Flight Technical Specifications
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An aeroplane can lift itself because the moving air generates a circulation around the aerofoil (aerodynamically shaped surface). It thereby produces a difference in air pressure, low above the aerofoil and high below it, according to
Bernoulli’s principle. The difference results in an upward pulling effect. The magnitude of this 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-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.
Supersonic Flight
The supersonic age that aviation entered after
World War II presented a number of new problems so revolutionary that aerodynamicists found themselves resorting to flight experimentation as dangerous and adventurous as any faced by early pilots. Neither complex mathematical analyses nor improvement of such research tools as the wind tunnel, in which models of planes are tested, could ensure completely satisfactory performance of an aircraft under the conditions encountered in supersonic flight.The Sound Barrier
The first formidable problem confronted by aerodynamicists is known popularly as the
sound barrier. It arose when planes attained the speed of sound (approximately 1,220 km/h, or about 760 mph, at sea level), scientifically termed Mach 1. An aeroplane about to break the sound barrier is on the verge of catching up with the pressure waves created by its own forward motion. The resulting distortion of the airflow at Mach 1 causes the formation of a shock wave, known as the compressibility shock, which greatly increases the drag of the plane. If the craft is not properly designed to cope with this abrupt change in the nature of the airflow, its control will be severely, if not disastrously, impaired.Noise Pollution
A major problem associated with supersonic aircraft is noise. Engine noise of supersonic transports is louder and more high-pitched than that of subsonic jets, already a serious annoyance to airfield workers and residents of communities near
airports. Medical concern, moreover, has been expressed about the effect of sonic booms, which are produced when the shock waves from an aircraft in supersonic flight pass over the listener. The shock front that travels with the aircraft extends over large vertical distances and reaches the ground with an impact that sounds like an explosion, even if a plane flies at maximum altitude. The shock wave may be so severe that it breaks windows on the ground far beneath the plane. Attempts are being made by designers and manufacturers to reduce both engine noise and sonic booms with aircraft such as the British Aerospace BAe 146, which entered service in 1983, and is one of the world’s quietest jet airliners. Regulations prevent supersonic flight over populated areas.The Heat Barrier
Among other serious problems associated with supersonic flight is the high temperature caused by the friction of the air against the outer surfaces of the aeroplane. This problem is sometimes known as the heat barrier. To withstand the high temperatures and pressures generated at supersonic speed, the structural materials must be more resistant to heat than the materials used for subsonic aircraft.
Titanium is an example of the type of heat-resistant, high-strength metal required in supersonic aircraft. The demands of the supersonic age for higher speeds, higher altitudes, and longer ranges have led not only to new aerodynamic designs, but also to research into new structural materials.
Mach Number, in aerodynamics and fluid mechanics, the ratio of the speed of an object, usually an aircraft, through a fluid (gas or liquid) to the speed of sound in the fluid. The Mach number, which is a pure number, without units, was named after the Austrian physicist and philosopher Ernst Mach. Speeds less than Mach 1 are subsonic, or less than that of sound; speeds about Mach 1 are transonic, or approximately equal to that of sound; and speeds above Mach 1 are supersonic, or greater than that of sound. An aircraft flying at Mach 2, for example, is travelling at twice (or 200 per cent) the speed of sound.
Compass, instrument that indicates direction, used by mariners, pilots, hunters, campers, and other travellers to enable them to get from one place to another. Two fundamental types of compass are used: the magnetic compass, which in a crude form was used as early as the 13th century; and the gyrocompass, a device developed at the beginning of the 20th century. In the magnetic compass, directions are obtained by means of one or more magnetic needles pointing in the general direction of the magnetic North Pole under the influence of the magnetic field of the earth. The gyrocompass, which is unaffected by the magnetism of the earth, consists of a gyroscope, with the spinning wheel on an axis confined to the horizontal plane so that its axle aligns itself with the north-south line parallel to the axis of the rotation of the earth, thereby indicating true north.
Magnetic Compass
In its simplest form this type consists of a magnetized needle mounted on a pivot at the centre of a fixed graduated card so as to permit the needle to swing freely in the horizontal plane. The mariner's compass, a large magnetic type used on board ships, has bundles of parallel magnetic needles attached to the underside of the compass card, which pivots about its centre in a glass-covered bronze bowl. The bowl is hung in gimbals, and hence the card retains a horizontal position despite the pitching and rolling of the ship (see
Navigation).In the liquid compass, which is the most stable type of mariner's compass, the bowl is filled with a liquid, usually a mixture of alcohol and water. The liquid helps to support the graduated card, which, in this type of compass, pivots about its centre and floats in the liquid, thereby reducing pivot friction and lessening the vibrations of the card caused by the motion of the vessel. Because of these advantages the liquid compass is used more often than the dry compass. In both types a black vertical line, known as the lubber's line, is drawn on the inner surface of the bowl. The course of the ship may be obtained by reading the number of degrees on the card opposite the lubber's line. The magnetic compass points towards the magnetic north only if the ship is free of magnetism and if no iron or steel objects are nearby. If the ship is magnetic and iron and steel objects deflect the magnetic needle, the error known as deviation occurs. To correct deviation, the compass is installed in a stand called a compensating binnacle, which is equipped with a system of magnets arranged to compensate for the disturbing influences.
In order to obtain readings of true north on the magnetic compass it must be corrected also for variation—the angle between the magnetic and true meridians. This angle (also called the magnetic declination) varies in amount, in direction from east to west of the true meridian principally with geographical position, and to some extent with time. The amount, direction, and annual change of the variation for most localities on the surface of the earth have been determined and these data are recorded on all charts. Transient, unpredictable changes in variation occur, mainly in the higher latitudes, as a result of magnetic storms (see
Geophysics).The ordinary mariner's compass is unreliable in aircraft because of the errors introduced by sudden turns and acceleration of the plane. To eliminate such errors, specially designed aircraft compasses have magnetic directional units stabilized against the motion of the craft by pendulums or gyroscopes. An important type of gyrostabilized magnetic compass, developed for aircraft and known as the Gyro Flux Gate, operates on the principle of magnetic induction. In this type the direction-sensitive magnetometer consists of induction coils with appropriate windings and excitation so that changes in direction are proportional to voltage induced by the magnetic field of the earth. The induced voltage may be used to activate direction-indicating elements at several locations on the aircraft by remote control.
Gyrocompass This device, incorporating one or more gyroscopes, is used in the navigation of all large vessels. Unaffected by magnetism and pointing to true north, the gyrocompass is not subject to the inherent errors of deviation and variation of the magnetic compass. It is equipped with correction devices for easterly drift resulting from the motion of the earth and for speed and course errors. In most ocean-going vessels the gyrocompass is connected electrically to a gyropilot, a device that automatically steers the ship, keeping it on course in accordance with signals from the gyrocompass.
Altimeter, mechanical or electronic device commonly used in aircraft to measure vertical height above the surface of the earth. Two main types of altimeter exist: pressure and radio. The more common pressure altimeter operates on the principle that atmospheric pressure decreases with an increase in altitude. Pointers on the graduated face of the altimeter dial connect through a system of gears and levers to an aneroid capsule, a hollow, metallic disc partially evacuated of air that expands and contracts slightly with changes in atmospheric pressure, that is, with altitude (see
Barometer). Radioaltimeters, radar devices modified to measure vertical distance only, beam a pulse of electromagnetic radiation downwards from the aircraft. A receiving antenna on the craft then detects the radio waves reflected by the surface of the earth. By measuring the time difference (t) between sending and receiving the pulse, the altitude (h) can be computed in the equation
where c is the speed of
light.Gyroscope, any rotating body that exhibits two fundamental properties: gyroscopic inertia, or "rigidity in space", and precession, the tilting of the axis at right angles to any force tending to alter the plane of rotation. These properties are inherent in all rotating bodies, including the earth itself. The term gyroscope is commonly applied to spherical, wheel-shaped, or disc-shaped bodies that are universally mounted, so as to be free to rotate in any direction; they are used to demonstrate these properties or to indicate movements in space. A gyroscope that is constrained from moving around one axis other than the axis of rotation is sometimes called a gyrostat. In nearly all its practical applications, the gyroscope is constrained or controlled in this way. The prefix gyro is customarily added to the name of the application, as, for instance, gyrocompass, gyrostabilizer, and gyropilot.
Gyroscopic Inertia The rigidity in space of a gyroscope is a consequence of Newton's first law of motion (see Mechanics), which states that a body tends to continue in its state of rest or uniform motion unless subject to outside forces. Thus, the wheel of a gyroscope, when started spinning, tends to continue to rotate in the same plane about the same axis in space. An example of this tendency is a spinning top, which has freedom about two axes in addition to the spinning axis. Another example is a rifle bullet, which, because it spins in flight, exhibits gyroscopic inertia, tending to maintain a straighter line of flight than it would if not rotating. Rigidity in space can best be demonstrated, however, by a model gyroscope consisting of a flywheel supported in rings in such a way that the axle of the flywheel can assume any angle in space. However, the model is moved about, tipped, or turned at the will of the demonstrator, the flywheel will maintain its original plane of rotation as long as it continues to spin with sufficient velocity to overcome the friction with its supporting bearings.
Gyroscopes constitute an important part of automatic-navigation or inertial-guidance systems in aircraft, spacecraft,
guided missiles, rockets, ships, and submarines. The inertial-guidance instruments in these systems comprise gyroscopes and accelerometers that continuously calculate the exact speed and direction of the craft in motion. These signals are fed into a computer, which records and compensates for course aberrations. The most advanced research craft and missiles also obtain guidance from so-called laser gyros, which are not really inertial devices but instead measure changes in counterrotating beams of laser light caused by changes in craft direction. Another advanced system, called the electrically suspended gyro, uses a hollow beryllium sphere suspended in a magnetic cradle. The remainder of this discussion deals with the conventional gyro.Precession When a force applied to a gyroscope tends to change the direction of the axis of rotation, the axis will move in a direction at right angles to the direction in which the force is applied. This motion is produced jointly by the angular momentum of the rotating body and the applied force. A simple example of precession can be seen in the rolling hoop: to cause the hoop to turn a corner, guiding pressure is not applied to the front or rear of the hoop, as might be expected, but against the top. This pressure, although applied about a horizontal axis, does not cause the hoop to fall over, but to precess about the vertical axis, with the result that the hoop turns and proceeds in a new direction.
Applications of the Gyroscope The characteristic of gyroscopic inertia and the force of gravity can be used to make the gyroscope function as a directional indicator or compass. Briefly, if a gyroscope is considered mounted at the equator of the earth, with its spinning axis lying in the east-west plane, the gyro will continue to point along this line as the earth rotates from west to east. So the east end will rise in relation to the earth, although it continues to point the same way in space. If a tube partially filled with mercury is attached to the frame of the gyro assembly, in such a way that the tube tilts as the gyro axle tilts, the weight of the mercury on the west or low side applies a force about the horizontal axis of the gyro. The gyro resists this force and precesses about the vertical axis towards the meridian. In the gyrocompass the controlling forces are applied automatically in just the right direction and proportion to cause the gyro axle to seek and hold the true meridian, that is, to point north and south.
Gyrocompasses are used in sea vessels all over the world. They are free from the vagaries of the magnetic compass; they indicate true, geographic north rather than magnetic north, and they have sufficient directive force to make practicable the operation of accessory equipment such as course recorders, gyropilots, and repeater compasses. The marine gyropilot has no gyroscope, but picks up electrically any divergence from the set course reference supplied by the gyrocompass; these signals are amplified and applied to the steering engine of the ship to cause the rudder to return the ship to its proper course.
Automatic Pilot The automatic pilot, or autopilot, detects variations from the selected flight plan of an aeroplane and supplies corrective signals to the ailerons, elevator, and rudder. A vertical gyroscope detects changes in pitch or roll, and a directional gyroscope detects changes in heading. The altitude is sensed by a barometric sensor. The speed with which these changes occur is determined by rate gyroscopes or accelerometers. The combination of displacement (how much) and rate (how fast) provides a very precise indication of the response needed. The gyroscopes transmit electrical signals to an electronic computer that combines and amplifies them. The computer then transmits corrective signals to servomotors attached to the control surfaces of the aircraft, which move to produce the desired response (see Servomechanism). An autopilot controller attached to the computer enables the pilot manually to execute manoeuvres, such as turns, climbs, and dives, that require a coordinated movement of the control surfaces. At the pilot's discretion, an assortment of navigation and radio aids can be coupled to the autopilot for automatic navigation. These include inertial navigation systems, Doppler radar navigation systems, and radio navigation beacons. Beams used in instrument landing systems (ILS), installed at airport runways, can also be coupled to the autopilot. In poor visibility, the ILS used with the autopilot directs the aircraft automatically to the desired glide path and aligns it with the runway. See Air Traffic Control; Navigation.
Flight Navigation System (VOR)
An omnirange station broadcasts radio beams that pilots within a radius of about 160 km (100 mi) may use for navigation. The VOR (Very High Frequency Omnidirectional Range) station uses a central aerial to broadcast a continuous reference signal and four variable signal aerials that produce a beam rotating at 1800 rpm. A pilot sets a desired course manually, then relies on electronic equipment to process the signals received from the VOR station. The aircraft’s receiver compares the phases of the signals to determine the bearing of the plane and indicates whether the plane is to the left or right of the desired course.