What a Sweep! (009)
~High Speed Wing Sweep~
There are two main reasons to sweep wings on an aircraft: Structural integrity and drag. For airplanes in general, a minimal amount of wing sweep is desired to improve low speed handling and allow for slower takeoff and landing speeds. A lot of airplanes, even slower aircraft, do have at least a very small amount of sweep for structural reasons. This will be discussed in a future Plane Truths article.
The main reason for wing sweep on aircraft designed to fly in the transonic and supersonic regions is all about reducing drag. Sweeping a wing backwards makes the wing hit the air at an angle. This exposes less frontal area of the wing to the oncoming air, reducing pressure drag. However, the bigger gain arises if we break the airflow across the wing into two components, parallel and perpendicular to the wing's chord. We will see that the perpendicular airflow going along the chord is slower than the plane is truly flying. It's this chordwise component of the airstream that is responsible for lift production. Sweeping a wing increases the critical Mach number, the speed at which air along some part of the aircraft goes supersonic. This "slower" airflow component allows the plane to fly faster before encountering supersonic shock waves that drastically increase drag. See Plane Truths article "Super Critical Condition (005)" for more on critical Mach numbers.
As with most gains in aircraft design, there's also an associated loss. Since less air is flowing directly across the chord, less lift can be generated at a given speed. The stall speed increases with sweep meaning controllability of the plane at low speeds suffers and the plane must achieve higher speeds on takeoff and landing. A faster speed requires longer runways and better stopping ability. These fast aircraft often have complex lift enhancement devices like flaps and slats that can be extended to increase lift and drag at low speeds. Having complex lift enhancement systems adds weight and operational complexity to the airplane. In the design process of an aircraft, lighter and simpler systems are always preferred.