aircraft is upright or inverted. Another example comes from sailboats, where the sail is a thin membrane with no path-length difference between one side and the other. For flight speeds near the speed of sound (transonic flight), airfoils with complex asymmetrical shapes are used to minimize the drastic increase in drag associated with airflow near the speed of sound. Such airfoils, called supercritical airfoils, are flat on top and curved on the bottom. Design features Aircraft wings may feature some of the following: A rounded leading edge cross-section A sharp trailing edge cross-section Leading-edge devices such as slats, slots, or extensions Trailing-edge devices such as flaps or flaperons (combination of flaps and ailerons) Winglets to keep wingtip vortices from increasing drag and decreasing lift Dihedral, or a positive wing angle to the horizontal, increases spiral stability around the roll axis, whereas anhedral, or a negative wing angle to the horizontal, decreases spiral stability. Aircraft wings may have various devices, such as flaps or slats that the pilot uses to modify the shape and surface area of the wing to change its operating characteristics in flight. Ailerons (usually near the wingtips) to roll the aircraft clockwise or counterclockwise about its long axis Spoilers on the upper surface to disrupt the lift and to provide additional traction to an aircraft that has just landed but is still moving. Vortex generators mitigate flow separation at low speeds and high angles of attack, especially over control surfaces. Wing fences to keep flow attached to the wing by stopping boundary layer separation from spreading roll direction. Folding wings allow more aircraft storage in the confined space of the hangar deck of an aircraft carrier Variable-sweep wing or "swing wings" that allow outstretched wings during low-speed flight (i.e., take-off and landing) and swept back wings for high-speed flight (including supersonic flight), such as in the F-111 Aardvark, the F-14 Tomcat, the Panavia Tornado, the MiG-23, the MiG-27, the Tu-160 and the B-1B Lancer warplanes Strakes to improve flight characteristics Chine, which may blend into the wing Leading-edge droop flap, a high-lift device Fairings, structures whose primary function is to produce a smooth outline and reduce drag. For example, flap track fairings Wings may have other minor independent surfaces. Applications and variants Besides fixed-wing aircraft, applications for wing shapes include: Hang gliders, which use wings ranging from fully flexible (paragliders, gliding parachutes), flexible (framed sail wings), to