Helicopter 4 : A type of rotorcraft, the support of which, in the air, is normally derived from airfoiles mechanically rotated about an approximately vertical axis.
Ilan Kelman (ed.), Vulnerability and Resilience Definitions and References, Version 2, 2007. http://www.islandvulnerability.org/docs/vulnres.pd f, (Vulnr and Resilience - Brit) {524}