A ROBOTIC flyer that looks like two giant moths on a stick may be the first
craft to cruise effortlessly through the Martian skies rather than inch across
its surface like Sojourner, which relayed back pictures of nearby rock clusters
and that familiar horizon. NASA hopes its next generation of landers will have a
flying companion to travel farther afield.
Despite the Red Planet鈥檚 lower gravity, it鈥檚 still tough designing a flying
machine for the rarefied Martian atmosphere. A conventional plane would never
get airborne because the atmosphere is less than 1 per cent the density of
Earth鈥檚. 鈥淣othing on Earth today could fly in that,鈥� says Bob Michelson, an
aerodynamics engineer at the Georgia Institute of Technology in Atlanta.
A conventional fixed-wing plane with huge propellers might do the job, but
the rock-strewn surface means it would somehow have to be launched in mid-air.
And its large propellers would have to spin so fast that its tips would break
the Martian sound barrier鈥攚hich would waste a lot of energy
(快猫短视频, 13 February 1999, p 22).
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Michelson reckons he has the answer鈥攃opy the way insects fly. His
Entomopter is a moth-like robot about a centimetre wide that flaps two pairs of
wings 30 times a second. He believes he can generate enough lift to fly on Mars
by scaling the Entomopter up to about a metre wingspan.
The wings are powered by a chemical 鈥渕uscle鈥�, like a battery except it pumps
out a constant stream of waste gas. The chemical muscle technology is classified
until a patent is granted, says Michelson.
But flapping wings don鈥檛 play by the same rules. Until recently scientists
were still baffled at how bumblebees manage to fly: bee wings are so sharply
angled they look as if they should cause a stall.
But in 1997 it was discovered that this doesn鈥檛 happen because insects
actually take advantage of the air spilling over their wings. They rotate their
wing tips as they flap, setting up circular vortices which rest on the wing and
increase its effective thickness and hence its lift.
Michelson says he can do the same thing with just a straightforward up and
down motion, by blowing exhaust gas from the chemical muscle out through tubes
under the flapping wings
(see Graphic).
鈥淭his should stabilise the vortex so it
stays on the wing longer and allows air to flow smoothly over the wing and the
vortex,鈥� he says. In tests, this produced 5 times as much lift as there was
without the gas. To reduce the number of moving parts, each wing pair moves in a
see-saw fashion in direct opposition to each other to cancel out any tilt. The
Entomopter鈥檚 body has springs connected to the wings that cause the wings to
rotate forwards and backwards as they flap. This also minimises the energy input
needed to keep the wings flapping.
One lesson that Michelson has taken on board from insect flight is resonance.
Insects make the most of their energy supply by vibrating at the same frequency
as they flap their wings, which makes them buzz.
Michelson鈥檚 moth will buzz at an ultrasonic frequency and so could be used
for echolocation. The oscillation is generated by the exhaust gas flowing past a
cavity鈥攃ausing it to vibrate like an ultrasonic flute.
Michael Dickensen, who works on insect flight at the University of
California, Berkeley, is sceptical about the Entomopter鈥檚 chances. 鈥淚 don鈥檛 know
how much manoeuvrability you鈥檇 have on Mars with that system.鈥� But a flying
robot might not need to turn on a coin or hover like an insect. 鈥淢aybe it can
make long, gentle turns,鈥� Dickensen says.