Steve Trimble The Army’s No. 1 aviation modernization priority is getting a reality check.
Steve Trimble
The Sikorsky Raider X, pictured escorting two Defiants, is designed with a coaxial rotor and a pusher propeller, which requires a large transmission. Credit: Sikorsky Concept
The U.S. Army’s project manager now says the original set of performance requirements for a future rotorcraft that represents the aviation branch’s top modernization priority are not compatible with the laws of physics.
Moreover, the original schedule set for the competitive flyoff of the Future Attack Reconnaissance Aircraft (FARA) also will lead to both industry teams using nonproduction-representative hardware, negating an acquisition strategy premised on minimizing airframe development during the engineering and manufacturing development (EMD) stage.
Speed and rotor disc requirements may be flexible
Bell and Sikorsky prototypes pass 50% completion milestone As the Bell 360 Invictus and Sikorsky Raider X teams pass a 50% completion milestone for their competitive prototypes, Army officials in the Program Executive Office for Aviation (PEO-A) are grappling with the need to amend requirements and strategies set in 2018 by the Army Futures Command.
The bottom line is that the Army’s requirements for speed, maximum takeoff weight, engine power and rotor disc diameter are incompatible with each other, says Col. Greg Fortier, the FARA project manager in the PEO-A office.
“There’s no version of the world that exists in physics, [except] maybe on a different planet, where the speed at range, endurance at range and payload all exist in a 14,000-lb. helicopter—not at what we’re asking [for],” Fortier said July 22 at the Army Aviation Industry Days. “Let’s just be real about that.”
The Army’s acquisition strategy also emphasizes focusing primarily on mission systems during the EMD phase, with the aircraft themselves transferred mostly intact from the competitive prototypes. But the timing for the flyoff, which begins in fiscal 2023, means the aircraft will require substantial modifications during the EMD phase, Fortier said.
“First flight by 2023 for industry meant that they have to go out with long-lead [orders] in 2019,” Fortier said. “Those are just facts. They’re unemotional. They are simply the brass tacks of the situation.
“As an industry partner getting ready to go fly for a competition in 2023, you’re going to bake in some extra structural margins,” Fortier added. “The aircraft is going to be a little heavier than you want it to be, and you’re not going to be final on your designs. So there’s going to be some modifications.”
The project management team also is reviewing the performance requirements to make realistic adjustments. The Army’s conditions include a minimum cruise speed of 180 kt., maximum takeoff weight of 14,000 lb., rotor disc diameter of 40 ft. and engine that generates 3,000 shp. That combination of metrics is impossible to design in a single rotorcraft.
“There’s no version of the world where you can go 180 [kt.] at 14,000 lb. on a 3,000-shp engine and a 40-ft. rotor disc,” Fortier said. “So we’ve got to figure out what that is and what it means and how we play that out.”
Some experts had anticipated that the Army’s original requirements for the FARA could not be met. In 2018, the Vertical Flight Society published an editorial in the organization’s magazine calling on the Army to make the contractual specifications realistic.
“Col. Fortier is right,” said Mike Hirschberg, executive director of the Vertical Flight Society. “When the Army published the FARA request for proposals in 2018, the technical community was skeptical that all the requirements could be met, particularly on such a short timeline. We made it clear that for FARA to be successful, the Army needed the willingness to compromise between cost, schedule and performance.”
That said, the project office is open to adjusting some of the performance requirements. The engine, GE Aviation’s T901 turboshaft, is likely nonnegotiable as the common engine for the FARA and upgrades to the Sikorsky UH-60M and Boeing AH-64E. But Fortier allowed for possible changes to the speed and rotor disc diameter.
“[We’re] learning a ton about speed, learning where the sensitivity is in speed,” Fortier said. “Learning that [adjusting between] 177 [and] 181 [kt.], what does that cost?” The same analysis is considering adjustments from 181 to 185 kt., he said.
The requirement to keep the rotor disc diameter under 40 ft. also could be changed. The Army set the limit to keep the rotor disc size of the FARA similar to the retired Bell OH-58D Kiowa Warrior, which was 35 ft. 4 in. in diameter. Keeping the rotor disc as small as possible is important for the FARA to fly in tight spaces.
“We don’t really want to go higher than that,” Fortier said. “But we understand the power of that if we can turn that knob along the way. Those are decisions that will be made by our senior leaders.”
The speed requirement for the FARA demands internal weapons bays and landing gear retraction systems, which led Bell to design the 360 Invictus with a single main rotor and stub wings. Credit: Bell Concept
The Army Futures Command set the original requirements to meet lofty performance goals for the FARA, which is considered a key piece of the Army’s multidomain operations strategy. By flying under 100 ft. and relatively fast for a helicopter, the FARA is expected to destroy an enemy’s outlying air defense radars, opening corridors for the Future Long-Range Assault Aircraft to insert ground troops deep into enemy territory.
The operating concept generated a set of top-down performance requirements, with apparently little regard for their feasibility in combination. Fortier cited the example of the approach to specifying the T901 engine—also known as the Improved Turbine Engine Program (ITEP)—for the FARA as an example of that approach.
“Ideally, if you’re going to design a feature future helicopter, you’re going to want to design the helicopter around the requirement, get the design and then figure out how much shaft horsepower you need to go from there,” Fortier said. “In our case, we’re powered by the ITEP. Again, all for the right reasons: for affordability across the branch and Army Aviation. So, now we’ve got a machine that needs to go fast with 3,000 shp, 40-ft. rotor disc, and a target weight of 14,000 lb. The design box just got very, very small.”
Perhaps anticipating a thrust challenge, Bell and Sikorsky added supplementary power units to their designs. But Fortier’s remarks suggest total thrust is still insufficient to meet the combination of payload, enudrance and speed requirements for the FARA.
The retreating blade stall phenomenon limits the forward speed of most conventional helicopters to around 175 kt. or less, but the operating concept for the FARA demanded a slightly faster helicopter. That led Bell to design the 360 Invictus with a single rotor and small wings. Sikorsky, meanwhile, proposed the Raider as an eight-blade coaxial rotor with a pusher-propeller configuration. Each of those design decisions, however, comes with a large weight penalty.
“It’s very counterintuitive, but 180 kt. is heavy. Why is it heavy? It’s heavy because you [have] to retract the [landing] gear,” Fortier said. “Maybe you’ve got to put some [share of the lift] on the wings if you’re one of the single-rotor helicopter designs. Or maybe you put eight rotor blades on because you have extra technology. Maybe those extra rotor blades are pretty big [and] pretty heavy. That transmission then becomes very heavy to get the speeds, and you’ve got a pusher-prop in the back. Again, nobody’s fault, right? It’s just physics at the end of the day.”
Despite the size, weight and performance challenges embedded into the FARA requirements, the project office believes competitive prototypes by both companies are off to a good start, along with GE’s first engine to test for the T901.
“There are a lot of things in the FARA program that keep me up at night,” Fortier said. “There are a lot of reasons why I wake up at 3:00 in the morning routinely. However, one of [them] is not [whether] we’re going to fly in 2023.”
The program also has eliminated all of the unknown design risks for the two FARA competitors, he said, adding: “We absolutely know all of the challenges that have to be accomplished.”
But that just means Fortier’s staff understands the complexity that lies ahead. In less than a decade, the Army must resolve the physics incompatibilities with the performance requirements, complete a competitive acquisition process and then execute a fast-paced development schedule with an aircraft that will require major modifications during the EMD phase. At the same time, the Army also is managing the acquisition the Future Long-Range Assault Aircraft and the modernization of the UH-60, AH-64 and the Boeing CH-47 Chinook.
“Most days, as I come to work, I think somebody’s going to jump out of my closet and say: ‘No, no, no, we’re just kidding. It’s really not this hard,’” Fortier said. “But it truly is really this difficult.”
