Tail Rotor Airfoils Stabilize Helicopters, Reduce Noise

Originating Technology/NASA Contribution

Given the eye-catching nature of space shuttle launches, deep-space imagery, and Mars exploration, it can be easy to forget NASA’s aeronautics efforts, which have a daily impact on life within the bounds of Earth’s atmosphere. Virtually every flying vehicle in operation today has benefited in some way from NASA advancements, and the helicopter is no exception. In fact, NASA’s involvement in rotorcraft research can be traced back to its predecessor, the National Advisory Committee for Aeronautics (NACA). NACA was founded in 1915, less than a decade after the first successful piloted rotorcraft flight in 1907, and made a number of contributions to rotorcraft development—including a series of airfoils that are still employed in some modern vehicles.

NASA was formed in 1958, and within a little more than a decade the Agency had begun a collaborative rotorcraft research program with the U.S. Army, establishing laboratories at Ames Research Center, Glenn Research Center (then known as Lewis Research Center), and Langley Research Center. These labs focused on enhancing the performance and safety of helicopters for both military and civilian use. This research improved helicopter airfoil designs, flight control systems, aerodynamics, rotor blade and aircraft body composition, and cockpit configuration.

Partnership

Among the many outcomes of the NASA-Army research partnership are advanced airfoils designed and wind-tunnel tested at Langley. Two of these airfoil designs—the RC(4)-10 and the RC(3)-10—were licensed and commercialized by Carson Helicopters (Spinoff 2007) as a superior replacement main rotor for the Sikorsky S-61 helicopter, allowing the helicopter to fly faster and carry heavier loads while offering a service life twice that of the original rotor.

Carson’s success with its NASA-derived airfoil caught the eye of Dean Rosenlof, general manager and aerospace engineer for Van Horn Aviation LLC (VHA), based in Tempe, Arizona. The company—founded by former Ames engineer Jim Van Horn, who worked on NASA rotorcraft research like the Rotor Systems Research Aircraft in the early 1980s—was looking for airfoil designs to expand its tail rotor blade product offerings, which include an aftermarket carbon composite tail rotor for the UH-1H (“Huey”) military helicopter. Rosenlof brought the Langley RC series of airfoils—the low-speed RC(4)-10 and the high-speed RC(5)-10—to Van Horn’s attention, and they determined these were precisely the designs the company was looking for.

The complex aerodynamics of a helicopter present a challenge to airfoil designers, who must consider a range of aerodynamic forces and how they influence the rotorcraft’s flight capabilities. Among the chief concerns is pitching moment, the twisting force exerted by the airfoil that pushes the noses of the rotor blades up or down. Because this force can interfere with pilot control and rotor blade stability, designers aim to create airfoils with minimal to zero pitching moment.

“The RC airfoils were exactly what we needed,” says Van Horn. “They are very attractive in that they are thin, light, laminar-flow airfoils with essentially zero pitching moment.”

VHA contacted Langley and discovered that the airfoils’ patent had expired, meaning the original NASA designs had entered the public domain.

“Langley encouraged us to take the designs, go forward, and be fruitful,” says Van Horn.

Product Outcome

A helicopter tail rotor serves two essential functions. It provides a counteracting force to the helicopter’s main rotor; without the sideways thrust produced by the tail rotor, the torque generated by the main rotor would spin the helicopter’s body in the opposite direction. The tail rotor also allows the pilot to steer the helicopter around its vertical axis by adjusting the pitch of the rotor blades. Using the design for the NASA RC(4)-10 airfoil, VHA crafted an updated aftermarket tail rotor for the popular Bell 206 series of helicopters.

“It’s an excellent airfoil, very stable, with very high stall margins,” says Van Horn. The company built upon the RC(4)-10 airfoil, employing corrosion-resistant composite material with a titanium root fitting, a swept tip, a nickel abrasion strip that reduces wear on the blades’ leading edges, and a new pitch bearing design. The result is a highly durable tail rotor blade—the Federal Aviation Administration (FAA) granted the VHA 206 tail rotor a 5,000-hour lifetime, twice that of the original equipment manufacturer blade—with a number of enhanced features. The airfoil possesses zero pitching moment within typical operating speeds, Van Horn explains, and while the NASA airfoil’s design already limits the turbulence that causes noise and drag, the inclusion of the swept tip further reduces these undesirable qualities. FAA-mandated testing demonstrated a 40-percent reduction in the overall sound exposure level (the amount of noise produced) for helicopters employing the VHA 206 tail rotor—a welcome improvement for pilots, passengers, and people on the ground. In addition, the airfoil’s high stall margins enhance helicopter performance at high altitude; VHA flew helicopters with the new tail rotor at the Leadville Airport in Colorado, the highest elevation airport in North America, and determined the NASA-derived blade delivered superior high-altitude performance compared to the existing model. These improvements stand to benefit helicopter performance for a wide range of missions, including law enforcement and homeland security, military training, aerial patrol of wildfires and pipelines, mosquito control, and emergency medical services.

The tail rotor received FAA certification in 2009, and VHA delivered its first shipment of the NASA-derived blades to customers that same year. Now the company plans to use the NASA airfoils as its go-to design for all future projects aimed at advancing rotorcraft performance, Van Horn says. He adds that VHA has helped ensure a solid base for its future by taking advantage of NASA research.

“Given the market size and that we could capture a reasonable market share, this puts our company on very firm footing for the next 10 to 20 years and will provide a steady income to allow us to grow at a reasonable rate and develop new products,” he says.

“I’ve been on both sides of the government-research-to-commercial-product equation, and it’s a great system. It gives us an advantage that other companies don’t have, because we were able to avail ourselves of this NASA technology.”