At what speed does air moving become warm from the friction of it moving across a surface?

Question

Say you stick your arm out the window of an airplane in the winter time. At certain speeds your arm will get colder faster from the wind chill. But at what speed of the airplane will the air moving across your arm actually begin to warm your arm?

Answer 1

There is a problem with your example. You are mixing heat loss from a heated surface (the arm at 98.6F or there abouts) in colder than average air - winter.
But friction is about raising the temp of a surface in air that is the same temperature. In theory, as soon as a moving body starts moving through the air, friction will raise the temperature slightly. Of course, the moving air also drags some of the heat away very quickly. Air liners expand with heat. The SST Concord expanded several inches.