Gliding flight

Source: Wikipedia, the free encyclopedia.
For the sport of soaring in gliders (sailplanes), see Gliding.

Gliding flight is heavier-than-air flight without the use of thrust; the term volplaning also refers to this mode of flight in animals.[1] It is employed by gliding animals and by aircraft such as gliders. This mode of flight involves flying a significant distance horizontally compared to its descent and therefore can be distinguished from a mostly straight downward descent like a round parachute.

Although the human application of gliding flight usually refers to aircraft designed for this purpose, most powered aircraft are capable of gliding without engine power. As with sustained flight, gliding generally requires the application of an

flying snake
can achieve gliding flight without any wings by creating a flattened surface underneath.

Aircraft ("gliders")

Main article:
Glider aircraft

Most winged aircraft can glide to some extent, but there are several types of aircraft designed to glide:

The main human application is currently recreational, though during the Second World War

paragliders. These two latter types are often foot-launched. The design of all three types enables them to repeatedly climb using rising air and then to glide before finding the next source of lift. When done in gliders (sailplanes), the sport is known as gliding and sometimes as soaring. For foot-launched aircraft, it is known as hang gliding and paragliding. Radio-controlled gliders
with fixed wings are also soared by enthusiasts.

In addition to motor gliders, some powered aircraft are designed for routine glides during part of their flight; usually when landing after a period of a powered flight. These include:

Some aircraft are not designed to glide except in an emergency, such as engine failure or fuel exhaustion. See list of airline flights that required gliding flight. Gliding in a helicopter is called autorotation.

Gliding animals

Further information: Flying and gliding animals and List of soaring birds

Birds

A number of animals have separately evolved gliding many times, without any single ancestor. Birds in particular use gliding flight to minimise their use of energy. Large birds are notably adept at gliding, including:

Like recreational aircraft, birds can alternate periods of gliding with periods of soaring in rising air, and so spend a considerable time airborne with a minimal expenditure of energy. The great frigatebird in particular is capable of continuous flights up to several weeks.[3]

Mammals

Patagia on a flying squirrel

To assist gliding, some mammals have evolved a structure called the patagium. This is a membranous structure found stretched between a range of body parts. It is most highly developed in bats. For similar reasons to birds, bats can glide efficiently. In bats, the skin forming the surface of the wing is an extension of the skin of the abdomen that runs to the tip of each digit, uniting the forelimb with the body. The patagium of a bat has four distinct parts:

  1. Propatagium: the patagium present from the neck to the first digit
  2. Dactylopatagium: the portion found within the digits
  3. Plagiopatagium: the portion found between the last digit and the hindlimbs
  4. Uropatagium
    : the posterior portion of the body between the two hindlimbs

Other mammals such as gliding possums and flying squirrels also glide using a patagium, but with much poorer efficiency than bats. They cannot gain height. The animal launches itself from a tree, spreading its limbs to expose the gliding membranes, usually to get from tree to tree in rainforests as an efficient means of both locating food and evading predators. This form of arboreal locomotion, is common in tropical regions such as Borneo and Australia, where the trees are tall and widely spaced.

In flying squirrels, the patagium stretches from the fore- to the hind-limbs along the length of each side of the torso. In the

aerofoil enabling them to glide 50 metres or more.[4] This gliding flight is regulated by changing the curvature of the membrane or moving the legs and tail.[5]

Fish, reptiles, amphibians and other gliding animals

In addition to mammals and birds, other animals notably

flying squid
also glide.

Flying fish taking off

The flights of flying fish are typically around 50 meters (160 ft),

updrafts created by a combination of air and ocean currents.[8][9]

Snakes of the genus

attitude control by "slithering" in the air.[16]

Flying lizards of the genus Draco are capable of gliding flight via membranes that may be extended to create wings (patagia), formed by an enlarged set of ribs.[17]

Gliding flight has evolved independently among 3,400 species of frogs[18] from both New World (Hylidae) and Old World (Rhacophoridae) families.[19] This parallel evolution is seen as an adaptation to their life in trees, high above the ground. Characteristics of the Old World species include "enlarged hands and feet, full webbing between all fingers and toes, lateral skin flaps on the arms and legs

Forces

Forces on a gliding animal or aircraft in flight

Three principal forces act on aircraft and animals when gliding:[20]

  • weight – gravity acts in the downwards direction
  • lift – acts perpendicularly to the vector representing airspeed
  • drag – acts parallel to the vector representing the airspeed

As the aircraft or animal descends, the air moving over the wings generates

lift. The lift force acts slightly forward of vertical because it is created at right angles to the airflow which comes from slightly below as the glider descends, see angle of attack. This horizontal component of lift is enough to overcome drag
and allows the glider to accelerate forward. Even though the weight causes the aircraft to descend, if the air is rising faster than the sink rate, there will be a gain of altitude.

Lift to drag ratio

Main article:
Lift to drag ratio
Drag vs Speed. L/DMAX occurs at minimum Total Drag (e.g. Parasite plus Induced)
Coefficients of Drag and Lift vs Angle of Attack. Stall speed corresponds to the Angle of Attack at the Maximum Coefficient of Lift

The lift-to-drag ratio, or L/D ratio, is the amount of lift generated by a wing or vehicle, divided by the drag it creates by moving through the air. A higher or more favourable L/D ratio is typically one of the major goals in aircraft design; since a particular aircraft's needed lift is set by its weight, delivering that lift with lower drag leads directly to better fuel economy and climb performance.

The effect of

polar curve
. These curves show the airspeed where minimum sink can be achieved and the airspeed with the best L/D ratio. The curve is an inverted U-shape. As speeds reduce the amount of lift falls rapidly around the stalling speed. The peak of the 'U' is at minimum drag.

As lift and drag are both proportional to the coefficient of Lift and Drag respectively multiplied by the same factor (1/2 ρair v2S), the L/D ratio can be simplified to the Coefficient of lift divided by the coefficient of drag or Cl/Cd, and since both are proportional to the airspeed, the ratio of L/D or Cl/Cd is then typically plotted against angle of attack.

Drag

Induced drag is caused by the generation of lift by the wing. Lift generated by a wing is perpendicular to the relative wind, but since wings typically fly at some small angle of attack
, this means that a component of the force is directed to the rear. The rearward component of this force (parallel with the relative wind) is seen as drag. At low speeds an aircraft has to generate lift with a higher angle of attack, thereby leading to greater induced drag. This term dominates the low-speed side of the drag graph, the left side of the U.

wind resistance, varies with the square of speed (see drag equation
). For this reason profile drag is more pronounced at higher speeds, forming the right side of the drag graph's U shape. Profile drag is lowered primarily by reducing cross section and streamlining.

As lift increases steadily until the critical angle, it is normally the point where the combined drag is at its lowest, that the wing or aircraft is performing at its best L/D.

Designers will typically select a wing design which produces an L/D peak at the chosen

stall
are also important.

Minimising drag is of particular interest in the design and operation of high performance glider (sailplane)s, the largest of which can have glide ratios approaching 60 to 1, though many others have a lower performance; 25:1 being considered adequate for training use.

Glide ratio

When flown at a constant speed in still air a glider moves forwards a certain distance for a certain distance downwards. The ratio of the distance forwards to downwards is called the glide ratio. The glide ratio (E) is numerically equal to the lift-to-drag ratio under these conditions; but is not necessarily equal during other manoeuvres, especially if speed is not constant. A glider's glide ratio varies with airspeed, but there is a maximum value which is frequently quoted. Glide ratio usually varies little with vehicle loading; a heavier vehicle glides faster, but nearly maintains its glide ratio.[21]

Glide ratio (or "finesse") is the

cotangent
of the downward angle, the glide angle (γ). Alternatively it is also the forward speed divided by sink speed (unpowered aircraft):

Glide number (ε) is the reciprocal of glide ratio but sometime it is confused.

Examples

Flight article Scenario
glide ratio
Eta (glider) Gliding 70[22]
Great frigatebird Soaring over the ocean 15–22 at typical speeds[23]
Hang glider
 Gliding 15
Air Canada Flight 143 (Gimli Glider)
fuel exhaustion
 12~
British Airways Flight 9
 Boeing 747-200B when all engines failed due to volcanic ash 15~
Paraglider High performance model 11
Helicopter in autorotation 4
Powered parachute with a rectangular or elliptical parachute 3.6/5.6
Space Shuttle unpowered approach from space after re-entry 4.5[24]
Wingsuit
 while gliding 3
Hypersonic Technology Vehicle 2 Equilibrium hypersonic gliding estimate[25] 2.6
Northern flying squirrel Gliding 1.98
Sugar glider (possum) Gliding 1.82[26]
Space Shuttle Supersonic 2 (at Mach 2.5)[24]
Space Shuttle Hypersonic 1.8 (at Mach 5), 1 (over Mach 9)[24]
Apollo CM
 Transonic 0.50 (at Mach 1.13)[27]
Apollo CM
 Reentry and hypersonic 0.368 avg (prior to 1st peak g), 0.41 (at Mach 6)[27]

Importance of the glide ratio in gliding flight

Polar curve showing glide angle for the best glide speed (best L/D). It is the flattest possible glide angle through calm air, which will maximize the distance flown. This airspeed (vertical line) corresponds to the tangent point of a line starting from the origin of the graph. A glider flying faster or slower than this airspeed will cover less distance before landing.[28][29]

Although the best glide ratio is important when measuring the performance of a gliding aircraft, its glide ratio at a range of speeds also determines its success (see article on gliding).

Pilots sometimes fly at the aircraft's best L/D by precisely controlling airspeed and smoothly operating the controls to reduce drag. However the strength of the likely next lift, minimising the time spent in strongly sinking air and the strength of the wind also affects the optimal

ballast
to increase the airspeed and so reach the next area of lift sooner. This has little effect on the glide angle since the increases in the rate of sink and in the airspeed remain in proportion and thus the heavier aircraft achieves optimal L/D at a higher airspeed. If the areas of lift are strong on the day, the benefits of ballast outweigh the slower rate of climb.

If the air is rising faster than the rate of sink, the aircraft will climb. At lower speeds an aircraft may have a worse glide ratio but it will also have a lower rate of sink. A low airspeed also improves its ability to turn tightly in the centre of the rising air where the rate of ascent is greatest. A sink rate of approximately 1.0 m/s is the most that a practical hang glider or paraglider could have before it would limit the occasions that a climb was possible to only when there was strongly rising air. Gliders (sailplanes) have minimum sink rates of between 0.4 and 0.6 m/s depending on the class. Aircraft such as airliners may have a better glide ratio than a hang glider, but would rarely be able to thermal because of their much higher forward speed and their much higher sink rate. (The Boeing 767 in the Gimli Glider incident achieved a glide ratio of only 12:1).

The loss of height can be measured at several speeds and plotted on a "

polar curve
" to calculate the best speed to fly in various conditions, such as when flying into wind or when in sinking air. Other polar curves can be measured after loading the glider with water ballast. As mass increases, the best glide ratio is achieved at higher speeds (The glide ratio is not increased).

Soaring

Main article: Lift (soaring)

Soaring animals and aircraft may alternate glides with periods of soaring in

lee waves, convergences and dynamic soaring. Dynamic soaring is used predominately by birds, and some model aircraft, though it has also been achieved on rare occasions by piloted aircraft.[31]

Examples of soaring flight by birds are the use of:

For humans, soaring is the basis for three air sports: gliding, hang gliding and paragliding.

See also

References

  1. ^ volplane. The Free Dictionary.
  2. ^ Blackburn, Ken. "Paper Plane Aerodynamics". Ken Blackburn's Paper Airplanes. Archived from the original on 1 October 2012. Retrieved 8 October 2012. Section 4.3
  3. ^ "Nonstop Flight: How The Frigatebird Can Soar For Weeks Without Stopping". NPR. Retrieved 2 July 2016.
  4. ISBN 0207144540
    .
  5. ^ "Sugar Glider Fun Facts". Drsfostersmith.com. Retrieved 22 June 2010.
  6. ^
    Greenwood Press
    .
  7. ^ Flying Fish, Exocoetidae National Geographic. Retrieved 10 August 2014.
  8. ^ a b c Kutschera, U. (2005). "Predator-driven macroevolution in flyingfishes inferred from behavioural studies: historical controversies and a hypothesis" (PDF). Annals of the History and Philosophy of Biology. 10: 59–77. Archived from the original (PDF) on 20 August 2007.
  9. ^
    doi:10.1111/j.1469-7998.1990.tb04009.x. Archived from the original
    (PDF) on 20 October 2013.
  10. ^ Fish, F. (1991). "On a fin and a prayer" (PDF). Scholars. 3 (1): 4–7. Archived from the original (PDF) on 2 November 2013.
  11. ^ Garland, T Jr.; Losos, J.B. (1994). "10. Ecological morphology of locomotor performance in squamate reptiles". Ecological Morphology: Integrative Organismal Biology (PDF). Chicago: University of Chicago Press. pp. 240–302. Retrieved 14 July 2009.
  12. JSTOR 1445288. Archived from the original
    (PDF) on 30 October 2006. Retrieved 15 July 2009.
  13. ^
    S2CID 4424131. Retrieved 14 July 2009. [dead link
    ]
  14. S2CID 84133938
    .
  15. Smithsonian Institution Press. pp. 14–15
    .
  16. ^ "Researchers reveal secrets of snake flight". 12 May 2005. Retrieved 27 November 2007.
  17. ^ "BBC Earth – Flying draco lizard". Retrieved 5 October 2021.
  18. ^ Emerson, S.B., & Koehl, M.A.R. (1990). "The interaction of behavioral and morphological change in the evolution of a novel locomotor type: 'Flying' frogs." Evolution, 44(8), 1931–1946.
  19. ^ Emerson, S.B., Travis, J., & Koehl, M.A.R. (1990). "Functional complexes and additivity in performance: A test case with 'flying' frogs." Evolution, 44(8), 2153–2157.
  20. ^ NASA: Three forces on a glider or gliding animal
  21. ^ Glider Flying Handbook, FAA Publication 8083-13, Page 3-2
  22. ^ Eta aircraft Archived 13 November 2017 at the Wayback Machine Eta aircraft performances plots – accessed 2004-04-11
  23. ^ Flight performance of the largest volant bird
  24. ^ a b c Space Shuttle Technical Conference pg 258
  25. S2CID 67827450
    .
  26. ISSN 1365-2907
    .
  27. ^ a b Hillje, Ernest R., "Entry Aerodynamics at Lunar Return Conditions Obtained from the Flight of Apollo 4 (AS-501)," NASA TN D-5399, (1969). p16
  28. ^ Wander, Bob (2003). Glider Polars and Speed-To-Fly...Made Easy!. Minneapolis: Bob Wander's Soaring Books & Supplies. pp. 7–10.
  29. ISBN 9780160514197
    .
  30. ISBN 0-07-069633-0
    . There are four main kinds of lift which the soaring pilot may use....
  31. ^ Reichmann, Helmut (2005). Streckensegelflug. Motorbuch Verlag.
    ISBN 3-613-02479-9
    .
  32. ^ [Report of use of wave lift by birds by Netherlands Institute for Ecology]
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