Centaur has landed

A UK seaplane contravenes traditional thinking by employing marine design and materials to produce a lighter, stronger aircraft which can operate on water and land. Christopher Sell reports.

In an age of progressive maritime technology, where Ellen MacArthur can use the latest technology to help her set a new round-the-world record, it’s clear that complementary industries could benefit from the advances that modern materials and hull design can offer.

Such thinking has seemingly been taken up by the traditional seaplane market with one UK company looking to break away from tried-and-tested thinking by adopting composites and marine technology to produce lighter, stronger seaplanes, which could operate in rougher weather and have greater access to shore berths around the world.

Seaplane design, according to James Labouchere, managing director of Salisbury-based Warrior (Aero Marine) has changed little since the 1930s and manufacturing technology is out of the 1940s. He explained that current seaplane designs use a mixture of bulbous noses, proud forebodies and pronounced ridges that seriously affect aerodynamic and hydrodynamic efficiency. But while seaplane design has stood still, marine designs have altered to the extent that the average speed of fast ferries has doubled in the past 20 years, while trimarans and catamarans have broken every available off-shore sailing record.

After working in various areas of the high-speed marine industry, including design and construction of fast multihulls, in 1992 Labouchere received the Royal Aeronautical Society’s Handley Page Award to develop a series of new-concept seaplane hulls.

This has resulted in the Centaur, a seaplane that contravenes traditional thinking by employing marine design and materials to produce a lighter, stronger aircraft able to carry a greater payload with a more efficient and controlled take-off. And as an added bonus the plane has an undercarriage, meaning it can operate on both sea and land.

‘It was significant that we had not read any seaplane design textbooks, and that turned out to be an asset,’ said Labouchere. ‘Had we done so it would have taken us in the wrong direction and we would have come up with a design that creates a great big bow wave — meaning problems as soon as the plane accelerated.’

In contrast, Warrior’s hull design cuts through the water, allowing it to travel twice as fast in displacement mode as conventional seaplanes. aerodynamic lift takes more than half of the aircraft’s weight off the water and with a tap on the back-stick energy is translated into flying mode. Furthermore, as the design cuts through the water it does not push a visible bow wave, thus avoiding the drag hump conventional seaplanes encounter during acceleration. This characteristic is unique to the Centaur.

This patented design means the plane is unconventionally stable, docile and efficient in rough water. Labouchere claimed that it can handle short, steep waves over 80 per cent larger than those tolerated by conventional seaplanes.

‘But it has been a long process,’ said Labouchere, ‘An amphibious seaplane with an undercarriage — so it operates on both land and sea — is extremely challenging because there are many more design criteria than with just a land plane,’ he said.

To achieve this, Warrior used a combination of computational flow dynamics (CFD) and privately-developed software which carried out performance configuration assessment during development and testing. The software allowed the team to simulate the whole aircraft’s geometry and look at the effect of adjustment configuration on take-off performance — based on empirical data taken from the model hull tests. The effects on lift and drag of hull geometry, flap geometry, weight, balance, power and tail surface definition could therefore all be reviewed graphically, allowing the design to be optimised. ‘So we have a very valuable optimisation tool, which traditional seaplane developers of the past have not had,’ said Labouchere. He believes the Centaur design holds many advantages over traditional seaplanes.

Owing to its fine bow and high length-to-beam ratio resulting in a negligible bow wave, it is able to handle twice as much rough water as conventional seaplanes. Moreover, the ability to fold its wings allows it to access five times as many coastal facilities as normal seaplanes. This ability is common in military aircraft, but unique to the Centaur in the commercial sector.

Another first is the material — a resin infusion composite — used in the manufacture of the plane, While this is not new, Labouchere claimed that there is no other certified composite seaplane on the market. Vinylester-epoxy laminating resin is an established composite in the marine and automotive industries and is impervious to bacteria, acidic fresh water and salt water. It also heralds good impact and shock-absorption properties, with good resistance to fatigue.

The composite will mean the plane has a longer life due to its resistance to corrosion. ‘Conventional seaplanes are made of aircraft-grade aluminium with a magnesium content which corrodes quickly in salt water,’ he said. Unlike marine-grade aluminium it cannot be welded as it will lose its flexibility. Instead it is glued or riveted together.

The flap and wings create down-wash on the tail.  This down-wash substantially balances the pitching effects of changes in power and flap setting. 

You are hard pushed to make a metal seaplane that survives 10 years before being rebuilt,’ said Labouchere. ‘That is in cool water, but in the tropics the salt eats into a plane much quicker and often it may not last more than four years,’ he said. The current way of dealing with tropical conditions is to progressively rebuild the plane, by replacing a section each season until it is totally upgraded.

He added that Warrior has been using cutting-edge manufacturing processes borrowed from the marine industry rather than the aviation industry. The reason for this is that the marine industry allows more freedom than the heavily-legislated aviation industry.

Will this mean there is a market for a costly seat in such a plane though? Labouchere thinks so, comparing the cost per seat for a seaplane to that of helicopter travel. Not only that, but the high performance of the design, which gives greater lift and greater load capacity, results in a much greater payload range reflected in lower cost-per-seat mile.

This, together with the fact that seaplanes account for about six per cent of general aviation suggests there is a sizeable market to capitalise upon. Indeed, the results of a recent study, not counting military and big carriers, suggests there are more active civil seaplanes than their helicopter counterparts.

Warrior is looking to fly the first passengers by mid-2006. Only then will we see if the seaplane market will benefit from hi-tech marine technology that will turn it into a more accessible, high-utility form of transport.

Watch the videos!

Flying: Aerodynamic interference between the wing and centre-section make the Centaur pitch correctly and safely with changes of flap and power setting.

Rough water take off: A progressive increase in power but otherwise no control input until trimming after take-off. The waves are breaking occasionally and scale to about 0.8m (2.5 ft).

With wings folded (hydraulically) it will taxi at over 20 knots in displacement mode. It will manoeuvre at 4 to 6 knots with an independent 360-degree thruster.