is the maximum top-to-bottom depth of an airfoil section. It is expressed as a percentage of the airfoil chord. The thickness of an airfoil affects the profile drag of an airfoil. The greater the thickness, the higher the profile drag. Profile drag is drag not produced by lift from an airfoil. High thickness ratios tend to cause stalling at lower lift coefficients. A forward location of maximum thickness tends to cause higher drag than an aft location.