Bell Textron has completed wind tunnel testing for its rotor-folding aircraft as part of the Defense Advanced Research Projects Agency (DARPA) Speed and Runway Independent Technology (SPRINT) program.
This testing phase follows the Stop/Fold rotor system assessment conducted at the Holloman High-Speed Test Track in New Mexico in 2023.
The wind tunnel program focused on validating the aircraft’s stability and control during the rotor fold and unfold sequence while in flight.
These evaluations and earlier rotor tests indicate that this aircraft’s development is now complete, and it is ready to proceed into the flight demonstration phase of the DARPA SPRINT program.
Rotor-folding aircraft
“Having completed folding rotor transition testing at Holloman Air Force Base last year, we are pleased to advance to this next phase of testing,” stated Jason Hurst, executive vice president of Engineering at Bell.
“The ongoing development of our Stop/Fold systems aims to enhance the capabilities of vertical lift aircraft for operations in various environments.”
In Phase 1B of the DARPA SPRINT program, the initiative aims to design, construct, and test an experimental aircraft (X-Plane).
This aircraft will demonstrate key technologies and integrated concepts for enhancing aircraft speed and achieving runway independence for future air mobility and combat operations.
The Stop/Fold configurations are designed to combine high-speed capabilities with the flexibility of runway-independent operation.
This approach aims to support Agile Combat Employment (ACE) from various locations, addressing mission requirements in environments with limited runways and significant distances.
It is also anticipated that this aircraft will benefit Special Operations, improving the effectiveness and safety of complex missions.
VTOL
Bell has unveiled updated renderings of its crewed and uncrewed aircraft concepts featuring the innovative “Stop/Fold rotor system.”
Initially introduced in 2021 for the US Special Operations Command’s High-Speed Vertical Take-Off and Landing (HSVTOL) project, the SPRINT design is geared toward fulfilling future US Air Force special operations aviation needs.
The Stop/Fold rotor system allows for vertical take-off, landing, and hovering capabilities while featuring a tilting rotor that can be retracted for improved aerodynamic efficiency during high-speed flight.
A traditional jet propulsion system complements this design for forward thrust.
This design approach contrasts with the V-22 Osprey’s larger proprotors, which are bulkier and used for vertical and forward flight.
Bell also has a legacy of working on tilting rotor technologies, including the V-280 Valor and the Future Long-Range Assault Aircraft (FLRAA), which is set to succeed the Army’s Black Hawk helicopters.
The latest renderings showcase the larger crewed Stop/Fold design and a redesigned uncrewed concept, although the smaller crewed variant is not displayed.
Notably, the uncrewed design features new top-mounted air intakes, enhancing vertical and low-speed flight airflow. The central fuselage and wingtip rotor pods have also been streamlined for better performance.
The crewed design concepts display a streamlined shape in the fuselage and wingtip rotor pods.
The fairings over the proprietor hubs have significantly increased in size. Additionally, the top-mounted air intake seen in previous renders is now absent, indicating that there may be a new flush intake on the top instead.
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