Advances in blade design
Rotorcraft OEMs tackle noise and speed issues through improvements to main disc and antitorque.
By David Bjellos
ATP/Helo. Gulfstream IVSP, Bell 407
Later phases of the British Experimental Rotor Programme used radically shaped blade tips for noise and vibration reduction and performance enhancement. A modified AgustaWestland AW101, alias Merlin HC3, is shown in flight test.
One of the primary concerns voiced by every manufacturer at Heli-Expo 2010 was the worry that helicopters were becoming marginalized due to noise and lack of speed. There has been almost no significant improvement in the past 4 decades with regard to increasing the forward speeds for traditional rotorcraft.
Noise issues are becoming so restrictive at many airports that they are actually interfering with fixed-wing aircraft movements, conflicting with the primary mandate for helicopters to “avoid the flow of fixed-wing traffic.” OEMs are innovating and taking a fresh look at blade design and function. Several technologies hold promise.
Blue Edge and Blue
Pulse Grouped under a single program called Bluecopter, Eurocopter has incorporated 2 specialized main rotor blade innovations called Blue Edge and Blue Pulse. Blue Edge has a double swept shape that modifies the trailing-edge vortex of a preceding blade, called blade-vortex interaction (BVI) in a reactive or passive mode.
Because of the relative speeds associated with the advancing blade during descent, BVI dissipates the disturbed air or vortex created and smoothes it, thus reducing noise by 3–4 dB. Trials have been undergoing validation since 2007 on an EC155—the Blue Edge design has reduced by half the noise profile associated with the rotor in a descent.
In conjunction with Blue Edge, Blue Pulse specifically addresses blade tip vortices through a technology known as piezoelectrics. The science of piezoelectrics is the ability of an object—notably crystals and ceramics—to generate an electric field when mechanical stress is applied. This is known as direct piezoelectric effect.
For Eurocopter’s demonstrator, an EC145, 3 trailing-edge flap modules dynamically or actively adjust the path of the rotor blade while moving at 15–40 times per second. This rapid “pulsing” action neutralizes blade slap associated with descent profiles and Eurocopter has reported a 5-dB reduction in noise.
Eurocopter Blue Edge is designed for interior and exterior noise reduction.
There are currently 2 technology programs under way to increase the forward speed of rotorcraft high speed vertical takeoff and landing (HS-VTOL) civil aircraft—the Sikorsky X2 and the Bell/Agusta BA609.
Each represents uniquely different approaches to achieving high-speed rotorcraft flight. The first is the coaxial X2 demonstrator by Sikorsky, built on its acquisition of Schweizer. The Sikorsky X2 uses a dual counter-rotating main rotor to counteract torque effect (and effectively neutralize retreating blade stall) and a pusher propeller for forward thrust.
Flight control is provided through fly-by-wire (FBW) systems on a Schweizer airframe, modified with off-the-shelf (OTS) components. Sikorsky gained valuable knowledge on FBW technology through its Cypher unmanned aerial system (UAS) program and expertise in composites from its RAH66 Comanche helicopter.
Blue Edge design has reduced cabin noise levels significantly.
Designed to validate the potential for 250-kt cruise flight, the X2 is scalable for small, attack-size aircraft up to large, heavy-lift rotorcraft which require higher enroute speeds. The aircraft is currently in the Sikorsky facility near West Palm Beach FL undergoing trials and continues to expand its flight envelope.
Blade technologies have advanced significantly for Sikorsky as well and are evident on the S92 and newly introduced S76D. Both employ a trailing-tip technology that aids in reducing noise from blade slap and much effort has been placed in reducing BVI through asymmetrical airfoil shape and trailing tip design.
Both aircraft have impressive forward speeds for traditional rotorcraft but fall well short of the 250-kt enroute goal. As X2 technology matures, we are certain to see market-specific models designed for civilian use. Hailed as a breakthrough for HS-VTOL operations almost a decade ago, the tiltrotor concept has been mired in design and cost issues, and its future is unknown.
The BA609 joint venture between Bell and AgustaWestland provided crucial R&D capital and the combined technical knowledge of the 2 companies—the prototype first flew in 2003. In Sep 2009, AgustaWestland Chairman Giuseppe Orsi announced that parent company Finmeccanica would consider buying the program outright to speed up production. The tiltrotor has considerable history.
First practical application of piezoelectrics was aboard submarines during WWI as sonar. A transducer emitted the classic “chirp” and 2 thin steel plates sandwiched a quartz crystal and a hydrophone combined to detect the return echo.
Measuring the time differential produced distance, and sonar became the de facto choice of ranging in underwater operations. Applying piezoelectrics to aviation is not new, but it is novel for rotorcraft noise mitigation. Bell Telephone labs developed an “AT crystal” in the 1940s which was considerably lighter than earlier designs for wireless (radio) perfected by Nikola Tesla and Guglielmo Marconi.
This modified crystal allowed the Allies to install significantly lighter radios in aircraft, increasing the effectiveness of airborne assaults through active real-time coordination. Using a variety of composite materials in its rotor blades, Eurocopter was able to incorporate piezoelectrics in active rotor system development and holds promise for continued noise mitigation and future applications, specifically FBW flight controls.
Bell produced a reciprocating-powered version in the 1950s, designated XV3, which was followed by the turbine-powered XV15. That technological growth allowed Bell to produce the V22 Osprey, a joint-venture between Bell and Boeing, which has entered service with the US military.
In Jul 2007 this author visited the Bell XworX facility and had the pleasure to fly with Chief Experimental Test Pilot Roy Hopkins in the BA609. Flying the tiltrotor was straightforward, thanks in large part to FBW technology. The complex transition (in terms of aerodynamic stability and controllability) from vertical flight to airplane mode (and back) could not be accomplished safely without digital flight controls, and was the primary drawback to commercial success for both the XV3 and XV15 prototypes.
Bell/AgustaWestland claims more than 80 orders for the BA609 from private operators, EMS, offshore and paramilitary. Without question, there remains a civilian need for HS-VTOL, but the future of the BA 609 remains clouded, due at least partly to economics.