Glasses

Scraping, fracturing, or breakage of the lenses require time-consuming and costly professional repair, though modern plastic lenses are almost indestructible and very scratch-resistant. Navy usage) or "paraffin budgie" (the latter term being mostly used in the UK offshore oil industry). The lenses themselves can also become greasy or trap vapour when eating hot food or swimming or walking in rain, reducing visibility significantly. Some common nicknames for helicopters are "copter", "chopper", "whirlybird", "windmill", "helo" (common U.S. Even though the late-20th century saw the creation of light frames, such as those made of titanium, very flexible frames, and new lens materials and optical coatings, glasses can still cause problems during rigorous sports. A helicopter should not be mistaken for an autogyro, which is a historical predecessor of the helicopter that gains lift from an unpowered rotor. Another unpopular aspect of glasses is their inconvenience. Marine Corps and will be the first mass produced tilt-rotor aircraft to enter service.

Some people who find that wearing glasses may look nerdy turn to contact lenses instead, especially under peer pressure. Hybrid types that combine features of helicopters and fixed wing designs include the experimental Fairey Rotodyne of the 1950s and the Bell Boeing Osprey, which is on order by the U.S. This conception probably comes from an era when most people were illiterate and the first people to wear glasses were those who did a lot of reading. Rotomotion is currently selling a line of small (less than 50 kg) rotorcraft UAVs, including an all electric helicopter. An example of halo effect is seen in the stereotype that those who wear glasses are intelligent or, especially in teen culture, even geeks and nerds. Some companies, notably Schweizer Aircraft Corporation in the USA, are developing remotely-controlled variants of light helicopters for use in future battlefields. In popular culture, glasses were all the disguise Superman and Wonder Woman needed to hide in plain view as alter egos Clark Kent and Diana Prince, respectively. In identifying conventional helicopters during flight it is helpful to know that when viewed from below, the rotor of a French, Russian, or Soviet designed helicopter rotates counter-clockwise, whilst that of a helicopter built in Italy, the UK or the USA rotates clockwise.

And of course John Lennon wore his round "granny glasses" from some of his time with the Beatles to his assassination in 1980. For this reason, good pilotage demands operation within safe flight regimes and avoiding hazardous conditions. Masaharu Morimoto wears glasses to separate his professional persona as a chef from his stage persona as Iron Chef Japanese. Each of these conditions is potentially fatal and recovery might not be possible. British comedic actor Eric Sykes, who became profoundly deaf as an adult, wears glasses that contain no lenses, but are in fact a bone-conducting hearing aid. The following is a list of some of the potential hazards:. Drew Carey continued to wear glasses for the same reason after getting corrective laser eye surgery. For helicopters the hazards are particularly acute since they are flying at relatively low altitude, with little time to react to a sudden event.

British soap star Anne Kirkbride had the same problem: her character on Coronation Street, Deirdre Barlow, became so well-known for her big frames that she was expected to wear them at social gatherings and in international tours, even though Kirkbride has always worn contact lenses. As with any moving vehicle, operation outside of safe regimes could result in loss of control, structural damage, or fatality. American Senator Barry Goldwater continued to wear lensless horn-rimmed spectacles after being fitted with contact lenses because he was not recognizable without his trademark glasses. The whirling rotor blades of a helicopter can cause large charges to build up on the airframe, large enough to cause injury to shipboard personnel should they touch any part of the helicopter as it approaches the deck. For some celebrities, glasses form part of their identity. A secondary purpose of the haul-down device is to equalize electrostatic potential between the helicopter and ship. Glasses can be a major part of personal expression, from the extravagance of Elton John and Dame Edna Everage, from Groucho Marx to John Denver to Lisa Loeb all the way to the varied professional personas of eyeglass-wearing knowledge workers. Navy implementation of this device, based on Beartrap, is called the "RAST" system (for Recovery Assist, Secure and Traverse) and is an integral part of the LAMPS MK III (SH-60B) weapons system.

On the other hand, many people are attracted to people who wear glasses, and glasses are available in a wide range of styles, materials, and even designer labels. The U.S. Contact lenses also provide much improved peripheral vision. This device was pioneered by the Royal Canadian Navy and was called "Beartrap". Glasses are often regarded as unattractive, and many people prefer to wear contact lenses for that reason. Tension is maintained on the cable as the helicopter descends, assisting the pilot with accurate positioning of the aircraft on the deck; once on deck locking beams close on the probe, locking the aircraft to the flight deck. Many people have several pairs of glasses, one for each task or distance, with specific glasses for reading, computer use, and television watching. Shipboard landing for some helicopters is assisted though use of a haul-down device that involves attachment of a cable to a probe on the bottom of the aircraft prior to landing.

Others have lenses specifically intended for use with computer monitors at a fixed distance. In the Royal Navy, landing on is usually achieved by lining up slightly astern and on the port quarter, as the ship steams into the wind and the aircraft captain slides across and over the deck. Some modern multifocal lenses give a smooth transition between these lenses, unnoticeable by most wearers. Navy it is commonly and properly referred to as the flight deck. These were originally separate lenses, as invented by Benjamin Franklin. In the U.S. Traditional multifocal lenses have two or three distinct viewing areas, each requiring a conscious effort of refocusing. A helicopter deck (or helo deck) is a helicopter pad on the deck of a ship, usually located on the stern and always clear of obstacles that would prove hazardous to a helicopter landing.

As people age, their ability to focus is lessened and many decide to use multiple-focus lenses, bifocal or even trifocal to cover all the situations in which they use their sight. The traditional low-tech system is to mount coloured chalk on the rotor tips, and see how they mark a linen sheet.
. Lenses can be ground to specific eyes, but in most cases standard off-the-shelf prescriptions suffice, but require custom-fitting to particular frames. The most common adjustment measurement system is to use a stroboscopic flash lamp, and observe painted markings or coloured reflectors on the underside of the rotor blades. Magnifying lenses for reading that are used to treat mild hypermetropia and presbyopia can be bought off the shelf, but most glasses are made to a particular prescription, based on degree of myopia or hypermetropia combined with astigmatism. Adjustment is difficult in part because measurement of the vibration is hard. Glasses can be very simple. Usually the feedback system uses a mass as a "stable reference" and a linkage from the mass operates a flap to adjust the rotor's angle of attack to counter the vibration.

None of these conditions is considered a disease. Some also use mechanical feedback systems to sense and counter vibration. As most people age the crystalline lens of the eye loses elasticity resulting in presbyopia, which limits their ability to focus on nearby objects. Most also have vibration dampers for height and pitch. Some would say "the rays of light converge at an imaginary point behind the retina." Astigmatism is a condition in which parallel rays of light entering the eye converge at two separate locations, either before and/or after the retina. To reduce vibration, all helicopters have rotor adjustments for height and pitch. Hyperopia is a condition in which parallel rays of light entering the eye do not converge prior to reaching the retina. An unadjusted helicopter can easily vibrate so much that it will shake itself apart.

Myopia is a condition in which parallel rays of light entering the eye converge before reaching to retina. Helicopters vibrate. Emmetropia, the condition of ideal focus is described as two parallel rays of light entering the eye and converging on or at the retina. The redesigns followed the closure of some city heliports and government action to constrain flight paths in national parks and other places of natural beauty. Glasses fitted with corrective lenses are a common means of correcting focus problems such as myopia (nearsightedness, short-sightedness) and hyperopia (farsightedness, long-sightedness). Urban communities have often expressed great dislike of noisy aircraft, and police and passenger helicopters can be unpopular. Virtual reality glasses and helmets have separate video screens for each eye and a method for determining the direction the head is turned. During the closing years of the 20th century designers began working on helicopter noise reduction.

One kind of electronic 3D spectacles uses electronic shutters. There are several reasons why a helicopter cannot fly as fast as a fixed wing aircraft. Another kind of 3D glasses uses polarized filters. The current record is around 400 km/h set by the Westland Lynx. 3D glasses made of cardboard and plastic are distributed at 3D movies. The single most obvious limitation of the helicopter is its slow speed. The classic 3D glasses have one red lens and one blue lens. While fixed-wing aircraft are generally designed so pilots sit on the left side of the aircraft, freeing up their right hand for dealing with radios, engine controls, and the like, helicopters are generally designed so pilots sit on the right side of the aircraft so they can keep their right hand (usually the strong hand) on the cyclic at all times, leaving the radios and engine controls for their left hand (usually the weaker hand).

Classic 3D glasses create the illusion of three dimensions when viewing specially prepared images. Small helicopters can be so unstable that it may be impossible for the pilot to ever let go of the cyclic while in flight. The illusion of three dimensions on a two dimensional surface can be created by providing each eye with different visual information. Changing collective will also cause a change in torque, which will require the pilot to adjust the foot pedals. Polarized sunglasses may present some difficulties for pilots in that reflections from water and other structures often used to gauge altitude may be removed, or instrument readings on liquid crystal displays may be blocked. Increasing collective will reduce rotor RPM, requiring an increase in throttle to maintain constant rotor RPM. Popular among fishermen and hunters, polarized sunglasses allow wearers to see into water when normally glare or reflected light would be seen. Moving the cyclic forward causes the helicopter to move forward, but will also cause a reduction in lift, which will require extra collective for more lift.

Polarization filters remove horizontal rays of light, which can cause glare. Adjusting one flight control on a helicopter almost always has an effect that requires an adjustment of the other controls. Light polarization is an added feature that can be applied to sunglass lenses. Hovering a helicopter has been compared to balancing yourself while standing on a large beach ball. Still, they offer the convenience of not having to carry both clear glasses and sunglasses to those who frequently go indoors and outdoors during the course of a day. When a hovering helicopter is nudged in one direction by a gust of wind, it will tend to continue in that direction, and the pilot must adjust the cyclic to correct the motion. Unfortunately, many car windshields protect the passengers from UV light, while not shielding from bright visible light, making photochromic lenses ineffective where they are most needed. Simply hovering requires continuous, active corrections from the pilot.

Glasses with photosensitive lenses, called photochromic lenses, become darker in the presence of UV light. In contrast, helicopters are very unstable. Sunglasses vary greatly and many offer more style than protection. Many small, fixed wing aircraft are stable enough that a pilot can let go of the controls while looking at a map or dealing with a radio, and the plane will generally stay on course. It is possible to have lenses that look very dark and yet offer little ultraviolet protection. If a gust of wind or a nudge to one of the controls causes a fixed wing aircraft to pitch, roll, or yaw, the aerodynamic design of the aircraft will tend to correct the motion, and the aircraft will return to its original attitude. Due to changes in the atmosphere, ultraviolet levels are much higher than in the past and ultraviolet protection for eyes and skin is even more important. Fixed wing aircraft are usually inherently stable.

Good sunglasses should also protect against ultraviolet light. It took inventors many years to recognize precession, and to learn how to arrange the cyclic's control system to overcome it. plano) lenses that are darkened to provide protection against bright visible light. The helicopter's control linkages rotate the pitching forces 90 degrees backwards against the rotor spin, to push on the sides of the rotor rather than its front and back. Sunglasses may be made with either prescription or non-prescription (i.e. For example, forward motion requires less lift at the front of the disk and more lift at the rear of the disk, so the pilot pushes the cyclic forward. Main article: Sunglasses. So control forces on the rotor are rotated 90 degrees before the desired motion.

Corrective glasses with plastic lenses can often be used in the place of safety glasses in many environments; this is one advantage that they have over contact lenses. This is called "gyroscopic precession". The pictured wraparound safety glasses are evidence of this style change with the close fitting nature of the wraparound dispensing with the need for side shields. This is because when one tries to tilt a spinning object (like a rotor), it moves at right angles to the direction of the force. Recent safety glasses have tended to be given a more stylish design, in order to encourage their use. A very peculiar feature of the cyclic is that the lift is made to occur 90 degrees of rotation before the direction of tilt. They may provide less eye protection than goggles, face shields or other forms of eye protection, but their light weight increases the likelihood that they will actually be used. (see Height-velocity diagram).

Some safety glasses are designed to fit over corrective glasses or sunglasses. Autorotation can allow a pilot to make an emergency landing if the engine failure occurs while the helicopter is traveling high enough or fast enough. The lenses of safety glasses can also be shaped for correction. A transmission connects the main rotor to the tail rotor so that all flight controls are available after engine failure. For example, those used in medicine may be expected to protect against blood splatter while safety glasses in a factory might have stronger lenses and a stronger frame with additional shields at the temples. This technique is known as autorotation. Safety glasses can vary in the level of protection they provide. The main rotor acts like a "windmill" and turns.

Although safety lenses may be constructed from a variety of materials that vary in impact resistance, certain standards suggest that they maintain a minimum 1mm thickness at the thinest point regardless of material. Helicopters are powered aircraft, but they can still fly without power by using the momentum in the rotors and using downward motion to force air through the rotors. Safety glasses are usually made with shatter-resistant plastic lenses to protect the eye from flying debris. On a helicopter, this can happen in any of three ways. Lenses made to conform to the prescription of an ophthalmologist or optometrist are called prescription lenses and are used to make prescription glasses. This condition is called aerodynamic stall. Glasses correcting for myopia will have negative diopter strengths. If the angle of attack of any wing, including rotor blades, is too high, the airflow above the wing separates causing instant loss of lift and increase in drag.

Over-the-counter reading glasses are typically rated at +1.00 to +3.00 diopters. And the angle of attack is decreased on the advancing blade to produce less lift, compensating for the faster airspeed over the blade. The power of a lens is generally measured in diopters. The angle of attack is increased on the retreating blade to produce more lift, compensating for the slower airspeed over the blade. Corrective lenses modify the focal length of the eye to alleviate the effects of shortsightedness (myopia), longsightedness (hyperopia) or astigmatism. To compensate for the added lift on the advancing blade and the decreased lift on the retreating blade, the angle of attack of the blades is regulated as the blade spins around the helicopter. Either of these designs offers dramatically better ability to withstand the stresses of daily wear and the occasional accident. As the blade swings to the other side of the helicopter, it moves at rotor tip speed minus aircraft speed and is called the retreating blade.

Other frames have spring-loaded hinges. As a helicopter moves forward, the rotor blades on one side move at rotor tip speed plus the aircraft speed and is called the advancing blade. For instance, it is now possible to purchase frames made of special memory metal alloys that return to their correct shape after being bent. If the pilot pushes the cyclic forward, then the helicopter tilts forward, and the rotor produces a thrust in the forward direction. Despite the increasing popularity of contact lenses and laser corrective eye surgery, glasses remain very common and their technology has not stood still. This causes the helicopter to tilt in the same direction as the cyclic. These designs were not immediately successful, however, and various styles with attached handles such as scissors glasses and lorgnettes remained fashionable throughout the 18th and into the early 19th century. When it is tilted, the links give a pitch-up at some azimuthal angle and a pitch-down at the opposite angle, hence creating a sinusoidal variation in blade angle of attack.

The modern style of glasses, held by temples passing over the ears, was developed in 1727 by the British optician Edward Scarlett. When the swashplate is not tilted, the blades are all at the collective angle. Girolamo Savonarola suggested that eyepieces could be held in place by a ribbon passed over the wearer's head, this in turn secured by the weight of a hat. The rotating section rotates with the rotor and is connected to blade pitch horns through pitch links, one link for each blade. Early eyepieces were designed to be either held in place by hand or by exerting pressure on the nose (pince-nez). The cyclic controls the angle of the stationary section of the swashplate, which in turn controls the angle of the rotating section of the swashplate. Over time, the construction of spectacle frames also evolved. The cyclic is similar to a joystick and is usually positioned in front of the pilot.

The first lenses for correcting astigmatism were constructed by the British astronomer George Airy in 1827. This variation in lift causes the rotor disk to tilt, and the helicopter to move during hover flight or change attitude in forward flight. The American scientist Benjamin Franklin, who suffered from both myopia and presbyopia, invented bifocals in 1784 to avoid having to regularly switch between two pairs of glasses. The cyclic changes the pitch of the blades cyclically, causing the lift to vary across the plane of the rotor disk. However, it was not until 1604 that Johannes Kepler published in his treatise on optics and astronomy, the first correct explanation as to why convex and concave lenses could correct presbyopia and myopia. Turbine engined helicopters, and some piston helicopters, use servo-feedback loop in their engine controls to maintain rotor RPM and relieves the pilot of routine responsibility for that task. Nicholas of Cusa is believed to have discovered the benefits of concave lens in the treatment of myopia (nearsightedness). The pilot manipulates the throttle to maintain rotor RPM and therefore regulates the effect of drag on the rotor system.

These early spectacles had convex lenses that could correct the farsightedness (presbyopia) that commonly develops as a symptom of aging. In many piston-powered helicopters, the pilot must manage the engine and rotor RPM. Bacon's published writings describe the magnifying glass (which he did not invent), but make no mention of glasses. In general, RPM must be maintained within a tight tolerance, usually a few percent. Other stories, possibly legendary, credit Roger Bacon with the invention. If the RPM is too high, damage to the main rotor hub from excessive forces could result. In 1738, a Florentine historian named Domenico Manni reported that a tombstone in Florence credited one Salvino d'Armato (died 1317) with the invention of glasses. If the RPM is too low, rapid descent with power, known as settling with power could result.

Based on this evidence, Redi credited another Dominican monk, Fra Alessandro da Spina of Pisa, with the re-invention of glasses after their original inventor kept them a secret, a claim contained in da Spina's obituary record. Helicopter rotors are designed to operate at a specific RPM. In 1676, Franciscus Redi, a professor of medicine at the University of Pisa, wrote that he possessed a 1289 manuscript whose author complains that he would be unable to read or write were it not for the recent invention of glasses, and a record of a sermon given in 1305, in which the speaker, a Dominican monk named Fra Giordano da Rivalto, remarked that glasses had been invented less than twenty years previously, and that he had met the inventor. RPM control is critical to proper operation for several reasons. The identity of the original inventor is unknown, although a possible source is the Arabs, who may have had magnifying lenses in the 10th century. The throttle control is a twist grip on the collective control. Glasses were possibly invented in northern Italy, most likely in the late 1280s. The throttle controls the absolute power produced by the engine that is connected to the rotor by a transmission.

. Simultaneously increasing the collective and adding power with the throttle causes a helicopter to rise. Sunglasses protect against high levels of visible and ultraviolet light. The collective control is usually a lever at the pilot's left side, near his leg. Safety glasses are a kind of eye protection against flying debris or visible and near visible light or radiation. When the angle of attack is increased, the blade produces more lift. Corrective spectacles have lenses shaped to correct vision abnormalities, such as myopia. The collective pitch control lever controls the collective pitch, or angle of attack, of the helicopter blades altogether, that is, equally throughout the 360 degree plane-of-rotation of the main rotor system.

Hydrophobic coatings designed to ease cleaning are also available, as are anti-reflective coatings intended to improve night vision and make the wearer's eyes more visible. Helicopters maneuver with three flight controls besides the pedals. Scratch-resistant coatings can be applied to most plastic lenses giving them similar scratch resistance to glass. More lift at the rear of the rotary wing will cause the aircraft to pitch forward, an increase on the left will cause a roll to the right and so on. Some plastics have a greater index of refraction than most types of glass, allowing thinner lenses for a given prescription. For pitch (tilting forward and back) or roll (tilting sideways) the angle of attack of the main rotor blades is altered or cycled during the rotation creating a differential of lift at different points of the rotary wing. Some plastics also have more advantageous optical properties than glass, such as better transmission of visible light and greater absorption of ultraviolet light. Yaw controls are usually operated with anti-torque pedals, on the floor in the same place as a fixed-wing aircraft's rudder pedals.

Glasses were originally made from glass, but many are now made from plastic (often polycarbonate or CR-39) due to the danger of breakage and the greater weight of glass lenses. Dual-rotor helicopters have a differential between the two rotor transmissions that can be adjusted by an electric or hydraulic motor to transmit differential torque and thus turn the helicopter. In hipster slang they are cheaters. Varying the pitch of the tail rotor alters the sideways thrust produced. Spectacles is often shortened to specs. For rotation about the vertical axis (yaw) the anti-torque system is used. Glasses are more often called eyeglasses in North American English, occasionally spectacles in British English, and (rarely) frames or lenses. In a helicopter, however, there often isn't enough airspeed for this method to be practical.

Historical types include the pince nez, monocle, and lorgnette. In a fixed-wing aircraft, this is easy: small movable surfaces are adjusted to change the aircraft's shape so that the air rushing past pushes it in the desired direction. Modern glasses are typically supported by pads on the bridge of the nose and by temples placed over the ears. Useful flight requires that an aircraft be controlled in all three dimensions (see flight dynamics). Special glasses are used for viewing three-dimensional images or experiencing virtual reality. Although this method is simple and eliminates precession, development of such helicopters ceased soon, because their extreme noise levels preclude both military and civilian use. Glasses, spectacles, or eyeglasses are frames bearing lenses worn in front of the human eyes, sometimes for purely aesthetic reasons but normally for vision correction or eye protection. The most unusual design is the roto-rocket principle, where the single main rotor draws power not from the shaft, but from its own wingtip jet nozzles, which are either pressurized from a fuselage-mounted gas turbine or have their own pulsejet combustion chambers.

Eyeglasses were a common part of the hipster persona, for example Dizzy Gillespie. The NOTAR system was developed in the United States and is used exclusively by McDonnel Douglas Helicopters, or MD Helicopters. Cheaters is used in the hipster argot. The NOTAR eliminates the tail rotor by conducting high-velocity air through the tail boom. Lenses is also sometimes used to refer to framed eyepieces, although it is not common. A recent development in helicopter technology is the NOTAR system, which stands for NO TAil Rotor. Frames is sometimes used to refer to framed eyepieces, although it is not common. V-22 Osprey tilting rotorcraft is similar, although its nacelles can be rotated, and shares some of the inherent technical problems of a cross system.

In contrast, glass eye refers to a cosmetic prosthetic artificial eye that replaces a missing eye. The U.S. Eye glasses or eyeglasses is a word used in North American English. The world's largest ever helicopter, the Soviet Mil-V-12 prototype, was a cross of two Mil Mi-6 turbine-rotor units built onto a modified Antonov cargo plane. Also in frequent use is the shortened form, specs. The 1930s German FW-61 helicopter was built to such design. Spectacles is widely used in Britain and occasionally in the US, in addition to use by professional opticians. Such helicopters are rare, because structural integrity of the wings is difficult to maintain against the amplified resonance of far off-board rotor-turbine units.

Compare with other meanings of the word glass. In the cross system, the rotary wing aircraft resembles a traditional fixed-wing airplane, with the two main rotors mounted at the extremities of its wings. Pair of glasses (or just glasses) is commonly used in Britain and in North America. These were placed at the corners of an equilateral triangle and all turned the same direction. A helicopter built by Juan de la Cierva had three main rotors. The main drawback of a waggon is limited agility in air and the need for a highly trained crew, as the large main rotors have long outreach beyond the fuselage and may easily hit nearby obstacles (in 2001, a South Korean army CH-47 Chinook crashed onto a bridge for that reason while being shown live on TV).

The rotors and turbines are located very high on top of the fuselage, making them less sensitive to damage and dirt. Waggon helicopters are practical for military logistical purposes, because entry and unloading is easily facilitated via the unobstructed front and rear ramps. A prime example is the Boeing CH-47 Chinook, that can carry 14 tons of payload. examples), the two main rotors are located at the front and rear extremity of a long, boxy fuselage that resembles a railway wagon.

In the flying-waggon or tandem rotor system (sometimes called "flying banana" for the peculiar shape of early U.S. Kamans have high stability and powerful lifting capability, thus the latest Kaman V-Max model is a dedicated sky crane design, used for construction works. During the Cold War the American Kaman company started to produce similar helicopters for USAF firefighting purposes. The contra-rotating rotors are located on top of the fuselage, close to each other.

The Kaman system of intermeshing rotors, which was developed in Nazi Germany for a small anti-submarine warfare helicopter, features two main rotors on separate, obliquely mounted axles. Another example is the Kamov Ka-26, a successful crop duster aircraft. Co-axial helicopters in flight are highly resistant to side-winds, which makes them suitable for shipboard use, even without a rope-pulley landing system. The co-axial design, where rotors are mounted on top of each other at the top of the fuselage and share a common main axle complex, was first built by Theodore von Karman and Asbóth Oszkár in 1918 and later became the hallmark of soviet Kamov design bureau (see for example the Kamov Ka-50 "Hokum").

These methods introduce even more mechanical complexity to the design and are usually relegated to specialized helicopter types. All of these systems are designed for the same purpose: the torques from each rotor have opposite signs, so the net effect on the vehicle is negligable. Such designs use two rotors which turn in opposite directions, or contra-rotate. There are alternatives to Sikorsky's layout, which save the weight of a tail boom and rotor.

In extreme cases, such as that of the Mil Mi-24, the wings are large enough to obstruct airflow down from the rotors, making the helicopter all but unable to hover. They are also used as external mounts for weapons. Many military helicopters, especially attack types, have short wings called stub wings to add lift during forward motion. Another reason for the angled vertical stabilizer is to make it possible to stage a successful high-speed, run-on landing, in case of the tail rotor failure or damage.

This is commonly known as slip-streaming and can make hovering turns difficult on windy days. At high speeds, it is possible for the vertical stabilizer to counteract the entire torque, leaving more power available for forward flight. To reduce this waste during cruise, the vertical stabilizer is often angled to produce a force which helps counter the main rotor torque. A tail rotor typically uses about 5 to 6% of the engine's power, and this power does not help the helicopter produce lift or forward motion.

The amount of power required to prevent a helicopter from spinning is significant. Notars adjust thrust by opening and closing a sliding circular cover near the end of the tail boom. Other helicopters use a NOTAR (an acronym meaning no tail rotor) design: they blow air through a long slot along the tail boom, utilizing the Coanda effect to produce forces to counter the torque. It is less efficient but the advantages are that less noise is generated, it's safer for people that may walk near it and there is less chance of the blades being damaged by objects because it's shrouded, unlike the traditional tail rotor.

The fenestron rotor system on the model EC120 helicopter uses a shaft driven system and gearbox to turn the fan. If the tail rotor is shrouded (i.e., a fan embedded in the vertical tail) it is called a fenestron. AH-64 Apache).
Sometimes the blades of a tail rotor are not separated by the same angle, but laid out in an X-shape, which is supposed to reduce the noise levels for military use (e.g.

The world's fastest helicopter, the Westland Lynx can perform aerobatic loops and rolls with this conventional rotor system. Almost all civilian helicopters have the main rotor and tail rotor system. The Mil Mi-26 can lift 27 metric tons, the Robinson R22 has a crew of two and a gross weight of 1300 lbs (590 kg). The world's largest and smallest series-produced helicopters follow this principle.

When the thrust from the tail rotor is sufficient to cancel out the torque from the main rotor, the helicopter will not rotate around the main rotor shaft. This rotor creates thrust which is in the opposite direction from the torque generated by the main rotor. At low speeds, the most common way to counteract this torque is to have a smaller vertical propeller mounted at the rear of the aircraft called a tail rotor. It is as follows: turning the rotor generates lift but it also applies a reverse torque to the vehicle, which would spin the helicopter fuselage in the opposite direction to the rotor.

The most common design is the Sikorsky-layout, which is used by approximately 95% of all helicopters manufactured to date. There are several possible design layouts for arranging a helicopter's rotors. The helicopter's rotor can simply be regarded as rotating wings, from where the military appellation of "rotary wing aircraft" originates. A helicopter makes use of the same principle, except that instead of moving the entire aircraft, only the wings themselves are moved in a circular motion.

However, the more the lift of the airfoil, the more drag that is caused. This pressure difference integrated over the airfoil area causes a net lift. Thus, by causing the air to flow faster over the top surface than the bottom, the airfoil causes a pressure difference directed upward. The higher the speed of a fluid, the lower the dynamic pressure (as opposed to static pressure) on the surface.

The longer path that the fluid (in this case air) must travel across the top surface equates to a higher speed. In conventional aircraft, the wing profile (called airfoil) is designed to have a shape where the bottom surface has a shorter path than the top surface. Turboshaft engines are the preferred powerplant for all but the smallest and least expensive helicopters today. The availability of lightweight turboshaft engines in the second half of the 20th century led to the development of larger, faster, and higher performance helicopters.

Improvements in fuels and engines during the first half of the 20th century were a critical factor in helicopter development. Igor Sikorsky is reported to have delayed his own helicopter research until suitable engines were commercially available. This is largely due to higher engine power density requirements when compared with fixed wing aircraft.
Reliable helicopters capable of stable hover flight were developed decades after fixed wing aircraft.

The Bell 47 designed by Arthur Young became the first helicopter to be licensed (in March 1946) for certified civilian use in the United States and two decades later the Bell 206 became the most succesful commercial helicopter ever built with more hours and set (and broken) more industry records than any other aircraft in the world. Mass production of the military version of the Sikorsky XR-4 began in May 1942 for the United States Army. Models such the Flettner FL 282 Kolibri were use in the Mediterranean Sea. Nazi Germany used the helicopter in combat during WWII in little numbers.

The German Focke-Wulf Fw 61 first flew with limited control achieving vertical and forward flight in 1934. A flight of the first fully controllable helicopter was demonstrated by Raúl Pateras de Pescara 1916 in Buenos Aires, Argentina. Developers such as Jan Bahyl, Oszkár Asbóth, Louis Breguet, Paul Cornu, Emile Berliner, Ogneslav Kostovic Stepanovic and Igor Sikorsky pioneered this type of aircraft, with Juan de la Cierva introducing the first practical autogiro in 1923 that was to be the basis for the modern helicopter. The first somewhat practical idea of a human carrying helicopter was first conceived by Leonardo da Vinci around 1490, but it was not until after the invention of the powered aeroplane in the 20th century that actual models were produced.

"Pao Phu Tau" was a 4th century book in China that described some of the ideas in a rotary wing aircraft. This toy eventually made its way to Europe via trade and has been depicted in a 1463 European painting. Since around 400 BC the Chinese had a flying top that was used as a children's toy. Speed and range limitations also constrain commercial applications.

For these reasons, helicopters are not economically viable for commercial transportation. The costs are due to inherent mechanical complexity and greater power requirements for a given gross weight. Helicopters suffer from significantly higher operating and maintenance costs compared with fixed wing aircraft. Unmanned helicopters are used in industrial and military applications in areas deemed dangerous for manned flight.

Helicopters have many uses, both military and civil, including troop transportation, infantry support, firefighting, shipboard operations, business transportation, casualty evacuation (including MEDEVAC, and air/sea/mountain rescue), police and civilian surveillance, carrying goods (some helicopters can carry slung loads, accommodating awkwardly shaped items), or as a mount for still, film or television cameras. . However these other configurations have considerably more cruise speed than a helicopter (270 km/h for a helicopter, 460 km/h for a tiltrotor, 900+ km/h for a vectored thrust airplane), giving each their place in the operational spectrum. Compared to other vertical lift aircraft like Tiltrotors (V-22 Osprey for example) and Vectored Thrust airplanes (AV-8 Harrier for example), helicopters are very efficient, carrying more than twice the payload, consuming less fuel in hover and costing considerably less to buy and operate.

Subject only to refuelling facilities and load/altitude limitations, a helicopter can travel to any location, and land anywhere with enough space (a diameter of length 1.5 times the rotor disk). The compensating advantage is maneuverability: helicopters can hover in place, reverse, and above all take off and land vertically. Compared to conventional fixed-wing aircraft, helicopters are much more complex, more expensive to buy and operate, relatively slow, have shorter range and restricted payload. The first stable, single-rotor, fully-controllable helicopter to enter large full-scale production was made by Igor Sikorsky in 1942.

The engine-driven helicopter was invented by the Slovak inventor Jan Bahyl. The word helicopter is derived from the Greek words helix (spiral) and pteron (wing). Helicopters are classified as rotary-wing aircraft to distinguish them from conventional fixed-wing aircraft. A helicopter is an aircraft which is lifted and propelled by one or more horizontal rotors (propellers).

Vortex ring state, a problem the V-22 Osprey was associated with. Operating within the shaded area of the height-velocity diagram. Low-G condition. Ground resonance.

Settling with power. Retreating blade stall. If this ring is augmented by terrain, wind, rain, or sea spray, the helicopter can lose enough lift to experience settling with power and hit the ground. In these, the downward wind from the rotor causes a circular vortex to form around the rotor.

Helicopters are susceptible to potentially disastrous vortex ring effects. Low or negative-G situations encountered in a semi-rigid system will result in blade flapping down until it hits the tail boom or other airframe structure, followed by rotor separation, causing a crash. Rotorhead design is a limiting factor on many helicopters. The adjustment is either by adjusting the angle of attack of the blades, or by engine-powered vacuum devices that suck air into the blades, adjusting the lift.

In most such designs, the lift is varied cyclically and according to the speed of the helicopter. Fully rigid rotors exist and create very responsive helicopters. The blades are made from composites which can bend without breaking. In some designs the hub is rigid.

At high speeds, the force on the rotors is such that they "flap" excessively and the retreating blade can reach too high an angle and stall. Conversely, the retreating blade flaps down, develops a higher angle of attack, and generates more lift. In consequence, rotor blades are designed to "flap" - lift and twist in such a way that the advancing blade flaps up and develops a smaller angle of attack, thus producing less lift than a rigid blade would. Because the advancing blade has higher airspeed than the retreating blade, a perfectly rigid blade would generate more lift on that side and tip the aircraft over.

Most rotors are not rigid. It is theoretically possible to have spiralling rotors, similar in principle to variable-pitch swept wings, which could exceed the speed of sound, but no presently known materials are light enough, strong enough, and flexible enough to construct them. It is possible for this blade to exceed the speed of sound, and thus produce vastly increased drag and vibration. The airspeed of the forward-going rotor blade is much higher than that of the helicopter itself.

In a moving helicopter, however, the speed of the blades relative to the air depends on the speed of the helicopter as well as on their rotational velocity. When the helicopter is at rest, the outer tips of the rotor travel at a speed determined by the length of the blade and the RPM. Unique to helicopters is vertical ring vortex which is when a helicopter in a hover or decent comes into contact with its own down wash causing imense turbulence and complete loss of lift. Any low rotor RPM flight condition accompanied by increasing collective pitch application will cause aerodynamic stall.

This is called retreating blade stall. With a low enough relative airspeed and a high enough angle of attack, aerodynamic stall is inevitable. As helicopter speeds increase, the retreating blade experiences lower relative airspeeds and the controls compensate with higher angle of attack. As helicopter speed increases, the advancing blades approach the speed of sound and generate shock waves that disrupt the airflow over the blade causing loss of lift.