The rotor blades look a lot like the wings of an aircraft. The thick profiles in the innermost part of the blade are usually designed specifically for wind turbines. Choosing profiles for rotor blades involves a number of compromises including reliable lift and stall characteristics, and the profile's ability to perform well even if there is some dirt on the surface. It includes glass fibre reinforced plastics, glass fibre reinforced polyester or epoxy materials. The carbon fibre or aramid reinforcing material is another possibility, but usually such blades are uneconomic for large turbines. Steel and aluminium alloys have problems of weight and metal fatigue respectively. They are currently only used for very small wind turbines.