How Spy Flies Will Work

Our country is at war in an unfamiliar territory, and a battle is about to begin. Enemy ground troops are positioning themselves to form an attack on our army, located just 2 miles (3.2 km) away. However, the enemy doesn't know that its every move is being monitored by robotic insects equipped with tiny cameras, flying overhead. These tiny robotic flyers, called micro air vehicles (MAVs), will be able to buzz over enemy territory nearly unnoticed by the enemy troops below. Few would even look twice at these dime-sized flying robots.

The U.S. Department of Defense is spending millions of dollars to develop these MAVs. They are the perfect way to keep soldiers out of harm's way during reconnaissance missions. Today, gathering reconnaissance during battle typically involves putting either small teams of soldiers or large aircraft in harm's way. At the same time, satellite imagery is not immediately accessible by a ground soldier.

The Defense Advance Research Projects Agency (DARPA) is funding several research teams to develop MAVs no larger than 6 inches (15 cm) in length, width and height. These tiny aircraft will be an order of magnitude smaller than any unmanned aerial vehicle (UAV) developed to date. One class of these MAVs is being designed to mimic the flying motions of certain insects, including flies, bees and dragonflies. In this article, we will focus on these bug-like MAVs. You will learn how flies fly, how machines can be built to mimic their movements and where these tiny aerial devices will be deployed.

Learning to Fly

Flies have a lot to teach us about aviation that can't be learned from studying fixed-wing aircraft. For years, there was little known about the mechanics of insect flight, yet they are the world's oldest group of aviators, sometimes called nature's fighter jets. You may have heard about how bumblebees can't fly according to conventional aerodynamics. That's because the principles behind insect flight are far different from those behind fixed-wing airplane flight.

"Engineers say they can prove that a bumblebee can't fly," said Michael Dickinson, a biologist at the University of California, Berkeley. "And if you apply the theory of fixed wing aircraft to insects, you do calculate that they can't fly. You have to use something different."

Dickinson is part of the Micromechanical Flying Insect (MFI) Project, which is developing small flying robots using the flight principles of insects. The project is in cooperation with DARPA. The MFI Project is proposing a robotic insect that is about 10 to 25 millimeters (0.39 to 0.98 inches) in width, which is much smaller than DARPA's size limit of 6 inches (15 cm), and will use flapping wings to fly. The project's goal is to recreate the flight of a blowfly.

If you read the article How Airplanes Work, you know that airplanes generate lift due to the air travelling faster over the top of the wing than along the bottom of the wing. This is called steady-state aerodynamics. The same principle cannot be applied to flies or bees, because their wings are in constant motion.

"Unlike fixed-wing aircraft with their steady, almost inviscid (without viscosity) flow dynamics, insects fly in a sea of vortices, surrounded by tiny eddies and whirlwinds that are created when they move their wings," said Z. Jane Wang, a physicist at Cornell University's College of Engineering. An eddy is whirlpool of air that is created by the wing, and the air in the eddy is flowing in the opposite direction of the main current of air.

The vortices created by insect wings keep the insects aloft. Dickinson's group outlines these three principles to explain how insects gain lift and stay airborne:

Delayed stall - The insect sweeps its wing forward at a high angle of attack, cutting through the air at a steeper angle than a typical airplane wing. At such steep angles, a fixed-wing aircraft would stall, lose lift and the amount of drag on the wing would increase. An insect wing creates a leading-edge vortex that sits on the surface of the wing to create lift.
Rotational circulation - At the end of a stroke, the insect wing rotates backward, creating backspin that lifts the insect up, similar to the way backspin can lift a tennis ball.
Wake capture - As the wing moves through the air, it leaves whirlpools or vortices of air behind it. When the insect rotates its wing for a return stroke, it cuts into its own wake, capturing enough energy to keep itself aloft. Dickinson says that insects can get lift from the wake even after the wing stops.

"It would be real spiffy if we could exploit these mechanisms, too, by building an insect robot. But you can't build them now based on known principles -- you have to fundamentally rethink the problem," Dickinson said. In the next section, you will learn how researchers are taking these principles and applying them to the creation of robotic flying insects.

Robobugs Prepare for Flight

There are at least two DARPA-funded MAV projects that have been inspired by the principles of insect flight. While Michael Dickinson is creating the micromechanical flying insect at Berkeley, Robert Michelson, a research engineer at the Georgia Institute of Technology, is working on the Entomopter. Let's take a closer look at both projects.

Entomopter

In July 2000, the United States Patent Office awarded a patent to Georgia Tech Research Corporation for Michelson's invention of the Entomopter, also called a multimodal electromechanical insect. The Entomopter is being designed for possible indoor operations, according to U.S. Patent Number 6,082,671. It will mimic the fight of an insect by flapping its wings to generate lift. In addition, researchers are studying ways for the Entomopter to navigate hallways and ventilation systems and crawl under doors.

Let's look at the basic parts of the Entomopter:

Fuselage - Just like in larger aircraft, this is the hull of the machine and houses the power source and primary fuel tank. All other components of the Entomopter are attached to the fuselage.
Wings - There are two wings, front and rear, which are pivotally coupled to the fuselage in an X configuration. These wings are made out of a thin film. Stiff but flexible veins are attached to the wings at the fuselage junction to give the wings the curve they need to generate lift on both the upstroke and the downstroke.
Reciprocating Chemical Muscle (RCM) - A compact, noncombustive engine is attached to the wings to create a flapping motion.
Sensors - There are sensors for looking forward, downward and sideways.
Camera - The prototype lacks a mini-camera, but the final version could carry a camera or an olfactory sensor. This sensor would detect odors, and the Entomopter would track the odors to their point of origin.
Surface steering mechanism - This aids in navigation when the Entomopter is used in ground missions.
Legs/feet - Also called surface locomotors, these parts provide anti-roll inertia and auxiliary fuel storage.

The Entomopter is powered by a chemical reaction. A monopropellant is injected into the body, causing a chemical reaction that releases a gas. The gas pressure that builds up pushes a piston in the fuselage. This piston is connected to the pivotally coupled wings, causing them to flap rapidly. Some of the gas is exhausted through vents in the wing and can be used to change the lift on either wing so the vehicle can turn. Currently, the Entomopter has a 10-inch (25-cm) wingspan. "The next step is to shrink the RCM device down to bug size," said Michelson.

In a vehicle the size of a house fly, every part must perform multiple tasks. For example, a radio antenna attached to the back of the vehicle may also act as a stabilizer for navigation. The legs could store fuel for adjustment of the vehicle's weight and balance during flight.

Micromechanical Flying Insect

The U.S. government has also invested $2.5 million in the Berkeley project to develop a robotic insect the size of a common housefly. The first major step toward getting this micromechanical flying insect (MFI) in the air was the development of Robofly, which gave researchers important insight into the mechanisms of insect flight.

In order to build the MFI, researchers performed experiments to learn how flies fly. One of the experiments involved building a pair of 10-inch (25-cm) robotic wings, called Robofly, which was made of Plexiglass and modeled after the wings of a fruit fly. The wings were immersed in a tank of mineral oil, which forces them to react like smaller, 1-millimeter-long fruit-fly wings beating rapidly in the air. Six motors -- three on each wing -- moved the wings back and forth, up and down and in a rotary motion. Sensors were attached to measure the force of the wings.

Eventually, the Robofly will be shrunk down to a stainless-steel microrobotic fly that is 10 to 25 millimeters (0.4 to 1 inch) in width and weighs roughly 43 milligrams (0.002 ounces). The wings will be made of a thin Mylar film. Solar power will run a piezoelectric actuator that will push the wings to flap. The robot's thorax will transform piezoelectric-actuator deflections into the large wing stroke and rotation required to achieve flight.

Although the robot does not yet fly, it's been reported that approximately 90% of the force required for lift has been achieved experimentally with a fully operational, two-wing structure. The next step will be to add a flight-control unit and communication unit for remote control. The researchers say that they are working on enabling controlled hovering by way of optical sensing and an onboard gyroscope.

Fly on the Wall

Considering the amount of money that the U.S. military is pumping into MAV (micro air vehicle) projects, it's likely that the first use of these robotic bugs will be as spy flies. DARPA envisions a spy fly that could be used for reconnaissance missions and controlled by soldiers on the ground. This small flying vehicle would not only relay images of troop movements, but it could also be used to detect biological, chemical or nuclear weapons. Additionally, the robotic insect would be able to land on an enemy vehicle and place an electronic tag on it so it could be more easily targeted.

In a 1997 report from DARPA concerning the development of MAVs, the authors wrote that advances in microtechnologies, including microelectromechanical systems (MEMS), would soon make spy flies a feasible idea. He pointed out that microsystems such as CCD-array cameras, tiny infrared sensors and chip-sized hazardous-substance detectors are being made small enough to integrate into a spy fly's architecture.

The military would like an MAV that has a range of approximately 6.2 miles (10 km), flies in day or night and can stay airborne for approximately one hour. DARPA officials say that the ideal speed for an MAV is 22 to 45 mph (35.4 to 72.4 kph). It would be controlled from a ground station, which would employ directional antennas and maintain continuous contact with the MAV.

Robotic flies could also be well-suited as a new generation of interplanetary explorers. The Georgia Tech Research Institute (GTRI) has received funding from the NASA Institute for Advanced Concepts (NIAC) to study the idea using the Entomopter as a flying Mars surveyor. In March 2001, NASA funded the second phase of the study in anticipation of future Mars micromissions.

Entomopters offer several advantages over larger surveyors. They would be able to land, takeoff, hover and perform more difficult maneuvers in flight. Their ability to crawl and fly also gives them an advantage in exploring other planets. Most likely, NASA would send dozens of these surveillance vehicles to explore other planets. Entomopter developer Rob Michelson said that the Mars version of the Entomopter would have to be sized up to have a wingspan of about 1 meter in order to fly in the thin atmosphere of Mars.

Researchers say that these tiny flying robots would also be valuable in the aftermath of natural disasters, such as earthquakes, tornadoes or landslides. Their small size and ability to fly and hover make them useful for searching for people buried in rubble. They could fly between crevices that humans and larger machines are unable to navigate. Other uses include traffic monitoring, border surveillance, wildlife surveys, power-line inspection and real-estate aerial photography.

Spy flies are yet another example of how technology is aiding humans in performing dangerous tasks, allowing the humans to stay out of harm's way. Military reconnaissance, searching for earthquake victims and traveling to other worlds are all hazardous activities -- flying microrobots would allow us to accomplish these tasks without actually being there.