A hang glider is an unpowered ultralight aircraft that is capable of being foot launched and is controlled via pilot weight shift and/or the actuation of aerodynamic control surfaces.  The definition of 'hang glider' per the international hang gliding commission, or Commission Internationale de Vol Libre (CIVL), which operates under the auspices of the Federation Aeronautique Internationale (FAI), is:
A glider capable of being carried, foot launched, and landed solely by
the use of the pilot's legs. 

There are four distinct classes of aircraft which are covered by the definition of 'hang glider':
  • Class 1 - Hang gliders having a rigid primary structure and controlled by weight shift only and which demonstrate consistent ability to safely take-off and land in nil wind conditions. 
  • Class 2 - Hang gliders having a rigid primary structure and controlled by deflection of aerodynamic control surfaces as the primary method of control in at least one major axis and which demonstrate consistent ability to safely take-off and land in nil wind conditions. 
  • Class 3 - Hang gliders having no rigid primary structure and which demonstrate consistent ability to safely take-off and land in nil wind conditions. 
  • Class 4 - Hang gliders which are unable demonstrate consistent ability to safely take-off and land in nil wind conditions but that otherwise are capable of being launched and landed by the use of the pilot's legs. 
Class 1 hang gliders are the delta shaped, cloth and metal aircraft that most people associate with the term 'hang glider', and are commonly called 'flex wings'.  Class 2 hang gliders, 'rigid wings', may look like Class 1 wings or they may have wings more similar to standard airplane wings.  Class 3 covers paragliders, which resemble advanced ram-air skydiving canopies (parachutes).  The remaining class serves to cover the gap between Class 2 hang gliders and ultralight gliders, which are defined elsewhere as gliders with a maximum empty weight of 440 lbs.  (220 kg. ) and without restrictions regarding wind conditions for launch and landing, nor any mention of the pilot's legs. 

The remainder of this discussion will pertain primarily to Class 1 hang gliders.  Class 2 and 4 wings, rigid wings and Paragliders are distinct enough from the other classes that separate discussions are warranted. 

Theory:
Class 1 hang gliders are called 'flex wings' because the design depends on the aircraft having a certain flexibility.  When the frame is tensioned, the straight leading edge tubes are forced into curves by the shape of the wing covering, thus holding the wing covering taut.  Except in the very middle, the sail is attached only to the leading edge, which allows the wingtips to rotate up or down relative to the leading edge.  This flexing is in response to loads placed on the wing by the pilot's weight, shifts of weight, and maneuvering.  The wingtip on the inside of a turn flexes up, for instance, which lowers its Angle of Attack.  This is important for making tight radius turns because if the angle of attack were fixed then the tip would stall, since it has very low airspeed; the tip would drop and the glider would slip sideways, losing excessive altitude as it turned.

The flexibility of the wing is what allows full control of the glider without using aerodynamic control surfaces such as ailerons, rudders, and so on.  Rigid wings, by contrast, require some form or combination of ailerons, rudder, or spoilers.

NOTE: Hang glider pilots primarily discuss airspeed and ground speed in terms of miles per hour (MPH) or kilometers per hour (km/h), unlike most forms of aviation, where knots is the preferred unit of measure. Climb and descent rate units used are feet per minute (FPM) or meters per second (m/s).

The wing of a hang glider generates lift when air flows across it from fore to aft, like any airfoil.  The amount of lift produced is not enough to overcome the force of gravity (climb), but is enough to slow the descent rate in still air to about 200 feet per minute (FPM).  If the glider flies in air rising at more than the glider's sink rate then the glider will gain altitude, or soar.  The stall speed of a hang glider is 18-22 mph, best glide is usually just under 30 mph, and top speeds are around 60 mph, though speeds of 80-100 mph can be achieved during aerobatic maneuvers.  At best glide speeds hang gliders have glide angles of between 9:1 and 14:1.  Sink rates vary from about 200 fpm just above stall speed (minimum sink) to as much as 1500 fpm for low performance wings at top speed.  Sink rate increases more or less linearly with airspeed and is the main topic when hang glider performance is discussed.  Most flying is done in the narrow range between minimum sink, when climbing in lift, and best glide, when going from point to point.  Airfoil profile, aspect ratio and percentage of double surface determine to a great extent the performance of a wing.  Thick-cambered low aspect (5. 5) wings with little or no double surface are slow and docile, usually with great low-speed characteristics.  Wide (>7. 5 AR), skinny 'blade wings' with 80% or more double surface are made for racing and require finesse to realize their potential.

A given wing planform's useful load capacity is a function of its area.  Glider models come in several sizes to cover the range of pilot sizes, and gliders are referred to by model and size, either in square feet or square meters.  The smallest racing gliders, for 100 lb.  pilots, are about 120 square feet, while 225 - 250 square foot gliders have been made for tandem (two-person) operation.  A 190 lb.  pilot might fly a 195 square foot beginner glider or a 150 square foot high performance model.  Wing loading determines sink rate and control authority.  The example pilot just mentioned would have a wing loading of about 1. 3, including harness, helmet, parachute, instruments, and glider weight.  At higher wing loadings a glider will fly, but will have an increased sink rate, and will also be more responsive to control inputs.  Conversely, a lightly loaded glider will have a better sink rate but will take longer to respond to control inputs.  Thus, a pilot can choose a model and size of glider suited to the type of flying desired.

The pilot hangs from a point near the center of the glider such that the glider trims, i. e.  flies 'hands off', at a speed between minimum sink and best glide.  The wing of the glider has 'twist' - the Angle of Attack is higher at the middle than at the wingtips, which are aft of the pilot due to the sweep angle of the wings.  When the pilot pulls the control bar (basetube), which is attached near the hang point, towards his or her body (actually pulling his or her body weight forward) the nose of the glider drops a few degrees and the glider speeds up.  Due to the twist in the wing, the wingtips produce much less lift than before, while the center continues producing lift.  The center of pressure of the lift produced by the wing as a whole moves forward and the glider tries to return to trim speed.  When the pilot 'pushes out' (body weight aft) the center of the wing starts to stall and the tips produce more lift, so a nose-down pitch moment is produced.  In this way the glider is self-correcting in pitch, or convergent.  Wings that are divergent are dangerous and unpleasant to fly, and so are not made.

The sweep of the wings also makes hang gliders stable in the yaw axis.  If the nose of the glider begins pointing away from the direction of travel, the wings meet the air at different angles, and the differential drag slows the forward wing while the aft wing speeds up, and the glider points back into the direction of travel.  A pilot can purposely maintain a crab angle via a small roll input to soar along a ridge or to compensate for a crosswind.

Hang gliders are stable in the remaining axis, roll, in a somewhat different way.  Most airplanes have dihedral, that is, their wingtips are higher than the root, which results in roll stability and a resistance to other than level flight.  When the plane rolls a little, the wing rolled toward becomes more horizontal, producing more force against gravity, since the lift vector is generally perpendicular to the span of the wing, while the other produces less, so the plane wants to roll back to level flight.  Hang gliders are made with anhedral - their wingtips are below the middle of the wing.  A hang glider will maintain level flight unless the pilot pulls his or her weight to the side (weight left=turn left) or is rolled by turbulence.  With the pilot's weight to one side, the glider banks in that direction, the frame flexes a bit, and the sail shifts the load a bit so that the angle of attack of the low wing increases slightly, and that of the high wing decreases.  The low wing slows down in response while the high wing speeds up, and a turn results.  The sweep of the wing plays a role in this action as well.  Most gliders have a predictable bank angle to which they will settle at a given airspeed.  Once in a turn a hang glider will generally keep turning until given a turn input in the opposite direction.  This is an important quality since much time is spent circling in lift.  Docile gliders can be made which prefer level flight, will return to level flight with little input, and will resist high bank angles, while high performance gliders roll in and out with very light input, will remain without input at at a range of moderate bank angles, and will happily bank steeper and steeper ('wrap in') without pilot correction.

Hang gliders are roll-yaw coupled, i. e.  a roll motion always results in a corresponding yaw change.  This is how a hang glider can be controlled with only input on two axes (pitch and roll) instead of the three axes of control required by airplanes and sailplanes, and how hang gliders can thus be controlled without aerodynamic control surfaces. 

Construction:
Hang gliders typically consist of a frame of aluminum tubing braced with stainless steel cables and covered with a dacron sail which forms the actual airfoil.  Curved battens, also usually aluminum, are inserted into the sail to help define and maintain the airfoil shape.  Hang gliders generally weigh 45-70 lbs.  (20-35 kg. ) and have a wingspan in the neighborhood of 30 feet (9. 2 m).  The wing may have a single surface curved like the top of an airplane wing or it may have an additional bottom surface, usually flat, extending rearward from the leading edge of the wing to some point on the underside of the upper surface; there may be a few battens inserted in this bottom surface.


Airframe:
The tubing used for the frame is almost exclusively 7075-T6 aluminum alloy or 6061-T6, or a combination of those two.  The 7075 is lighter and stiffer than 6061, but considerably more expensive.  An all 6061 glider would weigh on the order of 10 pounds (~15%) more than an all 7075 wing.  Aircraft grade aluminum is also used for the plates and brackets used in the frame.  Stranded stainless steel cable, primarily 7 x 7 3/32", with stainless steel tangs for attachment, makes up the rest of the frame, along with AN grade bolts and clevis pins.  Various small pieces of Delrin and UHMW plastics are used in the fittings as well.

The primary components of the glider's frame are the leading edge tubes, the keel, and the crossbars.  The leading edges and keel are joined with a plate or plates at the nose - the leading edges on either side of the keel, all in the same plane.  The crossbars are attached somewhere near the middle of the leading edge tubes and are hinged together at their other ends.  The center hinge of the crossbars is pulled rearward by a cable, which attaches to a point near the rear of the keel, spreading the leading edges and holding them apart in flight.

A tube called the kingpost rises vertically from the keel with cables from the top to points at the ends of the keel and the outboard ends of the crossbars.  The latter cables support the wings while not flying, i. e.  on the ground, and in the (unusual) event that the glider experiences a negative load during flight.  Smaller diameter cables run from the top of the kingpost to the trailing edge of the sail, two to five per wing, attaching near the batten pockets.  These cables, called 'reflex bridles' or 'luff lines', support the trailing edge if the glider is at an extremely low nose angle, and thus provide some extra nose-up pitching moment, that is, dive recovery.  Beginning in the mid-1990s high performance hang gliders began being produced which dispense with the kingpost by utilizing very strong crossbars and robust center hinge mechanisms which can withstand the negative loads encountered in ground handling, landing, and even the rare inverted attitude in flight.  Most of these "topless" designs use carbon composites for good strength to weight, but at least one employs riveted aluminum box construction.  In place of reflex bridles these gliders have various arrangements of internal tubes extending aft from the leading edges and/or crossbars to support the trailing edges.

The benefit of eliminating the kingpost and upper rigging is greatly reduced parasitic drag and correspondingly improved glide ratio, especially at speed.  The best kingposted racing hang gliders had glide ratios of just over 12:1; the kingpostless gliders had glide ratios of more than 14:1.  The inevitable trade-off was increased weight, cost and complexity.  A pair of aluminum tube crossbars weighs 2-3 pounds compared to 10-12 lbs.  for carbon composites.  Cost was more than doubled for the carbon bars as well.  'Topless' hang gliders became de rigeur on the competition circuit within one season.

Below the keel is a triangle of tubing known as the control bar, consisting of a 4'-5' control bar, or 'basetube', parallel to the wing plane and two ~5. 5' control legs, or 'downtubes', which join the ends of the basetube to a point at the middle of the keel.  Cables run from the corners of the triangle to the noseplate and aft of the keel (usually where the crossbar haulback cables and upper rear cable attach) to keep the control bar in a given position relative to the keel.  Stout cables run from the corners out to the leading edge / crossbar junctions to keep the wings in place under flight loads.

The tubing used for the control triangle may be either round or streamlined, and the basetube may have some bends so the the center section is offset forward a few inches; the bends allow more comfortable grip positions and the bar can be pushed back a few inches further, for more speed.  Downtubes are frequently bent or broken in poor landings - the corner of the triangle hits the ground and stops while the pilot, who is holding the downtubes, continues forward.  The compression loaded tube takes little force to bend.  Downtubes are made with thin-walled tubing, 1. 125" x . 058" is typical for round tubing, so that the tubing bends instead of the pilot's arm breaking.  Carbon composite downtubes, with very thin airfoil sections, and thus very low drag were introduced in 2000, as were streamlined composite basetubes.  The cost, about 4 times that of aluminum, limits the use of composite downtubes, though not so much for basetubes as they are less subject to damage.


Sail:
The fabric part of a hang glider, which makes up the wing surfaces, is known as the sail.  Standard dacron polyester sailcloth, used for making boat sails, is the primary material.  The cloth is coated/impregnated with resin (to limit porosity), weighs about 4 Oz. per square meter (manufacturer's units), and comes in a range of colors.  The fabric is woven in the ripstop style, where thicker strands are used every 1/4" or so to stop tears from spreading; the fabric would be much heavier if these strands were used for the whole cloth.  Dacron, like virtually all plastics degrades under exposure to ultraviolet light from the sun, so hang glider sails are rated for about 400 hours of UV exposure (4-8 years of average use), though wings typically last for 800 hours or more.  The main effects of UV exposure are to weaken the fibers and to break down the resin, colors fade as well.  The sail gets more porous, which reduces performance slightly, and the fabric begins to tear easily.  Since white reflects most frequencies of light, and thus has the longest UV resistance, that is the color most used for the upper surface of the wing.  The bottom surface of a hang glider wing is not a structural element, like the rest of the sail, and is somewhat protected from UV exposure by the upper surface, so colored panels are used, either simple two or three color layouts made as 'stock' patterns by manufacturers or custom patterns incorporating symbols, lettering, etc.

The fabric comes in widths of four to five feet, so multiple panels are sewn together to make up the wing surfaces, and are needed to get the wing's shape anyway.  The trailing edge of the wing is under a great deal of tension, so it is usually a slightly heavier material or is reinforced with extra layers, or both.  Tears in the trailing edge must be repaired before flying the glider again or total failure in flight could result.  The leading edge of the wing is under moderate tension and also takes the brunt of the oncoming airflow, which tends to make it deform at speed and perform poorly.  For this reason mid- to high-performance wings use a mylar coated dacron fabric on the leading edge, forming a pocket into which pliable sheets of mylar are inserted, from root to tip, to stiffen the leading edge so that it retains the smooth airfoil shape at higher speeds.  Very stiff carbon composite sheets are added by some competition pilots to help at very high speeds.  The mylar coated fabric ('scrim') is sometimes used for the entire top surface of racing gliders, because it seems to perform better, but the cost is extra weight and stiffer handling.  The mylar coated fabrics begin to delaminate after about 300-400 hours of UV exposure, especially in rocky, dusty areas, so full mylar sails don't last very long.  It is common to replace the mylar leading edge panels of a good glider once before the entire sail breaks down.  Generally, by the time a sail ages to the point that it needs replacement the design of the glider is so obsolete that it doesn't make sense to have a replacement sail made - newer used gliders are cheaper.  The fabrication of the sail is where the bulk of the labor in making a hang glider is invested.

Sail panels are either cut by hand or are cut by machine.  Beginning in the 1990s at least one manufacturer used a 40 foot long, six foot wide sail cutting machine to cut sail panels.  The surface of the table has small holes via which a vacuum is applied to hold the cloth in place while a computer-controlled apparatus moves along the length of the table, with a pen that draws lines and a blade wheel that cuts the cloth.  The patterns are made using various CAD programs, and the precision panel cutting helps greatly in making wings consistently.  The old (by hand) method was to follow mylar patterns laid on top of the material; the mylar changes size and shape as humidity varies, so each glider was different from the next.  The computer-controlled sail cutter also makes rather complex patterns and inlaid designs practical.

The hang glider sail has heavy-duty zippers in a few places in the bottom surface, such as at the root and crossbar/leading edge junctions, to allow access for inspection and maintenance.  A glider may have a piece of fabric-covered mylar covering the nose plates ('nosecone') to reduce drag, attached with velcro.  Double surface wings often incorporate tip fairings, which are plastic covers that attach to the open wingtips with velcro to clean up the airflow there.  As a performance option, some gliders can accommodate winglets, small (8"-15") vertically oriented fins at or near the wingtips; they may replace tip fairings or be velcroed to the top surface.  The purpose of winglets is to reduce parasitic drag by reducing the wingtip vortex which is a by-product of producing lift shared by all wings.  The winglet effectively adds span, about 60% of the winglet's height, and that combined with the added weight reduces roll rate, but the reduction is minor.  Properly oriented winglets reduce stall speed by about 1 mph, allowing better climb while circling, and winglets help reduce minor yaw variations at speed, giving an 'on rails' feeling.  Due to the sail's flexibility winglet orientation is inexact and hard to control, so the value of winglets is a point of contention.  An alternative to winglets is the vertical stabilizer, a fin which attaches to the rear of the keel, like a traditional airplane tail (NOT a rudder).  The Vertical stabilizer provides the yaw stability of winglets (more actually) without the drag reduction but also without as much effect on turn rate and response.


Remarks:
Hang gliders are mechanically quite simple, but the flexible wings, with their continuous reduction of camber from root to tip and the ever changing wing shape in flight, are astoundingly complex.  Designing hang gliders is as much art as science, where small changes can have profound effects and seemingly major changes can be hard to notice.  The hang gliding population numbers about 10,000 in the USA ca.  2001 (at best) and there are perhaps 10000-20000 more active pilots worldwide.  The number of new gliders manufactured in a given year is 3000-5000, and typical models retail for $3,000 to $7,000 in the USA.  There are three or four major manufacturers in the world, with one based in the USA.  The market is small, and this small scale means that research and development funds are limited, so advances happen very slowly.

Hang gliding, unpowered, foot-launched flight, is a unique, demanding pastime which is nevertheless well within the abilities of many more people than currently participate.  If that were to change, then the sport could make some profound advances, given the technology and materials available at the dawn of the 21st century. 

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