Sikorsky S-97 Raider

yasotay

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sferrin said:
I almost hope they buy this if only to get the industry to see the wisdom of putting money into R&D again. Look how it paid off for Lockheed with stealth.
Getting new blood into the system is good. Keeping them, these days, is much more challenging. So unless the Army maintains a robust rotorcraft R&D effort , past the FVL effort (fat chance that), I think the atrophy of the industry will be even faster. Investors want fast return on their dollar. Young folk seem not to have the brand loyalty that kept people with a company for a career. It is not the industrial world that I grew up in any more. ~400 aircraft may be a big order in the future. Hope industry can sustain at those levels with that financial and social dynamic.
 

TomcatViP

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Past FVL, the road will be simple for all actors involved: mass transportation. Plenty of work, plenty of money for all folks.
the mil industrial effort is focus driven: that it has to be mass production, cutting edge R&D or both. It is not there for itself, by itself. Worker migrations is a sane and beneficial byproduct and happens on both ends.

my 2 cents only.
 

GWrecks

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TomcatViP said:
Past FVL, the road will be simple for all actors involved: mass transportation. Plenty of work, plenty of money for all folks.
the mil industrial effort is focus driven: that it has to be mass production, cutting edge R&D or both. It is not there for itself, by itself. Worker migrations is a sane and beneficial byproduct and happens on both ends.

my 2 cents only.
Reminds me of the response I once heard to "You're building death machines, wringing us of our money, etc."

"Give it ten years or so and it'll be in the hands of ungrateful civilians like yours."
 

TomcatViP

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I might have lost something in translation, sorry.. My point was that cutting edge engineering design does not belong to any civilian/military said-so ambivalence. It belongs to the people (who in any ways pays for it ;) ). One day it's North American Aviation, another SpaceX... That's where the fruit is.
 

sferrin

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TomcatViP said:
I might have lost something in translation, sorry.. My point was that cutting edge engineering design does not belong to any civilian/military said-so ambivalence. It belongs to the people (who in any ways pays for it ;) ). One day it's North American Aviation, another SpaceX... That's where the fruit is.
Yep. Best efforts can't capture what's between each individual's ears.
 

yasotay

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Raider is flying. With FVL Cap Set 1 now competitive, news will likely be scant.
 

yasotay

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Raider #2 was completed and continues the flight program.
 

TomcatViP

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Damage to the helicopter included collapsed landing gear, structural cabin damage, and dynamic component damage, including rotor blade tip separation of all 8 rotor blades.
[...]
As he applied collective and got airborne the helicopter rolled quickly left and then right, continuing with 2-3 roll reversals of increasing roll attitudes eventually exceeding estimated 60⁰ angle of bank. [...]
the upper and lower rotors intermeshed about the 1 o’clock position (as viewed from the cockpit), creating a cloud of blade fragments and gray dust.
[...]
The time between when the helicopter got airborne and landed hard was approximately 5 seconds.
[...]
All 8 rotor blades remained attached to their respective hubs and all rotor blade ends had been separated from each blade at approximately the same location along the length of each blade.
[...]
Sikorsky engineers reported that telemetry data showed the [rear] propeller was turning but provided no thrust.
[...]
Control law changes were introduced in late 2015 to improve ground to air transitions. The key update was changing the switch functionality at the end of the feed forward command path, from a transient free switch to an asymmetric fader switch and reset to zero the proportional path stick filter. The change from a transient free switch to a fader switch eliminated the 40% stick to head attenuation and replaced it with the full 100% proportional signal. The results of these two changes was an unintended increase of the cyclic stick sensitivity by 2.5 times during the transition to flight.

While pitch, roll, and yaw axis all share the same architecture, the helicopter’s lower roll inertia as compared to the pitch and yaw inertia, made the problem most apparent in the roll axis.
[...]
The larger than expected roll response to the pilot roll stick input was the effect of a flight control system design error that resulted in unintended changes in the pilot input sensitivity in the roll axis during the transition from ground control mode to flight mode.

During the ensuing dynamic response, the left main landing gear momentarily came in contact with the ground and aircraft transitioned from flight mode back to ground mode. Shortly thereafter, flight mode reengaged which in turn reset the mode transition timeline and re-introduced the erroneous control sensitivity gain. At this point the roll oscillations became extreme and the counter rotating blade tips contacted each other due to the gyroscopic moments generated by the high resulting roll rates leading to loss of lift, and a hard landing occurred.
Full NTSB document here: https://dms.ntsb.gov/public/62000-62499/62323/624201.pdf
 

sferrin

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Whisper quiet like a flying chainsaw. The X-2 was noisy as a mo-fo as well.

 
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Orionblamblam

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Whisper quiet like a flying chainsaw. The X-2 was noisy as a mo-fo as well.
I keep hearing that the A-10, due to its high bypass turbofans, is quiet. But a decade or so ago when my little neck of rural Utah was a playground for various military pilots practicing for Afghanistan, there'd come this low, LOUD rumble that you could pick up *mnutes* before an A-10 would eventually trundle by. It might be quiet by F-15-on-afterburner standards, but it's still loud as hell. And the Raider and X-2 are not only loud (presumably due to high tip speed and low disk loading), they are also unique... and certainly no faster than the A-10. Slow and lumbering though the A-10 is, it can certainly outpace even a Raider on turboboost. So these things won't exactly be sneaking up on the enemy, so long as the enemy is out in the boonies. In urban areas maybe it doesn't matter so much, what with all the other noise, but I do wonder if maybe going a bit slower with a less exciting chopper might be a better approach for a lot of rural missions.
 

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sferrin

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Makes you wonder what the SB-1 Defiant will sound like. I'm liking the V-280 more and more.
 

TomS

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Makes you wonder what the SB-1 Defiant will sound like. I'm liking the V-280 more and more.
Tiltrotors aren't that quiet either. I've had MV-22s flying around my house and they have a rather distinctive sound you can hear at least as far out as the H-60s. None of these options are going to be Blue Thunder.
 

yasotay

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At slower speeds without the prop going the S-97 is relatively quiet compared to other helicopters. The V-280 in helicopter mode is as loud as any helicopter. In airplane mode with the RPM down to 85% it is very quiet and only starts to be noticable at about 500 meters out. There is a noticeable low frequency noise that can be acquired further out if it is quiet with little wind. So the trade off is that one is quiet at low speed and one is quiet at high speed. So my money is on S-97 (FARA) for a scout, and V-280 (FLRAA) for lift.

On a side note I read that the S-97 did not make it over 200 knots as planned in the demo due to a glitch in the vibration software. Getting vibratory loads under control will be one of the main challenges for Sikorsky.
 

Hood

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Any helicopter is going to be noisy.
In perspective, my office is next to a busy motorway and you can still clearly here approaching Merlins, Apaches, Bell 212s and (obviously) Chinooks, with enough advance warning to leave my desk and get to the window to catch them flying past.
Oddly the quietest aircraft to have flown by and sneak up on me is a C-130J flying low a couple of times.
 

TomS

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At slower speeds without the prop going the S-97 is relatively quiet compared to other helicopters. The V-280 in helicopter mode is as loud as any helicopter.
This is probably why my experience around the V-22 is what it is. They were usually in partially transitioned configuration (rotors at about 60-70 degrees from vertical) when they went over my house en route to the Pentagon. So more helicopter than airplane mode.
 
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Sundog

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I've heard the Osprey and it was kind of loud in helo mode, but I remember one of the quietest aircraft I've heard was the XV-15 in forward flight and hover. Granted, it's disk loading probably wasn't very high, comparatively speaking.
 

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Terrain hugging also helps to attenuate the noise. The idea is that the aircraft will be heard only briefly before an overflight and in sight and that, in its forward sector. If you can hear it passing 200m from your position but can't see it (low alt), you won't probably be a great danger to them en-route for their mission.
 

yasotay

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Sikorsky Evolves Raider Into FARA Armed Scout Contender

Jul 4, 2019

Graham Warwick | Aviation Week & Space Technology

On a hazy Florida morning, a sleek black helicopter makes a high-speed pass, close to 200 kt., the sound of its tail-mounted propeller unique for a rotorcraft. Seconds later, the noise changes as the prop bites into the humid air and the helicopter reverses down the runway and into a tail-high pirouette, circling with its nose laser-focused on a spot on the ground.

This is Sikorsky’s S-97 Raider, being put through its paces before a select group of suppliers the company wants on board to support its bid to build the U.S. Army’s next armed scout—the Future Attack Reconnaissance Aircraft (FARA).

Sikorsky is one of five companies chasing two contracts to build prototypes for a competitive flyoff in 2023. If successful, FARA would be the fourth instantiation of Sikorsky’s X2 high-speed helicopter configuration comprising the original 6,000-lb. technology demonstrator, the 11,500-lb. S-97 Raider and the 33,000-lb. Sikorsky-Boeing SB-1 Defiant demonstrator also now in testing.

- FARA to be slightly bigger, slower than S-97
- Raider addressing drag, vibration challenges

This line of aircraft has its origins in 2005 when, seeking to differentiate itself from other helicopter manufacturers and after analyzing a range of configurations, Sikorsky returned to the design of its XH-59 Advancing Blade Concept (ABC) coaxial compound helicopter demonstrator. First flown in 1973, this achieved 238 kt. in level flight.

To overcome the ABC’s drawbacks of complexity, vibration and high fuel consumption, Sikorsky applied technologies developed over the intervening years, including carbon-fiber blades and airframe, fly-by-wire flight controls, integrated propulsion system and active vibration control. The result was the X2 demonstrator, which in 2010 achieved 250 kt. in level flight with power still in hand.

The next step was to use the configuration “to make something useful,” says Steve Weiner, director of engineering sciences and “father of the X2.” The result was the S-97 light tactical helicopter, designed around the Army’s Armed Aerial Scout (AAS) requirements. Sikorsky and its supplier partners launched an industry-funded program to build two prototypes, the first of which flew in May 2015.

AAS was canceled in 2013, with the Army blaming budget sequestration. It then retired the Bell OH-58D Kiowa Warrior armed scout that AAS was to replace, and its role was transferred to the Boeing AH-64E Apache. But the Army acknowledges the attack helicopter is not best suited to armed reconnaissance, and in 2018 FARA emerged as an urgent requirement to field 200 new armed scouts beginning in 2028.

On paper, Sikorsky looks well placed to win FARA. The company, its parent Lockheed Martin and its industry partners have invested about $300 million in the Raider program, says Chris Van Buiten, vice president of Sikorsky Innovations. The second prototype has exceeded 200 kt. in flight testing, and FARA would be an evolution of the S-97 configuration, not an entirely new design.

But Sikorsky faces a challenge. To enable a competition, the Army has trimmed its requirements for FARA, setting a minimum speed of 180 kt.—fast for a conventional helicopter, but slow for the X2. That speed requirement pits Sikorsky’s coaxial-rotor compound design against single-main-rotor helicopters and brings the affordability of its more capable, but more complex configuration into sharp focus.


Propulsor reverse thrust allows the Raider to point its nose at a target. Credit: Lockheed Martin

Tim Malia, Sikorsky’s director of future vertical lift–light, describes the company’s driving priorities for FARA as a triangle, with schedule, performance and affordability as the three sides. The Army has made clear that “schedule is king,” he says, but to win the company must also get the other two right.

Sikorsky should be in a good position on schedule. The Raider has already logged more than 55 hr. of flight testing, reaching 207 kt. in level flight and pulling 2g in maneuvers. The first prototype was damaged in a hard landing in August 2017, but the second aircraft has been transferred to the FARA team, and flight testing refocused on reducing the risk in Sikorsky’s proposal.

The Raider was showcased to industry partners (see video), and the press, on June 25 with a demonstration at Sikorsky’s development flight center in West Palm Beach. This highlighted unique capabilities enabled by the compound-helicopter’s combination of rigid coaxial rotors and tail-mounted propulsor.

There are two 34-ft.-dia. contrarotating four-blade rotors. The blades are stiff and the hubs hingeless so the rotors can be mounted close together, reducing drag. This configuration enables higher airspeed because the lift is generated by the advancing blades on each side, allowing the pitch of the retreating blades to be reduced and avoiding the blade stall that limits the speed of conventional helicopters.

Rotor speed is scheduled with tip Mach number, and reduces to a minimum of 85% rpm as airspeed increases to prevent the advancing blades from going supersonic. The distance between the tips of the upper and lower rotors is actively sensed. Two-thirds of the tip gap is to allow for blade flexing during maneuvers and one-third is the safety margin, says Bill Fell, senior experimental test pilot.

In the August 2017 accident, a flight-control software error led to a roll rate on liftoff three times greater than intended.

Gyroscopic forces caused the rotor tips to collide. “We fixed the error in the code and we now completely understand the physics,” he says. “Our job is to make sure it never happens again.”

The Raider is powered by a single 2,600-shp General Electric YT706 turboshaft. In hover and at low speed, power goes to the rotors and the propulsor is declutched. As speed increases and the propulsor is engaged, power shifts to the six-blade, variable-pitch pusher propeller. At high speed, 90% of the power goes to the propulsor, says Fell.

Pilots increase or decrease propulsor thrust using a “beep” switch on the central collective lever. This moves propeller pitch through its range of +50 to –20 deg. The rotors can take up to 1,800 shp of power, the propeller 2,200 shp. The engine cannot provide full power to both simultaneously, so the integrated digital engine and flight controls automatically maintain the rotor power required for lift and limit the propulsor to the excess power available.


Coaxial rigid rotors provide increased agility, as well as higher speed. Credit: Lockheed Martin

The Raider can fly at up to 150 kt. with the propeller disengaged. This allows the helicopter to dash at high speed then declutch the propulsor to reduce the noise on approach to the target. There is no anti-torque tail rotor. As the prototype demonstrated during two passes, there is a significant reduction in acoustic detectability when the propeller is disengaged.

The propulsor offers other unique capabilities. Conventional helicopters tilt nose-down to accelerate and nose-up to decelerate. Using forward or reverse thrust on the propeller allows the Raider to slow down or speed up with a level fuselage attitude. This is valuable when carrying casualties or passengers, says Van Buiten. Tactically, using the propeller can prevent the aircraft popping up into the sights of enemy defenses when flying over a ridge during high-speed nap-to-the-earth flight.

By using reverse thrust on the propeller, counteracted by forward thrust on the rotors, the S-97’s fuselage can be pointed downwards. Fell showed how this allows sensors or weapons to dwell on the target as the Raider circled, nose down. Forward thrust on the propeller, balanced by rearward thrust on the rotors, allows the fuselage to be pointed upward. These are capabilities conventional single-rotor helicopters do not have, but the value of which must still be demonstrated to the customer.

Although speed is no longer the focus of testing, Sikorsky still hopes to achieve the Raider’s 220-kt. design goal. “We underestimated the drag,” says Fell, “but there are dials we can tweak.” Sikorsky is preparing to install a new set of rotor blades, redesigned to reduce drag and vibration at high speed—another area of concern with the rigid-rotor helicopter.

In addition to being stiff, the Raider’s blades are complex, with changing thickness and chord across five different airfoil sections from root to tip.

Fell says vibration in the Raider at more than 200 kt. is similar to that in the UH-60 Black Hawk at 150 kt. But the potential impact of vibration on crew fatigue, aircraft systems and component lives is a concern, although the S-97 is equipped with an active vibration control (AVC) system. The new blades have been redesigned to improve aerodynamic efficiency, but how they attach to the hingeless hubs also has been modified, says Fell, to reduce the transmission of vibratory forces from the blades into the airframe.

The importance of minimizing vibration was illustrated during the demonstration flight. Fell says the flight plan called for a 200-kt. high-speed pass, but maximum speed was held to 190 kt. “We planned 200 kt., but a new script file in the AVC did not work out and there was no reason to push it,” he says. AVC actively cancels vibration by introducing opposing forces into the airframe.

Mitigating vibration is one example of risk-reduction testing the Raider will perform for the FARA team, which is nearing its preliminary design review. Because the Army has reduced its speed requirement, Sikorsky’s FARA design will be “detuned” relative to the Raider, but will still have more growth potential than a single-rotor helicopter already at the limits of its capability at 180 kt, says Malia.

With a larger, 39-ft.-dia. rotor system, a higher, 14,000-lb. gross weight and a single, 3,000-shp General Electric T901 Improved Turbine Engine, the FARA will be slower than the Raider, he says. There will be other changes. The fuselage will be stretched to accommodate two 80-in. long internal bays for weapons, air-launched unmanned aircraft and other mission payloads. There will be no external stores on FARA, Malia says, and these bays will essentially replace the Raider’s passenger cabin.

The fuselage will be tuned to minimize vibration, he says. Sikorsky has already selected Swift Engineering to build the airframe for the FARA prototype. Aurora Flight Sciences built the Raider fuselage, but is now owned by Boeing, and Swift manufactured the airframe for the SB-1 Defiant. Suppliers of other long-lead components are also already on board, Malia says, to meet a schedule that calls for contract award in March 2020, flight in first-quarter fiscal 2023 and the Army flyoff in the fourth quarter of that year.

The landing-gear arrangement is different on the FARA design, and there are changes to the propulsor. On the Raider, when the propeller is disengaged a limited-slip clutch keeps it turning at 200 rpm. This is to avoid heat damage to a blade by the engine exhaust, which is located above the tail, just forward of the prop. On FARA, as on the Defiant, the exhaust design is different and the propulsor will stop completely. Sikorsky could also redesign the propeller blades to reduce noise in high-speed flight.

For Sikorsky, tailoring its X2 configuration to the less-demanding FARA requirements is crucial to making it affordable.

But the company wants to preserve the design’s inherent growth capability—particularly in speed—because it sees this as a differentiator. After decades of false starts, the Army urgently needs an armed scout. A conventional helicopter could do the job, but Sikorsky needs the service to take the long-term view and pick a configuration still at the beginning of its evolution.
 

yasotay

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Test pilots say Sikorsky’s S-97 Raider handles like a sportscar

Posted on July 8, 2019 by Eric Adams

During a 20-minute test flight of the prototype S-97 Raider at Sikorsky’s West Palm Beach, Florida, facility — conducted in front of four journalists in the first such public demonstration — the unconventional coaxial-rotor aircraft performed routines that would make any helicopter pilot jealous.


Sikorsky originally developed the S-97 Raider for the U.S. Army’s Armed Aerial Scout program, which was canceled in 2013. Now, Sikorsky is developing a prototype for the Army’s FARA program informed by the Raider’s design. Eric Adams Photo

For instance, the single-engine Raider, which Sikorsky is using as a testbed as it develops its entry for the U.S. Army’s Future Attack Reconnaissance Aircraft (FARA) competition, used reverse pitch on its rear propeller to maintain a nose-down attitude in a steady hover — as if targeting weapons or sensors to the ground, searching for injured hikers, or inspecting a landing zone. It’s a move no other helicopter can execute without drifting. Then it maneuvered briskly and perfectly above the runway, performing multiple tight patterns with an agility made possible by its rigid, stacked rotors, which rotate in opposition to each other, counteracting torque and negating the need for a tail rotor. Most other helicopters, with their hinged, flexing blades, can’t come close to that sort of precision.

Test pilot Christiaan Corry described the S-97 Raider’s performance characteristics in a way that seemed to suggest that he and his program colleague, Bill Fell, who commanded the flight, might actually have the best job in the world. “It really handles like a sportscar,” Corry said. “We don’t have an inverted oil or fuel system in this aircraft, but aerodynamically, it could fly inverted all day long. We demonstrated that in the simulator, and are proving every day that Raider can do so much in the air. The rotor system acts like a wing, and we can do these aerobatic maneuvers that just aren’t things helicopters could ever do before.”

The test seemed to validate that point. As the crew cycled through its test card on that characteristically sweltering Florida morning, we also saw Raider perform turns in half the distance of other helicopters and accelerate and brake with the fuselage completely level, thanks to the rear propeller that sits in place of the tail rotor. Whereas conventional helicopters must tilt their main rotor discs forward in order to accelerate — and backward to slow down — in the Raider, the rear propeller can be used to push the helicopter to faster speeds and also, with the blade’s pitch reversed, practically stop it on a dime. Engaging reverse pitch while simultaneously tilting the main rotors forward enables the nose-down hover. The Raider can also hover with its nose pointed skyward, by tilting the main rotors aft while generating forward thrust with the prop.


The rear prop on the S-97 Raider enables both high speed and unique maneuverability. Eric Adams Photo

Of course, this enhanced maneuverability is actually the lesser of the payoffs from the coaxial main rotors and rear prop. The main one is significantly faster forward flight. Developed from Sikorsky’s Advancing Blade Concept, the compound design’s dual rotors neutralize the stall tendency exhibited by retreating rotor blades in conventional helicopters as speed increases.

Because that instability effectively limits top speed, a helicopter with this design can fly far faster than any other. Raider’s predecessor, the X2 experimental helicopter that flew between 2008 and 2011, reached 250 knots, and Raider, which first flew in 2015, has hit 207 knots so far. (Raider’s successor is the larger and faster SB-1 Defiant being developed in partnership with Boeing through the Army’s Joint Multi-Role Technology Demonstrator program. It first flew in March.)

During the demonstration in front of the media, Fell and Corry streaked past us at 190 knots on multiple passes, with Raider behaving more like an airplane than a rotorcraft — and for that matter, sounding more like one, too. (In fact, it had a distinctly warbird-like vibe. Think Spitfire, not Black Hawk.) Although this was short of the planned 200 knots due to issues with the vibration control software, it was still staggeringly fast for a rotorcraft.

Raider’s bag of tricks goes even deeper. It can dive onto a target — also airplane-style — while using reverse pitch on the rear propeller to slow and extend the dive. The pilots can also disengage the prop and slow down the main rotors, generating a “quiet mode” when a dash of stealth is called for. Sikorsky demonstrated both of these, as well. In sum, all of Raider’s capabilities add up to a product that Chris Van Buiten, vice president of Sikorsky Innovations, feels confident meets or exceeds the Army’s requirements. “It’s well beyond the capabilities of the current fleet,” Van Buiten said. “The threat environment going in and out of combat will become more intense, so you need to be fast, maneuverable, and agile, and you need low-altitude flight so you can use the clutter to mask your signal-to-noise ratio.”


Sikorsky test pilot Bill Fell explained that aircraft rates are engineered through the flight controls to limit rotor tip convergence. “As a rule we allow two-thirds of the static separation for maneuvering and retain one-third as a safety factor,” he said. Eric Adams Photo

As Sikorsky’s senior experimental test pilot, Fell said Raider’s acceleration and maneuverability strengths will represent a massive upgrade to pilots. “When I was in Army flight school, we had this teetering rotor system that has relatively low control power,” he said. “Then you go to a Black Hawk or an Apache with its articulated rotor system, and there’s a jump in control power, but also a lag that the system creates between putting the input in and getting the aerodynamic response. But here, with the rigid rotor, it’s pretty instantaneous. The jump is just as big as it is going between teetering and articulated.”

He cited not just the demonstration we observed, but the facility’s ADS-33 slalom course, used to evaluate handling qualities. There, pilots can easily gauge their progress due to the visual cues of the course and the scripted parameters developed for the challenge. “What I found in this aircraft, even with no altitude hold and no position hold, I was able to do it better in this machine than in other aircraft with all of those extra features,” Fell said. “Because the response is so crisp and so instantaneous, you can just go up to exactly that spot. You can put it exactly where you want it.”

Of course, there remain challenges for the program — understanding everything from maintenance costs and reliability, to fully grasping the new configuration’s quirks and risks. The program suffered a setback in this area in 2017 when the first test model sustained a hard landing on the runway in Florida after the upper and lower counter-rotating blades made contact. Van Buiten said it was caused by a software problem that increased the sensitivity of the controls and led to excessive roll oscillations, and it would have impacted any helicopter equally. He noted that it has been fixed thoroughly enough to ensure no similar accidents can happen again, and Fell emphasized that the physics of that incident are “well-understood.”


The rear propeller on the S-97 Raider is geared much like a tail rotor. A clutch allows it to be disengaged from the system and either freewheel or brake. Eric Adams Photo

The crew certainly didn’t seem shy about testing the maneuverability of the aircraft during the demonstration, which of course shows their presumed confidence in the fix. But the routine highlighted not just the far end of combat agility; it also showed off the aircraft’s more subtle performance characteristics. Level acceleration and deceleration, for example, could make for smoother and more comfortable departures and approaches, especially if the helicopter is flying passengers or VIPs, as Sikorsky is indeed positioning it to do.

On the other hand, Fell has no problem doing things the old-fashioned way, even in the Raider. “When I go to leave a landing zone, if I push the nose over and accelerate out like a helicopter, those big rotors take a bigger bite of the air than the prop ever could,” Fell said. “So I really think the best way to accelerate out of that landing zone is still to dip the nose, use the rotor, and supplement it with the with the propeller.” 1
 
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