A SHORT AND DIFFICULT LIFE (from Engineering Magazine 2002).
Seaplanes, satellite dishes and spaghetti all have to undergo physical testing. Antonia Ward reports
It's not that one can hold Meccano solely responsible for creating a generation of engineers eager to
put things together and test how well they work, but the classic children's toy may have a lot to answer for.
Certainly it's the analogy that springs to mind when Mike Marlow, structural and mechanical group leader
at GKN Aerospace Test Services, begins to describe the test house at the company's facility in Cowes on
the Isle of Wight. Work at the facility is primarily focused on strength and fatigue testing for certification
of flight critical aircraft components and assemblies. The 2,970m2 test house - which is UKAS approved and
in fact was one of the first to be registered under the former NAMAS rules - incorporates a 22 x 16m strong floor.
The floor effectively functions as a structural member of the test rigs constructed on it, and it is in this
that Marlow sees some resemblance to the construction toys of his childhood. He explains how the strong floor
enables the team to create jigs very quickly in modular form: and there is space for up to ten rigs in the
test house. "It's a little like using a Meccano set - although a very big one," he grins, adding that there
is also the advantage of "a lot of reusable parts."
There is usually a lot going on in the test house, and as Marlow describes some current projects the breadth
of the facility's work becomes clear. The largest test rig currently in operation is engaged in static and
fatigue testing of an engine nacelle for the Lockheed Martin Alenia C27J military transport aircraft. Because
this series of the turboprop C27 is being fitted with larger Rolls-Royce AE2100D2 engines, tests are being
conducted on the nacelle primary load bearing structure and the inner wing, which has been strengthened to
support the new nacelle. The department is also currently carrying out static testing on a turbofan engine
outer fan duct, simulating the thrust and bending loads on the engine casing, while another test rig is
fatigue testing a forward engine mount.
Components under test have their entire lifetime - well, two lifetimes, actually, simulated within the test
house. "We can simulate around 3,000 flights - that's two complete lifetimes - in ten days," says Marlow. "We
put the component through the entire flight pattern - taxi out, take off, climb, level out, manoeuvre, descent,
landing, taxi in - with each sequence taking about 1.75 minutes." Two lifetimes are necessary because the
first set of flights tests the undamaged component, while the second tests the part after the team has
deliberately damaged it.
As one would expect from a facility located at Cowes, water is never far away, and elsewhere at GKN Aerospace
Services the Fluid Dynamics facility can be found. This is the world's only fully commercial site for testing
the ditching and floatation characteristics of both fixed wing and rotary wing aircraft. Famous aircraft, from
Concorde to the Saunders-Roe Princess flying boat, have been tested at the site.
The four tanks at the facility, which have a total area of 8,360m2, are set alongside a wind tunnel with a
4.2m section. This is capable of speeds of up to 30m per sec. There is a 76m long helicopter ditching and
flotation tank that employs a lightweight, unmanned carriage which runs on a monorail. David Eldrige, fluid
dynamics group leader, explains its capabilities. "We can generate large regular and irregular sea patterns,
with an equivalent wave height of 3.5m. The aim is to find the optimum ditching procedure for individual
helicopter makes - what speed, what altitude, for example. We then look at their static and dynamic stability
to determine the capsize boundary." The team is currently about to begin a series of ditching and flotation
tests on a new version of the Sikorsky S-92.
The fixed wing ditching tank, by comparison, is a somewhat smaller 54.8m, but can produce waves up to 9.1m in
length along and across the basin, simulating head and beam seas as required. Models are launched into the
water by a gravity powered catapult at a speed of up to 30m per sec. Often, Eldridge comments, the critical
factor in fixed wing aircraft ditching is the shape of the rear fuselage. "If this isn't compatible, you can
have the aircraft hitting the sea with an initial impact of, say, 5G but then skipping up into the air again
before stalling and then impacting for a second time at a fatal 15G." One of the team's tasks is to test
modifications that can stop this happening. These can be quite simple, adds Eldridge, but vital if they are
to make the aircraft 'porpoise' rather than take off again.
It's not just helicopter and fixed wing aircraft that come through Eldridge's doors, however - the 200m tank
has been used for testing everything from America's Cup yachts to US Navy hovercraft. Hull design is often
the issue here. Other work, for example, has involved water testing of catamarans that were being used as
river ferries, but were generating bow waves that were eroding riverbanks and damaging moored boats. Recently,
GKN has also been testing a 60knot powerboat design: trying to find the right hull shape to translate power
into speed.
Both Marlow and Eldridge agree that there is no universal substitute for physical testing - and that claims
in the 1980s that computer modelling techniques would eventually replace physical tests were overstated.
GKN Aerospace, in fact, is seeing a revival of activity with increased levels of work coming in from external
customers from throughout the aerospace, marine, defence and automotive sectors, including companies such as
Bell Helicopters, BMT Seatech, Saab and Boeing Douglas.
This article was published online in Engineering Magazine in 2002, subsequently lost
during a system upgrade and is copied here until
reproduced again on the
Engineering Magazine web site.