An aeroplane can lift itself because the wing, angled slightly downwards
towards the back, pushes air downwards as the wing is propelled forwards by
the engine. In reaction, the wing is pushed upwards, generating lift, as
predicted in the third law of motion formulated by Isaac Newton: that for
every action, there is an equal and opposite reaction. The magnitude of the lift
that is generated depends upon the shape of the aerofoil in cross-section, the
area and shape of the lifting surface, its inclination relative to the airflow, and
the airflow speed.
Lift
The lift developed on a wing or similar surface is directly proportional to the
plan area exposed to the airflow but proportional to the square of the speed of
the airflow. It is also approximately proportional to the inclination, or angle of
attack, of the aerofoil relative to the airflow
for angles typically in the range of plus and minus 14°. At greater angles the
airflow characteristics change rapidly, the
flow “breaks away”, and lift falls drastically. In these circumstances the
aerofoil is said to have “stalled”.
As an aeroplane flies on a level course, the lift contributed by the wing
and other parts of the structure counterbalances the weight of the plane.
Within limits, if the angle of attack is increased while the speed remains
constant, the plane will rise. If the angle of attack is decreased, that is,
the wing is inclined downward, the plane will lose lift and start to
descend. An aeroplane will also climb from level flight if its speed is
increased, and it will dive if its speed is decreased.
During the course of a flight, a pilot frequently alters the speed and angle
of attack of the aircraft. These two factors are often balanced against
each other. For instance, if the pilot wishes to increase speed and yet
maintain level flight, the angle of attack must be decreased to offset the
extra lift that is provided by the increase in the speed of the aircraft.
In preparing to land, the pilot must ease the plane down and at the same
time reduce its speed as much as possible. To compensate for the
considerable loss of lift resulting from the decrease in speed, the pilot
provides additional lift by altering the wing area, effective curvature, and
angle of attack. This is done through the use of high-lift devices called flaps,
large wing extensions located at the rear or trailing edge. Most flaps are
normally retracted into the wing during cruising flight. If extra lift is
wanted, the pilot extends the flaps outward and downward. Sometimes
high-lift devices are provided at the front, or leading, edge of a wing.
Drag
Factors that contribute to lift in flight also contribute to undesirable forces
called drag. Drag is the force that tends to retard the motion of the plane
through the air. Some drag is a result of the resistance of the air to objects
moving in it and is dependent upon the shape and smoothness of the surface. It
can be reduced by streamlining the aircraft. Some designs also incorporate
devices to reduce the drag owing to friction by maintaining the surface airflow
in so-called “laminar” form.
Another form of drag, however, known as induced drag, is a direct result of the lift
produced by the wing. Work has to be done to produce lift and the induced drag is
the measure of this. The expenditure of energy appears in the form of eddies, or
vortices, which form along the trailing edge and especially at the outer extremities,
or tips, of the wing.
Aeroplane designers conceive aircraft with the highest possible ratio of lift to drag,
which occurs when the drag resulting from the shape is equal to the induced drag
resulting from the lift. The lift-to-drag ratio is limited by factors such as speed and
acceptable weight of the airframe. A subsonic transport aircraft may have a lift-todrag ratio of about 20, while that of a high-performance sailplane may be twice this.
On the other hand, the extra drag that occurs when an aircraft flies at supersonic
speed reduces the achieved lift-to-drag ratio to less than 10.
Aeroplane, heavier-than-air craft that is usually propelled mechanically and
supported by the aerodynamic action of the airstream on fixed-wing surfaces.
Other types of aircraft that are heavier than air include the glider or sailplane,
which is similarly equipped with fixed-wing surfaces but is not self-propelled, and
rotary-wing aircraft, which are mechanically driven and supported by overhead
rotors, such as the Autogiro and Helicopters. Another type is the ornithopter,
which is lifted and propelled by flapping wings. Toy-sized ornithopters have been
developed, but large-scale experiments have been unsuccessful. For the history of
heavier-than-air craft, see Aviation
The term “aeroplane” generally denotes craft operated from land bases, but it
applies also to several other categories of aircraft, including the carrier-based
plane, the seaplane, and the amphibian. The principal variation in
configuration can be found in the landing apparatus.
The carrier-based plane is a type of land plane designed for use on an
aircraft carrier, and is fitted with a tail hook that engages a cable stretched
across the deck to arrest the plane after landing. The seaplane employs floats
instead of the wheel gear of the land plane. In the variety of seaplane known
as the flying boat, the fuselage is constructed as a hull, similar to that of a
seagoing vessel, and serves to keep the plane buoyant. The amphibian is
equipped with both wheel gear and hull or floats to permit operation with
equal effectiveness on land and water.
Before World War II, flying boats were used for military transports and
for intercontinental commercial service. These planes were limited to
low flying speeds and to low landing speeds in water. With the advent of
planes that fly and land much faster, to gain efficiency, large planes have
been limited to land-based operation. The amphibian, even slower
because of its double undercarriage, is less commonly employed than the
land plane. For light sports planes,
amphibious floats are available. Generally resembling conventional floats, they
have a recessed wheel located at the centre. The wheel does not extend far
enough to add much drag to the float in the water, but it protrudes far enough
to enable wheeled landings to be made on hard-surfaced runways or short-cut
grass.
Particular types of heavier-than-air craft include the VTOL (vertical take-off
and landing) and STOL (short take-off and landing) craft, and the
convertiplane. The VTOL craft is an aeroplane that can rise vertically, move off
horizontally, and then reverse the procedure for a landing. The term “VTOL”
is limited to describing aircraft with performance similar to that of
conventional aeroplanes but with additional vertical take-off and landing
ability. Several means are used to lift VTOL aircraft off the ground. The direct
downward thrust of jet engines is used in several designs, but the power
required is high. Rotating wings and ducted fans are also
used for direct lift, but they introduce drag into the horizontal flight.
Convertiplanes, combining the rotors of helicopters with the fixed
wings of aeroplanes, show promise for short-distance commercial
VTOL operation. They compete directly with helicopters, but can fly
faster.
The STOL craft is an aeroplane that takes off and lands very steeply,
thus requiring only a short runway. For a given payload, it is more
efficient in terms of fuel consumption and power requirements than a
VTOL craft. It is also capable of higher speeds and longer-range
flights than a helicopter. In September 1999 a solar-powered aircraft
completed its first test-flight in California. The aircraft has a
wingspan of 75 m (247 ft) and flies without a pilot. In the future it is
thought the plane could remain in continuous flight for up to six
months at a time and would be employed for scientific tests and
telecommunications projects. For lighter-than-air craft, see Airship;
Balloon
The present-day conventional aeroplane may be divided into four
components: fuselage, wings, tail assembly, and landing gear, or
undercarriage.
A Fuselage
In the early days of aviation, the fuselage was merely an open framework to
support the other components of the plane; the bottom of the airframe served
as the landing gear. Subsequently, the need for greater strength and better
performance resulted in the development of enclosed, box-like “strut-andwire” fuselages that decreased drag, and also provided protection for pilot
and passenger, as well as space for the payload. This “truss” structure was
gradually superseded by the monocoque (literally, single shell) fuselage. The
loads imposed on such a structure are carried primarily by the skin, rather
than by an internal framework, as in the trussed structure. It is the most
common fuselage presently in use. The outer shell also confers the possibility
of pressurizing the internal volume for high-altitude flight.
Wings
Assembling an Aeroplane Here, at a McDonnell-Douglas assembly line,
several large passenger planes are in production. In the foreground, the
wing infrastructure is fastened to a body section. Further on in the
assembly line, the tail section and engine mounts are added.Tom
Carroll/Phototake NYC
Although the single-winged plane, known as the monoplane, made its
appearance in the first decade of powered flight, early aeroplane
construction favoured the use of two wings (the biplane), and
occasionally even three or four. Multiple-wing planes have the
advantage of superior lift and relatively stronger construction, but the
monoplane has lower drag. Once the cantilever principle of wing
construction was developed, the dominance of the monoplane was
assured, although it did not become the design of choice until the 1930s.
Cantilever wings obtain their entire strength from internal structural
elements. Cantilever construction is employed in most present-day
aircraft, and external bracing is used only for some small, light planes.
The structure of a typical wing consists of a spar-and-rib framework enclosed
by a thin covering of metal sheet. Treated fabric, or, infrequently, bonded
plywood or resin-impregnated glass fibre are used for some small aircraft and
sailplanes. The spar, or beam, extends from the fuselage to the wing-tip. One
or more spars may be used in the wing, but the two-spar design is most
common. The ribs, normally at right angles to the spars, give the wing its
external shape. If the covering is of metal sheet, it contributes its own share of
strength to the wing. This “stressed-skin” type of wing is used in all large
planes, although there is an increasing use of high-strength reinforced plastic
skins and structure.
The size and shape of wings vary widely, depending on specific aerodynamic
considerations. Wings of many supersonic planes have a high degree of
sweepback (arrowhead tapering from the nose of the plane) and are as thin as
possible, with a knife-like leading edge. Such a shape helps to reduce the
shock of compression when the plane approaches the speed of sound. The
structural importance of the wing is dramatically demonstrated by the
development of the so-called “flying wing”, a craft in which fuselage and tail
are almost entirely eliminated.
Tail Assembly
The conventional type of tail assembly consists of two basic surfaces,
horizontal and vertical, each of which has movable sections contributing
to control of the craft and fixed sections to provide stability. The leading
section of the horizontal surface is known as the horizontal stabilizer,
and the rear movable section is known as the elevator. Sometimes the
whole surface can move and the elevator is eliminated. The stationary
section of the vertical surface is called the fin, and the movable section,
the rudder. Two vertical surfaces are used in some aircraft; in that case,
a double rudder is used. The V-shaped tail combines the rudder and
elevator functions in a single device. Tails vary in size according to the
type of aircraft. In some supersonic aircraft the horizontal tail is
replaced by a foreplane, or “canard”, located near the nose of the plane.
Landing Gear
Present-day landing gear is one of the most intricate of all aeronautical
mechanisms. Its components include the shock strut, a hydraulic leg
connecting the wheel with the wing or fuselage to absorb the shock of
landing; the retracting mechanism, which raises and lowers the gear; the
wheels; and the wheel brakes. There are a number of types of
undercarriage, but two are most commonly employed: the older twowheel gear and the nose or tricycle gear, which is now usual. The
former consists of two large wheels located forward of the centre of
gravity of the plane with a small wheel at the tail. A tricycle gear
consists of two large wheels or wheelgroups behind the centre of gravity
and a third wheel, called the nosewheel, in front of the two main
wheels. Landing is easier with the tricycle gear because braking and
manoeuvring are improved and the danger of nosing over is
diminished. Some large aircraft have more than two rear wheel groups.
Other forms of landing gear include a caterpillar tread for handling
heavy loads on poor landing fields, a swivelling gear for landing in
crosswinds, and a combination ski-wheel gear for use on ice and snow.
Components of modern aircraft necessary for flight control include devices
manipulated from the cockpit by the stick or wheel and by the rudder
pedals, and instruments that provide the pilot with essential information.
A Mechanical Controls
Basic Movements of an Aeroplane Bridgeman Art Library, London/New
York
Expand
The attitude of an aeroplane (its orientation relative to the horizon and to
the direction of motion) is conventionally determined by three control
devices, each of which provides for movement about a different axis. The
three devices include the movable sections of the tail, which are the
elevators and rudders; and the movable sections of the trailing (aft) edge of
the wing, known as ailerons. The control surfaces are operated from the
cockpit by means of a control stick or wheel column and rudder pedals.
Stick control is used in smaller, lighter aeroplanes, and the wheel, with its
greater leverage, is generally used in larger aircraft, as well as in some
small ones
Elevators provide for pitching movement around the lateral axis. A backward
pull on the control stick or wheel column raises the elevators, thereby
depressing the tail and lifting the nose of the plane for a climb. Forward
movement of the stick or column produces the opposite effect, making the
plane dive.
Ailerons, usually placed far out on the wing, control rolling movement around
the longitudinal axis. Leftward movement of the stick or wheel raises the left
aileron and lowers the right, thereby banking the plane to the left. The
reverse tilt occurs when the stick or wheel is moved to the right.
Rudders provide for turning movement around the vertical axis, in
coordination with the ailerons, changing the course of the plane to the left or
right. When the right rudder pedal is pressed, the rudder turns the plane to
the right around the vertical axis. Pressing the left pedal produces a left turn.
To ensure easier and more dependable handling of all control surfaces, a
number of secondary controls have been devised. Trim tabs are used on
rudders, elevators, and ailerons as a means of adjusting the equilibrium,
or trim, of the plane. Other
secondary controls include flaps (on trailing edges) and slots (on leading
edges) to increase lift for take-off or drag for landing, or to improve various
other flight characteristics. Spoilers are surfaces that normally lie flush with
the wing but can be raised to present a flat surface to the airstream and
“spoil” the lift of the wing. Somewhat similar surfaces are called air brakes
and extend at right angles to the fuselage or undersurface of the wing to slow
the speed of the plane. The control surfaces may be operated directly by pilot
effort or by a hydraulic or electrical power system. In the latter case the
pilot’s commands may be transmitted mechanically, by electrical signals
(“fly-by-wire”) or by optical signals (“fly-by-light”).
Instruments
Flight Control Panel The cockpit of a Concorde jet shows the complexity of
flight controls. Electronic and computerized equipment in the cockpit
provides information regarding navigation, speed, altitude, landing, and
engine performance.Albert Visage/Explorer/Photo Researchers, Inc.
Information required in flight is provided by various types of equipment,
which may be divided into four general categories: power-plant
instruments, flight instruments, landing instruments, and navigation aids.
Power-plant instruments indicate whether the engines are functioning
properly and include the tachometer, which shows the revolutions per
minute of each engine, various pressure gauges, temperature indicators,
and the fuel gauges. The primary flight instruments provide indications of
speed (the air-speed indicator), direction (the magnetic compass and the
directional gyro), altitude (the altimeter), and attitude (the rate-of-climb
and turn-and-bank indicators and the artificial horizon). Several of the
flight instruments, including the automatic pilot, utilize the gyroscopic
principle.
Landing instruments needed in poor visibility are of two types, the
instrument-landing system (ILS), providing direct signals to the pilot to
ensure a safe landing, and the ground-controlled approach (GCA), a system
employing radar equipment on the ground to guide the pilot solely by radiotelephonic advice. The ILS is widely used in civil aviation; the GCA system,
in military aviation. Both systems may also use the standard approach
lighting system (ALS), which guides the aeroplane the last few hundred
metres of the airway route to the runway.
Flying the Flyer
Orville Wright mans the controls
of the Wright Flyer in 1908, five
years after he made the world’s
first successful, sustained flight.
The Wright brothers’ experiments
with heavier-than-air flight had
launched Flyer I on December 17,
1903, near Kill Devil Hill in Kitty
Hawk, North Carolina. The first
flight lasted about 12 seconds, and
the plane travelled 36.5 m (120 ft)
at an altitude of roughly 3 m (9.9
ft) and an airspeed of 48 km/h (30
mi/h). Wilbur Wright made a
longer, 59-second flight later on the
same day.
Ornithopter Design Leonardo da Vinci designed
several flying machines. This one, called an
ornithopter, simulated the motion of a bird flying.
Assembling an Aeroplane Here, at a
McDonnell-Douglas assembly line, several
large passenger planes are in production. In
the foreground, the wing infrastructure is
fastened to a body section. Further on in the
assembly line, the tail section and engine
mounts are added
Henson and Stringfellow's "Aerial Steam Carriage" One of the biggest difficulties
faced by early would-be pilots was finding an engine that was both powerful and
light. Many models, such as the Henson and Stringfellow’s “Aerial Steam Carriage”
(shown above) might have flown as early as 1845 with adequate engines.
Unfortunately, the only engines available were steam engines, which were too weak
or too heavy for successful flight. It was not until the arrival of the compact,
relatively lightweight petrol engine that planes were able to get off the ground.
Culver Pictures/Courtesy of Gordon Skene Sound Collection.
All rights reserved.
Amelia Earhart
In 1932 Amelia Earhart became the first woman to make a solo
flight across the Atlantic Ocean. In 1937 she and a navigator,
Frederick Noonan, attempted a flight around the world.
Towards the end of their journey they disappeared somewhere
over the central Pacific Ocean; their fate remains a mystery.
Here, Amelia Earhart speaks of the aircraft’s rapid
transformation from a novel invention to an ordinary part of
everyday life.
Precursor to the famous Sopwith Camel fighter of World War I, the
Sopwith Pup was a light, manoeuverable aeroplane. It travelled at speeds
of 185 km/h (115 mph) and was among the first planes to use the new
aileron wing design. Ailerons are hinged flaps on the tips of wings used to
turn, or bank, the plane.Dick Hanley/Photo Researchers, Inc.
During the period before World War I the design of both the aeroplane
and its engine showed considerable improvement. Pusher biplanes—twowinged aeroplanes with the engine and propeller behind the wing—were
succeeded by tractor biplanes, with the propeller in front of the wing.
Only a few types of monoplanes were used.
Throughout World War II, aircraft became increasingly crucial factors
in military strategy and battles. The need to produce high-performance
military aeroplanes as rapidly as possible during the war served as the
impetus for many advances in aircraft design and production techniques.
Here, World War II military aircraft fly in formation.
Air power played a significant role in World War II. The vintage Fairey
Swordfish, shown here, contributed to Allied naval victories in 1940 and
1941. This torpedo bomber, although not as advanced as some of the other
bombers in use at the time, crippled an Italian fleet at Taranto and
inflicted severe damage on the German battleship Bismarck.Ian A.
Griffiths/Robert Harding Picture Library
The most significant development of all was jet propulsion. It was
originally proposed by Frank Whittle in Britain. However, it was Germany
that developed and flew the first jet-propelled aircraft, the Heinkel He 178,
powered by an HeS 3B engine developed by Hans von Ohain. The aircraft
first flew in August 1939, just one week before the outbreak of war.
PROPULSION
Two basic means are used to provide the thrust for an aeroplane in flight:
propellers or jet propulsion. In a propeller-driven aeroplane either a pistondriven internal-combustion engine or a turboprop engine is utilized to drive the
propeller, which thrusts the air backwards because it has aerofoil-shaped blade
sections cutting through the air in a screw-like fashion. In jet propulsion, the
forward thrust is provided by the discharge of high-speed gases through a rearfacing nozzle. Rocket engines, working on a similar principle, are occasionally
used.
An aircraft engine must satisfy a number of major design requirements, including
high reliability, long life, low weight, low fuel consumption, and low frontal area.
The most important factor is reliability. Long life is mainly an economic
consideration, of special interest in commercial aviation. The relative importance
of the other three requirements depends upon the type of plane that the engine is
intended to propel. Low weight and low fuel consumption are naturally
interdependent because the fuel itself is a weight factor. Low frontal area is
desirable as a means of minimizing the drag caused by the engine.
Piston Engines
The piston engine used in most propeller-driven aircraft is one of two types,
the reciprocating engine and the rotary engine. In the reciprocating engine,
heat energy is utilized to move pistons operating within cylinders. Cylinder
arrangement is generally in-line, horizontal-opposed, or radial, and either
air-cooling or liquid-cooling systems are used. Nearly all aircraft
reciprocating engines are petrol operated. In general, the advantages of the
reciprocating engine are reliability and fuel economy. The rotary engine
replaces the pistons by a single rotating one and hence has fewer ports. It is
claimed to produce lower vibration. Some engines of this type are becoming
available for use in small aircraft.
The compound engine consists of a reciprocating engine combined with an
exhaustgas turbine- that drives a supercharger, an air compressor in the
intake system of the engine. The supercharger compensates for the
decreasing density of the atmosphere at higher altitudes. The chief
advantage of the compound engine over the basic reciprocating engine is its
high power at altitude. The compound engine served as the chief engine in
United States military aircraft during World War II, before the advent of jet
propulsion. British high-performance reciprocating engines of that era used
mechanically driven superchargers.
Jet Engines
Most non-reciprocating aircraft engines operate on the principle of jet
propulsion, and include the turbojet, the turboprop, the ramjet, and the
rocket engine. The turbojet and its modifications, the turbofan and
turboprop, are gas turbine engines, in which the air that enters the intake of
the engine is first compressed in a compressor. Fuel is then added to burn
with the oxygen in the air, increasing the gas temperature and volume. The highpressure gases are then directed through a turbine, which drives the rotating
assembly of the engine. In the case of the turbojet the expansion is partial and
the residual gas, which is now at intermediate pressure, is accelerated by
expansion through a rear-facing nozzle, to produce a high leaving velocity and,
with it, the desired thrust.
Turboprop and turbofan engines extract most of the gas energy in the turbine,
the residual jet thrust being of secondary magnitude. Turboprop engines are
efficient for medium-sized planes at speeds up to about 480 to 640 km/h (300 to
400 mph). At higher subsonic speeds the turbofan is the preferred engine, as the
performance of a propeller drops to a low level of efficiency. Turbofan engines
use less fuel and are quieter than turbojets, but at higher, supersonic, speeds the
high exhaust velocity of the turbojet is necessary.
The ramjet engine is a jet engine in which the air compression needed for
combustion is obtained from the speed of forward motion alone. As in the
turbojet, its total power output is delivered as the jet thrust of its expelled gases.
Although the ramjet can be applied to piloted aircraft, its present rate of fuel
consumption is so prohibitively high that it is used only in guided-missile
applications.
The rocket engine carries its own oxidant as well as its fuel and, like the ramjet,
has its chief application in guided missiles. A solid-propellant rocket is used for
rocket-assisted take-off, supplementary initial power for heavily loaded
aircraft.
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