Mach number - Mach number

Japanese: マッハ数 - まっはすう（英語表記）Mach number

When the speed of an aircraft exceeds the speed of sound (the speed at which pressure changes propagate through the atmosphere), the surrounding air is compressed, its volume decreases, its temperature rises, and the pressure distribution around the aircraft, such as the wings and fuselage, changes drastically. The speed of sound is 340 m per second at an air temperature of 15°C, but since it changes depending on the air temperature, the speed of high-speed aircraft, which is affected by compressibility, is expressed based on the speed of sound. Mach 1 is the speed of sound, 1.2 to 5 is supersonic, and less than 1 is subsonic. Transonic is when the speed of an aircraft approaches the speed of sound, partially exceeds the speed of sound, and subsonic and supersonic speeds coexist, generating shock waves at the boundary between them. This causes a sudden increase in air resistance, inducing a shock wave stall, and the aircraft faces difficulties in control, such as buffeting (violent vibration), wing lock (sudden rotation), tuck-under (strong nose-down), and pitch-up (uncontrollable nose-up). At supersonic speeds, these phenomena disappear, but the pressure changes around the flying object accumulate in a conical surface (Mach cone) with the nose at its apex, and when that energy reaches the ground, it becomes an impact sound called a sonic boom. At high Mach speeds exceeding Mach 4, the effectiveness of the rudder and tail is reduced, making it necessary to control the aircraft's attitude by ejecting gas, etc. The apex angle of the Mach cone becomes sharper, and the temperature of the air on the aircraft's surface exceeds 500°C due to compression. This state is the hypersonic range. The Space Shuttle is launched at supersonic speeds, re-enters the atmosphere at hypersonic speeds, and lands at subsonic speeds.

(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:

飛行体の速度が音速(大気中を圧力の変化が伝播する速度)を超えると周囲の空気は圧縮され、体積が減り、温度が上昇し、主翼や胴体など飛行体周囲の圧力分布は一変する。音速は気温が15℃の時に毎秒340mだが、気温によって変化するので、圧縮性の影響を受ける高速飛行体の速度は音速を基準にして表現する。マッハ1が音速、1.2〜5が超音速、1未満が亜音速、飛行体の速度が音速に近づいて、部分的に音速を超え亜音速と超音速が共存し、その境界に衝撃波を発生する状態になったのが遷音速。空気抵抗が急増し、衝撃波失速を誘起し、バフェット(激しい振動)、ウイングロック(不意自転)、タックアンダー(強力な機首下げ)、ピッチアップ(操縦不能の機首上げ)などの操縦困難に直面する。超音速になるとこうした現象は消滅するが、飛行体周囲の圧力変化は、機首を頂点とする円錐面(マッハコーン)に蓄積され、そのエネルギーが地上に到達するとソニックブームと呼ばれる衝撃音となる。マッハ4を超えるような高マッハ飛行では舵や尾翼の利きが低下するため、ガスの噴出などで機体姿勢の制御が必要になる。マッハコーンの頂角が鋭くなり、機体表面の空気の温度は圧縮によって500℃を超えるようになる。この状態が極超音速域。スペースシャトルは超音速で打ち上げられ、極超音速で大気圏に再突入、亜音速飛行で着陸する。

(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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