GB2252131A

GB2252131A - Pressure measurement in turbofan engines.

Info

Publication number: GB2252131A
Application number: GB8826784A
Authority: GB
Inventors: Graham John Dadd
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1987-11-20
Filing date: 1988-11-16
Publication date: 1992-07-29
Anticipated expiration: 2008-11-16
Also published as: IT8948326A0; IT1231807B; GB2252131B; DE3844565A1; GB8826784D0

Description

1 PRESSURE MEASUREMENT IN TURBOFAN ENGINES 2252131 This invention relates

to the control of turbofan engines for aircraft and, in particular, to apparatus for measuring fan inlet total pressure (P1) and fan total pressure ratio ie the ratio between bypass duct entry total pressure (P2) and (P1).

P1 Is commonly used in digital engine controls for estimating control compensator parameter values and for schedules such as fuel limits, for example.

The regulation of thrust is one of the prime objectives of aircraft engine control. It is known that engine thrust is closely related to overall engine pressure ratio, and on conventional mixed turbofan engines with internal mixing this ratio is very closely associated with the value of P2/P1. Hence measurement of P2/P1 may be used as a thrust indicator. A method of P1 and P2/P1 measurement may involve sensing P1 (and P2) by means of two total pressure sensing probes, one protruding into the fan inlet and the other into the bypass duct. This method can give an accurate indication of P1 or P2/P1 (and hence thrust) but only provided that the airflow through the engine inlet Is undistorted. If the engine is presented with a swirling and distorted airflow such as may be the case when the aircraft is flying at high incidence, for example, then such a flow will give rise to random fluctuations in the sensed pressures even though the mean pressures on which thrust depends remain essentially constant. Any closed loop engine control system based on these fluctuating pressure 2 measurements would generate thrust changes which may be disconcerting to the crew and possibly hazardous to the aircraft. One solution to this problem would be to use a multiplicity of distributed pressure probes in the inlet.

However, such an arrangement would be unattractive because of its complexity and could represent an unacceptable icing and bird--;strike hazard.

The object of the present invention to provide non-intrusive apparatus for the accurate measurement of fan inlet total pressure and fan total pressure ratio which is tolerant of bulk swirl and distortion in the engine inlet flows and this is achieved by the measurement of inlet static pressure (PSI) instead of P1 combined with calibration data relating PSI and PI.

The invention therefore consists of apparatus for determining fan inlet total pressure in a turbofan aircraft engine having an inlet and a bypass duct wherein the apparatus comprises:

total pressure PB; and means for computing PI from the calibration data and said measured quantities.

The invention may further include means for determining fan total pressure ratio P2/P1 comprising means for storing calibration data relating bypass duct entry total pressure P2 with the measured quantities NL, T, PSI and PB and means for computing P2/P1 from the calibration data and said measured quantities.

It is easily possible to measure mean inlet static pressure PSI by non-intrusive means and such means may comprise means for measuring fan rotational speed NL; means for measuring inlet temperature T; means for measuring mean inlet static pressure PSI; means for measuring mean bypass duct entry pressure PB; means for storing calibration data relating fan inlet pi with the measured quantities NL, T, PSI and 3 at least two orifices in the inlet wall preferably equally spaced around the wall and which are connected together by an annular manifold. Preferably, the orifices are positioned upstream of the fan at a sufficient distance therefrom to be substantially unaffected by any static pressure distortion induced by the fan in close proximity thereto. The orifices and manifold are connected to a pressure transducer via one or more tappings in the manifold. The transducer may, for example, be a pressure - sensitive diaphragm.

Either the computed value of Pl or of the ratio P2/Pl may be used as an input to a closed loop engine control system to create (with other engine feedback signals) a fuel demand signal and a nozzle area demand signal for the engine and thereby control engine thrust.

The means for measuring bypass duct entry pressure PB may be configured in a similar fashion to the means described above for measuring inlet static pressure, but positioned at the bypass duct entry. Alternatively, the means may comprise two or more preferably equally spaced probes which radially traverse the duct and provide a measurement of a bypass duct Mach number parameter. By way of example, two embodiments of the invention will -now be described with reference to the drawings of which; Figure 1 is a schematic block diagram of a first embodiment of the invention which determines fan total pressure ratio; Figures 2 and 3 are sets of typical engine performance cutves which could form the basis of calibration data for use with the first embodiment; and 30 Figures 4 and 5 are sets of engine performance curves which form the basis of calibration data for use with a second embodiment. In Figures 2 to 5, the curves are plotted for various values of corrected fan speed NLIVO, expressed as a percentage of the design value of NL in conventional manner, 0 being the 4 inlet total temperature relative to the design inlet reference temperature T R for the engine.

In Figure I a data processor 1 receives data from an inlet pressure probe assembly 2, a bypass duct pressure probe assembly 3, a temperature sensor 4, mounted in the inlet in this embodiment, a calibration data store 5 and a fan speed sensor 6. The data processor I computes the fan total pressure ratio P2/P1 given the measured parameter values and the relevant data from the calibration data store and feeds this ratio to a closed loop engine control system 7. The control system 7 is configured to calculate a fuel demand signal on line 8 with reference to a pilot lever input on line 9 and the computed pressure ratio P2/P1.

The inlet pressure probe assembly 2 comprises four orifices drilled into the engine inlet wall and equally spaced around it. The orifices are linked by an annular manifold which is, in turn, connected to a pressure-sensitive diaphragm. The orifices are positioned upstream from the fan at a distance equal to one-quarter of the engine diameter, this separation being sufficient to avoid significant static pressure distortion induced by the fan. Using this probe assembly, the mean static pressure at the fan inlet can be accurately measured, even under condtions of distorted air flow.

The bypass duct entry pressure probe assembly is also configured to measure mean static pressure and comprises four orifices drilled into the bypass duct entry wall and equally spaced around it. All orifices communicate with a pressure sensitive diaphragm via an annular manifold.

The data store contains information obtained from the engine performance curves shown in Figures 2 and 3.

PSI is the measured mean inlet static pressure and PS2 is the measured mean bypass duct entry static pressure. The data in Figures 2 and 3 are based on fan performance which is obtained from engine test-bed trials for the engine type in j question in undistorted air flow conditions and is typical for turbofan engines operated under sea-level static conditions. The instrumentation in these trials measures mean total pressures Pl and P2 at the fan inlet and bypass duct entry respectively. For operation at high altitude and low Mach number, appropriate Reynolds number corrections are applied to these curves.

The data processor applies the computed value of pressure ratio P2/P1 to an input of the control system 7. The pilot-operated lever input (controlling engine throttle) on line 9 is used by the control system 7 to generate a demand signal for fan total pressure ratio using an appropriate function generator code. This signal is compared with the computed value of P2/Pl and the resulting difference signal is processed by a proportional-plus-integral control algorithm to produce a fuel demand signal for the engine which appears on line 8. The value of the fuel demand signal is constrained between two limits so that engine surge and flame-out are avoided.

A second embodiment of the invention comprises the same apparatus as described in the first embodiment for measuring fan rotational speed NL, inlet temperature T and inlet static pressure PS1 but has a different bypass duct pressure probe assembly. A data processor, data store and control system are also employed in a manner similar to that described for the first embodiment except that the data store contains different calibration data and the control system has a nozzle actuator output in addition to a fuel demand signal output.

The bypass duct pressure probe assembly of the second embodiment comprises two probes diametrically opposed across the engine circumference in the plane of the bypass duct entry. The probes, which are mounted in the bypass duct outer wall extend inwardly across the bypass duct and each probe consists of a front tube and a rear tube, the front tube being located upstream of the rear tube.

6 The front tubes of each probe are connected together by a front manifold and the rear tubes of each probe are connected by a rear manifold so that circumferentially averaged pressures are sensed. Each manifold is connected to a pressure-sensitive diaphragm. This bypass duct probes assembly facilities determination of a bypass duct Mach number parameter MNAV which is a function of the front and rear manifold pressures P F and P R MNAV also serves as an indicator of the working point of the fan at a given corrected fan speed (NL/e).

Although two diametrically opposite probes are sufficient for most purposes, in some applications is may be preferable to utilise three or more probes equally spaced around the bypass duct and similarly communicating with the front and rear manifolds.

The data processor calculates the ratio P2/PI from the measured values of PF, P R, NL, T and PS1 and from the information contained in the data store relating these parameters derived from the turbofan engine performance curves such as those shown in Figures 4 and 5. The computed value of P2/P1 and MNAV are fed into the control system 7 and used jointly to calculate demand signals for the fuel and nozzle actuators with reference to the pilot lever input 9 and to a demand signal for MINAV which is generated by the control system 7 using an appropriate function generator code, given measured values of NL and T.

The fuel demand and nozzle actuator output signals can be protected by limits imposed by the control system in order to prevent engine surge or flame-out. They can also be integrated with structural limiters to safeguard engine temperature and spool speeds.

For engine'fitted with variable inlet guide vanes on the high pressure compressor, the engine calibration data represented by Figures 2, 3 and 4 can be extended to reflect the effects of any guide vane positional errors provided this error measurement is available. Normally such errors only exist during engine manoeuvres.

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Claims

1. Apparatus for determining fan inlet total pressure in a turbofan aircraft engine having an inlet and a bypass duct wherein the apparatus comprises: means for measuring fan rotational speed NL; means for measuring inlet temperature T; means for measuring mean inlet static pressure PSI; means for measuring mean bypass duct entry pressure PB; means for storing calibration data relating fan inlet total pressure P1 with the measured quantities NL, T, PSI and PB; and means for computing P1 from the calibration data and said measured quantities.

2. Apparatus as claimed in Claim 1 further including means for determining fan total pressure ratio P2/P1 comprising means for storing calibration data relating bypass duct entry total pressure P2 with the measured quantities NL, T, PSI and PB and means for computing P2/P1 from the calibration data and said measured quantities.

3. Apparatus as claimed in Claim 1 or Claim 2 wherein the means for measuring mean inlet static pressure PSI comprise at least two orifices in the inlet wall, equally spaced around said wall and connected together by an annular manifold.

4. Apparatus as claimed in Claim 3 wherein the orifices are positioned upstream of the fan at a sufficient distance therefrom to be substantially unaffected by any static pressure distortion induced by the fan in close proximity thereto.

5. Apparatus as claimed in any preceding claim wherein the means for measuring mean bypass duct entry pressure PB comprise at least two orifices in the bypass duct entry wall, equally spaced around said wall and connected together by an annular manifold.

8

6. Apparatus as claimed in any of Claims 1 to 4 wherein the means for measuring bypass duct entry pressure PB comprise at least two probes protruding through the bypass duct entry wall and equally separated around it, each probe comprising a front and a rear tube, the front tubes of each probe being connected by a front manifold and the rear tubes of each probe being connected by a rear manifold.

7. Apparatus as claimed in any of Claims 3 to 6 incorporating a pressure-sensitive diaphragm connected to each manifold via one or more tappings in said manifold.

8. Apparatus as claimed in any preceding claim further comprising a closed loop engine control system for creating a fuel demand signal for the engine from the measurements of fan inlet total pressure or fan total pressure ratio.

9. Apparatus as claimed in Claim 6 further comprising a closed loop engine control system for creating a fuel demand signal for the engine and a nozzle area demand signal.

10. Apparatus substantially as herein described with reference to Figure 1.