Editing Flying wing (section)

==Design==

===Overview===
[[File:Northrop N-1M Udvar-Hazy.jpg|thumb|The [[Northrop N-1M]] on display at the [[National Air and Space Museum]]'s [[Steven F. Udvar-Hazy Center]] ]]

A flying wing is an [[airplane|aeroplane]] that has no definite [[fuselage]] or [[tailplane]], with its crew, payload, fuel, and equipment housed inside the main wing structure. A flying wing may have various small protuberances such as pods, [[nacelle]]s, blisters, booms, or [[vertical stabilizer]]s.
<ref name="Crane">Crane, Dale: ''Dictionary of Aeronautical Terms'', third edition, p. 224. Aviation Supplies & Academics, 1997. {{ISBN|1-56027-287-2}}.</ref>

A clean flying wing is sometimes presented as theoretically the most [[aerodynamics|aerodynamically]] efficient (lowest drag) design configuration for a fixed wing aircraft. It also would offer high structural efficiency for a given wing depth, leading to light weight and high [[fuel efficiency]].<ref>{{Cite journal |last=Weyl |first=A.R. |date=1945-03-01 |title=Stability of Tailless Aeroplanes |url=http://dx.doi.org/10.1108/eb031228 |journal=Aircraft Engineering and Aerospace Technology |volume=17 |issue=3 |pages=73–81 |doi=10.1108/eb031228 |issn=0002-2667|url-access=subscription }}</ref>

Because it lacks conventional stabilizing surfaces and the associated control surfaces, in its purest form the flying wing suffers from the inherent disadvantages of being unstable and difficult to control. These compromises are difficult to reconcile, and efforts to do so can reduce or even negate the expected advantages of the flying wing design, such as reductions in weight and [[Drag (physics)|drag]]. Moreover, solutions may produce a final design that is still too unsafe for certain uses, such as commercial aviation.

Further difficulties arise from the problem of fitting the pilot, engines, flight equipment, and payload all within the depth of the wing section. Other known problems with the flying wing design relate to [[Flight dynamics|pitch]] and [[Flight dynamics|yaw]]. Pitch issues are discussed in the article on [[tailless aircraft]]. The problems of yaw are discussed below.

===Engineering design===
A wing that is made deep enough to contain the pilot, engines, fuel, undercarriage and other necessary equipment will have an increased frontal area, when compared with a conventional wing and long-thin fuselage. This can actually result in higher drag and thus lower efficiency than a conventional design. Typically the solution adopted in this case is to keep the wing reasonably thin, and the aircraft is then fitted with an assortment of blisters, pods, nacelles, fins, and so forth to accommodate all the needs of a practical aircraft.

The problem becomes more acute at supersonic speeds, where the drag of a thick wing rises sharply and it is essential for the wing to be made thin. No supersonic flying wing has ever been built.

===Directional stability===
For any aircraft to fly without constant correction it must have [[directional stability]] in yaw.

Flying wings lack anywhere to attach an efficient vertical stabilizer or fin. Any fin must attach directly on to the rear part of the wing, giving a small moment arm from the aerodynamic centre, which in turn means that the fin is inefficient and to be effective the fin area must be large. Such a large fin has weight and drag penalties, and can negate the advantages of the flying wing. The problem can be minimized by increasing the wing sweepback and placing twin fins outboard near the tips, as for example in a low-aspect-ratio [[delta wing]], but given the corresponding reduction in efficiency many flying wings have gentler sweepback and consequently have, at best, marginal stability.

The aspect ratio of a swept wing as seen in the direction of the airflow depends on the yaw angle relative to the airflow. Yaw increases the aspect ratio of the leading wing and reduces that of the trailing one. With sufficient sweep-back, differential induced drag resulting from the tip vortices and crossflow is sufficient to naturally re-align the aircraft.

A complementary approach uses twist or wash-out, reducing the angle of attack towards the wing tips, together with a swept-back wing planform. The [[Dunne D.5]] incorporated this principle and its designer [[J. W. Dunne]] published it in 1913.<ref name="dunne">Dunne, J.W.; "The Theory of the Dunne Aeroplane", ''The Aeronautical Journal'', April 1913, pp.83-102. Reprinted in ''Flight'', 16 Aug to 13 Sept 1913.</ref> The wash-out reduces lift at the tips to create a bell-shaped distribution curve across the span, described by [[Ludwig Prandtl]] in 1933, and this can be used to optimise weight and drag for a given amount of lift.

Another solution is to angle or crank the wing tip sections downward with significant [[Dihedral (aircraft)#Anhedral|anhedral]], increasing the area at the rear of the aircraft when viewed from the side. When combined with sweepback and washout, it can resolve another problem. With a conventional elliptical lift distribution the downgoing elevon causes increased induced drag that causes the aircraft to yaw out of the turn ("adverse yaw"). Washout angles the net aerodynamic vector (lift plus drag) forwards as the angle of attack reduces and, in the extreme, this can create a net forward thrust. The restoration of outer lift by the elevon creates a slight induced thrust for the rear (outer) section of the wing during the turn. This vector essentially pulls the trailing wing forward to cause "proverse yaw", creating a naturally coordinated turn. In his 1913 lecture to the Aeronautical Society of Great Britain, Dunne described the effect as "tangential gain".<ref name="dunne"/> The existence of proverse yaw was not proved until NASA flew its [[Prandtl-D]] tailless demonstrator.<ref>{{cite journal |last1=Bowers |first1=Albion, H |title=On Wings of the Minimum Induced Drag: Spanload Implications for Aircraft and Birds |journal=NASA STI Programme |date=1 March 2016 |pages=11–12 |url=https://ntrs.nasa.gov/citations/20160003578 |access-date=4 August 2021}}</ref>

===Yaw control===
In some flying wing designs, any stabilizing fins and associated control rudders would be too far forward to have much effect, thus alternative means for [[Yaw (rotation)|yaw]] control are sometimes provided.

One solution to the control problem is differential drag: the drag near one wing tip is artificially increased, causing the aircraft to yaw in the direction of that wing. Typical methods include:
* [[Deceleron|Split ailerons]]. The top surface moves up while the lower surface moves down. Splitting the aileron on one side induces yaw by creating a differential air brake effect.
* [[Spoiler (aeronautics)|Spoilers]]. A spoiler surface in the upper wing skin is raised, to disrupt the airflow and increase drag. This effect is generally accompanied by a loss of lift, which must be compensated for either by the pilot or by design features that automatically compensate.
* [[Spoileron]]s. An upper surface spoiler that also acts to reduce lift (equivalent to deflecting an aileron upwards), so causing the aircraft to bank in the direction of the turn—the angle of roll causes the wing lift to act in the direction of turn, reducing the amount of drag required to turn the aircraft's longitudinal axis.

A consequence of the differential drag method is that if the aircraft maneuvers frequently then it will frequently create drag. So flying wings are at their best when cruising in still air: in turbulent air or when changing course, the aircraft may be less efficient than a conventional design.

===Related designs===
Some related aircraft that are not strictly flying wings have been described as such.

Some types, such as the [[Northrop X-216H|Northrop Flying Wing (NX-216H)]], still have a tail stabilizer mounted on tail booms, although they lack a fuselage.

Many hang gliders and microlight aircraft are tailless. Although sometimes referred to as flying wings, these types carry the pilot (and engine where fitted) below the wing structure rather than inside it, and so are not true flying wings.

An aircraft of sharply swept delta planform and deep centre section represents a borderline case between flying wing, [[blended wing body]], and/or [[lifting body]] configurations.