Why the Wing Sweep?
The wing sweep was designed in order to allow for a higher Mcrit (Critical Mach number) resolving the airflow component into two vectors, one parallel to the leading edge of the wing and one perpendicular. If you take a straight wing section and with a Mcrit of M.70 and sweep it back 30 degrees you can increase the Mcrit by about .108. Wings with a large degree of sweep which where thin and flexible became the aerodynamic answer.
Thin flexible wings produced a higher Mcrit however they also created a unique roll reversal issue. When banking right a wing twist affect from a single aileron design would produce a roll in the opposite direction.
The solution to this problem was resolved in the design of wing with two sets of ailerons. One set of inboard ailerons and one set of outboard ailerons. Inboard ailerons are used at both high and low airspeeds and strategically located in the region where very little wing twists exist. The outboard ailerons are used at low speeds and are located in a region where twist moment is limited and not produced. Differential spoilers where also designed to assist inboard ailerons at high airspeeds. The aerodynamic value of spoilers is realized in the fact that they do not produce a twist moment and can also be used symmetrically as speed brakes.
Not all airplanes utilize the multi aileron design. In the case of the B737 the wing is shortened, stiffened and created ridged enough in order to eliminate wing twist. Another solution to eliminate the twist effect is to design a wing, which utilizes spoilers as the main source of rolling the aircraft.
The issue with the thin wing design is the limitation they impose on an aircrafts ability to carry the maximum amount of fuel in order to be economical and efficient. The solution was found in being able to produce a thicker specifically at the root region with a double wing sweep like the B727.
Another solution was realized by designing a wing with the shape of it’s airfoil where the thickest part is farther aft thus reducing the amount of wing area which is affected by the flow separation at high Mach number.
Ultimately the aerodynamicist found that the real issue was not so much the supersonic flow as much as it was the location of the pressure wave being forward of the leading edge as a result of supersonic airflow. A wing was designed with a large radius on the leading edge followed by a relatively flat upper surface. By forcing the airflow go supersonic on the upper surface early followed by a flat surface the pressure wave disruption moves to the trailing edge.
The advantage of this new design is allows for a reduced wing sweep angle, which allows for a lighter wing. A thicker wing can also be produced which allows for more fuel storage.
