In aeronautics, transonic refers to the condition of flight in which a range of
velocities of airflow exist surrounding and flowing past an air vehicle or an
airfoil that are concurrently below, at, and above the speed of sound in the
range of Mach 0.8 to 1.0, i.e. 600–768 mph at sea level. This condition depends
not only on the travel speed of the craft, but also on the temperature of
the airflow in the vehicle's local environment. It is formally defined as
the range of speeds between the critical Mach number, when some parts of the
airflow over an air vehicle or airfoil are supersonic, and a higher speed,
typically near Mach 1.2, when the vast majority of the airflow is supersonic.
Between these speeds some of the airflow is supersonic, but a significant
fraction is not. Most modern jet powered aircraft are
engineered to operate at transonic air speeds. Transonic airspeeds see a rapid
increase of drag from about Mach 0.8, and it is the fuel costs of the drag
that typically limits the airspeed. Attempts to reduce wave drag can be seen
on all high-speed aircraft; most notable is the use of swept wings, but another
common form is a wasp-waist fuselage as a side effect of the Whitcomb area rule.
Severe instability can occur at transonic speeds. Shock waves move
through the air at the speed of sound. When an object such as an aircraft also
moves at the speed of sound, these shock waves build up in front of it to form a
single, very large shock wave. During transonic flight, the plane must pass
through this large shock wave, as well as contend with the instability caused
by air moving faster than sound over parts of the wing and slower in other
parts. Transonic speeds can also occur at the
tips of rotor blades of helicopters and aircraft. However, as this puts severe,
unequal stresses on the rotor blade, it is avoided and may lead to dangerous
accidents if it occurs. It is one of the limiting factors to the size of rotors,
and also to the forward speeds of helicopters side of the rotor, thus
possibly causing localized transonics). Condensation clouds
At transonic speeds, supersonic expansion fans form intense
low-pressure, low-temperature areas at various points around an aircraft. If
the temperature drops below the dew point a visible cloud will form. These
clouds remain with the aircraft as it travels. It is not necessary for the
aircraft as a whole to reach supersonic speeds for these clouds to form.
Typically, the tail of the aircraft will reach supersonic flight while the bow of
the aircraft is still in subsonic flight. A bubble of supersonic expansion
fans terminating by a wake shockwave surround the tail. As the aircraft
continues to accelerate, the supersonic expansion fans will intensify and the
wake shockwave will grow in size until infinity is reached, at which point the
bow shockwave forms. This is Mach one and the Prandtl–Glauert_singularity.
Transonic flows in astronomy and astrophysics
In astrophysics, wherever there is evidence of shocks, the flow close by
must be transonic, as only supersonic flows form shocks. Interestingly, all
black hole accretions are transonic. Many such flows also have shocks very
close to the black holes. The outflows or jets from young stellar
objects or disks around black holes can also be transonic since they start
subsonically and at a far distance they are invariably supersonic. Supernovae
explosions are accompanied by supersonic flows and shock waves. Bow shocks formed
in solar winds are a direct result of transonic winds from a star. It has been
long thought that a bow shock was present around the heliosphere of our
solar system. This was recently found not to be the case according to IBEX
data. See also
Anti-shock body Subsonic flows
Supersonic flows Hypersonic flows
Supersonic expansion fans References