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How does an Airplane Fly?
Forces on an Airplane in Flight
The four aerodynamic forces that act upon an airplane in flight are lift
(the upward acting force), weight (or gravity, the downward acting
force), thrust (the forward acting force), and drag (the air resistance or
backward acting force). These four forces are continuously battling each
other while an airplane is in flight.
Gravity opposes lift, thrust opposes drag. In order to
take off, the aircraft's thrust and lift must be suffucient
to overcome its weight and drag. In level flight at
constant speed, thrust exactly equals drag and lift
exactly equals the pull of gravity. To land, an aircraft's
thrust must be reduced safely below its drag, as its lift
is reduced to levels less than its weight.
Control Surfaces and
An airplane in flight moves around three axes of rotation:
longitudinal axis, lateral axis, and vertical axis. These axes are
imaginary lines that run perpendicularly to each other through
the center of gravity of the airplane. Rotation around the
longitudinal axis (the line from the nose of the plane to the
tail) is called roll. Rotation around the lateral axis (the line
from wingtip to wingtip) is called pitch. Rotation around the
vertical axis (the line from beneath to above the plane) is
called yaw. The pilot guides and controls the aircraft by
controlling its pitch, roll, and yaw via the control surfaces.
These include the ailerons, elevators, and rudder.
How an Airplane Generates Lift
a
Since lift is dependant on the motion of the air, it
increases as the speed of the air increases. Lift also
increases (to a point) as the angle that the wing
makes with the airflow (known as the angle of
attack) increases. Past a certain point, however,
increased angle of attack will cause the wing to
suddenly lose its lifting ability, or stall.
Ailerons
The ailerons on an airplane's wings control roll around the
longitudinal axis. They work together, simultaneously, tied to
the control wheel, or stick, in the cockpit. When the control
wheel is turned left, the aileron on the left wing goes up and
the one on the right wing goes down.
The ailerons alter the lifting ability of the wings slightly. When
an aileron is lowered, the lift on the outer portion of that wing
increases, causing that wing to rise a little. When an aileron is
raised, the lift on the outer portion of that wing is decreased
slightly, causing that wing to drop a little. Since the ailerons
on an airplane work together, their action causes the airplane
to roll.
Rudder
The rudder on the rear edge of the vertical fin on the airplane's
tail controls yaw around the vertical axis. It is connected to the
pedals at the pilot's feet. Pushing the right pedal causes the
rudder to deflect to the right. This makes the tail of the
airplane move toward the left, causing the nose to move to the
right. Pushing the left pedal makes the rudder deflect to the
left, the tail moves to the right, and the nose points to the left.
rudder left, tail right
rudder neutral (centered)
rudder right, tail left
Rudder Position
(As viewed from above)
Although the rudder pedals and control wheel in the cockpit are not
linked together, they must be used simultaneously to control the plane.
The pilot guides the airplane by careful and precise movements of the
control wheel and rudder pedals, as well as adjusting the thrust of the
aircraft.
How Does an Airplane Produce
Thrust?
Thrust is the force created by a power source that
overcomes the airplane's aerodynamic drag (its
resistance to passing through the air) and gives it
forward motion. This force can either "pull" or "push"
the aircraft forward, depending on the type of power
source used. Common types include reciprocating
(piston-powered) engines driving propellers, and jet
engines.
Reciprocating Engines with Propellers
A reciprocating engine is an internal-combustion
engine in which pistons moving back and forth act
upon a crankshaft to create rotational movement. (This
is the same type of engine that powers most family
cars.) A mixture of fuel and air is compressed by the
pistons, an electric spark causes the mixture to
explode, driving the pistons downward. This motion is
transferred to the crankshaft by connecting rods. The
rotating crankshaft turns the propeller.
. Thus, the propeller creates a propulsive force perpendicular
to its plane of rotation that moves the aircraft forward as a
reaction. Props can either "pull" the aircraft from their position
on the front of the wings or fuselage, or "push" it from behind,
or both.
After you are finished reading this article, you many wish
to view an expanded discussion on Piston Engines. Read
our two article set "How Engine Make Power".
Jet Engines
A jet engine is any engine that ejects a jet or stream of
gas or fluid, thereby obtaining thrust in reaction to the
ejection force. A jet aircraft engine obtains oxygen
from the atmosphere for the combustion of its fuel,
creating thrust in reaction to the rapid exhaust of the
combustion products. There are several types of jet
engines. Some are briefly described below.
Turbojet
A turbojet engine is a jet engine that incorporates a turbinedriven compressor to take in and compress air for the
combustion of fuel. The exhaust from the combustion drives
the turbine and creates the thrust-producing jet.
Basic Turbojet Engine
aileron neutral, normal lift
aileron lowered, increased lift
aileron raised, decreased lift
Aileron Position
(As
from the end of the wing)
Turboprop
A turboprop engine is a turbojet engine in which a portion of
the exhaust energy is used to drive a propeller. The engine's
thrust is therefore generated by a combination of the
propeller's thrust and the jet exhaust from the engine.
Basic Turboprop Engine
Ramjet
A ramjet engine is the simplest type of jet engine since it has no moving
parts. The engine is basically a specially-shaped duct open at both ends,
with the air necessary for combustion being compressed by the forward
motion of the engine. Fuel is sprayed into the airstream and the mixture
is ignited. The high-pressure air coming into the combustion chamber
keeps the reaction from going back toward the inlet.
Basic Ramjet Engine
Ramjet engines cannot operate under static conditions.
In order to function, they have to already be traveling
through the air at slightly over the speed of sound.
(The speed of sound is somewhat over 740 miles per
hour at sea level.) This means that the aircraft using
them must first get up to the required speed using
some other type of propulsion, then start the ramjets.
They can operate at up to five times the speed of
sound.