The Aviationist was among the outlets invited to attend a demonstration event of the X2 technology at the Sikorsky Development Flight Center. Here’s how it went.
After the U.S. Army’s FARA program was cancelled in 2024, Sikorsky has continued to work on its S-97 Raider demonstrator. As the Raider’s underlying X2 technology is now being proposed to NATO and South Korea for their Future Vertical Lift programs, on Feb. 11, 2025, The Aviationist was among the outlets who were invited to the Sikorsky Development Flight Center to attend a demonstration of the X2 technology.
The event included two days of activities, with the first one focused on a walkaround of the S-97, a simulator session and briefings about the X2 technology, while the second day was focused on the flight demonstration itself. Program officials, engineers and test pilots were all on hand to answer to questions and provide additional info.
Without further ado, let’s now delve into the details about the X2 technology and the S-97 Raider. This report will be divided into a four-part series, with the second one focusing and the walkaround of the Raider and the flight demonstration.
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The X2 technology
The S-97 Raider which Sikorsky displayed at its Development Flight Center in West Palm Beach, Florida, is the result of 20 years of development, started in 2005 with the X2 demonstrator. That demonstrator flew from 2008 to 2011 and reached a speed of 252 kts in level flight, way higher than a normal helicopter.
In fact, the X2 was not a standard helicopter, but a compound helicopter, with two coaxial counter-rotating main rotors and a prop rotor in the tail. This solution exploited the Advancing Blade Concept that Sikorsky developed in the 1970s, however at the time there were still some technical challenges to solve, and that took decades.
Sikorsky created the Advancing Blade Concept with the goal of significantly increasing helicopter flight speed. The concept is based on the rotor’s ability to develop lift on the blade that advances in the direction of flight, relieving of this duty the retreating blade that goes in the opposite direction. Using counter-rotating, rigid rotors with limited bending in the vertical direction, this design delayed the drag increase caused by the blades’ tip Mach number effects, thus enabling higher speeds.
The concept was first tested in NASA’s wind tunnel and led to the S-69 (also known as XH-59A) demonstrator, which reached 240 knots in level flight and 263 knots in a shallow dive with two auxiliary jet engines mounted on the sides. The demonstrator improved speed, altitude, and control without compromising hover efficiency, however, it had drawbacks such as heavy titanium rotors, high fuel consumption, rotor drag, heavy pilot workload and vibrations.
The technological limitations prevented further development at the time. Technology evolved through the years and, decades later, advancements such as composites rotor blades and fly by wire control system, together with a pusher propeller added for efficiency, solved some of the technical challenges, paving the way for the X2 technology demonstrator which proved out the efficiency and the viability of this technology.
Sikorsky viewed this technology as the perfect choice for the U.S. Army’s armed aerial scout which was supposed to replace the OH-58 Kiowa Warrior, the Future Attack Reconnaissance Aircraft (FARA) program, which was canceled in 2024. The program resulted in a new demonstrator, the S-97 Raider, on which the company applied all the lessons from the past and is continuing to learn new ones, as the team is completing engineering experiments on the aircraft on a regular basis to advance and mature the technology for the future.
The FARA program
The Future Attack Reconaissance Aircraft program was launched in 2018 to develop a successor to the Bell OH-58D Kiowa Warrior scout helicopter, retired in 2014, as part of the Future Vertical Lift program. This was the fourth time the service tried to replace the OH-58, and the service was also looking to replace nearly half of the AH-64 Apache fleet with the new FARA.
The U.S. Army aimed to field the new aircraft by 2028. In April 2019, design contracts were awarded to five manufacturers, which were to present their design by February 2020.
According to the mandatory requirements, FARA candidates were required to use the General Electric T901 engine, selected under the Improved Turbine Engine Program (ITEP), and the General Dynamics XM915 20mm cannon. The maximum dimensions were to not exceed 40 foot (12 m) for both rotor diameter and fuselage width, with an unspecified target gross weight and payload and an affordability goal.
The aircraft was supposed to meet a minimum speed of 180 knots and a maximum speed of 205 knots, range of 135 nm, 90-minute on station endurance, 4000 feet/95 F Hover Out of Ground Effect (HOGE). Another requirement was a pilot-optional capability that could be further exploited in future and a Modern Open Systems Architecture (MOSA) to be able to easily upgrade the aircraft.
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After that 2020 deadline, the Bell 360 Invictus and the Sikorsky Raider X were selected for the second phase of the FARA program. The two companies received contracts for detailed design, build and test of their air vehicle solutions and were expected to face a final fly-off competition in 2023, later delayed to 2024.
However, in 2024, the U.S. Army has unexpectedly announced the cancellation of the FARA, in an abrupt change of its plans for the 2030s’ Army aviation. The service said this was based on lessons learned and a sober assessment of the modern battlefield, which led to the decision to invest in unmanned assets.
This way, the Invictus and the Raider X suffered the same fate of the other helicopters that were supposed to replace the OH-58 in the last two decades, the RAH-66 Comanche and the ARH-70 Arapaho.
The S-97 Raider
The day before the flight demonstration, we had a chance to do a complete walkaround of the first S-97 Raider built, which flew for the first time in May 2015 and is now used as a static display. Sikorsky’s experimental pilot Bill Fell provided many insights in the design and characteristics of the Raider
“It’s a spectacular aircraft, it is absolutely the sportscar of helicopters, and it’s really a pleasure to fly,” Fell enthusiastically told us while describing his experience flying the Raider.
The rotor system is a critical element of the S97’s design, “some of the bread and butter” of the Advancing Blade Concept, as Fell described it. Unlike traditional helicopters, the X2 employs rigid rotors with an effective hinge offset significantly higher than conventional designs. This rigid rotor also have 50% less parts than an articulated rotor.
Whereas helicopters like the UH-60 Black Hawk have a hinge offset (the distance between the flapping hinge and the rotor mast, used as a measure of the rigidity of the rotor) of 12-18%, the X2 boasts a figure exceeding 40%, potentially reaching 45%. This increased rigidity enhances control power and reduces response time, allowing for precise maneuverability.
In conventional single-main-rotor helicopters, as speed increases beyond 150 knots, the retreating blade encounters aerodynamic limitations, experiencing reversed airflow (as the blade retreats, the airflow moves from the trailing edge to the leading edge, lowering the relative speed) and eventually suffering from retreating blade stall, one of the most hazardous flight conditions in helicopters where the blade doesn’t produce lift anymore.
This phenomenon imposes a speed constraint on traditional rotorcraft, as the issues on the retreating blade would cause an unbalanced lift distribution on the rotor. However, the X2’s coaxial rotor configuration mitigates this issue by utilizing the advancing blades on both rotors to maintain balanced lift distribution, effectively eliminating retreating blade stall as a limiting factor.
Normally, the two rotors move in unison and when the pilot pushes the stick forward, both rotors tilt accordingly. However, there are situations where independent movement is necessary. The flight control system (FCS) manages these adjustments while maintaining a consistent 30-inch tip separation. The rotors are designed to allow up to 20 inches of coning for maneuverability, but a third of the tip separation (10 inches) is reserved as a safety margin, ensuring the blades never come too close to one another.
Unlike conventional helicopters, which require a tail rotor to counteract torque effects, the X2 balances the torque between its two main rotors. In low-speed environments, differential pitch between the rotors enables yaw control, while at higher speeds, rudders take over to ensure coordinated turns. The fly-by-wire system automatically transitions between these control mechanisms, ensuring smooth handling without requiring additional pilot input.
The main gearbox plays a crucial role in transferring power efficiently between components. The lower rotor functions similarly to a conventional main rotor, with three hydraulic servos transmitting inputs via a swashplate mechanism to change the blades’ pitch. The upper rotor mirrors this setup but in an inverted configuration, utilizing pitch change rods that pass through the gearbox to deliver control inputs.
The swashplate for the upper rotor is located below the gearbox, while the swashplate for the lower rotor is located above the gearbox. The gearbox has a compact design and is fed by a single driveshaft. Additionally, aerodynamic efficiency is further enhanced by the streamlined fairing between the rotors, reducing drag.
Since the counter-rotating rotors cancel out torque effects, instead of a tail rotor, the S-97 features a pusher propeller that provides forward thrust. This is a critical component of the S-97’s high speed capability, which allowed the aircraft to reach 207 kts in level flight, while without the prop the S-97 “only” reaches 150 kts.
The pusher propeller is engaged via a clutch system and can be disengaged entirely, although when disengaged it still turns like a freewheel at 200 RPM to prevent damage to its composite blades, since the prop sits just behind the engine exhaust. The normal speed is around 2,000 RPM.
In conventional helicopters, a tail rotor typically consumes about 1/9th of the total power. In the S-97, more power can be allocated to the pusher propeller than to both main rotors combined, depending on the requirements and the situation. The aircraft’s flight control system (FCS) prioritizes rotor operation, as the main rotors are essential for maintaining lift, avoiding the risk of selecting too much power for the prop. The FCS looks primarily at two limitations, the available power and the torque limits.
The system allows for variable thrust output, including a significant negative pitch range, which can act as an airbrake. A switch on the collective allows the pilot to select the required power output. The acoustic signature of the S-97 changes fairly significantly with the prop, and this is also one of the considerations made to determine when to use it.
When the propeller is active, the aircraft exhibits different handling characteristics. During takeoff, the propeller can be used to assist in acceleration. Pushing the nose forward, similar to traditional helicopter flight, is initially more efficient as the main rotor gets “a much bigger bite of the air,” but as the propeller’s airflow efficiency increases with the speed, it functions almost like a turbo boost, enhancing acceleration.
The negative range of the prop allows multiple uses to increase the capabilities of the S-97. “I honestly think the negative range is of the best capabilities of that prop,” said Fell during the walkaround. As an example, during an aggressive assault landing where the helicopter approaches the landing zone at high speed, putting negative pitch on the prop allows to quickly decelerate without having to bring the nose up too much, which would cause a loss of situational awareness and visibility in one of the most delicate phases of the flight.
If the propeller is set to maximum deceleration capability when in a hover, the aircraft will rapidly begin moving backward. To counteract this, the pilot can engage a neutral setting, known as the zero-thrust button, effectively eliminating any thrust or drag from the prop.
The negative pitch in the hover, however, is not an undesirable setting, at it can be used to either fly backwards or balance the helicopter to hover with the nose down. Similarly, positive pitch enables to hover with the nose up. Tests have demonstrated the ability to maintain a nose-down pitch of 7-8° and a nose-up pitch of 20° during hover with the help of the prop. The ability to maintain these different attitudes in hover is particularly beneficial for armed aircraft, eliminating the need for articulated pylons to employ weapons, like the ones used by the AH-64 Apache.
The rudders and elevators are electrically actuated. The rudders, which are positioned further away from the tail boom compared to traditional designs, play a critical role in turn coordination, especially at high speed. The cabin of the S-97 can accommodate up to six passengers, making it adaptable for multiple mission profiles, including attack and troop transport roles.
The cockpit of the X2 is not as spacious as the one of a UH-60, with Fell describing it saying “it’s a fairly tight cockpit, but that’s the sportscar nature of this machine.” The doors are small, as it was decided as a safety measure to only have a smaller upper section of the door that could be opened.
The cockpit features sidearm controllers with a small, half inch, input range, enabling precise adjustments, with a single collective between the seats. Most of the time, the fly by wire system doesn’t require the pilot to hold the controls, as it is a full-authority system that requires only the necessary inputs to maintain control. Additionally, a tiered flight control system and attitude command modes for various flight conditions help reduce the pilots’ workload.
A difference that immediately catches the eye is the missing multitude of circuit breakers. Instead, the S-97 uses solid-state power controllers, which can be controlled through the cockpit displays, with just two traditional circuit breakers for redundancy on critical systems. Any issues are flagged on the displays, which provide alerts as needed.
Another advanced technology present of the S-97 is the smart pitot tubes. Unlike traditional systems that rely on pneumatic lines to measure airspeed through pitot tubes and static ports, these are solid-state probes that send electrical signals. This design eliminates the need for pneumatic systems, allowing for quicker replacement if something malfunctions.
Safety considerations and additional advantages
Typically, a helicopter is designed around specific requirements, such as the ability to hover while carrying a certain number of people. For example, if you need to hover with 12 people, the rotor size and power requirements are calculated accordingly. For the S-97, the key design consideration is the power needed to lift the aircraft and push it forward at 220 knots.
The result is that, in hover, there is significant excess power available, power that would otherwise be only used at higher speeds to power the propeller. This provides a substantial safety benefit, especially in multi-engine machines. If an engine fails, there’s a high likelihood that a helicopter based on the X2 technology can still hover on a single engine, giving it a strong safety margin.
The design of a helicopter based on the X2 technology has a safety advantage also on single engine machines. In fact, when the engine quits, autorotation is required to safely land, with the inflow of air through the rotor allowing the aircraft to descend at a controlled rate.
In the event of engine failure during level flight at high speed, the S-97’s engine will communicate with the flight control computer, which will immediately begin reducing the blade pitch on the prop. This reduces power demand and creates a decelerating effect on the aircraft, causing air to flow through the propeller and feed energy back into the main rotor. This deceleration helps maintain rotor RPM.
Also, this allows up to 15 seconds of time before needing to lower the collective and begin autorotation, while a traditional helicopter requires much more rapid intervention. This gives the pilot significantly more time to assess the situation and make decisions, evaluating the best options for an emergency landing. The glide ratio is approximately three times better than that of a typical helicopter, offering additional landing options.
Since there is no torque differential between the main rotors, the absence of the tail rotor is not a factor, and the rudders, which remain effective down to 40 knots, are used for yaw control. In the simulator, autorotation has been successfully demonstrated down to 30–35 knots.
While the main propeller of the S-97 is not considered a safety-of-flight element, as the aircraft can operate with or without it, the tail rotor of a traditional helicopter is a critical safety component. Also, the main propeller is not connected to the aircraft’s hydraulic system, while the tail rotor relies on hydraulics, making it more vulnerable to ballistic damage.
For new pilots, the X2 is actually considered easier to fly due to its higher stability and augmentation modes which assist the pilots. For an instance, pilots can start in attitude command mode, where the computer holds the altitude and assists with hover, and then they can gradually transition to more dynamic flight modes.
The fly-by-wire system (FBW) also simplifies an eventual transition to unmanned operations. In fact, since the FBW system already includes a comprehensive set of sensors and controls, adding unmanned capabilities requires only software modifications and minimal additional effort, compared to conventional aircraft that lack FBW systems.
The Raider X
While we’re here, it is worth looking at a brief recap of the aircraft that the was benefiting from the S-97 demonstrator.
The Raider X is a scaled-up version of the S-97 Raider, with a side-by-side cockpit to widen the fuselage and increase the payload carried in the internal weapon bays. Speaking about the payload, Lockheed Martin (which acquired Sikorsky in 2015) published a new concept art that showed for the first time the Raider X with its weapon bays open and the turret for the 20 mm cannon in front of the cockpit.
The Raider X is 20% larger than the S-97, with the main rotor going from a 34 feet (10.4 m) diameter to a 39 feet (11.9 m) diameter and the weight increased from 12,000 pounds to 14,000 pounds (5,445 to 6,350 kg). According to the company, Raider X is capable of low and high-speed maneuvers exceeding 70° bank angle.
The helicopter uses a single T901 engine that could propel it well over the 180 knots requirement, as the less powerful S-97 already reached 207 knots in level flight and 250 in a shallow dive during testing. The engine is mounted in a classic central position, with air intakes on both sides and the exhaust contained in the tail boom.
In late 2023, the Raider X prototype was said to be 98% complete, as the aircraft awaited the completion of the GE T901 Improved Engine Turbine Program (ITEP). The Raider X team immediately started the installation process of the new engine upon arrival in October, with the plan to allow the prototype, after a first ground testing phase, to take to the skies within a year.
Stay tuned for the next part of our reporting about the X2 Technology demonstration!
