The U.S. Defense Advanced Research Projects Agency (DARPA) recently announced that the high-speed tilt-rotor aircraft it developed in partnership with Bell Textron has officially received the "X-76" designation, becoming the latest member of the U.S. X-plane program. The aircraft is promoted by DARPA's "High Speed and Runway Independent Technology" (SPRINT) project, which aims to combine the short take-off and landing capabilities of helicopters with the high-speed cruise performance of jets on the same platform.
Since its inception with the Bell X-1 in 1946, the U.S. X-plane program has been mainly used to verify cutting-edge aerospace technology rather than mass-produced aircraft directly in service. These test platforms are usually very small in number and have a higher tolerance for test flight risks. The number of X-76 is not a simple extension, but a special response to the 250th anniversary of the founding of the United States in 2026, which has symbolic significance.
Currently, the X-76 has completed the concept and overall design stages. With the official X-machine number obtained, the project will enter the second stage of detailed design and prototype manufacturing. According to the plan, the third phase of flight testing is expected to start in 2027. Its technical positioning is to fill the capability gap between rotary-wing aircraft such as the V-22 "Osprey" and high-performance jet fighters.
The X-76 is designed to "take off and land from a field improvised site like a helicopter and fly at high speeds like a jet" to break the reliance of traditional platforms on runways. Traditional jets generally require a complete runway to operate, while rotorcraft can take off and land in the wild, but their high-speed performance is limited. The SPRINT program hopes to eliminate this trade-off with the X-76, providing new options for battlefield mobility and responsiveness.
The core problem faced by current tilt-rotor aircraft (such as the V-22) is that the large-size rotor not only provides lift and thrust, but also becomes a serious source of drag at high speeds, and the higher the speed, the greater the drag. Taking the V-22 as an example, its top speed is about 270 knots (311 mph, 500 km/h), which is limited by the aerodynamic characteristics of the rotor.
The ideal solution proposed by DARPA is to "get rid of" the drag caused by the rotor itself during high-speed level flight. The idea of the X-76 is to stop the rotor rotation after accelerating to a certain speed and fold the blades back onto the nacelle shape so that they adhere to the nacelle envelope with a streamlined shape. At the same time, the engine operating mode switches to full jet propulsion. To this end, the X-76 no longer uses the complex mechanical transmission and reduction gearbox of the V-22, but adopts a new propulsion configuration that can support two different sets of thermal cycles: one for vertical takeoff and landing hovering, and one for subsonic high-speed cruising.
During the vertical take-off and landing and hovering stages, the X-76 works in a traditional tilt-rotor manner, with a high-performance turbine core driving the rotor through a transmission shaft to provide lift. As the body accelerates, the wings begin to share the lift. When the speed increases to a critical range of about 150–200 knots (173–230 mph, 278–370 km/h), the transmission system cuts off the power input to the rotor, and the rotor blades “feather” at an angle and fold back, turning into a nacelle with a good aerodynamic shape.
In this process, the engine thrust is redirected to the conventional jet nozzle output at the rear of the nacelle, thereby entering pure jet propulsion mode. Without having to overcome the massive drag of the rotors, the X-76 is designed to reach target speeds in excess of 450 knots (518 mph, 833 km/h). The prototype is expected to carry a payload of about 1,000 pounds (454 kilograms) and has a target range of about 1,000 nautical miles (1,151 miles, 1,852 kilometers).
Lieutenant Colonel Ian Higgins of the U.S. Navy, DARPA SPRINT program manager, said, “For a long time, runways have been both an aid to acceleration and a shackle that constrains fighter aircraft’s actions. They bring speed but at the same time create critical vulnerabilities.” He emphasized that the goal of SPRINT is not only to build an X-machine, but also to provide a new "option" for combat command - to achieve sudden deployment, rapid reinforcement and life-saving high-speed response capabilities on a global scale without the need for any runways.