 It also had to operate from an elevated platform of perforated steel to reduce HGI.The Dassault Mirage IIIV VTOL research aircraft only ever operated vertically from a grid which allowed engine exhaust to be channelled away from the aircraft to avoid suckdown and HGI effects.Ventral strakes retroactively fitted to the P.1127 improved flow and increased pressure under the belly in low altitude hovering.