Technology Update

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March 2002
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A330 Tanker

Current military developments have underlined the crucial role played by air-to-air refueling (AAR). In Europe, several major aerospace and defense companies are involved in the design of a new strategic tanker aircraft for the UK Royal Air Force (RAF), based on the widebody Airbus A330. The companies have formed AirTanker to bid for the UK Ministry of Defence's Future Strategic Tanker Aircraft (FSTA) program to replace the RAF's TriStar and KC-10 (VC-10-based) tankers. The aircraft would have a dual refueling and transport role—cargo, personnel, or medevac (aero-medical evacuation).

Designated A330-200 MRTT by AirTanker, it would retain the commercial A330's two-crew cockpit but add a mission-systems operator console for a third crew member during air-to-air operations. In its tanker role, the aircraft would have a total fuel capacity of 139,090 L. Hose and drogue pods installed under each wing would have a flow rate of 1500 L/min, and a hose/drum unit (HDU) in the lower center fuselage would provide up to 2270 L/min. The aircraft could have a fuel-receiving capability via a probe for RAF service. Other equipment could include video monitoring and AAR rendezvous and operational lighting system with infrared capability.

In a cargo role, the aircraft would offer large lower- and optional upper-deck cargo capacity with semiautomatic cargo loading. The aircraft could also be used in a passenger role with cargo carried in the forward part of the upper deck and passengers aft. Engine choice would be two GE CF6-80E1, Pratt & Whitney 4000, or Rolls-Royce Trent 700 engines.

- Stuart Birch

Compressor blade repair at MTU

CLPC techniques used by MTU allow for the restoration of worn blade tips to their original length.

The economic and effective repair of worn aircraft engine compressor blades can be a major challenge, particularly for late-generation types that incorporate blisk (integrally bladed disc) technology. These can make conventional methods of blade replacement "practically useless," according to MTU Aero Engines. So they are using a new technique called Contained Laser Powder Cladding (CLPC). Developments in CLPC techniques now allow restoration of worn blade tips to their original length.

Traditionally, arc welding has been used to add length to blades abraded in service. However, it is a slow process. Laser Powder Cladding (LPC) is about 10 times as fast, says MTU, and the new CLPC technique also adds precision to the process. CLPC uses a mold to contain the weld puddle, which makes for a near-net-shape weld with no appreciable overhang.

MTU describes LPC as being a highly complex, multi-process technique. It depends very much on manifold parameters including laser power, focal length, powder feed and powder gas-flow rates, grain diameter, and material characteristics. Since both CO₂ and solid-state lasers are viable options, selection of a suitable source is an important consideration.

For industrial applications, the use of solid-state lasers has proved the best choice, according to the company. Powder used in the cladding process can be fed laterally or coaxially. Because blades requiring repair may vary in length up to 2 mm, free (uncontained) LPC coaxial powder nozzles have proved effective. Use of an external non-coaxial powder nozzle, therefore, would necessitate the repositioning of the entire laser head, within �0.2 mm, relative to the surface for every single blade. In industrial applications, elaborate fixturing or sophisticated sensors would be needed.

A laser head and compressor blade are arranged for mold-contained laser powder cladding.

But use of a coaxial powder head eliminates the need for such intricate provisions, states MTU. Due to the coaxial alignment of laser beam and powder stream, the two remain coincident over a relatively wide range, making this type of head better suited to compensate for differing blade lengths than a laterally arranged powder nozzle. Coaxial powder feed also gives better process stability and reduces the programming work required.

During the welding proper, the laser energy applied should be just strong enough to locally fuse the substrate and melt the powdered filler material. That typically means a laser power of 500-800 W and a deposition rate of 300-500 mm/min. The low heat input and non-turbulent gas flow keep the porous fraction in the tip buildup low.

MTU says that comparison of the two laser-powder cladding techniques (mold contained and uncontained) with conventional arc welding shows the laser method to be superior, although expertise in its application is essential.

- Stuart Birch

March 2002
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