I happen to fly a lot on business. For me, personally,
airplanes are one of the most amazing things that I see on a
daily basis. When I get on a 747, I am boarding a gigantic
vehicle capable of carrying 500 or 600 people. A 747 weighs up
to 870,000 pounds at takeoff. Yet it rolls down the runway
and, as though by magic, lifts itself into the air and can fly
up to 7,000 nautical miles without stopping. It is truly
incredible when you think about it!
If you have ever wondered what allows a 747 -- or any
airplane for that matter -- to fly, then read on! In this
edition of HowStuffWorks,
we will walk through the theory of flight and talk about the
different parts of a standard airplane, and then you can
explore tons of links to learn even more!
Aerodynamic Forces
Before we dive into how
wings keep airplanes up in the air, it is important that we
take a quick look at four basic aerodynamic forces: lift,
weight, thrust and drag.
Straight and Level
Flight
In order for an airplane to fly straight and
level, the following relationships must be true:
- Thrust = Drag
- Lift = Weight
If, for any reason, the
amount of drag becomes larger than the amount of thrust, the
plane will slow down. If the thrust is increased so that it is
greater than the drag, the plane will speed up.
Similarly, if the amount of lift drops below the weight of
the airplane, the plane will descend. By increasing the lift,
the pilot
can make the airplane climb.
Thrust
Thrust is an aerodynamic force that must
be created by an airplane in order to overcome the drag
(notice that thrust and drag act in opposite directions in the
figure above). Airplanes create thrust using propellers, jet
engines or rockets. In
the figure above, the thrust is being created with a
propeller, which acts like a very powerful version of a
household fan, pulling air past the blades.
Drag
Drag is an aerodynamic force that resists
the motion of an object moving through a fluid (air and water
are both fluids). If you stick your hand out of a car
window while moving, you will experience a very simple
demonstration of this effect. The amount of drag that your
hand creates depends on a few factors, such as the size of
your hand, the speed of the car and the density of the air. If
you were to slow down, you would notice that the drag on your
hand would decrease.
We see another example of drag reduction when we watch
downhill skiers in the Olympics. You'll notice that, whenever
they get the chance, they will squeeze down into a tight
crouch. By making themselves "smaller," they decrease the drag
they create, which allows them to move faster down the hill.
If you've ever wondered why, after takeoff, a passenger jet
always retracts its landing gear (wheels) into the body of the
airplane, the answer (as you may have already guessed) is to
reduce drag. Just like the downhill skier, the pilot wants to
make the aircraft as small as possible to reduce drag. The
amount of drag produced by the landing gear of a jet is so
great that, at cruising speeds, the gear would be ripped right
off of the plane.
Weight
This one is the easiest. Every object on
earth has weight (including air). A 747
can weigh up to 870,000 pounds (that's 435 tons!) and still
manage to get off the runway. (See the table below for more
747 specs!)
Lift
Lift is the aerodynamic force that holds an
airplane in the air, and is probably the trickiest of the four
aerodynamic forces to explain without using a lot of math. On
airplanes, most of the lift required to keep the plane aloft
is created by the wings (although some is created by other
parts of the structure).
747-400
Facts
- Length: 232 feet (~ 71 meters)
- Height: 63 feet (~ 19 meters)
- Wingspan: 211 feet (~ 64 meters)
- Wing area: 5,650 square feet (~ 525 square
meters)
- Max. takeoff weight: 870,000 pounds (~
394,625 kilograms)
- Max. landing weight: 630,000 pounds (~
285,763 kilograms)
(explains why planes may need to
dump fuel for emergency landings)
- Engines: four turbofan
engines, 57,000 pounds of thrust each
- Fuel capacity: up to 57,000 gallons (~
215,768 liters)
- Max. range: 7,200 nautical miles
- Cruising speed: 490 knots
- Takeoff distance: 10,500 feet (~ 3,200
meters)
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A Few Words About Fluid
A
principal concept in aerodynamics is the idea that air is a
fluid. Like all gases, air flows and behaves in a similar
manner to water and other liquids. Even though air, water and
pancake syrup may seem like very different substances, they
all conform to the same set of mathematical relationships. In
fact, basic aerodynamic tests are sometimes performed
underwater.
Another important concept is the fact that lift can
exist only in the presence of a moving fluid. This is also
true for drag. It doesn't matter if the object is stationary
and the fluid is moving, or if the fluid is still and the
object is moving through it. What really matters is the
relative difference in speeds between the object and the
fluid.
Consequently, neither lift nor drag can be created in space
(where there is no fluid). This explains why spacecraft don't
have wings unless the spaceship spends at least some of its
time in air. The space
shuttle is a good example of a spacecraft that spends most
of its time in space, where there is no air that can be used
to create lift. However, when the shuttle re-enters the
earth's atmosphere, its stubby wings produce enough lift to
allow the shuttle to glide to a graceful landing.
Popular (and Imperfect) Explanations of Lift
Creation
If you read any college-level aerodynamics
textbook, you will find plenty of mathematical methods for
calculating lift. Unfortunately, none of these explanations
are particularly satisfying unless you have a Ph.D. in
mathematics. (Click here
for a demo of an online aerodynamics textbook from Stanford
University.)
There are many simplified explanations of lift that appear
on the Internet
and in some textbooks. Two of the most popular explanations
today are the Longer Path explanation (also known as
the Bernoulli or equal transit time explanation)
and the Newtonian explanation (also known as the
momentum transfer or air deflection
explanation). While many versions of these explanations are
fundamentally flawed, they can still contribute to an
intuitive understanding of how lift is created.
The Longer Path
Explanation
What is it?
The Longer Path explanation
holds that the top surface of a wing is more curved than the
bottom surface. Air particles that approach the leading edge
of the wing must travel either over or under the wing. Let's
assume that two nearby particles split up at the leading edge,
and then come back together at the trailing edge of the wing.
Since the particle traveling over the top goes a longer
distance in the same amount of time, it must be traveling
faster.
Bernoulli's equation, a fundamental of fluid dynamics,
states that as the speed of a fluid flow increases, its
pressure decreases. The Longer Path explanation deduces that
this faster moving air develops a lower pressure on the top
surface, while the slower moving air maintains a higher
pressure on the bottom surface. This pressure difference
essentially "sucks" the wing upward (or pushes the wing
upward, depending on your point of view).
Why is it not entirely correct?
There are
several flaws in this theory, although this is a very common
explanation found in high school textbooks and even
encyclopedias:
- The assumption that the two air particles described
above rejoin each other at the trailing edge of the wing is
groundless. In fact, these two air particles have no
"knowledge" of each other's presence at all, and there is no
logical reason why these particles should end up at the rear
of the wing at the same moment in time.
- For many types of wings, the top surface is longer than
the bottom. However, many wings are symmetric (shaped
identically on the top and bottom surfaces). This
explanation also predicts that planes should not be able to
fly upside down, although we know that many planes have this
ability.
Why is it not entirely
wrong?
The Longer Path explanation is correct in
more than one way. First, the air on the top surface of the
wing actually does move faster than the air on the bottom --
in fact, it is moving faster than the speed required for the
top and bottom air particles to reunite, as many people
suggest. Second, the overall pressure on the top of a
lift-producing wing is lower than that on the bottom of the
wing, and it is this net pressure difference that creates the
lifting force.
The Newtonian
Explanation
What is it?
Isaac Newton stated that for
every action there is an equal, and opposite, reaction
(Newton's Third Law). You can see a good example of this by
watching two skaters at an ice rink. If one pushes on the
other, both move -- one due to the action force and the other
due to the reaction force.
In the late 1600s, Isaac Newton theorized that air
molecules behave like individual particles, and that the air
hitting the bottom surface of a wing behaves like shotgun
pellets bouncing off a metal plate. Each individual particle
bounces off the bottom surface of the wing and is deflected
downward. As the particles strike the bottom surface of the
wing, they impart some of their momentum to the wing, thus
incrementally nudging the wing upward with every molecular
impact.
Note: Actually, Newton's theories on fluids were developed
for naval warfare, in order to help decrease the resistance
that ships encounter in the water -- the goal was to build a
faster boat, not a better airplane. Still, the theories are
applicable, since water and air are both fluids.
Why is it not entirely correct?
The
Newtonian explanation provides a pretty intuitive picture of
how the wing turns the air flowing past it, with a couple of
exceptions:
- The top surface of the wing is left completely out of
the picture. The top surface of a wing contributes greatly
to turning the fluid flow. When only the bottom surface of
the wing is considered, the resulting lift calculations are
very inaccurate.
- Almost a hundred years after Newton's theory of ship
hulls, a man named Leonhard
Euler noticed that fluid moving toward an object will
actually deflect before it even hits the surface, so it
doesn't get a chance to bounce off the surface at all. It
seemed that air did not behave like individual shotgun
pellets after all. Instead, air molecules interact and
influence each other in a way that is difficult to predict
using simplified methods. This influence also extends far
beyond the air immediately surrounding the wing.
Why is it not entirely wrong?
While
a pure Newtonian explanation does not produce accurate
estimates of lift values in normal flight conditions (for
example, a passenger jet's flight), it predicts lift for
certain flight regimes very well. For hypersonic
flight conditions (speeds exceeding five times the speed
of sound), the Newtonian theory holds true. At high speeds and
very low air densities, air molecules behave much more like
the pellets that Newton spoke of. The space shuttle operates
under these conditions during its re-entry
phase.
Unlike the Longer Path explanation, the Newtonian approach
predicts that the air is deflected downward as it passes the
wing. While this may not be due to molecules bouncing off the
bottom of the wing, the air is certainly deflected downward,
resulting in a phenomenon called downwash. (Click here
for more on downwash.)
How Lift is Created
Pressure Variations Caused By
Turning a Moving Fluid
Lift is a force on a wing (or
any other solid object) immersed in a moving fluid, and it
acts perpendicular to the flow of the fluid. (Drag is the same
thing, but acts parallel to the direction of the fluid flow).
The net force is created by pressure differences brought about
by variations in speed of the air at all points around the
wing. These velocity variations are caused by the disruption
and turning of the air flowing past the wing. The measured
pressure distribution on a typical wing looks like the
following diagram:

|
A. Air approaching the top surface of the wing is
compressed into the air above it as it moves upward. Then,
as the top surface curves downward and away from the
airstream, a low-pressure area is developed and the air
above is pulled downward toward the back of the wing.
B. Air approaching the bottom surface of the wing
is slowed, compressed and redirected in a downward path. As
the air nears the rear of the wing, its speed and pressure
gradually match that of the air coming over the top. The
overall pressure effects encountered on the bottom of the
wing are generally less pronounced than those on the top of
the wing.
C. Lift component
D. Net force
E. Drag component
When you sum up all the pressures acting on the wing (all
the way around), you end up with a net force on the wing. A
portion of this lift goes into lifting the wing (lift
component), and the rest goes into slowing the wing down
(drag component). As the amount of airflow turned by a
given wing is increased, the speed and pressure differences
between the top and bottom surfaces become more pronounced,
and this increases the lift. There are many ways to increase
the lift of a wing, such as increasing the angle of attack or
increasing the speed of the airflow. These methods and others
are discussed in more detail later in this article.
It is important to realize that, unlike in the two popular
explanations described earlier, lift depends on significant
contributions from both the top and bottom wing surfaces.
While neither of these explanations is perfect, they both hold
some nuggets of validity. Other explanations hold that the
unequal pressure distributions cause the flow deflection, and
still others state that the exact opposite is true. In either
case, it is clear that this is not a subject that can be
explained easily using simplified theories.
Likewise, predicting the amount of lift created by wings
has been an equally challenging task for engineers and
designers in the past. In fact, for years, we have relied
heavily on experimental data collected 70 to 80 years ago to
aid in our initial designs of wings.
Calculating Lift Based on Experimental
Test Results
In 1915, the U.S. Congress created
the National Advisory Committee on Aeronautics (NACA -- a
precursor of NASA). During the 1920s and 1930s, NACA conducted
extensive wind tunnel tests on hundreds of airfoil shapes
(wing cross-sectional shapes). The data collected allows
engineers to predictably calculate the amount of lift and drag
that airfoils can develop in various flight conditions.
The lift coefficient of an airfoil is a number that
relates its lift-producing capability to air speed, air
density, wing area and angle of attack -- the angle at
which the airfoil is oriented with respect to the oncoming air
flow (we'll discuss this in greater detail later in the
article). The lift coefficient of a given airfoil depends upon
the angle of attack.
 Image courtesy NASA The lift-curve slope of a NACA
airfoil |
Here is the standard equation for calculating lift using a
lift coefficient:
L = lift Cl = lift
coefficient (rho) = air density V =
air velocity A = wing
area
|
As an example, let's calculate the lift of an airplane with
a wingspan of 40 feet and a chord length of 4 feet (wing area
= 160 sq. ft.), moving at a speed of 100 mph (161 kph) at sea
level (that's 147 feet, or 45 meters, per second!). Let's
assume that the wing has a constant cross-section using an
NACA 1408 airfoil shape, and that the plane is flying so that
the angle of attack of the wing is 4 degrees.
We know that:
- A = 160 square feet
- (rho) = 0.0023769 slugs / cubic foot (at sea
level on a standard day)
- V = 147 feet per second
- Cl = 0.55 (lift coefficient for NACA 1408 airfoil
at 4 degrees AOA)
So let's calculate the lift:
- Lift = 0.55 x .5 x .0023769 x 147 x 147 x 160
- Lift = 2,260 lbs
Try your hand at airfoil
design on NASA's Web site using a virtual wind tunnel.
Calculating Lift Using Computer
Simulations
In the years since NACA's
experimental data was collected, engineers have used this
information to calculate the lift (and other aerodynamic
forces) produced by wings and other objects in fluid flows. In
recent years, however, computing power has increased such that
wind tunnel experiments can now be simulated on an average personal
computer.
Software packages, such as FLUENT,
have been developed to create simulated fluid flows in which
solid objects can be virtually immersed. The applications of
this type of software range from simulating the air flowing
over a wing, to mapping the airflow through a computer case to
ensure that there is enough cool air passing over the CPU
to prevent the computer from overheating.
Interesting Things about Wings
These
interesting facts about wings are useful in developing a more
detailed understanding of how they work.
Wing Shape
The "standard"
airfoil shape that we examined above is not the only shape for
a wing. For example, both stunt planes (the kind that fly
upside down for extended periods of time at air shows) and supersonic
aircraft have wing profiles that are somewhat different
than you would expect:
The upper airfoil is typical for a stunt plane, and the
lower airfoil is typical for supersonic fighters. Note that
both are symmetric on the top and bottom. Stunt planes
and supersonic jets get their lift totally from the angle of
attack of the wing.
Angle of Attack
The angle
of attack is the angle that the wing presents to oncoming air,
and it controls the thickness of the slice of air the wing is
cutting off. Because it controls the slice, the angle of
attack also controls the amount of lift that the wing
generates (although it is not the only factor).
 Zero angle of attack
|
 Shallow angle of attack
|
 Steep angle of attack
|
Flaps
In general, the
wings on most planes are designed to provide an appropriate
amount of lift (along with minimal drag) while the plane is
operating in its cruising mode (about 560 miles per hour, or
901 km per hour, for the Boeing 747-400). However, when these
airplanes are taking off or landing, their speeds can be
reduced to less than 200 miles per hour (322 kph). This
dramatic change in the wing's working conditions means that a
different airfoil shape would probably better serve the
aircraft.
To accommodate both flight regimes (fast and high as well
as slow and low), airplane wings have moveable sections called
flaps. During takeoff and landing, the flaps are
extended rearward and downward from the trailing edge of the
wings. This effectively alters the shape of the wing, allowing
the wing to turn more air, and thus create more lift. The
downside of this alteration is that the drag on the wings also
increases, so the flaps are put away for the rest of the
flight.
Slats
Slats
perform the same function as flaps (that is, they temporarily
alter the shape of the wing to increase lift), but they are
attached to the front of the wing instead of the rear. They
are also deployed on takeoff and landing.
Rotating Surfaces
Given
what we know so far about wings and lift, it seems logical
that a simple cylinder would not produce any lift when
immersed in a moving fluid (imagine a plane with wings shaped
like cardboard paper-towel tubes). In a simplified world, the
air would just flow around the cylinder evenly on both sides,
and keep right on going. In reality, the downstream air would
be a little turbulent and chaotic, but there still would be no
lift created.
However, if we were to begin rotating the cylinder, as in
the figure below, the surface of the cylinder would actually
drag the surrounding layer of air around with it. The net
result would be a pressure difference between the top and
bottom surfaces, which deflects the airflow downward. Newton's
Third Law states that if the air is being redirected downward,
the cylinder must be deflected upward (sounds like lift to
me!). This is an example of the Magnus Effect (also
known as the Robbins Effect), which holds true for
rotating spheres as well as cylinders (see any similarities to
curveballs
here?)
Believe it or not, in 1926, Anton Flettner built a ship
named the Bruckau that used huge spinning cylinders instead of
sails to power itself across the ocean. Click here
to learn about Flettner's Rotorship.
Blown Surfaces
Let's take
our cylindrical wing from the above examples and find another
way to create lift with it. If you've ever held the back of
your hand vertically under the faucet, you may have noticed
that the water did not simply run down to the bottom of your
hand and then drip off. Instead, the water actually runs back
up and around the side of your hand (for a few millimeters)
before falling into the sink. This is known as the Coanda
Effect (after Henri Coanda), which states that a fluid
will tend to follow the contour of a curved surface that it
contacts.
In our cylinder example, if air is forced out of a long
slot just behind the top of the cylinder, it will wrap around
the backside and pull some surrounding air with it. This is a
very similar situation to the Magnus Effect, except that the
cylinder doesn't have to spin.
The Coanda Effect is used in specialized applications to
increase the amount of additional lift provided by the flaps.
Instead of just altering the shape of the wing, compressed air
can be forced through long slots on the top of the wing or the
flaps to produce extra lift.
Believe it or not, in 1990, McDonnell Douglas Helicopter
Co. (now known as MD
Helicopters, Inc.) removed the tail rotors from some of
its helicopters and replaced them with cylinders! Instead of
using a conventional tail rotor to steer the aircraft, the
tail boom is pressurized and air is blown out through long
slots exactly like the figure above.
More Airplane Parts
The wing is obviously
the most important part of an airplane -- it's what gets the
airplane in the air! But airplanes have a lot of other
characteristic parts designed to control the plane or get it
moving. Let's examine the parts you find in a typical airplane
by looking at a Cessna 152.
Probably the most important parts of an airplane, after the
wing, are the propeller and engine. The
propeller (or, on jet aircraft, the jets) provides the
thrust that moves the plane forward. (Check out How Gas
Turbine Engines Work to learn about jet engines.)
A propeller is really just a special, spinning wing. If you
looked at the cross section of a propeller, you'd find that a
propeller has an airfoil shape and an angle of attack. Just by
looking at the propeller pictured above, you can see that the
angle of attack changes along the length of the propeller --
the angle is greater toward the center because the speed of
the propeller through the air is slower close to the hub. Many
larger propeller aircraft have more elaborate three-blade or
four-blade props with adjustable pitch mechanisms.
These mechanisms let the pilot adjust the propeller's angle of
attack depending on air speed and altitude.
The landing gear is also essential during take-off
and landing.
 Front landing gear
|
 Rear landing gear
|
The Cessna 152 has fixed landing gear, but most planes have
retractable landing gear to reduce drag while in flight.
Controlling the Direction
The tail of the
airplane has two small wings, called the horizontal and
vertical stabilizers, that the pilot uses to control
the direction of the plane. Both are symmetrical airfoils, and
both have large flaps on them that the pilot controls with the
control stick to change their lift characteristics.
 Horizontal tail wing
|
 Vertical tail wing
|
With the horizontal tail wing, the pilot can change
the plane's angle of attack, and therefore control whether the
plane goes up or down. With the vertical tail wing, the
pilot can turn the plane left or right.
The plane's main wing is 40 feet (~ 12 m) long from end to
end, and about 4 feet (~ 1.2 m) wide. On the inner portion of
the wing, there are flaps used during takeoff, landing and
other low-speed situations. On the outer ends, there are
ailerons used to turn the plane and keep it level.
The plane also has two different sensors mounted on the
wing:
For more information on airplanes, flight dynamics and
other other related topics, check out the links on the next
page!