Interesting.

Big John
`````````````````````````````````````````````````` `````````````````````````````````


Flying circles around the helicopter

Hope on a rope

ECUADOR, 1956. A small aircraft skims dangerously low over the
rainforest, making tight circles above a narrow canyon. The pilot is
Nate Saint, a missionary from the Mission Aviation Fellowship. He
wants to show the Waodani people in the remote settlement below that
he is friendly. Gifts are a universal language. Now all he has to do
is drop them into a small clearing.

Keeping one hand on the joystick, he reels a basket loaded with
machetes and cooking pots out of the plane on a long line. When enough
rope is paid out, Saint's tight circular flight path combines with the
forces of gravity and drag to hold the basket almost motionless in the
air. He lets out more line, lowering the basket until it hovers a
metre above the ground.

The Waodani understand, and reach into the basket for the gifts. They
even leave some in return - a feather head dress, some smoked meat and
a parrot which Saint's son later adopts as a pet.

Although Saint's "bucket drop" technique, perfected over the orange
groves of California, proved invaluable for making contact, it has
been largely ignored - until now.

Almost 50 years after Saint's flight, Pavel Trivailo and a team of
engineers at the Royal Melbourne Institute of Technology in Australia
are exploring the same basic principles to devise a more sophisticated
air delivery system. They are working on an automated device that will
allow them to pick up and put down loads - including people - with
hardly a jolt. If their system is successful, it could speed up
rescues at sea, make cargo or aid delivery far easier and help collect
injured people from otherwise inaccessible regions of jungle or
mountain.

Of course, helicopters have been successfully performing all these
tasks for decades. So why bother developing an alternative now?

The problem is that helicopters have limited range, speed and cargo
capacity. A Lockheed C-130 transport aircraft, for instance, can carry
twice as much cargo, fly three times as fast and travel five times as
far as the biggest helicopter. This could make a major difference when
performing a rescue or trying to reach a remote disaster site. In war
zones the complex rotor systems of helicopters make them more
vulnerable than fixed-wing planes. And since rotors generate limited
lift, helicopters cannot fly to high altitudes where the air is thin.

Air forces around the world routinely use planes to parachute people
or supplies to the ground. But even parachuting can be difficult when
aiming for a small clearing, and while it is possible to collect loads
or people from the ground and winch them into an aircraft, the process
can be rather violent.

The most widely employed collection system, the Fulton Skyhook, was
originally used by the US Navy for recovering downed pilots. The
pilot's harness was linked onto a line hanging from a helium balloon
hundreds of metres in the air. A low-flying recovery aircraft would
snatch the line with a special yoke, and the pilot was jerked aloft
and winched in through the plane's cargo door. Although the force of
the snatch has been described as being "like a kick in the pants", it
would be less than the force skydivers feel as their parachutes open.
The technique has been employed in numerous covert and military
operations and was even used in the James Bond film Thunderball, but
since a fatal accident in 1996, the technique is now only used to
recover equipment.
Ever decreasing circles

Trivailo believes his trailing cable system will be rather more
gentle, and far safer. His interest in developing the system stemmed
from simulations he carried out on the behaviour of trailing lines in
space and beneath the sea. Then a few years ago, he decided to use his
technique to find the best way to control a line dangling in air.

Initial simulations confirmed what Saint and others had already shown
in practice: to lower a load on the end of a line, you adopt a
circular flight path and then simply pay out the line (see Diagram).
At first the line and load simply whip round behind the aircraft, but
when sufficient length is paid out - typically several hundred metres
- the combined effect of air resistance, gravity, the cable's
elasticity and damping is such that the load begins to move in smaller
and smaller circles. Eventually the whole thing reaches a stable
configuration in which the load hardly moves, and can be lowered to
the ground by flying at lower altitude or by feeding out more line.

While Saint flew the plane and lowered the cable by himself, Trivailo
wanted to create a more sophisticated system that did not rely on
human judgement to calculate the precise position of the load at the
end of the line. So he developed a smart controller that monitors and
precisely adjusts the position of the load by paying out or reeling in
the line automatically. His aim was to make the whole process far
quicker, easier and safer.

To start with, Trivailo's team ran computer simulations that
calculated several hundred different configurations in which the end
of the trailing line remained stationary. This told them which factors
- cable length and thickness, aircraft speed and turn radius, for
instance - were most important for stabilising the line. They found
the optimal configuration was an aircraft moving at relatively high
rotational speed trailing a long cable with high drag.

Once Trivailo had found the best set-up, he had to develop a system to
control the winch that adjusts the cable length. Because of the
complexity of the problem - for example, the exact values for drag are
difficult to calculate - and various approximations such as the
representation of the line as a series of masses linked by springs,
Trivailo realised that he could not use a conventional simulation that
calculated the optimal length of line every few seconds. Instead, he
decided to use fuzzy logic to generate sets of rules based on known
behaviour instead of exact values.

For example, if the end of the cable is too high and is not being
lowered then the controller must pay out more. The fuzzy logic
controller achieves this using feedback from sensors that detect the
position of the end of the cable and that detect the aircraft's flight
path. The system that this creates can manage the position of the end
of the line on a millisecond-by-millisecond basis, giving a finer
degree of control than a human could achieve. And unlike the Fulton
Skyhook, this system should not create extreme or sudden forces that
could damage the payload.

The sensitivity of the trailing cable system could allow it to be used
for complicated manoeuvres such as recovering an injured sailor from a
yacht. The line would be lowered until it hung just above the vessel
and the stretcher then hooked into a harness at the end. Then the line
would be winched in until the stretcher was clear of masts, and the
aircraft would increase the size of the circles it flew as the
stretcher was brought into the plane.
Special delivery

In one simulation, Trivailo's team found that by using the optimum
flight configuration and the fuzzy logic controller, a small aircraft
could deliver a 25-kilogram payload to hover half a metre above the
ground on the end of a 3000-metre line. Deployment took just 10
minutes. The end of the cable could even be secured on the ground
while passengers or payload were attached.

So far, real-life testing has not been as ambitious as the
simulations. The team has begun by trying out their optimal flight
configurations with a small radio-controlled aircraft. Now they are
fitting GPS navigation, an intelligent control system and cable winch
to a larger remote-controlled plane with a 3.5-metre wingspan. They
are also designing instruments that will allow them to download data
from sensors on the aircraft and the cable end to control the winch
and the aircraft's flight path automatically. When they start tests
later this month, they believe they will be able to use the plane to
precisely position a small load.

There is still much to do. One of the problems highlighted in
simulations is that altering the weight of the system - picking up or
releasing a payload - destabilises the end of the line. In some
simulations, the line dropped so rapidly that the load struck the
ground. Trivailo hopes adjustments to the software that controls the
cable will solve this, but further modelling is needed before the
system can be tested for real. He also wants to model how sudden gusts
of wind will affect the stability of the cable.

So how long before Trivailo's system is tried out in planes big enough
to pick up people safely? If the military decides the system would be
useful to pick up downed pilots or collect troops from behind enemy
lines it could develop the technology relatively quickly, says
Trivailo, but civilian researchers must gain approval from aviation
authorities before tests can begin. This could lead to significant
delays. "With a focused effort the technology could be developed
within two to three years," he says, but it might be five years before
the concept gets its first test with human cargo.

The system could find many uses. As well as rescue and military
operations, a commercial cable system would allow light planes or even
pilotless aircraft to retrieve or deliver packages almost anywhere.
Remote settlements and scientific bases could receive vital supplies
without the need for an airstrip or the risks associated with
parachute drops. Payloads could include fragile items such as
scientific specimens, medical supplies or living creatures.

Trivailo believes the technology has a more urgent application in
firefighting. Wildfires have become more severe in recent years but
many of the US fire service's water tankers are due for retirement.
Trivailo's cable system could help take the pressure off, since it
would allow almost any large aircraft to collect water and spray it
from the air or deliver it to remote sites without the need to land.

The idea could even find uses within the weaving industry, says Chris
Rahn, an engineer at Pennsylvania State University who works on
similar problems. "The toughest part is getting a stable system. If
they can do that, they will have beaten one of the hardest
challenges."

As the 50th anniversary of Saint's remarkable flight approaches, the
full potential of the technique he pioneered is finally becoming
clear. Saint could never have imagined the possibilities that were
dangling right there on the end of his rope.
From issue 2497 of New Scientist magazine, 30 April 2005, page 35
Printable version Email to a friend RSS Feed

Cover of latest issue of New Scientist magazine