The compound helicopter has been a concept in play for decades since conceived of in the mid-1930s. However, its practical application has proved elusive for military and civilian market service as no prominent product has seen large scale adoption since that time. The compound helicopter is called such due to the use of a conventional rotor system for traditional hovering flight and a propulsion unit added for faster-than-normal horizontal flight. In essence, the design is a bridge between the standard helicopter concept and a fixed-wing aircraft - offering up the benefits of both designs in a single package. In the modern world of aviation, the compound design is attempting to find its place once more as new military and civilian concepts are being forged by various industry players.
For the American military, it is seeking to develop a compound helicopter product that can consistently exceed 230 miles per hour while displaying inherently strong range, reliability, and survivability qualities for the modern battlefield. Funding initiatives have been enacted to prove a design sound for possible large-scale future application. Additionally, its fleet of venerable, yet aging, Sikorsky H-60 "Blackhawk" family will eventually be in need of a successor and several aviation concerns have thrown their hat into the ring to help develop a viable compound helicopter solution as a result.
Two avenues in the process have emerged - development of a whole new helicopter system or modification of the existing H-60 family product. For Piasecki Aircraft Corporation, this has become the latter as it has installed is proprietary Vectored Thrust-Ducted Propeller (VTDP) unit onto the tail stem of an existing Sikorsky SH-60 Seahawk, replacing the entire tail unit of the original (including its tail rotor). Wing mainplanes have also been added to the lower fuselage sides for the control necessary at the speeds encountered, speeds to go beyond that of the basic Seahawk helicopter (about 170 mph maximum). These changes have produced what was originally known as the YSH-60F, later becoming the X-49A in May of 2003. It carries the nickname of "Speedhawk" as an homage to its Seahawk roots.
Origins of the X-49 lay in an earlier technology demonstration initiative headed by the United States Navy (USN) hence the use of a Seahawk for the conversion process. The pair of General Electric T700 turboshaft engines were retained and the Piasecki unit added as well as the aforementioned wing mainplanes. The product was eventually passed on to the United States Army in 2004 and the finalized X-49A prototype completed a first flight on June 29th, 2007, going on to log nearly 100 hours of flight time while undertaking various airborne tests.
The compound helicopter approach was also the focus of the abandoned Lockheed AH-56 Cheyenne attack helicopter product which proved it a faster helicopter attack platform than anything in existence at the time. Piasecki also holds a history in such work dating back to its testing of the compound Piasecki 16H-1A during the 1960s which managed to clock speeds in excess of 225 miles per hour.
The X-49 continues active testing and development as of 2015. It features a listed maximum speed of 167 miles per hour (though it has surpassed the 207 mph measure in testing) with a range out to 440 miles, a service ceiling of 19,000 feet, and a rate-of-climb of 700 feet per minute. Beyond its crew of three, it can seat up passengers in the existing Seahawk cabin at midships. The aircraft utilizes a four-blade main rotor assembly, Fly-By-Wire (FBW) control system, and several aerodynamic refinements along its structure. The product has successfully met all of its Phase I milestones.