Thursday, March 17, 2011

Planes can fly!

It all started out with that blasted xkcd comic! It came up at random and it got my fingers itching to know the answer. So as is my way, i spent an afternoon digging up information, when i finally hit on this gem.

There are a couple of things that need to be noted here :
1. Bernoulli effect : "when the speed of horizontal flow through a fluid increases, the pressure decreases" quoted from wisegeek . I especially liked the analogy about a fluid flowing through a narrowing pipe. It speeds up, but considering that there's no change in mass or gravity, the pressure behind the fluid must have to increase compared to that in front to push the fluid faster.

2. Cambered airfoil : basically this means an unsymmetrical airfoil.

3. To avoid the suspense :-
  • airplanes CAN fly upside down
  • lift is not because of airfoil shape (What a world do we live in when we can't even begin to trust our own school textbooks!), though it does on ;)
  • airfoils can be symmetrical, asymmetrical and even flat :D
4. Two criteria are essential to an airfoil :-
  • It should have a sharp trailing edge
  • the trailing edge should be aimed diagonally downward
Now to the explanation.
Oh wait, first off, forget what you read in school about airfoil shape being responsible for anything....hmm, next i'll have to find out what the different shapes are really for!
One answer that I know of is : the camber is there to prevent stall (reduced lift from upper surface compared to lower surface) and to allow a plane to fly at lower speeds

Two explanations of the lift
1. Newton's 3rd law and Coanda effect
2. Bernoulli effect
According to the source that I've cited, both explanations are equally viable and are not competing in any way...they're just both right!
So, on to the first :-
When an airfoil (remember that this can even be a plank of plywood, though it wouldn't be a very good one) goes through the air at a positive angle of attack, both the upper and lower surface of the airfoil are responsible for the lift; a greater portion of which is from the upper surface. The air along the upper surface is stuck to the surface or attached to it because of the Coanda effect (fluid or gas stream will hug a convex contour when directed at a tangent to that surface) and hence it flows along the upper surface and since the trailing edge of the wing is aimed downwards (see point 4 above), the air is pushed downwards resulting in a downwash. By Newton's 3rd law, this produces an upward force on the airfoil. Now, to the lower surface. At the +ve AoA (fancy abbr for angle of attack), the air is pushed downwards by the lower surface, hence the air pushes upwards. These two forces, combine to produce lift. YAY!

Without the downward deflection of air, the AoA is irrelevant since lift won't be created.

The leading edge of the airfoil splits the airflow current into two, sending one over the upper edge, and another below the lower edge. If the airfoil has a +ve AoA OR if the airfoil has a classic shape (like we've always been taught), then the stream of air travelling above the airfoil will be well over the upper edge.
Enter Bernoulli...The air flow on the upper edge, since it travels in an arc well over the upper edge, causes the creation of a pocket of low pressure. Air therefore rushes in to fill this low pressure area. However, the air flow along the lower edge, since it collides with it, causes a region of high pressure to develop. This high pressure region slows the air stream on the lower edge down. The pressure differential generates a lift.
But the difference in air velocities over the upper and lower edge is caused because of the pressure difference, and not the other way around.

Now that that is all sorted out, lets move on to the airfoil shapes....
1. The classic shape
From above, we see that the classic shape is good in the case we want large lift even at low speeds. Also, it reduces drag significantly.
2. A flat wing
This would require a much larger AoA to generate the required lift. Though, it would still develop the pressure differential and downwash in the same explained manner. But the increased AoA would mean a greater drag.
Helicopter blades are nearly flat. This is preferred because drag is almost not an issue for them. What is required is a maneuverable AoA and airspeed. This is encouraged by nearly flat or tear-drop shaped (symmetrical) blades.
3. symmetrical wings
A symmetrical or tear drop shaped wing (like most planes do), will resemble the classic shape when positioned appropriately. This is why planes can fly upside down. Both lifting phenomenon still work. 

A really good explanation is quoted below from

"Airfoil sections are of two basic types, symmetrical and nonsymmetrical.
Symmetrical airfoils have identical upper and lower surfaces. They are suited to rotary-wing applications because they have almost no center of pressure travel. Travel remains relatively constant under varying angles of attack, affording the best lift-drag ratios for the full range of velocities from rotor blade root to tip. However, the symmetrical airfoil produces less lift than a nonsymmetrical airfoil and also has relatively undesirable stall characteristics. The helicopter blade (airfoil) must adapt to a wide range of airspeeds and angles of attack during each revolution of the rotor. The symmetrical airfoil delivers acceptable performance under those alternating conditions. Other benefits are lower cost and ease of construction as compared to the nonsymmetrical airfoil"

Other sources were :

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