So, the field is complete – kinda.
Boeing has finally released some coy animations of their contender for the US Army’s Future Attack and Reconnaissance Aircraft (FARA) project. The timing has been tight: in March, the US Army down-selects just two vendors from the five proposals to award deign and production contracts to in order to produce prototype aircraft for a live ‘fly-off’ in 2023. Whereas the proposals from other manufacturers have been publicly released as animations or even full scale mock-ups at US Army events, such as the annual Association of the US Army (AUSA) conference held last fall, Boeing has, to leverage an analogy from Hollywood, only given us a number of atmospherically lit and knowingly edited ‘teaser trailers.’
Therefore, due to the limited nature of the images released, it’s hard to be definitive about what Boeing’s FARA competitor is. However, we can make some judgments about what it isn’t.
First, it’s clearly not a tilt rotor, such as the V-22 Osprey pictured above. We can hence deduce that absolute speed is not Boeing’s priority. This is sensible and appropriate, as the US Army has mandated a relatively modest threshold speed requirement of 180kts (only some 10kts faster than the venerable CH-47 Chinook) - significantly slower than the target speed of 250kts+ for the Future Long Range Assault Aircraft (FLRAA), another part of the Future Vertical Lift program.
Tilt rotors are an example of a high speed platform that has the ability to hover – Boeing has clearly understood that much of a Scout helicopter’s role is to move slowly, or even hover, to use its sensors (including the crew’s eyesight) to detect and classify potential threats or hazards. Tilt rotors are, comparatively, poor in the low speed flight regime as they are invariably “under-rotored” with high disc loading (increasing rotor downwash) as the rotors are compromised by the conflicting needs of providing enough disc area to sustain a credible hover whilst keeping the rotors to a small enough size to keep the wing to a containable size leaving enough gap between the blade tips and the fuselage when in ‘airplane’ mode. Tiltrotors also have issues with forward and sideways weapon arcs when in forward flight – not ideal for a platform in the Scout and Attack roles.
With the decision not to go tilt rotor, other approaches to increasing speed and manoeuvrability are required – and these all involve design techniques known as ‘compounding.’ There are two generic compound types.
Firstly, there is ‘lift-compounding.’ Bell’s ‘Comanche lite’ Invictus FARA proposal (illustrated above) exploits this design technique. Fitting a traditional looking wing to a rotorcraft may appear counter-intuitive, but it produces several significant advantages. The key gain is the lift that the wing produces in forward flight, especially at high speed. This lift reduces the amount of lift the rotor system is required to generate, enabling the angle of attack (AoA, angle between the air flow and the pitch angle of the blade) to be reduced. This has two benefits; firstly, reducing AoA reduces the amount of engine power required to maintain the rotor at optimum rotational speed – this has benefits in engine life, fuel consumption and frees up more power for speed or powering high energy sub-systems. Secondly, reducing lift on the advancing blade enables a commensurate reduction in pitch on the retreating blade – avoiding a condition called “retreating blade stall’ where elements of the retreating blade have too high AoA in an attempt to equalise lift across the disc and ‘stall’. Retreating blade stall is one of the key factors in limiting forward flight speed in rotorcraft. A wing also, clearly, gives you real estate to mount weapons, sensors and extra fuel tanks.
However, it’s not all good news. What you gain at high speed, you lose in the hover. The wing obstructs the rotor downwash, reducing hover efficiency by a significant margin. Wings also increase weight, radar cross section and visual signature. Aerodynamic studies also hint at the generation of a significant amount of interference drag where the wing attaches to the fuselage. Drag is not normally a huge issue for relatively slow-moving rotorcraft, but if you’re trying to be that bit quicker (180-200kts vs 140-160kts) then it starts to become an issue as the required power increases – and that’s not good for a single engine aircraft.
Looking at the first ‘teaser shot’ released, from the head-on aspect, it could be considered that the Boeing FARA has wings. However, the most recent video gives a clearer view, and although fitted with small (possibly retractable) stub wings for weapons, there does not seem to be an appreciable lift generating wing on the fuselage. That’s not exactly the case for the tail.
It’s now clear from the latest video released, “For the Future.” that Boeing’s FARA uses the “other” major method of compounding - ‘thrust compounding.’ This usually requires the use of a propulsor propeller mounted at the rear of the fuselage – though the Airbus X3 Racer concept aircraft had twin ‘tractor’ propellers ‘pulling’ the aircraft to a higher speed (though, technically, the X3 was a hybrid compounded design with both wing and propellers). Thrust compounding literally ‘pushes’ the aircraft to a higher speed, potentially offloading the rotor in required pitch / AoA and opening the door to slowing the rotor down when established in the cruise.
This configuration has none of the disadvantages of having a wing; it is efficient in the hover and low speed regimes – important for a Scout aircraft – and it has a lower radar and visual signature.
The Boeing design has a tail mounted propulsor at the rear of the fuselage and a significant horizontal stabiliser and noticeably down-angled conventional tail rotor. This arrangement may be an attempt to keep the aircraft in a level attitude in both high speed flight and in the hover. At high speed, a level attitude is more aerodynamically efficient – requiring less power and fuel to achieve the same speed/range. Even my old ride, the venerable CH-47 Chinook, employed some fairly rudimentary techniques (using an extra control jack to ‘tilt’ the rotors in a speed-scheduled manner) to achieve a level floor at cruise speed. The tilted tail rotor, conversely, may help direct thrust towards the ground in the hover, helping to keep the aircraft level for better crew look-out, sensor picture and, ultimately, weapon release. It also probably ‘buys back’ a few percentage points in torque required to hover.
However, thrust compounded rotorcraft are, essentially, helicopters trying to fly fast. Therefore, they are good at the ‘traditional’ helicopter parts of the spectrum – hover and low speed agility – but they face significant challenges going at speeds approaching that of a tilt rotor. They are also significantly challenging in terms of technical integration, especially regarding the aerodynamic interaction between the conventional tail rotor and propulsor and vibration.
The FLRAA competitors are a case in point: Bell’s offering, the V-280 Valor, exploits and improves upon tiltrotor technology matured by the V-22 Osprey program. It has already exceeded all of the test requirements laid down by the US Army, exceeding 300kts airspeed and even flying in an optionally manned mode.
By contrast, its competitor, the Sikorsky/Boeing SB>1 Defiant, a coaxial rotor, thrust compounded design, has been plagued with design and development issues and has only recently exceeded 100kts in flight, despite not having to cater for a tail rotor due to the counter-torque effect of the coaxial main rotors. It is significantly behind the V-280, largely due to the fact that no thrust compounded helicopter has entered series production, therefore there is a commensurately smaller pool of knowledge to draw from. It might explain why Boeing has reportedly conducted wind tunnel testing on a thrust-compounded concept of the AH-64 Apache, which was trailed a few years back – trying to garner as much information to de-risk the FARA design.
Moving on from the basic aircraft configuration, there appears to be a lot of conventional tech on display. A multi-barrelled nose gun, a nose mounted EO/IR (possibly head-tracked to the pilot) and the usual Missile and Radar warning sensors and communications antennae. What is intriguing though is that, to my mind at least, no definitive shot of a two-man cockpit. It’s clearly not a side-by-side cockpit, and if it is two crew a tandem arrangement – but, have Boeing offered the US Army something their competitors haven’t – a single pilot machine? Why would they consider doing it?
Firstly, the Army is not unique in struggling to recruit, train and retain sufficient pilots. The civil aviation market is booming again, and as swathes of pilots trained in the Cold War hit mandatory retirement age there are vacant slots in airlines all over North America and beyond. Secondly, people are expensive. Even in barracks, soldiers need to be paid, clothed, fed and housed – that’s before the costs of professional training and, in the case of aircrew in particular, staying in flying practice. Even when they leave, the cost doesn’t end due to pensions, medications and, potentially, resettlement training or College. Thirdly, the military is finding it increasingly hard to entice well-qualified and fit young people to serve. Many are more attracted to the good salaries and workstyles of IT and other technology industries.
All these factors, plus the inexorable growth in the use of Unmanned platforms and Artificial Intelligence, will lead to a gradual shrinkage in the size of the US military, and the Army in particular, over coming decades. Finally, the ‘Scout’ role as originally envisioned was always a single-seat mission. Growing out of observers in Piper, Stinson and Taylorcraft light aircraft in World War Two, the Scout helicopter truly came of age in the Vietnam war with the OH-6 Cayuse (better known as the Loach) and the OH-58 Kiowa both being single pilot aircraft, though often also crewed by a door gunner if weight and space permitted.
It would be a bold move by Boeing to go single pilot. I would expect the design to have an optionally manned mode, regardless of the number of seats, and to be ready to accept any number of options for Manned / Unmanned Teaming (MUM-T) operations with either self-deployed or third party launched UAVs. The role of the Scout, after all, is mainly to find the enemy and hand-off targets to other assets – in the Army context FIRES (via artillery or long-range rocket) or a partnered AH-64E. In the future ‘Combat Cloud’ we should expect the FARA to also interface with other assets, such as F-35 or Naval Gunfire Support (NGS).
Boeing are promising a ‘full reveal’ imminently. Once they do, it will be up to the US Army to decide which two of the five designs get the lucrative prototype contracts. Then, finally, the race to finally replace the OH-58 Kiowa will be on the home straight. It will be interesting to see how the politics of it plays out, and whether The Hill will direct the Army to split the FARA and FLRAA awards to maintain some breadth in the industrial base and keep at least two manufacturers in the military helicopter business.
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