Handling with precision

A very low profile, battery powered tractor unit for manoeuvring helicopters and aircraft on a hanger deck being used by the Royal Navy is setting standards in digital controllability and adaptability. A portable console controls its movements allowing the operator to stand a safe distance away. And because all four wheels are separately controlled, it steers better than previous designs and is part of a growing family of products that are designed to safely take hold of and manoeuvre literally anything that is heavy and mounted on wheels. It is produced by Douglas Equipment in Cheltenham, a company with a well established reputation for building conventional aircraft towing tractors of various sizes and capabilities. The unit has a very low profile unit, only 350mm high and 215mm in the middle with 70mm ground clearance. It can nonetheless lock onto and tow up to 25tonnes if the object has wheels in contact with the ground. Its initial intended function is moving a Lynx Mark8 helicopter from a hanger on a moving vessel likely to be pitching in various directions. This makes the control critical as the ship motion could easily lead to the helicopter being damaged. It uses four solid polyurethane wheels: two drive wheels and two idlers to steer. The drive wheels are each powered by 1.2kW separately excited DC hub motor controlled by pulse width modulation. The torque on each drive motor is proportioned according to the required steering angle. Douglas Equipment, design engineer, Paul Channon, says: "This allows the drive wheels to be driven through an electronic differential and, you can set up full Ackerman steering, so that all the wheels trace out circles of different radii without scuffing. As a result, the turning circle is a smooth 2.5m, close to the width of the tractor – 2.56m. "Maximum tractive force is 2,268N and electromagnetic brakes are automatically applied when there is no power." Batteries are two conventional 24V traction battery packs giving a total of 205Ah. Duration on one charge is 3,500m pulling an aircraft weighing 25,000kg or 3.5hour continual movement pulling an 18,000kg load. Before starting, the operator identifies the aircraft type to be moved and selects it on the controller. This sets the maximum tractive effort within strict parameters laid down by the aircraft manufacturer for the particular aircraft. A particular ingenious innovation is the way the towing arms latch onto the nose or tail wheels. Almost all helicopters, even Russian ones, have a hollow shaft to allow the attachment of a conventional tow bar. What Douglas has devised is an adaptor plate with a pin that goes into this hollow shaft. The adaptor plate, regardless of exactly what height it is at may then be grasped by a hexagonal array of spring loaded pins on the inside of each of the arms. These are pressed in by the arms being moved together under hydraulic pressure until the pins that have not gone into the holes are pushed back. Hydraulic pressure is 196bar and pin clamp pressure is 10.3MPa. A number of navies now use or have ordered the handlers. The Royal Navy intends to eventually have 70. Channon says: "Anything on wheels, we will move it. The handler was equally good at moving boat trailers, generators and compressors."