Flying wing

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A Northrop YB-49 flying wing.
A Northrop YB-49 flying wing.

Flying wing is the generic designation given for a fixed-wing aircraft configuration which is capable of stable, controllable flight without the aid of lifting surfaces other than the main wing itself, that is, without auxiliary surfaces such as "tails" and "canards". In it strictest sense, the Flying Wing also lacks a fuselage, or has only a rudimentary fuselage pod barely extending from the wing itself. In this layout, most of the payload is transported inside the main wing, the latter comprising most of its structural volume.

The less restrictive designation of "tailless aircraft" includes the flying wing aircraft, but also all aircraft without stabilizers or canards, but with a full-length payload bearing fuselage such as delta aircraft.

Historically, the flying wing has been defended by many as potentially the most efficient aircraft configuration from the point of view of aerodynamics and structural weight. Such a notion usually comes from the idea that the absence of any aircraft components other than the wing should naturally provide those benefits. On the other hand, the aircraft's wing should be able to provide flight stability and control "by itself", a requirement which in principle imposes additional constraints to the wing design problem. Therefore, the expected gains in weight and drag reduction may be partially or wholly negated due to design compromises needed to provide stability and control.

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[edit] Design issues

Although the discussion about flying wing aircraft usually surrounds the balance between aerodynamic/weight performance and stability/control requirements, those are by no means the only important design factors. Other issues such as (but not limited to) high-speed compressibility effects, useful payload volume, cabin pressurization, satisfaction of certification safety requirements and industrial/commercial risk must also come into play to determine whether or not the flying wing configuration is the best solution for a given aircraft mission.

One must distinguish between the differing types of flying wings and their use. The three general types are forward swept wings, straight wings and rear swept wings. Then one must define their use as to speed and wing loading.

A forward swept wing will not stall a wing tip and there is really no need for reflex as the elevator provides the reflex for whatever speed is necessary. A moving weight then comes into play in adjusting the amount of reflex which gives the best drag ratio, to give optimal flight characteristics making the flying wing far more efficient.

[edit] Stability and control

Flight stability can be loosely defined as the ability of an aircraft to recover its original flight state after suffering a transitory external force perturbation, such as imposed by wind gusts or arbitrary pilot control input. In recent years, automatic flight control systems have been used to achieve that goal with relatively few limitations (see relaxed stability), but at the expense of system sophistication, as in the case of most modern fighter aircraft. However, the "traditional" way of providing flight stability is to design the aircraft aerodynamic and mass distribution characteristics in such a way that it is inherently stable when in flight (see dihedral).

As a first requirement for that, the aircraft must be statically stable: whenever it is disturbed from its flight path, aerodynamic forces and moments must arise in directions that tend to bring the aircraft back to its original flight state. A classical simplified example is that of an arrow in flight. The arrow is built such that its center of gravity (CG) is positioned at a point which is ahead of the point where the resultant of aerodynamic forces is applied, its aerodynamic center (AC). Whenever the arrow is misaligned to the relative wind due to some disturbance, the increase in resultant aerodynamic force (lift) on the AC produces a moment around the CG which tends to make the arrow realign to the relative flow.

Like the arrow, any aircraft can be made statically stable, regardless of its external configuration, provided its aerodynamic center (AC) lies behind the center of gravity (CG). The main difference between the two scenarios is that, unlike the arrow, the aircraft must continuously generate a lift force that opposes its weight, making sustained flight possible. Therefore, an aircraft in flight has an ever present moment component around its CG, due to the lift force it generates at some point behind it. An opposing moment must be generated to achieve balance, in a process generically referred to as "trimming".

For conventional, "tailed" aircraft configurations, the trimming moment is provided by auxiliary lifting surfaces: horizontal and vertical ("fins") stabilizers, usually rear-mounted on relatively long fuselages. The forces produced by such surfaces can be varied either by adjusting their incidence or through the use control surfaces, in the form of movable trailing edge sections such as elevators and rudders. That arrangement provides a quite convenient way of producing trimming and control moments, since relatively small forces on the tail surfaces are required due to the fuselage "lever arm" available.

Flying wings must provide trimming and control moments without such a surface/fuselage set. That is by no means impossible, but the central question is whether it can be done while preserving an aerodynamic efficiency level high enough not to reduce or annul the gains expected from the absence of the fuselage.

There are two well known solutions for the longitudinal trimming of flying wings:

  1. Usage of wing section airfoil shapes which produce some amount of "nose-up" pitching moment. Usually called "reflex" airfoils, they generally rely on some level of reduced or even reversed aerodynamic loading (i.e. "lifting down") on the trailing edge region to produce the effect of a "built-in tail". Although there are viable designs which fully rely on this kind of solution, like the Fauvel and Marske Aircraft series of sailplanes, the use of such airfoils is generally regarded as not very efficient. The requirement to generate the favorable pitching moment causes impacts such as increased drag and reduced maximum lift, believed by many not to be an advantageous tradeoff for the overall design. On the other hand, there is probably no published work definitely proving those disadvantages based on actual facts and data, for a wide set of application scenarios.
  2. Adequate "tailoring" of wing planform and spanwise lift distribution shape to produce the required "nose-up" trimming moment. The most usual solution of this kind is to use a backward swept wing with appropriate chord and twist distributions to control the spanwise load. In simple terms, the wing tips may be thought to be working partially as horizontal stabilizers, displaced backwards from the center of gravity by the application of wing sweep. This kind of solution is frequently associated to the use of low or null pitching moment airfoils. Successful aircraft such as the Northrop flying wings and the Horten series of sailplanes and fighters have applied this kind of design solution in the past.

[edit] History

The Horten Ho 229, produced by Nazi Germany during World War II.
The Horten Ho 229, produced by Nazi Germany during World War II.
The US-produced B-2 Spirit, a strategic bomber capable of intercontinental missions.
The US-produced B-2 Spirit, a strategic bomber capable of intercontinental missions.

The flying wing configuration has been seriously considered by many aircraft designers since the early years of aviation, probably due to its natural appeal as a minimal design solution. Some examples of early flying wing aircraft are the Dunne series of biplane aircraft and Waldo Waterman's Whatsit.

The configuration was studied extensively in the 1930s and 1940s, when it was seen as a natural solution to the problem of building an airliner large enough to carry a reasonable passenger load and enough fuel to cross the Atlantic in regular service. The flying wing's potentially large internal volume and low drag made it "a natural" for this role, and was studied in depth by Jack Northrop and Cheston L. Eshelman in the United States, and Alexander Lippisch and the Horten brothers in Germany, where Hugo Junkers had in 1910 patented a wing-only air transport concept.

Junkers started work in 1919 on his "Giant" JG1 design, intended to seat passengers within thick wings, but in 1921 the Allied Aeronautical Commission of Control ordered the incomplete JG1 destroyed for exceeding post-war size limits on German aircraft. Junkers conceived futuristic flying wings for up to 1,000 passengers; the nearest this came to realization was in the 1931 Junkers G-38 34-seater Grossflugzeug airliner, which admittedly had a short fuselage ending in a double tail, but for the restconsisted mainly out of one large thick-cord wing which provided space for fuel, engines and even two passenger cabins (inside the leading edge of the inboard wing panels). The biggest land plane of its day, and nicknamed 'The Flying Hotel' one G-38 entered service with Lufthansa; It was not a comercial succces however. Only two passenger planes were built although Mitsubishi in Japan acquired the licencing rights and built several (Some sources state 6) G-38s, as bombers under the designation Ki-20 or Type 92. Of Junkers own G38's one was grounded, the other was put to military use as a heavy transport and destroyed by a British air raid in Athens in 1941.

Several late-war German military designs were based on the flying wing concept (or variations of it) as a proposed solution to extend the range of the otherwise very short-range jet engined aircraft. Most famous of these would be the Horten Ho 229 fighter. This aircraft, first flown in 1944, combined a flying wing, or Nurflügel, design with twin jet engines. The surviving prototype remains in storage with the Smithsonian Institute in an unrestored state.

After the war, a number of experimental designs were based on the flying wing concept, but a number of problems arose. Some general interest remained until the early 1950s, when the concept had been proposed as a design solution for long range bombers. Such trends culminated in the Northrop YB-35 and YB-49, which did not enter production. Those designs did not necessarily offer a great advantage in range and presented a number of technical problems, leading to the adoption of "conventional" solutions like the Convair B-36 and the Boeing B-52.

Interest in the flying wing configuration was renewed in the 1980s as a way to design aircraft with low radar reflection cross-sections. Stealth technology relies on shapes which only reflect radar waves in certain directions, thus making the aircraft hard to detect unless the radar receiver is at a specific position relative to the aircraft - a position that changes continuously as the aircraft moves. The tailplanes and engine intakes of a conventional jet, and especially its round fuselage, reflect radar in all directions, while the flat and nearly-horizontal surface of a flying wing only reflects radar in a couple of specific directions. In addition, if the edges of the wings are straight rather than curved, then they only reflect radar at angles perpendicular to these straight segments, rather than in all directions. This approach eventually led to the Northrop B-2 Spirit stealth bomber. In this case, the aerodynamic advantages of the flying wing are not the primary needs. However, modern computer-controlled fly-by-wire systems allowed for many of the aerodynamic drawbacks of the flying wing to be minimized, making for an efficient and stable long-range bomber.

Apparently, the flying wing concept still remains at its best in the slow-to-medium speed range, and there has been continual interest in using it as a tactical airlifter design. Boeing continues to work on paper projects for a Blended Wing Body Lockheed C-130 Hercules sized transport with better range and about 1/3rd more load, while maintaining the same size characteristics. A number of companies, including Boeing, McDonnell Douglas and de Havilland did considerable design work on flying-wing airliners, but to date none have entered production.

[edit] See also

[edit] External links

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