With the AT-04, Airship Technologies claims to have solved all the technical problems which have dogged the development of previous airships.
The envelope of a non-rigid ship must be strong, light, tear-resistant, and an effective gas container. In the 1980s, a team lead by Roger Munk, Airship Technologies’ chief executive and technical director, pioneered single-skin coated polyesters. These needed repainting with a polyurethane coating annually to prevent degradation.
Airship Technologies now uses a laminated fabric with a woven polyester core bonded to Milar on the inside and Tedlar film on the outside. It does not degrade, needs no painting or maintenance, and is about 10 times as strong as cotton.
Stronger envelopes, based on sailcloth technology using Kevlar or Spectra composites between Milar films, are being developed.
‘Because an airship hull is loaded in tension it can exploit the increased tensile strength of modern materials to the full,’ says Munk.
Metal, used for the fins, nose-cone and gondola of airships until the 1970s, was heavy, tended to corrode and had a big radar signature, which prevented airships being used as early warning platforms. The gondola’s front and rear, propulsion ducts and nose cone are now moulded in Kevlar. The gondola’s sides and the tailfins are made from unidirectional glass fibre skins on a core of Nomex, producing a very light yet strong structure.
The R100 and R101 had ground crews of up to 500, who would literally throw them into the air on takeoff and hold them down for landing.
Munk developed the propulsion duct, a vectored thrust system to swivel the propellers up or down so it could climb quickly on take-off or be forced downwards for landing.
Because petrol engines would not operate at high levels of tilt, Munk mounted them in the gondola, with a driveshaft powering the propellers through a Westland Lynx tail rotor gearbox.
Diesel Air is developing highly efficient diesel engines small and tolerant enough to be mounted in the duct, eliminating the driveshaft and cutting vibration in the gondola. One of the AT-04’s three engines will be on the tail, a more effective position as the airflow is less turbulent there.
Fly by light
Munk sought to replace the ‘pretty lousy’ conventional airship controls, operated by pulleys and mechanical cables, with some form of powered control. A ‘fly by wire’ system would have been susceptible to electro-magnetic interference, so Munk went straight to ‘fly by light’, using fibre optics to send signals to control surfaces.
Instead of having two horizontal fins with elevators and two vertical ones with rudders, the AT-04 will have four fins in an X formation, each with a ‘ruddervator’. The ‘X’ rather than cruciform arrangement solves the problem of the bottom vertical fin having to be smaller to improve ground clearance.
When an airship is landed but not moored to its mast, its controls are ineffective. A large ground crew is needed to stop it from drifting broadside on to the wind.
An engine to give yaw control was needed. It had to produce 400lb of thrust and switch from full left to full right thrust in 0.5 seconds. The solution was a small, forward- facing gas turbine engine in the nose, with a vane to direct the thrust left or right.
This allowed an inexperienced pilot to take off and land with total precision, says Munk. ‘We realised we had everything taped. Until we fixed these things, getting airships accepted was an impossible task.’