If you’ve ever had a flight delayed by a snow-covered runway, you’ll know that cold weather can be the enemy of air travel. But far from the ground, ice has even more serious implications. As an aircraft flies through clouds at sub-zero temperatures, super-cooled water droplets can form layers of ice on aerodynamic surfaces like wings, propellers, or jet intakes. This can increase drag and significantly reduce lift, making the plane difficult to control.
Systems that pipe hot air to melt ice from vulnerable areas have been used for decades, but they are heavy, inefficient and require constant maintenance. They also require more power when used with the carbon fibre reinforced polymer (CFRP) composites that are increasingly common for passenger aircraft (e.g. Boeing 787 is >50wt% CFRP). A new paper, published in Carbon [DOI: 10.1016/j.carbon.2018.04.039], reports on an alternative system for anti-icing and de-icing on composite aircraft – a lightweight resistance heater, based on carbon nanotubes (CNTs).
The researchers from Queen's University Belfast start by drawing out a ‘CNT web’ – a continuous film of horizontally-oriented CNTs. Though highly conductive along the draw direction, the web’s electrical resistance can be controlled by stacking individual layers. CNT web stacks of 10-40 layers were produced and cured with a glass fibre resin to provide structural support, and copper foil connected the samples to a power supply. The team also made a series of CFRP / glass fibre stacks with equivalent resistances for comparison. Samples with 10, 20, 30 and 40 layers of CNT web were found to have very similar resistance to CFRP samples with 4, 8, 12 and 16 layers, respectively.
The CNT heaters achieved higher heating and cooling rates than the equivalent CFRP heaters – e.g. after 30s, the CNT-40 sample had reached 95°C, while CFRP-12 reached 39°C. This would indicate that CNT heaters could de-ice with less time and energy than CFRP heaters. The CNT stacks were also significantly more thermally-uniform than CFRP – the authors attributed this to the presence of a large temperature gradient within the CFRP, as a result of its thickness (CFRP-16 was ten times thicker than CNT-40).
Tests of the anti-icing performance of the carbon heaters showed that half of the CFRP stacks were effective, while only CNT-10 failed to achieve results. De-icing requires additional energy to overcome the latent heat of melting, and samples were mounted vertically to represent leading edges. Under simulated flight conditions, a CNT-40 heater de-iced the test surface in 15s, compared to 25s for the best performing CFRP heater.
The authors say that compared to the current state-of-the-art systems, their CNT-heater “possesses negligible weight, rapid and uniform heating, efficient energy consumption, higher compatibility with CFRP, and can be tuned… to achieve rapid anti-icing/de-icing”. They are expected to publish further papers in the months to come.
Xudan Yao, Stephen C. Hawkins, Brian G. Falzon. “An advanced anti-icing/de-icing system utilizing highly aligned carbon nanotube webs” Carbon 136 (2018) 130-138. DOI: 10.1016/j.carbon.2018.04.039