Originally Posted by henra
There are two effects whose significance varies with the Mach Number.
The Sweep of the leading edge helps from M1 to the point where the Mach cone angle is the same as the sweep. That is where a shock wave emanating from one point of the leading edge will hit a neighbouring section of the LE.
At the same time there is the relative thickness perpendicular to the LE. As this is thicker the Mach induced flow detachment on the upper side will have a steeper angle and occur earlier and thus increase drag.
Okay, but if you take the relative thickness into account, and the aircraft is below M2.1, a higher relative thickness (as is the case with increased wing sweep) should also produce larger drag figures.
But on the graph the drag is lower for a higher swept wing below M2.1.