When an aircraft passes through clouds containing supercooled water droplets, the leading edge’s surface will gradually accumulate ice. Ice surface roughness is an important parameter affecting the local convective heat transfer coefficient and the water collection coefficient, which in turn affect the ice’s shape. However, because the surface roughness of aircraft icing is a transient value varying in time and space, it is extremely difficult to measure with existing methods in real time. In this study, a noncontact ultrasonic pulse-echo (UPE) technique is applied to characterize the ice roughness of an airfoil model’s surface. A multilayer model with equivalent bead-like roughness profiles is established to study the effects of changes in ice roughness on ultrasonic echo signals. A series of simulations indicated that ice roughness can be measured quantitatively and effectively in the range of [11.6, 120] μm. Based on these simulations, an experimental UPE device was developed to measure echo signals on top of the ice corresponding to surface roughness. The results show that for both the regular and irregular surface roughness samples, the maximum relative error in the roughness is less than 15%. Meanwhile, we designed and supplemented the experiment with the NACA-0012 airfoil model to realize the online measurement of ice roughness in an icing research tunnel.

The accumulation of ice in critical regions on the surfaces of modern aircraft is a serious threat to flight safety, and so for aircraft to be able to fly safely in all weathers, they must be capable of reliable early warning of icing and online measurement of ice thickness. In this paper, an ultrasonic pulse-echo technique combined with signal processing is proposed for icing early warning and ice-thickness measurement. The early-warning sensitivity and ice-thickness measurement ability are verified through both simulations and experiments. The simulation results show that (i) the proposed signal processing method can identify ice and water effectively and (ii) the minimum icing warning and precise measurement thickness of the ice layer decrease with increasing center frequency of the excitation pulse; for a given center frequency, the icing-warning thickness is approximately a third of the initially measured ice thickness. The experimental results show that a probe-type sensor with a center frequency of 7.5 MHz can measure 0.24-mm-thick ice, and the early-warning time is 8.5 s ahead of the accurate measurement time. A flush-mounted sensor cannot measure the ice thickness because of the roughness of the ice surface, but it can still provide early warning of icing.