Sikorsky's X2 tackles this through a coaxial contra-rotating rotor system, the advancing blades on opposite sides producing the lift and allowing the retreating blades to be offloaded. Eurocopter resolves it by removing from the main rotor the need to provide thrust and all of the lift. Propulsion is shifted to the propellers and lift partially offloaded to the wing, allowing rotor rpm to be reduced. Rotor rpm is automatically slowed by up to 15% as forward speed increases, Ferrier says. This reduction in rotor rpm allows a significantly higher forward velocity before the aircraft reaches its never-exceed speed.
Primary objectives of the X3 program are to develop and validate the H3 concept in terms of aircraft control and trim strategies, anti-torque control, propulsion systems, variable rotor speed and power-system management, Ferrier says.
The aircraft first flew September 6, 2010, with testing performed at the Istres flight test center in southern France. Initial flights took the aircraft to 180 kt., the speed limit set by the initial transmission. After upgrading the gearboxes, flights resumed in March 2011, achieving a sustained maximum speed of 232 kt. in May that year, “while using less than 80% of available power,” Ferrier says. Speed will be increased gradually throughout the test program. Current cruise speed is 180 kt. Operational specifications for the future H3 aircraft include a high cruise speed in excess of 220 kt.
To produce an experimental concept demonstrator as rapidly as possible, but at minimum cost, Eurocopter borrowed heavily from its existing helicopters. The fuselage is from the AS365N3 Dauphin and the entire rotor system, from swashplate to blades, is from the EC155. The main gearbox is from the new EC175, but instead of a driveshaft to an anti-torque tail-rotor it has two shafts going to left and right reduction gearboxes to drive the two wingtip propellers, supplied by Germany's MT-Propeller. The twin Rolls-Royce Turbomeca RTM322 engines, rated at 2,300 shp each, are from the NH90.
The X3 has no tail-rotor. The aft section consists of a wide horizontal stabilizer with twin vertical stabilizers. At low speed, rotor anti-torque compensation is provided by differential pitch angle between the left and right propellers, directly controlled by the pilot putting in the appropriate foot pedal movement. At high speed, anti-torque is provided by computer-controlled fin flaps.
Since the rotors on French-designed helicopters turn clockwise, as viewed from the cockpit, the advancing blade comes from the left, causing the nose to yaw to the left as power is applied. The pilot instinctively applies right pedal to counter this. As right pedal input is applied, propeller pitch increases on the left side while decreasing on the right, bringing the nose back around to the front.
Our initial “walk around” at Grand Prairie revealed two additional features designed to help with torque control. The twin vertical stabilizers each have a narrow aileron at the trailing edge. These move automatically above 80 kt. to balance propeller torque at high speed. A series of metal tabs just forward of the vertical ailerons acts as vortex generators to improve their efficiency.
The aircraft also has a strake along the starboard side of the tailboom. As the retreating blade passes over the tail, the downwash accelerates around and under the boom, creating a low-pressure area. Just as airflow over a wing provides lift, the low-pressure area on the boom tries to pull the tail to the right. The strake breaks up the airflow, increasing anti-torque control as well as reducing the power required for hovering.
As with any experimental aircraft, the X3 is set up to record every aspect of flight. Cameras are located both on the fuselage and in the cabin, including a 3-D camera on the front panel focused on the pilots and flight-test engineers.
The instrument panel was based on the EC155, but greatly expanded to provide a wide array of readings for every system in the aircraft. For the purposes of our flight, the primary instruments were two multi-function displays on each side of the panel showing standard flight parameters such as airspeed and altitude, as well as a single display that showed only the true airspeed, propeller pitch and vertical climb speed in ft./min. Another highlights propeller pitch and both rotor and propeller torque.