Toy gliders

Glider (aircraft)

Aircraft designed for operation without an engine

For other uses, see Glider (disambiguation).

A glider is a fixed-wing aircraft that is supported in flight by the dynamic reaction of the air against its lifting surfaces, and whose free flight does not depend on an engine.[1] Most gliders do not have an engine, although motor-gliders have small engines for extending their flight when necessary by sustaining the altitude (normally a sailplane relies on rising air to maintain altitude) with some being powerful enough to take off self-launch.

There are a wide variety of types differing in the construction of their wings, aerodynamic efficiency, location of the pilot, controls and intended purpose. Most exploit meteorological phenomena to maintain or gain height. Gliders are principally used for the air sports of gliding, hang gliding and paragliding. However some spacecraft have been designed to descend as gliders and in the past military gliders have been used in warfare. Some simple and familiar types of glider are toys such as paper planes and balsa wood gliders.

Etymology[edit]

Glider is the agent noun form of the verb to glide. It derives from Middle English gliden, which in turn derived from Old English glīdan. The oldest meaning of glide may have denoted a precipitous running or jumping, as opposed to a smooth motion. Scholars are uncertain as to its original derivation, with possible connections to "slide", and "light" having been advanced.[2]

History[edit]

Main article: Early flying machines

Early pre-modern accounts of flight are in most cases difficult to verify and it is unclear whether each craft was a glider, kite or parachute and to what degree they were truly controllable. Often the event is only recorded a long time after it allegedly took place. A 17th-century account reports an attempt at flight by the 9th-century poet Abbas Ibn Firnas near Cordoba, Spain which ended in heavy back injuries.[3] The monk Eilmer of Malmesbury is reported by William of Malmesbury (c. 1080–1143), a fellow monk and historian, to have flown off the roof of his Abbey in Malmesbury, England, sometime between 1000 and 1010 AD, gliding about 200 metres (220 yd) before crashing and breaking his legs.[4] According to these reports, both used a set of (feathery) wings, and both blamed their crash on the lack of a tail.[5]Hezârfen Ahmed Çelebi is alleged to have flown a glider with eagle-like wings over the Bosphorus strait from the Galata Tower to Üsküdar district in Istanbul around 1630–1632.[6][7][8]

19th century[edit]

Main article: Early flying machines

The first heavier-than-air (i.e. non-balloon) man-carrying aircraft that were based on published scientific principles were Sir George Cayley's series of gliders which achieved brief wing-borne hops from around 1849. Thereafter gliders were built by pioneers such as Jean Marie Le Bris, John J. Montgomery, Otto Lilienthal, Percy Pilcher, Octave Chanute and Augustus Moore Herring to develop aviation. Lilienthal was the first to make repeated successful flights (eventually totaling over 2,000) and was the first to use rising air to prolong his flight. Using a Montgomery tandem-wing glider, Daniel Maloney was the first to demonstrate high-altitude controlled flight using a balloon-launched glider launched from 4,000 feet in 1905.[9]

The Wright Brothers developed a series of three manned gliders after preliminary tests with a kite as they worked towards achieving powered flight. They returned to glider testing in 1911 by removing the motor from one of their later designs.

Development[edit]

Main article: Glider (sailplane)

In the inter-war years, recreational gliding flourished in Germany under the auspices of Rhön-Rossitten. In the United States, the Schweizer brothers of Elmira, New York, manufactured sport sailplanes to meet the new demand. Sailplanes continued to evolve in the 1930s, and sport gliding has become the main application of gliders. As their performance improved, gliders began to be used to fly cross-country and now regularly fly hundreds or even over a thousand of kilometers in a day,[10] if the weather is suitable.

Military gliders were developed during World War II by a number of countries for landing troops. A glider – the Colditz Cock – was even built secretly by POWs as a potential escape method at Oflag IV-C near the end of the war in 1944.

Smallest glider in the world - BrO-18 "Boružė" (Ladybird), constructed in Lithuaniain 1975

Development of flexible-wing hang gliders[edit]

Main article: Hang gliding

Foot-launched aircraft had been flown by Lilienthal and at the meetings at Wasserkuppe in the 1920s. However the innovation that led to modern hang gliders was in 1951 when Francis Rogallo and Gertrude Rogallo applied for a patent for a fully flexible wing with a stiffening structure. The American space agency NASA began testing in various flexible and semi-rigid configurations of this Rogallo wing in 1957 in order to use it as a recovery system for the Gemini space capsules. Charles Richards and Paul Bikle developed the concept producing a wing that was simple to build which was capable of slow flight and as gentle landing. Between 1960 and 1962 Barry Hill Palmer used this concept to make foot-launched hang gliders, followed in 1963 by Mike Burns who built a kite-hang glider called Skiplane. In 1963, John W. Dickenson began commercial production.[11]

Development of paragliders[edit]

Main article: Paragliding

January 10, 1963 American Domina Jalbert filed a patent US Patent 3131894 on the Parafoil which had sectioned cells in an aerofoil shape; an open leading edge and a closed trailing edge, inflated by passage through the air – the ram-air design.[12] The 'Sail Wing' was developed further for recovery of NASA space capsules by David Barish. Testing was done by using ridge lift.[13] After tests on Hunter Mountain, New York in September 1965, he went on to promote "slope soaring" as a summer activity for ski resorts (apparently without great success).[14] NASA originated the term "paraglider" in the early 1960s, and ‘paragliding’ was first used in the early 1970s to describe foot-launching of gliding parachutes. Although their use is mainly recreational, unmanned paragliders have also been built for military applications e.g. Atair Insect.

Recreational types[edit]

The main application today of glider aircraft is sport and recreation.

Sailplane[edit]

Main article: Glider (sailplane)

Gliders were developed from the 1920s for recreational purposes. As pilots began to understand how to use rising air, gliders were developed with a high lift-to-drag ratio. These allowed longer glides to the next source of 'lift', and so increase their chances of flying long distances. This gave rise to the popular sport known as gliding although the term can also be used to refer to merely descending flight. Such gliders designed for soaring are sometimes called sailplanes.

Gliders were mainly built of wood and metal but the majority now have composite materials using glass, carbon fibre and aramid fibers. To minimise drag, these types have a fuselage and long narrow wings, i.e. a high aspect ratio. In the beginning, there were huge differences in the appearance of early-sailplanes. As technology and materials developed, the aspiration for the perfect balance between lift/drag, climbing ratio and gliding speed, made engineers from various producers create similar designs across the world. Both single-seat and two-seat gliders are available.

Initially training was done by short 'hops' in primary gliders which are very basic aircraft with no cockpit and minimal instruments.[15] Since shortly after World War II training has always been done in two-seat dual control gliders, but high performance two-seaters are also used to share the workload and the enjoyment of long flights. Originally skids were used for landing, but the majority now land on wheels, often retractable. Some gliders, known as motor gliders, are designed for unpowered flight, but can deploy piston, rotary, jet or electric engines.[16] Gliders are classified by the FAI for competitions into glider competition classes mainly on the basis of span and flaps.

A class of ultralight sailplanes, including some known as microlift gliders and some as 'airchairs', has been defined by the FAI based on a maximum weight. They are light enough to be transported easily, and can be flown without licensing in some countries. Ultralight gliders have performance similar to hang gliders, but offer some additional crash safety as the pilot can be strapped in an upright seat within a deformable structure. Landing is usually on one or two wheels which distinguishes these craft from hang gliders. Several commercial ultralight gliders have come and gone, but most current development is done by individual designers and home builders.

Hang gliders[edit]

Modern 'flexible wing' hang glider.

Main article: Hang gliding

Unlike a sailplane, a hang glider is capable of being carried, foot launched and landed solely by the use of the pilot's legs.[17]

  • In the original and still most common designs, Class 1, the pilot is suspended from the center of the flexible wing and controls the aircraft by shifting their weight.
  • Class 2 (designated by the FAI as Sub-Class O-2) have a rigid primary structure with movable aerodynamic surfaces, such as spoilers, as the primary method of control. The pilot is often enclosed by means of a fairing. These offer the best performance and are the most expensive.
  • Class 4 hang gliders are unable to demonstrate consistent ability to safely take-off and/or land in nil-wind conditions, but otherwise are capable of being launched and landed by the use of the pilot's legs.
  • Class 5 hang gliders have a rigid primary structure with movable aerodynamic surfaces as the primary method of control and can safely take-off and land in nil-wind conditions. No pilot fairings are permitted.

In a hang glider the shape of the wing is determined by a structure, and it is this that distinguishes them from the other main type of foot-launched aircraft, paragliders, technically Class 3. Some hang gliders have engines, and are known as powered hang gliders. Due to their commonality of parts, construction and design, they are usually considered by aviation authorities to be hang gliders, even though they may use the engine for the entire flight. Some flexible wing powered aircraft, Ultralight trikes, have a wheeled undercarriage, and so are not hang gliders.

Paragliders[edit]

A paraglider taking off in Brazil

Main article: Paragliding

A paraglider is a free-flying, foot-launched aircraft. The pilot sits in a harness suspended below a fabric wing. Unlike a hang glider whose wings have frames, the form of a paraglider wing is formed by the pressure of air entering vents or cells in the front of the wing. This is known as a ram-air wing (similar to the smaller parachute design). The paraglider's light and simple design allows them to be packed and carried in large backpacks, and make them one of the simplest and economical modes of flight. Competition level wings can achieve glide ratios up to 1:10 and fly around speeds of 45 km/h (28 mph).

Like sailplanes and hang gliders, paragliders use rising air (thermals or ridge lift) to gain height. This process is the basis for most recreational flights and competitions, though aerobatics and 'spot landing competitions' also occur. Launching is often done by jogging down a slope, but winch launches behind a towing vehicle are also used. A Paramotor is a paraglider wing powered by a motor attached to the back of the pilot, and is also known as a powered paraglider. A variation of this is the paraplane, which has a motor mounted on a wheeled frame rather than the pilot's back.

Comparison of gliders, hang gliders and paragliders[edit]

There can be confusion between gliders, hang gliders, and paragliders. Paragliders and hang gliders are both foot-launched glider aircraft and in both cases the pilot is suspended ("hangs") below the lift surface. "Hang glider" is the term for those where the airframe contains rigid structures, whereas the primary structure of paragliders is supple, consisting mainly of woven material.

ParaglidersHang glidersGliders/Sailplanes
Undercarriage pilot's legs used for take-off and landing pilot's legs used for take-off and landing aircraft takes off and lands using a wheeled undercarriage or skids
Wing structure entirely flexible, with shape maintained purely by the pressure of air flowing into and over the wing in flight and the tension of the lines generally flexible but supported on a rigid frame which determines its shape (note that rigid-wing hang gliders also exist) rigid wing surface which totally encases wing structure
Pilot position sitting in a harness usually lying prone in a cocoon-like harness suspended from the wing; seated and supine are also possible sitting in a seat with a harness, surrounded by a crash-resistant structure
Speed range
(stall speed – max speed)
slower – typically 25 to 60km/h for recreational gliders (over 50km/h requires use of speed bar),[18] hence easier to launch and fly in light winds; least wind penetration; pitch variation can be achieved with the controls faster maximum speed up to about 280 km/h (170 mph);[19] stall speed typically 65 km/h (40mph);[19] able to fly in windier turbulent conditions and can outrun bad weather; good penetration into a headwind
Maximum glide ratio about 10, relatively poor glide performance makes long distance flights more difficult; current (as of May 2017[update]) world record is 564 kilometres (350 mi)[20]about 17, with up to 20 for rigid wings open class sailplanes – typically around 60:1, but in more common 15–18 meter span aircraft, glide ratios are between 38:1 and 52:1;[21] high glide performance enabling long distance flight, with 3,000 kilometres (1,900 mi) being current (as of November 2010[update]) record[22]
Turn radius tighter turn radius[citation needed]somewhat larger turn radius[citation needed]even greater turn radius but still able to circle tightly in thermals[23]
Landing smaller space needed to land, offering more landing options from cross-country flights; also easier to carry to the nearest road longer approach and landing area required, but can reach more landing areas due to superior glide range when flying cross-country, glide performance can allow glider to reach 'landable' areas, possibly even a landing strip and an aerial retrieve may be possible but if not, specialized trailer needed to retrieve by road. Note some sailplanes have engines that remove the need for an out-landing, if they start
Learning simplest and quickest to learn teaching is done in single and two-seat hang gliders teaching is done in a two-seat glider with dual controls
Convenience packs smaller (easier to transport and store) more awkward to transport and store; longer to rig and de-rig; often transported on the roof of a car often stored and transported in purpose-built trailers about 9 metres long, from which they are rigged. Although rigging aids are used, sailplane wings are heavy. Some frequently used sailplanes are stored already rigged in hangars.
Cost cost of new is €1500 and up,[24] cheapest but shortest lasting (around 500 hours flying time, depending on treatment), active second-hand market[25]cost of new glider very high (top of the range 18m turbo with instruments and trailer €200,000) but it is long lasting (up to several decades), so active second-hand market; typical cost is from €2,000 to €145,000[26]

Military gliders[edit]

Main article: Military glider

Military gliders were used mainly during the Second World War for carrying troops and heavy equipment (see Glider infantry) to a combat zone. These aircraft were towed into the air and most of the way to their target by military transport planes, e.g. C-47 Dakota, or by bombers that had been relegated to secondary activities, e.g. Short Stirling. Once released from the tow near the target, they landed as close to the target as possible. Advantages over paratroopers were that heavy equipment could be landed and that the troops were quickly assembled rather than being dispersed over a drop zone. The gliders were treated as disposable leading to construction from common and inexpensive materials such as wood, though a few were retrieved and re-used. By the time of the Korean War, transport aircraft had also become larger and more efficient so that even light tanks could be dropped by parachute, causing gliders to fall out of favor.

Research aircraft[edit]

Main article: Experimental aircraft

Horten Ho IV flying wing sailplane prone seating glider

Even after the development of powered aircraft, gliders have been built for research, where the lack of powerplant reduces complexity and construction costs and speeds development, particularly where new and poorly understood aerodynamic ideas are being tested that might require significant airframe changes. Examples have included delta wings, flying wings, lifting bodies and other unconventional lifting surfaces where existing theories were not sufficiently developed to estimate full scale characteristics.

Unpowered flying wings built for aerodynamic research include the Hortenflying wings, the scaled glider version of the Armstrong Whitworth A.W.52 jet powered flying wing.

Lifting bodies were also developed using unpowered prototypes. Although the idea can be dated to Vincent Justus Burnelli in 1921, interest was nearly non-existent until it appeared to be a solution for returning spacecraft. Traditional space capsules have little directional control while conventionally winged craft cannot handle the stresses of re-entry, whereas a lifting body combines the benefits of both. The lifting bodies use the fuselage itself to generate lift without employing the usual thin and flat wing so as to minimize the drag and structure of a wing for very high supersonic or hypersonic flight as might be experienced during the re-entry of a spacecraft. Examples of type are the Northrop HL-10 and Martin-Marietta X-24.

The NASA Paresev Rogallo flexible wing glider was built to investigate alternative methods of recovering spacecraft. Although this application was abandoned, publicity inspired hobbyists to adapt the flexible wing airfoil for modern hang gliders.

Rocket gliders[edit]

Me 163B on display at the National Museum of the USAF

Main article: Rocket glider

Rocket-powered aircraft consume their fuel quickly and so most must land unpowered unless there is another power source. The first rocket plane was the Lippisch Ente, and later examples include the Messerschmitt Me 163 rocket-powered interceptor.[27] The American series of research aircraft starting with the Bell X-1 in 1946 up to the North American X-15 spent more time flying unpowered than under power. In the 1960s research was also done on unpowered lifting bodies and on the X-20 Dyna-Soar project, but although the X20 was cancelled, this research eventually led to the Space Shuttle.

NASA's Space Shuttle first flew on April 12, 1981. The Shuttle re-entered at Mach 25 at the end of each spaceflight, landing entirely as a glider. The Space Shuttle and its Soviet equivalent, the Buran shuttle, were by far the fastest ever aircraft. Recent examples of rocket glider include the privately funded SpaceShipOne which is intended for sub-orbital flight and the XCOR EZ-Rocket which is being used to test engines.

Rotary wing[edit]

Main article: Rotor kite

Most unpowered rotary-wing aircraft are kites rather than gliders, i.e. they are usually towed behind a car or boat rather than being capable of free flight. These are known as rotor kites. However rotary-winged gliders, 'gyrogliders', were investigated that could descend like an autogyro, using the lift from rotors to reduce the vertical speed. These were evaluated as a method of dropping people or equipment from other aircraft.

Unmanned gliders[edit]

Paper airplane[edit]

Main article: paper airplane

A paper plane, paper aeroplane (UK), paper airplane (US), paper glider, paper dart or dart is a toy aircraft (usually a glider) made out of paper or paperboard; the practice of constructing paper planes is sometimes referred to as aerogami (Japanese: kamihikōki), after origami, the Japanese art of paper folding.[28]

Model gliders[edit]

Main article: model glider

Model glider aircraft are flying or non-flying models of existing or imaginary gliders, often scaled-down versions of full size planes, using lightweight materials such as polystyrene, balsawood, foam and fibreglass. Designs range from simple glider aircraft, to accurate scale models, some of which can be very large.

Larger outdoor models are usually radio-controlled gliders that are piloted remotely from the ground with a transmitter. These can remain airborne for extended periods by using the lift produced by slopes and thermals. These can be winched into wind by a line attached to a hook under the fuselage with a ring, so that the line will drop when the model is overhead. Other methods of launching include towing aloft using a model powered aircraft, catapult-launching using an elastic bungee cord and hand-launching. When hand-launching the newer "discus" style of wing-tip hand-launching has largely supplanted the earlier "javelin" type of launch.

Glide bombs[edit]

Main article: glide bomb

A glide bomb is a bomb with aerodynamic surfaces to allow a gliding flightpath rather than a ballistic one. This allows the bomber aircraft to stand off from the target and launch the bomb from a safe distance. Most types have a remote control system which enables the aircraft to direct the bomb accurately to the target. Glide bombs were developed in Germany from as early as 1915. In World War II they were most successful as anti-shipping weapons. Some air forces today are equipped with gliding devices that can remotely attack airbases with a cluster bomb warhead.

See also[edit]

References[edit]

  1. ^FAA Glider handbookArchived 2009-02-06 at the Wayback Machine
  2. ^Liberman, Anatoly. "An Addendum to “Ten Scandinavian and North English Etymologies”", from alvíssmál 7. 1997. 101–4.
  3. ^Lynn Townsend White, Jr. (Spring, 1961) "Eilmer of Malmesbury, an Eleventh Century Aviator: A Case Study of Technological Innovation, Its Context and Tradition", Technology and Culture2 (2), pp. 97–111 [100–101].
  4. ^White, L., Jr., Eilmer of Malmesbury, an Eleventh Century Aviator. Medieval Religion and Technology. Los Angeles: University of California Press, 1978, Chapter 4.
  5. ^Lynn Townsend White, Jr. (Spring, 1961). "Eilmer of Malmesbury, an Eleventh Century Aviator: A Case Study of Technological Innovation, Its Context and Tradition", Technology and Culture2 (2), pp. 97–111 [98 & 101].
  6. ^Who is Hezarfen Ahmet Çelebi?Archived 2016-01-21 at the Wayback Machine
  7. ^Hezârfen Ahmed Çelebi "The First Man to Fly"Archived 2016-01-21 at the Wayback Machine
  8. ^Çelebi, Evliya (2003). Seyahatname. Istanbul: Yapı Kredi Kültür Sanat Yayıncılık, p. 318.
  9. ^Harwood, Craig S. and Fogel, Gary B. Quest for Flight: John J. Montgomery and the Dawn of Aviation in the West, University of Oklahoma Press 2012.
  10. ^"FAI list of people with 1000km diplomas". Retrieved 24 May 2019.
  11. ^The Fédération Aéronautique Internationale Hang Gliding Diploma (2006) for the invention of the modern hang glider: FAI Award: The FAI Hang Gliding DiplomaArchived 2011-05-18 at the Wayback Machine
  12. ^"History of Paragliding". Archived from the original on 2009-09-13. Retrieved 2009-02-15.
  13. ^"Pilot Profile: David Barish, the Probable Inventor of the Paraglider". Archived from the original on 2010-06-08. Retrieved 2009-03-10.
  14. ^David Barish, The Forgotten Father of ParaglidingArchived 2010-10-29 at the Wayback Machine
  15. ^Schweizer, Paul A: Wings Like Eagles, The Story of Soaring in the United States, pages 14–22. Smithsonian Institution Press, 1988. ISBN 0-87474-828-3
  16. ^"Definition of gliders used for sporting purposes in FAI Sporting Code". Archived from the original on 2014-01-06. Retrieved 2013-01-01.
  17. ^"FAI Sporting Code Section 7"(PDF). Archived from the original(PDF) on 2009-03-19. Retrieved 2009-03-13.
  18. ^"Technical data for Advance Omega 8". Advance AG. Archived from the original on 2013-05-30. Retrieved 2011-10-22.
  19. ^ abFlight Manual of Scheicher ASW27b. Alexander Schleicher GmbH & Co. 2003.
  20. ^"FAI Paragliding record". Fédération Aéronautique Internationale. Archived from the original on 2011-05-09. Retrieved 2010-11-30.
  21. ^"Handicap list 2008"(PDF). Deutsche Meisterschaft im Streckensegelflug. Deutscher Aero Club. Archived from the original(PDF) on 2009-02-24. Retrieved 2008-08-07.
  22. ^"FAI records". Fédération Aéronautique Internationale. Archived from the original on 2011-09-11. Retrieved 2010-11-30.
  23. ^Stewart, Ken (1994). The Glider Pilot's Manual. Airlife Publishing Ltd. p. 257. ISBN .
  24. ^"Brochures Ozone". Ozone France. Archived from the original on 2013-10-27. Retrieved 2011-10-21.
  25. ^"Typical set of classified ads for paragliders". Archived from the original on 2012-03-30. Retrieved 2011-10-22.
  26. ^"Typical set of classified ads for gliders". Archived from the original on 2010-12-06. Retrieved 2011-01-18.
  27. ^The Me 163 was powered by an unstable fuel mix and landing with fuel left caused several accidents
  28. ^"Aerogami - Event Description". Pragyan. Retrieved 21 March 2019.
Sours: https://en.wikipedia.org/wiki/Glider_(aircraft)

LET’S GO FLYING!

Article by
Dave Pecota

I’m sure almost all of you played with balsa wood gliders at some time in your youth, just as I did.   Whether it was a toy that your mom or dad bought you for “being good” … a party favor … or something you spent your allowance on in order to have some fun with friends on sunny, summer’s day. 
 

We just opened the packages … slid the wings and tails into the slots … and we were ready to go flying.   Few of us even bothered to read the assembly instructions.   We all knew what airplanes looked like.   Assembly was simple and easy … almost intuitive.

Some of the gliders, we merely tossed into the air.   Some we shot skyward with rubber band catapults.   Others had rubber-band “motors” that we wound furiously and released to fly off under their own power. (If they were rubber-powered AND had an "undercarriage" ... wheels & struts ... they took on the moniker "ROG" because they could actually "Rise Off the Ground".)

Many youngsters loved to fly toy airplanes, but … like me … lacked the building skills necessary to assemble those marvelously complicated balsa wood stick & tissue kits.   So these “ready-to-fly” (RTF) balsa wood toys provided an easy way for us to enter the realm of flight.

We soon discovered that we could alter the way the gliders flew by moving the wings forward and back … or by adding weight to the nose … or by changing the shape of the wings and tail with a piece of sandpaper … or even by winding more and more knots into the rubber-band motors.

Unknowingly, we were actually learning about the basics of flight in almost the same way Wilber & Orville Wright did ... experimentation.  (As the legend goes, it was the gift of a rubber-band powered helicopter toy that first piqued the Wright’s interest in flying.) 

Many, many times, our best flights ended with the airplanes landing on a neighbor’s rooftop, or in a tree or disappearing totally from sight.   But a quick trip to the store could easily replenish our “air force”.  They seemed to be available everywhere, with lots of company choices.   There were company names like American Junior, North Pacific, Guillow, Comet, Testors, Champion and Top Flite.   And many others I can’t remember. 


Ready-to-Fly Balsa Wood Toys
 

Fond memories indeed.  And for me, the start of what turned out to be a 35 year career in aviation. 

Although hand-made airplane-like (or bird-like) flying toys appeared in the 1800’s, it’s unclear exactly when company-made RTF toy airplanes first became available.   Some model airplane kits reportedly appeared as early as 1910.   1911 issues of “Aircraft” magazine (about “real” airplanes) had numerous ads from several manufacturers for model airplanes in kit and RTF form.   Most of these were expensive to buy.   In a time when $20-25 per week was a really good, working-class salary, the Ideal Model Aeroplane Co. (which became the Ideal Toy Co.) advertised airplane kits for $4-6.   RTF versions sold for as much as $20.   Most of the Ideal RTF airplanes were “factory built” examples of their kit aircraft. 

From 1914-20, Ideal offered wood and fiber board RTF gliders for 45 cents.  Though not inexpensive by any means, these can probably be considered some of the fore-runners of our “toy” airplanes.

In the 1920’s and 30’s, balsa wood became more readily available and the number of simple RTF toy gliders increased.   Certainly the Charles Lindbergh phenomenon also boosted sales of toy and model airplanes of all types.  However, most were still only available from hobby shops, finer toy stores or through mail order.  Many of the companies that would become household names in the toy and model airplane world … American Junior Aircraft Co., the Paul K Guillow Co., the Cleveland Model & Supply Co., the Testor Corporation and Comet Model Airplane & Supply Co. … all had their beginnings in this period.

During World War 2, balsa wood was considered to be a “strategic material”, so toy airplane production was reduced dramatically.   However, American Junior Aircraft founder Jim Walker cleverly developed a launching platform for his folding wing balsa gliders.  This provided the Army with a quick and effective system for gunnery practice.   As a result, American Junior Aircraft received significant supplies of balsa and over 120,000 Walker gliders met their doom for the war effort.       

After the war, balsa once again became plentiful.   As the post-war economy … and family “production” … boomed, dozens of companies now competed in the toy airplane market.   The number and variety of toy airplanes was truly dazzling.   New and important entrants into the RTF glider market included North Pacific Products, Pactra Chemical Co. and Top Flite.

For many of us, great airplane names like Hornet, Interceptor, Super Ace, Super Saber, Space Kadet, Ceiling Walker, Skeeter and Sleek Streek became integral parts of our everyday vocabulary.

 

In 1953, the Paul K.Guillow Co. introduced the Jetfire glider, which was the first of its type to be mass-produced and packaged in high-speed machinery.   This allowed Guillow to meet the production quantity and unit price demands of the now-flourishing “chain stores”.  The mass-marketing success of Guillow and a slowing economy spelled the end for many of the smaller companies in the 1950’s and 60’s.   Some, like American Junior Aircraft, North Pacific and Comet disappeared into larger companies.   Others just disappeared.   By the 1970’s, only a few players were left in the game.

Currently, Guillow is the dominant manufacturer of wooden toy gliders in the US.  This is understandable in my view, since they are the most akin to the gliders of the distant past.  Kids can fly ‘em right out of the package … or learn the intricacies of modification and trimming for increased performance.  Several other companies produce flying toy airplanes using plastic and foam, but their flight performance and durability are … to put it kindly … disappointing.  Likewise, contemporary laser-cut wood gliders from Asia also seem to “miss the mark” on quality and “fly-ability” in my view.

In this era of internet auctions and antique malls, vintage gliders can often be found if one is persistent.  However, many gliders are now finding their way into collections verses being used as “toys” for a weekend’s flying fun … so prices are rising accordingly.   

 

 

Please read more about these flying toys in the following pages:

SeaplaneHi-FlierInterceptorAmerican JuniorGuillowMiami WoodNorth PacificTestorsOther FlyersPromo/SpecialtySpirit of St. LouisComet
 

 

Author’s note – This article is provided for the entertainment of readers.  To my knowledge, no definitive book(s) on the subject currently exist.  However, the information was sourced as much as possible from original catalogs and ads, period newspaper articles and first-hand data from company representatives.   The opinions expressed are not necessarily endorsed by the website management.

I welcome any comments, questions or corrections.

David C “Dave” Pecota

[email protected] 

Sours: http://www.oldwoodtoys.com/new_page_2.htm
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Basics of toy glider physics

fore,       aft, port, starboard and chord on a toy balsa glider

Throwing the problem
Forces and torques
Stability
Dampering the oscillation
Weight
Aerodynamic yield of the wing
Stability 2
Angle of attack
Cambered airfoils
Thick airfoils
Airfoil Algebra
Acknowledgments





The aim of this text is to provide some understanding in the physics of an airplane, to help in the design and tuning of little paper or balsa gliders.

The drawings do not always match real-world proportions, angles or air streams. Their purpose is to make you understand the principles.



Throwing the problem


The purpose of an airplane is to transport something through the air, be it some cargo, people, or just itself. The aircraft uses the air to fly, somehow like a boat uses water to float upon.

Let's start by focusing on the load that the aircraft transports; just the mass of it. Let's imagine a little ball of lead; a mass almost concentrated in a dot. Or say a coin...



a mass dot



We can throw it some distance away, like if it was a cannonball.



parabolic         trajectory



But that's not what we want. We want to build something around the coin that will allow it to hang in the air on a slow and straight path.



trajectory



The most important part of an airplane are the wings. Some aircraft are just one big wing with nothing else... So lets take a postcard (or a rectangular sheet of balsa wood) and latch the coin in the middle of it.



nonsense



Then we gently throw this assembly like we would do with a toy glider and... the result is quite awful. One probable outcome is that the rectangle rotates at a fast rate around its longest axis and "flies" towards the ground with a straight angle. (Note that it didn't fall vertically towards the ground... we had a sideways displacement... this is encouraging!)







You may have the intuition that a solution could be to hang the coin below the sheet of balsa, like a pilot hangs below a delta glider. Maybe try this out using a rod of wood or ropes. Yet you won't get the desired result. The thing will probably fall to the ground making random movements. At best it will fall gently like a parachute. No flight...







You may have yet another intuition: that you should add a tail. A little rod that extends rearwards from the wing, with little surfaces at the end, like the tail of an arrow. You try to launch this like a toy glider... and still it's a catastrophe.







Placing the coin lower than the wing is good. Using a tail is good. But you didn't have the physics of flight in mind...



Forces and torques


When you hold a sheet of cardboard or balsa wood in your hand, with your arm extended, and you shear it through the air like a wing (with a slight upward tilt of the leading edge), it is pushed upwards by an aerodynamic lift force. The wing wings...







The air shearing along the wing, exerts relative pressure and suction forces all around the surfaces of the wing... Every single little portion of surface, experiences a tiny aerodynamic force. The force is different everywhere, and, what's even worse, it constantly changes... It's like if little bees were pushing and pulling everywhere on the wing. You easily can get the impression that no clear understanding can be build out of this mess...

Yet scientists have ways to extract key information out of such mess. The whole "mesh of bees", pushing and pulling all around the wing, can be summarized (averaged) to two things: one aerodynamic forceand one aerodynamic torque. The force is what tends to push the wing upwards. The torque is what tends to make it turn around its main axis. (Remember that experiment above, when the wing fell sideways to the ground while turning at a fast rate around its main axis. The torque is responsible for the rotation.) When you hold a piece of cardboard in your hand and shear it through the air around you, you will easily feel the lift force but not the rotation torque, because that torque is quite weak. Anyway it is key for us.







The aerodynamic force "seemingly grasps" the wing at about 1/3 from the leading edge. So, our first reaction would be to put the coin at 1/3 from the leading edge, expecting that now the force caused by the weight of the coin will be aligned with the aerodynamic force:







Nice try. But there are two bold errors. The first one is that you forgot to take into account the mass of the wing. The wing and the coin are a whole. If you want the center of mass of that whole to be 1/3 from the leading edge, you have to place the coin further away than 1/3 length.







The other bold error is that the two forces (lift and weight) are not aligned. So they cannot compensate each other exactly. The force caused by the weight will always be directed vertically towards the ground. We can only change the aerodynamic force, by rotating the whole system counterclockwise. Now the wing flies slightly towards the ground. That's what happens in the real world: in still air, gliders slowly fly towards the ground.







Nice work. But the postcard and the coin still won't fly correctly. That's because we forgot the aerodynamic torque. It's still there and it tends to make the thing flip upwards.







What can we do to counteract that torque? The most obvious proposal would be to add a tail to the postcard. It would be a little horizontal wing yet slightly turned so that it creates and aerodynamic force towards the sky, that counterbalances the torque from the main wing.







You indeed can compensate the torque that way... But if you try this out, by gluing a toothpick behind the postcard and a little square of paper at the end of the toothpick, you will neverget a proper flight. Sure you did counterbalance the torque... but you created a highly unstable system. In other words: that thing can fly if you add a computer to it, with captors and actuators, and the computer constantly adapts the angle of the tail in order to keep flying straight. It's like the broom you hold vertically on your finger. You can hold it vertically quite a long time... with great efforts. (The Wright Brother's Flyer from 1903 was unstable too and the pilot constantly had to correct it's attitude. That's why flying a replica of the Flyer requires autorisations and a special training.)

We need another way to compensate the aerodynamic torque... The way that's used in almost every airplane is to place the center of mass of the system further ahead. The coin is placed even more forward. That way a torque appears, because the force of the weight is not aligned with the aerodynamic force.







You have to place the coin so that the center of mass of the whole is 1/4 from the leading edge. Nowyou have a real chance to get the thing to fly correctly. Just a postcard with a coin you taped underneath... if you made sure that the center of mass of the whole is 1/4 from the leading edge:



center of mass         1/4 from the leading edge



Remember that 1/4th chord forever. Since I use it, I never more had to tune the center of mass of a little glider. I always get a good flight from the first throw on, provided the center of mass is at 1/4 chord.

Possibly use a toothpick to place the weight further frontwards, ahead of the leading edge, in order to get the center of mass at 1/4th chord. But, keep in mind that the shape of the weight may cause an aerodynamic instability.

There are several ways to throw such a postcard glider:
  • Hold it loosely by the trailing edge and let it hang towards the ground, like a pendulum. Just open your fingers. The glider should accelerate while falling then bend its trajectory towards a stable flight.
  • Hold it by the nose but with your wrist turned towards you. Your fingers point towards your face while gently pinching the nose of the glider. Your eyes see the trailing edge. Make a forward movement with your arm while releasing the glider. Try to make sure that your fingers release the glider in its stable flight attitude and at the correct flight speed.
  • Glue a tiny vertical piece of balsa wood on the belly of the postcard, so you can hold it like you would a conventional balsa glider.
Don't be confused if you never manage to get a flight, even an awful and oscillating one. I don't always make successful flying postcards either. Just try to make another one with a different shape, possibly after reading the next chapter.



Stability


Throwing that coined postcard may still not yield a flight... because, though we have an equilibrium of the forces and torques, that equilibrium can be a tiny little bit unstable. No fuss... the trick to make it stable is very simple. Just make sure that the trailing edge is a tad upward. If you are using a postcard, bend it very slightly (hardly visible). If you are using a sheet of balsa wood, taper the underside of the trailing edge, using some sandpaper. That way you make what is called a "flying wing airfoil profile" (guess why...)



flying wing         airfoil profile



You should get a straight flight.

You may have to correct a tendency to turn, by slightly warping the rear ends of the wings, like you would do with a toy balsa or paper glider... Or cut out ailerons and bend them. Bend them only upwards, in order to keep a globally upwards trailing edge.







It is very important to insist on the fact that the equilibrium of this whole system is "stable". That's why you get a stable flight path, on a straight line, with the system not accelerating nor decelerating. The concept is called "self-regulation". Actually, the glider is constantly slightly changing speed, course and attitude. But, the mechanisms of the stability will bring the system back towards the position of equilibrium. For example, if the speed of the glider increases, the torque that tends to lift the nose upwards will gain slightly and the glider will both lift more and drag more... which makes the speed decrease. In shorter words: an increase of speed ultimately leads to a decrease in speed. Conversely, a decrease of speed ultimately leads to an increase of speed. That's how you get stability in an otherwise quite random system.

If you built such an elementary cardboard glider and it flies more or less correctly, you will probably notice that while flying it oscillates constantly around its main axis. The nose slightly pitches up and down constantly. It oscillates... This does not hamper flight, though it is not desirable because it makes the glider loose energy (drag more). The oscillation is part of the equilibrium but we will try to damper it down.

Yet another problem you will encounter with such simplified airplanes is that they can fly correctly on some distance (if you threw them well) then suddenly they go berserk and fall to the ground. They went out of stability... Their flight equilibrium is fragile! In the real world, when engineers build a new airplane, they have to prove that it will never go out of stability. More precisely, they have to prove that, if the airplane comes to get in any possible position and speed that disallows proper flight and stability, the aircraft will anyway always tend to get back towards proper flight and stability. The requirements are different for each kind of aircraft. Unpiloted model airplanes need to be firmly stable, in order to go on flying properly whatever the gusts of wind. Airplanes with pilots will rather tend to have a "neutral" equilibrium. This means that if you put them in a given position, they will tend to stay in that position. Military airplanes like fighter airplanes, can be slightly unstable because this increases their reactivity. Military pilots are trusted to be constantly focused on the attitude of the airplane and they are trained to cope instantly with all kinds of instabilities. (Modern fighter planes can be very easy to pilot, because they have onboard computers hat control everything...)

You may get yet another chance at getting the postcard to fly by slightlybending the outer triangles. Try to bend the two trailing triangles upwards... Try to bend the two leading ones downwards or maybe upwards... Try different triangle sizes and shapes...





While you can make a proper glider using almost no maths, if you want to master all the problems of stability you will need high levels of Physics and Mathematics. After the Wright's Flyer in 1903, it took about 40 years before airplanes could be build that were perfectly stable and reliable in all circumstances. This problem of stability is key to almost every technological endeavor, be it a chemical plant, an artificial forest, a windmill, a rocket or a political or economic system. Many industrial accidents occur in developing countries, because to build a machine is easy, yet to build a stable one requires extended knowledge in several branches of Mathematics. Complex yet stable systems have been developed using few or no maths, like the boomerang, but this often required centuries of experimenting and many casualties...

Now back to our glider. It flies... It's a minimalistic shape to get flight... But it has many problems. It does not fly very far... It oscillates while flying...



Dampering the oscillation


Let's first try to cope with the oscillation. Many solutions exist... It's much fun for model airplane builders, to conceive flying wings that do not oscillate, thanks to minute details in the shape of the wings. But the common solution will do: let's use a tail. Glue a thin rod of wood to the postcard and at the end of it glue a horizontal little surface of paper. (The mass of this tail forces you to put the coin even further ahead, in order to keep the center of mass of the whole at 1/4th chord of the wing.)



flying postcard         with a horizontal tail



That little surface at the tail has only one purpose: to damper down the natural oscillation of the system. (You will see such little surfaces used in old clockworks, inside churches or museums.) Its purpose is by no way to act like the tail of an arrow. It absolutely not has the purpose to impose the angle at which the wing travels through the air. If it would impose that angle, superseding the self-regulation mechanism discussed above, there would be no more stable equilibrium, hence no flight! That's why the surface of that tail has to be very little, just enough to damper the oscillation.

Conversely, the angle at which the wooden rod emerges from the postcard, and the angle of the little surface in the flow or air, have almost no importance. Amongst toy glider builders, it is often tensely discussed what angle the tail should have, compared to the wings. A friend of mine makes much fun of this, as no angle at all perfectly does the job. I suppose that the optimal layout is to have the wings have some angle compared to the rod, but I never checked this out...



flying postcard         with a horizontal tail



A quite natural rule is that if you put the horizontal tail closer to the wing, you have to increase its surface. This is up to you. A big horizontal tail surface, close to the wing, will be heavier, will create more drag and will aerodynamically interfere with the wing. On the other extreme, a very long tail with an infinitesimal surface at its end, will be fragile or will cause a problem with the center or mass... The compromise is in-between. Building an airplane is the art of computing out compromises...



flying postcard         with a horizontal tail




Weight


Many beginners are puzzled by the weight that their glider should have. Actually, this is maybe the least important parameter of the system. Most gliders will fly correctly if they are made two, three, sometimes even ten times heavier. As long as the center of mass keeps being at 1/4th chord, you do what you want... Remember this system is self-regulating. If you increase the weight, the glider will simply fly faster, in order to generate enough lift to keep that weight in the air. If you decrease the weight, the glider will fly slower.

Of course there are practical limits. If the weight is so heavy that the wings bend away under the aerodynamic forces, you won't get a proper flight... A glider may be able to fly correctly but the high speed makes it won't survive many landings... On the opposite, a glider can also be too lightweight. At very low speed, the air will simply no more follow the wing profile correctly. Hence you have no more aerodynamic lift... (This depends on the chord of the wing. The longer the chord, the more you are allowed to decrease the speed. Conversely, if you want to make a glider that flies really slow, use wings with a strong chord.)

Even if the wings don't bend and the wing chord is not too short for the flight speed, you still may experience very different behaviors with different weights. I'd say that this is because your glider is "borderline". It is stable by chance, on a narrow margin of flight speeds. There is nothing wrong with this. I often build balsa gliders that behave that way. Just, there is no chance that the glider would be certified for manned flights...



Aerodynamic yield of the wing


This postcard glider is kind of a hybrid between a glider and a parachute... It does fly but with a steep angle towards the ground.

Let's talk a little bit about air pressure and depressure. Imagine a loose and heavy piston in the middle of a closed cylinder that contains air. If you place the cylinder vertically, the weight of the piston will make it descend slightly towards the ground. The air below the cylinder will be slightly compressed and the air above the cylinder is slightly depressed. That keeps the piston aloft, like hovering.







If there is air leaking along the piston, the piston will slowly fall to the bottom of the cylinder.

The postcard glider can be viewed in much the same way. Throw the postcard, yet between two vertical walls, in such a way that the walls close the port and starboard sides of the postcard. This is hardly doable but let's suppose that, like the piston in the cylinder, the sides of the postcard are airtight closed by the walls and this causes no friction.







The air that shears above the postcard creates a depressure and the air that shears under the postcard creates a pressure. That's what keeps the postcard in the air, just like the piston stayed hovering inside the cylinder.



air pressure on         airfoil



But here we have something very different from the piston, namely the fact that the leading edge and the trailing edge of the postcard are open. Hence you may fear that the pressure below leaks towards the depressure above... This does not happen, because the postcard is moving through the air and this dynamic system sustains the difference of pressure despite the open leading and trailing edges. If it wasn't for the frictions and turbulences (some of which are necessary to get lift and flight...), the postcard can stay flying at a constant altitude between the walls, just like the piston does inside the cylinder.

But what if we remove the walls? Then, the air will massively leak between the upper and down side of the postcard, along the port and starboard edges of the postcard. The pressurized air from underneath constantly filling the depression above the wing.



air pressure on         wing causing wing tip vortexes



One first thought may be that this is not so bad. There is a loss of lift force... well let's compensate for this, by using a huger postcard... Actually, the problem is that the exchange of air makes the air turn in a massive whirl (two whirls, one on each side of the wing) and the rotation energy that those whirls contain... is drawn from the postcard. The whirls "suck" the postcard backwards and this makes that it quite quickly falls to the ground. Poor flight... The potential energy the postcard had, due to its height above the ground, is quickly transformed into whirl energy (which ultimately becomes simply heat).

You got it: we need to prevent that sideways leak of pressure. To the least we must decrease it. Yet we cannot have airplanes and gliders fly only between walls... Your first thought could be to have the postcard carry its own "walls". You would glue two other postcards aside of it, vertically, acting as embedded walls. The problems is that this creates a massive drag on itself... It's not a good solution.

A far better solution is to use a wide postcard. You still get those massive bleeds of air between the upside and the downside, but, as the length of the sides decreased compared to the width of the wing, the loss is proportionally less severe. Closer to the center of the wing, things will be like if flying between walls.



nearly a common         glider




vortexes by         rectangular and trapezoidal wings



That's one reason why modern gliders have very wide wings, with a very narrow chord especially at the tips. But do not forget that toy gliders fly slowly, hence they need chord. I once was very puzzled by this. I build a very neat little balsa glider with extremely narrow and wide wings, nearly like a modern sport glider. I expected it to have a very good yield... but it almost parachuted to the ground.

To further regulate and decrease the whirls, you can cut rounded wing tips at the ends of the wings. The decrease of the turbulences and vibrations can even be felt in your fingers if you shear such a wing through the air.



toy glider with         dihedral



Such a wide wing with tapered wing tips would be what sea birds have (but the overall shape of their wings is ways more sophisticated than what I drew above).

The other simple way to cope with the turbulences would be the trapezoidal wings. With the delta wing being an extreme. Birds that live in cities and forest, making short and agile flights, tend to use the delta shape (include the tail and rotate 180°).

The trapezoidal wing can be felt as ideal. It is widely used and proven, it is close to optimal for the mechanical constraints, but it is not as perfect as you may expect. At every zone of say the downside of the wing, the air will slightly move towards the zone aside of it that's closer to the wing tips. Because, that zone creates less overall pressure because it has less surface. On the whole length of the downside, there is constant movement of the air towards the wing tips. While reciprocally, on the upside, there is a constant movement of air away from the wing tips. Those opposite movements meet behind the wings and create whirls that ultimately unite in two huge whirls similar to those spoken above for a rectangular wing.



vortexes by         trapezoidal wings



Trapezoidal wings have some disadvantages, like the fact that their stalls are deadlier, because the stall tends to occur on the whole surface of the wing at once. On rectangular wings, the strong vortexes at the wing tips, ensure a correct air flow there, even when the center part of the wing is in a severe stall. And... remember the chord: if the ends of a trapezoidal wing have a too narrow chord for the flight speed of the glider...

Maybe try a trapezoidal wing with rounded wing tips...

Yet another optimal shape would be an almost rectangular wing with "winglets". To see beautiful such wings look at photographs of flying eagles. For example by using this image search page: http://www.google.be/images?q=eagle&biw=1272&bih=625. How do winglets work? I can propose an explanation. Look at the 4:1 aspect ratio wing below. Let's arbitrarily decide that the outermost squares cause the worst turbulence and braking. Let's color them in red. They make half of the wing surface, so for sure this is not an optimal wing.



rectangular wing with turbulent outer zones



A wing with a sixth of the chord, with both end squares considered red zones, has proportionally much more efficient surface. It's a "high aspect ratio wing".



high aspect ratio wing with few outer turbulent         zones



Now, we can create the same wing shape as the awful rectangle above but using efficient high aspect ratio wings. We "simply" put six of them one after the other.



 



Of course this only makes sense if we separate them vertically, creating an "hexaplane" structure.







But... biplane, triplane and for sure hexaplane wing structures are complex and not optimal. So, we try to get the best of both systems. We'll use a plain surface for the most, inside part. And, separate little wings (winglets) for the outer parts.







This implies that those winglets each end separately, as in the six-plane structure. That way we have a straight rectangular wing for most of the surface yet with much less "red surface".









Stability 2


If you try out a glider with wide wings, it will probably succeed but you will notice a problem: the glider tends to turn itself to the right or to the left. Even if it stays flying on a straight path, its nose starts heading aside. It flies like a crab... Simple problem, simple solution: we add a vertical tail. But it's mandatory that you do not oversize its surface. A tiny surface, much like the horizontal tail, will do. A huge vertical surface would cause instabilities and you would get ugly flights. (The common solution to use a tail with more surface without creating instabilities is to reverse the tail: either place the vertical surface below the horizontal surface or use and inverted V tail (a /\ tail...).)



glider with         traditional tail



Oh, yeah, I forgot about the slight upward bend of the trailing edge. Now that we're using that tiny horizontal tail surface, it is most often no more needed...

There remains one big potential source of instability. If you launch this glider on a long flight, it may steadily tend to turn in a given direction and then the turn rate will augment towards a catastrophe (I rarely encounter that problem...) One common way to cope with this is to place the center of mass of the glider a little lower. That regulates the problem by the "pendulum effect".



toy glider with         center of mass placed lower



Or you can use a positive dihedral (bend the wings slightly upward). Sometimes, the conceivers of an airplane have no other choice than to place the wings above the fuselage. A good example is the British "Harrier" VTOL fighter plane. But the strong pendulum effect is unwanted. In order to decrease it, the wings will have a negative dihedral...



toy glider with         dihedral



Actually, the concept of stability can be perceived in a broader way. If you build a glider with a huge vertical tail (which should bring stability), a strong dihedral or the wings high above the center of gravity (which should bring stability), you probably will have the glider fly a "Dutch roll", that is oscillating in a corkscrew path. The Dutch roll... will be very stable. You dohave stability... but not the kind of stability that is required for a standard airplane. (If you don't succeed in inducing the Dutch roll that way, try to put the center of gravity slightly backwards.)

Now your glider should have an efficient and stable flight. It should glide far, on a straight line, with no oscillations. In order to further better it, you can slightly streamline the wing airfoil profile, using some high grain sandpaper.



toy glider with         dihedral




Angle of attack


One more sophisticated tuning is the AOA; the angle of attack of the wings. Try it out with the piece of cardboard that you hold in your hand and shear through the air. If you hold it parallel to the flow of air, there is no lift force. The more you pitch its leading edge upward, the higher the lift force. Till some high angle where the cardboard rather brakes than lift. The angle between the stream of air and the surface of the cardboard is the AOA.



AOA



One obvious reason to tune the AOA is that this has a direct impact on the flight speed. If the AOA is low, the wings lift less, hence a higher speed is needed to get the appropriate lift force to counterbalance the weight of the glider. The self-regulation mechanism will (should) ensure that this speed is attained. On the opposite, a high angle will lead to a low speed of flight.

Some airplanes, like the flying flee, are driven (you don't pilot a flying flee, you drive it...) by changing the incidence of the main wing (the angle compared to the fuselage). The wing can rotate a few degrees around its main axis. The yoke that the pilot holds in his hands, is coupled to a system of levers that slightly rotates the wing. The flying flee has no horizontal tail surface...

Yet far out most airplanes are piloted using the horizontal tail. The angle and curvature of the tail surface changes according to the position the pilot gives to the yoke (and to the trim). This leads the airplane as a whole to pitch up or down, which changes the angle the wings travel through the air (the AOA). I was a little bit provocative on purpose, a while above, by telling that the horizontal tail had no purpose at all regarding the angle the glider travels through the air. Of course it has, in most cases. But never forget that the tail should always be little, in order not to supersede the self-regulation. (Airplanes do exist that have such a huge horizontal tail that actually they have two pairs of wings, but precise rules have to be followed to ensure stability.)

There is a second reason why you want to tune the AOA: there exist two optimal angles. The one that's optimal for you depends on what specific performance you want: that your glider flies the longest distance or that it stays the longest time in the air.

If you want your glider to fly far, then you need the highest aerodynamic yield. What you need is not the least wing drag... (You would get that by using an AOA of zero, hence no lift at all, hence only drag.) Rather, you need that the ratiobetween the wing lift and the wing drag be as high as possible. In other words: as much possible lift per unit drag. It seems that you get that by using an AOA of about 7°. I did not verify...

On the other hand, if you want the glider to stay the longest time in the air, then you need it to fly slow. Hence you need a strong lift from the wings, as long the drag is not too high. It seems that you get that by using an AOA of about 14°. I didn't verify either... But here we have a problem: a flat airfoil profile like we're using, will simply "stall" at an AOA of 14°. It is so that the air has to follow a strange path around the leading edge. Once this is exaggerated, due to the strong AOA, the air stream will detach from the wing and there will be no more proper lift.



detachment of         air stream above wing



One solution would be to bend the leading edge downward, to allow/force the air to follow the shape of the airfoil. Let's call it a beak. (That's what the slats on the wings of airliners are for, to allow a high AOA, in order to be able to take off and land at a lower speed.) Yet be careful: while such a beak can save the day at a strong AOA, it is useless and it causes drag at a low AOA. (That's why airliners retract the slats once in flight.)



Air stream         follows the leading edge



If you seek performance, then you want the fuselage to travel through the air with the least drag. Hence parallel to the stream. So simply use the wing incidence that matches your needs, always make sure that the center of mass is at 1/4th chord and tune the horizontal tail (the elevator) to get the glider to travel at the wished angle through the air.

About the elevator... Don't make some errors. You want it to travel through the air without generating a high drag and without stalling. (That's why the tails of many airplanes have a delta shape; it's the shape that's the most tolerant to steep angles of attack.) You need an elevator that has enough surface to exert its desired control force while having only a few degrees of AOA. See for example the glider below. A beginner may claim that he managed to build a glider whose wings have a very steep AOA. And indeed the glider flies. But... it flies with the wings at a normal AOA and the rear tail bluntly braking. The fuselage and the nose are braking too, by the way. The incidenceof the wings is 30°, as this is the manufactured angle between the fuselage and the wing plane, but the AOA in flight is just a few ° and the aerodynamic yield of the whole is bad.








Cambered airfoils


We only talked about flat airfoils.



flat thin         airfoil



Now about cambered airfoils.



cambered thin         airfoil



One first advantage of cambered airfoils is that they lift more. Hence they allow to use wings with less surface, or reciprocally they allow to fly slower with a given wing surface. (FoilSim is a great Java applet developped by NASA: http://www.grc.nasa.gov/WWW/K-12/airplane/foil3.html)

(This property is used by ailerons and flaps. When the aileron is lowered at the end of a wing, this increases both the AOA and the camber of the wing, hence its lift force. This makes the airplane roll. The sideways movements of the yoke command the ailerons. The flaps, on the other hand, are deployed simultaneously on both sides of the wings. They strongly increase the curvature and allow to take off and land at a lower speed.)

The second advantage of the camber is that a slight camber allows for an even better aerodynamic yield. The lift to drag ratio is slightly better than that of a flat airfoil. (Yet a strong camber will brake a lot; it creates a lot of drag. This is not a problem when an airplane takes off with powerful engines and especially when it is landing. Flaps are often designed to increase the drag when they are fully deployed, to ensure a safe landing path. On most little airplanes, the flaps will not increase the wing surface, only its camber. On the B-52 bomber, the flaps cause no camber but they increase the surface of the wings. On modern airliners, the flaps both increase the curvature and the surface of the wings.)

So, if you want a glider designed for distance, you would use a slight camber, that yields the maximum efficiency. If you want a glider that stays in the air for a long while, you would use a stronger camber, that yields a lot of lift and allows to fly slower, hence stay longer in the air even if the increased drag decreases the distance.

But... are you sure that you need a camber? It can poison your glider... You can get the same lift with a flat profile, simply by using a wider chord. That compensates for the fact that the flat profile lifts less. And... there is an elephant of a good reason why you may want and benefit from a wider chord. Remember that when the chord is little and the flight speed is low, the air will no more follow the airfoil properly. By using a wider chord, you get a cleaner behavior at a given low speed. That's why butterflies have flat wings.

Reciprocally, at very high speeds, some camber will allow a shorter wing chord and this is much desirable because the problem with long chords and high speed is that strong and useless turbulence appear on the upside of the trailing part of the wing. You wantless chord, in order to have less wing surface braked by turbulence.

One way to talk about it is that under a given   chord x speed   product you get no turbulence at all. That makes the air won't follow the wing profile. (Unless you artificially produce turbulence, say using turbulators, or widely flapping wings like little insects.) You need to be above a given   chord x speed.   But, when your   chord x speed   becomes very high, you get a lot of turbulence, useless to get a proper air path and that create a lot of drag.

Yet another advantage of flat profiles is that they have a neat behavior. Their stability is quite neutral and constant at any AOA. They simply lift proportionally to the AOA. Cambered airfoils tend to have a more constant lift, less dependent of the AOA. When using highly cambered airfoils, for example (don't), the AOA has few impact on the lift (but don't, the drag is tremendous). Common cambered airfoils lift even at a slightly negative AOA. (Consider that the AOA lifts and that the camber lifts, the lift force of the wing being the sum of the two.) But the real problem with cambered airfoils is that they are more unstable. Hence, either you mustuse a flying wing airfoil (with the end of the camber inverting to a short and slight upward camber) (exaggerated in the picture below) or you must use an appropriate horizontal tail. (That's why when a conventional airplane looses its tail, no recovery is possible and the airplane will tumble to the ground and crash.)








Thick airfoils


Whether they were flat, had a beak or were cambered, till now we talked only about thin profiles.

Why use a thick profile? The thicker an object, the more it brakes when passing through the air...

Early airplanes all had thin wings. This is desirable for very slow aircraft. The air won't move correctly around a thick profile, at very low airspeed. Yet the problem with thin profiles is the difficulty to get stiff wings. More exactly: stiff and lightweight. By using steel spars or plain wooden plates, the wings would be very stiff yet far too heavy. Instead, a huge structure was used made out of ropes and rods, to get something very lightweight and rigid. That was the biplane shape, like shown on these pictures: https://www.google.be/search?tbm=isch&q=early+biplane. But those ropes and rods cause drag. And two wings one above each other is not aerodynamically optimal.

By using a thick wing, you can put spars trough it, that have a strong diameter, hence that can be both stiff and reasonably lightweight. You still get a heavier wing, but you have no ropes and rods flapping in the airstream. The overall result is better. And... if the wing is not too thick, it won't drag much more than a thin wing in normal circumstances of flight, with a regular AOA.

(The WWII Spitfire fighter plane used quite thin wings anyway, in order to be able to fly at high speed at low altitude. In such circumstances, the wings were more a hinder than a necessary part of the airplane. Much littler wings would have allowed better performances. Yet the wings being quite flat, the impact of the problem was reduced because they would brake less when almost parallel to the airstream. (This caused yet another problem: the wings being less rigid, due to their flatness, at high speed they would twist when the pilot used the ailerons. That was vicious: say when the aileron went down, that would twist the wing to a lower AOA, hence the lift of the wing decreased instead of increasing and the plane rolled the other side.))

Thick wing... When something thick travels through the air, you must streamline it. The fuselage of an airliner would be a good illustration for this. It's a tube yet the fore part is rounded and the aft is a soft cone.







The more progressive the transitions between the shapes, the better the result.







For a wing profile, one tries to use an optimal aerodynamic shape, typically with the spar in the thickest part.



standard thick         flat airfoil



Most wings use such a shape. But... a toy glider is an extreme, as we already stated. Again, it's about turbulence. The fore part tends to avoid creating turbulence, while the aft part will eagerly trigger them. That's why the high-speed World War II P-51 Mustang fighter plane used an airfoil profile with a longer fore part and a shorter aft part; in order to minimise the part of the wing that creates the dreaded turbulences.



P-51 Mustang         schematic airfoil



(Modern gliders have yet another way to shorten the aft part of the wing: the airfoil as a whole is kept short. Their wings have a short chord and that's one reason why they are so wide.)

On the other hand, a slow toy glider, that dearly needs some turbulence for the wings to wing, may benefit from the longest possible aft part.







But... thick airfoils are reportedly not good at very low speed. That would be because the leading edge of thin airfoils scrapes a little un-aerodynamically trough the air and creates turbulence. Those turbulence then ensure that the stream of air correctly follows the upper side of the airfoil. Some airfoil profiles exist that have a thin leading edge, then swell aerodynamically to contain a spar. The airfoil below would be a thin one, with both a curvature and a beak, and with a swell to contain a spar.








Airfoil Algebra


All the thick airfoil profiles drawn till now are flat. But of course they can have a camber and a beak. You "bend" the thick airfoil to match the desired camber. You "bend" its fore part to follow the desired beak curve.

Say you want to make a glider that must stay the longest possible time in the air. Low speed, high lift. Hence a long chord, a beak, some thickness in order to have lighter wings, a long aft part. You get this:



airfoil         generation



Now a common warning to beginners. Suppose you get excellent results with the composite airfoil depicted above (then tell me, I never tried it out). You love it. Your glider flies wonderfully. You get another kind of wood that's more rigid and you decide to make another wing, with a thinner airfoil, say two times thiner. But you want to keep the same aerodynamic quality! If you just take the above shape and you flatten it, halving every height, you're a beginner. Your glider will nothave the same characteristics. Let's explain why. The flight characteristics are mostly imposed by the flat wing part, with the beak. The flesh you put around it is a minor detail. When you halved the airfoil, you changed the shape of the beak. Hence you changed the behavior of the wing. You mustkeep the same flat profile. But you are allowed to make the flesh around it thinner. So you get this:



airfoil         modification



This reasoning also yields that when you talk about wing incidence or angle of attack, the reference plane of the airfoil will be that of the virtual thin profile it contains. The picture below show the thick airfoil from above laying flat in the airstream:



airfoil with         null AOA



But wait a minute. A rounded leading edge may be no good at very low speed. So let's use a sharp one:






Acknowledgments


I wish to thank:
  • The pioneers of flight, who made this dream reality and whose research is still ongoing.
  • My friend Jacques Donneux, for his thrive and dedication. He learned building gliders to many children.
  • My friends Didier Bizzarri and Yves-Dominique Franck, whose advice and data I have been thoroughly using.
  • The many experimenters and model glider builders that made data available on the Internet.
  • The NASA, for their excellent online documentation and tools.
  • My friend Frédéric Cloth, who hosts this page and many more.
  • Frank Armbruster for pointing out two misused technical terms.


Eric Brasseur  -  November 21 2009  till  October 12 2014

Sours: http://www.ericbrasseur.org/glider_physics.html

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