In summer 2015 a research team led by Jayant Sirohi, an associate professor in the Department of Aerospace Engineering and Engineering Mechanics, tested an experimental helicopter rotor design in a wind tunnel, generating the first publicly available performance data on a cutting-edge, dual-rotor design.
This spring Sirohi received a five-year, $550,000 grant from the U.S. Army, U.S. Navy, and NASA to continue research on the rotor system. It also designates Sirohi’s research team, which includes collaborators at the University of Maryland at College Park, as a Vertical Lift Research Center of Excellence.
Specifically, the grant will fund the research and development of techniques that can measure “dynamic inflow,” a term for control inputs to the rotor that affect the surrounding air, which in turn affects forces generated by the rotor.
“It’s mostly an experimental project, where we’ll actually measure perturbations,” Sirohi said. “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.”
What makes the rotor system special is its design: instead of one rotor, it has two stacked rotors that turn in opposite directions, a setup known as a coaxial, counter rotating rotor system. The opposing spins of the rotors boosts helicopter 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, you have a more efficient forward flight.”
According to wind tunnel test data collected in 2015, the experimental rotor had a lift to drag ratio that was 1.5 times greater than a single-rotor helicopter. Sirohi said that the gains could potentially be even greater with rotor blades optimized to improve lift.
The rotor system, which was built entirely by graduate students on Sirohi’s research team, is about five times smaller than a full-scale prototype helicopter. Sirohi said much of the research will focus on adapting devices and measuring techniques to the small scale.
“Our problem is, since we’re doing everything on a model scale, 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 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 life size. But Sirohi said it’s likely that the data collection methods could influence other research too.
“The measurements will be used to make a full analytical model of this system, but more importantly we’ll be developing some tools along the way,” Sirohi said. “Some experimental techniques, some analytical tools that we may be able to use in other types of research, not restricted to helicopters.”
The speed and efficiency offered by the coaxial, counter rotating rotor system has attracted industry to the design. Currently, two aerospace companies are partnering to develop a prototype coaxial, counter-rotating rotor helicopter that the U.S. military could use in is next-generation fleet. In contrast, the system developed by Sirohi’s team is the only coaxial, counter rotating rotor system in academia and government laboratories. That means that while industry prototypes remain company secrets, the knowledge gained from Sirohi’s experimental rotor is open to anyone.
This can help inform other research, as well as lead to insights about the physics of a novel rotor system.
“While these [industry] teams are more interested in getting a prototype flying, we are looking at the fundamental physics of these systems,” Sirohi said.