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Sky-high Goals for Oils

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A deadly helicopter crash last year has focused attention on the design and maintenance of helicopter transmissions and gearboxes. On March 12, 2009, a Sikorsky S-92A helicopter carrying off-shore oil workers ditched in the North Atlantic off Newfoundland, shortly after the pilot reported problems with the main engine gearbox. Of the 18 men aboard, only one survived. It was later found that a titanium mounting stud attaching the main engines gearbox filter bowl had sheared off, permitting the gear oil to escape. Severe damage ensued, and when the gears could not drive the tail rotors, the craft pitched out of control, said a preliminary report from Canadas Transportation Safety Board.

Investigation into the incident is ongoing, and is expected to result in more stringent testing and safety measures. Thats important, because helicopters often are the only option when it comes to medical evacuations, access to remote areas, search and rescue operations, firefighting and offshore services. For example, almost 1 million U.S. flights occur each year over the Gulf of Mexico alone, according to the Federal Aviation Administration.

Aviation gearboxes and their lubricants present unique demands for designers, who must keep an eye on cost as well as performance needs. Besides low weight, high speed and high power ratings, these requirements include high reliability and safe operation under all conditions, pointed out Michael Weigand. He noted that aviation safety begins with rigorous air laws called Certification Specifications, which address the design, development, manufacture and maintenance of each type of aircraft. These laws are overseen in the United States by the FAA, and on the continent by the European Aviation Safety Agency.

Weigand, who works in aviation transmission research and development at the Vienna University of Technology in Austria, has been striving to harness tribology to improve the air-worthiness of gearboxes. At the universitys laboratories, researchers use large-scale tests and sophisticated rigs to examine helicopter rotors and gearboxes, as well as those from fixed-wing aircraft. The laboratory includes soundproof test cells, an FZG test bench, and this year will install a 300kW universal test stand to dynamically assess aviation gearbox performance.

Helicopter requirements are truly unique, the engineering professor explained to the International Colloquium Tribology, held in January at Germanys Technische Akademie Esslingen. In a fixed-wing aircraft, each jet engine has a large gearbox called the power take-off unit, sometimes known as a banana gearbox because of its shape. This supplies energy to the planes many hydraulic pumps, generators and compressors.

One of the most exciting new designs for fixed-wing aircraft, Weigand pointed out, is the geared turbofan adopted by engine builder Pratt & Whitney, where a planetary gear stage reduces the fan speed to improve the bypass ratio and energy efficiency (which should have strong appeal for cost-conscious airlines). Jets normally have an auxiliary power unit as well, with a gearbox serving the slat and flap actuation systems on the wings and also ground operation of pumps and generators.

By comparison, he went on, helicopters need to transmit power from their turbines to the main rotor overhead as well as back to the tail rotor. The configuration can vary, as the turbine may be located behind or beside the engine, and rotor shafts may be in tandem, coaxial or even intermeshing. The main engine gearbox and tail rotor gearbox are connected directly to their rotors with forged hollow drive shafts. For the housing, aluminum is preferred for its high strength and corrosion resistance.

An even more intricate design can be seen in tilt-rotor or tilt-wing aircraft, an extreme challenge for transmission technology, said Weigand. Here, the gearboxes must pivot along with the engines during the crafts conversion from vertical lift mode to aircraft mode. Additionally, safety requires each turbine to have a mechanical connection to both rotors, in case one or the other engine should fail during flight.

In Europe and North America, Weigand noted, air laws require rigorous experimental tests and qualifications for drivetrain components, beginning with motor-driven test benches, then ground test vehicles, moving up next to flight testing, and years more of scrutiny to validate the time required between overhauls.

These tests include endurance runs, tests with one engine inoperative, tests with oil under low pressure, and in low-temperature and high-temperature regimes. Gears are examined for tooth root bending, flank pressure and load distribution, fretting, micropitting and other flaws.

Durability and scoring resistance are among the factors that are very much affected by the lubricant, as is load-carrying capacity. Most gears are case-hardened or nitrided and ground, and more recently are being superfinished to achieve a high surface quality thats resistant to fretting and micropitting, but lubricants play a role here too.

Tribology challenges for aviation gears include the effects of centrifugal forces on the oil, the high-speed application of gears and bearings, and oil condition as it ages, Weigand said. In terms of temperature ranges, the oils need to be able to perform from at least -40 degrees C to over 50 C.

Gearbox oils also face very different needs on the input and output sides of the design, he continued. On the input side, the gears tend to be lightly loaded, and move at very high speeds. For this, a thin oil with no extreme-pressure agents would work well. But on the output side, the gears are highly loaded and move at lower speeds, which points to a thick, EP-additized oil.

The lubrication of helicopter gears is also complicated by the desire of many operators to use a single oil to lubricate both the turbine engine and the main gearbox, Weigand said. This tends to lead to a lop-sided compromise, one where the synthetic turbine oil is selected rather than a gear oil that would better serve the gearboxes.

Both commercial and military operators tend to prefer using a single lubricant because the former can more easily and widely source the turbine oil, and the latter can simplify their procurement and inventory needs, Weigand explained. Thats practical, but not optimal when it comes to lubrication, he said, pointing to research by Giuseppe Gasparini and others at helicopter manufacturer AgustaWestland. He concurred with them that gearbox oils in the range of 5 to 9 centiStokes, with specific additives for improving the load-carrying

abilities and ensuring protection against corrosion, micropitting and other bearing-contact issues, would be better. But he also acknowledged that separate sumps and lubrication systems would add weight, and thereby reduce payloads for helicopter operators.

Finally, he pointed out that one of the most difficult Certification Specifications requires a gearbox to perform for 30 minutes after total loss of oil. The gears are meant to continue rotating without oil during this run-dry time, he said, to allow pilots time to land.

The gearbox involved in the Newfoundland crash did not pass this test, but was certified for service by both the FAA and EASA under an exemption that doesnt require run-dry capability. Until the crash, the possibility of total gear oil loss was considered highly remote compared to other issues that might lead to a drop in oil pressure, such as a failed pump.

A little over a month before the crash, Sikorsky directed all S-92A operators to replace the titanium studs on their gear filter bowls with steel ones, but the Canadian helicopter had not completed the service yet. Since then, all S-92As have done so, Sikorsky reports, and it has taken additional steps such as redesigning the filter bowl to improve maintainability and beefing up the main gearboxs durability.

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