Aeroplane

I

Introduction

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.

II

Principles of Flight

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.

A

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.

B

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-to-drag 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.