Flying Circles Around The Helicopter
David Hambling

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 gentler, 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. 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 judgment 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 stabilizing 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 realized 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 maneuvers 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 - destabilizes 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 modeling 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 pilot-less 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