Advanced rotor designs break
conventional helicopter speed restrictions
A fast future awaits for rotorcraft with coaxial rotor systems and pusher propellers.
AVX was granted a primary utility patent for coaxial rotors and dual ducted fans, covering all aspects, technical functions and configuration.
To address the retreating blade stall problem, the blade is actually feathered in forward flight as it rotates around the helicopter becoming the retreating blade and then the AOA is increased as it continues to rotate becoming the advancing blade.
As the 2 rotor systems rotate in opposite directions there is an advancing blade on both sides producing lift without the increased drag of a conventional retreating blade. Forward thrust is provided primarily from the pusher propeller, reducing the need for high blade angles in flight, which decreases the AOA on all blades.
As the propeller does its work, lower lift requirements from the main rotor system allow the main rotor rpm to be reduced at high airspeeds. Lower rotor rpm reduces blade tip speed, which in turn prevents compressibility and blade tip damage and vibration. Without a tail rotor (which makes up a significant amount of a helicopter's noise signature) and throwing in an advanced composite rotor blade design, the X2 is one quiet machine.
Helicopters of the past have not always been neighborhood friendly, broadcasting their arrival long before the aircraft was even visible due to blade slap noise.
According to Bredenbeck, the X2 has conventional helicopter controls—collective, cyclic and pedals. Turning at a hover is achieved through differential torque control with the pedals. The pilot can take off by adding forward cyclic which lowers the nose, similar to a conventional helicopter. But, says Bredenbeck, if the pilot really needs to move out he pushes a beep switch on the collective, quickly increasing propeller rpm, maintains a level attitude, and the X2 accelerates very quickly.
The collective increases blade pitch, the cyclic controls the rotor system like a conventional helicopter, and the propeller provides forward thrust. Rudders on the vertical surfaces become effective above 150 kts, and in forward flight the X2 performs like an airplane. When the pilot wishes to descend, he lowers the collective, propeller rpm is reduced, and the pilot can land accordingly.
Additional benefits of the propeller are that it can be used as a brake to slow the helicopter very quickly or used to accelerate the aircraft rapidly in the event of a tactical need. During ground operations the propeller is disengaged by a clutch system to avoid ground personnel accidents and increase safety. Sikorsky plans to integrate all the features of the X2 in the new S97 Raider currently under development.
The advanced vertical lift aircraft proposed by AVX Aircraft of Benbrook TX will be equipped with a stacked coaxial system, negating the need for an antitorque system. AVX Pres & Chief Engineer Troy Gaffey says that not only is there 15% additional power available for vertical lift (because the drive train does not have to power an antitorque system), but operations on the ground and in the air are safer without a tail rotor, which can be dangerous for personnel on the ground and may hit obstructions while hovering near trees.
According to Gaffey, additional efficiencies are gained by the mutual integration of coaxial rotor systems. This results in a 6% increase of lift at sea level, increasing to possibly 20% during hot-and-high operations.
AVX aircraft will also use lower rotor rpm during high-speed flight to minimize compressibility and decrease the aircraft's noise signature. Dual ducted fans will provide forward thrust and offload the lift requirement of the coaxial rotor system in forward flight.
During a hover, the fans use differential thrust to pivot the aircraft about the vertical axis for hovering turns. Recently, AVX received a patent which covers all aspects of coaxial rotors and dual ducted fans, as well as the relationship between the rotors and fans plus a number of internal controls and components.
Eurocopter's X3 toured the US recently, demonstrating the high-speed capability of the highly modified EC155 fuselage with dual tractor propellers and wings. Noticeably missing is the tail rotor no longer required because of differential thrust provided by 2 propellers.
Continuing with advanced helicopter designs, Eurocopter's X3 demonstrator uses the single main rotor system of the EC155 and 2 tractor propellers mounted on short wings on either side of the fuselage for forward thrust.
To reduce compressibility and retreating blade stall occurrences, rotor rpm is reduced in flight as the wings provide 40% of the vertical lift and the propellers provide the majority of forward thrust. The 2 propellers can be stopped and declutched on the ground to increase passenger safety—much like the X2—and are also used to accelerate the aircraft quickly on takeoff.
As the sole stakeholder in the AW609, AgustaWestland expects the tiltrotor to meet multiple mission requirements in the future. With a maximum cruise altitude of 25,000 ft in pressurized comfort, it far surpasses conventional rotorcraft capabilities.
An alternative way to break the slow cruise speed in a rotorcraft is to tilt the rotors horizontally. This has been done by tilting the wing and rotor pod together, which is more efficient at a hover, but the configuration finally chosen by Bell for the XV15 was a stationary wing with engine/rotor pods rotating for transitions between helicopter to airplane mode. This technology was carried forward in the military V22 Osprey and the Bell Agusta BA609.
Tiltrotor aircraft are still helicopters when the prop/rotors are positioned vertically—therefore, they can take off vertically and, in the V22's case, pick up an external load and fly away at much higher speeds. The tiltrotor can not only take off vertically—the prop/rotors can be tilted forward slightly on takeoff providing STOL capability and allowing increased takeoff gross weights.
Once in the air, the pilot rotates the props forward, the aircraft accelerates and the wings begin to provide lift just like airplanes, but now with very large prop/rotors producing forward thrust. To decrease noise, reduce unnecessary thrust and save fuel, the pilot reduces the RPM of the proprotors and the aircraft now flies like an airplane, higher, faster and much more fuel efficiently than a conventional helicopter.
Last fall, AgustaWestland acquired full ownership of the civilian tiltrotor program from Bell Helicopter, which covered transfer of all intellectual properties, prototypes, certification credits and production tooling and rigs for the now designated AW609.
According to AgustaWestland Senior VP Marketing Roberto Garavaglia, the AW609 program is on track for FAA certification in the first half of 2016 with projected deliveries beginning in 2017. The AW609 will be fitted with a state-of-the-art fully digital glass cockpit and avionics suite. The pressurized tiltrotor's maximum speed is 275 KIAS with a ceiling of 25,000 ft.
AW609's pressurized cabin puts it in a class of its own, since it allows the aircraft to fly higher, avoiding lower weather phenomena, and provides increased fuel efficiency and higher cruise speeds—things conventional rotorcraft cannot match.
Future commercial applications
Aircraft demonstrators like the X2 and X3 are already breaking the paradigm of slow helicopters, but much work is still to be done to meet the needs of the future US military FVL program. The target first delivery date has already slipped from 2025—as originally discussed—to 2030.
There is no doubt the technical advances made in helicopter design for military use will trickle down to commercial applications. All manufacturers acknowledge that high-speed commercial rotorcraft have a place in EMS, law enforcement and VIP transport, but that for now they must be vetted through the trials and tribulations of defense procurement funding. Many in the helicopter world eagerly await the arrival of faster rotorcraft today but they'll have to wait patiently for the technology of tomorrow.
Jay Chandler has written for Pro Pilot magazine since 1995 and has flown for the FAA, military and Part 91 and 135 operators.