In designing a high-speed wing, you need to consider first the requirement for economical high-speed performance in the cruise configuration. However, you also have to consider the restraints on the design of the need to keep airfield performance within acceptable limits and the need to give the structural people a reasonable task. There are several interactive design areas of a wing. Some are purely for lift, some are a compromise between lift and speed, and some are purely for speed. For the high-speed requirements of a wing, the design would focus on sweep, thickness, and chamber. The degree of sweep, thickness, and chamber used for the final highspeed wing design depends on their many interactive compromises that culminate in directly fulfilling the wing’s high-speed requirement and inversely the wing’s lift and structural requirements.
The swept-wing design increases its Mcrit speed because it is sensitive to the (airflow) airspeed vector normal to the leading edge for a given aircraft Mach number. A swept wing makes the velocity vector normal (perpendicular) to the leading edge a shorter distance than the chordwise resultant . Since the wing is responsive only to the velocity vector normal to the leading edge, the effective chordwise velocity is reduced (in effect, the wing is persuaded to believe that it is flying slower than it actually is). This means that the airspeed can be increased before the effective chordwise component becomes sonic, and thus the critical number is raised.
To optimize the lift design on a swept wing, you would need to examine and develop the lift design areas of the clean wing and add high lift devices to the clean wing to a degree that satisfies our main lift concern, that of adequate airfield performance.
What advantages does a jet aircraft gain from a swept wing?
The advantages a jet aircraft gains from the swept wing are (1) high Mach cruise speeds and (2) stability in turbulence.
1. High Mach cruise speeds. The swept wing is designed to enable the aircraft to maximize the high Mach speeds its jet engines can produce. The straight-winged aircraft experiences sonic disturbed airflow, resulting in a loss of lift at relatively low speeds. Therefore, a different wing design was required for the aircraft to be able achieve higher cruise speeds. The swept-wing design delays the airflow over the wing from going supersonic and, as such, allows the aircraft to maximize the jet engine’s potential for higher Mach cruise speeds. Additionally, the swept wing is also designed with a minimal chamber and thickness, thereby reducing profile drag, which further increases the wing’s ability for higher speeds.
2. Stability in turbulence. Ironically, a disadvantage of the swept wing is its poor lift qualities, which lends itself to an advantage in that it is more stable in turbulence compared with a straight-winged aircraft. This is so because the swept wing produces less lift and therefore is less responsive to updraughts, which allows for a smoother, more stable ride in gusty conditions.
What disadvantages does a jet aircraft suffer from a swept wing?
Because the swept wing is designed for high cruise speeds, it suffers from the following disadvantages as a consequence:
1. Poor lift qualities are experienced because the sweep-back design has the effect of reducing the lift capabilities of the wing.
2. Higher stall speeds are a consequence of the poor lift qualities of a swept wing.
3. Speed instability is the second consequence of poor lift at lower speeds for the swept-wing aircraft. Speed is unstable below minimum drag speed (VIMD) because the aircraft is now sliding up the back end of the jet drag curve, where power required increases with reducing speed. This means that despite the higher coefficient of lift (CL) associated with lower speeds, the drag penalty increases faster than the lift; therefore, the lift-drag ratio degrades, and the net result is a tendency to progressively lose speed. Thus speed is unstable because of the drag penalties particular to the swept wing.
4. A wing-tip stalling tendency is particular to a swept-wing aircraft mainly because of the high local CL loading it experiences. Uncorrected (in the design), this effect would make the aircraft longitudinally unstable, which is a major disadvantage.
Where does a swept wing stall first, and what effect does this have on the aircraft’s attitude?
A simple swept and/or tapered wing will stall at the wing tip first if not induced/controlled to stall at another wing section first by the designer. This is so because the outer wing section experiences a higher aerodynamic loading due to the wing taper, which causes a greater angle of incidence to be experienced to a degree where the airflow stalls at the wing tips. The boundary layer span-wise airflow, also a result of sweep, further contributes to the airflow stalling at the wing tips. A stall at the wing tip causes a loss of lift outboard and therefore aft (due to the wing sweep), which moves the center of pressure inboard and therefore forward; this produces a pitch-up tendency that continues as the wing stalls progressively further inboard. A wing-tip stall is resolved in the wing design with the following better aerodynamic stalling characteristics:
1. Greater chamber at the tip; this increases airflow speed over the surface, which delays the stall.
2. Washout or twist, which creates a lower angle of incidence at the wing tips and delays the effect of the outboard wing loading that causes the stall.