From Wikipedia, the free encyclopedia
Although rockets and missiles also
travel through the atmosphere, most are not considered aircraft because they do
not have wings and rely on rocket thrust as the primary means of lift.
Flying model craft and stories of manned
flight go back many centuries, however the first manned ascent - and safe
descent - in modern times took place by hot-air balloon in the 18th century.
Each of the two World Wars led to great technical advances. Consequently the
history of aircraft development can be divided into five eras:
§ Inter-war,
sometimes called the Golden
Age, 1919 to 1938.
§ Postwar
era, also called the jet age, 1946 to the present day.
Lighter than air – aerostats
Aerostats use buoyancy to float
in the air in much the same way that ships float on the water. They are
characterized by one or more large gasbags or canopies, filled with a
relatively low-density gas such as helium, hydrogen, or hot air, which is less dense than the
surrounding air. When the weight of this is added to the weight of the aircraft
structure, it adds up to the same weight as the air that the craft displaces.
Small hot-air balloons called sky lanterns date back
to the 3rd century BC, and were only the second type of aircraft to fly, the
first being kites.
A balloon was originally
any aerostat, while the term airship was used
for large, powered aircraft designs – usually fixed-wing[citation
needed] – though none had yet been
built. The advent of powered balloons, called dirigible balloons, and later of
rigid hulls allowing a great increase in size, began to change the way these
words were used. Huge powered aerostats, characterized by a rigid outer
framework and separate aerodynamic skin surrounding the gas bags, were
produced, the Zeppelins being the
largest and most famous. There were still no fixed-wing aircraft or non-rigid
balloons large enough to be called airships, so "airship" came to be
synonymous with these aircraft. Then several accidents, such as the Hindenburg
disaster in 1937, led to the demise
of these airships. Nowadays a "balloon" is an unpowered aerostat,
whilst an "airship" is a powered one.
A powered, steerable aerostat is called
a dirigible. Sometimes this term is
applied only to non-rigid balloons, and sometimes dirigible balloon is regarded as the
definition of an airship (which may then be rigid or non-rigid). Non-rigid
dirigibles are characterized by a moderately aerodynamic gasbag with stabilizing
fins at the back. These soon became known as blimps. During
the Second World War, this shape was widely
adopted for tethered balloons; in windy weather, this both reduces the strain
on the tether and stabilizes the balloon. The nickname blimp was adopted along with the
shape. In modern times, any small dirigible or airship is called a blimp,
though a blimp may be unpowered as well as powered.
Heavier than air – aerodynes
Heavier-than-air aircraft must find some
way to push air or gas downwards, so that a reaction occurs (by Newton's laws
of motion) to push the aircraft upwards. This dynamic movement through the air
is the origin of the term aerodyne. There
are two ways to produce dynamic upthrust: aerodynamic lift, and powered lift in the
form of engine thrust.
Aerodynamic lift involving wings is the most common, with fixed-wing
aircraft being kept in the air by the
forward movement of wings, and rotorcraft by
spinning wing-shaped rotors sometimes called rotary wings. A wing is a flat,
horizontal surface, usually shaped in cross-section as an aerofoil. To fly, air must flow over the
wing and generate lift. A flexible wing is a wing made of fabric or
thin sheet material, often stretched over a rigid frame. A kite is
tethered to the ground and relies on the speed of the wind over its wings,
which may be flexible or rigid, fixed, or rotary.
With powered lift, the aircraft directs
its engine thrust vertically downward. V/STOL aircraft,
such as the Harrier Jump Jet and F-35B take off
and land vertically using powered lift and transfer to aerodynamic lift in
steady flight.
A pure rocket is not usually regarded as
an aerodyne, because it does not depend on the air for its lift (and can even
fly into space); however, many aerodynamic lift vehicles have been powered or
assisted by rocket motors. Rocket-powered missiles that obtain aerodynamic lift
at very high speed due to airflow over their bodies are a marginal case.
Fixed-wing
The forerunner of the fixed-wing
aircraft is the kite. Whereas a fixed-wing aircraft relies on its forward
speed to create airflow over the wings, a kite is tethered to the ground and
relies on the wind blowing over its wings to
provide lift. Kites were the first kind of aircraft to fly, and were invented
in China around 500
BC. Much aerodynamic research was done with kites before test aircraft, wind
tunnels, and computer modelling programs became available.
The first heavier-than-air craft capable
of controlled free-flight were gliders.
A glider designed
by Cayley carried out
the first true manned, controlled flight in 1853.
Besides the method of propulsion, fixed-wing aircraft are in general characterized
by their wing
configuration. The most important wing characteristics are:
§ Wing
support – Braced or cantilever, rigid, or flexible.
§ Location
of the horizontal stabilizer, if any.
A flying wing has no
fuselage, though it may have small blisters or pods. The opposite of this is a lifting body, which has no wings, though
it may have small stabilising and control surfaces.
Most fixed-wing aircraft feature a tail
unit or empennage incorporating
vertical, and often horizontal, stabilising surfaces.
Some people consider wing-in-ground-effect vehicles
to be fixed-wing aircraft. Others do not. These craft "fly" close to
the surface of the ground or water. An example is the Russian ekranoplan (nicknamed
the "Caspian Sea Monster"). Man-powered
aircraft also rely on ground
effect to remain airborne, but this
is only because they are so underpowered — in theory, the airframe is capable
of flying much higher.
Rotorcraft
An
autogyro.
Rotorcraft, or rotary-wing aircraft, use
a spinning rotor with aerofoil section blades (a rotary wing) to provide lift. Types include helicopters, autogyros, and various hybrids such as gyrodynes and
compound rotorcraft.
Helicopters have a
rotor turned by an engine-driven shaft. The rotor pushes air downward to create
lift. By tilting the rotor forward, the downward flow is tilted backward,
producing thrust for forward flight. Some helicopters have more than one rotor
and a few have rotors turned by gas jets at the tips.
Autogyros have
unpowered rotors, with a separate power plant to provide thrust. The rotor is
tilted backward. As the autogyro moves forward, air blows upward across the
rotor, making it spin. This spinning increases the speed of airflow over the
rotor, to provide lift. Rotor kites are
unpowered autogyros, which are towed to give them forward speed or tethered to
a static anchor in high-wind for kited flight.
Other methods of lift
X-24B
lifting body, specialized glider
§ A lifting body is the
opposite of a flying wing. In this configuration the
aircraft body is shaped to produce lift. If there are any wings, they are too
small to provide significant lift and are used only for stability and control.
Lifting bodies are not efficient: They suffer from high drag, and must also
travel at high speed to generate enough lift to fly. Many of the research
prototypes, such as the Martin-Marietta
X-24, which led up to the Space Shuttle, were lifting bodies (though
the shuttle itself is not), and some supersonic missiles obtain
lift from the airflow over a tubular body. The flat bodies of recent jet
fighters also produce lift, as in the F-14 Tomcat's "pancake".
§ Powered lift types rely
on engine-derived lift for vertical takeoff and landing (VTOL). Most types transition to fixed-wing lift for horizontal
flight. Classes of powered lift types include VTOL jet aircraft (such as the
Harrier jump-jet) and tiltrotors (such as
theV-22 Osprey), among others.
Gliders are
heavier-than-air aircraft that do not employ propulsion once airborne. Take-off
may be by launching forward and downward from a high location, or by pulling
into the air on a tow-line, either by a ground-based winch or vehicle, or by a
powered "tug" aircraft. For a glider to maintain its forward air
speed and lift, it must descend in relation to the air (but not necessarily in
relation to the ground). Some gliders can 'soar' - gain height from updrafts
such as thermal currents. The first practical, controllable example was
designed and built by the British scientist and pioneer George Cayley, whom many recognise as the
first aeronautical engineer.[2] Common
examples of gliders are sailplanes, hang gliders and paragliders.
Balloons drift with
the wind, though normally the pilot can control the altitude, either by heating
the air or by releasing ballast, giving some directional control (since the
wind direction changes with altitude). A wing-shaped hybrid balloon can glide
directionally when rising or falling; but a spherically shaped balloon does not
have such directional control.
Kites are aircraft[3] that are
tethered to the ground or other object (fixed or mobile) that maintains tension
in the tether or kite line; they rely on virtual or real
wind blowing over and under them to generate lift and drag. Kytoons are balloon-kite hybrids
that are shaped and tethered to obtain kiting deflections, and can be
lighter-than-air, neutrally buoyant, or heavier-than-air.
Lockheed
Martin F-22A Raptor
Airbreathing
jet engines take in air, burn fuel with
it in a combustion
chamber, and accelerate the exhaust rearwards at high speed to
provide thrust. Turbojet and turbofan engines use
a spinning turbine to drive one or more fans, which provide thrust. Anafterburner may be
used to inject extra fuel into the hot exhaust, especially on military
"fast jets".
Jet engines can provide much higher
thrust than propellers, and are efficient at higher altitudes, being able to
operate above 40,000 ft (12,000 m). They are also much more
fuel-efficient at normal flight speeds than rockets. As a consequence, nearly all high-speed and
high-altitude aircraft use jet engines.
Use of a turbine is not absolutely
necessary: Other designs include the pulse jet and ramjet. These mechanically simple designs cannot work when
stationary, so the aircraft must be launched to flying speed by some other
method. Other variants have also been used, including the motorjet and
hybrids such as the Pratt
& Whitney J58, which can convert between turbojet and ramjet
operation.
A helicopter obtains
lift from a powered rotary wing or rotor, which acts much like an upward-pointing propeller. Forward
propulsion is provided by angling the rotor disc slightly forward so that a
proportion of its lift is directed forward to provide thrust. The rotor may,
like a propeller, be powered by a variety of methods such as a piston engine or
turbine. Experiments have also used jet nozzles at rotor blade tips.
[edit]Other methods of propulsion
§ Rocket-powered
aircraft have occasionally been
experimented with, and the Messerschmitt Komet fighter
even saw action in the Second World War. Since then, they have been restricted
to research aircraft, such as the North American
X-15, which traveled up into space where air-breathing engines
cannot work (rockets carry their own oxidant). Rockets have more often been
used as a supplement to the main powerplant, typically for the rocket-assisted
take off of heavily loaded aircraft,
but also to provide high-speed dash capability in some hybrid designs such as
the Saunders-Roe
SR.53.
§ The ornithopter obtains
thrust by flapping its wings. It has found practical use in a model hawk used to
freeze prey animals into stillness so that they can be captured, and in toy
birds.
[edit]General
construction
The parts of an aircraft are generally
divided into three categories:
§ The airframe comprises the mechanical
structure and associated equipment.
§ The propulsion system comprises the engine or
engines and associated equipment.
§ The avionics comprise the electrical
flight control and communication systems.
The main parts of the airframe are the
fuselage, wing and tail.
The fuselage is an
aircraft's main body section containing the crew cockpit or flight
deck, and any passenger cabin or cargo hold. In single- and
twin-engine aircraft, it will often also contain the engine or engines. The
fuselage also serves to position control and stabilization surfaces in specific
relationships to lifting surfaces, required for aircraft stability and
maneuverability.
The wings of an aircraft produce lift.
Many different styles and arrangements of
wings have been used on
heavier-than-air aircraft, and some lighter-than-air craft also have wings.
Most early fixed-wing aircraft were biplanes, having wings stacked one above
the other. Most types nowadays are monoplanes, having one wing each side.
Wings also vary greatly in their shape viewed from above.
Development of an effective set of
flight controls was a critical advance in the development of aircraft. Early
efforts at fixed-wing aircraft design succeeded in generating sufficient lift
to get the aircraft off the ground, but, once aloft, the aircraft proved
uncontrollable, often with disastrous results. The development of effective
flight controls is what allowed stable flight.
The undercarriage or landing
gear, is the structure that supports an aircraft on the ground and allows it to taxi, take off, and land. In the typical
undercarriage, wheels are used, but skids, floats, or a combination of these and
other elements can be used, depending on the surface. Many aircraft have
undercarriage that retracts into the wings and/or fuselage to decrease drag
during flight.
Flying boats are
supported on water by their fuselage and hence have no undercarriage, except
for amphibians,
which have retractable undercarriage allowing them to take off from and alight
on both land and water.
Powered aircraft have one or more
engines. Most aircraft engines are either lightweight piston engines or gas turbines. The fuel is usually kept in
tanks around the vehicle. Most aircraft store the fuel predominantly in the
wings, but may have additional fuel tanks elsewhere.
The flight envelope of an aircraft
refers to its capabilities in terms of airspeed and load
factor or altitude.[5][6] The term
can also refer to other measurements such as maneuverability. When a plane is
pushed, for instance by diving it at high speeds, it is said to be flown
"outside the envelope", something considered unsafe.
The Boeing 777-200LR is the
longest-range airliner, capable of flights of more than halfway around the
world.
The maximal total range is the distance
an aircraft can fly between takeoff and landing, as limited by fuel capacity in
powered aircraft, or cross-country speed and environmental conditions in
unpowered aircraft. The range can be seen as the cross-country ground speed
multiplied by the maximum time in the air.
Ferry range means the
maximum range an aircraft can fly. This usually means maximum fuel load,
optionally with extra fuel tanks and minimum equipment. It refers to transport
of aircraft for use on remote location.
The combat range is the maximum range an
aircraft can fly when carrying ordnance. The combat radius is
somewhat less.
The fuel time limit for powered aircraft
is fixed by the fuel load and rate of consumption. For unpowered aircraft, the
maximum flight time is limited by available daylight hours, weather conditions,
and pilot endurance.
Flight
dynamics is the science of air
vehicle orientation and control in three dimensions. The three critical flight
dynamics parameters are the angles of
rotation in three dimensions about the
vehicle's center of mass, known as pitch, roll, and yaw (quite different from their
use as Tait-Bryan angles).
§ Roll is a
rotation about the longitudinal axis (equivalent to the rolling or heeling of a ship)
giving an up-down movement of the wing tips measured by the roll or bank angle.
§ Pitch is a
rotation about the sideways horizontal axis giving an up-down movement of the
aircraft nose measured by the angle of attack.
§ Yaw is a
rotation about the vertical axis giving a side-to-side movement of the nose
known as sideslip.
A fixed-wing
aircraft increases or decreases the
lift generated by the wings when it pitches, respectively, nose up or down by
increasing or decreasing the angle of attack. A fixed-wing aircraft usually
"banks" to change the horizontal direction of flight. To maintain
direction, efficiency, and controllability of flight the sideslip angle must
remain near zero, though there are instances when an aircraft may be
deliberately "sideslipped," for example, a slip in a
fixed-wing aircraft.
Besides lift, the other main aerodynamic
force on an aircraft is drag opposing
its motion through the air. An aircraft is usually streamlined from nose to
tail to reduce drag.
Flight control
Aerospace
engineers develop control systems for a
vehicle's orientation (attitude) about its center of mass. The control systems
include actuators, which exert forces in various directions, and generate
rotational forces or moments about the aerodynamic
center of the aircraft, and thus
rotate the aircraft in pitch, roll, or yaw. For example, a pitching moment is a
vertical force applied at a distance forward or aft from the aerodynamic center
of the aircraft, causing the aircraft to pitch up or down. Control systems are
also sometimes used to increase or decrease drag, for example to slow the
aircraft to a safe speed for landing.
The major distinction in aircraft types
is between military aircraft, which includes not just
combat types but many types of supporting aircraft, and civil aircraft, which include all
non-military types.
A military aircraft is any fixed-wing or rotary-wing aircraft
that is operated by a legal or insurrectionary armed service of any type.[8] Military
aircraft can be either combat or non-combat:
§ Non-Combat
aircraft are not designed for combat as their primary function, but may carry
weapons for self-defense. Non-combat roles include search and rescue,
reconnaissance, observation, transport, training, and aerial refueling. These aircraft are often
variants of civil aircraft such as the Douglas DC-3 airliner.
Civil aircraft divide into commercial and general types, however there are
some overlaps.
Commercial
aircraft include types designed for
scheduled and charter airline flights, carrying both passengers and cargo. The larger passenger-carrying types are often referred
to as airliners, the largest of which are wide-body
aircraft. Some of the smaller types are also used in general aviation, and some of the larger
types are used as VIP aircraft.
General aviation is a
catch-all covering other kinds of private (where the
pilot is not paid for time or expenses) and commercial use, and involving a
wide range of aircraft types such as business jets (bizjets), trainers, homebuilt, aerobatic types, racers,gliders, warbirds, firefighters, medical transports, and cargo transports,
to name a few. The vast majority of aircraft today are general aviation types.
Experimental
Experimental aircraft are one-off
specials, built to explore some aspect of aircraft design and with no other
useful purpose. The Bell X-1 rocket
plane, which first broke the sound barrier in level flight, is a famous
example.
A model
aircraft, weighing six grams.
Model
A model aircraft is a small unmanned
type made to fly for fun, for static display, for aerodynamic research or for
other purposes. A scale model is a
replica of some larger design.
