I am desigining an aircraft, which shall fly up to Mach=0.9 in cruise flight in an altitude of 10 km.
So when I think of which airfoil to choose, I have to consider aspects like
- max. lift coefficient required
- max. necessary/allowed angle of attack
But what comes first in my mind is, that I have to prevent shock waves forming on my airfoil, because drag will dramatically increase. So actually, this is my first demand for the airfoil.
NACA 64006 is a supercritical airfoil. In order to prevent Mach=1 on the airfoil, I would need a swept wing with an angle of more than 50° to reach Ma_crit=0.88 .
I am wondering now, if this is normal that I need such a high sweep angle? Or did I choose the wrong airfoil. So normal commerical aircraft have angles between 30 to max. 40° ?
So the only way to have less sweep angle: find another airfoil (I think 64006 is already one of the best free available airfoils) or I accept higher values of drag coefficient ?
How do airliners tackle the problem? I think I read that they allow mach numbers a little bit higher until the drag divergence velocity?