Kent Cooper brings us a routine to draw an arc based on the arc length. Start by loading the LISP file, then type in the command ARCL.
Kent has noted that a couple of similar routines exist among the Cadalyst CAD Tips (Nos. 4218 and 3392), but this submission is different in a few ways. By default, this routine will always start at the last point, and the user will then provide the radius, length, and direction. You do not need to know the center point, and the last point is used as the starting point for the next arc if you are drawing multiple arcs.