GB2318961A

GB2318961A - De-icing unit for air pressure sensor

Info

Publication number: GB2318961A
Application number: GB9722812A
Authority: GB
Inventors: Josef Wenzl; Robert Schneider
Original assignee: MTU Motoren und Turbinen Union Muenchen GmbH
Current assignee: MTU Aero Engines GmbH
Priority date: 1996-11-02
Filing date: 1997-10-30
Publication date: 1998-05-06
Anticipated expiration: 2017-10-30
Also published as: DE19645164C1; GB9722812D0; GB2318961B; ES2148049B1; ES2148049A1

Abstract

A de-icing unit 7 for an air pressure sensor intended for use on aircraft comprises a carrier member 8 with a coaxial air pressure line 5 running through it, the carrier member supporting at least one annular PTC heating element 9. The heating elements are constrained by a flange 11 formed on the carrier so that heat is conducted into the carrier warming the air in the line. The mouth 4 of the line opens into the mass of air to be measured. The other end 20 leads to a pressure sensor (not shown). Current is supplied to the elements via connections (15a, b, figure 1b). The elements and connections are secured to the carrier by a nut 14. The elements are surrounded by a casting compound 19, within housing 2.

Description

2318961 Air pressure sensor having a de-icing heating unit The invention

relates to an air pressure sensor for aircraft and air- breathing flight propulsion units, having an electrical de-icing heating unit and a pressure line which opens out facing an air current or air mass to be measured.

Such sensors are used, for example, to determine the static air pressure in the air inlet of jet engines. The air pressure measured with the sensor and pressure probe is supplied in the form of an electrical signal to a power plant regulating unit so that, together with other operating parameters, the power plant can be operated within the permissible limits. If the power plant is used to propel aeroplanes, extreme environmental conditions have to be taken into account when measuring the air pressure. When an aeroplane flies through clouds, the air humidity and low temperature prevailing there may cause ice to be formed in the air inlet, so that the sensor ices up and supplies incorrect pressure values to the probe.

In order to prevent the resulting disturbances, such sensors are provided with de-icing heating units. Electrical de-icing heating units are known 2 for the purpose which are either switched on and off manually or are switched on automatically under the control of a temperature sensor if the air temperature falls below a specific. level. The disadvantage here is that such de-icing heating units have a high current requirement because they have to be designed for the most unfavourable icing conditions in order reliably to prevent the formation of ice when the humidity is high and the temperature is low. The drawback of manually switched heating units is the attention which has to be paid in order to adjust the heating in time. De-icing heating units controlled by temperature sensors require increased construction costs for the additional temperature sensor, and the reliability of the system as a whole is lacking owing to the additional component. The de-icing heating unit having PTC heating elements disclosed in DE 27 46 342 C3 avoids such problems because the necessary heating capacity is regulated automatically in accordance with the temperature at the heating element. Owing to the positive temperature coefficient (PTC), the electrical resistance of the heating element drops as the temperature falls, so that, as the current strength increases, an increased heating capacity is provided. The PTC heating element - also known as a PTC resistor - comprises a semi-conductive 3 and ferroelectric material. Above a temperature which depends on the substance composition, the effect of the ferroelectricity becomes active. As this happens, the individual crystallites of the material lose their orientation, which in turn, in a narrow temperature range, leads to an exponential increase in the resistance. which is expressed by the positive temperature coefficient. By a suitable choice of the parameters, such as temperature coefficient and nominal temperature, the de-icing heating unit can be designed in accordance with the conditions in which the air pressure sensor is used, so that the heating element automatically regulates the temperature of the sensor and maintains it at a predetermined level so that the sensor can be prevented from icing up.

On the basis of the above, the problem of the invention is to provide an air pressure sensor of the type indicated in the precharacterising clause of Claim 1 which, even when used in icy conditions, prevents, with the help of a de-icing heating unit, the formation of ice which would falsify the measurements. The sensor is to be heated automatically with minimum current consumption.

4 The problem is solved in accordance with the invention in that the deicing heating unit has a heating unit carrier, inside which extends a portion of the pressure line, which portion is surrounded concentrically by at least one annular PTC heating element, the heating unit carrier having a flange which is formed coaxially between the heating element and the mouth of the pressure line and against the annular end face of which the heating element lies flat in heat-conductive connection.

The invention has the advantage that the largearea contact of the heating element against the annular end face of the heating unit carrier ensures undisturbed heat transfer into the pressure line. The concentric design of the heattransferring flange also ensures uniform heat distribution, especially in the pressure line mouth region where the risk of ice formation is at its greatest. As a result of using heating elements having positive temperature coefficients (PTC), the temperature of the sensor is regulated automatically, so that no additional apparatus is required for temperature regulation and a sensor having compact dimensions can be made available. Depending on the number and the capacity of the heating elements, very rapid heat supply into the -. 5 pressure line formed inside the heating unit carrier is possible so that rapid ice formation can be prevented if icy conditions suddenly occur. Owing to the position of the flange at a slight distance from the mouth region, the rear portion of the pressure line is also protected from internal ice formation.

The rotationally symmetrical design of the heating unit carrier together with the flange permits not only uniform temperature distribution but also economical manufacture. In order to ensure that heat is transported rapidly into the region of the line on the side where the mouth is arranged, the heating unit carrier is produced from a heatconductive material, such as, for example, copper. In order to prevent oxidation inside the pressure line, for example in the case of transoceanic use, the inner wall of the pressure line is preferably provided with a seawater-resistant coating, for example a layer of nickel. The design of the flange and the heating element with substantially the same outside diameter results in a compact structure.

Claims

The design of the heating unit carrier in accordance with the features of

Claims 6 and 7 gives a spindle-like configuration of the heating

6 unit carrier and the position of the flange relative to the mouth of the pressure line is to be selected to be in the first or middle third for optimum heat distribution. Owing to the design of the pressure line as a through-bore inside the heating unit carrier, one end forms the mouth of the pressure line while the other end can be provided with a connection for the portion of the pressure line leading to the pressure probe.

An important factor for the smooth functioning of the sensor is the current connection of the deicing heating unit, which connection always ensures a reliable and trouble-free current supply to the de-icing heating unit even under extreme mechanical conditions, such as vibration, acceleration and jolting. According to the features of Claims 9 and 10, the heating unit carrier is provided with a current connection, for example the earth connection, the heating element annular face remote from the mouth being provided with the opposite-pole current connection. The contact face between the flange and the heating element end face nearest the mouth thus acts not only as a heat transfer face but also as an electrical contact face, which further simplifies the structure of the sensor.

7 In order to be able to minimise the heating capacity to be installed while maintaining the necessary de-icing capacity, the de-icing heating unit is surrounded by heat insulation, casting in plastics material, for example polyimide, on the one hand preventing unnecessary heat loss and on the other hand protecting the inside of the sensor and, at the same time, especially, the current connections from mechanical damage and corrosion. In a further development of the sensor, the latter is inserted in a housing, for example of aluminium, for protection against mechanical damage.

A preferred embodiment of the invention is described hereinafter with reference to the appended drawings.

Figure la is a longitudinal section through an air pressure sensor having a line connection, Figure 1b is a longitudinal section through the sensor according to Figure la showing the current connection and 8 Figure 2 shows the characteristic line of a heating element of the de-icing heating unit.

The air pressure sensor 1 shown in Figure la is a component of an air pressure measuring device (not shown in further detail) of a jet engine for aeroplanes. The sensor 1 is used to determine the static air pressure of the air current in the air inlet and is connected via a pressure line to a pressure pick-up. In the embodiment according to Figure la, the sensor 1 accommodated in a cylindrical housing 2 sits in the wall 3 of an air inlet channel, the mouth 4 of a pressure line 5 terminating flush with the surface of the inner wall 3 so that the static pressure inside the air inlet channel can be measured exactly. The air pressure occurring inside the pressure line 5 and corresponding to the static air pressure of the air current S to be measured is further conveyed for evaluation via a pressure connection 6 arranged on the sensor and via a further pressure line to a pressure pick-up (not shown).

In order to be able to measure exact pressure values even under icy conditions, that is to say, at high air humidity and at temperatures below 100C, the mouth 4 and the pressure line 5 9 extending inside the sensor 1 have to be prevented from icing up. For that purpose, the sensor 1 is provided with a de-icing heating unit 7. The deicing heating unit 7 is composed of a pin-shaped, rotationally symmetrical heating unit carrier 8 and of eight disc-shaped PTC heating elements 9. The pressure line 5 is formed inside the heating unit carrier 8 as a coaxial through-bore which exits at the mouth end in a pin-like end 10a of the heating unit carrier 8. In a front third of the heating unit carrier 8, but at an axial distance from the mouth 4, the heating unit carrier 8 has a radially and axially extending flange at the annular end face 12 of which remote from the mouth 4 heating elements 9 are arranged coaxially with one another. At the second, likewise pin-shaped end 10b of the heating unit carrier 8, the heating elements 9 surround the pressure line 5 concentrically, the heating element 91 nearest the mouth lying flat against the end face 12 in heat-conductive connection. The second end 10b has an external thread 13 so that, by means of a nut 14, the heating elements 9 are pressed in the axial direction against the flange 11 and secured, so that optimum gapless heat transfer takes place between the heating elements 9, on the one end, and the flange 11, on the other hand.

As further shown in Figure lb, for its current supply the de-icing heating unit 7 has two current connections 15a and 15b, each of which is in the form of an annular plate having a soldering lug. A first of the two current connections 15a and b arranged coaxially between the nut 14 and the heating elements 9 is in contact with the last heating element 911 arranged nearest the nut, while the second current connection 15b is separated by an insulating disc 16 from the first current connection 15a, and its inside is in radial contact with the second end 10b. By means of the heating unit carrier 8 connected as earth there is thus a flow of current via the heating element 91 arranged nearest the flange as far as the heating element 911 arranged nearest the nut and to the first current connection 15a, an electrically insulating sleeve 17 being arranged radially between the heating elements 9 and the heating unit carrier 8.

11 In the region of the flange 11 and the front end 10a of the heating unit carrier 8, the heating unit carrier is surrounded by polyimide heat insulation 18, while leaving a gap for.the mouth 4, so that heat loss via the adjoining wall 3 is prevented. The front region of the housing 2, which region is stepped in accordance with the design of the heating unit carrier 8, sits with the front region of the de-icing heating unit 7 in a cylindrical recess in the wall 3. To ensure that the sensor 1 is held securely, it is screwed to the wall 3. In the rear region of the sensor 1 remote from the mouth 4, the sensor is filled inside the housing 2 with a casting compound which has a channel 20 for connecting the pressure line 5 to the pressure connection 6. Finally, the housing 2 has a lid 21 with which the de-icing heating unit 7 is secured in the housing 2 by means of the casting compound 19.

Figure 2 shows the pattern of the characteristic line of the PTC heating elements 9 also known as PTC resistors. The PTC resistor resistance R is shown as a function of the PTC resistor temperature T. In the region of the sharp increase in resistance, the ohmic resistance depends on the temperature T in accordance with the following equation:

12 R=ROJ(T-TO) In the equation, a represents a temperature coef f icient and RO is the resistance in Ohms at the temperature TO. The temperature coefficient a is not a function of the temperature and is a material constant. With the selected heating elements, which have a maximum switch-on capacity of approximately 35 W, the reference temperature TA at which the temperature coefficient becomes positive is approximately 1200C. Owing to the then rapidly increasing resistance, this is approximately the maximum occurring temperature of the de-icing heating unit.

13 CLAIMS 1. Air pressure sensor (1) for aircraft and air-breathing flight propulsion units, having an electrical de-icing heating unit (7) and a pressure line (5) which opens out facing an air current or air mass to be measured, characterised in that the de-icing heating unit (7) has a heating unit carrier (8), inside which extends a portion of the pressure line (5), which portion is surrounded concentrically by at least one annular PTC heating element (9), the heating unit carrier (8) having a flange (11) which is formed coaxially between the heating element (9) and the mouth (4) of the pressure line (5) and against the annular end face (12) of which the heating element (9) lies flat in heat-conductive connection.
2. Sensor according to Claim 1, characterised in that the heating unit carrier (8), with the flange (11), is formed to be rotationally symmetrical.
3. Sensor according to Claim 1 or 2, characterised in that the heating unit carrier (8) is produced from a heat-conductive material.

14
4. Sensor according to any one of the preceding Claims, characterised in that the heating unit carrier (8) is formed in one piece.
5. Sensor according to any one of the preceding Claims, characterised in that the flange (11) and the heating element (8) have substantially the same outside diameter.
6. Sensor according to any one of the preceding Claims, characterised in that the heating unit carrier (8) has, adjoining the flange (11) and on the side where the heating element (9) is arranged, a pin-like end (10b) which is surrounded by the heating element (9) at least over an axial partial region.
7. Sensor according to any one of the preceding Claims, characterised in that the heating carrier (8) has, adjoining the flange (11) and on the side remote from the heating element (9), a pin-like end (10a) having the mouth (4) of the pressure line (5).
8. Sensor according to any one of the preceding Claims, characterised in that the pressure line (5) is formed as a coaxial throughbore inside the heating unit carrier (p).
9. Sensor according to any one of the preceding Claims, characterised in that the deicing heating unit (7) is provided with a current supply, the heating unit carrier (8) being connected to a current connection (15b) and the annular face of the heating element (911), which face is remote from the flange (11), being connected to the opposite-pole current connection (15a).
10. Sensor according to Claim 8, characterised in that an electrically insulating sleeve (17) is arranged between the heating element (9) and the heating unit carrier (8).
11. Sensor according to any one of the preceding Claims, characterised in that the deicing heating unit (7) has several disc-shaped PTC heating elements (9) which, abutting one another coaxially, are in planar contact with one another.
12. Sensor according to any one of the preceding Claims, characterised in that the de- 16 icing heating unit (7) is surrounded by heat insulation (18).
13. Sensor according to Claim 11, characterised in that the material of the heat insulation (18) is a castable plastics material, especially a polyimide.
14. Sensor according to any one of the preceding Claims, characterised in that the pressure line (5) has a corrosion-resistant, especially a seawater-resistant, inner coating.
15. Sensor according to any one of the preceding Claims, characterised in that the sensor (1), while leaving a gap for the mouth (4) of the pressure line (5), has a housing (2) having connections (15a, b, and 6) for the current supply and the pressure line, respectively.
16. Sensor according to any one of the preceding Claims, characterised in that, in order to secure the de-icing heating unit (7) in the housing (2) of the sensor (1), the sensor is filled with a casting compound (19).
17. Sensor according to any one of the preceding Claims, characterised in that the region 17 of the sensor (1) nearest the mouth is designed to be cylindrical or rotationally symmetrical at least in its outer contour.