The wetted aspect ratio is a good indication of the aerodynamic efficiency of an aircraft. It is a better measure than the aspect ratio. It is defined as:

where Sw is the wetted surface of the whole aircraft in contrast to the wing area used for the definition of the aspect ratio.

A good example of this is the Boeing B-47 and Avro Vulcan. Both aircraft has very similar performance although they are radically different. The B-47 has a high aspect ratio wing, while the Avro Vulcan is a low aspect ratio blended wing body. They have, however, s very similar wetted aspect ratio.