|
|
By Joseph Kays
Since the Wright Brothers first floated over the sand dunes at Kittyhawk, bigger and faster has always been the goal of aerospace engineers. From the DC-3 to the 747 to the Concorde, engineers have strived for aircraft that can carry more cargo farther in less time.
But one of the hottest trends in aircraft design today is to develop smaller planes, known as micro air vehicles, or MAVs, with wingspans of less than six inches that travel at just 20 to 30 miles per hour.
"In the development of micro air vehicles, many things we've learned about aircraft design are useless," says Wei Shyy, chair of UF's Department of Aerospace Engineering, Mechanics and Engineering Science (AeMES). "Aerospace engineering education usually deals with high-speed phenomena. Here we go to the other extreme."
Like so many technological advancements, the push to develop MAVs is coming from the military. Military planners envision a soldier of the future pulling an MAV from his belt like a grenade, launching it and, within minutes, receiving video images of an enemy battalion over the next hill or chemical analysis of poisonous gases inside a factory target. MAVs also could serve important civilian roles, such as flying into a nuclear reactor, assessing the risk at a chemical spill or combing crumbled freeways for earthquake survivors.
Everything about MAVs is open to new thinking, Shyy says. Researchers at universities, government laboratories and private businesses around the country are taking novel approaches to virtually every aspect of the aircraft, including control and navigation, the propulsion system and wing design. One research team is even working on an MAV that looks more like a giant insect than an airplane. The U.S. Department of Defense has earmarked millions of dollars over the next decade to develop the micro aircraft and its components.
Nowhere was this experimentation more apparent than at the first Micro-Aerial Vehicle Flyoff coordinated by AeMES Professors Raphael Haftka and Edward Walsh last April at a field near UF's Gainesville campus. Five teams - with entries ranging in size from 24 to 48 inches and 1.27 to 4.98 pounds - attempted to launch planes, have them peer inside a box 600 hundred yards away and report back to the launch site on its contents. Some never got off the ground; others crashed during flight. Only two planes, one built by a California aeronautics company and one of the UF entries, successfully completed the "mission."
"The technological reality is that today's mission-capable MAVs are not yet palm size," says Richard J. Foch, head of vehicle research for the U.S. Naval Research Laboratory. "But the ability to perform a video reconnaissance mission with air vehicles significantly smaller and less costly than those currently being employed by the military was effectively demonstrated by this competition."
Foch adds that engineers unimpressed by the size or performance of the entries in the 1997 competition have until May 9 to design a smaller, cheaper, better-performing entry for the 1998 competition, also to be held at UF.
Mimicking Mother Nature
From a design standpoint, the good news about MAVs is that as they get smaller they are more easily able to withstand crash landings.
"A squirrel can fall from a tree without any harm, even if it doesn't land on its feet," says UF aerospace engineering associate Professor Bruce Carroll. "A cat can do that only by carefully landing on its feet, while a human is unlikely to escape injury. In the MAV competition, some entrants crashed several times without any apparent damage."
The bad news is that their small dimensions and modest speeds make it much harder for MAVs to maintain the lift needed for controlled flight.
"In very small airplanes like this, the aerodynamics are worse than a Boeing 747," says Shyy.
Just how poor can be measured through a mathematical expression known as "Reynolds Number." While it has applications in virtually every engineering field, in aerospace engineering Reynolds Number is used to measure the drag, or viscosity, of air as it flows over the wings. The lower the number, the greater the drag.
Reynolds Numbers for large aircraft are typically in the millions, but the small size and low speed of the MAVs currently being developed cause their Reynolds Numbers to be between 1,000 and 100,000.
To small objects that fly in this Reynolds Number range, the air may seem more like thick syrup. Viscous forces begin to dominate, says UF aerospace researcher David A. Jenkins.
But small birds and insects overcome these limitations all the time, so the UF team has looked to nature for guidance.
"Large birds move their wings sparingly," Shyy says. "Smaller birds and insects flap their wings very fast. They also often change the shape of their wings during the flapping cycle.
"Since MAVS are in the size range of small birds," Shyy continues, "the factors that induced birds and insects to rely on moving, adaptive wings may also apply to MAVs."
Sailboats also offer an example of a vehicle that employs a flexible, adaptive surface to take full advantage of the wind, Shyy says.
The UF researchers are looking at both the initial shape of the wing and at ways to adapt that shape to changing aerodynamic conditions.
MEMS
To imitate birds' wings on MAVs, UF researchers are focusing on micro-electro-mechanical-systems, or MEMS, to create a more favorable airflow pattern around the wings.
MEMS do for mechanical systems what the integrated circuit did for electrical systems. Manufacturers employ much of the same masking and etching technology used in the semiconductor industry to carve out microscopic sensors, motors, valves and other mechanical devices from raw silicon.
MEMS already are used in such everyday applications as automobile airbags, where they both detect an impact and trigger inflation of the airbag, and they could be employed for other elements of MAV design, where many systems will have to do multiple jobs to meet the size constraints.
First, however, the UF researchers want to use them to sense when air begins to separate from the wing surface and react by modulating the airflow pattern around the wing and/or changing the shape of the wing to fill that void. Their research is supported by a $500,000 grant from the Air Force Office of Scientific Research.
"As air travels over the leading edge of the wing at slow speed, it is easy for it to lose contact with the wing surface, causing the aircraft to lose lift," Carroll says. "We are looking at MEMS systems that allow us to constantly adjust the effective shape of the wing surface to maintain better lift."
To modulate the air flow, Carroll and a team including Shyy, AeMES researchers Norman Fitz-Coy and Andrew Kurdila and electrical and computer engineering Professor Toshi Nishida are trying to create spinning vortices of air at the leading edge of the wing that will cause the air to roll over the wing surface and prevent the undesirable separation.
Much of the research on MAV design will be conducted in UF's recently modified wind tunnel, where airfoils and full-size MAVs of many shapes and sizes will be tested to determine the ideal aerodynamic shape.
But the true test of the theories being explored in the UF labs will come in future MAV competitions. With a 24-inch wingspan, UF had the smallest entry in the 1997 competition. The team hopes to have a 15-inch MAV for the 1998 competition and to achieve the desired 6-inch wingspan by the 1999 competition.
Shyy says one of the most attractive features of MAV design is that the aircraft are small enough to allow faculty and students to put their hands on the entire system. In fact, several students in the senior aerospace design course taught by Edward Walsh are designing MAVs for their class projects.
"It's nice to be able to do most of the work ourselves," Shyy says. "With full-size planes, we have to rely on Boeing or Lockheed-Martin to test our theories because they are the only ones with facilities big enough."
Wei Shyy, Professor and Chair, Department of Aerospace Engineering, Mechanics and Engineering Science, (352) 392-6416, wss@tiger.aero.ufl.edu
Bruce Carroll, Associate Professor, Department of Aerospace Engineering, Mechanics and Engineering Science, (352) 392-4943, bfc@aero.ufl.edu
David Jenkins, Associate Engineer, Department of Aerospace Engineering, Mechanics and Engineering Science,(352) 392-6105, daj@aero.ufl.edu
Related web site: http://www.aero.ufl.edu/~bfc/html/mav_mems.htm