HUP-3 / US NavyUS Navy HUP 3 helicopter model. Versions of the HUP built for the U. S. Army were designated H 25 Army Mule. The tandem overlapping rotor configuration was a development by Piasecki and was used in future helicopter designs by the company and successors including the H 21, HRB 1 CH 46, and CH 47. The design featured two three bladed 35 foot diameter rotors in tandem in which blades could be folded for storage in ships hangars. The HUPs were powered by