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F/A-18 flying at transonic speed
F/A-18 flying at transonic speed

Transonic is an aeronautics term referring to the condition in which a range of velocities of airflow exist surrounding and flowing past an air vehicle or an airfoil. Air flow velocities are concurrently below, at, and above the speed of sound at the pressure and temperature of the airflow of the air vehicle's local environment (about mach 0.8–1.2). 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 air foil are supersonic, and a higher speed, typically near Mach 1.2, when all of the airflow is supersonic. Between these speeds some of the airflow is supersonic, and some is not.

Most modern jet powered aircraft are engineered to operate with as high a transonic air speed as possible, before their air foils experience the onset of transonic wave drag, which is prevalent and really defines the beginning of the transonic speed ranges. The importance of transonic wave drag lies in the fact that it is both an unpredictable and non-linear phenomena. That is the behavior of an airfoil, or an airframe, is very difficult to predict at the onset of transonic wave drag. Also the rate of increase in drag is almost never linearly related to an increase in speed. In the transonic region an air foil’s speed may increase by say 2%, but the increase in drag (in the transonic region) may be 8%. Worst of all in the transonic region for an airfoil an increase in speed that goes from a 2% to 3% increase; can yield an increase in transonic drag that rises from 8% to 16%. That is how non-linear the phenomenon is. Attempts to combat 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 contending with the instability caused by air moving faster than sound over parts of the wing and slower in other parts. The difference in speed is due to Bernoulli's principle.

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 (as this speed is added to the forward-sweeping (leading) side of the rotor, thus possibly causing localized transonics).

Interesting facts

Transonic flow patterns on an airfoil showing flow patterns at and above critical Mach number.
Transonic flow patterns on an airfoil showing flow patterns at and above critical Mach number.
  • At transonic speeds intense low-pressure areas form at various points around an aircraft. If conditions are right (i.e. high humidity) visible clouds will form in these low-pressure areas as shown in the illustration; these are called Prandtl-Glauert singularities. 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.

Transonic flow patterns on F-16 fighter aircraft
Transonic flow patterns on F-16 fighter aircraft

Transonic flow patterns on F-16 fighter aircraft
Transonic flow patterns on F-16 fighter aircraft

Transonic Flows in Astronomy and Astrophysics

In Astrophysics, wherever there is evidence of shocks (standing, propagating or oscillating), the flow close by must be transonic as only supersonic flows form shocks. Interestingly all the black hole accretions are transonic (S.K. Chakrabarti, ApJ, 1996, v. 471, p. 237), many of the 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 explosion is accompanied by super sonic flows and shock waves. Bow shocks formed in solar winds around the earth is a direct result of transonic wind from the sun.

See also

Other Flow Regimes

Theory of transonic astrophysical flows: Sandip K. Chakrabarti, World Scientific Publishers , Singapore (1990)

Text from Wikipedia is available under the Creative Commons Attribution/Share-Alike License; additional terms may apply.

Published in July 2009.

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