In the air

An aircraft is a machine that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.

The human activity that surrounds aircraft is called aviation. Crewed aircraft are flown by an onboard pilot, but unmanned aerial vehicles may be remotely controlled or self-controlled by onboard computers. Aircraft may be classified by different criteria, such as lift type, propulsion, usage and others.

(Source photo: facebook.com)

History

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 can be divided into five eras:

Pioneers of flight, from the earliest experiments to 1914.
First World War, 1914 to 1918.
Aviation between the World Wars, 1918 to 1939.
Second World War, 1939 to 1945.
Postwar era, also called the jet age, 1945 to the present day.

(Clyde Cessna in "Silverwings", 1913)

(Photo of Boeing Stratocruiser courtesy of www.aviation-history.com, with permission.)

Flight characteristics

Flight envelope

The flight envelope of an aircraft refers to its capabilities in terms of airspeed and load factor or altitude.The term can also refer to other measurements such as maneuverability. When a craft is pushed, for instance by diving it at high speeds, it is said to be flown "outside the envelope", something considered unsafe.

Range

The Boeing 777-200LR is the longest-range airliner, capable of flights of more than halfway around the world.

The range is the distance an aircraft can fly between takeoff and landing, as limited by the time it can remain airborne.

For a powered aircraft the time limit is determined by the fuel load and rate of consumption.

For an unpowered aircraft, the maximum flight time is limited by factors such as weather conditions and pilot endurance. Many aircraft types are restricted to daylight hours, while balloons are limited by their supply of lifting gas. The range can be seen as the average ground speed multiplied by the maximum time in the air.

Flight dynamics

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 around three axes about the vehicle's center of mass, known as pitch, roll, and yaw (quite different from their use asTait-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.

Flight dynamics is concerned with the stability and control of an aircraft's rotation about each of these axes.

Stability

The empennage of a Boeing 747–200

An aircraft that is unstable tends to diverge from its current flight path and so is difficult to fly. An aircraft which is very stable tends to stay on its current flight path and is difficult to manoeuvre. So it is important for any design to achieve the desired degree of stability. Since the widespread use of digital computers, it is becoming increasingly common for designs to be inherently unstable and to rely on computerised control systems to provide artificial stability.

A fixed wing is typically unstable in pitch, roll and yaw. Pitch and yaw stabilities of conventional fixed wing designs need horizontal and vertical stabilisers, which act in a similar way to the feathers on an arrow. These stabilizing surfaces allow equilibrium of aerodynamic forces and to stabilise the flight dynamics of pitch and yaw.They are usually mounted on the tail section (empennage), although in thecanard layout, the main aft wing replaces the canard foreplane as pitch stabilizer. tandem and Tailless aircraft rely on the same general rule to achieve stability, the aft surface being the stabilising one.

A rotary wing is typically unstable in yaw, requiring a vertical stabiliser.

A balloon is typically very stable in pitch and roll due to the way the payload is hung underneath.

Control

Flight control surfaces enable the pilot to control an aircraft's flight attitude and are usually part of the wing or mounted on, or integral with, the associated stabilizing surface. Their development was a critical advance in the history of aircraft, which had until that point been uncontrollable in flight.

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 two main aerodynamic forces acting on any aircraft are lift supporting it in the air and drag opposing its motion. Control surfaces or other techniques may also be used to affect these forces directly, without inducing any rotation.

Impact and use

In general, aircraft have positive properties as they permit long distance, high speed travel and are often reasonably efficient. In addition to their usefulness, they have some environmental impacts. They generate some atmospheric pollution, are relatively noisy compared to other forms of travel and high altitude aircraft generate contrails, which experimental evidence suggests may alter weather patterns.

Aircraft are produced in several different types optimized for various uses; military aircraft, which includes not just combat types but many types of supporting aircraft, and civil aircraft, which include all non-military types, experimental and model.

Military

Boeing B-17E in flight

A military aircraft is any aircraft that is operated by a legal or insurrectionary armed service of any type. Military aircraft can be either combat or non-combat:

Combat aircraft are aircraft designed to destroy enemy equipment using its own armament. Combat aircraft divide broadly intofighters and bombers, with several in-between types such as fighter-bombers and ground-attack aircraft (including attack helicopters).
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.

Most military aircraft are powered heavier-than-air types. Other types such as gliders and balloons have also been used as military aircraft; for example, balloons were used for observation during the American Civil War and World War I, and military gliders were used during World War II to land troops.

Civil

Agusta A109 helicopter of the Swiss air rescue service

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 passengers, mail and other cargo. The larger passenger-carrying types are the 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, gliders, warbirds and hot air balloons to name a few. The vast majority of aircraft today are general aviation types.

Experimental

An experimental aircraft is one that has not been fully proven in flight, or one that carries an FAA airworthiness certificate in the "Experimental" category. Often, this implies that new aerospace technologies are being tested on the aircraft, although the term also refers to amateur- and kit-built aircraft; many of which are based on proven designs.

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.

(Source: wikipedia.com)