Rotor & Wing International
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X2 From the Pilot's View

Sikorsky’s experimental test pilot for the X2 details the technology that is influencing the S-97 Raider and SB>1 Defiant.

Imagine cruising low level in a helicopter in excess of 200 kt and then hammering the brakes, coming to a hover in less than half a mile. Now consider you can do this at maximum gross weight and in 6,000 feet, 95-deg F (6K95°) conditions.

This off-the-chart performance comes with equally impressive handling qualities due to the characteristics of the rigid rotor, such as greater control power and decreased rotor response lag. These fundamentals make this helicopter fly more like a tactical jet aircraft than a classic rotorcraft.

You might be thinking this is marketing exaggeration. But I don’t need to use my imagination to conjure up this scenario as an experimental test pilot for Sikorsky; I have experienced this performance firsthand.

I have flown as a command pilot on every flight of the S-97 Raider, and prior to that I was the backup telemetry pilot for the X2 technology demonstrator, a Collier-award-winning aircraft that now has a place in the Smithsonian’s National Air & Space Museum.

Similar performance is available on all X2 technology aircraft. X2 refers to “times two,” in reference to roughly double helicopter-like speeds. The incredible hover performance is achieved due to how the power is managed for the aircraft. Usually, helicopter power requirements are calculated based on “hover power required” as the highest demand condition. In X2 aircraft, power is based on the highest demand, which is now maximum speed, and so excess hover power is the result. So you are probably thinking you’d need a giant fuel tank to cruise at these high speeds. Not true. The X2 lift-over-drag performance, proven in flight test, is closer to a turboprop airplane than a helicopter.

As the lead project pilot for both the Raider and SB>1 Defiant, the Sikorsky-Boeing demonstrator being built as part of the U.S. Army’s joint multi-role technical demonstration program, I have spent hundreds of hours in simulation testing these configurations. We proved the physics with the X2 demonstrator and now are proving the scalability with both Raider and Defiant.

Sikorsky, in partnership with Boeing, is developing the SB>1 Defiant for the U.S. Army’s joint multi-role technology demonstration program.Photo courtesy of Sikorsky

Raider is a single-engine, 10,000- to 12,000-lb, 34-ft rotor diameter, 7-ft diameter propeller aircraft with a crew of two and room for six passengers. Raider has a fly-by-wire flight control system, active vibration control and retractable landing gear. Two multi-function displays and a control and display unit (CDU) provide control for all systems except the radios. A basic Garmin stack of radios provides communication capability. Solid state power controllers are activated through the CDU rather than traditional breakers to aid in providing a clean uncluttered cockpit. Environmental control provides cockpit cooling as higher speed aerodynamics necessitates a tightly buttoned-up fuselage.

I cannot say enough about fly-by-wire systems — they are like an “easy button” for pilots. With a press of a button on the cyclic, the aircraft can fly like a sports car for nap-of-the-earth combat flying or like an airliner for instrument flying. The system provides the backbone for a variety of features that can be added via software to include automatic landings for brownout, fully coupled approaches or Sikorsky’s Matrix Technology autonomous system.

In Raider, the flight control mode is currently rate command attitude hold, aka the “sports car” mode. The Raider cyclic is a side stick spring loaded to a center detent position whereby the pilot commands a rate proportional to stick displacement. The maximum displacement represents the maximum rate the team has decided to allow the aircraft to achieve. This differs from a limited authority hydro-mechanical system where full displacement is seldom seen as it represents the limits of the control cube. The benefit this provides is to allow the pilot to use the full range of the cyclic and consistently achieve a predictable rate with finely-tuned stick sensitivity.

My first exposure to this phenomenon was in the Boeing-Sikorsky RAH-66 Comanche, which was easy to fly, up to the significant limits of the machine. The ability to slap the stick to the stop and get an eye-watering turn rate that is both controllable and within all the dynamic system and structural limitations will make aces out of relatively junior aviators. We have demonstrated this capability in all our fly-by-wire aircraft and are developing the same capability on Raider.

The collective in Raider is unique in that there is only one, and it’s positioned between the pilot and co-pilot. This replicates the modern business jet model with power in the center and sticks outboard. That’s right — the pilot in the left seat flies with the cyclic in the left hand and the collective in the right hand. The Raider is an experimental aircraft, and this configuration represents part of the experiment.

If the fly-by-wire control system can fly autonomously, we can also make it easy to fly in an alternate configuration thereby providing designers more flexibility in cockpit design. The X2 relies primarily on the advancing blades of each disc for lift and the propeller for thrust as such the collective programs down as you go faster. The fly-by-wire system automates the collective. At around 100 kt, the pilot feels a gentle tug down from the collective, essentially the flight controls saying, “I got this.” The system then manages the collective to the optimal spot for the current airspeed.

In high-speed flight, X2 flies like an airplane with nearly all the power going to the prop. Climbs and descents are made by raising or lowering the nose for the desired vertical speed. The prop is adjusted via a thumb beeper on the collective. Push forward to go faster and aft to slow down. There is incredible deceleration capability via negative pitch on the prop. Negative prop pitch is used on approaches to keep the nose low until touchdown, greatly improving terminal area safety. Prop pitch also provides the ability to hover nose up or nose down adding tactical options. While the collective is automated to fly like an airplane in high speed flight, the pilot can override this and fly like a traditional helicopter moving the collective to climb or descend. The final feature of the prop is that the pilot can disengage it via a clutch. Once disengaged, the prop either slows down or stops to reduce the acoustic signature and improve ground safety. The prop is self-contained and is not required for flight. Without the prop, Raider has already demonstrated flight out to 150 kt with power to spare.

X2 aircraft do not require hydraulic lines from the main gearbox-driven hydraulic pumps down the tailboom to the prop, as is required for tail rotors. The independence of the prop simplifies the hydraulic system and improves reliability.

The Sikorsky S-97 Raider is Sikorsky’s proposed high-speed scout and attack helicopter.Photo courtesy of Sikorsky

In flight test, we build up slowly based on conservative limits and progress from the known to the unknown while methodically looking at data and making improvements where necessary. In flying Raider during its first flights in 2015, I frequently heard “knock it off” over the radio as the engineers watching 1,000-plus aircraft parameters observed a parameter from a low fidelity test go over the limit.

Component limitations are based on three primary sources of information: ground test events, an “iron bird” test vehicle or via analytical means. Analytical limits are the lowest fidelity and consequently have the largest knock downs, whereas the other test-based limits tend to have less conservatism based on the rigorous exercise of actual componentry. When the “knock it off” call happens, it sets in motion a detailed review to determine how to proceed. We are then required to conduct a higher fidelity test to raise the limit, strengthen the part or tune the dynamics in a different way. The iterative nature of this exploration is time-consuming.

So far, Raider has a solid feel through the flight envelope. The last four flights were clear of any “knock it off” calls. We have flown to 150 kt with the propeller disengaged, flown slaloms turns at 45- to 50-deg angles of bank, and accelerated to 120 kt with the propeller engaged.

The control in these maneuvers is as crisp as any aircraft I have flown. Rolling to a steep bank angle requires a single lateral input to the cyclic. I hit the roll rate I want, hold the input until reaching the desired bank and then let go of the cyclic, allowing the system to maintain the bank angle. When I want to roll out of the turn I put an input in the opposite direction until leveling off. I also can hit that level point quickly with little effort as you only have to get close and the flight controls will “tidy it up” to wings level.

These are not autopilot features, but attributes of a full-authority system that greatly reduces pilot workload. To accelerate out of the hover, I can either lower the nose like a standard helicopter, add propeller thrust or a combination of both, all of which have been flown in Raider. In low-speed-flight, Raider is agile and the lag-free control response gives it a true sports car feel. In low speed, we have been to 25 to 30 kt in all cardinal directions, with more to follow.

The first Raider prototype was poised to exceed 180 kt last summer when an incident resulted in a program speed bump. Its flight status is expected to resume soon, followed by return to flight with speed objective demonstration over the summer. RWI