Classification of Mach Regimes
While the terms "subsonic" and "supersonic" in the purest verbal sense refer to speeds below and above the local speed of sound respectively, aerodynamicists often use the same terms to talk about particular ranges of Mach values. This occurs because of the presence of a "transonic regime" around M=1 where approximations of the Navier-Stokes equations used for subsonic design actually no longer apply, the simplest of many reasons being that the flow locally begins to exceed M=1 even when the freestream Mach number is below this value.
Meanwhile, the "supersonic regime" is usually used to talk about the set of Mach numbers for which linearised theory may be used, where for example the (air) flow is not chemically reacting, and where heat-transfer between air and vehicle may be reasonably neglected in calculations.
In the following table, the "regimes" or "ranges of Mach values" are referred to, and not the "pure" meanings of the words "subsonic" and "supersonic".
Generally, NASA defines "high" hypersonic as any Mach number from 10 to 25, and re-entry speeds as anything greater than Mach 25. Aircraft operating in this regime include the Space Shuttle and various space planes in development.
| Regime | Mach | mph | km/h | m/s | General plane characteristics |
|---|---|---|---|---|---|
| Subsonic | <0.8 | <610 | <980 | <270 | Most often propeller-driven and commercial turbofan aircraft with high aspect-ratio (slender) wings, and rounded features like the nose and leading edges. |
| Transonic | 0.8-1.2 | 610-915 | 980-1,470 | 270-410 | Transonic aircraft nearly always have swept wings, delaying drag-divergence, and often feature design adhering to the principles of the Whitcomb Area rule. |
| Supersonic | 1.2-5.0 | 915-3,840 | 1,470-6,150 | 410-1,710 | Aircraft designed to fly at supersonic speeds show large differences in their aerodynamic design because of the radical differences in the behaviour of flows above Mach 1. Sharp edges, thin aerofoil-sections, and all-moving tailplane/canards are common. Modern combat aircraft must compromise in order to maintain low-speed handling; "true" supersonic designs include the F-104 Starfighter and BAC/Aérospatiale Concorde. |
| Hypersonic | 5.0-10.0 | 3,840-7,680 | 6,150-12,300 | 1,710-3,415 | Cooled nickel-titanium skin; highly integrated (due to domination of interference effects: non-linear behaviour means that superposition of results for separate components is invalid), small wings, see X-51A Waverider |
| High-hypersonic | 10.0-25.0 | 7,680-16,250 | 12,300-30,740 | 3,415-8,465 | Thermal control becomes a dominant design consideration. Structure must either be designed to operate hot, or be protected by special silicate tiles or similar. Chemically reacting flow can also cause corrosion of the vehicle's skin, with free-atomic oxygen featuring in very high-speed flows. Hypersonic designs are often forced into blunt configurations because of the aerodynamic heating rising with a reduced radius of curvature. |
| Re-entry speeds | >25.0 | >16,250 | >30,740 | >8,465 | Ablative heat shield; small or no wings; blunt shape |
Read more about this topic: Mach Number
Famous quotes containing the word mach:
“Physics is experience, arranged in economical order.”
—Ernst Mach (1838–1916)