My Science School

A loudspeaker works like a reversed microphone. Electric current flows into a coil of wire, turning it into an electromagnet. This attracts the coil to another magnet inside the loudspeaker, causing the coil to vibrate. This vibrates a diaphragm at the same frequency as the original sound, pushing air in front of it to carry the sound to the ears of the listeners. Many loudspeakers can be connected together, so that sound is heard all around a large outdoor or indoor space.

A loudspeakers (loud-speaker or speaker) is an electroacoustic transducer which converts an electrical audio signal into a corresponding sound.

A loudspeaker consists of paper or plastic moulded into a cone shape called ‘diaphragm.’ When an audio signal is applied to the loudspeaker’s voice coil suspended in a circular gap between the poles of a permanent magnet, the coil moves rapidly back and forth due to Faraday’s law of induction. This causes the diaphragm attached to the coil to move back and forth, pushing on the air to create sound waves.

Voice coil, usually made of copper wire, is glued to the back of the diaphragm. When a sound signal passes through the voice coil, a magnetic field is produced around the coil causing the diaphragm to vibrate. The larger the magnet and voice coil, the greater the power and efficiency of the loudspeaker.

The coil is oriented co-axially inside the gap; the outside of the gap being one pole and the centre post (called as the pole piece) being the other. The gap establishes a concentrated magnetic field between the two poles of the permanent magnet. The pole piece and backplate are often a single piece, called the pole plate.

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Inside a microphone is a metal disc, called a diaphragm. When a sound wave hits the sensitive diaphragm, it makes it vibrate at the same frequency. This causes a wire coil, beneath the diaphragm, to move up and down. As the coil comes near to a magnet below, it creates a pulse of electric current in the wire. The pattern of these pulses matches the pattern of the sound wave. The pulses can be sent along a wire to a loudspeaker, to be turned back into sound, or they can be recorded on a tape or compact disc.

When you speak, sound waves created by your voice carry energy toward the microphone. Remember that sound we can hear is energy carried by vibrations in the air. Inside the microphone, the diaphragm (much smaller than you'd find in a loudspeaker and usually made of very thin plastic) moves back and forth when the sound waves hit it. The coil, attached to the diaphragm, moves back and forth as well.

The permanent magnet produces a magnetic field that cuts through the coil. As the coil moves back and forth through the magnetic field, an electric current flows through it.

The electric current flows out from the microphone to an amplifier or sound recording device. Hey presto, you’ve converted your original sound into electricity! By using this current to drive sound recording equipment, you can effectively store the sound forever more. Or you could amplify (boost the size of) the current and then feed it into a loudspeaker, turning the electricity back into much louder sound. That's how PA (personal address) systems, electric guitar amplifiers, and rock concert amplifiers work.

Dynamic microphones are just ordinary microphones that use diaphragms, magnets, and coils. Condenser microphones work a slightly different way by using a diaphragm to move the metal plates of a capacitor (an electric-charge storing device) and generate a current that way. Most microphones are omnidirectional, which means they pick up sound equally well from any direction. If you're recording something like a TV news reporter in a noisy environment, or a rare bird tweeting in a distant hedgerow, you're better off using a unidirectional microphone that picks up sound from one specific direction. Microphones described as cardioid and hypercardioid pick up sounds in a kind of "heart-shaped" (that's what cardioid means) pattern, gathering more sound from one direction than another. As their name suggests, you can target shotgun microphones so they pick up sounds from a very specific location because they are highly directional. Wireless microphones use radio transmitters to send their signals to and from an amplifier or other audio equipment (that's why they're often called "radio mics").

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A compact disc (CD) has a plastic surface on which sounds are stored in binary code as very small holes, called pits, and flat areas, called lands. These can be “read” by a laser beam. The laser beam scans across the surface of the disc. When the light falls on a pit, it is scattered, but when it falls on a land, it is reflected back to a light-sensitive detector. This in turn causes a pulse of current to pass to a loudspeaker, which converts it back into sound.

If you have read the HowStuffWorks article How CDs Work, you know that the basic idea behind data storage on a normal CD is simple. The surface of the CD contains one long spiral track of data. Along the track, there are flat reflective areas and non-reflective bumps. A flat reflective area represents a binary 1, while a non-reflective bump represents a binary 0. The CD drive shines a laser at the surface of the CD and can detect the reflective areas and the bumps by the amount of laser light they reflect. The drive converts the reflections into 1s and 0s to read digital data from the disc. See How CDs Work for more information.

Normal CDs cannot be modified -- they are read-only devices. A CD-R disc needs to allow the drive to write data onto the disc. For a CD-R disk to work there must be a way for a laser to create a non-reflective area on the disc. A CD-R disc therefore has an extra layer that the laser can modify. This extra layer is a greenish dye. In a normal CD, you have a plastic substrate covered with a reflective aluminum or gold layer. In a CD-R, you have a plastic substrate, a dye layer and a reflective gold layer. On a new CD-R disc, the entire surface of the disc is reflective -- the laser can shine through the dye and reflect off the gold layer.

When you write data to a CD-R, the writing laser (which is much more powerful than the reading laser) heats up the dye layer and changes its transparency. The change in the dye creates the equivalent of a non-reflective bump. This is a permanent change, and both CD and CD-R drives can read the modified dye as a bump later on.

It turns out that the dye is fairly sensitive to light -- it has to be in order for a laser to modify it quickly. Therefore, you want to avoid exposing CD-R discs to sunlight.

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By putting collars with radio transmitters onto wild animals, naturalists have been able to track their movements, night and day, adding enormously to our knowledge of animal behaviour. The collars do not interfere with the animals’ normal lives. As well as learning about animal migrations and hunting patterns, naturalists are also able to discover more about the life span of animals in the wild, which may differ enormously from that of those kept in zoos and wildlife parks.

Since a protracted durable the tightlipped animals are studied by man, creating use of the many a method. Of course, within the starting it had been the employment of the fundamental explanation that helped them study animals. Folks would watch them, follow their tracks, creating interpretations etc. Those were the times of the co–existence for man and animal. The diversity of the kingdom is exploited so as that each little and enormous animals is tracked and monitored victimization constant system. Application of geoinformatics (remote sensing, Geographic system (GIS) associate degreed GPS) has enjoying an progressively vital role in conservation biology and life management by providing means that for grouping point and habitats data of life. Another advantage of the system is that the facility to integrate non–spatial knowledge directly, purpose knowledge collected from the sphere, GPS knowledge of life observance, pugmarks, scats, pellets etc. are fed directly and might generate a separate layer. But the trendy research goes on the far side the radio signals. It helps researchers to urge additional precise answers to the targeted queries concerning environs, migration patterns among others. And these answers are quantitative and analytical. Also, the advancement in technology has helped scientists to try to analysis victimization additional non–invasive means that and besides create the invasive ways safer. Each time a GPS radio collars tries to record a location it records data on the date, time and latitude. This data is then utilized to calculate the gap between locations, travel speed, location methods, direction, daily activity levels, home ranges, and analysis of spatial and temporal variations in behavior.

Recent technologies have helped solve the matter of untamed life following. Some electronic tags provide off signals that are picked up by radio devices or satellites whereas alternative electronic tags may embody deposit tags. Scientists will track the movement and locations of the labeled animals. These electronic tags will offer a good deal of information. Also, owing to their size and weight, electronic tags could produce drag on some animals, fastness them down. However, they're costlier than the low–tech tags that are not electronic.

Tracking an animal by radio involves 2 devices. A VHF receiver picks up the signal, a bit like a home radio picks up a station's signal. The receiver is sometimes during a truck, an ATV, or an airplane. To stay track of the signal, the soul follows the animal victimization the receiver. A transmitter attached to the animals sends out a proof within the type of radio waves, even as a radio station does. A soul would possibly place the transmitter around associate degree animal’s ankle, neck, wing, carapace, or dorsal fin. This approach of victimization radio following is accustomed track the animal manually however is additionally used once animals are equipped with alternative payloads.

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A video recorder stores television sound and pictures on a magnetic tape. It receives the electric signal that comes through a cable or aerial into the machine, then records it on tape in much the same way as a tape recorder does, although the video recorder makes diagonal tracks so that more information can be held on the tape. A record - replay head in the video recorder enables the information on tape to be sent to a television set.

Video tape recorder, also called Video Recorder, electromechanical device that records and reproduces an electronic signal containing audio and video information onto and from magnetic tape. It is commonly used for recording television productions that are intended for rebroadcasting to mass audiences. There are two types of video tape units: the transverse, or quad, and the helical.

The transverse unit uses four heads rotating on an axis perpendicular to the direction of 2-inch (5-centimetre) tape. The transverse format achieves a 1,500-inch-per-minute head-to-tape speed necessary for high picture quality. For broadcast industry needs, an audio track, control track, and cue track are added longitudinally. These units follow the standards of the North American Television Standards Commission—i.e., the electron beam sweeps 525 horizontal lines at 60 cycles per second.

The helical unit, designed for home and amateur use, uses half- or three-quarter-inch tape traveling around a drum in the form of a helix. There are various forms of these recorders: the playback deck can play back recorded programs but cannot record or erase; the video-record deck can record directly from a camera but cannot record off-the-air programs; the TV-record deck has an antenna and tuner for recording off-the-air programs. Portable reel-to-reel or cassette recorders are also produced.

Videotape has many uses in sport. For example, it may be used for an “action replay”, to check what really happened in a fast-moving sport. Athletes are also able to study videotape in order to see where they are making errors and so improve their technique.

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Television technology uses electric signals through cables or ultra-high frequency (UHF) radio waves to transmit pictures and sound to a television set, which acts as a receiver. The signals come into the television through a cable or an aerial. The picture signals are divided into three — one each for red, green and blue. In the television, there is an electron gun for each colour, which fires electron beams (also known as cathode rays) onto the screen. The screen is covered with chemicals called phosphors. The electron beams scan rapidly across the screen, causing tiny dots of phosphors to glow red, green and blue. Viewed with normal vision, from a distance, the dots blur into a full-colour picture.

Most people spend hours each day watching programming on their TV set, however, many people might wonder how in fact television works. There are many parts to this process and many technologies that are involved. Following are the most important processes and technologies involved in making television work.

Main Elements of the TV Process

There are many major elements that are required in order for TV to work. They usually include a video source, an audio source, a transmitter, a receiver, a display device, and a sound device.

Video Source

The video source is the image or program. It can be a TV show, news program, live feed or movie. Usually the video source has already been recorded by a camera.How TV Works?

Audio Source

Besides the video source, we also need the audio source. Practically all movies, TV shows and news programs have some sought of audio. Audio source can be in the form of mono, stereo or digitally processed to be later played back with surround sound.

Transmitter

The transmitter is necessary for broadcast television companies that broadcast a free signal to viewers in their area. The transmitter transmits both the video and audio signals over the air waves. Both audio and video signals are electrical in nature and are transformed into radio waves which can then be picked up by receivers (your TV set). A transmitter not only transmits one channels audio or video signal, but in most cases many different channels.

Receiver (TV set)

A receiver is usually integrated in your TV set and this receiver is able to grab radio waves (the transmitted signal) and process these radio waves back to audio and video electric signals that can now be played on your TV set.

Display Device

A display device is usually a TV set, but can also be just a monitor. The display device is able to receive electrical signals (usually sent from the receiver) and turn these electrical signals to a viewable image. Most standard TV sets incorporate a cathode ray tube (CRT), however new display devices can include LCD (liquid crystal display) and Plasma (gas charged display) display devices among others.

Sound Device

While most sound devices are built into your TV set in the form of speakers. Audio signals are obviously needed to match up with the video being shown to the viewer. Many newer TV sets have outputs to send the TV sound to high quality speakers that reproduce sound much better. Since audio signals can include surround sound technology, the TV set is able to send audio signals to the proper speakers located around your room.

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The way in which we see the world has been greatly influenced by photography. We are used to seeing printed images that we could never see with our naked eyes, either because they happen too fast, or because a special camera lens has allowed an extraordinary view to be taken.

Macro-photography is a way of photographing very small objects by using special macro lenses. Used for both still and moving pictures, macro-photography has transformed our knowledge of the way that living things, such as insects, behave.

Macro photography is extreme close-up photography, usually of very small subjects and living organisms like insects, in which the size of the subject in the photograph is greater than life size (though macro-photography technically refers to the art of making very large photographs). By the original definition, a macro photograph is one in which the size of the subject on the negative or image sensor is life size or greater. However, in some uses it refers to a finished photograph of a subject at greater than life size.

The ratio of the subject size on the film plane (or sensor plane) to the actual subject size is known as the reproduction ratio. Likewise, a macro lens is classically a lens capable of reproduction ratios of at least 1:1, although it often refers to any lens with a large reproduction ratio, despite rarely exceeding 1:1.

Apart from technical photography and film-based processes, where the size of the image on the negative or image sensor is the subject of discussion, the finished print or on-screen image more commonly lends a photograph its macro status. For example, when producing a 6×4 inch (15×10 cm) print using 35formet (36×24 mm) film or sensor, a life-size result is possible with a lens having only a 1:4 reproduction ratio.

Reproduction ratios much greater than 10:1 are considered to be photomicrography, often achieved with digital microscope (photomicrography should not be confused with microphotography, the art of making very small photographs, such as for microforms).

Due to advances in sensor technology, today’s small-sensor digital cameras can rival the macro capabilities of a DSLR with a "true" macro lens, despite having a lower reproduction ratio, making macro photography more widely accessible at a lower cost. In the digital age, a “true” macro photograph can be more practically defined as a photograph with a vertical subject height of 24 mm or less.

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Moving pictures, or movies, do not really have moving images at all. They are simply a series of still photographs, shown rapidly one after the other. Our brains are not able to distinguish the individual images at that speed, so we see what appears to be a moving picture.

Film, also called movie or motion picture, is a visual art-form used to simulate experiences that communicate ideas, stories, perceptions, feelings, beauty or atmosphere, by the means of recorded or programmed moving images, along with sound (and more rarely) other sensory stimulations. The word “cinema”, short for cinematography, is often used to refer to filmmaking and the film industry, and to the art form that is the result of it.

The moving images of a film are created by photographing actual scenes with a motion-picture camera, by photographing drawings or miniature models using traditional animation techniques, by means of CGI and computer animation, or by a combination of some or all of these techniques, and other visual effects.

Traditionally, films were recorded onto celluloid film through a photochemical process and then shown through a movie projector onto a large screen. Contemporary films are often fully digital through the entire process of production, distribution, and exhibition, while films recorded in a photochemical form traditionally included an analogous optical soundtrack (a graphic recording of the spoken words, music and other sounds that accompany the images which runs along a portion of the film exclusively reserved for it, and is not projected).

The movie camera, film camera or cine-camera is a type of photographic camera which takes a rapid sequence of photographs on an image sensor or on a film. In contrast to a still camera, which captures a single snapshot at a time, the movie camera takes a series of images; each image constitutes a “frame”. This is accomplished through an intermittent mechanism. The frames are later played back in a movie projector at a specific speed, called the frame rate (number of frames per second). While viewing at a particular frame rate, a person’s eyes and brain merge the separate pictures to create the illusion of motion.

Since the 2000s, film-based movie cameras have been largely (but not completely) replaced by digital movie cameras.

A spacesuit is all that stands between an astronaut on a space walk and the emptiness of space. It must supply all his or her needs. There is no breathable atmosphere in space, so a spacesuit supplies oxygen to the astronaut.

Most spacesuits have very specific purposes. First and foremost, they must keep the wearer alive. They serve—in actuality—as individual custom spaceships of a sort, designed for considerably differing environments.

Today, the United States “sports” two specific space suits. One, the Assured Crew Escape System (ACES) suit, is the bright orange contraption familiar to many through viewing shuttle launches on television. Visible during the crew’s “waving walkout” to the Astros Van and their subsequent trip to the launch pad, they are no longer needed with the ending of the space shuttle program (boo hiss!). The suits are now being evaluated for use with future programs—including NASA ’s Orion capsule development—and modified capability to perform spacewalks in the event of a capsule emergency need (e.g., a solar panel that won’t deploy).

Designed to protect the crew only during the ascent (liftoff) and entry (coming back to earth) phases of flight, the ACES suit’s essence was to provide an oxygen source (although a brief one of about 10 minutes) and helmet-suited pressurization for a relatively short period of time, in hopes of enabling the crew to parachute to safety in the event of a bailout emergency. Note that this very purpose went unfulfilled during the Columbia (2003) tragedy and was not even available during Challenger (1986). In fact, the Challenger disaster was one of the key reasons for the original development of this type of spacesuit.

The spacewalking suit, also known as the Extra-Vehicular Activity (EVA) suit, is easily recognizable to even the most casual space buff. Bright white, with identifying stripes of solid, dashed, and angled/hashed red and white, this suit truly was an astronaut’s spaceship. Worn outside of a vehicle while working in the vacuum of outer space, this suit’s design was a bit more extravagant than that of the ACES.

Within the helmet, headphones and a microphone enable the astronaut to communicate with crew members and mission control. All the joins in the spacesuit must be absolutely airtight. Inside, the spacesuit is pressurized, like a deep-sea diver’s suit. A specially treated dark visor protects the astronaut’s eyes from the glare of the Sun, while lights can illuminate dark areas.

A camera may be fixed to the astronaut’s shoulder, so that other crew members and the ground crew can watch what is being done. The temperature, pressure and oxygen levels inside the suit are monitored by a control pack on the astronaut’s front or back. Under the outer suit, a body suit contains pipes through which cool liquid flows to protect the astronaut from the heat of the Sun. The visor and outer layer of the spacesuit must be tough enough not to be torn or cracked by tiny meteorites that may bounce off the astronaut.

Suits are made of artificial materials that offer maximum protection, such as nylon, Kevlar and Dacron. The astronaut is completely sealed within his or her suit, so urine is collected inside for disposal later! On Earth, a spacesuit can be as difficult to walk in as a suit of armour, but in the weightlessness of space, the pull of gravity is not a consideration.

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The space shuttle has many uses and, because it is reusable, has made it possible to pursue some space activities that would otherwise have been too expensive. It is used to launch satellites and to make repairs to existing satellites. The shuttle can also be used as a laboratory, in which to carry out experiments that are only possible in zero gravity.

The space shuttle was NASA’s space transportation system. It carried astronauts and cargo to and from Earth orbit. The first space shuttle flight took place April 12, 1981. The shuttle made its final landing July 21, 2011. During those 30 years, the space shuttle launched on 135 missions.

The space shuttle carried as many as seven astronauts at a time to and from space. In all, 355 people flew on the shuttle. Some of them flew more than one time. During its history, the space shuttle flew many different types of missions. It launched satellites and served as an orbiting science laboratory. Its crews repaired and improved other spacecraft, such as the Hubble Space Telescope. The shuttle also flew missions for the military. On its later missions, the space shuttle was mostly used to work on the International Space Station.

The space shuttle had three main parts. The first part was the orbiter. The orbiter was the large, white space plane where the crew lived and worked. It was the only part of the shuttle that flew into orbit. The orbiter also had a payload bay for carrying cargo into orbit. Five different orbiters took turns flying into space. The second part of the shuttle was the external tank. This was the large orange fuel tank that was attached to the bottom of the orbiter for launch. The third part was actually two pieces. A pair of white solid rocket boosters provided most of the thrust for the first two minutes of a shuttle launch. The solid rocket boosters were long and thin.

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A rocket needs enormous power to escape from the Earth’s gravity. The velocity required to achieve this is called the escape velocity, which is about 49,000km/h (29,000mph). The rocket’s power comes from burning liquid hydrogen and oxygen. Each stage of a rocket is a fuel tank, which is jettisoned when its fuel is used up. After all, carrying an empty fuel tank will only use up more fuel? Only the top stage of the rocket, called the payload, makes the whole journey and brings the crew back to Earth.

The study of rockets is an excellent way for students to learn the basics of forces and the response of an object to external forces. All rockets use the thrust generated by a propulsion system to overcome the weight of the rocket. For full scale satellite launchers, the weight of the payload is only a small portion of the lift-off weight. Most of the weight of the rocket is the weight of the propellants. As the propellants are burned off during powered ascent, a larger proportion of the weight of the vehicle becomes the near-empty tankage and structure that was required when the vehicle was fully loaded. In order to lighten the weight of the vehicle to achieve orbital velocity, most launchers discard a portion of the vehicle in a process called staging. There are two types of rocket staging, serial and parallel.

In serial staging, there is a small, second stage rocket that is placed on top of a larger first stage rocket. The first stage is ignited at launch and burns through the powered ascent until its propellants are exhausted. The first stage engine is then extinguished, the second stage separates from the first stage, and the second stage engine is ignited. The payload is carried atop the second stage into orbit. Serial staging was used on the Saturn V moon rockets. The Saturn V was a three stage rocket, which performed two staging maneuvers on its way to earth orbit. The discarded stages of the Saturn V were never retrieved. The other type of staging is called parallel staging.

In parallel staging, several small first stages are strapped onto to a central sustainer rocket. At launch, all of the engines are ignited. When the propellants in the strap-ons are extinguished, the strap-on rockets are discarded. The sustainer engine continues burning and the payload is carried atop the sustainer rocket into orbit. Parallel staging is used on the Space Shuttle. The discarded solid rocket boosters are retrieved from the ocean, re-filled with propellant, and used again on the Shuttle.

Some launchers, like the Titan III's and Delta II's, use both serial and parallel staging. The Titan III has a liquid-powered; two stage Titan II for a sustainer and two solid rocket strap-ons at launch. After the solids are discarded, the sustainer engine of the Titan II burns until its fuel is exhausted. Then the second stage of the Titan II is burned, carrying the payload to orbit. The Titan III is another example of a three stage rocket. "Serial" brings the rocket back to its original serial configuration and "Parallel" brings back a parallel configuration.

While they cannot fly all the way to orbit, there are two stage model rocket kits available. You can study the flight characteristics of a two stage model rocket by using the Rocket-Modeler II simulation program. And you can use the Circular Orbit simulation program to investigate the velocity and altitude requirements for specific orbit.

At lift-off, the space shuttle has two rocket boosters. These are jettisoned when the shuttle reaches a height of 43km (27 miles). The shuttle usually remains in orbit around the Earth for about seven days, although it can continue for 30 days. When it returns to Earth, the shuttle lands on a runway, in a similar way to an ordinary aircraft. The rocket boosters are reattached to it, so that it is ready for another mission.

Space shuttle, also called Space Transportation System, partially reusable rocket-launched vehicle designed to go into orbit around Earth, to transport people and cargo to and from orbiting spacecraft, and to glide to a runway landing on its return to Earth’s surface that was developed by the U.S. National Aeronautics and Space Administration (NASA). Formally called the Space Transportation System (STS), it lifted off into space for the first time on April 12, 1981, and made 135 flights until the program ended in 2011.

The U.S. space shuttle consisted of three major components: a winged orbiter that carried both crew and cargo; an external tank containing liquid hydrogen (fuel) and liquid oxygen (oxidizer) for the orbiter’s three main rocket engines; and a pair of large, solid-propellant, strap-on booster rockets. At liftoff the entire system weighed 2 million kilograms (4.4 million pounds) and stood 56 metres (184 feet) high. During launch the boosters and the orbiter’s main engines fired together, producing about 31,000 kilonewtons (7 million pounds) of thrust. The boosters were jettisoned about two minutes after liftoff and were returned to Earth by parachute for reuse. After attaining 99 percent of its orbital velocity, the orbiter had exhausted the propellants in the external tank. It released the tank, which disintegrated on reentering the atmosphere. Although the orbiter lifted off vertically like an expendable rocket launcher, it made an unpowered descent and landing similar to a glider.

The space shuttle could transport satellites and other craft in the orbiter’s cargo bay for deployment in space. It also could rendezvous with orbiting spacecraft to allow astronauts to service, resupply, or board them or to retrieve them for return to Earth. Moreover, the orbiter could serve as a space platform for conducting experiments and making observations of Earth and cosmic objects for as long as about two weeks. On some missions it carried a European-built pressurized facility called Spacelab, in which shuttle crew members conducted biological and physical research in weightless conditions.

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Seaplanes and flying boats have floats instead of wheels, so that they can land on water. In the 1930s, flying boats were often larger and more luxurious than ordinary aircraft, as they could be made larger without the expense of creating longer runways at airports around the world. Instead, they took off and landed at sea, taxiing in and out of existing harbours.

In aviation, a water landing is, in the broadest sense, an aircraft landing on a body of water. Some aircraft such as floatplanes land on water as a matter of course. The phrase “water landing” is also used as a euphemism for crash-landing into water an aircraft not designed for the purpose, an event formally termed ditching. In this case, the flight crew knowingly makes a controlled emergency landing on water. Ditching of commercial aircraft is a rare occurrence.

Seaplanes, flying boats, and amphibious aircraft are designed to take off and alight on water. Alighting can be supported by a hull-shaped fuselage and/or pontoons. The availability of a long effective runway was historically important on lifting size restrictions on aircraft, and their freedom from constructed strips remains useful for transportation to lakes and other remote areas. The ability to loiter on water is also important for marine rescue operations and fire-fighting. One disadvantage of water alighting is that it is dangerous in the presence of waves. Furthermore, the necessary equipment compromises the craft's aerodynamic efficiency and speed.

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An airship is a cigar-shaped balloon, filled with a gas. Nowadays, this is usually helium, as it cannot catch fire, unlike the hydrogen used in earlier airships. Beneath the balloon, a cabin (often called a gondola) and engines are suspended. In the 1930s, the Germans developed airships called Zeppelins, although the tragic crash of the Hindenburg in the USA in 1937 really spelled the end of the age of the airship.

Airship, also called dirigible or dirigible balloon, a self-propelled lighter-than-air craft. Three main types of airships, or dirigibles (from French diriger, “to steer”), have been built: nonrigids (blimps), semirigids, and rigids. All three types have four principal parts: a cigar-shaped bag, or balloon, that is filled with a lighter-than-air gas; a car or gondola that is slung beneath the balloon and holds the crew and passengers; engines that drive propellers; and horizontal and vertical rudders to steer the craft. Nonrigids are simply balloons with cars attached by cables; if the gas escapes, the balloon collapses. Semirigids likewise depend on the internal gas to maintain the balloon’s shape, but they also have a structural metal keel that extends longitudinally along the balloon’s base and supports the car. Rigids consist of a light framework of aluminum-alloy girders that is covered with fabric but is not airtight. Inside this framework is a number of gas-filled balloons, each of which can be filled or emptied separately; rigids keep their shape whether they are filled with gas or not.

The usual gases used for lifting airships are hydrogen and helium. Hydrogen is the lightest known gas and thus has great lifting capacity, but it is also highly flammable and has caused many fatal airship disasters. Helium is not as buoyant but is far safer than hydrogen because it does not burn. The gas-containing envelopes of early airships used cotton fabric impregnated with rubber, a combination that was eventually superseded by synthetic fabrics such as neoprene and Dacron.

The first successful airship was constructed by Henri Giffard of France in 1852. Giffard built a 160-kilogram (350-pound) stem engine capable of developing 3 horsepower, sufficient to turn a large propeller at 110 revolutions per minute. To carry the engine weight, he filled a bag 44 metres (144 feet) long with hydrogen and, ascending from the Paris Hippodrome, flew at a speed of 10 km (6 miles) per hour to cover a distance of about 30 km (20 miles).

In 1872 a German engineer, Paul Haenlein, first used an internal-combustion engine for flight in an airship that used lifting gas from the bag as fuel. In 1883 Albert and Gaston Tissandier of France became the first to successfully power an airship using an electric motor. The first rigid airship, with a hull of aluminum sheeting, was built in Germany in 1897. Alberto Santos-Dumont, a Brazilian living in Paris, set a number of records in a series of 14 nonrigid gasoline-powered airships that he built from 1898 to 1905.

Air traffic controllers have screens on which they can see the planes in their sector. It is their job to see that planes are kept safely apart and guided appropriately during take-off and landing. When aeroplanes are near enough, the air traffic controllers can speak to them directly, but they cannot be expected to speak all the languages of international pilots. For this reason, to make communications as safe and clear as possible, all instructions and discussions take place in English all over the world.

Air traffic controllers use an aircraft’s registration mark when calling it by radio. As one letter can sound rather like another, words are used instead, each one standing for the letter that begins it.

Until controller-pilot data link communication (CPDLC) comes into widespread use, air traffic control (ATC) will depend upon voice communications that are affected by various factors. Aircraft operators and air traffic management (ATM) providers, like pilots and controllers, are close partners in terms of “productivity” for enhancing the airport and airspace flow capacity; operators and ATM should also be close partners in terms of “safety” or risk management.

Communication between controllers and pilots can be improved by the mutual understanding of each other’s operating environment. This briefing note provides an overview of various factors that may affect pilot-controller communication. It may be used to develop a company awareness program for enhancing pilot-controller communications.

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