How planes generate lift!
Back to cool questions about flight
Any textbook that talks about airplanes
always discusses the four forces affecting a plane: lift, weight, thrust and
drag: Three of those forces are obvious to us. For example, everyone weighs
himself or herself on the bathroom scale every morning, so weight is easy to
understand. Weight is what keeps
us on the ground.

Household fans produce thrust (at least a
little) in the same way a propeller does, so that concept is familiar. 
Anyone who has ever stuck his or her hand out
the window of a car on the freeway has felt the effects of drag. 

But lift 
there's something uncommon. If lift were easy we could stick out our arms and
fly like a bird! But obviously it is not that easy. To understand lift you need
to understand how wings work.
If you read an aerodynamics textbook, you will find many different explanations
of lift: Bernoulli's
principle, vortex circulation, fluid
dynamics, etc. are discussed. None of these explanations are particularly
satisfying unless you have a Ph.D. in mathematics. However, it is possible to
get a good basic understanding of lift using the simple idea of action and reaction, or Newton's 3rd law.
The
following is from HowStuffWorks.com.

Let's say that you were to build an
airplane's wing out of flat sheets plywood  no fancy airfoils, just a flat
wing. A typical small airplane, like the one shown here, has wings that are
40 feet long and about 4 feet wide: 
Say you made the wing for this plane out of five 4'x8' sheets of plywood
arranged as a 40' x 4' sheet. If you were to angle the plywood wing slightly
(giving it an "angle of attack") and move the wing through the air,
you can imagine that the wing would cut a "slice of air". The action
of the wing is much like a knife cutting a slice from a block of cheese. The
following figure shows the slice of air being cut off of the larger block of
air by a slightly angled plywood wing:
In this picture the wing is moving from right to left, and it is cutting off a
slice of air as it goes. The thickness of the slice is controlled by the angle
of the wing. If the front of the wing is 4 inches higher than the back, the
slice is 4 inches thick.
What's interesting is that, by thinking about what is happening to the slice,
you can actually learn a lot about lift. You know that rocket engines are
reaction engines. That is, they "throw" mass in one direction and
take advantage of the equal and opposite reaction. See the rocket engine
article for a complete explanation. The simplest way to think of a wing is
this: A wing works by exactly the same action/reaction principle. The mass that
a wing is throwing is the weight of the airslice it is carving off. The
airslice has mass, and in the process of getting carved it is
"thrown" downward. The wing reacts to the airslice's mass being
thrown, and it rises.
That may be hard to believe, so let's look and see if this action/reaction idea
actually works. Let's assume a flat plywood wing that is 40 feet long and 4
feet wide. Let's assume that its front edge is 4 inches higher than the back
edge, so a 4inch slice is being carved off. Let's also assume the wing is
slicing through the air at the rate of 100 miles per hour.
First let's figure out the mass of the air that this wing is carving off. 100
MPH is equivalent to 147 feet per second. So in one second the wing carves off
a slice of air 147 feet long, 40 feet wide and 4 inches thick. That's about
1,955 cubic feet of air.
Air really does have weight, and one cubic foot of air weighs about 35 grams.
Therefore, the 1,955 cubic foot slice of air that the wing produces every
second weighs almost 160 pounds!
Force = mass * acceleration. The upward force on the wing is determined by
figuring out the acceleration that the wing applies to the 160 pound slice. The
wing is definitely accelerating it: as the wing is cutting off the slice, it
moves it downward by 4 inches. It takes the wing 4 feet (the width of the wing)
to move the air downward by 4 inches.
Since the wing is moving at 146 feet per second, it takes only 0.027 seconds to
move 4 feet. Therefore, the slice is moving downward at a velocity of 4 inches
per 0.027 seconds (12.3 feet per second) once the wing passes by, and it took
0.027 seconds to accelerate it from zero to that speed. So the air in the slice
is being accelerated at a rate of about 448 feet/second2.
Now we can calculate the upward force from the wing:
Upward force = 160 pounds * 448 feet/second2 / 32
feet/second2 = 2,240 pounds!
Wow! There is 2,240 pounds of upward force being generated by the slice of air
this wing carves off. The 32 feet/second2
in the divisor, by the way, is the acceleration that earth's gravity generates.
To calculate pounds of upward force we divide by gravity's acceleration.
2,240 pounds of upward force is a lot. The interesting thing to note is that
the small airplane pictured above has wings that are about 40 feet long and 4
feet wide, it cruises about about 100 MPH, and it weighs about 2,000 to 2,200
pounds once it is loaded with fuel and passengers. What an amazing coincidence!
So, why aren't all wings simply big flat sheets of plywood? There are three
reasons:
1. A big flat sheet of plywood carving off a slice of air would create a
tremendous amount of drag. The triangleshaped area behind the wing is a big
vacuum, and as the vacuum sucks in air to replace the slice it creates drag.
2. It would be hard to make a 40foot long sheet of plywood that has any
strength.
3. If you run the equations we just ran using the parameters of a 747, you will
find that you need to carve off about a 6footthick slice of air to get the
lift you need. That's not so unreasonable when you consider how wide a 747's
wing is  the angle of attack would be about 20 degrees. But it would still
create a lot of drag. One interesting thing to note  the small plane weighs
2,000 pounds and has 40 foot wings, while the 747 weighs 800,000 pounds and has
211 foot wings. The 747 weighs 400 times as much but its wings are only 5 times
longer. That indicates there might be another force at work.
Vocabulary
Aerodynamics: the study of the motion of air and the effect of
moving air on obstructions
Angle of Attack: The angle formed by the tilt
of the flying disk and the line parallel to the ground.
Angular
Momentum: A rotating body's resistance to change in its
orientation and rate of rotation. This is similar to gyroscopic inertia.
Bernoulli's
principle: The pressure in a fluid decreases as the speed of
the fluid increases.
Drag: The
force that resists the motion of the aircraft through the air.
Fluid: a substance tending to flow and conform to the outline of its
container
Force: A measurable push or a pull in a certain
direction.
Lift: the
force that allows for upward motion on a flying object.
An upward force resulting from decreasing the pressure on the top of an object
by increasing the velocity of the air flowing over the top of it. This is the
force that opposes the weight force of the plane. The difference between the
lower pressure on top of the wing, and the higher pressure beneath the wing,
results in lift.
Newton's Third Law: for every action there is
an equal and opposite reaction.
Pressure: the force exerted over a surface
divided by its area
Thrust: a force
created by the engine that pushes an aircraft through the air. Thrust acts
against the force of drag.
Weight: the
force of gravity acting on an object. The weight force pulls an aircraft toward
the Earth. Weight is balanced by lift.
Wind tunnel: an enclosure in which a steady current of air can be
maintained for the purpose of testing lift and friction
Velocity: the rate of change of displacement of a moving body
with time