THE SWEPT WING
A STANDARD FOR TRANSPORT CATEGORY AIRCRAFT
MOST OF THE training aircraft you will fly use the rectangular or semi-tapered wing design. They provide stable and safe platforms for flying slow or even gliding when needed. One of the most prolific and utilized designs in Transport category aircraft, however, is the swept wing.
Since the birth of aviation, the goal has always been to go faster, farther, and higher. The swept-wing design helped push those goals beyond what was thought to be the limit. The entire design of a swept wing is a trick. The goal is to trick the airplane into believing it is flying slower than it actually is. It accomplishes this by allowing relative wind to strike the airfoil at an angle. This “tricks” the wing into believing it is flying at a speed slower than the actual true airspeed. Thus, the overall drag created is lower, allowing higher and faster flight.
The premise of the whole trick has everything to do with compressibility. The upper, curved portion of the wing acts like half of a venturi; the still-undisturbed air above the wing is the other half. This accelerates the relative wind over the wing. As an airfoil travels through the air at subsonic speeds, the air flowing over the wing might actually exceed the speed of sound, or Mach 1.0. This creates a shock wave over the wing that is drastically detrimental to performance, destroying lift and dramatically increasing drag. The speed at which these shock waves become apparent and critical is known as critical Mach. By tricking the wing into believing it is flying more slowly, the wing operates farther away from this critical point at higher speeds.
Using a thin, low-cambered wing increases the critical Mach number, allowing higher-speed flight. The downside here is low-airspeed flight. A swept, thin, low-cambered wing might be great for high-speed flight, but how about during takeoff and landing? To counteract these pitfalls, the leading and trailing edges are equipped with high-lift devices. Leading- and trailing-edge flaps increase the aircraft’s overall ability to produce lift by increasing wing area and reenergizing the local airflow. Combine the high-speed benefits of the swept wing with the low-speed generosity of Fowler flaps and leading-edge slats, and you have the efficient and proven design installed on almost every Transport category aircraft.