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Transonic (or transsonic) flow is air flowing around an object at a speed that generates regions of both subsonic and Supersonic speed airflow around that object. The exact range of speeds depends on the object's critical Mach number, but transonic flow is seen at flight speeds close to the speed of sound (343 m/s at sea level), typically between Mach number 0.8 and 1.2.
The issue of transonic speed (or transonic region) first appeared during World War II. Pilots found as they approached the sound barrier the airflow caused aircraft to become unsteady. Experts found that can cause large-scale Flow separation downstream, increasing drag, adding asymmetry and unsteadiness to the flow around the vehicle. Research has been done into weakening shock waves in transonic flight through the use of Anti-shock body and supercritical airfoils.
Most modern jet engine powered aircraft are engineered to operate at transonic air speeds. (2025). 9781606506837, Momentum Press Engineering. ISBN 9781606506837 Transonic airspeeds see a rapid increase in drag from about Mach 0.8, and it is the fuel cost 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 , but another common form is a wasp-waist fuselage as a side effect of the Whitcomb area rule.
Transonic speeds can also occur at the tips of Rotorcraft blades of helicopters and aircraft. This puts severe, unequal stresses on the rotor blade and may lead to accidents if it occurs. It is one of the limiting factors of the size of rotors and the forward speeds of helicopters (as this speed is added to the forward-sweeping leading side of the rotor, possibly causing localized transonics).
After World War II, major changes in aircraft design were seen to improve transonic flight. The main way to stabilize an aircraft was to reduce the speed of the airflow around the wings by changing the chord of the plane wings, and one solution to prevent transonic waves was swept wings. Since the airflow would hit the wings at an angle, this would decrease the wing thickness and chord ratio. Airfoils' wing shapes were designed flatter at the top to prevent shock waves and reduce the distance of airflow over the wing. Later on, Richard Whitcomb designed the first supercritical airfoil using similar principles.
One of the first methods used to circumvent the nonlinearity of transonic flow models was the hodograph transformation. This concept was originally explored in 1923 by an Italian mathematician named Francesco Tricomi, who used the transformation to simplify the compressible flow equations and prove that they were solvable. The hodograph transformation itself was also explored by both Ludwig Prandtl and O.G. Tietjen's textbooks in 1929 and by Adolf Busemann in 1937, though neither applied this method specifically to transonic flow.
Gottfried Guderley, a German mathematician and engineer at Braunschweig, discovered Tricomi's work in the process of applying the hodograph method to transonic flow near the end of World War II. He focused on the nonlinear thin-airfoil compressible flow equations, the same as what Tricomi derived, though his goal of using these equations to solve flow over an airfoil presented unique challenges. Guderley and Hideo Yoshihara, along with some input from Busemann, later used a singular solution of Tricomi's equations to analytically solve the behavior of transonic flow over a double wedge airfoil, the first to do so with only the assumptions of thin-airfoil theory.
Although successful, Guderley's work was still focused on the theoretical, and only resulted in a single solution for a double wedge airfoil at Mach 1. Walter Vincenti, an American engineer at Ames Laboratory, aimed to supplement Guderley's Mach 1 work with numerical solutions that would cover the range of transonic speeds between Mach 1 and wholly supersonic flow. Vincenti and his assistants drew upon the work of Howard Emmons as well as Tricomi's original equations to complete a set of four numerical solutions for the drag over a double wedge airfoil in transonic flow above Mach 1. The gap between subsonic and Mach 1 flow was later covered by both Julian Cole and Leon Trilling, completing the transonic behavior of the airfoil by the early 1950s.
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 are a direct result of transonic winds from a star. It had been long thought that a bow shock was present around the heliosphere of the Solar System, but this was found not to be the case according to IBEX data published in 2012..
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