Lamb-wave and impedance based ice accretion sensing on airfoil structures

Icing is a well known phenomenon in aviation. It happens, when an aircraft is flying in a freezing air mass containing water droplets which are then accreting to the surface of the aircraft. If the ice accretion gets too severe, it reduces the aerodynamic performance and increases the weight which may lead to a catastrophic failure of the aircraft. Recognizing an ice accretion is essential to initiate countermeasures against icing e.g. switching on ice protection systems or leaving the icing zone. By now, this mostly relies on the pilot looking to the front fuselage to detect ice accretion, which may be too late. To overcome this, ice accretion sensors can be used. Two types of ice accretion sensors have therefore been developed and tested at the German Aerospace Center (DLR). The first type is based on sending ultrasonic lamb waves through an icing prone structure, e.g. the leading edge of an airfoil. If an ice accretion happens, this will alter the waveguide characteristics of the structure. This change in the waveguide parameters hence influences the sensor signal, which can be detected by the sensor electronics. For the lamb wave sensor, piezoelectric transducers are therefore applied on the structure at a certain distance since the lamb wave signal needs a minimum amount of traveling time. The second sensor type also uses piezoelectric transducers and is based on measuring the impedance of the transducer. Due to the electromechanical coupling via the piezoelectric effect, the measured impedance of the transducer also contains the characteristics of the structure where it is applied to. Since an ice accretion will change these structural characteristics, measuring the impedance generates an obtainable sensor signal. Both of these sensor principles have been tested in a icing wind tunnels on a typical airfoil structures. There, the impedance sensor was mounted on three different positions in chord direction. The lamb wave based sensor has been mounted on the leading edge and further downstream positions to detect an ice accretion along the flow. During the icing tests multiple icing runs have been performed. It could be shown, that both sensors are capable of detecting the beginning of the ice accretion and the presence of ice on the structure. The lamb wave sensor reacts nearly instantaneously to the presence of ice. According to preliminary data, the impedance sensor needs a slightly larger ice thickness to produce a signal, but can handle larger ice thicknesses compared to the lamb wave sensor.