Flying wing , also tailless aircraft or flying wing is the designation for aircraft with a special structural design, in which on a separate elevator unit and also to a rudder will be omitted. Instead of a conventional fuselage cross-section, flying wings often have a smooth transition between the fuselage and the wings . With the so-called “pure flying wing”, all important components such as drive, fuel, cargo and crew are located within the wing.
"Pure" flying wings represent a subgroup of the tailless aircraft . "Tailless" aircraft differ from conventional aircraft in the lack of a detached horizontal stabilizer, whereby a detached fuselage and a conventional vertical stabilizer can be present. Delta-winged birds are often designed as "tailless".
Discussion Tailless vs. Flying wing
The distinction between tailless and flying wings is not sharply delineated; Often both terms are used synonymously in German-speaking countries. Internationally they are called "Flying Wing" - French "Ailes Volantes" - Spanish "Ala Volante" etc.).
A clear separation does not seem to make sense either, since then the Northrop XB-35 would have been a flying wing, whereas its further development YB-49 with vertical stabilizing fins would only have been "tailless". This also applies to the use of winglets , which could be viewed both as an extension of wing aerodynamics and as vertical stabilization surfaces.
The same applies to the definition of the so-called delta wing, as for example, the aerodynamics of the first delta wing "Delta I" by Alexander Lippisch with its relatively high stretching more of a so-called Brettnurflügels corresponded as example of a Dassault Mirage or Convair F-106 with very low stretch. An example of a Delta flying wing is the McDonnell Douglas / General Dynamics A-12 project.
Last but not least, the “pure flying wing” requires a certain size so that at least the pilot can be accommodated in the wing. In this respect, even most of the hoarding constructions would not have been pure flying wings.
An overview of most of the tailless and flying wing aircraft can be found in the list of tailless aircraft .
Origin and Pioneers
The concept of the flying wing was born on February 16, 1876 , when the French engineers Alphonse Pénaud and Paul Gauchot amphibiously applied the patent for "an airplane or flying device", which is powered by two propellers and has all the properties of a flying wing as we know it today. having. Only with the end of the First World War and the beginning of civil aviation did the concept gain momentum.
The first technical implementation of the flying wing idea goes back to a publication by Friedrich Ahlborn published in 1897 , in which the stable flight properties of the seeds of the climbing plant Zanonia macrocarpa are described. The Austrian Ignaz "Igo" Etrich developed the first flying wing based on the model of this flying seed in 1903 , received a patent for it in 1905 , and in 1906 first flew a manned glider with this wing shape. In England, the painter and aviation pioneer José Weiss also designed gliders based on this principle.
1910 was Nurflügelpatent of Hugo Junkers logged. This patent includes the accommodation of engines, payload and fuel within the wing, but does not describe the omission of a separate horizontal stabilizer, which is a common feature of all tailless aircraft. The Junkers patent specification was implemented with the Junkers G 38 , which had a conventional box tail.
Before the First World War , flying wings by René Arnoux (flying board) and John William Dunne ( swept wing ) were sometimes quite successful. When Rhön competition in 1921 it was the flying wing glider world sailors , probably the first of a pure on his only flying gliding beyond glider has taken.
The flying wing idea was continued in Germany by Alexander Lippisch , the Horten brothers , in Switzerland by Alexander Leo Soldenhoff , in the USA by Jack Northrop , in the Soviet Union by Boris Iwanowitsch Tscheranowski , in France by Charles Fauvel and in Great Britain by Geoffrey TR Hill and the Handley Page Aircraft Company . During the Second World War , the development of tailless types in Germany and the USA underwent significant further development, which led, for example, to the Me 163 and the Northrop N-9M as well as the Horten H IX .
Immediately after the end of the war, the victorious powers took up these concepts and the De Havilland DH.108 Swallow , the Northrop X-4 , the Chance Vought F7U Cutlass were created. The Northrop YB-35 and its further development Northrop YB-49 only flew after the end of the war. The AW52 test aircraft was built at Armstrong Whitworth . Nevertheless, the pure flying wing was never really ready for series production until the introduction of the Northrop B-2 .
After the Second World War, there were also two manufacturers of flying wing gliders in board design, namely the company Survol from Charles Fauvel and the "Pioneer series" from the American Jim Marske or today (2017) Kollman, which were supplied as a construction kit Composites. However, the flying wing concept was only able to fully establish itself with the weight-controlled hang-gliders.
There are three basic construction approaches:
1. Strongly backward swept flying wings
One of the golden rules of aerodynamics says that for a stable flight the part of the total horizontal surfaces flying ahead must provide a higher specific lift than the part flying behind. In addition, the center of gravity must be in front of the aerodynamic neutral point of the aircraft for stable flight. In normal aircraft, but also in tandem or duck aircraft, this is achieved by a higher angle of attack of the surface ahead. With the swept flying wing, this effect is achieved by twisting the wing, which allows the outer wing to fly with a smaller angle of attack. The backward-swept flying wings can be divided into three sub-groups.
- " Pure" flying wings without vertical stabilization surfaces and a more or less strong sweep and twist of the wing.
Nurflügel by the Brothers Horten have no vertical surfaces and produce the stability to high and transverse axis by a bell-shaped distribution of the buoyancy force and a strong sweepback. The bell buoyancy distribution is intended to compensate for the negative turning moment . In the literature in the 1980s and 1990s, however, the view that favorable flight characteristics can only be achieved in tailless aircraft by means of the bell-shaped lift distribution was considered a myth. On the one hand, all hoarding flying wings were still subject to a negative turning moment and, according to the school opinion, the bell distribution has the major disadvantage that it would cause considerable additional induced drag. However, it should be noted here that the zero lift required for optimal elliptical lift distribution on the wing tip can only be achieved over the entire working range if this is dimensionless, i.e. the wing depth is zero (e.g. Do 228 ) or the wing tip always flies with 0 ° Angle of attack ( Aero-Isoclinic-Wing ). In addition, the sweep effect and winglets even increase the lift load on the outer wing. In this respect, Ludwig Prandtl published a further assessment a few years after his proclamation on the optimal elliptical lift distribution (1920), which said that an optimal lift distribution with zero lift at the wing tip is only possible through a bell-shaped distribution of lift. The since 2015 ongoing project Prandtl-D ( Preliminary Research Aerodynamic Design to Lower drag , German about: preliminary investigation for aerodynamic design to reduce air resistance ) of the NASA tries to show that a complete compensation of the adverse yaw by appropriate twisting and shaping of the wingtips and the Elevons is possible after all. Geoffrey TR Hill took a special route in which the wing tips were attached so that they could be completely rotated, like a pendulum elevator. Flying wings based on the Northrop concept do not have a bell-shaped but an optimal elliptical lift distribution and work with expansion flaps at the wing tips that increase resistance on one side to compensate for the negative turning moment or to achieve a rudder effect. However, due to the great depth of the arch and the lack of elevons on the outer wing of the XB 35 and YB 49 (where the spreading rudders were attached), the lift distribution could never have been elliptical.
- Swept flying wings with elliptical lift distribution and additional vertical stabilization surfaces or winglets on the wing tips (e.g. Armstrong Whitworth AW52 or Akaflieg Braunschweig SB 13 ).
Here a rudder effect is achieved by folding the wing tips into the vertical, so to speak. Here, too (depending on the arrow), both normal flaps and resistance-increasing expansion flaps can be attached. If the winglets are viewed as an extension of the span that is folded into a vertical position, a zero lift can be generated here regardless of the angle of attack, which counteracts the formation of tip vortices. On the other hand, there is even increased (induced) lift in the transition area between wing and winglet, which in turn contradicts Prandtl's theory of elliptical lift distribution with zero lift at the end of the wing ...
- Creation of vertical stabilization surface through spatial geometry such as wing tips angled downwards (e.g. Dunne D6, Weltensegler , DFS 39 , DFS 40 or Boeing Bird of Prey ).
With this variant of the swept flying wing, the aileron deflections of the elevons attached to the downward-hanging wing tips also act like a meaningful rudder deflection. In German-speaking countries, the term " Weltensegler-Knick " or, more recently, "MV concept" has become common for this concept .
2. Minimally swept back flying wings, high stretched delta wings as well as unearthed and minimally swept forward flying wings
The Lippisch Delta III , Mihail Stabiloplan IV or Tscheranowski BITsch 8 constructions could not achieve their stability around the transverse axis due to the low lever arm of the outer wing in relation to the center of gravity by twisting the wing, but required an inherently stable wing profile. In principle, they belong to the board-only wings. However, even a minimal backward sweep improves the effectiveness of elevons .
Unswept flying wings are wings with a straight t / 4 line (25% of the profile depth) (e.g. Fauvel AV.36 ). If, on the other hand, the leading edge of the wing is straight, the t / 4 line is swept slightly forward (e.g. Marske Pioneer II ). The board flying wing creates its longitudinal stability through special, stable flying wing profiles ( S-flapping profile ), which generate a (positive) torque around the transverse axis when generating lift, which corresponds to the nod created by the center of gravity and the (negative) profile moment of the normal profiles counteracts the aircraft nose, which in conventional aircraft must be compensated for by the horizontal stabilizer . The stability around the yaw axis is usually ensured by a vertical stabilizer on a more or less rudimentary fuselage or on the wing trailing edges (e.g. Fauvel AV.22 or AV.36). The stability around the longitudinal axis, on the other hand, can be achieved by a V-shape, just like with a normal aircraft. The light aircraft “ Facet Opal ” by Scott Winton, the Noin “Choucas” and the latest development “ Pioneer IV ” by Jim Marske are considered to be the highlights of this concept .
3. Forward-swept flying wings
This concept has so far only been used very rarely, as it harbors a number of additional problems (the forward-swept wing is unstable around the vertical axis ) and in any case requires an oversized rudder unit (e.g. Cornelius XFG-1 ). In addition, the wing must be designed to be extremely torsionally rigid in order to ensure a stable flight attitude. Aeroelastic deformations at the wingtips are unacceptable, especially with the forward swept flying wing.
Compared to conventional aircraft - in which only the wings and not the entire aircraft body provide lift - flying wings are characterized by their shape-specific maximized lift properties. Because of this, the power of the engines is used more economically. Since, in contrast to classic aircraft, the payload, which is subject to gravity, is not separated from the lift-generating wings, lower structural forces arise. Thus, the aircraft can be constructed more easily. The pressure point-fixed or inherently stable wing profiles required for the construction of flying wings correspond roughly to the profiles that are optimized for high-speed flight in the subsonic area and are at least equal to these in the area between best gliding and high-speed flight.
In the case of military aircraft, the lower radar profile is also decisive.
With a stably designed flying wing, regardless of whether it is swept back or not swept back, there is another advantage - if elevons are used:
When initiating a curve (as with normal aircraft), an aileron deflection is first necessary in order to adopt the desired bank angle. Then the extra lift required for turning must be generated by an elevator deflection. In addition, the negative turning moment and the increased resistance of the (faster) wing on the outside of the curve must be compensated for by an appropriate rudder deflection.
With correctly designed elevons, the situation arises that ideally the rudder on the outside of the curve remains in the zero position, whereas the rudder on the inside of the curve deflects very strongly upwards in order to generate the desired moments around the longitudinal and transverse axis. This corresponds to a very differentiated aileron deflection on a normal aircraft - a trick that is normally used as a constructive countermeasure against the negative turning moment . This effect can be intensified if the elevons are subdivided and the deflections are superimposed differently. Thus, on a rudder deflection u. U. can be completely dispensed with. In addition, the wing on the inside of the curve has the advantage of being largely relieved of the generation of lift ( flap deflection ). This in turn makes overtaking in turns almost impossible. The situation is similar in straight flight, where both elevons are deflected upwards for slow flight. Vertical surfaces therefore serve more to dampen yaw than to stabilize around the vertical axis.
The disadvantages of a flying wing are the lower lift capacity in slow flight, due to the elevators attached to the trailing edge of the wing, which noticeably "de-arch" the profile with increasing angle of attack. For this reason, the use of conventional high-lift aids (e.g. flaps , Fowler flaps ) is only possible to a very limited extent with the arrow flying wing. A torque-free Flap, the least extended over about 60% semi-span, was first successfully implemented, however, in 1989 the ultralight glider "Flair 30" by Günther Rochelt and also comes in light sailors aériane swift ( S wept W ing with I nboard F lap T rim ) is used.
With board flying wings, lift-increasing flaps are even completely impossible because of their elevator effect. In addition, an optimal elliptical lift distribution is practically only possible for one airspeed, unless elevons or trim tabs ( Flettner rudder ) are used along the entire trailing edge of the wing or it is worked with a shift in the center of gravity.
Furthermore, flight stability is a far more difficult problem to solve in the design of tailless aircraft than in conventional designs. The necessary aerodynamic stabilization around the transverse axis cannot be achieved by the horizontal stabilizer on the long lever arm, but has to be provided by the wing itself. This requires either a special wing geometry, or a specific twist , and / or a torque-free profiling, which under certain circumstances causes a higher air resistance of the wing in large parts of the speed range. On the other hand, the lack of a fuselage and the lack of a horizontal stabilizer reduce the aerodynamic drag, so that for certain requirements (e.g. cruising at high subsonic speeds with a low lift coefficient) a flying wing aircraft can also be aerodynamically significantly more favorable.
In pure flying wings, the center of gravity is often only just before the neutral point in order to keep the losses due to elevator deflections as low as possible. There is thus the risk that the flying wing, for example in the event of a stall , will not stabilize itself by tilting the fuselage nose caused by the distribution of its own weight, but will “rock up” and become uncontrollable. This effect can be intensified if the pilot tries to counteract stalling. If the flying wing is controlled by external elevons, the result is a sudden increase in the lift load on the outer wing due to the pressing down, which even favors the stall. On the other hand, correctly designed tailless with a safe center of gravity, a very good-natured stall behavior is attested (e.g. Me 163 or Horten H III). This is also one of the reasons why flying wing designs are very popular with model pilots.
With a sufficient center of gravity, flying wings are very safe to fly; strictly speaking, the flying wing problem does not lie in the inadequate flight stability, but in the low damping around the vertical and transverse axis, which is normally caused by a tail unit on a long lever arm. The NASA project Prandtl-D-Wing from 2015 shows that with the correct lift distribution (similar to hoarding ) the advantages of a flying wing can outweigh it, even if it is designed to be “stable”, i.e. digital flight stabilization can be dispensed with.
For a flying wing that is to fly at great altitude, there is also the problem of pressurized ventilation of the aircraft cabin. In conventional aircraft with an approximately circular cross-section, the structure is loaded almost evenly. However, the aircraft cabin of a flying wing differs greatly from the ideal shape of the circle (it resembles an elongated ellipse or a rectangle). The resulting higher structural load must be stiffened by additional material and thus compensated for the higher weight.
Opportunities for improvement
In their book, Nickel / Wohlfahrt explicitly refer to the “golden rule of lift distribution”. This reads: "What is locally lacking in the angle of attack can be compensated for by a greater surface depth in this area". They use an example to illustrate this: on a Fauvel AV.36, a deepening of the inner wing in the elevator area would considerably reduce the deformation of the lift distribution caused by a pulled elevator. As a result, since then, in many model constructions, sudden increases in depth have been built into the rudder area, as can also be seen on the picture of the model wing. The designs by Jim Marske "Pioneer III and IV" also show such a depth jump in the elevator area. Theoretically, a design is even possible in which the lift distribution is no longer deformed at all (see also Putilow Stal-5 and Senkow BP-1 )
With swept flying wings, a problem occurs that the Horten brothers called the “center effect ” (better known today as the swept effect). Because the flow flows away in the direction of the span when the wing is swept backwards, the center wing does not provide the same lift coefficient as the rest of the wing. After the Horten brothers had recognized this, they compensated for this with a significantly deepened central wing with simultaneous deburring of the so-called t / 4 line, which led to the characteristic "hydrangea tail", as it is e.g. B. in the H IV or even more pronounced in the Ho 229 can be seen. A positive side effect of this design was a significantly increased usable volume of the center wing, which from an aerodynamic point of view is the only similarity to the B-2 bomber .
Flying wings used for military purposes like the B-2 completely dispense with vertical control surfaces such as rudder units or winglets in order to minimize radar reflections by avoiding right angles . The stabilization of these aircraft then takes place in addition to the already existing stability around the vertical axis due to the wing sweep, through flap systems that increase the resistance on one side at the wing tips (horizontal spreading flaps or spoilers) and a digital flight control consisting of a large number of flaps along the entire trailing edge of the wing as well as one large number of attitude sensors. However, the machine is designed to be aerodynamically unstable, which means that it cannot fly if the computer or sensor fails. The crash of the B-2 "Spirit of Kansas" is due to a sensor failure.
- With a few exceptions, hang-gliders are designed with flying wings. The absence of a tail protruding backwards allows them to start on foot from a steep slope. It is controlled by shifting your weight, partly supported by a roll spoiler.
- Paragliders create their stability around the transverse axis through an extremely low center of gravity. A tailless design is inherent in the system here.
Compared to conventional aircraft with tail units, there are only a few types of pure flying wings with a large payload:
- the aircraft of the Horten brothers before, during and after the Second World War, especially Horten H IX (Horten Ho 229 / Gotha Go 229)
- the SB 13 glider of the Braunschweig Academic Aviation Group
- the ultralight glider (glider) Swift
- the Northrop N-1M 1940
- the Northrop N-9M from 1942 (propeller engines)
- the Northrop YB-35 from 1946 (propeller engines)
- the Northrop YB-49 from 1947 (jet engines)
- the Armstrong Whitworth AW52 (jet bomber)
- the strategic stealth bomber Northrop B-2 of the United States Air Force from 1982
- the Lockheed Martin RQ-170 reconnaissance drone , developed since 2004, presumably in use since 2007
- the unmanned, solar-powered long-haul aircraft: Pathfinder , Centurion and Helios .
- Rudolf Storck among others: Flying Wings. The historical development of the world's tailless and flying wing aircraft. Bernard and Graefe, Bonn 2003, ISBN 3-7637-6242-6 , extensive type documentation with drawings and photos.
- Karl Nickel, Michael Wohlfahrt: Tailless Airplanes. Their design and properties. Birkhäuser Verlag, Basel u. a. 1990, ISBN 3-7643-2502-X ( Flugtechnische Reihe 3). Comprehensive textbook covering all types of flying wings.
- Reimar Horten , Peter F. Selinger: Nurflügel, The history of the Horten aircraft 1933-1960. 5th unchanged edition. H. Weishaupt Verlag, Graz 1993, ISBN 3-900310-09-2 . - History of their development, richly illustrated, numerous type sketches.
- Diploma thesis on the topic (PDF; 7.6 MB).
- Robert weld metal: wing tips. Self-published by Robert Schweissgut, Weissenbach / Tirol 2004. - Numerous flying wing models (hoarding, arrow and board concepts) are presented here.
- Ludwig Prandtl: About hydrofoil smallest induced drag. Zeitschrift für Flugtechnik und Motorluftschiffahrt, 1933, pp. 305–306
- Uwe W. Jack: hoarding flying wing jets. High tech in World War II. PPVMEDIEN, Bergkirchen 2015. ISBN 978-3-95512-084-9 .
- K. Nickel, M. Wohlfahrt: Tail-less aircraft. Pp. 1, 4, 6.
- Rudolf Storck: Flying Wings. P. 14.
- Philippe Ballarini, "Alphonse Pénaud (1850–1880) - Brillant et tragique", Aerostories, no 12, octobre, novembre, decemebre 2002
- Storck: Flying Wings. P. 372.
- K. Nickel, M. Wohlfahrt: Tail-less aircraft. P. 553.
- K. Nickel, M. Wohlfahrt: Tail-less aircraft. P. 554.
- PRANDTL-D No. 3 Takes Flight , NASA project to compensate for the negative turning moment
- Anatol Johansen: X-48 B: Boeing makes Hitler's dream of a flying wing come true. Welt Online, October 15, 2010, accessed May 6, 2017 .