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Technology update
Simulating aircraft icing
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Malvern Instruments' Spraytec particle analyzer is operated by Glenn Howard at Aerospace Composite Technologies' Icing Research Wind Tunnel facility.
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Aviators and aerospace engineers have battled long and hard against the effects of icing. Dealing with its effects on flying surfaces has involved the development of a wide variety of solutions, icing's potential dangers if not satisfactorily countered, are every bit as real to the largest and most modern aircraft as they were decades ago to frail, under-powered biplanes. Icing remains one of the major aviation weather hazards, and this decade has seen fatal accidents attributed to loss of control resulting from icing problems. Consequently, investment in terms of finance, and intellectual and engineering resources, continues to be focused on developing and testing more effective deicing and anti-icing solutions.
Aerospace Composite Technologies (ACT) in the UK has specialized for more than half a century in the development and provision of aircraft ice-protection systems and transparencies. It recently commissioned a new Icing Research Wind Tunnel (IRWT) at its London (Luton) Airport base. The tunnel will have double the capacity of the company's previous facility, and is equipped with the latest test and measuring equipment. Particularly significant is the installation of a new particle sizing system that allows the rapid and accurate determination of droplet size, enabling the simulated flight icing conditions in the tunnel to be closely controlled and adjusted much more rapidly than was previously possible.
Malvern Instruments' Spraytec system operating in the Icing Research Wind Tunnel at Aerospace Composite Technologies.
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ACT's Spraymat electrothermal ice protection systems are fitted to a range of both civilian and military aircraft. The systems have applications on wing leading edges, horizontal and vertical stabilizers, helicopter tail and main rotor blades, engine air intakes, propeller spinners, oil coolers, air inlets, and weapons systems. They are designed to be applied to aircraft currently in service, or integrated into structural components during the manufacture of new fleets. All of this requires extensive testing and parts certification. The company's capabilities and responsibilities stretch from design and building to final certification. The company uses its new IRWT for testing extreme weather conditions and temperatures while other testing required for aircraft part certification, including lightning strike and bird strike, is carried out elsewhere, but all under ACT's control.
The company is participating in international efforts to standardize results from icing wind tunnels, as it is known that there are variations between tunnels, depending on the tools used for measuring. This has a direct bearing on the future development of aircraft testing, said Glenn Howard, ACT's Test Facilities Manager responsible for wind tunnel operations at Luton: "The icing conditions required for full aircraft certification may not easily be found for natural flight icing trials, so one solution would be the use of tunnel studies. However, this requires that appropriate icing conditions can be correctly simulated and are measurable and reproducible from facility to facility. Different tunnels must therefore produce the same results irrespective of the tools being used."
The new tunnel is designed primarily for testing the external structures of fixed and rotating wing aircraft, and can be used to demonstrate performance across the full range of international icing requirements. This includes simulation of flight into supercooled liquid water droplets, ice crystals, and mixed conditions, in static air temperatures down to below -30°C. Simulated rain and snow conditions will also be available. The tunnel can be used for nonaviation test work. Key elements of the tunnel include a maximum air-mass flow of more than 47 kg/sec; a large standard working section 508 x 762 x 1670-mm long at Mach 0.32 and a small standard working section 178 x 305 x 1000-mm long at Mach 0.62. The tunnel has extensive data and visual recording capabilities.
Among aerospace customers using the company's IWT facilities are SAAB AB; the Bell/ Boeing V-22 Osprey Tiltrotor Team; Rolls-Royce Aerospace; British Aerospace Regional Aircraft; and CASA. According to Glenn Howard, the rate-limiting step until now has been the time taken to measure the size of water droplets, ice particles, and supercooled liquid water: "Clouds are created in the tunnel using airblast water atomizers that produce a spectrum of droplets to simulate natural conditions. For complete control and validation of results it is necessary to know the largest droplet size as well as the median value. Traditionally, droplet measurement is performed using the long-established oil slide method in which an oil-coated glass slide is exposed to the water cloud for droplet capture. The slide is then placed under a microscope, and the image is magnified and photographed, then counted and measured. All this is both labor-intensive and time-consuming. It also means that there is a long delay in feeding back the data necessary for the correct adjustment of wind tunnel conditions." This is critical because droplet size measurement is essential to ensure that icing conditions in the tunnel conform to testing specifications and that effective simulation of specific environmental conditions is being achieved. When ice particles generated by milling an ice block are blown into the tunnel, the particle size is determined by sieving. Again, this is a laborious technique and one that gives no useful information about particle-size distribution.
To overcome the measurement bottleneck, ACT has incorporated a particle-size analyzer into its new tunnel. Called Spraytec, designed and manufactured by Malvern Instruments, it operates as a continuous measurement system, measuring droplet or particle size, and producing a stream of traceable data that enable the immediate adjustment of conditions before and during testing. According to ACT, the system significantly reduces the time taken to achieve controlled conditions, with benefits both in cost savings and in improving the work rate of the tunnel. Sampling ability, continuous measurement capability, the time taken to process data, repeatability and accuracy of results are all essential elements of the new tunnel. The Spraytec system uses established laser diffraction technology called low-angle light scattering (LALLS) for the real-time measurement of particle or droplet size. It was developed specifically to overcome the fundamental problems associated with measuring continuous sprays, including wetness, high concentration, and the short duration types.
Particle size analysis using laser diffraction involves a particle passing through a laser beam, causing light to be scattered at an angle that is inversely proportional to its size. The scattered light is collected by a detector and analysis of the ensuing diffraction pattern enables calculation of the particles' size distribution in a given sample. To be sufficiently robust for continuous operation on an experimental test rig such as in a wind tunnel, the Spraytec system has been designed to have large clearances between the laser and detector modules. This allows large volume sampling. The use of sealed modules and air-purgeable optics minimizes contamination by the measured sprays. In addition, separation of the control and electronics units allows these to be placed outside the testing area. Spraytec measures up to 2500 times per second. It can be used effectively to monitor all types of tunnel conditions and will completely replace both the oil-slide method and sieving at ACT. The Spraytec system allows tunnel conditions to be more closely and more rapidly controlled and ensures that for every sample, there is now a recorded history of the test, together with comprehensive data for immediate or future analysis. ACT regards this as an important step towards certification procedures and method validation, for which permanent data records are desirable. It also opens up the possibilities for more fundamental research for the development of novel testing conditions, different spray spectra, and new atomizers.
Stuart Birch
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