Flight refuelling probe tested at 396 bar and temperature range of -55 /+145°C

Citation

(2005), "Flight refuelling probe tested at 396 bar and temperature range of -55 /+145°C", Aircraft Engineering and Aerospace Technology, Vol. 77 No. 2. https://doi.org/10.1108/aeat.2005.12777baf.002

Flight refuelling probe tested at 396 bar and temperature range of -55 /+145°C

Flight refuelling probe tested at 396 bar and temperature range of -55 /+145°C

Keywords: Aircraft, Fuels

Flight Refuelling Ltd has successfully delivered the first telescopic re-fuelling probe to Boeing for testing on the unusual V-22 Osprey tilt rotor aircraft. Flight Refuelling is also conducting its own series of demanding structural and environmental tests using a new purpose-built test facility in Dorset, UK. In order to fully test the probe under real world conditions a range of climates from arctic cold to desert heat have had to be recreated in the test chamber. WYKO Fluid Power Engineering Services was called upon to help design, build and supply a hydraulic power pack that could deliver military grade oil at 396 bar over a temperature range of -55/+145°C to actuate the probe in these conditions (Plates 1-3).

Plate 1 WYKO hydraulic power pack used to test flight refuelling probe at 396 BAR and temperature range of -55/+145°C

Plate 2 WYKO hydraulic power pack used to test flight refuelling probe at 396 BAR and temperature range of -55/+145°C

Plate 3 WYKO hydraulic power pack used to test flight refuelling probe at 396 BAR and temperature range of -55/+145°C

Mark White, senior project engineer at WYKO Fluid Power Engineering Services describes how it was achieved.

The Osprey is designed to take off and land like a helicopter, but, once airborne, its engine nacelles can be rotated to convert the aircraft to a turboprop airplane capable of high- speed, high-altitude flight. The wings also fold in on themselves allowing the aircraft to be stowed and deployed from aircraft carriers and other vessels without the need for a runway. The probe is just over 9ft long when extended and so had to be retractable in order for the aircraft to be stowed easily. Once retracted, the probe is completely flush with the nose section of the fuselage.

In order to actuate the probe and satisfy the stringent test criteria it was necessary to develop a hydraulic power pack unit that could supply oil to the test chamber in exactly the same state as it would be in its most extreme working environments. This posed a series of challenges to the WYKO engineers as it meant the specifications were also extreme; the power pack had to deliver oil at a flow rate between 1 and 6l/min at a pressure of 396 bar and at an adjustable temperature range of -55 to 145°C. It also had to be capable of pumping two different military specification hydraulic oils.

The system is designed around three separate hydraulic circuits that are connected to a common reservoir. Two auxiliary circuits are used to condition the oil in the common reservoir prior to the main circuit delivering the oil to the test facility. These auxiliary circuits consist of a “hot” and a “cold” circuit that, as the name suggests, either heat or cool the oil depending on the system requirement. A temperature probe coupled to a Eurotherm temperature controller continually monitors oil in the reservoir. This in turn outputs a “cool” or “heat” signal to a PLC that controls the system.

The PLC decides which auxiliary circuit to run depending on the ambient temperature and the required setpoint temperature for the current test. In basic terms the cool circuit is designed to control temperature below around 25°C and to “superchill” whilst the hot circuit controls above that temperature and obviously heats. As the temperature passes through a predetermined set temperature (ambient) the PLC shuts down one circuit and starts up the other.

The fundamental issue here is with the characteristic of the hydraulic fluids being used. The viscosity of military spec fluid can be around 2 mm²/s at 130°C and 4500 mm²/s at -45°C. At the high temperature it behaves much like water at room temperature and at the low temperature it takes on the consistency of sticky toffee and is extremely difficult to pump.

Every hydraulic component and pipefitting had to be checked for compatibility with the extremes of temperature and pressure. The same was true for the materials; both electrical and mechanical used in the construction. It was not possible for instance to find a gear pump that could operate over the entire temperature and viscosity range of the fluids used. Hence the use of two auxiliary circuits which switch in and out as necessary.

A check ball piston pump close- coupled to an inverter-controlled motor carries out the final delivery of the conditioned hydraulic fluid to the thermal chamber. This pump is fed by a boost pump that is enabled whenever the temperature falls below around 10°C. The boost pump, also inverter- controlled, is necessary to provide sufficient suction pressure to the main pump at low temperatures. At elevated temperatures, it is not necessary and is disabled. The fluid passes from the main pump and across a pressure filter and out to the temperature controlled chamber where the test rig is housed. The fluid is continually monitored for pressure, temperature and flow as it continues its journey.

Inverters are used to control three of the four motor pump sets used on the rig and provide control of pump speeds. This was achieved through the use of reference tables that enable the PLC to send the correct speed signal to each inverter and hence its pump motor to cope with the variation of viscosity as temperature varied. The values took some time to establish as the characteristics of each fluid were explored during development. It was found on reaching -39°C the viscosity of one fluid doubled in the space of 5°C!

The action of cooling the oil to extremely low temperatures was an area that demanded a considerable amount of research and development time. The original concept involved the use of liquid Nitrogen as the coolant in the heat exchanger but the -195°C input temperature and the nature of the medium was found to be too aggressive when passing through the heat exchanger and caused localised freezing of the oil and hence a lamina flow effect that resulted in a reduced chilling ability of the system. A refrigerated cooling system unit was eventually chosen as it provided superior controllability.

One of the main hurdles to overcome having reached the desired setpoint temperature, was to then maintain it, not only through closed loop control of temperature but also through adequate insulation materials to prevent heat loss or gain. Thermal breaks had to be designed into brackets that minimised conduction of heat and suitable insulation found that could cope with the extremes of temperature variation.

The complexities of achieving the end results are mainly hidden by a simple operator control setup. Access to predetermined functions at set temperatures is provided through a standard HMI display. Through this, an operator can also carry out manual overriding of functions and adjust the reference tables and set values programmed into the system during testing. Overall, there has been an emphasis on providing a reliable and accurate system with clear visual display of pressures and temperatures through the use of gauges that provide information on the systems status.

For further details contact. WYKO Industrial Services. Tel: +44 (0)121 5086341; Fax: +44 (0)121 5086333; E-mail: marketing@wyko.co.uk