Story ID: 133InflowAINOct16EditedByAY_NM
Texas Researchers Study Coaxial Inflow
By: Mark Huber
Caption: Such a system could provide significant gains in a helicopter's lift-to-drag ratio.
Word Count: 564

Aerospace researchers at the University of Texas at Austin and associated collaborators have received a five-year grant from the Pentagon and NASA to study the dynamic inflow of air associated with contra-rotating coaxial main rotor systems like that found on the Sikorsky S-97 Raider. Dynamic inflow refers to the control inputs to the rotor that affect the surrounding air, which in turn affects the forces generated by the rotor. The grant will fund the research and development of techniques that can measure dynamic inflow. The research team includes collaborators from the University of Maryland at College Park.

“It’s mostly an experimental project, where we’ll actually measure perturbations,” said University of Texas aerospace engineering associate professor Jayant Sirohi. “It’s one of the few times it’s been done for any rotor system and we’ll be doing it for the first time on this special rotor system.” Last year Sirohi and his team began wind-tunnel testing the coaxial rotor design to accumulate performance data.

That data showed that the coaxial system they developed had a lift-to-drag ratio that was 1.5 times greater than that of a single-rotor helicopter. Sirohi said that the gains could potentially be even greater with rotor blades finely tuned to improve lift.

Sirohi points to the inherent advantages a coaxial system has over traditional single-stack rotors; stacked rotors turning in opposite directions boost the helicopter’s speed and efficiency by canceling out forces that cause uneven lift on single-rotor helicopters. “You don’t have to get rid of the extra lift on the advancing side of the rotor, because the two rotors cancel their rolling moments,” Sirohi said. “So the aircraft remains in level flight, and because you can make more efficient use of the rotor disk, forward flight is more efficient,” he said.

Graduate students on Sirohi’s research team are building the 20-percent-scale rotor system and much of the research is necessarily focused on adapting devices and measuring techniques to the small scale. “We’re doing everything on a model scale, so it gets difficult to fit everything in that small volume. And we’re also spinning much faster than a full-scale helicopter rotor,” Sirohi said. In addition to developing experimental tools, the team is working on fine-tuning the rotor system controls, so that variables such as pitch can be carefully adjusted. Once the measurements are collected, they will be used to make a digital model of the system scaled up to actual size. But Sirohi said it’s likely that the data-collection methods could influence other research beyond helicopters. “The measurements will be used to make a full analytical model of this system, but more important we’ll be developing some tools along the way. Some experimental techniques, some analytical tools that we may be able to use in other types of research, not restricted to helicopters,” he said.

Sirohi acknowledged that both Sikorsky and AVX are focused on developing helicopters with contra-rotating main rotor systems as part of the Defense Department’s ongoing competition for the Future Vertical Lift program, but much of their work is classified and product-specific, while his team is more focused on the basic science. “While these [industry] teams are more interested in getting a prototype flying, we are looking at the fundamental physics of these systems,” Sirohi said.