How Low Can You Go?

By James T. McKenna | October 1, 2005

University of Maryland researchers are among many teams exploring low Reynolds number rotors and other aerodynamics needed to make very small rotorcraft a practical reality.

For six years, a small group of researchers and doctoral students at the University of Maryland have been plumbing the depths of just how low rotorcraft can go.

The researchers and students at the school's Alfred Gessow Rotorcraft Center have been plumbing the subject of micro aerial vehicles, systems so small that--even shrouded in a thin, protective cage--they could fit inside a basketball. The Maryland vehicle, Micro Coaxial Rotorcraft (MICOR, for short), weighs less than half a pound, depending on its payload.


At that scale, everything can become a challenge. Selection of a powerplant, for instance. Development and installation of flight controls. Even the aerodynamics of the vehicle's rotors--for it is the rotorcraft center.

But don't be deceived into thinking of the group's projects as child's play, even though some of the students look to a middle-aged eye like they should be clutching a little-league bat, not a doctoral degree. No, this is serious stuff, with serious people interested in it. This group's work has captured the interest of some at the U.S. Defense Advanced Research Projects Agency. Today, the researchers and students work with the support of $5 million in funding from the Army Research Office.

For instance, as the students and faculty explain it, the scale of these micro aerial vehicles (the center has three flying prototypes now) means they "operate in a unique aerodynamic regime involving low Reynolds number flow conditions."

Many readers may grasp that. It's way over my head. Just getting a definition of "Reynolds number" is a challenge. (As you can now tell, I'm not an engineer.) One resource explained that "Reynolds number" is "the dimensionless ratio of the inertial force (U2/L) to the viscous force ( U/L2) in the Navier-Stokes equations, where U is a characteristic velocity, L is a characteristic length, and is the kinematic viscosity of the fluid."

I see.

Another resource explained that this number "dominates the viscous effects by defining the size of the boundary layers," adding that "almost all aerodynamic flows occur at high Reynolds number, which implies viscous phenomena are limited to narrow boundary layers." Ah, so low Reynolds numbers would be unique.

Indeed, in searching for the optimum configuration of a micro aerial vehicle's rotor blades, the Maryland researchers have charted some new territory, aerodynamically speaking. The better rotors at this scale, they've found, are not necessarily what you'd like to see on your helicopter. One bit of research indicated, for instance, that a blunt, even squared off leading edge might have some promise.

This work at the Gessow Center kicked off in 1999, when two graduate students, Paul Samuel and Jayant Sirohi, had a brainstorm about building a micro helicopter. They put the idea to Darryll Pines, a professor at the center who now specializes in micro aerial vehicle development (as well as rotorcraft structural prognostics and health management; he's also a program manager at DARPA.)

Pines backed their idea with funding and morale support.

"The project is focused on the development of hovering MAVs and supporting technologies," Samuel said, "and is expected to yield systems that will be used by warfighters and emergency response units in the near future for missions including over the hill/around the corner surveillance, urban warfare, biological and chemical agent detection, and other missions of this nature."

The systems are expected to be man portable and expendable (hence inexpensive), he added, and have a high degree of autonomy.

Samuel and his colleagues now are working on incorporating enough computing power within their weight and space constraints to permit basic stability augmentation. They've added a six-degree-of-motion inertial measurement unit and an avionics package to permit operation of the vehicle from a computer ground station instead of a hobby-shop radio control box. They've also upgraded the transmission.

In addition to the coaxial design, the Gessow group has a prototype with a single rotor and vanes to guide its downwash for directional control.

The group at the Gessow Center is far from alone. Universities throughout the United States and the world have active micro aerial vehicle research and development programs.

"This is a very large area of interest to the military right now," Samuel said. "MAVs are considered a hot technology, and a man-portable, autonomous and expendable hovering MAV that can be simply pulled out of a pack and sent to inspect a building for threats is considered the Holy Grail."

Where military interest goes, funding (and academia) will follow.

As a measure of just how hot a technology this is, specialists in the field convened last month near Garmisch-Partenkirchen, Germany for the First U.S.-European Micro Aerial Vehicle (MAV) Technology Demonstrations. They were held in conjunction with the First U.S.-European Micro Aerial Vehicle Workshop.

Noting that micro aerial vehicles "are ideally suited for deployment in intelligence, surveillance and reconnaissance missions," organizers of the events challenged attendees to compete in a particular scenario:

"A little known Swedish terrorist group has been infiltrated by the Swedish Secret Police and a plot to place an explosive device of an indeterminate nature into a train terminal has been uncovered. A member of the group will place the explosive device in the terminal, and is willing to sacrifice himself by setting off the device if he suspects he's been followed or detected. The police learn the delivery is already under way but do not believe the terrorist has yet entered the building. Police or military patrols are out of the question."

Competing teams portrayed units called in to use micro aerial vehicles to covertly identify the terrorist before he enters the terminal, so snipers can neutralize the courier outside the terminal.

"Unfortunately," the organizers note, "the terminal is in the open, about 0.5 km from the nearest buildings." Not only will the snipers have to shoot from that distance, the teams will have to deploy their vehicle at that stand-off distance.

Terms of the competition were stringent. Since the terrorist was expected to reach the terminal within 30 min., contending vehicles had to provide surveillance around the terminal for that long.

To win, a team must ID the terrorist at least 60 sec. before he entered the terminal.

Competing micro aerial vehicles could weigh no more than 1.1 lb. (500 grams), though 0.33 lb. (150 grams) was preferred. They would have to fly at least 10 min., with the goal 30, with a range of at least 0.3 mi. (0.5 km.) and a noise level of no more than 60 dB at 15 m.

Demonstrations were conducted in daylight with winds gusting up to 20 kt.

Vehicles had to be tele-operated or fully autonomous and had to sustain flight out of ground effect "while requiring the Earth's atmosphere as a medium of interaction to achieve lift" (that is, no pogo sticks). They were permitted to land and takeoff autonomously within the arena if desired. No tethers were permitted.

Any form of propulsion was acceptable if judged to be safe.

The incentive for competing teams, besides pride, was up to $300,000 in research contracts.

Child's play?

Receive the latest rotorcraft news right to your inbox