To control the airplane, the lines must remain in tension. Centripetal Acceleration is generally sufficient to maintain line tension if the airplane is properly "trimmed" (adjusted), but sometimes additional features such as rudder offset and engine offset are added to provide extra tension. It is of interest to note, that when a control line model does a loop, it no longer flies on the edge of a hemisphere, but traverses the edge of a cone, a planar path, and the motion of the model produces no centripetal acceleration. In the condition of flying a loop, other factors must therefore provide the line tension, such as engine offset, or lead-out rake. Weight in the outboard wing tip is usually used to balance the weight of the lines. Top aerobatics models typically have a large number of adjustable features like tip weight boxes, adjustable rudder offset, adjustable line sweep, and adjustable elevator and flap controls. Some aerobatics models use a variable rudder system (commonly called the Rabe rudder after its inventor, Al Rabe) to vary the rudder offset during flight. The adjustment of the various adjustable features on a modern stunt model can become quite complex.