Piasecki Vectored Thrust Ducted Propeller (VTDP) installed on a Cobra. Piasecki is always installing this device on something at Government expense, including most recently a contract to create the VTDP Blackhawk for the US Airforce!. . <a href="http://avia.russian.ee/vertigo/piasecki_cobra-r.html" target="_blank">Compound Cobra.</a>