What is the reason for the poor low-speed characteristics of sweptback wings?

Question

I know there are advantages of swept-back wings delaying shock-wave allowing a aircraft to fly faster. However, what are the disadvantages. I know one of them is that they have very poor low speed characteristics, however I do not know the reason. Another disadvantage will be on the F-100 Super Sabre Dance, do not understand these either.

Can any kind soul explain the disadvantages?

Answer 1

Disadvantages of wing sweep

• Lift curve slope is reduced by the cosine of the quarter chord sweep angle. This means more angle of attack for takeoff, which requires a longer take-off run and a longer landing gear to avoid a tail strike on rotation.
• If you rotate the airplane, the tips of backward swept wings come down when the aircraft rotates for take-off. This also might drive the requirement for a longer landing gear.
• Bending moments in the wing will become torsion moments when you change the sweep angle. And you will need to change it at least in the wing root where the wing connects to the fuselage. This translates into a heavier structure.
• If your sweep angle and aspect ratio both are large enough, the wing will show nasty stall characteristics. The boundary layer is swept towards the tips and causes earlier separation when the wing stalls, and the airplane will pitch up or roll uncontrollably. Wing fences help, but cannot completely remedy this.
• Wing sweep causes a yaw-induced rolling moment, so less dihedral is needed. On high wing airplanes this requires to use anhedral. Unfortunately, this rolling moment varies with lift coefficient, so your yaw-induced rolling moment on a swept aircraft is lower than ideal at high speed and higher at low speed.
• For flying wings, sweep will let the aircraft center pitch up and down when the wing flexes. This creates a powerful interaction between the fast period mode (which is only moderately damped in flying wings) with the wing bending mode, resulting in flutter.

In short, when having a choice, the clever airplane designer avoids sweep whenever he/she can. But sweeping a wing back is still better than forward sweep.

Low Speed Characteristics

An airfoil initially accelerates the air which flows over its top surface and decelerates it again over its rear part. On swept wings, this acceleration-deceleration only affects the orthogonal speed component, so the speed component in span direction remains unaffected. This is the reason for the higher Mach capability of swept wings, but also causes the air to flow first inwards and then outwards while transversing the wing's upper surface.

On top, friction decelerates the air flowing around a body, such that a layer of decelerated air surrounds each surface of an airplane. The thickness of this boundary layer increases with flow length, and on a swept wing this friction will initially mostly affect the orthogonal flow component. At around mid chord you will find air which has been decelerated mostly in its orthogonal speed component (since this component was so high over the forward part) and now will be subject to more deceleration of the orthogonal component, so that only the spanwise component will be left over the rear part of the boundary layer. Now this boundary layer will only flow off in span direction, such that a massive increase of slow, low-energy air will be collecting towards the tips.

A thick boundary layer will cause early flow separation, so when the angle of attack is increased, the flow at the tips of a backward swept wing will separate first. This will cause lift loss, and since the tips are also the rearward part of the wing, will shift the aerodynamic center forward. This in turn will make the aircraft pitch up, which aggravates the stall condition. If the separation happens asymmetrically, the aircraft will roll in addition to pitching up. In case of the F-100, the tail surfaces were too small to stop the pitch-up, so once the aircraft crossed into the stall region, it would uncontrollably pitch up more and stall completely.

Modern swept-back aircraft have an angle-of-attack limiter which will prevent the aircraft from flying into the stall region. Also, wing fences help to keep the spanwise flow in check, and vortex generators help to energize the flow such that the early flow separation at the wing tips is sufficiently delayed to avoid the uncontrollable pitch-up. The F-100 lacked all those remedies.

Wing fences on a MiG-17 (picture source)

Vortex generators on a Boeing 737 wing (picture source)

Answer 2

Wing sweep improves the performance by delaying the shock waves and accompanying aerodynamic drag rise caused by fluid compressibility at high (near sonic) speeds. However, there are some disadvantages associated with the wing sweep like:

• Wing sweep reduces the slope of the lift curve and the maximum lift coefficient of the wing. This means that the swept wing aircraft must fly at a higher AOA to achieve maximum lift.

Source: code7700.com

• The wing sweep induces spanwise flow along the wing, which means that the air has to travel longer distance over the wing. This means a thicker boundary layer and more chances of flow separation.

Source: boldmethod.com

• The spanwise flow on swept wings increases the effective angle of attack of wing segments relative to its neighboring forward segment; the result being the rear segments of the wing stall first (tip stall for rearward swept wings and root stall for forward swept wings). If the resulting nose-up pressure (due to the fact that the lift vector moves forward) is not corrected, the aircraft pitches up further, extending the stalled region, resulting in a vicious cycle, resulting in complete stall of aircraft.

This is the reason for the 'Sabre dance' experienced in F-100 Super Sabre. This is further exacerbated by the already high angles of attack during takeoff and landing, as already noted.

Source: code7700.com

• Wing bending moments cause torsion in case of swept wings. This requires strengthening of wings, or a complicated design. This affects the forward swept wings to a larger degree than the rearward swept wings , and is one of the main reasons the forward swept wings are not used much.

• One important consideration due to wing sweep is the high speed aileron reversal. In case of streamwise angles of twist resulting from wing bending and aileron pitching moment add together, resulting a large amount of nose down wing twist, which reduces aileron effectiveness. Due to this swept back wings have low aileron reversal speeds (while swept forward wings have higher values). This problem is not experienced in wings with constant sweep.