Air resistance - aerodynamic drag

Japanese: 空気抵抗 - くうきていこう（英語表記）aerodynamic drag

The air resistance of an aircraft that flies using lift is composed of induced resistance and form resistance. Induced resistance is air resistance caused by lift generation. It is very small at high speeds and large at low speeds. Form resistance consists of pressure resistance that generates vortices behind the aircraft and friction resistance caused by the viscosity of the air. It increases almost in proportion to the square of the speed. In a streamlined aircraft, the proportion of pressure resistance is very small. When induced resistance and form resistance are equal, air resistance is at its minimum and the lift-to-drag ratio is at its maximum. Induced resistance can be reduced by using a large wingspan, so large wingspans are used for human-powered aircraft and gliders. Small wings called winglets that are installed on the wingtips of recent jet passenger aircraft also reduce induced resistance. A well-known method of reducing friction resistance is the laminar flow wing, which keeps the flow in a thin area affected by viscosity called the boundary layer in a laminar flow. Riblets, which are lined with thin grooves along the flow used in yachts for America's Cup races, special tape attached to the surface of skaters' clothes, and swimsuits made of special materials are known as similar resistance reduction methods. During transonic and supersonic flight, shock waves are generated by the compressibility of air, which adds wave resistance. Swept-back wings, delta wings, and area rules that smooth the cross-sectional area distribution by narrowing the joint between the wing and the fuselage are technologies that reduce wave resistance.

(Tsuruo Torigai, former Managing Director of the Japan Aircraft Development Corporation, Engineer (Aircraft Division) / 2007)

Source : "Chiezo" published by Asahi Shimbun Publications Co., Ltd. About Chiezo

Japanese:

揚力を利用して飛行する航空機の空気抵抗は、揚力発生に伴う誘導抵抗と形状抵抗とで構成される。誘導抵抗は揚力発生に伴う空気抵抗。高速飛行では非常に小さくなり、低速で大きくなる。形状抵抗の内容は飛行体の後方に渦を発生する圧力抵抗と、空気の粘性によって発生する摩擦抵抗。速度のほぼ2乗に比例して大きくなる。流線型の飛行体では圧力抵抗の占める比率は非常に小さい。誘導抵抗と形状抵抗が等しくなったときに空気抵抗が最小、揚抗比最大となる。誘導抵抗は大きな翼幅を使えば小さくなるから、人力飛行機やグライダーでは大きな翼幅が使われる。近年のジェット旅客機の翼端に立てられているウイングレットという小翼も誘導抵抗を減らすもの。摩擦抵抗の低減手段には、境界層と呼ばれる粘性の影響をうける薄い領域内の流れを広く層流に保つ層流翼が有名。アメリカスカップ・レース用のヨットに使われた流れに沿って細い溝を並べるリブレット、スケート選手の衣服の表面に張り付ける特殊テープ、特殊素材の水着などが同種の抵抗低減手段として知られている。遷音速、超音速飛行では、空気の圧縮性によって衝撃波が発生し、造波抵抗が加わる。後退翼やデルタ翼、胴体の主翼との結合部をくびれさせ断面積分布をスムースにするエリアルールは造波抵抗を減らす技術。

(鳥養鶴雄元日本航空機開発協会常務理事　技術士（航空機部門）／ 2007年)

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