Sikorsky And Boeing Did “Hundreds Of Trade Studies” And Consulted Extensively With The Army To Refine Their SB>1 Defiant Compound Helicopter.

By Sydney J. Freedberg Jr.

Sikorsky and Boeing did “hundreds of trade studies” and consulted extensively with the Army to refine their SB>1 Defiant compound helicopter into the new Defiant-X, officially unveiled today. What’s the difference? The companies were cagey about details, and they said they may make further tweaks. But between their guarded statements and the keen eyes of fellow reporters on the conference call, we can highlight a few visible changes in the images released today:

Defiant-X has beefed up landing gear, having gained a nose wheel that the SB>1 Defiant lacked. (SB>1 had two large wheels forward and a small wheel on the tail – check out these close-ups). This almost certainly adds weight, but, the companies said, it will improve stability on rugged frontline landing zones.

Defiant-X gets rid of the prominent exhaust outlet that SB>1 had below its engine. That helps “reduce thermal signature,” the companies said in a statement. In other words, the new design sheds its engine heat in a different and less detectable way, making it stealthier against enemy infrared sensors and harder to target. The companies said they’re still deciding which specific engine to install. They’ll have to find the best balance of raw power, ease of maintenance, and heat.

Finally, Defiant-X has a more sharply angled nosecone and a pronounced ridge on the back half of the fuselage. Such “mold line” changes improve the aircraft’s “aerodynamic handling,” the companies said. There’s no word on whether they also improve its speed, a high priority for the Army – and an area where the SB>1 has lagged its rival, the Bell V-280 Valor.

The Sikorsky-Boeing team is competing with Bell to build the Future Long-Range Assault Aircraft (FLRAA), intended to replace the Cold War UH-60 Black Hawk with something faster, longer-ranged, and capable of carrying heavier loads. (FLRAA, in turn, is part of a wider Future Vertical Lift initiative to develop new aircraft from scout choppers to mini-drones). Both teams are already flying FLRAA prototypes, officially known as Joint Multi-Role Tech Demonstrators.

Bell’s took flight first. Their V-280 tiltrotor – a smaller, sleeker sibling to the battle-proven V-22 Osprey – celebrated the third anniversary of its first flight on Dec. 18. As of that day, Bell told me, it had flown more than 150 times for more than 200 total flight hours, reaching 350 mph (305 knots) in level flight.

The Sikorsky-Boeing SB>1 Defiant, by contrast, was delayed by manufacturing issues early on, notably with the gearbox and with the ultra-rigid rotor blades required by its compound helicopter design, which has dual helicopter-style rotors up front but a pusher propeller at the tail. SB>1’s been flying for a bit less than two years – first flight was March, 2019 – and has flown 31 times, for a total of 26 flight hours. It’s reached 242 mph (211 knots) in level flight, and the companies are aiming to max out around 288 (250 knots). Both the companies and Army officials have said there’ll be enough data to refine the design and make a fair comparison with Bell’s V-280.

Flight hours aren’t the only metric, Boeing’s Heather McBryan told reporters. “We’ve really leveraged all of the test data we have today as we move forward, so not only the test article [i.e. the aircraft], but our Power System Test Bed, our SIL [System Integration Lab], and our wind tunnel testing.” Those ground-based tests add up. The Power System Test Bed, basically a stripped-down Defiant bolted to the ground, has racked up 135 hours, while the System Integration Lab, which does computer simulations, has exceed 1,500.

What’s more, both the Sikorsky-Boeing team and Bell are now executing Competitive Demonstration & Risk Reduction (CDRR) contracts with the Army to refine the design. “During CDRR, we’ve done hundreds of trade studies and worked very closely with our Army partner,” Sikorsky’s Jay Macklin said.

Sikorsky and Boeing argue that their design – while admittedly slower than the V-280 – is a better fit for the Army’s actual tactical needs. Since a compound helicopter is still basically a helicopter, they argue, Defiant looks and behaves much more like the Army’s existing aircraft, requiring fewer changes to training, tactics, and even infrastructure such as hangars. Defiant’s bigger and faster than the current UH-60, but because its length and rotor diameter are almost the same, it fits in the same size landing spot.

V-280 is a tiltrotor, with two huge rotors mounted on an airplane-like wing, which makes it much wider an existing helicopters. But the aircraft is much shorter nose-to-tail. So, Bell execs have told me, the V-280 fits in the same landing spot as UH-60: It just has to turn 90 degrees. So you could still fit the same number of aircraft on, say, a soccer field; you’d just have to fly in from a different angle. But there will be spaces just wide enough for a UH-60 or Defiant to fly through – very, very carefully – where a V-280 can’t go.

Sikorsky and Boeing also argue that Defiant handles better than the V-280 in tight quarters at low altitudes, a critical consideration for Army pilots. Bell, not surprisingly, disagrees. Finally, Sikorsky and Boeing say Defiant can carry loads suspended under the fuselage – what’s called sling-loading – over longer distances. (See the dramatic artists’ conception below). V-280 has demonstrated the ability to sling-load objects, but it’s not been tested to its max weight or range. And neither company has released actual numbers for the amounts its aircraft can sling-load over various distances, so it’s impossible to make an independent assessment.

It’s equally difficult for reporters to assess claims like agility and how well the autonomy software assists the pilots, or to see which aircraft is cheaper and easier to maintain, or to check out the Modular Open Systems Architecture (MOSA) interfaces meant to make them easier to upgrade. Of course, the Army will get all that hard data as it decides between the two competitors. The service has scheduled its final choice for 2022, with the winning design entering service in combat units by 2030 – if the service’s post-COVID budget still can afford it.

This news was originally published at Breaking Defense.