Transonic
The flow around a wing passes from the mode Subsonique to the transonic mode when a zone appears in which local speed becomes equal to the speed of sound. The Mach number corresponding, lower than 1, is called the Mach criticizes.
For the clearness of exposed it is convenient to position on the fixed wing as in a blower which produces a flow of air around a fixed model.
Origin of the critical Mach
So that a wing develops a Portance, it is necessary that the pressures on the under-surface are higher than those on the suction face. In this case the flow is accelerated with the Extrados and is slowed down with the Intrados. Thus a flow of which speed “ad infinitum”, far from the wing, is lower than that of the sound can locally generate supersonic speeds on the suction face.
Shock waves into transonic
When this critical Mach is exceeded, while following the flow of the upstream towards the downstream one notes that it passes on the suction face of subsonic to supersonic through a range of relaxation then returns to subsonic through a Shock wave. It is a right, normal shock with the wing at the points where it meets this one, through which speed is reduced but not its direction.
It increases the trail in two manners. It there with the trail of wave due to the kinetic loss of energy in any shock wave. With this one a trail related to the right shock is added. The Boundary layer which was in a supersonic environment runs up brutally against a subsonic zone, which creates an separation whose consequences are similar to those of the Décrochage.
The increase speed ad infinitum increases the width of the supersonic zone and the shock moves towards the trailing edge. When all the points of a profile reached Mach 1, there is no more separation, the trail falls brutally and the difficulties of piloting disappear: the plane passed the " wall of the son" and enters the Supersonique.
Increase in the Mach number critical
The phenomena related to the transonic flow modify the bearing pressure, the trail and the moment of balance in plate but it is especially the separation which poses serious problems of piloting, even of safety. One pushes back the critical Mach to move back the threshold where the trail becomes too important (it is the Mach of divergence of trail , Mach number for which the slope dCx/dMach reaches 10%). There exists for that two techniques, which are generally used simultaneously:
Supercritical profile
The profiles of supercritical wings qualified of can be thicker than the first profiles adapted to high speeds (advantages in behavior in inflection, torsion and fuel capacity).
Sweptback wing
The angulation towards the back of the wing increases the aerodynamic cord of the profile, which decreases the effective relative thickness of the profile. The wing “sees” only the component of the relative wind in the direction of the profile, the other component running out along the aerofoil.
MO
It is the Mach number Maximum Operational taking into account the characteristics of the profile (in 2D) and the wing (in 3D). For the airliners equipped with supercritical profiles, MO is about 0.85 to 0.88.
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