25.Features of flow of an aerofoil section in a transonic flow.

Transonic airfoils are designed to achieve the desired thickness and lift coefficient without developing strong shock waves on the upper surface.

The larger the t/c and the larger the Cl, the lower the freestream Mach number must be in order to avoid shocks and the accompanying drag increase.

Actually, one need not entirely avoid supersonic flow to avoid high transonic drags. By keeping the maximum local Mach number to something less than about 1.2 and trying to keep the maximum speeds forward of the airfoil "crest", a low drag airfoil with some supercritical flow can be designed.

One way to keep the upper surface Cp's from getting too low is to carry positive pressures on the lower surface. This can be done easily near the trailing edge. This technique of aft-loading together with minor changes to the airfoil nose can lead to achievable low drag Mach numbers as much as 0.06 above conventional sections. These are called "supercritical" airfoils and are discussed in more detail in the section on transonic airfoil design.

26.The aerodynamic characteristics of an aerofoil section in a transonic flow.

27.The aerodynamic characteristics of a wing in a transonic flow.

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 (965-1236 km/h). 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.Today, at transonic speeds wings are swept to delay drag rise.

Early transonic military aircraft such as the Hawker Hunter and F-86 Sabre were designed to fly satisfactorily faster than their critical Mach number. They did not possess sufficient engine thrust to reach Mach 1.0 in level flight but could be dived to Mach 1.0 and beyond while remaining controllable. Modern passenger-carrying jet aircraft such as Airbus and Boeing aircraft have maximum operating Mach numbers slower than Mach 1.0.

28.Critical Mach number of a finite-span wing, relation of a critical Mach number to an angle of attack, geometrical characteristics of an aerofoil section and wing.

In aerodynamics, the critical Mach number (Mcr) of an aircraft is the lowest Mach number at which the airflow over some point of the aircraft reaches the speed of sound.

For all aircraft in flight, the airflow around the aircraft is not exactly the same as the airspeed of the aircraft due to the airflow speeding up and slowing down to travel around the aircraft structure. At the critical Mach number, local airflow in some areas near the airframe reaches the speed of sound, even though the aircraft itself has an airspeed lower than Mach 1.0. This creates a weak shock wave. At speeds faster than the Critical Mach number the drag coefficient increases suddenly, causing dramatically increased drag,[2] and, in an aircraft not designed for transonic or supersonic speeds, changes to the airflow over the flight control surfaces lead to deterioration in control of the aircraft.