Brüel & Kjær Sound & Vibration Measurement A/S

Vibration testing of fuel probes


Aerial Refuelling

The American Bell-Boeing V-22 Osprey is a multimission, military, tilt rotor aircraft with both a vertical and short takeoff and landing capability. It combines the functionality of a conventional helicopter with the high-speed cruise performance of a long-range turboprop aircraft. But even though it is long-range, some missions require in-flight refuelling.

The V-22 Osprey can be equippped with a fuel probe that engages a para-drogue dragged behind the tanker aircraft. Obviously, aerial refuelling is a hazardous business, so attention to safety and integrity is paramount. And this includes the dynamic response to vibration of the probe. Leaders in this field are Cobham plc in partnership with TRaC UK.

The Partnership
Founded in 1934 as Flight Refuelling Limited (FRL), Cobham plc is a British manufacturing company based in Wimborne Minster, Dorset, England. It is organised into four core technology divisions – Mission Systems, Defence Systems, Avionics and Surveillance, and Aviation Services. The Cobham Mission Systems Division claims to be the world market leader for air-to-air refuelling. 

TRaC is a market leading UKAS accredited test company comprising three Centres of Excellence and three regional test laboratories in the UK and a partner facility in Japan. TRaC offers unrivalled testing, regulatory and compliance services to the defence and commercial sectors.

Unlike for TRaC, Vibration testing is not one of Cobham's core competences, so in an unusual step to get these skills Cobham agreed to TRaC investing in the creation of a complete vibration test facility at Cobham’s Wimborne site, offering total control of development, pre-qualification and full qualification test planning.

TRaC equipped the new facility with vibration, shock, and climatic test capabilities, including a 160 kN LDS V984 electromagnetic shaker with slip-table, an LDS V875, and a range of M&P controllers. Damon Close, one of TRaC’s most experienced vibration test engineers, was placed in the role of Test Laboratory Manager. Damon’s responsibilities include the hands-on management of testing at Cobham’s Wimborne site, and he provides a single point of contact for all environmental requirements, ranging from fixture design to building bespoke vibration and fatigue test solutions.

The Challenge

One of the most technically challenging aspects of Cobham’s vibration testing is the physical size of the equipment under test, where hardware can be in excess of 5 m long, 1 m in diameter and weigh up to 650 kg.

The large overturning moments and off-centred masses of refuelling probes do not lend themselves to straightforward vibration test setups. By their very geometry, the dynamic response to vibration is an inherent issue that not only requires careful consideration by Cobham during design phase, but requires significant consideration by TRaC prior to and during testing. To overcome these difficulties, TRaC initially designs and analyses bespoke aluminium alloy fixtures to replicate in-service mounting. These fixtures need to be designed with sufficient structural rigidity to overcome the large bending moments and the effects of the vibration regime itself. However, they are sufficiently lightweight that the vibration capacity of the shaker is not compromised.

Secondly, horizontal-axis testing is performed on an LDS 9-bearing, guided slip-table, specified by TRaC at the investment phase, which is capable of reacting to the high overturning moments associated with test hardware of this size.

During recent, similarly demanding qualification testing on Cobham’s refuelling probe for the V22 Osprey, TRaC also utilised a variety of techniques to monitor the response of the refuelling probe during vibration. These included conventional accelerometers located at various points in the system, coupled with a high-speed video camera to track the motion at the tip of the probe.

Lasting Relationship

Having reached its sixth anniversary in August, 2010, the environmental test partnership between TRaC and Cobham plc’s Cobham Mission Equipment, has proven to be a remarkable success for both parties, and has become a TRaC benchmark for excellence in environmental test partnerships. 

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