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The famous P-791 prototype airship - built last decade for a military transport programme which eventually came to nothing - is to give birth to new, mighty commercial versions of itself with Canadian financial backing.

A hybrid air vehicle isn't a conventional lighter-than-air dirigible, whose weight is entirely supported by the buoyancy of its lifting gas. A hybrid's gas only counteracts part of its weight: in order to get airborne, the SkyTug will swivel its propellors downwards for vertical thrust. Once it starts flying forward its hull will generate dynamic lift just as an aeroplane's wing does, allowing the props to be swivelled horizontal for greater cruising speed.

According to Aviation Capital, the fully vertical-takeoff-and-landing (VTOL) capable SkyTug will provide "greater payload and range at a fraction of the cost of a helicopter". The larger ships that will follow apparently won't be fully VTOL - they'll require something of a run-up on the ground to generate dynamic lift and get airborne, rather as an aeroplane does.

However the big ships aren't expected to need massive runways: they'll be able to land and take off from "unimproved surfaces and water". This will be achieved by the cunning air-cushion undercarriage which is such a novel feature of the P-791: rather than wheels or skids, the airships will move about on the ground supported on air blown down through big skirt assemblies, just like a hovercraft.

The fact that a hybrid ship is heavy helps to avoid one of the main headaches of unloading an airship at a place where there's no way of taking on water ballast. A traditional airship struggles to manage such operations, as once the payload is gone the ship becomes uncontrollably buoyant and is likely to soar up through its "pressure height" - the point at which the lifting gas has expanded to fill the entire envelope - and lose most of its helium via the automatic safety valves (or burst). Helium being expensive, this is not ideal.

P-791 style ships have another ace up their sleeve for unloading, in that the hover undercarriage can be switched from blow to suck, thus sticking the ship firmly down on the ground (or sea) even if it might have become a bit flighty during unloading. This is also handy for preventing the vessel being blown around by the wind.

The P-791 itself was originally built by Lockheed in 2006 as a contender for the US military's proposed "Walrus" project, intended to deliver a brobdingnagian airship capable of hauling an entire US army combat battalion of 500+ troops - plus all their vehicles, heavy weapons and other kit - in one lift. But a heavy-lift airship needs a lot of gas, and this in turn means that "pressure height" can't be too high or the envelope will have to be unfeasibly huger still.

Thus the Walrus was specced for 10,000 feet maximum altitude, which was realistically a show-stopper for the military - even shoulder-launched missiles can reach that high, and the ship would have been a very easy target.

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