Airplane Parts
Since the Wright brothers first flew in 1903, people have
created a multitude of airplane types. But every one of them has
dealt with the same four forces--lift, weight, thrust, and drag.
Lift is the hardest to understand, so lets tackle it first.
We used a query response (I say lift you say weight, I say
thrust you say drag) of the four forces to get the kids settled
when we taught the Aviation Immersion to 3rd, 4th and 5th graders
at Cherry Creek Challenge School.
Picture from: Plane Math
http://www.planemath.com/activities/pmenterprises/forces/forces2.html
(See Internet Resources Guide)
Remember, the four forces work on the aircraft and against each
other weight acts against lift. If my airplane weights 1700 pounds,
in the simplest sense, Id need 1700 pounds of lift generated by the
wings to get it off the ground.OK somebody do me a favor a jump up
out of your chair. Thanks! Now, can anyone tell me why he/she didnt
float away?You need a fluid (air acts like a fluid) and motion. You
need air and you need the wing to be moving through the air (or air
to be moving over the wing).***So, if the lift off speed of a small
aircraft is 50 kts, will it try to fly in a strong wind? You bet it
will thats why we always tie airplanes down!Laws/principals
proposed by Bernoulli & Newton are used to explain lift.
(although neither of them proposed the theories for that
reason)
Newtons Second Law states that a force will cause a change in
velocity and a change in velocity will generate a force. Also, the
net flow of air around the wing is turned down resulting in an
equal and opposite upward force (NewtonsThird Law).
From NASAs Glenn Research Center: Lift occurs when a flow of gas
is turned by a solid object.(Newtons Second Law a force will cause
a change in velocity and a change in velocity will generate a
force.) The flow is turned in one direction, and the lift is
generated in the opposite direction, according to Newton's Third
Law of action and reaction. For an airfoil, both the upper and
lower surfaces contribute to the flow turning. Neglecting the upper
surface's part in turning the flow leads to an incorrect theory of
lift.For more detail, visit the Glenn Research Center
site:http://www.grc.nasa.gov/www/k-12/airplane/right2.html
Illustration from Plane Math: http://www.planemath.com/ (See
Internet Resources)
Kite or How to send your wife to Home Depot to get a 4 x 8 sheet
of plywood on a windy day.
Understanding a Venturi tube is essential to understanding lift.
As velocity in the constriction increases, pressure must decrease.
Also: Make two stacks of books, three or four tall. Place them next
to each other with a small gap between. Place a sheet of paper over
the books. Move the two stacks close enough to each other that the
paper doesnt sag down into the gap. Now blow between the books,
under the paper. You might expect the wind to blow the paper up,
but it the lower pressure will suck the paper down into the
gap.Venturi tubes describe what happens over a wing. A wing acts
like half a venturi tube.Weve covered how Newtons Laws of Motion
are used to explain lift. Lets talk now about how Bernoullis
Principle helps.
When moving air encounters an obstacle--a person, a tree, a
wing--its path narrows as it flows around the object. Even so, the
amount of air moving past any section of the path must be the same,
because mass can be neither created nor destroyed. The air must
speed up where the path narrows, in order to have the same mass
flowing through it. So air speeds up where its path narrows and
slows down where it widens. One of the many simple illustrations of
Bernoullis Principle. Here a couple more follow.Weve covered how
Newtons Laws of Motion are used to explain lift. Lets talk now
about how Bernoullis Principle helps.
The air above a wing tends to move faster than the air below it.
According to Bernoulli's Principle, slower air has higher pressure
than faster air. That means that the air pressure pushing up on the
bottom of the wing is greater than the pressure pushing down, so
the wing goes up.
A wing is shaped and tilted so the air moving over it moves
faster than the air moving under it. Bernoullis Principle says that
as air speeds up, its pressure goes down. The faster-moving air
above exerts less pressure on the wing than the slower-moving air
below. The result is an upward push on the wing--lift!
Illustration from How Things Fly (See Internet Resources)
Bernoullis Principal explains how pressure variation around a
wing results in a net aerodynamic force. Air moving more quickly
over the top of the wing creates lower pressure above.
For more detail, visit the Glenn Research Center
site:http://www.grc.nasa.gov/www/k-12/airplane/right2.html
The super-simple explanation!FRICTION DRAG: How Things Fly:
Friction is the resistance to motion that occurs when two things
rub together. Air rubbing against the surface of an airplane
creates a force of resistance, known as friction drag. NASA Glenn
Research Pages: One of the sources of drag is the skin friction
between the molecules of the air and the solid surface of the
aircraft. Because the skin friction is an interaction between a
solid and a gas, the magnitude of the skin friction depends on
properties of both solid and gas. For the solid, a smooth, waxed
surface produces less skin friction than a roughened surface. For
the gas, the magnitude depends on the viscosity of the air and the
relative magnitude of the viscous forces to the motion of the flow,
expressed as the Reynolds number. PRESSURE DRAG OR FORM DRAG: How
Things Fly: Air flowing past an object pushes harder against the
upstream side than against the downstream side. This pressure
difference between front and back creates a backward force called
pressure drag. Streamlining an object can dramatically reduce
pressure drag. NASA Glenn Research Pages: We can also think of drag
as aerodynamic resistance to the motion of the object through the
fluid. This source of drag depends on the shape of the aircraft and
is called form drag. As air flows around a body, the local velocity
and pressure are changed. Since pressure is a measure of the
momentum of the gas molecules and a change in momentum produces a
force, a varying pressure distribution will produce a force on the
body. We can determine the magnitude of the force by integrating
(or adding up) the local pressure times the surface area around the
entire body. The component of the aerodynamic force that is opposed
to the motion is the drag; the component perpendicular to the
motion is the lift. Both the lift and drag force act through the
center of pressure of the object. VORTEX DRAG OR INDUCED DRAG:How
Things Fly: The higher-pressure air below a wing spills up over the
wing tip into the area of lower-pressure air above. The wing's
forward motion spins this upward spill of air into a long spiral,
like a small tornado, that trails off the wing tip. These wing tip
vortices create a form of drag called vortex drag. Tilting the
airplane's wings upward makes the vortices stronger and increases
vortex drag. Vortices are especially strong during takeoff and
landing, when an airplane is flying slowly with its wings tilted
upward. NASA Glenn Research Pages: There is an additional drag
component caused by the generation of lift call induced drag. This
drag occurs because the flow near the wing tips is distorted as a
result of the pressure difference from the top to the bottom of the
wing. Swirling vortices are formed at the wing tips, and there is
an energy associated with these vortices. The induced drag is an
indication of the amount of energy lost to the tip vortices. The
magnitude of induced drag depends on the amount of lift being
generated by the wing and on the wing geometry. Long, thin
(chordwise) wings have low induced drag; short wings with a large
chord have high induced drag.
This Gulfstream IV is making lift. Lift rolls off from underside
the wings at the wing tips making wing tip vortices. Wing tip
vortices of larger aircraft are a problem to smaller aircraft.Why
the twist? As a propeller blade spins, its tip slices through the
air faster than the part near its hub. This rotary motion, combined
with the airplane's forward motion, changes the effective direction
of the oncoming air at different points along the propeller blade.
Twisting the blade makes it meet the air at about the same angle
across its entire length. This provides the most thrust and the
least drag.
The angle of attack of the propeller (the angle at which the
blade meets the oncoming air or relative wind) is greater near the
hub because thats where its spinning the slowest.
The wings generate most of the lift to hold the plane in the
air. To generate lift, the airplane must be pushed through the air.
The jet engines, which are located beneath the wings, provide the
thrust to push the airplane forward through the air. Some airplanes
use propellers for the propulsion system instead of jets.To control
and maneuver the aircraft, smaller wings are located at the tail of
the plane. The tail usually has a fixed horizontal piece (called
the horizontal stabilizer) and a fixed vertical piece (called the
vertical stabilizer). The stabilizers' job is to provide stability
for the aircraft, to keep it flying straight. The vertical
stabilizer keeps the nose of the plane from swinging from side to
side, while the horizontal stabilizer prevents an up-and-down
motion of the nose. (On the Wright brother's first aircraft, the
horizontal stabilizer was placed in front of the wings. Such a
configuration is called a canard after the French word for "duck").
At the rear of the wings and stabilizers are small moving sections
that are attached to the fixed sections by hinges. In the figure,
these moving sections are colored brown. Changing the rear portion
of a wing will change the amount of force that the wing produces.
The hinged part of the vertical stabilizer is called the rudder; it
is used to deflect the tail to the left and right as viewed from
the front of the fuselage. The hinged part of the horizontal
stabilizer is called the elevator; it is used to deflect the tail
up and down. The outboard hinged part of the wing is called the
aileron; it is used to roll the wings from side to side. Most
airliners can also be rolled from side to side by using the
spoilers. Spoilers are small plates that are used to disrupt the
flow over the wing and to change the amount of force by decreasing
the lift when the spoiler is deployed.The wings have additional
hinged, rear sections near the body that are called flaps. Flaps
are deployed downward on takeoff and landing to increase the amount
of force produced by the wing. On some aircraft, the front part of
the wing will also deflect. Slats are used at takeoff and landing
to produce additional force. The spoilers are also used during
landing to slow the plane down and to counteract the flaps when the
aircraft is on the ground. The next time you fly on an airplane,
notice how the wing shape changes during takeoff and landing.The
fuselage or body of the airplane, holds all the pieces together.
The pilots sit in the cockpit at the front of the fuselage.
Passengers and cargo are carried in the rear of the fuselage. Some
aircraft carry fuel in the fuselage; others carry the fuel in the
wings.
Why are the wheel pants shaped the way they are?
A vertical stabilizer, or tail fin, keeps the airplane lined up
with its direction of motion. Air presses against both its surfaces
with equal force when the airplane is moving straight ahead. But if
the airplane pivots to the right or left, air pressure increases on
one side of the stabilizer and decreases on the other. This
imbalance in pressure pushes the tail back into line.
Like the vertical stabilizer, the horizontal stabilizer helps
keep the airplane aligned with its direction of motion. If the
airplane tilts up or down, air pressure increases on one side of
the stabilizer and decreases on the other, pushing it back to its
original position. The stabilizer also holds the tail down,
counteracting the tendency of the nose to tilt downward--a result
of the airplane's center of gravity being forward of the wing's
center of lift.
To help make turning easier, an airplane is usually less stable
along its roll axis than along its pitch and yaw axes. Several
factors help the pilot keep the wings level: the inclined mounting
of the wings, the position of the wings above or below the
fuselage, the swept-back shape of the wings, and the vertical
stabilizer. As an airplane rolls, it tends to slip to the side,
changing the direction of relative wind on the wings and tail.
These design features help the pilot restore the airplane to its
upright position.
Flaps change a wing's curvature, increasing lift. Airplanes use
flaps to maintain lift at lower speeds, particularly during takeoff
and landing. This allows an airplane to make a slower landing
approach and a shorter landing. Flaps also increase drag, which
helps slow the airplane and allows a steeper landing approach.
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