Micro Air Vehicles or MAVs have proven themselves in military situations and
are now poised for consideration in domestic applications. Going beyond MAVs,
the U.S. Dept. of Defense Advanced Research Projects Agency (DARPA) is pursuing
the next generation of even smaller craft dubbed Nano Air Vehicles (NAVs).
Creating ever smaller, pilotless and even autonomous flying vehicles requires a
shopping list of sophisticated technologies. Continued improvement of existing
technologies or technology breakthroughs are among the issues confronting
developers of these crafts.
Surprisingly, many of the same systems found on the largest
airplanes are also found on some of the smallest, including the infamous black
box. The interaction among the flight systems is even more critical in the
smaller aircraft adding to the complexity of the design problem.
"We are really looking at a very interdisciplinary type of
activity, especially the control theory is completely redefined," says George
Huang, Ph.D., a professor and chair of the Dept. of Mechanical and Materials
Engineering in the College of Engineering and Computer Science at Wright State
University, Dayton, OH. Huang and other university researchers are striving to
take MAVs to an even smaller scale. In partnership with Wright Patterson Air Force Base,
Huang is working to solve the flight problems of lightweight MAVs, which weigh
only grams. A few manufacturers have solved these issues on larger MAVs
currently used by the U.S. Air Force, Navy, Army and other defense
organizations in a variety of missions.
AeroVironment Inc., a California company that develops unmanned
aircraft systems, shrunk its highly successful 4-lb Raven Unmanned Air Vehicle
(UAV) to a 1-lb MAV called the WASP III. "It had to be small and to be packaged
in a very, very small area and volume such that it could be carried into the
field by the operators who have to carry not just a UAV system, but also all
the other things that they need to do their job," says Gabriel Torres, project
manager and technical lead for the development of AeroVironment's WASP Project.
The propulsion system for the WASP III is heavily
optimized for efficiency enabling it to carry a rather heavy infrared (IR)
night vision camera (approximately 2-lb payload) for 45 minutes. "You can
increase the endurance of an airplane by doing two things, decreasing its
weight or increasing its efficiency," says Torres. "Obviously, when you have a
fixed payload that you have to carry, you can't do much about the first one."
As a result, the engineering development involved an
extensive amount of testing and optimization of the propulsion and battery
systems for the electrically powered airplane. "The battery is the latest
technology in lithium polymer cells," says Torres. The propulsion system and
propeller are optimized for performance, while generating a minimum amount of
One design issue AeroVironment engineers confronted was
field serviceability. "For all of our systems, and WASP is no exception, we
make a very careful, deliberate decision in the design process to make sure
that the system is completely repairable in the field for the things that are
going to be possibly damaged," says Torres. "We have parts in every one of our
airplanes that are considered frangible - they're meant to break to relieve
stress so that the expensive parts don't break."
For the fixed wing WASP III, the plastic propeller
provides stress relief and may break to prevent damage to the engine.
AeroVironment engineers developed an innovative method to quickly replace a
propeller in less than five seconds, without using tools. "A custom-designed
hub for the propeller allows you take it in and out very, very quickly," says
Ducted Fan MAVs
Vertical Take-Off and Landing (VTOL) aircraft using a
ducted fan design avoid the propeller problem of a fixed-wing airplane but, at
this point, is a much heavier aircraft. A ducted fan system draws air into the
duct creating a region of low pressure around the duct that causes aerodynamic
lift. DARPA has ducted fan data dating back to at least the 1950s, but a viable
vehicle eluded reality until this decade. "The ducted fan system itself is just
a completely unstable system, so it takes a very sophisticated flight control
system and very fast processing rates to close the loop on those flight
controls to keep it stable enough to employ a sensor," says Vaughn Fulton,
senior unmanned aerial systems program manager, Honeywell's Defense and Space
For Honeywell's T-Hawk MAV, the solution employs its
microelectromechanical systems (MEMS) technology. "Micro but very capable
flight mission computers and inertial sensors together in a very small
package," says Fulton.
"Prior to that, the ducted fan vehicles had trouble because the size of the
mission computers necessary to run these very complex flight controls was 20,
30 pounds worth of avionics and LRUs (line replaceable units)," he says.
In addition, Honeywell leveraged its design capability in
engines, the nacelles that go around engines and the airflow associated with
the turbojet engine components to improve a ducted fan aircraft performance.
"We drove a several fold magnitude increase in the efficiency of a ducted fan
system," says Fulton.
Cameras are another system area Honeywell engineers
address in the development process. Initially, the cameras chosen for the MAV
did not provide acceptable situational awareness. "We changed the field of view
to match better the experimentation at the infantry level, how they want to
employ the vehicle, at what range they were employing it, what altitude, their
standoff ranges," says Fulton. "All of that drove additional specifications on
VTOL with a Twist
Aurora Flight Sciences initiated its GoldenEye 50 program
as a technology validation aircraft. The project allowed Aurora to investigate various system and
subsystems details of a ducted fan VTOL aircraft with two movable wings.
"The key figure of merit, the thing that you are
interested in, is the ability to lift payload and fuel," says Tom Clancy, chief
technical officer and vice-president of engineering, Aurora Flight Sciences. A
rather complex set of trade-offs involve the duct sizing associated with the
thrust loading of the lift fan and the power to weight of the engine.
Networking plays an important role in GoldenEye 50's
control. Aurora Flight Sciences uses a network architecture for communications
both inside and outside of the MAV.
Because of the networked architecture, the one operator
and one aircraft with a point to point link between them of a typical
radio-controlled UAV changes dramatically. Clancy explains, "It's a whole
different paradigm, where you have multiple users doing different things and
information being exchanged with different nodes of the UAV system all