Editing Mach number (section)

== Classification of Mach regimes ==
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The terms ''subsonic'' and ''supersonic'' are used to refer to speeds below and above the local speed of sound, and to particular ranges of Mach values. This occurs because of the presence of a ''transonic regime'' around flight (free stream) M = 1 where approximations of the [[Navier-Stokes equations]] used for subsonic design no longer apply; the simplest explanation is that the flow around an airframe locally begins to exceed M = 1 even though the free stream 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.

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.

{| class="wikitable"
|-
! rowspan=2 | Regime
! colspan=5 | Flight speed
! rowspan=2 | General plane characteristics
|-
! (Mach)
! (knots)
! (mph)
! (km/h)
! (m/s)
|-
! style="background-color: #FFFFFF;" | [[Subsonic aircraft|Subsonic]]
| <0.8
| <530
| <609
| <980
| <273
| Most often propeller-driven and commercial [[turbofan]] aircraft with high aspect-ratio (slender) wings, and rounded features like the nose and leading edges.
The subsonic speed range is that range of speeds within which, all of the airflow over an aircraft is less than Mach 1. The critical Mach number (M<sub>crit</sub>) is lowest free stream Mach number at which airflow over any part of the aircraft first reaches Mach 1.  So the subsonic speed range includes all speeds that are less than M<sub>crit</sub>.
|-
! style="background-color: #00ff00;" | [[Transonic]]
| 0.8–1.2
| 530–794
| 609–914
| 980–1,470
| 273–409
| Transonic aircraft nearly always have [[swept wing]]s, causing the delay of drag-divergence, and often feature a design that adheres to the principles of the Whitcomb [[area rule]].
The transonic speed range is that range of speeds within which the airflow over different parts of an aircraft is between subsonic and supersonic. So the regime of flight from M<sub>crit</sub> up to Mach 1.3 is called the transonic range.
|-
! style="background-color: #FF8181;" | [[Supersonic]]
| 1.2–5.0
| 794–3,308
| 915–3,806
| 1,470–6,126
| 410–1,702
| The supersonic speed range is that range of speeds within which all of the airflow over an aircraft is supersonic (more than Mach 1). But airflow meeting the leading edges is initially decelerated, so the free stream speed must be slightly greater than Mach 1 to ensure that all of the flow over the aircraft is supersonic. It is commonly accepted that the supersonic speed range starts at a free stream speed greater than Mach 1.3.
Aircraft designed to fly at supersonic speeds show large differences in their aerodynamic design because of the radical differences in the behavior of flows above Mach 1. Sharp edges, thin [[aerofoil]] sections, and all-moving [[tailplane]]/[[canard (aeronautics)|canards]] are common. Modern [[combat aircraft]] must compromise in order to maintain low-speed handling.
|-
! style="background-color: #FF4242;" | [[Hypersonic]]
| 5.0–10.0
| 3,308–6,615
| 3,806–7,680
| 6,126–12,251
| 1,702–3,403
| The [[North American X-15|X-15]], at Mach 6.72, is one of the fastest crewed aircraft. Cooled [[nickel]]-[[titanium]] skin; highly integrated (due to domination of interference effects: non-linear behaviour means that [[Superposition principle|superposition]] of results for separate components is invalid), small wings, such as those on the Mach 5 [[Boeing X-51|X-51A Waverider]].
|-
! style="background-color: #FF0303; color: #FFFFFF;" | High-hypersonic
| 10.0–25.0
| 6,615–16,537
| 7,680–19,031
| 12,251–30,626
| 3,403–8,508
| The [[NASA X-43]], at Mach 9.6, is one of the fastest aircraft. 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 [[Atmospheric reentry#Blunt body entry vehicles|blunt configurations]] because of the aerodynamic heating rising with a reduced [[Radius of curvature (mathematics)|radius of curvature]].
|-
! style="background-color: #C00000; color: #FFFFFF;" | [[Re-entry]] speeds
| >25.0
| >16,537
| >19,031
| >30,626
| >8,508
| [[Ablative heat shield]]; small or no wings; blunt shape. Russia's [[Avangard (hypersonic glide vehicle)|Avangard]] is claimed to reach up to Mach 27.
|}