AMA Scale competition rules 6.2.C, Proof of Scale, for Radio Controlled Sport Scale (Sportsman and Explert):

A plastic model of the type available commercially, unmodified and unpainted may be used. The use of a plasitc model as proof of Accuracy of Outline will require the deletion of two (2) 8 1/2 X 11" pages, or their equivalent as provided in paragraph 4.5, below.

Scott