Airships get a lift

Hybrid airships could in future become a common sight thanks to research at Cambridge University to model and optimise their behaviour and associated systems.

Traditional airships use helium for lift, giving an average density for the entire craft slightly less than that of air. They are, however, very difficult to handle on the ground, tending to exhibit jelly-like behaviour and are hard to fix and tie down. They also have a tendency to fly away in a crosswind.

Hybrid airships are different in that they also borrow concepts from aircraft. In the same way that an aircraft's wings create lift when moving forward or when air moves over them, the hybrid airship has a hull shaped to create lift.

The major advantage of hybrid airships is that they are heavier than air, making them much easier to handle on the ground.



Dynamic behaviour

Dr Fehmi Cirak, a lecturer in structural engineering at Cambridge, is leading the research. He specialises in computational research to develop models and algorithms for structures and fluids that, for this project, will simulate all the factors that affect the airship's dynamic behaviour.

'Our job will be computational modelling. In this and most other industries, people are moving away more and more from doing experiments and building prototypes. we basically want to accomplish the entire design process on the computer,' said Cirak.

The project will use data gathered from a 40m-long prototype hybrid airship to improve the design of subsequent larger vehicles. Design and production decisions and eventual US Federal Aviation Administration approval of these airships will be influenced by the computational modelling and simulation being undertaken in this project.

The overall structural design of the craft is led by UK specialist engineering consultant Tensys Dynamics. The Cambridge team will provide the real-time dynamic modelling of the flexible hull structure and the various internal and external gas masses and aerodynamic loads.

'There are many interesting research areas in connection with this airship, and we identified the landing system as a very interesting one for this project,' said Cirak. 'It works in a similar way to a hovercraft.

'Imagine an inflated doughnut, with a large fan in the centre. It can either blow or suck air, allowing the vehicle to hover like a hovercraft or to grip to the ground after landing.' This means the airship can be pulled to the ground on landing or pushed away on take-off.

Take-off and landing present a number of challenges for the hybrid. 'To take off, it needs to attain a certain speed, like aircraft. It cannot just ascend like balloon,' said Cirak.

'Landing systems need to consider the effects of large membrane deformations and fluid-like interaction — in other words, handling its jelly-like behaviour. On landing, the airship comes in with a certain forward speed and at some point — say around 6mph (10kph) — you suddenly bring it to a halt. This is a new way of landing that needs to be considered.'



Varying conditions

According to Cirak, another important factor to model is the way the shape of the envelope changes under varying loading conditions.

'As far as possible, the shape and the pressure difference between inside and out needs always to remain the same for structural reasons,' he said. 'As it goes up, the surrounding air density and pressure decreases and the differential pressure on the hull has the tendency to increase. To counteract this, on the ground, you fill the hull with about 80 per cent helium and 20 per cent air in ballonets (internal air cells that are also known as bladders). As it goes up, you pump out this air to decrease the internal density and pressure, to control the differential pressure on the hull.

'But the ballonets can also cause some problems during the landing. If they are not carefully positioned and properly fixed, they can destabilise the entire hull and cause structural failure.'

Work is due to start on the design of a large commercial craft for freight operations. The Cambridge team believes its expertise in the computation of complex interactions between flexible structures and their environment is crucial for the success of this project.