Multiple Design Point Airfoils
One of the difficulties in designing a good airfoil is the requirement
for acceptable off-design performance. While a very low drag section is
not too hard to design, it may separate at angles of attack slightly away
from its design point. Airfoils with high lift capability may perform very
poorly at lower angles of attack.
One can approach the design of airfoil sections with multiple design points
in a well-defined way. Often it is clear that the upper surface will be
critical at one of the points and we can design the upper surface at this
condition. The lower surface can then be designed to make the section behave
properly at the second point. Similarly, constraints such as Cm0 are most
effected by airfoil trailing edge geometry.
When such a compromise is not possible, variable geometry can be employed
(at some expense) as in the case of high lift systems.
Transonic section at a range of CL's, from Drela.