Questions about flight at hypersonic speeds, typically defined as above Mach 5.
The hypersonic flight regime is a subset of the supersonic range dealing with speeds high enough that compression heating (where the air in front of the aircraft can't get out of the way fast enough, so it's compressed by the aircraft flying into it and heats up dramatically as a result) starts to become severe enough to ionize the air around the aircraft. This typically occurs around mach 5, which, therefore, serves as a useful starting point for the hypersonic range. Very few aircraft, other than dedicated experimental and scientific aircraft, routinely fly at hypersonic speeds, due not only to the extreme drag and severe thermal load imposed by hypersonic flight (which generally restrict hypersonic flights to extremely high altitude, to prevent the aircraft from being destroyed by aerodynamic and thermal effects), but also the fact that these aircraft must generally use either rocket or scramjet propulsion (turbofan engines are essentially useless above mach 2, turbojets max out in the vicinity of mach 3, and even the ramjet can't go much beyond - conveniently - mach 5); rockets are extremely inefficient and loud and use huge amounts of fuel and oxidizer, while scramjets, though more efficient than rockets, are still quite inefficient compared to other types of jet engines, and, moreover, are still in their infancy.
For more information, see Wikipedia.