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Sunday, September 1, 2013

Not Just Folding Wings and Tails

An insight into one manufacturer’s approaches to the issue of marinization of military helicopters reveals the depths to which OEMs must go to protect aircraft against saltwater ingress and corrosion.

By Andrew Drwiega, International Bureau Chief

Sikorsky CH-53 through a Bell-Boeing V-22 at Marine
Corps Air Station (MCAS) Miramar in San Diego, Calif.
Folding blades and tail sections are perhaps the most physical attributes of a helicopter, especially in the military, that has been prepared for service at sea, usually for ship-borne operations where hanger space is at a premium. However focusing on just those two elements ignores the accumulation of experience that has been gained in terms of extending the lifetime of aircraft that are continually exposed to the threat of saltwater ingestion.

Corrosion in aircraft operating in the maritime environment is a particular problem that manufacturers have worked at reducing. One of the main problems is that where joints in the construction of the airframe are located, each poses a potential access point for water ingestion and therefore corrosion.

Seawater contains around 3.5 percent sodium chloride and is usually more corrosive than fresh water due to its higher conductivity and the penetrating power of the chloride ion through surface films on metal.

The different alloys of aluminum used in the construction of helicopters have varying strengths, with A1-clad aluminum being the strongest. Anodizing aluminum will also give it a protective coating. Primers and paints also include chromate coating.

Bell-Boeing V-22 flying around MCAS
Miramar.
 
According to Tim Healy, senior international maritime director at Sikorsky Aircraft, the company’s expertise in building Sea Hawks over 30 years has resulted in an ongoing continuous improvement process. “We have learned that marinization advances do not stay exclusively in the maritime environment. The UH-60M is a much more marinized and protected aircraft today than was the original Sea Hawk.” Healy adds that every advance is taken across to all of Sikorsky’s airframes including commercial S-76s, the more recent S-92s as well as military Sea Hawk Romeo and Sierra models. The CH-53K will also benefit from the experience gained by Sikorsky as well as the maintainers within the U.S. Marine Corps.

Electrical connector showing
corrosion from marine
environments.
Protection is layered in through the preparations involved in the type of paint and coatings used, to the materials used in construction and how they are manufactured. “We have evolved now to high-speed machining which has allowed us to create much larger components [monolithic structures]. These are not only stronger and lighter, but they are less susceptible to corrosion in the maritime environment as there are fewer places for ingestion of salt water,” Healy explains. “A lot of these techniques are bettering our requirements to balance needs across aircraft types – strength, cost, corrosion – and they can now be stronger without adding mass. We can engineer out some of the connecting pieces which can be failure points when it comes to structural integrity.”

This engineering involves good drainage and ventilation as well as avoiding crevices and water traps. It is about material selection, good access for maintenance, and the use of corrosion inhibitors and sealants. The replacement of sheet metal components by high-speed machined airframe components takes away opportunities for water ingestion. This helps eliminate a number of historic problem areas including mating surfaces, which were prone to crevice corrosion, and holes in the sheet metal, which allowed corrosion and the gradual elimination of dissimilar fasteners prone to galvanic corrosion.

MH-60R Sea Hawk from HSM-70 raises a fuel line
for an in-flight refueling from the USS Roosevelt.
Sikorsky’s fly-by-wire Superhawk MH-92 has a tail rotor pylon and rotor blade fold system (with manual fold back-up). The undercarriage is strengthened which allow the helicopter to operate up to Sea State 6. It also uses the Rapid Assist Secure and Traverse (RAST) deck handling system. It has anodized protection on internal components. Corrosion inhibiting sealants are also used improve water integrity.

The physical differences occur in the design and configuration of the landing gear and its footprint on the deck. The difference in tail structure of the Black Hawk verses Sea Hawk, for example, where the tail wheel on the Army version is at the end of the tail boom and has a heavy duty shock absorber. The Sea Hawk’s twin-tail wheels are at the root of the tail boom and are taller to allow space for the drum radome.

Sikorsky’s international maritime chief engineer, Reshma Shanku, added that the objective is to eliminate points that provide the potential for water to get trapped. The use of sealants and polyurethane paint does help in corrosive protection.

Deteriorating glass-fiber composites on
a helicopter fuel tank.
Industry tests have shown that polyester polyurethane paint exhibits better short-term resistance to initiation of overgrowth and surface damage by fungi than lacquer-based paint. Humid environments can cause the growth of fungi, which can have a direct effect on the airframe, or its presence can result in damage through persistent cleaning.

But individual users can specify the treatment that they require in addition to the basic aircraft. Said Shanku: “There are specific things that are available for individual users. There is a range of coatings that can be sprayed on the inside of a tail boom or the on the floor of an aircraft and all have different properties optimized for various environments.” Black Hawk and Sea Hawk helicopters have a different material protecting the inside of the aircraft.

 

 

Flotation Equipment

Sikorsky’s Shanku said that flotation equipment was a natural consideration for any aircraft that was to perform in the maritime environment. Calculations are made not only on the weight of the aircraft, but also on the time it needs to stay afloat to allow the crew to escape in a variety of sea conditions. “We do egress testing to ensure the crew can get safely out of our helicopters,” she stated. Location is important: the Sea Hawk flotation devices are housed in the stub wings. She added that no decision has yet been made on the flotation equipment for the U.S. Marine Corps CH-53K.

GKN Aerospace supplies flotation devices for both military and civil helicopters designed to FAR/JAR 27 and 29 including the AgustaWestland’s AW101, Sea King and Sea Lynx, as well as Sikorsky’s UH-60, S-70A and S-92, together with a wide variety of Bell helicopters and even Russian Mi-8/17s. GKN conducts computer simulated stability analysis followed by scale model testing before installation. The materials used for the manufacture of flotation devices also need to be light and reliable.

Over Water Equipment

A representative from Eurocopter provided a few points about preparing helicopters for operating over water: Operating at sea (ships, oil rigs, etc.), the helicopter needs flotation gear, anti-corrosion measures, AIS (automatic identification system – to know where ships are); a kind of maritime IFF, a radar (for weather as a minimum), and communications suitable for maritime channels (maritime VHF).It should have the tie down points so it can be secured to a deck through bad weather or ship rolling.If the aircraft is to operate from the ship, it needs folding rotors and, ideally, wheels rather than skids. A naval aircraft should have a deck lock such as the Harpoon system.

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