A glider or platform rocker is a type of rocking chair that moves as a swing seat, where the entire frame consists of a seat attached to the base by means of a double-rocker four-bar linkage.
A rocking chair
is a type of chair with two curved bands (also known as rockers) attached to the bottom of the legs, connecting the legs on each side to each other. The rockers contact the floor at only two points, giving the occupant the ability to rock back and forth by shifting his/her weight or pushing lightly with his/her feet. Rocking chairs are most commonly made of wood.
Many find rocking chairs soothing because of the gentle motion. Rocking chairs are also comfortable because, when a user sits in one without rocking, the chair automatically rocks backwards until the sitter's center of gravity is met, thus granting an ergonomic benefit with the occupant kept at an unstressed position and angle. Varieties of rockers include those mounted on a spring base (or platform) called "platform rockers" and those with swinging braces commonly known as gliders.
The word rocking chair comes from the verb to rock
. The first known use of the word rocking chair was in 1766.
Though American inventor Benjamin Franklin is sometimes credited with inventing the rocking chair, historians actually trace the rocking chair's origins to North America during the early 18th century (when Franklin was a child). They were originally used in gardens and were just ordinary chairs with rockers attached. It was in 1725 that early rocking chairs first appeared in England. The production of wicker rocking chairs reached its peak in America during the middle of the 18th century. These wicker rockers, as they were popularly known, were famous for their craftsmanship and creative designs.
Michael Thonet, a German craftsman, created the first bentwood rocking chair in 1860. This design is distinguished by its graceful shape and its light weight. These rocking chairs were influenced by Greek and Roman designs as well as Renaissance and colonial era artistry. During the 1920s, however, folding rocking chairs became more popular in the US and in Europe. They were handy for outdoor activities and travel purposes. By the 1950s, rocking chairs built by Sam Maloof, a US craftsman, became famous for their durability and deluxe appearance. Maloof's rocking chairs are distinguished by their ski-shaped rockers.
Rocking chairs are sometimes associated with maturity and class. They are also often associated with parenting, as the gentle rocking motion can soothe infants.
President John F. Kennedy made the P&P Chair Company rocker famous. The President was prescribed swimming and use of a rocking chair by his physician in 1955 because the President suffered from lingering back problems. The president so enjoyed his rocker he brought it on Air Force One when he traveled around the country and the world. He bought additional rockers for Camp David and the Kennedy estates, and gave them as gifts to friends and heads of state. The Kennedy Rocking Chair is shaped, stem-bent and assembled while green according to the original design.
Media related to Rocking chairs at Wikimedia Commons
A swing is a hanging seat, usually found at playgrounds for children, a circus for acrobats, or on a porch for relaxing. The seat of a swing may be suspended from chains or ropes. Once a swing is in motion it continues to oscillate like a pendulum until external interference or drag brings it to a halt. Swing sets are very popular with children.
On playgrounds, several swings are often suspended from the same metal or wooden frame, known as a swing set, allowing more than one child to play at a time. Such swings come in a variety of sizes and shapes. For infants and toddlers, swings with leg holes support the child in an upright position while a parent or sibling pushes the child to get a swinging motion. Some swing sets include play items other than swings, such as a rope ladder or sliding pole.
For older children, swings are sometimes made of a flexible canvas seat, of rubberized ventilated tire tread, of plastic, or of wood. A common backyard sight in modern America is a wooden plank suspended on both sides by ropes from a tree branch. Older children can go much higher, sometimes over 15 feet (5 m) above the ground. In the United States, some adults go to the extreme and compete in the NSA, "National Swing Association", where competitors reach heights over 20 feet, while leaping at the peak of their swing.
Tire swings are a form of swing made from a whole tire. These are often simply a new or used tire hanging from a tree on a rope. On commercially developed playground swingsets, oversized new tires are often reinforced with a circular metal bar to improve safety and are hung on chains from metal or wooden beams. They may hang vertically or hang flat, suspended from three or more points on one side. These flat type of swings can hold three or more children. Pumping is achieved by using one or two of the three chains attached to the swing, and two (or more) children can pump in turn. Tire swings can also be used in spinners, where the occupants use their feet to propel the tire.
Natural swings may be created by lianas (creeper plants) in a subtropical wild forest like Aokigahara forest near Mount Fuji.
Rope swings are swings created by tying one end of a length of rope to a tree branch, bridge, or other elevated structure. A knot or loop is usually put on the other end to prevent fraying and help the swinger stay on. Rope swings are often situated so that those swinging on them can let go and land in water deep enough to cushion the fall and to be swum around in.
The incorporation of a short board such as a skateboard in which the rider stands is called swing boarding. It is made safer by the use of an attached board and a harness for the rider.
Toddler swings are swings with a bucket shape with holes for the child's legs, or a half-bucket shape and a safety belt, that is intended to reduce the likelihood of a very young child from falling out.
Porch swings are a type of swinging bench primarily intended for adults. Porch swings are permanently mounted to the roof of a porch. The seat typically is large enough to seat about three people. Porch swings are an alternative to using rocking chairs or gliders outdoors.
Canopy swings are similar to porch swings, but they are hung on a separate frame and are usually portable. They derive their name from the canopy installed for sun protection.
Kiiking is a sport played in Estonia whereby players attempt to rotate 360 degrees around the spindle on a long swing made of seat and rigid steel bars in place in ropes or chains.
Hammock swings are portable bed swings made of lightweight material, often attached to a couple of trees in the backyard or to a hammock stand.
Estonian traditional swing
Three boys are on a tire swing
Swing made out of tyres in East Timor
Fresco of St Proculus on a swing, 7th century, South Tyrol
People have been known to fall off swings, injuring themselves.
The United States Patent and Trademark Office was disparaged in 2002 for issuing a patent to a five-year-old boy who claimed to have invented swinging sideways as a new form of entertainment. His father, a patent lawyer who wanted to show his son how the patent system worked, had told the boy that he could file a patent application on anything that the boy invented. The patent was rescinded on reexamination.
A four-bar linkage
, also called a four-bar
, is the simplest movable closed chain linkage. It consists of four bodies, called bars or links, connected in a loop by four joints. Generally, the joints are configured so the links move in parallel planes, and the assembly is called a planar four-bar linkage
If the linkage has four hinged joints with axes angled to intersect in a single point, then the links move on concentric spheres and the assembly is called a spherical four-bar linkage
. Bennett's linkage
is a spatial four-bar linkage with hinged joints that have their axes angled in a particular way that makes the system movable.
Planar four-bar linkages are important mechanisms found in machines. The kinematics and dynamics of planar four-bar linkages are important topics in mechanical engineering.
Planar four-bar linkages are constructed from four links connected in a loop by four one degree of freedom joints. A joint may be either a revolute
, that is a hinged joint, denoted by R, or a prismatic, as sliding joint, denoted by P. The planar quadrilateral linkage is formed by four links and four revolute joints, denoted RRRR. The slider-crank linkage is constructed from four links connected by three revolute and one prismatic joint, or RRRP. The double slider is a PRRP linkage.
Planar four-bar linkages can be designed to guide a wide variety of movements.
Planar quadrilateral linkage, RRRR or 4R linkages have four rotating joints. One link of the chain is usually fixed, and is called the ground link
, fixed link
, or the frame
. The two links connected to the frame are called the grounded links
and are generally the input and output links of the system, sometimes called the input link
and output link
. The last link is the floating link
, which is also called a coupler
or connecting rod
because it connects an input to the output.
Assuming the frame is horizontal there are four possibilities for the input and output links:
Some authors do not distinguish between the types of rocker.
The Grashof condition for a four-bar linkage states: If the sum of the shortest and longest link of a planar quadrilateral linkage is less than or equal to the sum of the remaining two links, then the shortest link can rotate fully with respect to a neighboring link.
In other words, the condition is satisfied if S
is the shortest link, L
is the longest, and P
are the other links.
The movement of a quadrilateral linkage can be classified into eight cases based on the dimensions of its four links. Let a, b, g and h denote the lengths of the input crank, the output crank, the ground link and floating link, respectively. Then, we can construct the three terms:
The movement of a quadrilateral linkage can be classified into eight types based on the positive and negative values for these three terms, T1
, and T3
The cases of T1
= 0, T2
=0, and T3
=0 are interesting because the linkages fold. If we distinguish folding quadrilateral linkage, then there are 27 different cases.
The figure shows examples of the various cases for a planar quadrilateral linkage.
The configuration of a quadrilateral linkage may be classified into three types: convex, concave, and crossing. In the convex and concave cases no two links cross over each other. In the crossing linkage two links cross over each other. In the convex case all four internal angles are less than 180 degrees, and in the concave configuration one internal angle is greater than 180 degrees. There exists a simple geometrical relationship between the lengths of the two diagonals of the quadrilateral. For convex and crossing linkages, the length of one diagonal increases if and only if the other decreases. On the other hand, for nonconvex non-crossing linkages, the opposite is the case; one diagonal increases if and only if the other also increases.
[, or design, of four bar mechanisms is important when aiming to produce a desired output motion for a specific input motion. In order to maximize cost and efficiency, a designer will choose the simplest mechanism possible to accomplish the desired motion. When selecting a mechanism type to be designed, link lengths must be determined by a process called dimensional synthesis. Dimensional synthesis involves an iterate-and-analyze
methodology which in certain circumstances can be an inefficient process; however, in unique scenarios, exact and detailed procedures to design an accurate mechanism may not exist.
The time ratio (Q
) of a four bar mechanism is a measure of its quick return and is defined as follows:
With four bar mechanisms there are two strokes, the forward and return, which when added together create a cycle. Each stroke may be identical or have different average speeds. The time ratio numerically defines how fast the forward stroke is compared to the quicker return stroke. The total cycle time () for a mechanism is:
Most four bar mechanisms are driven by a rotational actuator, or crank, that requires a specific constant speed. This required speed (ωcrank
)is related to the cycle time as follows:
Some mechanisms that produce reciprocating, or repeating, motion are designed to produce symmetrical motion. That is, the forward stroke of the machine moves at the same pace as the return stroke. These mechanisms, which are often referred to as in-line
design, usually do work in both directions, as they exert the same force in both directions.
Examples of symmetrical motion mechanisms include:
Other applications require that the mechanism-to-be-designed has a faster average speed in one direction than the other. This category of mechanism is most desired for design when work is only required to operate in one direction. The speed at which this one stroke operates is also very important in certain machine applications. In general, the return and work-non-intensive stroke should be accomplished as fast as possible. This is so the majority of time in each cycle is allotted for the work-intensive stroke. These quick-return
mechanisms are often referred to as offset
Examples of offset mechanisms include:
With offset mechanisms, it is very important to understand how and to what degree the offset affects the time ratio. To relate the geometry of a specific linkage to the timing of the stroke, an imbalance][ angle (
β) is used.
This angle is related to the time ratio, Q
, as follows:
Through simple algebraic rearrangement, this equation can be rewritten to solve for β
Timing charts are often used to synchronize the motion between two or more mechanisms. They graphically display information showing where and when each mechanism is stationary or performing its forward and return strokes. Timing charts allow designers to qualitatively describe the required kinematic behavior of a mechanism.
These charts are also used to estimate the velocities and accelerations of certain four bar links. The velocity of a link is the time rate at which its position is changing, while the link's acceleration is the time rate at which its velocity is changing.
Both velocity and acceleration are vector quantities, in that they have both magnitude and direction; however, only their magnitudes are used in timing charts. When used with two mechanisms, timing charts assume constant acceleration. This assumption produces polynomial equations for velocity as a function of time. Constant acceleration allows for the velocity vs. time graph to appear as straight lines, thus designating a relationship between displacement (ΔR
), maximum velocity (vpeak
), acceleration (a
), and time(Δt
). The following equations show this.
Given the displacement and time, both the maximum velocity and acceleration of each mechanism in a given pair can be calculated.
Slider-crank mechanisms involve both rotational and linear motion. For most of these mechanisms, a crank rotates at constant speed in order to repeatedly move an object in a linear motion to perform some task. This device is a simple way to convert rotational motion to linear motion.
With engines, for example, a crank continuously rotates which forces many pistons to move linearly back and forth through cylindrical chambers.
There are two types of slider-cranks: in-line and offset. There are also two methods to design each type: graphical and analytical]
An in-line crank slider is oriented in a way in which the pivot point of the crank is coincident with the axis of the linear movement. The follower arm, which is the link that connects the crank arm to the slider, connects to a pin in the center of sliding object. This pin is considered to be on the linear movement axis. Therefore, to be considered an in-line
crank slider, the pivot point of the crank arm must be in-line
with this pin point. The stroke() of an in-line crank slider is defined as the maximum linear distance the slider may travel between the two extreme points of its motion. With an in-line crank slider, the motion of the crank and follower links is symmetric about the sliding axis. This means that the crank angle required to execute a forward stroke is equivalent to the angle required to perform a reverse stroke. For this reason, the in-line slider-crank mechanism produces balanced motion.
This balanced motion implies other ideas as well. Assuming the crank arm is driven by a constant force and therefore constant velocity, the time it takes to perform a forward stroke is equal to the time it takes to perform a reverse stroke.
The graphical method of designing an in-line slider-crank mechanism involves the usage of hand-drawn or computerized diagrams. These diagrams are drawn to scale in order for easy evaluation and successful design. Basic trigonometry, the practice of analyzing the relationship between triangle features in order to determine any unknown values, can be used with a graphical compass and protractor alongside these diagrams to determine the required stroke or link lengths.
When the stroke of a mechanism needs to be calculated, first identify the ground level for the specified slider-crank mechanism. This ground level is the axis on which both the crank arm pivot-point and the slider pin are positioned. Draw the crank arm pivot point anywhere on this ground level. Once the pin positions are correctly placed, set a graphical compass to the given link length of the crank arm. Positioning the compass point on the pivot point of the crank arm, rotate the compass to produce a circle with radius equal to the length of the crank arm. This newly drawn circle represents the potential motion of the crank arm. Next, draw two models of the mechanism. These models will be oriented in a way that displays both the extreme positions of the slider. Once both diagrams are drawn, the linear distance between the retracted slider and the extended slider can be easily measured to determine the slider-crank stroke.
The retracted position of the slider is determined by further graphical evaluation. Now that the crank path is found, draw the crank slider arm in the position that places it as far away as possible from the slider. Once drawn, the crank arm should be coincident with the ground level axis that was initially drawn. Next, from the free point on the crank arm, draw the follower link using its measured or given length. Draw this length coincident with the ground level axis but in the direction toward the slider. The unhinged end of the follower will now be at the fully retracted position of the slider. Next, the extended position of the slider needs to be determined. From the pivot point of the crank arm, draw a new crank arm coincident with the ground level axis but in a position closest to the slider. This position should put the new crank arm at an angle of 180 degrees away from the retracted crank arm. Then draw the follower link with its given length in the same manner as previously mentioned. The unhinged point of the new follower will now be at the fully extended position of the slider.
Both the retracted and extended positions of the slider should now be known. Using a measuring ruler, measure the distance between these two points. This distance will be the mechanism stroke, ().
To analytically design an in-line crank slider and achieve the desired stroke, the appropriate lengths of the two links, the crank and follower, need to be determined . For this case, the crank arm will be referred to as L2
, and the follower link will be referred to as L3
. With all in-line slider-crank mechanisms, the stroke is twice the length of the crank arm. Therefore, given the stroke, the length of the crank arm can be determined. This relationship is represented as:
is found, the follower length (L3
) can be determined. However, because the stroke of the mechanism only depends on the crank arm length, the follower length is somewhat insignificant. As a general rule, the length of the follower link should be at least 3 times the length of the crank arm. This is to account for an often undesired increased acceleration yield]
[, or output, of the connecting arm.
With an offset slider-crank mechanism, an offset distance is introduced.
This offset distance is referred to as L1
and is the fixed distance between the crank arm pivot point and the slider axis. This offset distance means that the slider-crank motion is no longer symmetrical about the sliding axis. In addition, the required crank angles of the forward and reverse strokes are no longer equivalent. An offset slider-crank provides a quick return when a slower working stroke is desired.
With offset slider-cranks, the stroke is always twice the crank length, and as the offset distance increases, the stroke also becomes larger.
The potential range for the offset distance can be written in relation to the other mechanism lengths, L2
, as the equation:
The design of an in-line crank slider mechanism involves finding the two link lengths, L2
, and an appropriate offset distance,L1
, in order to achieve the wanted stroke,(ΔR4)max
, and imbalance angle, β
The analytical method for designing an offset crank slider mechanism is the process by which triangular geometry is evaluated in order to determine generalized relationships among certain lengths, distances, and angles. These generalized relationships are displayed in the form of 3 equations and can be used to determine unknown values for almost any offset slider-crank. These equations express the link lengths, L1, L2, and L3
, as a function of the stroke,(ΔR4)max
, the imbalance angle, β
, and the angle of an arbitrary line M
. Arbitrary line M
is a designer-unique line that runs through the crank pivot point and the extreme retracted slider position. The 3 equations are as follows:
With these relationships, the 3 link lengths can be calculated and any related unknown values can be determined.
Glastonbury chair is a 19th-century term for an earlier wooden chair, usually of oak, possibly based on a chair made for Richard Whiting, the last Abbot of Glastonbury, England. The Glastonbury chair was known to exist since the Early Middle Ages, but seems to have disappeared from use in part of the Later Middle Ages; it re-emerged in use in Italy by the 15th century AD.
It was made originally in Britain from a description brought back from Rome in 1504 by Abbot Richard Beere to Glastonbury Abbey, and was produced for or by John Arthur Thorne, a monk who was the treasurer at the abbey. Arthur perished on Glastonbury Tor in 1539, hung, drawn and quartered alongside his master, Richard Whiting, the last Abbot of Glastonbury, during the dissolution of the monasteries. The Abbot sat on a Glastonbury chair during his trial at Bishop's Palace, Wells, where one of the two original surviving examples (illustrated) can still be seen, together with other chairs of this age and later reproductions.
The second chair remained in St John's Church in Glastonbury until it found its way by an unknown route into the collection of Horace Walpole's Gothic pile Strawberry Hill in Twickenham, Middlesex. When the contents were sold in 1842, the then vicar of Glastonbury, the Reverend Lionel Lewis, made an impassioned speech telling the bidders the chair belonged in Glastonbury. Nobody bidding against him, Lewis took the chair back to Glastonbury where it is extant in St John's Church.
Gordon Browning was the last maker of the Glastonbury chair. A consummate worker in wood, he delighted in telling the story of the history of the chair he exported around the world. Browning had lived in Glastonbury for 80 years by the time of his death, with a brief absence during the Second World War, when he was employed making aircraft frames in Bristol. His son Clive said, "When my father was asked if he had lived all his life in Glastonbury, he loved to say - not yet."
The Glastonbury chair design has become popular with reenactors, owing to its simple construction, wide availability of plans, and the opportunity for extensive decorative carving. As a result, there are likely more chairs of this pattern in existence now than there ever were in period.
The chair does not fold. Although it is frequently assumed to do so, especially when made with circular tenons, triangular frames remain rigid even if they are joined by bearings. If the tenons are tusked then the chair may be quickly dismantled for shipping. Chairs for re-enactment are usually made in this way, but there is no evidence in period that they were regarded as being especially portable.
A glider or sailplane is a type of glider aircraft used in the sport of gliding. It has rigid wings and an undercarriage. Some gliders, known as motor gliders, are also used for gliding and soaring, but have engines which can be used for extending a flight and, for some types, for take-off. Aircraft such as hang gliders and paragliders are foot-launched and so are described in separate articles, though their differences from sailplanes are covered below. Glider aircraft that are used for purposes other than recreation, for example for military purposes, do not soar.
Sports gliders benefit from creating the least drag for any given amount of lift, and this is best achieved with long, thin wings and a fully faired narrow cockpit. Aircraft with these features are able to climb efficiently in rising air and can glide long distances at high speed with a minimum loss of height in between.
Although most gliders do not have engines, there are a few that do. (see Motor glider). The manufacturers of high-performance gliders often list an optional engine with a retractable propeller that can be used to sustain flight, if required; these are known as 'self-sustaining' gliders. Some have enough thrust to launch themselves before the engine is retracted and are known as 'self-launching' gliders. There are also 'touring motor gliders' which can self launch and switch off the engine in flight without retracting their propellers.
Sir George Cayley's gliders achieved brief wing-borne hops from around 1849. In the 1890s Otto Lilienthal built gliders using weight shift for control. In the early 1900s the Wright Brothers built gliders using movable surfaces for control. In 1903 they successfully added an engine.
After World War I gliders were built for sporting purposes in Germany and in the United States. Germany's strong links to gliding were to a large degree due to Post-WWI regulations forbidding the construction and flight of motorised planes in Germany, so the country's aircraft enthusiasts often turned to gliders and were actively encouraged by the German government.
The sporting use of gliders rapidly evolved in the 1930s and is now the main application. As their performance improved, gliders began to be used for cross-country flying and now regularly fly hundreds or even thousands of kilometers in a day if the weather is suitable.
Early gliders had no cockpit and the pilot sat on a small seat located just ahead of the wing. These were known as "primary gliders" and they were usually launched from the tops of hills, though they are also capable of short hops across the ground while being towed behind a vehicle. To enable gliders to soar more effectively than primary gliders, the designs minimized drag. Gliders now have very smooth, narrow fuselages and very long, narrow wings with a high aspect ratio and winglets.
The early gliders were made mainly of wood with metal fastenings, stays and control cables. Later fuselages made of fabric-covered steel tube were married to wood and fabric wings for lightness and strength. New materials such as carbon-fiber, fiber glass and Kevlar have since been used with computer-aided design to increase performance. The first glider to use glass-fiber extensively was the Akaflieg Stuttgart FS-24 Phönix which first flew in 1957. This material is still used because of its high strength to weight ratio and its ability to give a smooth exterior finish to reduce drag. Drag has also been minimized by more aerodynamic shapes and retractable undercarriages. Flaps are fitted to the trailing edges of the wings on some gliders to minimize the drag from the tailplane at all speeds.
With each generation of materials and with the improvements in aerodynamics, the performance of gliders has increased. One measure of performance is the glide ratio. A ratio of 30:1 means that in smooth air a glider can travel forward 30 meters while losing only 1 meter of altitude. Comparing some typical gliders that might be found in the fleet of a gliding club – the Grunau Baby from the 1930s had a glide ratio of just 17:1, the glass-fiber Libelle of the 1960s increased that to 39:1, and modern flapped 18 meter gliders such as the ASG29 have a glide ratio of over 50:1. The largest open-class glider, the eta, has a span of 30.9 meters and has a glide ratio over 70:1. Compare this to the infamous Gimli Glider, a Boeing 767 which ran out of fuel mid-flight and was found to have a glide ratio of only 12:1, or to the Space Shuttle with a glide ratio of 4.5:1.
Due to the critical role that aerodynamic efficiency plays in the performance of a glider, gliders often have aerodynamic features seldom found in other aircraft. The wings of a modern racing glider have a specially designed low-drag laminar flow airfoil. After the wings' surfaces have been shaped by a mold to great accuracy, they are then highly polished. Vertical winglets at the ends of the wings are computer-designed to decrease drag and improve handling performance. Special aerodynamic seals are used at the ailerons, rudder and elevator to prevent the flow of air through control surface gaps. Turbulator devices in the form of a zig-zag tape or multiple blow holes positioned in a span-wise line along the wing are used to trip laminar flow air into turbulent flow at a desired location on the wing. This flow control prevents the formation of laminar flow bubbles and ensures the absolute minimum drag. Bug-wipers may be installed to wipe the wings while in flight and remove insects that are disturbing the smooth flow of air over the wing.
Modern competition gliders carry jettisonable water ballast (in the wings and sometimes in the vertical stabilizer). The extra weight provided by the water ballast is advantageous if the lift is likely to be strong, and may also be used to adjust the glider's center of mass. Moving the center of mass toward the rear by carrying water in the vertical stabilizer reduces the required down-force from the horizontal stabilizer and the resultant drag from that down-force. Although heavier gliders have a slight disadvantage when climbing in rising air, they achieve a higher speed at any given glide angle. This is an advantage in strong conditions when the gliders spend only a small amount of time climbing in thermals. The pilot can jettison the water ballast before it becomes a disadvantage in weaker thermal conditions. Another use of water ballast is to dampen air turbulence such as might be encountered during ridge soaring. To avoid undue stress on the airframe, gliders must jettison any water ballast before landing.
Most gliders are built in Europe and are designed to EASA Certification Specification CS-22 (previously Joint Aviation Requirements-22). These define minimum standards for safety in a wide range of characteristics such as controllability and strength. For example, gliders must have design features to minimize the possibility of incorrect assembly (gliders are often stowed in disassembled configuration, with at least the wings being detached). Automatic connection of the controls during rigging is the common method of achieving this.
The two most common methods of launching sailplanes are by aerotow and by winch. When aerotowed, the glider is towed behind a powered aircraft using a rope about 60 meters (about 200 ft) long. The glider pilot releases the rope after reaching the desired altitude. However, the rope can be released by the towplane also. Winch launching uses a powerful stationary engine located on the ground at the far end of the launch area. The glider is attached to one end of 800–1200 metres (about 2,500–4,000 ft) of cable and the winch rapidly winds it in. The glider can gain about 900–3000 feet of height with a winch launch (about 300 – 900 metres), depending on the head wind. Less often, automobiles are used to pull gliders into the air, by pulling them directly or through the use of a reverse pulley in a similar manner to the winch launch. Elastic ropes (known as bungees) are occasionally used at some sites to launch gliders from slopes, if there is sufficient wind blowing up the hill. Bungee launching was the predominant method of launching early gliders. Some modern sailplanes can self-launch with the use of retractable engines and/or propellers, which can also be used to sustain flight once airborne (see motor glider).
Once launched sailplanes try to gain height using thermals, ridge lift, lee waves or convergence zones and can remain airborne for hours. This is known as 'soaring'. By finding lift sufficiently often experienced pilots fly cross-country, often on pre-declared tasks of hundreds of kilometers, usually back to the original launch site. Cross-country flying and aerobatics are the two forms of competitive gliding. For information about the forces in gliding flight, see lift-to-drag ratio.
Pilots need some form of control over the glide slope to land the glider. In powered aircraft, this is done by reducing engine thrust. In gliders, other methods are used to either reduce the lift generated by the wing, increase the drag of the entire glider, or both. Glide slope is the distance traveled for each unit of height lost. In a steady wings-level glide with no wind, glide slope is the same as the lift/drag ratio (L/D) of the glider, called "L-over-D". Reducing lift from the wings and/or increasing drag will reduce the L/D allowing the glider to descend at a steeper angle with no increase in airspeed. Simply pointing the nose downwards only converts altitude into a higher airspeed with a minimal initial reduction in total energy. Gliders, because of their long low wings, create a high ground effect which can significantly increase the glide angle and make it difficult to bring the glider to Earth in a short distance.
Early glider designs used skids for landing, but modern types generally land on wheels. Some of the earliest gliders used a dolly with wheels for taking off and the dolly was jettisoned as the glider left the ground, leaving just the skid for landing. A glider may be designed so the center of gravity (CG) is behind the main wheel so the glider sits nose high on the ground. Other designs may have the CG forward of the main wheel so the nose rests on a nose-wheel or skid when stopped. Skids are now mainly used only on training gliders such as the Schweizer SGS 2–33. Skids are around 100mm (3 inches) wide by 900mm (3 feet) long and run from the nose to the main wheel. Skids help with braking after landing by allowing the pilot to put forward pressure on the control stick, thus creating friction between the skid and the ground. The wing tips also have small skids or wheels to protect the wing tips from ground contact.
In most high performance gliders the undercarriage can be raised to reduce drag in flight and lowered for landing. Wheel brakes are provided to allow stopping once on the ground. These may be engaged by fully extending the spoilers/air-brakes or by using a separate control. Although there is only a single main wheel, the glider's wing can be kept level by using the flight controls until it is almost stationary.
Pilots usually land back at the airfield from which they took off, but a landing is possible in any flat field about 250 metres long. Ideally, should circumstances permit, a glider would fly a standard pattern, or circuit, in preparation for landing, typically starting at a height of 300 metres (1,000 feet). Glide slope control devices are then used to adjust the height to assure landing at the desired point. The ideal landing pattern positions the glider on final approach so that a deployment of 30–60% of the spoilers/dive brakes/flaps brings it to the desired touchdown point. In this way the pilot has the option of opening or closing the spoilers/air-brakes to extend or steepen the descent to reach the touchdown point. This gives the pilot wide safety margins should unexpected events occur.
In addition to an altimeter, compass, and an airspeed indicator, gliders are often equipped with a variometer, turn and bank indicator and an airband radio (transceiver), each of which may be required in some countries. An Emergency Position-Indicating Radio Beacon (ELT) may also be fitted into the glider to reduce search and rescue time in case of an accident.
Much more than in other types of aviation, glider pilots depend on the variometer, which is a very sensitive vertical speed indicator, to measure the climb or sink rate of the plane. This enables the pilot to detect minute changes caused when the glider enters rising or sinking air masses. Both mechanical and electronic 'varios' are usually fitted to a glider. The electronic variometers produce a modulated sound of varying amplitude and frequency depending on the strength of the lift or sink, so that the pilot can concentrate on centering a thermal, watching for other traffic, on navigation, and weather conditions. Rising air is announced to the pilot as a rising tone, with increasing pitch as the lift increases. Conversely, descending air is announced with a lowering tone, which advises the pilot to escape the sink area as soon as possible. (Refer to the variometer article for more information).
Gliders' variometers are sometimes fitted with mechanical devices such as a "MacCready Ring" to indicate the optimal speed to fly for given conditions. These devices are based on the mathematical theory attributed to Paul MacCready though it was first described by Wolfgang Späte in 1938. MacCready theory solves the problem of how fast a pilot should cruise between thermals, given both the average lift the pilot expects in the next thermal climb, as well as the amount of lift or sink he encounters in cruise mode. Electronic variometers make the same calculations automatically, after allowing for factors such as the glider's theoretical performance, water ballast, headwinds/tailwinds and insects on the leading edges of the wings.
Soaring flight computers, often used in combination with PDAs running specialized soaring software, have been designed for use in gliders. Using GPS technology in conjunction with a barometric device these tools can:
After the flight the GPS data may be replayed on computer software for analysis and to follow the trace of one or more gliders against a backdrop of a map, an aerial photograph or the airspace.
Because collision with other gliders is a risk, the anti-collision device FLARM is becoming increasingly common in Europe and Australia. In the longer term, gliders may eventually be required in some European countries to fit transponders once devices with low power requirements become available.
To distinguish gliders in flight, very large numbers/letters are sometimes displayed on the fin and wings. Registrations on narrow fuselages are difficult to read. These numbers were first added for use by ground-based observers in competitions, and are therefore known as "competition numbers" or "contest IDs". They are unrelated to the glider's registration number, and are assigned by national gliding associations. They are useful in radio communications between gliders, so glider pilots often use their competition number as their call signs.
Fibreglass gliders are white in color after manufacture. Since fibreglass resin softens at high temperatures, white is used almost universally to reduce temperature rise due to solar heating. Color is not used except for a few small bright patches on the wing tips; these patches (typically bright red) improve gliders' visibility to other aircraft while in flight (and are a requirement for mountain flying in France). Non-fibreglass gliders (those made of aluminum and wood) are not subject to the temperature-weakening problem of fibreglass, and can be painted any color at the owner's choosing; they are often quite brightly painted.
There is sometimes confusion about gliders, hang gliders and paragliders. In particular paragliders and hang gliders are both foot-launched. The main differences between the types are:
Eight competition classes of glider have been defined by the FAI. They are:
A large proportion of gliders have been and are still made in Germany, the birthplace of the sport. In Germany there are several manufacturers but the three principal companies are:
Germany also has Stemme and Lange Aviation. Elsewhere in the world, there are other manufacturers such as Jonker Sailplanes in South Africa, Sportinė Aviacija in Lithuania, Allstar PZL and PZL-Bielsko in Poland, HpH in the Czech Republic and AMS Flight in Slovenia.
The Garden Egg chair was designed by Peter Ghyczy in 1968. It was manufactured by Reuter Products. The chair was designed for both indoor/outdoor use, although as a design icon and collectable it is rarely used outdoors. The chair lid lifts and closes, when closed are theoretically waterproof. The Egg chair has been re-inroduced in 2001 by Ghyczy Novo. The Garden Egg Chair is known under several names; “seftenberger ei, pod chair, l’œuf en garden(egg)chair.” Elastogran/Reuter produced the plastic Polyurethane. Peter Ghyczy his job was to start a design centre in order to show industrial costumers the potential of this plastic material. The Garden Egg Chair is one of the first chairs which was made with Polyurethane. For a long time the chair was produced by the East German company VEB-Synthese-Werk but since 1998 the chair is produced in The Netherlands.
Peter Ghyczy (1940) left his motherland Hungary in 1956 because of the revolution and moved to West Germany. Here he finished his high school and studied at the Düsseldorf Art Academy and University of Achen. After graduating he started as head of the design department at Elastogran/Reuter were he developed the Garden Egg Chair in 1968. The in his opinion traditional way of working made him to leave the company in 1972. He moved to The Netherlands and started his own company.
A glider or platform rocker is a type of rocking chair that moves as a swing seat, where the entire frame consists of a seat attached to the base by means of a double-rocker four-bar linkage. The non-parallel suspension arms of the linkage cause the chair to simulate a rocking-chair motion as it swings back and forth.
Gliders are used as alternatives to porch swings, and are also popular as nursery furnishing for assisting parents in feeding newborn babies. Because pinch points are moved away from the floor, a glider is marginally safer for pets and toddlers.
Early patents described different mechanisms for glider chairs, such as rails and four-bar linkages supported by springs. Patents using a swinging seat suspended from a four-bar linkage appeared in 1939, and this is now the general configuration used by most glider chairs.
In the southern United States, porch gliders were referred to as divans. Especially popular was the "basket weave" pattern in the hot non- air conditioned South of the 1950s and 1960s.][
The primary glider manufacturers in North America are Canadian companies Dutailier and Shermag.
A rocking chair or rocker is a type of chair with two curved bands (also known as rockers) attached to the bottom of the legs, connecting the legs on each side to each other. The rockers contact the floor at only two points, giving the occupant the ability to rock back and forth by shifting his/her weight or pushing lightly with his/her feet. Rocking chairs are most commonly made of wood.
Many find rocking chairs soothing because of the gentle motion. Rocking chairs are also comfortable because, when a user sits in one without rocking, the chair automatically rocks backwards until the sitter's center of gravity is met, thus granting an ergonomic benefit with the occupant kept at an unstressed position and angle. Varieties of rockers include those mounted on a spring base (or platform) called "platform rockers" and those with swinging braces commonly known as gliders. Some rocking chairs do fold. glider chair
A rocking chair or rocker is a type of chair with two curved bands (also known as rockers) attached to the bottom of the legs, connecting the legs on each side to each other. The rockers contact the floor at only two points, giving the occupant the ability to rock back and forth by shifting his/her weight or pushing lightly with his/her feet. Rocking chairs are most commonly made of wood.
Many find rocking chairs soothing because of the gentle motion. Rocking chairs are also comfortable because, when a user sits in one without rocking, the chair automatically rocks backwards until the sitter's center of gravity is met, thus granting an ergonomic benefit with the occupant kept at an unstressed position and angle. Varieties of rockers include those mounted on a spring base (or platform) called "platform rockers" and those with swinging braces commonly known as gliders. Some rocking chairs do fold.
A four-bar linkage, also called a four-bar, is the simplest movable closed chain linkage. It consists of four bodies, called bars or links, connected in a loop by four joints. Generally, the joints are configured so the links move in parallel planes, and the assembly is called a planar four-bar linkage.
If the linkage has four hinged joints with axes angled to intersect in a single point, then the links move on concentric spheres and the assembly is called a spherical four-bar linkage. Bennett's linkage is a spatial four-bar linkage with hinged joints that have their axes angled in a particular way that makes the system movable. Linkage
Mechanical engineering is a discipline of engineering that applies the principles of engineering, physics and materials science for analysis, design, manufacturing, and maintenance of mechanical systems. It is the branch of engineering that involves the production and usage of heat and mechanical power for the design, production, and operation of machines and tools. It is one of the oldest and broadest engineering disciplines.
The engineering field requires an understanding of core concepts including mechanics, kinematics, thermodynamics, materials science, structural analysis, and electricity. Mechanical engineers use these core principles along with tools like computer-aided engineering, and product lifecycle management to design and analyze manufacturing plants, industrial equipment and machinery, heating and cooling systems, transport systems, aircraft, watercraft, robotics, medical devices, weapons, and others. Glider
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