Dr. Jayant Sirohi has focused his research on determining the most effective and efficient way to build tiny unmanned aircraft that can fly off and complete missions on its own. For the past two years, Sirohi has been developing, building and testing prototypes to find the most efficient design for unmanned, multi-mission capable micro aerial vehicles. These micro-helicopters would primarily be used for indoor surveillance, but filling that role could mean anything from recording video inside a room to sampling the air in a building that’s potentially dangerous due to fire or fumes.
Sirohi began working on micro-helicopters during his post doctoral research at the University of Maryland, asking a question that might seem, at first, deceptively simple: If you take a big helicopter and shrink it down exactly the way it is – completely miniaturizing it without making any adjustments – what happens to its performance?
“We find that it gets worse. It’s more inefficient,” Sirohi said. “If you want something that can fly around and hover inside a room, it’s not the best idea to take a big helicopter and shrink it down.” The primary reason a helicopter becomes less efficient as it shrinks is that the Reynolds number decreases as the scale decreases. The lower Reynolds number, a measure of the viscosity of air relative to the other forces acting on it, means the air appears thicker to whatever is passing through it, so the drag increases disproportionately compared to the lift.
So Sirohi set out to determine what a micro-helicopter would look like that is just as efficient as its much bigger brothers. “Does it look like a real helicopter or does it look like something that can flap its wings?” Sirohi asks. Of course, the answer also depends on what you want the micro-helicopter to do. A fixed wing, such as that on an airplane, has significant advantages over a rotary wing for some missions, or for parts of a mission.
“The advantages of fixed wing are that they are more efficient in forward flight, they cruise faster, and they’re more sensitive to gusts,” Sirohi said. “The rotary wing can hover in one small spot, but inherently the efficiency is lower, so you’d run out of batteries if you tried to move from one place to another in rotary wing mode. It’s not really ideal for flying over a long distance.”
But a rotary wing is essential for hovering, which is necessary for missions in small, confined spaces, so Sirohi wanted to develop a prototype that could morph from a fixed wing to a rotary wing and back again. Such an aircraft could travel the long distance necessary to get to its target area and then transform into a rotary wing to enter a building, navigate the corridors and complete its mission before traveling the long distance back to the launch area. Sirohi and his students design and build their electric battery-powered prototypes using off-the-shelf electronics and motors at the WRW, where one of his undergraduates has recently come up with an airframe design he will build when he returns in the fall.
Another project Sirohi has been researching is developing a microhelicopter with flexible rotor blades. “The idea is to have a blade that can be rolled up almost like a tape measure so you can change the diameter of the rotor,” Sirohi said. “If you have a large diameter rotor, the power it requires to hover is lower. If it’s smaller, it can go through smaller spaces.”
Having the best of both worlds, then, would be ideal, but this concept presents its own set of challenges. “When you have a rotor blade that has no stiffness and can be rolled up, it’s not stable,” Sirohi said. “It starts fluttering and makes this buzzing noise. It has a poor performance, so we’re looking at ways to make this rotor blade stable and improve its performance so it’s as good as a conventional rotor blade.”
Sirohi said he enjoys his work because of the hands-on element which combines all the areas of aerospace engineering. “It’s got aerodynamics and structural dynamics,” he said. “It’s very mechanically intense, and I like doing things with mechanisms and experimenting a lot.”
If Sirohi is successful in realizing these concepts, the applications could be tremendously helpful in a range of situations, from assessing the safety of interior spaces or surveying structurally unsound buildings such as those following an earthquake.
“There could be a fire in a building or on a ship and you want to go in there and see what’s happening in there,” Sirohi provides as an example. “You could send in drones to take a sample of the air and see what gases are in there, or see the layout of a building without endangering the life of a firefighter.”
In addition to the lab at WRW, Sirohi spends a good amount of time at his Rotor Whirl Test Facility on the Pickle Research campus, where he’s been collaborating with Bell Helicopter to test reduced-scale, advanced technology helicopter rotors. He is also working on developing mission-adaptable rotors that can change their characteristics mid-mission to become more efficient for that particular job.
“Sometimes you need to trade off between certain performances,” Sirohi said. “If you want to reduce the noise from the rotor, you would reduce the efficiency. If you want to go into a mode where you have very good fuel efficiency, you would not have enough thrust for maneuvering. The way you get around this is by changing the speed or the geometry of the rotor and adapting it in different ways during the course of the mission.”
Source: University of Texas at Austin