鈥淩ECONNAISSANCE videos look the same whether they are taken from an aircraft
the size of a jumbo or one the size of your hand,鈥� Stephen Morris tells his
audience. 鈥淭his is the kind of thing you can do.鈥� He turns to face a giant video
screen as it flickers into life. And there on the screen is Morris in a park in
Palo Alto, California, launching an aircraft so small that it disappears from
view within seconds.
Live action
Abruptly, the image changes to show the view from the aircraft鈥檚 camera. The
on-screen Morris can鈥檛 see the aircraft, so he flies it using these same images.
The live Morris turns back to commentate on the action. 鈥淭his is looking down on
a Silicon Graphics complex which was recently built near the park,鈥� he says as
the aircraft circles above an impressive complex of buildings and landscaped
gardens. Equipment on the roof and people leaving the building are clearly
visible. 鈥淭he on-board camera and transmitter have a range of about a mile,鈥� he
adds. As the aircraft turns, the camera pans across to a road. A car pulls out
and the aircraft follows it, occasionally switching to a telephoto lens for a
better view. The driver, of course, is utterly unaware of being watched.
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In the darkened room, the audience of leading aerospace engineers,
aerodynamics experts and military specialists ponder the possibilities. The head
weapons analyst from the US Naval Air Warfare Center in China Lake, California
watches quietly from the back row. Morris, who runs his own aerospace company
called MLB in Palo Alto, is one of the growing band of engineers working on a
new generation of minuscule aircraft鈥攅ven smaller than the one he has just
been demonstrating. Micro air vehicles (MAVs) have a wingspan of less than 15
centimetres. They are designed to exploit jet engines the size of shirt buttons,
highly efficient electric motors or even entirely new forms of propulsion. Some
will fly like insects.
The driving force behind this effort is the need to fill a surveillance blind
spot left by today鈥檚 military satellites and spy planes. MAVs are needed to give
soldiers a view of what is over that next ridge, or who is hiding in that
bombed-out building. The craft will have to be light and small enough to fit in
a soldier鈥檚 backpack, yet capable of carrying a solid-state camera, infrared
sensor or radar detector on flights of several kilometres. Fitted with
electronic noses, the craft could even track individuals by their scent
alone.
But these tiny flying machines are still several steps away from reality.
Last month, the scientists and engineers attempting to build them met at the
Georgia Institute of Technology in Atlanta to compare notes. This is where
Morris鈥檚 home video made such an impact.
Building an aircraft smaller than 15 centimetres is easy enough鈥攁 paper
plane would qualify. The big barrier is making them do something useful, like
taking video pictures and transmitting them back to base. Even Morris鈥檚 plane is
way over the limit, with a wingspan of almost 80 centimetres. Next week, many of
the pioneers who attended the conference will meet again in Florida to compete
in the first MAV competition. Their task will be to fly 600 metres and transmit
back an image of a target. The smallest aircraft to do this will win.
Sense and avoid
For the moment these vehicles will be piloted from the ground. But the goal
is to make them autonomous, controlled by an on-board computer that will use the
sensory data they collect to fly them around hazards such as trees and
buildings. Perhaps most challenging of all, MAVs must carry a power supply that
will keep them in the air for long enough to complete their mission. About 30
minutes is the minimum.
Engineers are already well on the way to developing jet engines small enough
to power MAVs. Alan Epstein, an aerospace engineer at the Massachusetts
Institute of Technology, is building a jet engine little more than a centimetre
across. And John Sherbeck, an engineer at M-Dot, a technology company in
Phoenix, Arizona, has a slightly larger design. Both are scaled-down versions of
conventional jets and work using rapidly spinning turbines to compress incoming
air. Fuel is then added and burnt. The exhaust gases provide the thrust, and
also drive a rear turbine that is connected by a shaft to the compressor blades,
forcing in more air.
The engines will have to be powerful enough to propel a MAV at more than 300
kilometres an hour. Speed is crucial to avoid buffeting by gusts of wind that
would make the craft useless as a camera platform, and might even prevent it
flying at all.
Microjets push engineering to its limits, says Sherbeck. Placing a turbine
just a couple of centimetres across in the palm of his hand, he points out that
existing machine tools can shape the tiny structure only to within a fraction of
a millimetre. The gap between the turbine and its casing is of similar
dimensions, so any small change in the size of the turbine due to stresses or
temperature changes can lead to disaster. Despite these challenges, Sherbeck is
confident that he鈥檒l have a working microengine within six months.
But Epstein and Sherbeck鈥檚 tiny turbojet engines are unnecessarily complex,
according to Rob Michelson, an electronics engineer at the Georgia Tech Research
Institute (GTRI). His alternative is to use pulse jet engines, which work on
much simpler principles. A pulse jet consists of a hollow tube with a flapper
valve at the front to admit air, a hole in the side for injecting fuel and a
pair of electrodes to create a spark. The exhaust gases created by burning the
fuel force the engine forwards. At the same time, the expanding gases push open
the flapper valve at the front, allowing a small burst of air into the tube
before it snaps shut. The air mixes with fuel ready for the next spark (see
Diagram).
Flying bombs
Pulse jet engines were used to power the V-1 鈥渇lying bombs鈥� that were
launched on London from across the Channel during the Second World War. They
fell out of favour as conventional power plants because they are inefficient.
But for MAVs this drawback may be outweighed by the advantage of having only one
moving part.
Michelson has already built a demonstrator, a small brass tube about the size
of a fat fountain pen. 鈥淭his engine will produce a tiny ultrasonic sound,鈥� he
says, tossing the device up and down like a little baton. He acknowledges,
however, that an engine that could drive an MAV is still some time away. For a
start, the tube would have to be much lighter. Michelson says carbon composite
materials would be ideal.
Some means of metering the air and fuel entering the engine will also be
needed, as well as a way of varying the pulse rate to control the thrust. Tiny
machines carved out of silicon could do the trick, Michelson believes. Such
microelectromechanical machines (MEMs) are already being made to do a variety of
tasks, from measuring acceleration to controlling airflow (鈥淚nvasion of the
micromachines鈥�, New Scientist, 29 June 1996, p 28). But there are
problems here, too. Existing silicon devices can only operate at relatively
modest temperatures鈥攔arely more than a few tens of degrees Celsius. Pulse
jet engines will run much hotter. 鈥淲e will need MEMs devices that withstand the
heat,鈥� admits Michelson.
Internal combustion engines are not the only option. Rob Roglin, a research
engineer who works with Michelson at the GTRI, is designing a disposable
military surveillance MAV powered by a jet of carbon dioxide released from a
cartridge similar to the type used in soda siphons. Another propulsion option is
to use electric motors to turn propellers. These motors can be made
incredibly small. Engineers at the Microtechnology Institute in Mainz, Germany,
have built a twin-rotor helicopter about 1 centimetre long, driven by a pair of
motors. Last September, much to the amusement of the world鈥檚 media, the helicopter
made an indoor flight while tethered to a power source on the ground.
Small electric motors can even be bought off the shelf. This is the option
preferred by Matt Keennon, an electrical engineer with Aerovironment, an
innovative technology company based in Simi Valley, California. Using
commercially available parts, which Keennon says can be bought for a few hundred
dollars 鈥渋f you know where to look鈥�, he has built two electric-powered aircraft
almost small enough to fit in the palm of your hand. Their ailerons and rudder
are remotely controlled, but speed is limited. For the moment, Keennon will have
to live with the problems caused by high winds.
It鈥檚 not just propulsion specialists who are being taxed by the extremely
small scale of MAVs. Aerodynamics poses major problems too. With large aircraft,
aerodynamics is all about a smooth flow of air. But as flying objects become
smaller, the viscosity of air becomes increasingly important until, for the
smallest insects, flying is more like swimming through honey. The wing sizes of
MAVs fall uncomfortably between these two extremes, and little is known about
aerodynamics at this scale鈥攁lthough this is changing (see 鈥淏reaking the
laws of flight鈥� New Scientist, 18 November 1995, p 55).
Aerodynamicists generally assume that the layer of air in contact with a wing
must be 鈥渟tuck鈥� to it by friction. By contrast, air several centimetres away
from the wing moves freely. Between these two is the 鈥渂oundary layer鈥�, which
behaves in such complex ways that some scientists spend their entire careers
studying it.
For large wings, the behaviour of the boundary layer is well understood,
allowing aerodynamicists to predict the flight of large birds and aircraft with
confidence. Any pilot knows that if an aircraft鈥檚 wing makes too large an angle
with the air flow, the boundary layer becomes separated from the wing, resulting
in the catastrophic loss of lift known as a stall. Likewise, pilots understand
that if the aircraft levels out, the boundary layer reattaches and lift
returns.
But for a wing that is only a few centimetres long, things are far less
predictable. Microwings are much more susceptible to boundary layer separation,
and even small changes in the angle of flight can result in a catastrophic loss
of lift. Worse, the boundary layer does not always reattach when the wing
returns to level flight. This kind of problem threatens to make MAVs very
difficult to control.
But all is not lost. Bob Englar, an aerodynamics expert at GTRI, says that
with minor modifications the amount of lift produced by a wing can be controlled
without changing the angle it makes with the air flow. The trick, he says, is to
blow air through a narrow slit that runs the length of the trailing edge of the
wing (see
Diagram). This generates a thin boundary layer that sticks
to the rounded contour of the trailing edge so that the flow points towards the
ground. The reaction to this redirection of air flow is a force that lifts the
wing. The phenomenon is known as the Coanda effect, after the Romanian physicist
who discovered it in the 1930s.
The principle was tested in the 1970s on US Navy A-6 aircraft, when engineers
designed a system to redirect a small amount of exhaust from the aircraft鈥檚 jet
engine. According to Englar, who headed the programme, the technique increased
lift by 140 per cent, cut takeoff speed by one-third and takeoff distance by
two-thirds.
Blowing hot
MAVs may be the next to benefit. With the extra lift that blowing can
generate, MAVs should easily be able to carry useful payloads, something that
today鈥檚 model aircraft cannot. But the big advantage is the possibility of
adjusting the lift by changing the amount of blowing, so the MAV鈥檚 angle of
attack need never be changed. This could eliminate the problem that small
aircraft have with boundary layer separation, says Englar. What鈥檚 more, by
changing the blowing on one wing relative to the other, the aircraft will turn.
This eliminates the need for ailerons and flaps. 鈥淢AVs could be controlled
without any moving parts,鈥� he says. Englar is planning to put a small aerofoil
the size of an MAV wing through its paces in a wind tunnel at the GTRI.
Of course, the 15 centimetre wing span is the maximum size for MAVs; smaller
would be better. At much smaller scales, scientists are hoping to borrow some of
the tricks of flight that insects use. Michelson is already trying to mimic the
beating motion of insect wings, and he is not the first. Hirofumi Miura and Isao
Shimoyama, mechanical engineers at the University of Tokyo, have already built a
small beating machine like a butterfly that is powered by rubber.
If the aerodynamics and propulsion problems can be sorted out, and MAVs
actually take to the air, the question then arises of how to control them. Their
small size will severely limit the length of the radio antennas they can use,
which in turn makes microwaves, with wavelengths of a few centimetres, most
suitable. This is a mixed blessing. Microwaves can carry large amounts of
data鈥攅nough for live video pictures; for example鈥攂ut they do not
pass through walls and so can only be used when the vehicle is in sight. The
ramifications of this are enormous. If MAVs are to fly out of sight鈥攂ehind
buildings, say, and even inside them鈥攖hen the machines will have to
navigate themselves. Autonomous aircraft are already flying, but making the
control systems small enough is a tough extra task for MAV designers.
Last year, Michelson organised a competition to find out just how well
autonomous aerial vehicles could perform. At the Walt Disney EPCOT centre near
Orlando in Florida, he laid out a dump of partially buried drums of make-believe
toxic waste, and challenged teams to locate it, map the drums and bring back a
sample from the area. The winner was a helicopter built at the Charles Stark
Draper Laboratory in Cambridge, Massachusetts. The Draper small autonomous
aerial vehicle (DSAAV) is the size of a milk crate with a rotor almost 2 metres
across, which is huge compared to the vehicles that Michelson, Englar and
Epstein have in mind. It navigates using Global Positioning System satellites,
together with on-board gyroscopes. But Paul DeBitetto, a member of the Draper
team, is quick to admit that none of the existing devices is suitable for MAVs.
The gyroscopes weigh a mere 15 grams, but this is still at least 10 times too
heavy for a MAV. And even the smallest GPS receivers are heavier than a complete
MAV and would consume all the available power and more.
To land automatically, the DSAAV has to know its altitude to within
centimetres, and it can glean this information from its modified GPS system,
says DeBitetto. MAVs will be more demanding still. But rather than add even more
sensors to achieve this extra precision, the trick will be to use data from the
existing sensors鈥攖he video cameras and radar devices, for example鈥�
and to use these to navigate and determine altitude. The question then becomes
how to do this effectively. 鈥淚t鈥檚 not a question of on-board processing power
but of what you look for, what clues you need to navigate in real time,鈥� says
Sam Blankenship, a physicist at the GTRI who organised the MAV conference.
Autonomy may still be a huge obstacle on the route to MAVs, but according to
Jim McMichael there is larger one still鈥攁nd that is power. McMichael is an
important figure. As programme manager for MAVs at the US Defense Advanced
Research Projects Agency in Arlington, Virginia, McMichael is the person that
researchers must impress if they want funding from the Department of Defense.
鈥淧ower problems could be the biggest potential showstopper,鈥� he says. The
problem is to pack enough energy into the volume and weight that a MAV can
carry.
There is no shortage of potential solutions. In the short term, fossil fuels
are probably the most promising power source because they have the highest
energy densities: 1 gram of petrol combined with air provides 13.1 watt-hours of
energy. A lithium battery of the same weight provides 0.3 watt-hours and a
rechargeable Nicad only 0.03 watt-hours. Not surprisingly the favoured options,
at least in the short term, are internal combustion engines. Electrical power
could be drawn from them using alternators. Alternatively, thermocouples could
turn heat from burning fuel into electrical power.
Wing power
But battery power is bouncing back. At last month鈥檚 conference, researchers
from the battery companies AstroPower and Ultralife announced a new lithium
battery that can be recharged by sunlight. The battery comes as a thin, flexible
sheet that could be used to cover the surface of a MAV. Whether it could provide
enough power to keep the craft aloft has still to be proved.
Another option is to leave the power source on the ground. At the Naval
Postgraduate School in Monterey, California, researchers are looking at ways to
beam power to an MAV using microwaves. As well as equipment aboard the MAV, this
would require a panoply of ground-based gear, from microwave dishes and aiming
devices to some way of transporting them around.
Probably the most important lesson that engineers are learning is that
traditional approaches to aircraft design simply do not apply to the tiny craft
they are aiming for. 鈥淢AVs have a high surface to volume ratio so we need some
surface functionality,鈥� says McMichael. In other words, building a shell and
stuffing it with equipment will not work: each of the craft鈥檚 structures must do
as many jobs as possible. Batteries that form the aircraft鈥檚 skin are just one
way of putting this idea into practice.
Despite the problems, McMichael is bullish about MAVs. He expects to see
examples flying within a year or so, with more advanced concepts taking to the
air within four years.
Inevitably, they will appear first on the battlefield, which is where Roglin
is targeting his carbon dioxide-powered MAV. He reckons his aircraft will fit
into a canister the size of a hand grenade, with its wings folded and spring
loaded. Pull the pin and the carbon dioxide jet will catapult the can several
hundred metres into the air. Next, the casing falls away and the aircraft鈥檚
wings spring into shape. The soldier controls the vehicle using a hand held
video display the size of a Gameboy. Two buttons turn the aircraft left or right
while a third opens a valve allowing carbon dioxide to escape, to boost
altitude.
During the flight, batteries power an on-board video camera which sends
images to the hand-held display. When the batteries and carbon dioxide run out,
the soldier discards the plane. 鈥淲e should be able to build them using
off-the-shelf components costing a few hundred dollars each,鈥� says Roglin, a
bargain by military standards.
The lesson from all this is simple. Next time you hear a high-pitched buzz
from above, or see what looks like a bird circling overhead, take a closer look.
Even an insect fluttering into the room may not be what it seems. You never know
who may be watching.