This invention relates to a de-icing system and method. More specifically, but not exclusively, it relates to a system and method for detecting and removing ice build-up on a domed protective housing mounted on an aircraft.

In modern aircraft, sensitive monitoring equipment, such as infrared (IR) cameras and laser 'pointers' are often mounted on the exterior of the aircraft. Such equipment must be protected from the harsh exterior environment and this is often achieved by use of transparent domed structures that do not inhibit the 'view' from such cameras or the transmission of the laser. However, ice and moisture can build-up on such protective domed housings at altitude thereby inhibiting the view for the camera and the transmission of the laser.

Accordingly, there is a requirement for a system to detect and remove ice and moisture from a domed transparent housing that does not impair the transmissivity of the structure itself.

Commercially available systems exist for removing ice from aircraft wing leading edges. Generally, these use wide-band electro-mechanical vibrations induced by a pulsed electromagnet embedded into the aircraft wing. Such a device is disclosed in U.S. Patent Number 6,102,333. The electro-mechanical de-icer described in this document uses electro-magnets that require a compliant surface that can deform sufficiently to remove ice-build-up. Additionally, such a system does not independently detect the presence of ice; it assumes the presence of ice and acts to remove it accordingly.

A further system is described in US Patent 2012/0074262 entitled "DE-ICING SYSTEM FOR A FIXED OR ROTARY AIRCRAFT WING". The system disclosed therein includes the use of ultrasonic vibration induced by piezo-electric actuators to remove ice build-up from the wings of rotary or fixed wing aircraft. However, the system as described cannot be used to detect the presence of ice. Furthermore, the surface of the dome cannot be obscured, for example by heating filaments embedded in the structure as this would block the laser / IR camera. Additionally, electrothermal de-icing at the base of the dome is also not possible as this method requires excessive electrical power to maintain temperatures above the external dew point to prevent moisture settling, or to melt ice on the surface.

According to the present invention there is provided a de-icing system for a protective housing mounted on fixed or rotary wing aircraft, the system comprising means for generating ultrasonic frequencies in the housing and means for sensing the ultrasonic frequencies induced in the housing, an ultrasound frequency range being predetermined and selected such that structural resonance of the housing is induced by the generating means, the means for sensing ultrasonic frequencies comprising means for maintaining the ultrasound frequency such that structural resonance of the housing is maintained and thereby causes the breakdown of ice existing on the housing and prevents build-up of additional ice on the housing.

According to the invention there is further provided a method of de-icing a protective housing forming part of the exterior of an aircraft comprising the steps of generating ultrasonic frequencies in the protective housing; monitoring the frequency induced in the housing; and adjusting the induced frequency until structural resonance of the housing is achieved.

The present invention provides a solution that maintains a complex curved structure free from ice and moisture whilst maintaining the transparent nature of the structure. Additionally, the present invention enables detection of ice build-up as well as ice removal.

The invention will now be described with reference to the following diagrammatic drawings in which:

Figure 1 is a cross-sectional diagrammatic drawing of a system in accordance with one form of the invention where a series of piezo-electric devices are mounted at the boundary 2 of a protective housing 2, the housing 1 being, for example a dome; and Figure 2 is a circuit diagram showing a phase locked loop for measuring frequency vibrations sensed by at least one of the piezo-electric devices of Figure 1 and the output to further piezo-electric devices generating the ultrasonic frequencies required to induce structural resonance in the housing.

The de-icing system in one form of the invention comprises a series of piezo-electric actuators positioned at the base (equator) 2 of a hemi-spherical dome surface 1. Several of the piezo-electric actuators are used to induce vibrations in the ultrasonic frequency range (20kHz - 200kHz), while one sensing piezo-electric device is used to measure the frequency of vibration of the dome 1 by measuring the induced displacements of the dome surface at its base. The vibrations measured by the sensing piezo-electric device are used to lock the other driving piezo-electric actuators into a high amplitude resonance, inducing structural resonance of the dome.

Structural resonance is continually induced to inhibit ice build-up. Higher power resonances may be further induced by the piezo-electric actuators to remove existing ice build-up.

The frequency of the induced structural resonance measured by the sensing piezo-electric device can be used to indicate that the structural character of the dome surface has changed, thereby suggesting the presence of ice build-up. This change may be used to automate the system, ensuring it is only in use when required by the external environment.

It has been found that high frequency vibrations are required to generate a sufficiently high surface acceleration, such that ice delaminates from the surface of the dome and falls off. These same surface accelerations also prevent ice from building up on the surface, and can prevent moisture from settling on the surface, frequencies over 24kHz, stable resonances in hemispherical domed sapphire structures were found to prevent ice build-up, remove ice build-up and detect ice in the manner described above.

However, strong structural resonances of 23kHz, 44kHz, 75kHz and 93kHz may be used in the specific structure described and achieve the above results. In this manner it can be shown that different frequencies or ranges of frequencies of resonance may be used to achieve the desired results and the invention is not limited to the use of the frequencies described above. The exact frequency position of each resonance may be over a relatively narrow band. For example at the 44kHz resonance, the peak of the resonance was at 44.2kHz, started at about 44.15kHz, and tailed off at about 44.35kHz). This indicates that the proposed system may be termed a high-Q. system - a very efficient means of energy transfer.

The amount of energy the piezo-electric actuators and device required is around 10W to remove ice / prevent ice build-up - much less than for some form of heating system.

Additionally, it is possible to measure the vibrations in the dome 1 using a high frequency microphone instead of a piezo-electric device. However, it should be noted that use of a piezo-electric device enables resonance to be detected, and can therefore be used to drive the other piezo-devices inducing the vibration towards a resonance, thereby making the system self-resonating.

In this way, the de-icing system and method induces the excitation of structural resonances of a hemi-spherical domed structure in the ultrasonic frequency range to inhibit ice buildup, delaminate ice build-up, detect the frequency of ultrasonic structural resonances for these purposes, and to determine the presence of ice build-up on the dome 1 surface from the frequencies detected.

Whilst the embodiment above relates to a hemispherical sapphire dome 1, it will be appreciated that structural resonance can be induced in any structure, even a flat plate, providing the boundaries 2 of the structure are fixed in position and it is free to move between those boundaries 2.