Review On :-
1) Forces acting on the Aircraft (Four
2) Bernoulli’s Principle
3) Airplane Parts
4) Debate whether Airplane can stay
stationary in air?
• Force – a push or a pull acting
on a body.
• As a plane flies it is in the center
of 4 forces.
– Weight, lift, drag and thrust
• Two natural forces being exerted
– Weight and drag
• A pilot needs to overcome
weight and drag to
• Two forces a pilot needs to
create to overcome weight and
– Lift and thrust
• Lift & thrust are required to
keep the airplane in the air
• Lift is the upward force on a plane
– Various parts of a plane help to
• But most of the lift is created
by the wings
• The magnitude of lift depends on
the shape, size and velocity
– For example, the faster the
plane goes the greater the lift
• The lift that is produced by the
wings must be greater than the
weight of plane to leave the
• Weight is defined as the
downward force of gravity
– Force is always directed
toward the center of the earth
• Weight is distributed throughout
• The magnitude of the weight
depends on the mass of the
plane plus the fuel, the people
• A pilot must overcome weight by
lift to get the plane in the air
• Thrust is defined as the forward push
that gets the plane into the air
– Thrust is artificially created and used to
overcome drag and to sustain lift
• This force is provided by the propeller or jet
• Thrust is also used to accelerate and
• Drag is a resistance force
created by the plane’s
movement through the air
– The force of the air pushes
against the plane, therefore
slowing the plane down
• The magnitude of drag
depends on the shape, air
quality and velocity
• Drag increases as air speed
– A pilot must overcome drag with
thrust to gain speed
Engines (either jet or propeller) typically provide the
thrust for aircraft. When you fly a paper airplane, you
generate the thrust.
• Principal concept in aerodynamics is the idea that
air is a fluid
– Air has mass, therefore it has weight
• Because it has weight, it exerts pressure
– Air flows and behaves in a similar manner to other
– Air has molecules which are constantly moving
• Lift can exist only in the presence of a moving fluid
– Faster moving fluids exert less force on surfaces they
are flowing along
As an airplane moves forward, the airflowAs an airplane moves forward, the airflow
splits up into two separate flowssplits up into two separate flows
Before We Begin…
• Air flowing around the wing experiences a change in speed and each
change in speed is accompanied by a change in pressure
– Airflow going under the wing encounters a sloping surface
• Slows airflow down and slow moving air maintains a higher pressure on the bottom
– Airflow going over the wing encounters the up/down sloping
• Slows the airflow down, then it speeds it up; with the faster moving air a lower
pressure develops on the below surface
– Air going over must travel farther, so its average speed is greater
than the speed of the air below
• Result: A reduction in sidewise pressure which
occurs at the top, exerting a lifting force on the
• Pressure imbalance produces an overall
Bernoulli’s Principle: Air moving over the wing moves
faster than the air below. Faster-moving air above
exerts less pressure on the wing than the slower-
moving air below. The result is an upward push on
Conservation of Energy
Bernoulli principle derived from the Law of Conservation
• A fluid under pressure has potential energy.
– Energy can be stored in pressurized air
– The higher the pressure the greater the potential energy
• Moving fluids have both potential energy and kinetic energy.
– Total energy must remain constant,
so its potential energy decreases,
and which means its pressure
decreases as well
– When the air’s speed and
motional energy increase, the
pressure and pressure energy must decrease to compensate
• Speed increases over the wing because the airflow converts some
of its pressure energy into kinetic energy
Bernoulli’sTheory in Action
Air speeds up in the constricted space between
the car & truck creating a low-pressure area.
Higher pressure on the other outside pushes
The distance traveled is the same.
Equal distances in equal times means
the air is traveling at same speed.
There’s no net force=no lift.
The curved shape is a longer distance
so the air is traveling faster. Equal
distances traveled in equal times. No
net force=no lift.
The air on top is traveling faster. It
exerts less force. When 2 forces
are combined they do not cancel
each other out. Therefore there is
some net force upward.
Shape of the Wing
theAmount of Lift Created
– The faster the wing moves through the air the more air is forced over
• So a plane must maintain ample velocity to keep the upward lifting force
– If it slows down too much—lift decreases—plane descend
• Density of air
– The denser the air the more lift (colder air is more dense; air density
changes with altitude)
• Planes climb better in winter.
• Shape of wing
• Angle of attack (its tilt relative to the wind)
– Downside: increases drag
• Provides side to side control of airplane.
• Controls yaw.
• Used for maneuvers in the air and for taxiing
The RUDDER controls
YAW. On the vertical
tail fin, the rudder
swivels from side to
side, pushing the tail in
a left or right direction.
A pilot usually uses the
rudder along with the
ailerons to turn the
Rudder Controls Yaw
controls PITCH. On
the horizontal tail
elevator tilts up or
or increasing lift on
the tail. This tilts the
nose of the
airplane up and
• Located at the top of the trailing edge
of the wings.
• Controls roll.
• Move up and down to control the
direction of wind blowing over and
Ailerons Control Roll
control ROLL. On the
outer rear edge of
each wing, the two
ailerons move in
up and down,
decreasing lift on
one wing while
increasing it on the
other. This causes
the airplane to roll to
the left or right.
• A frame with wheels that allow the plane to
takeoff and land.
• Some airplanes have retractable landing
gear. Landing Gear
• The front landing gear when the plane
has three wheels to land.
• The short answer is - No, they cannot stop in mid-air.
Even though it appeared as the plane was not moving,
it is unlikely that its forward motion was completely
stopped. At a distance, very large planes can appear to
be nearly stationary, especially at the relatively low
speeds found during a landing approach. This effect can
be made much more apparent depending on the angle
from which you observe the plane - from the front or
rear, the apparent motion would be very small. The
distance from you to the plane also makes a
Whether Flight Can stay Stationary
Whether Flight Can stay Stationary
in Air? – Cont.
• Air must be flowing past the wings in order to keep
the aircraft aloft.
• In very unusual circumstances, it could conceivably
remain stationary relative to the ground—if it were
flying directly into a very powerful headwind, for
example. That doesn't actually happen in practice,
• But - it would take a very, very fast wind to keep
any airliner in the sky completely stationary - more
than 100 mph, probably. In a wind of that speed,
planes would not be taking off or landing.
• If there were no wind at all, then the ground
speed and the airspeed would be the same.
When a plane flies into the wind, the airspeed
is higher because of the added speed of the
blowing wind across the wings. If the wind were
blowing very hard, then the plane might have
sufficient airspeed to maintain the needed lift
to remain in the air, while the ground speed
could be very slow.
Whether Flight Can stay Stationary
in Air? – Cont.
Notas do Editor
Now going back to the simple ingredients for creating lift – air and motion. Something needs to get that wing moving through the air before it can create lift. That’s where thrust comes in.
A wing is shaped and tilted so the air moving over it moves faster than the air moving under it. Bernoulli’s 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)
The sideward tug you feel on your car when you pass a large truck going in the opposite direction is caused by air pressure. The passing vehicles form a constriction that speeds up the flow of air, reducing the air pressure between them. (It makes no difference which is moving--the air or the vehicles. The result is the same.) The higher air pressure on the other side of the car pushes it toward the truck during the split-second as they pass.
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.