GB2587612A

GB2587612A - Ice detection system and method

Info

Publication number: GB2587612A
Application number: GB1913802.3A
Authority: GB
Inventors: Rodriguez Rodriguez Anxo; J Neat Matthew; Basheer Mohamedneyaz; Siddique Omar
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2019-09-25
Filing date: 2019-09-25
Publication date: 2021-04-07
Anticipated expiration: 2039-09-25
Also published as: GB2587612B; GB201913802D0

Abstract

A method of detecting ice accumulation on a region of a surface has the steps of: using a light source 52 to project a beam or pattern, such as a grid (fig.5,154), of light onto the region forming a light feature; using a camera 56 to capture an image of the region; using a detection unit, coupled in communication with the camera, to compare a position of the light feature in the captured image with a baseline position; the detection unit determines whether ice has accumulated on the surface region based on the comparison. Preferably the camera is positioned opposing the surface region so that ice accumulates towards the camera, a projection axis P is inclined relative to an imaging axis I, and a height of ice h can be determined based on distance S1,S2 measured between the image light feature position from the baseline. The surface may be a variable stator vane (fig.7,204) and/or an outer annulus (fig.7,208) of a gas turbine engine or a testing rig (fig.7,200), with a recess (fig.7,216) in a penny platform (fig.7,212) for the light source and/or camera.

Description

Ice detection system and method

Field of the disclosure

The present disclosure relates to an ice detection system and a method of detecting ice on a surface. In particular, it relates to systems and methods performed in a gas turbine engine or testing rig, and even more particularly to systems and methods to detect the build-up of surface ice on the outer annulus of a gas turbine engine and on stator vanes of a gas turbine engine or testing rig.

Background

The build-up of ice in a gas turbine engine can pose a hazard. If ice builds up on a surface to above a critical height, the ice may shed from the surface and pass through the engine.

Ice can damage engine components, and where the engine is an aircraft engine, damage to engine blades can cause an in-flight engine shutdown. Damaged components also need to be replaced, which can be time-consuming and costly.

Summary

According to a first aspect there is provided a method of detecting ice accumulation on a region of a surface comprising the steps of: directing a beam or pattern of light from a light source onto the region to project a light feature onto the region; capturing an image of the region with a camera; and, comparing, via an ice detection unit communicatively coupled to the camera, a position of the light feature in the captured image with a baseline position of the light feature, the ice detection unit determining, based on the comparison, whether ice has accumulated at a corresponding position on the region of the surface.

By determining ice accumulation through image comparison, the ice detection method may be carried out remotely, i.e. there is no need for a physical sensor to be present on the surface of interest. This is particularly useful in applications where the surface of interest needs to be free of physical sensors in order to perform its function, e.g. for aerodynamic reasons, where the surface of interest is a gas-washed surface in a gas turbine engine.

Optionally, a pattern of light may be directed onto the region to project a plurality of light features onto the region, and the step of comparing a position of the light feature with a baseline position may comprise comparing positions of a subset of the plurality of light features with respective baseline positions.

The subset may be a proper subset, or may be all light features.

Using a plurality of light features, ice accumulation may be monitored at multiple points across the region of the surface of interest. This may be used to establish points in the region that are particularly susceptible to ice accumulation. In some examples, ice monitoring may be restricted to susceptible points. For example, the location of susceptible points can be established using a testing rig, then when the method is later performed in a real-world scenario (e.g. in a gas turbine engine) light features may only be projected onto points in the region corresponding to susceptible points. This may improve efficiency by only monitoring points at which ice is likely to accumulate, and disregarding points at which ice is unlikely to accumulate.

Optionally, the pattern may define a grid of lines extending over the region, and the subset of the plurality of light features may comprise intersections of the grid lines.

The intersections of grid lines may be easier to track using image processing software compared to isolated points of light on a region of a surface.

Optionally, the method may further comprise the step of calculating a height of ice at the position on the surface corresponding to at least one of the light features.

This may enable the extent of ice accumulation to be determined at a point, or points, in the region. This information may be useful to further determine the risk of an ice shedding event where the method is performed in a gas turbine engine. Further, determining the height of ice at multiple points across the region may enable a volume of ice to be determined (see below). Again, this information may be used to further determine the risk of an ice shedding event where the method is performed in a gas turbine engine.

Optionally, the camera may be positioned generally opposing the region of the surface, such that ice accumulates along a direction generally towards the camera. The camera may be separated from the region along an imaging axis and the light source may be offset from the camera so that the beam or pattern of light is projected along a projection axis inclined with respect to the imaging axis. A position of the respective light features in the image may be a function of the height of ice accumulation; and, the step of calculating the height of ice may comprise: determining a distance between the baseline position of the light feature and the position of the light feature in the captured image; and, determining the height of the ice based on the measured distance.

For example, the ice detection unit may determine the height of the ice trigonometrically, using a look-up table or neural net, or using any other suitable method of determination based on the measured distance.

Optionally, the method may further comprise calculating a volume of ice on a region of the surface based on one or more calculated heights at one or more respective positions on the surface. The method may further comprise calculating a volume of ice on a region of the surface based on one or more calculated heights at one or more respective positions on the surface.

According to a second aspect there is provided an ice detection system for detecting the accumulation of ice on a region of a surface, the system comprising: a light source arranged to direct a beam or pattern of light onto the region to project a light feature onto the region; a camera positioned to capture an image of the region; and an ice detection unit configured to: receive a captured image; compare a position of the light feature in the captured image to a baseline position of the light feature; and, determine, based on the comparison, whether ice has accumulated at a corresponding position on the region of the surface.

Such a system may enable ice accumulation to be determined remotely, i.e. there may be no need for a physical sensor to be present on the surface of interest. This may be particularly useful in applications where the surface of interest needs to be free of physical sensors in order to perform its function, e.g. for aerodynamic reasons, where the surface of interest is a gas-washed surface in a gas turbine engine.

Optionally, the light source may be arranged to direct a pattern of light onto the region to project a plurality of light features onto the region, and the ice detection unit may be configured to compare positions of a subset of the plurality of light features with respective baseline positions.

Using a plurality of light features, ice accumulation may be monitored at multiple points across the region of the surface of interest. This may be used to establish points in the region that are particularly susceptible to ice accumulation. In some examples, ice monitoring may be restricted to susceptible points. For example, the location of susceptible points can be established by using the system in a testing rig, then when the system is later used in a real-world scenario (e.g. in a gas turbine engine) light features may only be projected onto points in the region corresponding to susceptible points. This may improve efficiency by only monitoring points at which ice is likely to accumulate, and disregarding points at which ice is unlikely to accumulate.

Optionally, the ice detection unit may be further configured to calculate a height of ice at the position on the surface corresponding to at least one of the light features.

This may enable the extent of ice accumulation to be determined at a point, or points, in the region. This information may be useful to further determine the risk of an ice shedding event where the system is used in a gas turbine engine. Further, determining the height of ice at multiple points across the region may enable a volume of ice to be determined (see below). Again, this information may be used to further determine the risk of an ice shedding event where the system is used in a gas turbine engine.

Optionally, the camera may be positioned generally opposing the region of the surface, such that ice accumulates along a direction generally towards the camera. The camera may be separated from the region along an imaging axis and the light source may be offset from the camera so that the beam or pattern of light is projected along a projection axis inclined with respect to the imaging axis. A position of the respective light features in the image may be a function of the height of ice accumulation; and.

wherein the ice detection unit may be configured to: determine a distance between the baseline position of the light feature and the position of the light feature in the captured image; and, determine the height of the ice based on the measured distance.

Optionally, the ice detection unit may be configured to calculate a volume of ice on a region of the surface based on one or more calculated heights at one or more respective positions on the surface.

Optionally, the light source may comprise a diffractive optical element.

Diffractive optical elements may produce a pattern of light features from a single light source while retaining a small form factor (typically on the order of millimetres). This may make them particularly suitable for applications where the environment is highly space-constrained, e.g. gas turbine engines, and testing rigs for gas turbine engines.

Optionally, the light source may be a laser light source.

Optionally, the system may be integrated into a gas turbine engine, and the surface may comprise a variable stator vane of the gas turbine engine and / or an outer annulus of the gas turbine engine.

Optionally, the gas turbine engine may include a variable stator vane comprising a penny platform, and wherein the light source and / or camera may be disposed in a recess in the penny platform.

Optionally, the system may be integrated into a testing rig designed to replicate part of a gas turbine engine, and the surface may comprise a variable stator vane of the testing rig and / or an outer annulus of the testing rig.

Optionally, the testing rig may include a variable stator vane comprising a penny platform, and wherein the light source and / or camera may be disposed in a recess in the penny platform.

The penny platform has been identified as a surface within the gas turbine engine / testing rig that may be readily modified to include a camera or a light source. A variable stator vane may be removed from an engine / rig and modified to include a camera or a light source in its penny platform. The variable stator vane may then be reinserted into the engine / rig.

This means that the modification does not need to take place in situ in the engine / rig, which may simplify the modification process.

According to a third aspect there is provided a gas turbine engine for an aircraft comprising: an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor; a fan located upstream of the engine core, the fan comprising a plurality of fan blades; and, optionally, a gearbox that receives an input from the core shaft and outputs drive to the fan so as to drive the fan at a lower rotational speed than the core shaft, wherein the gas turbine engine comprises the ice detection system of the second aspect.

Optionally, the turbine may be a first turbine, the compressor may be a first compressor, and the core shaft may be a first core shaft. The engine core may further comprise a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor; and, the second turbine, second compressor, and second core shaft may be arranged to rotate at a higher rotational speed than the first core shaft, wherein the ice detection system may be positioned in the first compressor, in the second compressor, or between the first compressor and the second compressor.

By determining ice accumulation and optionally calculating ice height and/or or ice volume, the methods and systems disclosed herein may provide for remote detection of ice. An ice accumulation signal relating to the detection of ice, one or more ice heights and/or an ice volume may be output by the method, and the ice detection unit may be configured to output such an ice accumulation signal. Such a signal may be used to determine performance parameters of the gas turbine engine or test rig, and/or to control operation of the gas turbine engine (e.g. mitigation actions such as increasing the engine idle speed), for example based on a determination of a threshold accumulated ice height or accumulated ice volume, to prevent excessive ice accumulation.

As noted elsewhere herein, the present disclosure may relate to a gas turbine engine. Such a gas turbine engine may comprise an engine core comprising a turbine, a combustor, a compressor, and a core shaft connecting the turbine to the compressor. Such a gas turbine engine may comprise a fan (having fan blades) located upstream of the engine core.

Arrangements of the present disclosure may be particularly, although not exclusively, beneficial for fans that are driven via a gearbox. Accordingly, the gas turbine engine may comprise a gearbox that receives an input from the core shaft and outputs drive to the fan so as to drive the fan at a lower rotational speed than the core shaft. The input to the gearbox may be directly from the core shaft, or indirectly from the core shaft, for example via a spur shaft and/or gear. The core shaft may rigidly connect the turbine and the compressor, such that the turbine and compressor rotate at the same speed (with the fan rotating at a lower speed).

The gas turbine engine as described and/or claimed herein may have any suitable general architecture. For example, the gas turbine engine may have any desired number of shafts that connect turbines and compressors, for example one, two or three shafts. Purely by way of example, the turbine connected to the core shaft may be a first turbine, the compressor connected to the core shaft may be a first compressor, and the core shaft may be a first core shaft. The engine core may further comprise a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor. The second turbine, second compressor, and second core shaft may be arranged to rotate at a higher rotational speed than the first core shaft.

In such an arrangement, the second compressor may be positioned axially downstream of the first compressor. The second compressor may be arranged to receive (for example directly receive, for example via a generally annular duct) flow from the first compressor.

The gearbox may be arranged to be driven by the core shaft that is configured to rotate (for example in use) at the lowest rotational speed (for example the first core shaft in the example above). For example, the gearbox may be arranged to be driven only by the core shaft that is configured to rotate (for example in use) at the lowest rotational speed (for example only be the first core shaft, and not the second core shaft, in the example above).

Alternatively, the gearbox may be arranged to be driven by any one or more shafts, for example the first and/or second shafts in the example above.

The gearbox may be a reduction gearbox (in that the output to the fan is a lower rotational rate than the input from the core shaft). Any type of gearbox may be used. For example, the gearbox may be a "planetary" or "star" gearbox, as described in more detail elsewhere herein. The gearbox may have any desired reduction ratio (defined as the rotational speed of the input shaft divided by the rotational speed of the output shaft), for example greater than 2.5, for example in the range of from 3 to 4.2, or 3.2 to 3.8, for example on the order of or at least 3, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1 or 4.2. The gear ratio may be, for example, between any two of the values in the previous sentence. Purely by way of example, the gearbox may be a "star" gearbox having a ratio in the range of from 3.1 or 3.2 to 3.8. . In some arrangements, the gear ratio may be outside these ranges.

In any gas turbine engine as described and/or claimed herein, a combustor may be provided axially downstream of the fan and compressor(s). For example, the combustor may be directly downstream of (for example at the exit of) the second compressor, where a second compressor is provided. By way of further example, the flow at the exit to the combustor may be provided to the inlet of the second turbine, where a second turbine is provided. The combustor may be provided upstream of the turbine(s).

The or each compressor (for example the first compressor and second compressor as described above) may comprise any number of stages, for example multiple stages. Each stage may comprise a row of rotor blades and a row of stator vanes, which may be variable stator vanes On that their angle of incidence may be variable). The row of rotor blades and the row of stator vanes may be axially offset from each other.

The or each turbine (for example the first turbine and second turbine as described above) may comprise any number of stages, for example multiple stages. Each stage may comprise a row of rotor blades and a row of stator vanes. The row of rotor blades and the row of stator vanes may be axially offset from each other.

Each fan blade may be defined as having a radial span extending from a root (or hub) at a radially inner gas-washed location, or 0% span position, to a tip at a 100% span position. The ratio of the radius of the fan blade at the hub to the radius of the fan blade at the tip may be less than (or on the order of) any of: 0.4, 0.39, 0.38 0.37, 0.36, 0.35, 0.34, 0.33, 0.32, 0.31, 0.3, 0.29, 0.28, 0.27, 0.26, or 0.25. The ratio of the radius of the fan blade at the hub to the radius of the fan blade at the tip may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of from 0.28 to 0.32. These ratios may commonly be referred to as the hub-to-tip ratio. The radius at the hub and the radius at the tip may both be measured at the leading edge (or axially forwardmost) part of the blade. The hub-to-tip ratio refers, of course, to the gas-washed portion of the fan blade, i.e. the portion radially outside any platform.

The radius of the fan may be measured between the engine centreline and the tip of a fan blade at its leading edge. The fan diameter (which may simply be twice the radius of the fan) may be greater than (or on the order of) any of: 220 cm, 230 cm, 240 cm, 250 cm (around 100 inches), 260 cm, 270 cm (around 105 inches), 280 cm (around 110 inches), 290 cm (around 115 inches), 300 cm (around 120 inches), 310 cm, 320 cm (around 125 inches), 330 cm (around 130 inches), 340 cm (around 135 inches), 350cm, 360cm (around 140 inches), 370 cm (around 145 inches), 380 (around 150 inches) cm, 390 cm (around 155 inches), 400 cm, 410 cm (around 160 inches) or 420 cm (around 165 inches). The fan diameter may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of from 240 cm to 280 cm or 330 cm to 380 cm.

The rotational speed of the fan may vary in use. Generally, the rotational speed is lower for fans with a higher diameter. Purely by way of non-limitative example, the rotational speed of the fan at cruise conditions may be less than 2500 rpm, for example less than 2300 rpm. Purely by way of further non-limitafive example, the rotational speed of the fan at cruise conditions for an engine having a fan diameter in the range of from 220 cm to 300 cm (for example 240 cm to 280 cm or 250 cm to 270cm) may be in the range of from 1700 rpm to 2500 rpm, for example in the range of from 1800 rpm to 2300 rpm, for example in the range of from 1900 rpm to 2100 rpm. Purely by way of further non-limitative example, the rotational speed of the fan at cruise conditions for an engine having a fan diameter in the range of from 330 cm to 380 cm may be in the range of from 1200 rpm to 2000 rpm, for example in the range of from 1300 rpm to 1800 rpm, for example in the range of from 1400 rpm to 1800 rpm.

In use of the gas turbine engine, the fan (with associated fan blades) rotates about a rotational axis. This rotation results in the tip of the fan blade moving with a velocity U. The work done by the fan blades 13 on the flow results in an enthalpy rise dH of the flow. A fan tip loading may be defined as dH/U2, where dH is the enthalpy rise (for example the 1D average enthalpy rise) across the fan and Utip is the (translational) velocity of the fan tip, for example at the leading edge of the tip (which may be defined as fan tip radius at leading edge multiplied by angular speed). The fan tip loading at cruise conditions may be greater than (or on the order of) any of: 0.28, 0.29, 0.30, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39 or 0.4 (all values being dimensionless). The fan tip loading may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of from 0.28 to 0.31, or 0.29 to 0.3.

Gas turbine engines in accordance with the present disclosure may have any desired bypass ratio, where the bypass ratio is defined as the ratio of the mass flow rate of the flow through the bypass duct to the mass flow rate of the flow through the core at cruise conditions. In some arrangements the bypass ratio may be greater than (or on the order of) any of the following: 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5 or 20. The bypass ratio may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of form 12 to 16, 13 to 15, or 13 to 14. The bypass duct may be substantially annular. The bypass duct may be radially outside the engine core. The radially outer surface of the bypass duct may be defined by a nacelle and/or a fan case.

The overall pressure ratio of a gas turbine engine as described and/or claimed herein may be defined as the ratio of the stagnation pressure upstream of the fan to the stagnation pressure at the exit of the highest pressure compressor (before entry into the combustor). By way of non-limitafive example, the overall pressure ratio of a gas turbine engine as described and/or claimed herein at cruise may be greater than (or on the order of) any of the following: 35, 40, 45, 50, 55, 60, 65, 70, 75. The overall pressure ratio may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of from 50 to 70.

Specific thrust of an engine may be defined as the net thrust of the engine divided by the total mass flow through the engine. At cruise conditions, the specific thrust of an engine described and/or claimed herein may be less than (or on the order of) any of the following: 110 Nkg-ls, 105 Nkg-ls, 100 Nkals, 95 Nkg-ls, 90 Nkg-ls, 85 Nkg-ls or 80 Nkals. The specific thrust may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of from 80 Nkg-ls to 100 Nkg-ls, or 85 Nkg-ls to 95 Nkg-ls. Such engines may be particularly efficient in comparison with conventional gas turbine engines.

A gas turbine engine as described and/or claimed herein may have any desired maximum thrust. Purely by way of non-limitafive example, a gas turbine as described and/or claimed herein may be capable of producing a maximum thrust of at least (or on the order of) any of the following: 160kN, 170kN, 180kN, 190kN, 200kN, 250kN, 300kN, 350kN, 400kN, 450kN, 500kN, or 550kN. The maximum thrust may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds). Purely by way of example, a gas turbine as described and/or claimed herein may be capable of producing a maximum thrust in the range of from 330kN to 420 kN, for example 350kN to 400kN. The thrust referred to above may be the maximum net thrust at standard atmospheric conditions at sea level plus 15 degrees C (ambient pressure 101.3kPa, temperature 30 degrees C), with the engine static.

In use, the temperature of the flow at the entry to the high pressure turbine may be particularly high. This temperature, which may be referred to as TET, may be measured at the exit to the combustor, for example immediately upstream of the first turbine vane, which itself may be referred to as a nozzle guide vane. At cruise, the TET may be at least (or on the order of) any of the following: 1400K, 1450K, 1500K, 1550K, 1600K or 1650K. The TET at cruise may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds). The maximum TET in use of the engine may be, for example, at least (or on the order of) any of the following: 1700K, 1750K, 1800K, 1850K, 1900K, 1950K or 2000K. The maximum TET may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of from 1800K to 1950K. The maximum TET may occur, for example, at a high thrust condition, for example at a maximum take-off (MTO) condition.

A fan blade and/or aerofoil portion of a fan blade described and/or claimed herein may be manufactured from any suitable material or combination of materials. For example at least a part of the fan blade and/or aerofoil may be manufactured at least in part from a composite, for example a metal matrix composite and/or an organic matrix composite, such as carbon fibre. By way of further example at least a part of the fan blade and/or aerofoil may be manufactured at least in part from a metal, such as a titanium based metal or an aluminium based material (such as an aluminium-lithium alloy) or a steel based material. The fan blade may comprise at least two regions manufactured using different materials. For example, the fan blade may have a protective leading edge, which may be manufactured using a material that is better able to resist impact (for example from birds, ice or other material) than the rest of the blade. Such a leading edge may, for example, be manufactured using titanium or a titanium-based alloy. Thus, purely by way of example, the fan blade may have a carbon-fibre or aluminium based body (such as an aluminium lithium alloy) with a titanium leading edge.

A fan as described and/or claimed herein may comprise a central portion, from which the fan blades may extend, for example in a radial direction. The fan blades may be attached to the central portion in any desired manner. For example, each fan blade may comprise a fixture which may engage a corresponding slot in the hub (or disc). Purely by way of example, such a fixture may be in the form of a dovetail that may slot into and/or engage a corresponding slot in the hub/disc in order to fix the fan blade to the hub/disc. By way of further example, the fan blades maybe formed integrally with a central portion. Such an arrangement may be referred to as a bladed disc or a bladed ring. Any suitable method may be used to manufacture such a bladed disc or bladed ring. For example, at least a part of the fan blades may be machined from a block and/or at least part of the fan blades may be attached to the hub/disc by welding, such as linear friction welding.

The gas turbine engines described and/or claimed herein may or may not be provided with a variable area nozzle (VAN). Such a variable area nozzle may allow the exit area of the bypass duct to be varied in use. The general principles of the present disclosure may apply to engines with or without a VAN.

The fan of a gas turbine as described and/or claimed herein may have any desired number of fan blades, for example 14, 16, 18, 20, 22, 24 or 26 fan blades.

As used herein, cruise conditions have the conventional meaning and would be readily understood by the skilled person. Thus, for a given gas turbine engine for an aircraft, the skilled person would immediately recognise cruise conditions to mean the operating point of the engine at mid-cruise of a given mission (which may be referred to in the industry as the "economic mission") of an aircraft to which the gas turbine engine is designed to be attached. In this regard, mid-cruise is the point in an aircraft flight cycle at which 50% of the total fuel that is burned between top of climb and start of descent has been burned (which may be approximated by the midpoint -in terms of time and/or distance-between top of climb and start of descent. Cruise conditions thus define an operating point of, the gas turbine engine that provides a thrust that would ensure steady state operation (i.e. maintaining a constant altitude and constant Mach Number) at mid-cruise of an aircraft to which it is designed to be attached, taking into account the number of engines provided to that aircraft. For example where an engine is designed to be attached to an aircraft that has two engines of the same type, at cruise conditions the engine provides half of the total thrust that would be required for steady state operation of that aircraft at mid-cruise.

In other words, for a given gas turbine engine for an aircraft, cruise conditions are defined as the operating point of the engine that provides a specified thrust (required to provide -in combination with any other engines on the aircraft -steady state operation of the aircraft to which it is designed to be attached at a given mid-cruise Mach Number) at the mid-cruise atmospheric conditions (defined by the International Standard Atmosphere according to ISO 2533 at the mid-cruise altitude). For any given gas turbine engine for an aircraft, the mid-cruise thrust, atmospheric conditions and Mach Number are known, and thus the operating point of the engine at cruise conditions is clearly defined.

Purely by way of example, the forward speed at the cruise condition may be any point in the range of from Mach 0.7 to 0.9, for example 0.75 to 0.85, for example 0.76 to 0.84, for example 0.77 to 0.83, for example 0.78 to 0.82, for example 0.79 to 0.81, for example on the order of Mach 0.8, on the order of Mach 0.85 or in the range of from 0.8 to 0.85. Any single speed within these ranges may be part of the cruise condition. For some aircraft, the cruise conditions may be outside these ranges, for example below Mach 0.7 or above Mach 0.9.

Purely by way of example, the cruise conditions may correspond to standard atmospheric conditions (according to the International Standard Atmosphere, ISA) at an altitude that is in the range of from 10000 m to 15000 m, for example in the range of from 10000 m to 12000 m, for example in the range of from 10400 m to 11600 m (around 38000 ft), for example in the range of from 10500 m to 11500 m, for example in the range of from 10600 m to 11400 m, for example in the range of from 10700 m (around 35000 ft) to 11300 m, for example in the range of from 10800 m to 11200 m, for example in the range of from 10900 m to 11100 m, for example on the order of 11000 m. The cruise conditions may correspond to standard atmospheric conditions at any given altitude in these ranges.

Purely by way of example, the cruise conditions may correspond to an operating point of the engine that provides a known required thrust level (for example a value in the range of from 30kN to 35kN) at a forward Mach number of 0.8 and standard atmospheric conditions (according to the International Standard Atmosphere) at an altitude of 38000ft (11582m). Purely by way of further example, the cruise conditions may correspond to an operating point of the engine that provides a known required thrust level (for example a value in the range of from 50kN to 65kN) at a forward Mach number of 0.85 and standard atmospheric conditions (according to the International Standard Atmosphere) at an altitude of 35000 ft (10668 m).

In use, a gas turbine engine described and/or claimed herein may operate at the cruise conditions defined elsewhere herein. Such cruise conditions may be determined by the cruise conditions (for example the mid-cruise conditions) of an aircraft to which at least one (for example 2 or 4) gas turbine engine may be mounted in order to provide propulsive thrust.

According to an aspect, there is provided an aircraft comprising a gas turbine engine as described and/or claimed herein. The aircraft according to this aspect is the aircraft for which the gas turbine engine has been designed to be attached. Accordingly, the cruise conditions according to this aspect correspond to the mid-cruise of the aircraft, as defined elsewhere herein.

According to an aspect, there is provided a method of operating a gas turbine engine as described and/or claimed herein. The operation may be at the cruise conditions as defined elsewhere herein (for example in terms of the thrust, atmospheric conditions and Mach Number).

According to an aspect, there is provided a method of operating an aircraft comprising a gas turbine engine as described and/or claimed herein. The operation according to this aspect may include (or may be) operation at the mid-cruise of the aircraft, as defined elsewhere herein.

The skilled person will appreciate that except where mutually exclusive, a feature or parameter described in relation to any one of the above aspects may be applied to any other aspect. Furthermore, except where mutually exclusive, any feature or parameter described herein may be applied to any aspect and/or combined with any other feature or parameter described herein.

Brief description of the drawings

Embodiments will now be described by way of example only, with reference to the Figures, in which: Figure 1 is a sectional side view of a gas turbine engine; Figure 2 is a close up sectional side view of an upstream portion of a gas turbine engine; Figure 3 is a partially cut-away view of a gearbox for a gas turbine engine; Figure 4 schematically shows a side cross-sectional view of an ice detection system in accordance with an embodiment of the invention; Figure 5 schematically shows a view from a camera of an ice detection system when a grid is projected onto a surface from an angle; Figure 6 schematically illustrates baseline and captured image positions for light features projected onto a surface from an angle; and, Figure 7 schematically shows a testing rig for an ice detection system in accordance with an embodiment of the invention.

Detailed description

Aspects and embodiments of the present disclosure will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art Figure 1 illustrates a gas turbine engine 10 having a principal rotational axis 9. The engine comprises an air intake 12 and a propulsive fan 23 that generates two airflows: a core airflow A and a bypass airflow B. The gas turbine engine 10 comprises a core 11 that receives the core airflow A. The engine core 11 comprises, in axial flow series, a low pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, a low pressure turbine 19 and a core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine 10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. The bypass airflow B flows through the bypass duct 22. The fan 23 is attached to and driven by the low pressure turbine 19 via a shaft 26 and an epicyclic gearbox 30.

In use, the core airflow A is accelerated and compressed by the low pressure compressor 14 and directed into the high pressure compressor 15 where further compression takes place. The compressed air exhausted from the high pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture is combusted. The resultant hot combustion products then expand through, and thereby drive, the high pressure and low pressure turbines 17, 19 before being exhausted through the core exhaust nozzle to provide some propulsive thrust. The high pressure turbine 17 drives the high pressure compressor 15 by a suitable interconnecting shaft 27. The fan 23 generally provides the majority of the propulsive thrust. The epicyclic gearbox 30 is a reduction gearbox.

An exemplary arrangement for a geared fan gas turbine engine 10 is shown in Figure 2. The low pressure turbine 19 (see Figure 1) drives the shaft 26, which is coupled to a sun wheel, or sun gear, 28 of the epicyclic gear arrangement 30. Radially outwardly of the sun gear 28 and intermeshing therewith is a plurality of planet gears 32 that are coupled together by a planet carrier 34. The planet carrier 34 constrains the planet gears 32 to precess around the sun gear 28 in synchronicity whilst enabling each planet gear 32 to rotate about its own axis.

The planet carrier 34 is coupled via linkages 36 to the fan 23 in order to drive its rotation about the engine axis 9. Radially outwardly of the planet gears 32 and intermeshing therewith is an annulus or ring gear 38 that is coupled, via linkages 40, to a stationary supporting structure 24.

Note that the terms "low pressure turbine" and "low pressure compressor" as used herein may be taken to mean the lowest pressure turbine stages and lowest pressure compressor stages (i.e. not including the fan 23) respectively and/or the turbine and compressor stages that are connected together by the interconnecting shaft 26 with the lowest rotational speed in the engine (i.e. not including the gearbox output shaft that drives the fan 23). In some literature, the "low pressure turbine" and "low pressure compressor" referred to herein may alternatively be known as the "intermediate pressure turbine" and "intermediate pressure compressor". Where such alternative nomenclature is used, the fan 23 may be referred to as a first, or lowest pressure, compression stage.

The epicyclic gearbox 30 is shown by way of example in greater detail in Figure 3. Each of the sun gear 28, planet gears 32 and ring gear 38 comprise teeth about their periphery to intermesh with the other gears. However, for clarity only exemplary portions of the teeth are illustrated in Figure 3. There are four planet gears 32 illustrated, although it will be apparent to the skilled reader that more or fewer planet gears 32 may be provided within the scope of the claimed invention. Practical applications of a planetary epicyclic gearbox 30 generally comprise at least three planet gears 32.

The epicyclic gearbox 30 illustrated by way of example in Figures 2 and 3 is of the planetary type, in that the planet carrier 34 is coupled to an output shaft via linkages 36, with the ring gear 38 fixed. However, any other suitable type of epicyclic gearbox 30 may be used. By way of further example, the epicyclic gearbox 30 may be a star arrangement, in which the planet carrier 34 is held fixed, with the ring (or annulus) gear 38 allowed to rotate. In such an arrangement the fan 23 is driven by the ring gear 38. By way of further alternative example, the gearbox 30 may be a differential gearbox in which the ring gear 38 and the planet carrier 34 are both allowed to rotate.

It will be appreciated that the arrangement shown in Figures 2 and 3 is by way of example only, and various alternatives are within the scope of the present disclosure. Purely by way of example, any suitable arrangement may be used for locating the gearbox 30 in the engine 10 and/or for connecting the gearbox 30 to the engine 10. By way of further example, the connections (such as the linkages 36, 40 in the Figure 2 example) between the gearbox 30 and other parts of the engine 10 (such as the input shaft 26, the output shaft and the fixed structure 24) may have any desired degree of stiffness or flexibility. By way of further example, any suitable arrangement of the bearings between rotating and stationary parts of the engine (for example between the input and output shafts from the gearbox and the fixed structures, such as the gearbox casing) may be used, and the disclosure is not limited to the exemplary arrangement of Figure 2. For example, where the gearbox 30 has a star arrangement (described above), the skilled person would readily understand that the arrangement of output and support linkages and bearing locations would typically be different to that shown by way of example in Figure 2.

Accordingly, the present disclosure extends to a gas turbine engine having any arrangement of gearbox styles (for example star or planetary), support structures, input and output shaft arrangement, and bearing locations.

Optionally, the gearbox may drive additional and/or alternative components (e.g. the intermediate pressure compressor and/or a booster compressor).

Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of compressors and/or turbines and/or an alternative number of interconnecting shafts. By way of further example, the gas turbine engine shown in Figure 1 has a split flow nozzle 18, 20 meaning that the flow through the bypass duct 22 has its own nozzle 18 that is separate to and radially outside the core exhaust nozzle 20. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct 22 and the flow through the core 11 are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed flow nozzle. One or both nozzles (whether mixed or split flow) may have a fixed or variable area. Whilst the described example relates to a turbofan engine, the disclosure may apply, for example, to any type of gas turbine engine, such as an open rotor (in which the fan stage is not surrounded by a nacelle) or turboprop engine, for example. In some arrangements, the gas turbine engine 10 may not comprise a gearbox 30.

The geometry of the gas turbine engine 10, and components thereof, is defined by a conventional axis system, comprising an axial direction (which is aligned with the rotational axis 9), a radial direction (in the bottom-to-top direction in Figure 1), and a circumferential direction (perpendicular to the page in the Figure 1 view). The axial, radial and circumferential directions are mutually perpendicular.

A side cross-sectional view of an ice detection system 50 in accordance with an embodiment of the invention is schematically shown in Figure 4.

The system 50 comprises a camera 56. The camera 56 is positioned opposing a surface 54 along an imaging axis I such that it is able to capture an image of a region of the surface 54.

In this example, the imaging axis I is parallel to a normal of the surface 54, but in other examples the imaging axis may be inclined with respect to such a normal. For simplicity, the invention will be described with respect to a planar surface, but it will be understood that the invention can be applied to a surface of arbitrary geometry.

The system 50 further comprises a light source 52. The light source 52 is disposed opposing the surface 54 along a projection axis P such that it projects a pattern of light features about the projection axis P over a region of the surface 54. The light source is oriented relative to the camera 56 so that the projection axis P is inclined with respect to the imaging axis I. The light source may project light along a plurality of projection axes Pl, P2, each of which may be inclined with respect to the imaging axis I. The camera 56 is communicatively coupled to an ice detection unit 58. The ice detection unit 58 receives images captured by the camera 56.

For simplicity of disclosure, the invention will be described with respect to comparative analysis of a captured image relative to a baseline image in order to determine ice accumulation. However, it will be appreciated that ice accumulation may be determined based on any suitable correlation of data extracted from a captured image and pre-stored information, such as information relating to an expected position of light features within the image, or information correlating determined locations of light features within a captured image with ice data such as data relating to ice height.

The ice detection unit 58 comprises a storage section 60 capable of storing a baseline image. The baseline image is an expected view of the light features on the region of the surface 54 from the camera 56 in the absence of ice. The baseline image can be a reference image taken by the camera 56 upon initial installation of the system 50, or it can be a computer-generated image that is generated using input information about the relative distances and angles between the light source 52, surface 54 and camera 56 (e.g. from CAD / CAM schematics of the installation environment) or measurements taken of the relative distances and angles between the light source 52, surface 54 and camera 56 once the system 50 has been installed in an environment.

Light features from the light source 52 have respective baseline positions corresponding to their position in the baseline image. As shown in Figure 4, a first light feature from the light source 52 has a first baseline position 62 and a second light feature from the light source 52 has a second baseline position 64.

In use, the camera 56 captures images of the region of the surface 54. This can be a continuous process (e.g. video capture), or discrete images can be captured at predetermined intervals, or on-command based on user input or other trigger (such as by a controller that has determined a change in a performance parameter of a gas turbine engine). The ice detection unit 58 receives these captured images and compares the positions of the light features in each captured image to their respective baseline positions in the baseline image stored in the storage section 60.

As shown in Figure 4, when a layer of ice 66 has formed over the region of the surface 54, the positions of the light features in a plane normal to the imaging axis I, as captured by the camera 56, differ from their respective baseline positions in such a plane. In the example shown in Figure 4, the first light feature has a first captured image position 68 within a respective plane and a second light feature has a second captured image position 70 within the respective plane.

From the perspective of the camera 56 the first light feature has shifted along the surface 54 by a first distance Si between the baseline image and the captured image, and the second light feature has shifted along the surface 54 by a second distance 32 between the baseline image and the captured image. That is, both light features have moved towards the light source 52 from the perspective of the camera 56. In reality, the respective light features are being reflected from the surface of the layer of ice 66.

The distance Si is in a direction along the surface 54 between the first baseline position 62 and a projected origin point 0 of the light source 52, which is an orthogonal projection of the position of the light source 52 along the imaging axis onto the surface 54. The distance 32 is in a direction along the surface 54 between the second baseline position 64 and the projected origin point 0.

Using the distances Si and 32, the height h of the ice can be calculated trigonometrically.

If an ice layer of constant height over the entire region of the surface is assumed, this can be calculated using a single light feature from the formula: Si tan a where a is the angle between the orthogonal line L (the orthogonal line L being parallel to the imaging axis I) and a line intersecting the first baseline position 62 and the first captured image position 68 (said line, in this case, corresponding to a first projection axis P1). This gives the height of the ice at the first captured image position 68.

However, in most cases the ice layer is not of constant height over the entire surface, but instead varies from point to point. In such cases it is desirable calculate the height at a number of different positions over the surface so that a relief map can be generated. This can be used to identify regions on the surface that are particularly susceptible to ice build-up.

In the example shown in Figure 4, the height of the ice at the first captured image position can be calculated using the formula above. The height of the ice at the second captured image position 70 can be calculated using the formula: S2 h -tan(a + fl) where S2 is the distance between the second captured image position 70 and second baseline position 64 along the surface 54, a is the angle between the orthogonal line L (the orthogonal line L being parallel to the imaging axis I) and a line intersecting the first baseline position 62 and the first captured image position 68 (said line, in this case, corresponding to the first projection axis P1), and 15' is the angle between a line intersecting the second baseline position 64 and the second captured image position 70 (said line, in this case, corresponding to a second projection axis P2), and the line intersecting the first baseline position 62 and the first captured image position 68.

More generally, the height hi of the ice at a position on the surface corresponding to a given captured image position i can be calculated using the formula: Si where Si is the distance between the ith captured image position and ith baseline position along the surface 54, a is the angle between the orthogonal line L and a line intersecting the first baseline position 62 and the first captured image position 68 (said line, in this case, corresponding to the first projection axis PO, and fl, is the angle between a line intersecting h -hi = tan(a + Pi) the ith baseline position and the ith captured image position, and the line intersecting the first baseline position 62 and the first captured image position 68.

Once heights are calculated at a plurality of positions over the surface to form a relief map, a volume of ice may be calculated by taking the calculated heights h at various x and y positions (i.e. captured image positions over the surface, see the coordinate system of Fig. 5) to produce 3D vectors of each position having respective x, y and h values.

The x and y values, like the height values, are converted from pixel lengths in the captured image to real lengths by calibration of the system, e.g. in some examples, 100 pixels in the captured image may correspond to 1 mm on the surface. These vectors form the vertices of a reconstructed ice model polyhedron, but volume estimation from these vectors requires further calculation.

These vectors may be used in the standard 'Convex Hull' algorithm, implemented with the standard python command ConvexHull from the SciPy library. The algorithm produces a 3D shape that bounds all these points in a polyhedron and calculates the total volume of that bound shape, as well as the vertices of this new polyhedron.

Once the (x,y,h) vectors are calculated, in theory any volume estimation technique can be used in practice. However, convex hull has been found to provide a reasonably accurate volume estimation in a short amount of time.

In some examples, a plurality of light features are projected by the light source (e.g. by passing the light through a diffractive optical element) such that the plurality of light features form a grid. Figure 5 shows a view from a camera of the pattern formed when the light source above a planar surface projects a grid of orthogonally-oriented lines onto the planar surface from an angle.

In Figure 5, a light source 152 suspended above a planar surface projects a square grid 154 onto the planar surface. The planar surface coincides with the surface of the page in Figure 5, with the light source 152 being disposed a distance above the page. A coordinate system is indicated on the page, with the x-direction corresponding to a horizontal direction across the surface, and the y-direction corresponding to a vertical direction along the surface, as viewed in Figure 5.

The grid 154 is formed of a leading vertical line 156 and a trailing vertical line 158. The leading vertical line 156 and the trailing vertical line 158 are parallel to one other, and to a notional reference vertical line R which is aligned with the light source 152. A first vertical line 160 and a second vertical line 162 are projected between the leading vertical line 156 and the trailing vertical line 158, the first vertical line 160 and second vertical line 162 being parallel with the leading vertical line 156 and the trailing vertical line 158 and the reference vertical line R. The grid is further formed of outer horizontal lines 164 and 166 that connect the ends of the leading and trailing vertical lines 156 and 158. First and second horizontal lines 168, 170 are projected between the outer horizontal lines 164, 166.

While the lines 164, 166, 168, 170 are described as "horizontal" lines of the grid 154, these lines are distorted when the grid 154 is projected onto a surface at an angle as shown.

These lines would be substantially horizontal if the grid 154 were projected directly beneath the light source 152 (i.e. along an axis normal to the surface), but as the grid is projected at an angle the lines splay out at respective angles. In the example shown in Figure 5, the outer horizontal lines 164 and 166 are each deflected a splay angle cl) from a notional horizontal centreline C which is perpendicular to the reference line R and intersects the light source 152. The first and second horizontal lines 168, 170 are closer to the notional centreline C, and so have smaller splay angles than the outer horizontal lines 164, 166. The lines 164, 166, 168, 170 are therefore not parallel on the surface.

Using a pattern, such as that formed by the grid 154, the height of ice build-up on the surface can be measured at a number of positions over the surface. For example, taking the first vertical line 160, a baseline first vertical line position can be established by capturing a baseline image of the grid 154 on the surface in the absence of ice. Following ice build-up, an image can be captured showing a captured image first vertical line position. The captured image first vertical line position can be compared to the baseline first vertical line and a 4x-shift of the first vertical line between the image can be established. The 4x-shift of the first vertical line is the distance that the line has moved in the x-direction, i.e. towards the reference line R. The height of the ice at the captured image first vertical line position can be calculated using the techniques described above with reference to Figure 4. This process can be repeated as necessary, e.g. for the second vertical line, to establish the height of ice at multiple positions over the surface.

The grid 154 comprises a plurality of intersection points 172, 174, 176 and 178, respectively the intersection of the first horizontal line 168 and the first vertical line 160, the intersection of the first horizontal line 168 and the second vertical line 162, the intersection of the second horizontal line 170 and the first vertical line 160, and the intersection of the second horizontal line 170 and the second vertical line 162. These intersection points can be used as light features for the purposes of ice detection and ice height calculation.

For these intersection points, it is possible to measure a Ay-shift between their respective baseline positions and their respective positions in the captured image. Two intersection points are illustrated in Figure 6, with the hollow circles 180 and 182 representing baseline intersection point positions and dark circles 184 and 186 representing captured image intersection point positions. An angle Li( can be measured between the notional centreline C and a line intersecting a captured image intersection point position and its respective baseline intersection point position, as shown. Using the grid pattern, this angle can be measured easily for intersection points, as it corresponds to the angle between the horizontal line on which the intersection point lies, and the centreline C. Naturally, intersection points falling on the centreline C do not exhibit a Ay-shift.

The height of the ice at the captured image intersection point can be calculated from the Ay-shift and the angle tp using the formula: where the angle (a+13) defines the angle at which the captured image intersection point is projected from a line orthogonal to the surface (i.e. a line parallel to the imaging axis I) that intersects the light source, e.g. orthogonal line L in Figure 4.

However, height calculation using this method requires more information about the light feature position on the ice surface. It is generally difficult to measure the angle lp accurately for light features not located at an intersection. Additionally, this method is much more sensitive to calibration and measurement errors than from using vertical line-shifts because the shift in the y-direction is typically less than the shift in the x-direction, unless the splay angle (I) is very large. Further, the denominator in the formula is smaller than that used for Ax-shifts because of the extra tangent term, which amplifies measurement and calibration errors more so than with vertical line measurement.

Nevertheless, the use of a grid, and hence the ability to calculate height from both Ax-shifts and Ay-shifts, is still beneficial in that it can provide a greater number of light features for use in ice detection than a series of non-intersecting lines, without increasing the projected line density.

Example locations

A number of surfaces within a gas turbine engine have been identified through testing as being particularly susceptible to ice build-up. In particular, ice generally accretes on an outer annulus of the compressor (e.g. compressor 15, shown in Fig. 1) and the inlet and outlet swan-neck ducts leading to / from the compressor, where the temperature is between 0-43°C.

However, these regions of the engine are highly space-constrained. As a result, there are no immediately apparent locations that an ice detection system could be located without unacceptably impacting essential engine components.

In one aspect, the present invention provides an ice detection system that is integrated into a gas turbine engine.

Figure 7 schematically shows a perspective view of testing rig 200 designed to replicate part of a variable stator vane mechanism for use in a compressor section of a gas turbine engine. The testing rig 200 comprises a pair of variable stator vanes 202, 204. The variable stator vanes 202, 204 are supported for rotation about respective axes between an inner annulus 206 and an outer annulus 208.

The right hand side of Figure 7 shows a more detailed perspective view of the variable stator vane 204. The variable stator vane 204 comprises a blade 210. The variable stator vane 204 further comprises an inner penny platform 212 and an outer penny platform 214 either side of the blade 210. The term "penny platform" represents a generally circular rotating surface that is flush with a surrounding structure On this case the respective annulus of the stator vane ring) and supports a rotating structure (in this case the vane). A recess 216 is formed in the inner penny platform 212 and a camera 218 of an ice detection system is disposed in the recess 216. A trench 220 is formed in the blade 210, and a through-hole is formed in the outer penny platform 214 so that power and / or data cables can run from outside the outer annulus 208, through the through-hole, along the trench 220 to the camera 218. Locating the camera 218 in the inner penny platform 212 has been found to provide a wider field-of-view of the outer annulus 208 compared to other potential camera positions.

A light source of the ice detection system is provided in a recess in the inner annulus 206 adjacent the inner penny platform 212. In alternative arrangements, the structured light source may be provided in an inner penny platform of an adjacent variable stator vane. In such arrangements, a similar trench and through-hole arrangement to that described above can be provided to run power and / or data cables from outside the outer annulus to the structured light source. Providing the light source in the inner annulus, or an adjacent inner penny platform, increases the amount of separation between the camera and light source (e.g. compared to co-locating the camera and light source within the same inner penny platform), which in turn increases the deviation of light feature positions from their baseline positions upon ice formation. The trenches may be omitted entirely in examples where the camera and / or light source are powered by batteries. In such examples, communication between the ice detection unit and the camera can be performed using wireless communication modules.

The light source may comprise a compact laser unit. A diffractive optical element may be provided over the laser unit to diffract the laser light into a plurality of beams of light forming a grid pattern over a region of the surface of the outer annulus 208 and / or one of the variable stator vanes 202, 204. Preferably, blue or green laser light is used, as this has been found to achieve a high degree of reflection from ice crystals.

The camera 218 is positioned and angled so as to capture an image of a region of the outer annulus and / or a variable stator vane. An ice detection unit communicatively coupled to the camera 218 (either through a data cable, or wirelessly) is configured to compare the positions of light features in images captured by the camera to their respective positions in a baseline image captured by the camera in the absence of ice. The ice detection unit may be configured to trigger an alarm if one or more of the light features deviates more than a predetermined distance from its respective baseline position, said alarm indicating (e.g. to a pilot of an aircraft in which the ice detection system is located) that there is a risk of ice shedding occurring.

The ice detection unit may additionally or alternatively be configured to perform ice detection, ice height calculation and ice volume calculation methods as described above with reference to Figures 4-6. The ice detection unit may comprise a processor and a machine readable medium, such as a memory (e.g. a hard disk or solid state disk) comprising instructions that, when executed by the processor, cause execution of the ice detection, ice height calculation and ice volume calculation methods as described above with reference to Figures 4-6.

It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

For example, the grid has been discussed above with respect to "horizontal" and "vertical" lines for simplicity. However, it should be noted that the orientation of the grid is arbitrary, and the grid could be projected onto the surface with its lines at any orientation in practice (e.g. at a 45° angle). Moreover, the grid need not be a regular grid, nor include parallel / perpendicular lines, it will still be possible to precisely track the angular position of intersection points provided that they lie at the intersection point of two (or more) lines.

While the grid in the examples above is formed by the simultaneous direction of light features onto the surface, the grid could alternatively formed by repeatedly scanning a single light feature over the surface at predetermined intervals, e.g. using a raster scan. Alternatively, the lines of the grid could be formed by passing the light from the light source through one or more slits.

Further, while the examples above describe multiple light features, a single light feature (either moving or stationary) could be used in the limit.

As a further example, while the specific embodiment is described with reference to a "camera" that captures an image of the region of the surface, this term is intended to be interpreted broadly to include any type of imaging device capable of capturing an image, e.g. still image cameras and video cameras.

Claims

CLAIMS1. A method of detecting ice accumulation on a region of a surface (54) comprising the steps of: directing a beam or pattern of light from a light source (52, 152) onto the region to project a light feature onto the region; capturing an image of the region with a camera (56, 218); and, comparing, via an ice detection unit (58) communicatively coupled to the camera (56, 218), a position of the light feature in the captured image with a baseline position of the light 10 feature, the ice detection unit determining, based on the comparison, whether ice has accumulated at a corresponding position on the region of the surface.
2. The method of claim 1, wherein a pattern of light is directed onto the region to project a plurality of light features onto the region, and the step of comparing a position of the light feature with a baseline position comprises comparing positions of a subset of the plurality of light features with respective baseline positions.
3. The method of claim 2, wherein the pattern defines a grid (154) of lines extending over the region, and the subset of the plurality of light features comprise intersections of the grid lines.
4. The method of any preceding claim, further comprising the step of: calculating a height of ice at the position on the surface corresponding to at least one of the light features.
5. The method of claim 4, wherein the camera is positioned generally opposing the region of the surface, such that ice accumulates along a direction generally towards the camera; wherein the camera is separated from the region along an imaging axis and the light source is offset from the camera so that the beam or pattern of light is projected along a projection axis inclined with respect to the imaging axis, whereby a position of the respective light features in the image is a function of the height of ice accumulation; and, wherein the step of calculating the height of ice comprises: determining a distance between the baseline position of the light feature and the position of the light feature in the captured image; and, determining the height of the ice based on the measured distance.
6. An ice detection system for detecting the accumulation of ice on a region of a surface, the system comprising: a light source (52, 152) arranged to direct a beam or pattern of light onto the region to project a light feature onto the region; a camera (56, 218) positioned to capture an image of the region; and, an ice detection unit (58) configured to: receive a captured image; compare a position of the light feature in the captured image to a baseline position of the light feature; and, determine, based on the comparison, whether ice has accumulated at a corresponding position on the region of the surface.
7. The system of claim 6, wherein the light source (52, 152) is arranged to direct a pattern of light onto the region to project a plurality of light features onto the region, and the ice detection unit (58) is configured to compare positions of a subset of the plurality of light features with respective baseline positions.
8. The system of claim 7, wherein the pattern defines a grid (154) of lines extending over the region, and the subset of the plurality of light features comprise intersections of the grid lines.
9. The system of any of claims 6-8, wherein the ice detection unit (58) is further configured to: calculate a height of ice at the position on the surface corresponding to at least one of the light features.
10. The system of claim 9, wherein the camera (56, 218) is positioned generally opposing the region of the surface, such that ice accumulates along a direction generally towards the camera; wherein the camera is separated from the region along an imaging axis and the light source is offset from the camera so that the beam or pattern of light is projected along a projection axis inclined with respect to the imaging axis, whereby a position of the respective light features in the image is a function of the height of ice accumulation; and, wherein the ice detection unit (58) is configured to: determine a distance between the baseline position of the light feature and the position of the light feature in the captured image; and, determine the height of the ice based on the measured distance.
11. The system of any of claims 6-10, wherein the light source (52, 152) comprises a diffractive optical element.
12. The system of any of claims 6-11, wherein the light source (52, 152) is a laser light source.
13. The system of any of claims 6-12, wherein the system is integrated into a gas turbine engine, and the surface comprises a variable stator vane (204) of the gas turbine engine and / or an outer annulus (208) of the gas turbine engine.
14. The system of claim 13, wherein the gas turbine engine includes a variable stator vane (204) comprising a penny platform (212), and wherein the light source and / or camera (56, 218) is disposed in a recess (216) in the penny platform.
15. The system of any of claims 6-12, wherein the system is integrated into a testing rig (200) designed to replicate part of a gas turbine engine, and the surface comprises a variable stator vane (204) of the testing rig and / or an outer annulus (208) of the testing rig.
16. The system of claim 15, wherein the testing rig (200) includes a variable stator vane (204) comprising a penny platform (212), and wherein the light source and / or camera (56, 218) is disposed in a recess (216) in the penny platform.
17 A gas turbine engine (10) for an aircraft comprising: an engine core (11) comprising a turbine (19), a compressor (14), and a core shaft (26) connecting the turbine to the compressor; a fan (23) located upstream of the engine core, the fan comprising a plurality of fan blades; and, optionally a gearbox (30) that receives an input from the core shaft (26) and outputs drive to the fan so as to drive the fan at a lower rotational speed than the core shaft, wherein the gas turbine engine comprises the ice detection system of any of claim 6-14.
18. The gas turbine engine according to claim 17, wherein: the turbine is a first turbine (19), the compressor is a first compressor (14), and the core shaft is a first core shaft (26); the engine core further comprises a second turbine (17), a second compressor (15), and a second core shaft (27) connecting the second turbine to the second compressor; and, the second turbine, second compressor, and second core shaft are arranged to rotate at a higher rotational speed than the first core shaft, wherein the ice detection system is positioned in the first compressor, in the second compressor, or between the first compressor and the second compressor.