フラップ (Flaps)

フラップとは？

Flaps are movable surfaces hinged to the trailing edge (and sometimes the leading edge) of an aircraft's wing. When deployed, they increase the wing's camber (curvature) and, on some designs, its effective surface area. This produces substantially more lift at any given airspeed, allowing the aircraft to fly safely at the lower speeds required for takeoff and landing without exceeding the wing's stall angle.

機能と目的

At cruise altitude, flaps are fully retracted so the wing is optimized for low-drag, high-efficiency flight. As the aircraft descends and decelerates for landing, the crew progressively extends the flaps through discrete settings (e.g., Flaps 1, 5, 10, 15, 20, 25, 30, 40 on the Boeing 737). Each step increases lift, allowing a lower approach speed, but also increases drag — which is desirable during descent since it helps control the rate of descent without requiring excessive thrust reduction.

For takeoff, flaps are set to an intermediate position (e.g., Flaps 5 or 10 on most Boeing jets) that provides additional lift to reduce the required runway length while adding less drag than a full landing flap setting. The optimal flap setting for takeoff depends on runway length, obstacle clearance requirements, aircraft weight, and ambient temperature.

種類とバリエーション

• Plain flap: The simplest type — a hinged section of the trailing edge that rotates downward. Low lift gain but mechanically simple. Common on small general aviation aircraft.
• Split flap: Only the lower surface hinges down; upper surface remains fixed. Creates significant drag, used on older designs like the Douglas DC-3.
• Slotted flap: A gap between the flap and the wing allows high-energy air from the lower surface to re-energize the boundary layer on the flap's upper surface, delaying stall. Used on most modern commercial jets in single or double-slotted configurations.
• Fowler flap: Slides aft and then rotates downward, increasing both camber and wing area. The Boeing 737 uses single-slotted Fowler flaps; the 747 and 777 use triple-slotted Fowler flaps for even greater lift.
• Leading-edge devices: Slats (Airbus A320, Boeing 737) or Krueger flaps (Boeing 747 inboard leading edge) extend from the leading edge to increase the stall angle of attack.

主な事例

The Boeing 777 uses a complex triple-slotted Fowler flap system that extends the trailing-edge chord by approximately 25%, enabling the large aircraft to land at relatively modest approach speeds despite its high maximum landing weight. The Airbus A350 uses a simpler single-slotted Fowler flap enabled by its high-aspect-ratio composite wing, achieving competitive approach speeds with fewer moving parts and lower maintenance burden.

関連コンポーネント

Flaps work together with winglets to manage the wing's overall aerodynamic performance. The landing gear is typically deployed at similar flight phases as the flaps, both serving to configure the aircraft for the slower speeds of approach. The increased lift from flaps is closely linked to the fundamental concept of lift generation — the Bernoulli and circulation effects that keep the aircraft airborne.