Happier landings

To combat the hazards of deck turbulence, aerodynamics experts are
working on ways to smooth the airflow. Jon Excell reports.

The conditions in which sea-based military aircraft can operate are pretty limited: extreme waves, high winds, and the swirling currents of air created by the shape of the ships often conspire to make conditions too dangerous for flight. Now, novel methods of smoothing airflow over warships are being considered by, among others, the MoD and the US Navy.

NASA aerodynamics expert Dr John Lamar and engineers from the US office of Naval Research are investigating the installation of devices called columnar vortex generators (CVGs) on ships.

Lamar’s investigation forms part of a wider NATO research and technology organisation (RTO) task group looking at devices to improve the airflow over ships conducting air operations at sea. He said that the turbulent vortices generated by the body shape of some warships can make air maneouvres at sea dangerous and difficult. But by installing CVGs at certain locations around the edges of a ship, he said that these vortices could be diverted, creating a smoother airflow over the take-off and landing decks.

A CVG is essentially a cylinder cut along one lengthwise edge with that edge rolled in giving the device a helical shape. Lamar explained that he first became aware of the concept around 14 years ago, but it was only on further reflection that he realised it could have benefits to flight-deck aerodynamics.

The purpose of a CVG in such an application would be to channel turbulence away from the take-off-and-landing area of a warship. Lamar explained that CVGs mounted on the edges and the bow of an the flight deck of an aircraft carrier would capture the turbulent airflow from the edges and effectively move it away from the path of an approaching aircraft.

Even in good weather the turbulence generated by the shape of a ship can make take-off and landing tricky. ‘Pilots encounter turbulence several miles downstream of the carrier and it gives them a bump. It’s great that these guys are able to train to get over this, but it would be better if they could have a smooth airflow,’ said Lamar.

The turbulence of primary interest to Lamar’s team is that associated with the bow and side edges of aircraft-carrier decks. He claimed that the addition of CVGs along the edges of the deck and the bow of, for instance, a Nimitz-class carrier, would promote smooth flow over the entire flight deck. he also explained how by using a combination of wind tunnel testing and computational fluid dynamics (CFD) computer simulation, the team has been able to optimise helix angles and CVG shapes. Tests carried out on a simulated helicopter carrier in a 14ft x 22ft wind tunnel indicate that CVGs could make a big difference. ‘Until recently all we had was qualitative smoke, but now we have real measurements that show the benefits of the combination of bow and side- edge CVGs.’

Lamar claimed that the ski-jump style ramps on many aircraft carriers generate particularly harsh turbulence, citing the example of the Kusnetsov-class Russian ski-jump carrier. He said that according to a 1998 NATO report, this vessel’s turbulence was reduced by rounding the edges of the jump and adding further devices at the junction between it and the rest of the ship.

His words of caution are equally valid for the UK Navy’s two hotly-anticipated new Future carriers. ‘From what I’ve seen of the proposed UK ski-jump carriers, they are going to have the same problems,’ he claimed. He added that frigates built for the UK navy, which for stealth reasons are much more slab-sided than their predecessors, will also present new flow problems.

Another member of the RTO task group, Dr Doug Greenwell, a lecturer in aerodynamics at the University of Bristol, is concentrating his efforts on the problems encountered by helicopters. Greenwell’s team is using a small amount of DSTL funding to look at the flow over the back of frigates on to helicopter flight decks where a big turbulent buffeting flow comes off the superstructures.

‘Today, helicopters are almost the primary weapon system and you’ve got to be able to operate them in all kinds of conditions,’ said Greenwell. ‘the problem is that you have a messy airflow over the back of the ship, and it can be a problem getting the helicopter safely back into that flow.’

Greenwell is disappointed with the small amount of funding his team is receiving for what he claims is a significant problem. He explained that flight testing is currently the only method of analysing the effects of deck turbulence and the amount of flight-testing carried out is restricted by both time and cost. This, he claimed, has led to an overly conservative estimate of the conditions helicopters are able to fly in. ‘Quite often the performance envelope is set on the basis that we had something similar on a ship that looked a bit like that a few years ago so we’ll play safe and set the limit here. This severely limits the operational effectiveness of a warship,’ he said.

Greenwell added that Lamar’s CVGs may not be the ideal solution. ‘the trouble with vortex generators is that they’re big bits of kit that have to be stuck on the side of the ship, and the navy tends to not like doing that,’ he claimed. His team is therefore looking at different, less obtrusive porous devices with flow-through cells such as fences, netting, meshes and grids.

Whatever technology is finally adopted, real applications on ocean-going vessels are some way off. The task group is due to wrap up its work up in about 18 months and even after that a considerable amount of optimisation is likely to be required before a solution makes its full debut.

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