マッハ数 (M: Mach Number)

マッハ数とは？

Mach number (M) is the ratio of an aircraft's true airspeed to the local speed of sound in the surrounding medium. Named after Austrian physicist Ernst Mach, it is the fundamental parameter for characterizing flight in compressible flow regimes. At Mach 1, an aircraft travels at exactly the speed of sound; below Mach 1 is subsonic flight; above is supersonic. The speed of sound varies with temperature — at sea level on a standard day (15°C) it is approximately 340 m/s (661 knots), while at 35,000 ft (–56°C) it drops to about 295 m/s (573 knots).

仕組み

Flight regimes are classified by Mach number:

• Subsonic (M < 0.8): Airflow around the aircraft is entirely below the speed of sound. Standard aerodynamic principles apply.
• Transonic (M 0.8–1.2): Mixed flow — some airflow over the wings accelerates to supersonic while the freestream remains subsonic. Shockwaves form on wing surfaces, causing wave drag, buffet, and control anomalies. Most jet airliners cruise in this regime (M 0.78–0.90).
• Supersonic (M 1.2–5.0): Shockwaves anchor ahead of the aircraft. Wave drag is a dominant concern. Specialized wing designs (delta, swept, ogival) manage shockwave behavior.
• Hypersonic (M > 5.0): Extreme aerodynamic heating dominates. Relevant to spacecraft reentry and experimental vehicles.

Modern jet airliners are assigned a Mach operating limit (MMO) — the maximum speed before dangerous compressibility effects occur. The Boeing 777's MMO is Mach 0.89; the 787's is Mach 0.90.

航空における重要性

Mach number determines the entire aerodynamic design philosophy of an aircraft. The flight envelope has a Mach boundary that cannot be exceeded in normal operations. High-Mach aircraft require swept or delta wings to delay the onset of shockwave formation, thinner airfoil profiles, and area-ruled fuselages. Autopilots and flight management systems actively monitor Mach number and automatically reduce thrust or lower the nose if MMO is approached.

実際の影響

The Concorde cruised at Mach 2.04 (~2,179 km/h) at 60,000 ft, reducing London–New York flight time to under 3.5 hours versus 7+ hours for subsonic jets. The SR-71 Blackbird flew at Mach 3.3+, requiring titanium construction to withstand aerodynamic heating above 300°C. Today, Boom Supersonic's Overture aims for Mach 1.7 commercial service by the late 2020s, while standard long-haul jets like the Boeing 787 cruise at Mach 0.85 to optimize the fuel burn–speed tradeoff.