And, to reinforce PBL's point, while that Boeing diagram may be principally thinking about Mach effect from, say, ).5 through to 0.8, there are aerofoils where the Mach effect is perceptible all the way down to very low numbers - 0.2 or lower - a speed regime where Mach effects are not normally expected.
(p-pinfinity)
/(1/2rhoV^2)/Cpi=1/[1-M^2]^.5+ [M^2/1+{1-M^2}^.5]*(cpi/2)
or maybe just 1/[1-M^2]^.5
no further comments
