RAIDER X and INVICTUS Graduate to Next Phase
As MON predicted on 9 March (https://www.monch.com/mpg/news/air/6782-fara-an-embarrassment-of-riches.html), the US Army has shortlisted Bell’s 360 INVICTUS and Sikorsky’s RAIDER-X prototype designs to go forward to the next phase of the Future Attack Reconnaissance Aircraft (FARA) competition to select a new-generation armed scout helicopter,.
The service’s 25 March statement gives no reason for the selection of these two, nor for the rejection of the offerings from the other teams offering solutions. It merely reiterated its need for an aircraft capable of operating in complex airspace and degraded environments against peer and near-peer adversaries with networked air defences.
“The Future Attack Reconnaissance Aircraft is the Army’s number one aviation modernization priority and is integral to effectively penetrate and dis-integrate adversaries’ Integrated Air Defense Systems,” stated Dr Bruce D Jette, Assistant Secretary of the Army for Acquisition, Logistics and Technology.
This carries echoes of Desert Storm almost 30 years ago, when APACHEs fired the opening shots with HELLFIRE missiles against Iraqi air defence radars. The Army also wants a dedicated reconnaissance attack aircraft, compact enough to hide in radar clutter and to negotiate the urban canyons of megacities.
Bell and Sikorsky are offering very different aircraft. The 360 INVICTUS is essentially a conventional helicopter design with a wing. Bell did not offer a FARA candidate based on its advanced tiltrotor technology, because it decided that it was not possible to package a tiltrotor that met the requirements within the stipulated maximum 40ft rotor span. RAIDER-X is the latest in a line of more avantgarde coaxial rotor compound helicopters, whose lineage stretches back to the 240kt S-69 (XH-59) Advancing Blade Concept (ABC) technology demonstrator of the 1970s and 80s.
ABC is a method of eliminating the retreating blade stall problem that limits conventional helicopters’ speed. The solution reduces the pitch on each blade as its arc carries it backwards with respect to the helicopter’s direction of travel, offloading it so that it neither stalls nor generates much lift or drag. ABC, therefore, uses only the advancing blades to generate lift.
More recently, RAIDER-X is a descendent of the X2 that proved new rotor blades, hub drag reduction, fly-by-wire, active vibration control and integrated auxiliary propulsion technologies, the latter a pusher propeller connected to the drive system through a clutch, completing its flight test programme in 2008-2011. It is also a cousin to the S-97 RAIDER six-place light tactical prototype and the larger SB-1 DEFIANT, which is a prospective BLACK HAWK replacement sized for 12 fully-equipped soldiers, both of which are in flight test.
Hazarding another educated guess, MON expects RAIDER-X to prevail in the FARA competition, largely because of advantages that its configuration provides over a conventional helicopter. The pusher propeller allows it to accelerate and decelerate hard while remaining level, giving it more options in confined spaces and minimising threat exposure. The propeller also allows very fine control of speed in the dive. Sikorsky also says that its very stiff main rotor system enables it to turn much more tightly. Finally, its claimed cruise speed of 200kt, which the S-97 has achieved, is 20kt higher than that of the INVICTUS. [MON will examine INVICTUS in a little more depth in the next several days. – Editor]
Peter Donaldson in London for MON