Similarities With COMANCHE not Necessarily Coincidental
As a single-rotor helicopter, Bell’s candidate for the US Army’s Future Attack Reconnaissance Aircraft (FARA) requirement is more conventional in configuration than its Sikorsky RAIDER-X competitor, yet the 360 INVICTUS is an interesting flying machine in its own right – and closely tailored to the US Army’s requirement.
With its stealthy, diamond-shaped cross-section, tandem two-seat cockpit and ducted fan tail-rotor canted at a jaunty angle – which counters the natural tendency of a tail-rotor whose thrust axis is not perfectly aligned with the centre of gravity to put a roll angle on in the hover – the 360 INVICTUS bears a strong visual resemblance to the Boeing-Sikorsky RAH-66 COMANCHE. Bell’s promotional material even uses the Army’s ‘quarterback of the battlefield’ metaphor that became familiar during the LHX competition and then the COMANCHE programme. In biology, this would be called ‘convergent evolution’.
Bell stresses that the aircraft will exceed the 180kt speed requirement at maximum continuous power and full mission gross weight: it will also maintain Level 1 handling qualities up to 4,000ft in altitude and 95°F in the hover. With a 1,400lb payload, it boasts a combat radius of 135nm with more than 90 min on station.
INVICTUS’ single rotor has a lower profile than that of the coaxial system on the RAIDER-X. The four-blade rotor head is derived from the five-blade head of the 16-20 seat Bell 525 RELENTLESS, which uses a fully-articulated design. This means that bearings, rather than flexures, allow for all the pitching, flapping and lead-lag motions the blades must make.
Judging by published drawings of the 525 head, these appear to consist of a single large elastomeric bearing per blade, each positioned inside one of the rings making up the head structure and supporting the closed end of the U-shaped blade grip that passes through the ring, the blade bolting to the other end of the grip.
The powerplant is unusual in that the 3,000shp-class GE901 turboshaft is supplemented by a 588shp Pratt & Whitney PW207D1 that, in addition to its electrical power-generating duties, also provides extra mechanical power for the rotors when needed. Bell says it is used to boost cruise and dash speeds and hover capability, so it appears it will be in use much of the time. Perhaps this little-and-large approach allows both engines to operate in their most efficient load ranges more of the time, minimising fuel consumption.
Unencumbered by weapons, the wing is purely for lift, offloading the main rotor at high speed and leaving more control margin for manoeuvring. It may mean that the retreating blades operate farther from their stall limit at high speeds, allowing the aircraft to fly faster, complementing the blade shaping that minimises retreating blade stall at high speeds. Weapons and other stores are carried inside the fuselage in a large bay and are deployed from fold-out launchers.
Bell is pushing the INVICTUS as the as low-cost, low-risk option for the FARA requirement, which is borne out by its conservative design and the reuse of commercial helicopter technology. First flight is expected in the Autumn of 2022,
Peter Donaldson in London for MON