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Light as Air

Date: 17.10.19
Light-weighting is a key objective for all aircraft manufacturers. Making a plane lighter reduces fuel use, lowering operator costs and the negative impact of flying on the environment. And this is where composites are coming into their own.
FUEL IS THE NUMBER ONE EXPENSE

for airlines, so it’s no surprise that they are continually looking for ways to lighten the load. A figure brought to the forefront by Norwegian economist Bharat P Bhatta’s proposed pay-as-you-weigh pricing mode l for passenger fares, is his estimate that a reduction of one kilogram of weight could result in a fue l saving of 3,000 US dollars per year.


Innovative ways to lower weight

Maybe not going the whole hog of charging passengers depending on their weight, carriers have resorted to ever more inventive ways of lowering aircraft weight.

These have included American Airlines serving one less olive in its inflight salads, British Airways lighter inflight magazine and Virgin Atlantic’s radical rethink of economy meals.*


All of these measures add up to lower operator costs, less fuel and a reduction in CO2 emissions. As effective as these in-cabin changes may be, potentially greater savings can be made in the fundamental design of the plane. One option for this is a shift from metal components to composites.

Composite bearings

Trelleborg Sealing Solutions has a focus on composites at its facility in Albany New York in U.S. as well as in Rotherham,England, where the unique Orkot® material is manufactured.


“Traditionally, Orkot® has been used in bearings for ships and hydropower plants,” says Brian Bowen, who is heading up a project to transfer this proven technology to aerospace applications. “Metal bearings feature in the landing gear shock absorbers of planes and if these can be successfully replaced with composite bearings instead, a huge potential saving can be made by operators through light-weighting.”


Not just lighter in weight

Replacing a steel/ bronze bearing with a composite bearing saves weight due to the specific density of the materials used. The specific density of steel is eight grams per centimeter cubed and bronze is 8.75. Polymer bearings have a specific density of just 1.25 grams per centimeter cubed; a typical saving of 7.5 in volume per unit.

“Composite bearings have other advantages too,” continues Brian. “Friction is low, running dry or lubricated, and constant. There’s no surface coating to wear off, and when running dry, there’s no maintenance.


“The composite bearings are easier to fit than metal ones and their flexible base material spreads load more effectively, allowing a larger contact area. They also have no edge loading, are impact resistant and over sized repair parts require no tooling. All in all the composite bearing is a promising alternative to metal ones.”


Eliminating metallic wear particles

A less obvious but important additional benefit of composite bearings, that has been identified by Trelleborg Sealing Solutions, is the exclusion of metallic particles from the hydraulic system.


Brian explains: “We’ve observed that customers who use metallic bearing shells have an increased risk of metallic wear particles in the shock absorber hydraulic fluid. Over time these metallic wear particles can scratch the counter surfaces of the shock absorber slider, and more critically, can damage the soft polymer seals. If the dynamic seal caps become scratched, then this can lead to a reduction in the service life of the shock absorber. Composite bearings do not generate such abrasive wear particles, extending the life of seals and the system itself.”


However, despite all these benefits, the biggest advantage of the composite bearing remains its lighter weight. To prove this, Trelleborg Sealing Solutions undertook tests to verify the potential weight saving from switching from metal to composite bearings.


Huge savings per year

“The total saving from just the re placement of the bearings  in the landing gear shock absorbers is 20 kilograms per plane. If we go back to Bharat P Bhatta’s estimated fuel saving of 3,000 US dollars per year per kilo, that equates to 60,000 US dollars per year per plane.


“And the shock absorber bearings are not the only application we’re looking at for Orkot®. There are also the LG pin bearings, landing gear actuation bushings, door hinges and cargo handling rollers and wear pads,” concludes Brian.

A350 LANDING GEAR QUALIFIED

To support our customers in proving seals for critical applications, Trelleborg Sealing Solutions invested in the world’s most powerful test bench for hydraulic rod seals. This unique facility allows testing to be carried out that has previously never been possible and was utilized to qualify the seals for the landing gear of the Airbus A350. The sealing system has successfully passed 20,000 full landing cycles plus additional tests for ice scraping and water spraying

The world’s most powerful test bench for hydraulic rod seals

Location: Trelleborg’s research and development center in Stuttgart, Germany

Weight: 18 ton / Power: 260 kW

Type: Floating-mount rod seal test bench

Test capabilities: Long-term endurance and development tests for complete sealing configurations in a single test construction.

Simulation capabilities: Patterns of movements and pressure patterns in sinus waves, trapezoidal forms and freely modeled patterns. Replicates the pressure between the primary and secondary seals and can simulate different atmospheric temperatures between -60 °C and +90 °C/ -76 °F and +194 °F.

Aerospace test capabilities: Check parameters such as braking or lateral forces on landing gear and simulate life-like knocking caused by uneven runways.

Size capabilities: Seals 100 and 400 millimeters/ 4 to 16 inches in diameter

Speed capabilities: Up to 1 meter/ 3.3 feet per second

Frequency capabilities: Can generate up to 10 Hz

Novel feature: The Lateral force cylinder is mounted at the bottom of the bench that is capable of exerting forces of up to 225 kN, It can place permanent radial loads on the seals, or, depending on the stroke, exert dynamic loads.

Results: Can be fully documented, allowing customers to provide their clients with proof of performance related to prescribed parameters.

Installation: Specially constructed so that vibrations can be isolated from the surrounding building.

Sustainability: Set up in a separate room so that energy otherwise lost through emissions can be used in an environmentally friendly manner to reheat the building.