Editing Flying wing (section)

===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>