Correctness for Misconceptions about Aircraft - 2

1. The upper surface of a wing will deflect air, but the lower surface is horizontal, so it has little effect. INCORRECT.

Incorrect, but for an interesting reason.

If a thin flat wing deflects air downwards, the air above the wing and the air below the wing are equally deflected. Both the upper and lower surfaces create the lifting force. If we then make this wing thicker and streamlined, the total amount of deflected air and the lifting force remain the same... but the air below the wing APPEARS less deflected, and the air above the wing appears more deflected. Also, the pressure below the wing APPEARS to provide less lift.

This happens because a thick wing must push air out of its way, and as the flowing air splits up and down to make a space for the oncoming wing, air below the wing takes a straighter path. It takes a straighter path because the thickness of the wing bends air upwards at the same time as the tilt of the wing bends air downwards. This has no effect on the lifting force, since the air above the wing takes a more curved path, so THE PRESSURE DIFFERENCE REMAINS THE SAME AS IT WAS FOR A THIN WING.

The thick wing is making us confused. The thick wing SEEMS to get more lift from the curved streamlines above the wing than from the straight streamlines below, but this is an illusion. The thick wing distorts the streamlines. Examine the streamlines surrounding a thin wing to see the truth. The lift comes from the DIFFERENCE between the two flows, and changing the thickness of the wing will alter the appearance of the air flows without changing the difference or changing the lifting force.


2. The Bernoulli effect pertains to the shape of the wing, while Newton's laws pertain to the angle of attack. INCORRECT.


Incorrect because Newton's laws pertain to all features of the wing; both to wing shape and attack angle. Exactly the same thing is true of Bernoulli's equation: angle of attack is critical, but wing shape has effects too. Wings don't violate Newton's laws, and wings in conventional flight (slower than the speed of sound) don't violate Bernoulli's equation.

3.In order to generate lift, the upper surface of an airfoil must be more strongly curved than the lower surface? INCORRECT.


Incorrect, since lift can be generated by symmetrical airfoil such as those used on acrobatic aircraft. Lift can also be generated by thin fabric airfoils, by sheets of paper (paper airplanes), by tilted pieces of flat plywood, or by "super-critical" airfoils which are more curved on the BOTTOM than the top.


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