The RPU (Rubber Power Unit) system is a modular, Velcro-attached power module for rubber-powered balsa models, designed to be reused across different aircraft. Built from simple materials like skewers or bamboo, balsa, piano wire, and a plastic airscrew, it comes in three sizes—RPU 20, RPU 30, and RPU 40—suitable for wingspans from 30 up to…