X-49 Speedhawk Vectored Thrust Ducted Propeller (VDTP)

As of early 2002 the X-49 designator had been skipped because DARPA requested the X-50A designation for the Dragonfly Canard Rotor/Wing demonstrator. DARPA reportedly wanted the number 50 under the theory that the CRW would be the first true 50/50 marriage of helicopters and high speed fixed wing aircraft.

The X-49 was an experimental compound helicopter developed to demonstrate the ability to increase the speed of existing helicopters to 200kt (360km/h). In October 2000, a demonstration contract was awarded by the US Naval Air Systems Command to Piasecki Aircraft. A Sikorsky YSH-60F was modified as a testbed for the Vectored Thrust Ducted Propeller (VTDP) system. During 2004 the VTDP program (now X-49A) was transitioned from the US Navy to the US Army's Aviation Applied Technology Division. The X-49A made its first flight on June 29, 2007.

The Piasecki compound helicopter used a vectored thrust propeller instead of a tail rotor. It also has short wings at the bottom of the fuselage for lift in forward flight. The propeller provided vectored thrust when the aircraft is in hover and acts as a rear propeller in forward flight. The vectored thrust in forward flight allowed a much more horizontal level of attack for the aircraft. There did not appear to be any new technology here, but the concept was evaluated by the Army. Piasecki believed the concept will allow increased speed and range and will reduce fatigue loads and resultant operation and support costs. Piasecki believes this concept is easily scalable and has significant growth potential.

All initial Phase I contract milestones were accomplished and the results met or exceeded all program objectives. The first phase of Speedhawk flights was limited to the existing Seahawk flight envelope, but after a drag clean-up (including rotorhub fairing and retracting gear) and installation of a third engine to help drive the VTDP, Piasecki had plans to push the X-49A up to 200kt. The VTDP is a descendant of the “Ring-Tail” ducted propeller used on the Piasecki 16H-1 Pathfinder, which first flew in 1962. The Pathfinder utlimately achieved a speed of 225mph.

In combination with a lifting wing, this technology unloads the rotor, allowing the helicopter to fly 50% faster, twice as far, is more maneuverable and reduces vibration and fatigue loads, improving reliability and reducing life cycle costs. This Army Advanced Technology Demonstration (ATD) program demonstrated potential improvements in speed, range, survivability and reliability, addressing the Army’s Future Force requirements for greater rotorcraft operational reach and sustainability.

The SpeedHawk (X-49A) design, development and flight test program was a primary example of Piasecki Aircraft Corporation [PiAC] engineering, manufacturing, and systems test capabilities. This program, initiated as an SBIR Phase I effort has resulted in the current Army Sponsored Advanced Technology Demonstration (ATD) program. Successful execution of this program has demonstrated PiAC’s core capabilities to rapidly design, fabricate, assemble’ and accomplish the systems qualification tests necessary to achieve the test program objectives within very limited budgets.

A `compound` aircraft is an aircraft that includes features of both fixed wing aircraft and rotary wing aircraft. Although the advantages of a compound helicopter are well known, no compound helicopters have been placed in regular operation in commercial or military fleets. For attack helicopter missions, compound helicopters such as the AH-56A Cheyenne (propulsor and wing) and S-67 Blackhawk (wing) have been developed in response to the perceived need for greater speed and range. For utility and troop transport missions, compounds such as the X-49A Speedhawk (propulsor and wing) have been developed. Other recent studies, such as Sikorsky’s X2 demo (propulsor) have also centered on the compound helicopter configuration as potentially desirable.

A compound aircraft offers several advantages over a conventional helicopter. Those advantages include achieving higher flight speeds and delayed onset of retreating blade stall and leading blade compression effects. By compounding the rotor, the rotor may be offloaded at higher speeds, with advantages in reduced power and potentially reduced loading on the rotor dynamic components. A compound helicopter typically achieves higher speed, better cruise efficiency, and can operate at higher altitudes. The wing can also provide a convenient mounting location for external stores and improve the maneuver performance in forward flight. Typically, compound helicopters pay penalties in hover performance due to increased download and power losses associated with the auxiliary propulsor, plus the extra weight of the propulsor and main wing. Operationally, the wing can be an issue for storage and transport, and can impede the egress of passengers in some circumstances.

The compound aircraft includes the elements of a helicopter, including at least one main rotor and a mechanism to overcome the torque response of the rotating main rotor. The compound aircraft also includes elements of a fixed-wing aircraft, such as a wing. The wing may be equipped with ailerons, flaps or a combination of flaps and ailerons known as `flaperons.` The compound aircraft may be equipped with a separate thrust mechanism to drive the aircraft forward, such as a propeller in a ducted fan. Through the use of appropriate vanes or sectors that change the configuration of the duct, the ducted fan may serve as the mechanism to overcome the torque response of the rotating rotor blades and to provide yaw control.

Although highly effective for conventional helicopter types, a vectored thrust ducted propeller tail assembly configuration is particularly effective for use in compound helicopters in which the rotor is unloaded in the high forward speed range with lift being provided by the fixed wing and forward propulsion being provided by the thrust of the shrouded propeller.

Yaw agility is of great importance in military rotary wing aircraft in which the aircraft must be turned to the proper heading in aiming its armament. Under combat conditions the ability to quickly change heading to the direction in which the armament must be aimed is often critical, hence military requirements necessitate military aircraft being able to establish an angular turning acceleration that will turn the aircraft 180 degrees in either direction within a few seconds under all flight conditions including those involving low rotor shaft torque and a power off mode of autorotation. Also, combat maneuvering agility is greatly improved by an ability to establish a high rate of acceleration or deceleration of aircraft speed along the line of flight.

Combat maneuverability for military rotary wing aircraft also involves an ability to rapidly accelerate or decelerate the forward motion of the aircraft. The ring tail configuration of shrouded propeller ring tail aircraft have the capability of rapid acceleration while in the forward high speed mode through rapidly increasing propeller pitch but forward speed deceleration is dependent upon aircraft drag which is a fixed amount and must be supplemented if a retardation rate of speed greater than that provided by the aircraft drag is required for combat maneuvering.