Swept wing

Describes the deviation of a wing sweep angle in degrees from the vertical plane of the axis in the plan view. One differentiates the leading edge sweep, and the sweep of the Hinterkantenpfeilung at t / 4 (25 % of the wing chord t).

The idea for Tragflächenpfeilung in connection with the supersonic flight was in 1935 by Adolf Busemann from.

• 2.1 Examples

Positive sweep

With infinite extension

In the idealized case of a swept wing of infinite aspect ratio the leading edge, the trailing edge and the sweep of t/4-Linie is the same. There is thus only a sweep angle, and the wing has furthermore no escalation (). The flow velocity can then be divided into a component perpendicular (normal) to the wing and a component tangential to it ( Fig. 1). It may then, for example, using the Euler equations to show that only the vertical component of the velocity is effective for the flow around the blade. The tangential component has no influence. Decoupling of the two directions allows the treatment of the ( infinite) swept wing as a 2D problem, which is a considerable simplification of the problem. Obtaining a two-dimensional (but not flat ) the flow field, which ( Figure 2 ) is apparent in the top view of the curved parallel but shifted current lines. The flow parameters are dependent in this case, only two variables (e.g. X and Y), but all the components of the velocity vector have non-zero values ​​. Figure 3 shows that the pressure distribution of a pure 2D bill in the normal section (nearly) corresponds exactly with the ( transformed ) of the pressure distribution profile section.

The reduction of the effective flow velocity results in a reduction of the buoyancy, the Auftriebsgradienten and the ( pressure ) resistance. Since the corresponding conversion formulas (in fact the sliding angle) containing the cosine of the sweep angle, these effects are also referred to as cosine effects beta. When swept wing also the characteristic impedance decreases ( pressure resistance ) from more than the lift and so increases in transonic flow onto the glide ratio of the wing. The critical Mach number and the Mach number of the resistance increase also increase.

The curvature of the streamlines of the boundary layer at the edge resulting in three-dimensional velocity profile in the boundary layer. This necessarily have a turning point, which makes the boundary layer smoothly unstable. Occur here on cross- flow vortices, leading to a cross- flow instability. Therefore usually occurs on swept wings on the transition from the laminar to the turbulent state of the boundary layer over the fanning the lateral flow instabilities and not over the fanning of the two-dimensional Tollmien -Schlichting waves. Due to this additional instability to the laminar-turbulent transition near the wing leading edge takes place. Hydrofoil usual sweep flow around almost vollturbulent.

Finite extension

In the finite wing the sweep leads to a change of the lift distribution.

• Positive sweep (> 0 ) leads to a ca- cant in the outdoor area and a reduction in the wing root.
• Negative sweep (<0 ) leads to a ca- rise at the wing root and a reduction in the outdoor area.

This deformation of the lift distribution leads to an increase in induced drag, which must be prevented by appropriate skewing and depth distribution.

When positive swept wing, it also comes in a deterioration of Abreißverhaltens because that is achieved CAMAX here first at the wing tip and the stall there ( both in the area of the ailerons as well as the " back " part of the wing ) occurs first. Another negative effect is the outflow of boundary layer material towards the wing tip, which leads there to a Grenzschichtaufdickung and a correspondingly greater detachment slope. Suitable counter-measures are here the use of boundary layer fences, saw teeth of the leading edge (see phantom II F-4 ), the twist of the blade and the adjustment of the profile. A positive swept wings also lead to increased stability and direction to a positive slide -roll moment.

The three-dimensionality of the finite wing leads to a local Entpfeilung the isobars at the wing root and near the edge of the sheet. The isobars must be for reasons of symmetry, for example, at the wing root perpendicular to the plane of symmetry. In order for a real wing loses the benefits of the sweep in these areas. To compensate for this drawback, the concept of attempting " straight isobars " implement, in which the profile shape is adapted locally in these areas so as that over the entire range even Isobarenverlauf is achieved. Another effect of the swept wing is the lower gusts sensitivity. This results from the reduced buoyancy increase, which is directly proportional to the gust load factor.

The sweep does not need to run constantly at a piano. Either the individual wing sections are swept differently, or the wing can be tilted ( tiltrotor ).

Negative sweep

The sweep is usually positive (both edges of the wings are pulled backward ), but there is since the beginning of the practical use of sweepback also constructions with negative sweep. As shown in the picture, the air flow running at this geometry wings from the fuselage rather than away from the body, as in conventional designs. This allows the air flow to wing tips and underlying control surfaces may be much slower before the laminar flow breaks off ( stall, Eng. Stall ) and thus the buoyancy is lost. As a result, an extraordinary maneuverability can be achieved when the carrier and the control surfaces are made ​​in a much steeper angle to the air flow. The aircraft has enough airflow over the control surfaces of lateral and vertical rudder as much lower airspeed. Thus, the use of this wing geometry at extremely maneuverable interceptors declared.

Although it has been quite some research during World War II on aircraft with negative Tragflächenpfeilung, it was hardly possible to derive the material loads safely at high speeds. In recent times there by fiber composites (including composite materials called ) ( inter alia carbon fiber reinforced plastic), the technical requirements, wings to construct negative sweep, the torsional and shear forces can also withstand high, which allows an application of the negative sweep in the high performance glider. Slow-flying gliders, predominantly two-seater, however, are for many decades with this wing geometry in use. That is because that the wing root, that is, the connection to the fuselage is put backwards so that the second seat will fit it.

Examples

• Front and rear edges of the wings are swept negative: Grumman X -29 (Experimental Aircraft )
• HFB 320 (Civil machine )
• Junkers Ju 287 (prototype)
• Schleicher ASK 13 ( Glider )
• Schleicher K 7 ( glider)
• Sukhoi Su -47 (Experimental Aircraft )
• FTAG Esslingen E11 ( two-seater glider for the study of extreme Vorpfeilung )

Application

The extent of the tapering of the wings depends on the expected air flow velocity around the foils. Here is a compromise between a high lift at low speeds for the start (low sweep ) compared to the low flow resistance and low turbulence at cruising speed must ( high sweep ) are found to be with the aim of a laminar flow of air over all control surfaces in all anticipated flight attitudes reach. If one draws the air pressure and each corresponding velocities in a coordinate system, the result is within the lines of an imaginary area in which the aircraft can be safely used. This envelope, called the flight envelope is different for each aircraft model and depends on many factors, off to a large extent on the wing geometry and thus the sweep.

As a simplified correlations the following basic configurations shall be given. Airplanes for which the vast fields of application are at low altitude and at rather low speeds should be equipped with no sweep. Transport aircraft that are at high altitudes quickly (ie transonic ) flight, but close to sea level tend to be in the medium speed range, receive an average sweep.

The Concorde, the only high-altitude very fast ( Mach 2, ie supersonic ) flew, had a high sweep.