoining takes place ...unless the wing is tilted so it produces zero lifting force. In other words, air parcels are permanently split by the wing, and the greater the split, the more lift is produced. Second: the air above a wing greatly outraces the air below, and these two velocities correctly predict the lifting force via Bernoulli's equation. If instead we measure the upper and lower surface of a wing, we'll find that the difference in path length is too small. If we use the path length differences to try to calculate the lifting force, our answer will be too small by several times. (This makes perfect sense of course: if air parcels never rejoin behind the wing, then it becomes pointless to measure the surface path differences.)

2. BERNOULLI-NEWTON PERCENTAGES ERROR Some authors say that Newton's laws predict only part of the lifting force, and Bernoulli's equation predicts the rest. This is wrong. Actually, Bernoulli's equation is based on Newton's laws. Bernoulli's equation predicts 100% of the lifting force. But Newton's laws also predict 100% of the lifting force. Is this insane? Nooooo... because there is no fight between Bernoulli and Newton, they are just two different ways of analyzing a single complicated situation. The only force applied to a wing is the pattern of surface pressures. And the only force applied to the wing is caused by deflected air and momentum-change of the mass-bearing air parcels. Bernoulli and Newton have no fight with each other (but textbook authors sure do!)

3. THE WING SHAPE ERROR Some authors insist that a wing must be curved on top and flat below. Or in other words, wings must be cambered or they won't create lift. This is wrong. Un-cambered wings fly just fine, and are used on high-performace acrobatic aircraft. Also, any plane can fly upside-down, even if this means that the flat side of the airfoil is then positioned on top.

4. THE NO-DEFLECTION ERROR Some textbook diagrams show that air isn't deflected by a wing. They show the air approaching the wing horizontally and leaving horizontally. This is wrong. In real wings the air curves upwards to meet the oncoming airfoil, and it's deflected downwards by the airfoil's trailing edge. If air wasn't deflected like this, then the air above an airfoil would move as fast as the air below, and the lifting force would be zero. Additionally... air behind a real wing will keep flowing downwards long after the wing has passed by. In real aircraft the air ahead far ahead of the aircraft is undisturbed, but the air behind the aircraft contains a downwards-moving "wake."