Patent Application
20200049022
The present disclosure concerns an aircraft gas turbine engine.
Conventional aircraft gas turbine engines comprise an engine core having a compressor, combustor and turbine, as well as a bypass duct comprising a turbine driven fan. A 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 recent years, there has been a steady progression toward aircraft engines with higher bypass ratios.
Engines are typically supported at a front end by one or more bifurcation supports, or one or more outlet guide vanes located rearward of the fan. An arrangement in which the core engine is supported by outlet guide vanes is described in US patent application US20110192166.
As bypass ratios increase, the outlet guide vanes become longer (i.e. have greater radial extent), which increases the flexibility of the guide vanes for a given cross-section and material. However, high structural rigidity is desirable to counteract loads induced by gyroscopic forces during aircraft manoeuvres such as take-off, and vibrational loads during flight. Consequently, as bypass ratios increase, structural weight and/or surface area of the OGVs in a conventional gas turbine engine must also increase, to provide the necessary rigidity. This in turn results in a heavier engine, and/or increased aerodynamic drag in the bypass nacelle.
According to a first aspect there is provided a gas turbine engine comprising: a bypass duct cowl;
an engine core housing defining an engine core inlet;
a bypass fan; and
a plurality of outlet guide vanes extending between a radially inner surface of the bypass duct cowl, and a radially outer surface of the engine core housing to define an outlet guide vane span, the outlet guide vanes being configured to support the engine core housing relative to the bypass duct cowl;
wherein the bypass fan and an engine core inlet define a bypass ratio between 10 and 17; and a ratio of the outlet guide vane span to a bypass fan radius is between 0.45 and 0.55.
It has been found that by providing a high bypass ratio gas turbine engine with relatively short OGVs, a relatively stiff forward mounting structure can be provided, while minimising the weight and aerodynamic drag of the OGVs.
The fan outlet guide vanes, bypass duct cowl and bypass fan may be arranged such that the condition
is satisfied, where SPANOGV is the span of the outlet guide vane, CHORDOGV is the chord of the outlet guide vane, Xa is the distance between the bypass nacelle inlet aerodynamic centre of pressure and the centre of the outlet guide vane, Fi is the maximum intake upload for which the engine is certified, ROGV is a distance from the engine axis of the tip of the outlet guide vanes in the radial plane, and K is a proportionality constant accounting the youngs modulus of the OGV, such that x is equates to the OGV tip deflection. In practice it will be desirable to keep this deflection in the range +/−20 mm for normal flight loads. This can be achieved by tuning the stiffness of the OGV assembly by adapting the Span, Chord, thickness, and outer radius of the OGV system.
A ratio of the inner radius of the outlet guide vanes and an outer radius of the outlet guide vanes may be between 0.5 and 0.6.
A ratio of an axial distance between an inlet to the bypass duct cowl and a trailing edge of the outlet guide vanes, and an outer radius of the outlet guide vanes may be between 1 and 1.5.
Each outlet guide vane may have an aspect ratio of between 2 and 8.
A ratio of the outer radius of the outlet guide vanes to the radius of the fan may be equal to or greater than 1, and may be between 1 and 1.2.
The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein.
An embodiment will now be described by way of example only, with reference to the Figures, in which:
With reference to
The gas turbine engine 10 works in the conventional manner so that air entering the intake 12 is accelerated by the fan 13 to produce two air flows: a first air flow A into the low pressure compressor 14 and a second air flow B which passes through a bypass duct defined by an interior of the nacelle 21 to provide propulsive thrust. The low pressure compressor 14 compresses the air flow directed into it before delivering that air to 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 combusted. The resultant hot combustion products then expand through, and thereby drive the high and low-pressure turbines 17, 18, before being exhausted through the nozzle 20 to provide additional propulsive thrust. The high 17, and low 18 pressure turbines drive respectively the high pressure compressor 15, low pressure compressor 14 and fan 13, each by suitable interconnecting shaft. A reduction gearbox (not shown) may be provided to link the fan 13 to the low pressure turbine 18.
Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. By way of example such engines may have an alternative number of interconnecting shafts (e.g. two) and/or an alternative number of compressors and/or turbines. Further the engine may comprise a gearbox provided in the drive train from a turbine to a compressor and/or fan.
Together, the compressors 14, 15, combustor 26 and turbines 17, 18 define a core, through which the first airflow A flows through an engine core inlet 34. The core is surrounded within a core housing arrangement comprising an inner core housing 22 and an outer core housing 23. The inner core housing provides an air-tight gas path for the core airflow A, while the outer core housing provides structural support for the core and the fan 13.
The core and fan 13 are supported by front and rear mounting arrangements 24, 25. The front mounting arrangement 24 comprises a plurality of outlet guide vanes (OGVs) 26, which are shown in more detail in
A plurality of OGVs are provided, which are distributed circumferentially around the engine 10. A radially inner end (“root”) 27 of each OGV 25 is mounted to an outer surface 28 of the outer core housing 23, while a radially outer end (“tip”) 29 of each OGV 26 is mounted to an inner surface 30 of the nacelle 21.
The OGVs act as the supporting structure for the front portion of the engine, that is to say that the weight of the engine and any in flight loads are supported by the OGVs. In contrast, in most conventional engines, additional support structure in the form of bifurcations is generally necessary. Consequently, the OGVs must be designed to be both structurally and aerodynamically efficient.
Several geometric properties can be described for each OGV 26, which are generally alike. A chord CHORDOGV can be defined by a distance between leading 31 and trailing edges 32 at a mid-span position. A span SPANOGV can be defined as a distance between the root 27 and the tip 29 of the respective OGV 26 at the mid-chord position.
From these geometric properties, further geometric properties can be defined. For example, an aspect ratio of the OGV can be determined by the following equation:
In this case, the aspect ratio is between 4 and 8. This provides a relatively stiff, strong OGV, which can react large loads without bending or failure.
Referring once more to
A fan radius FOGV is defined by a distance from the longitudinal axis 11 and an aerodynamic tip 33 of the fan 13. Similarly, an OGV radius ROGV is defined by a distance from the longitudinal axis 11 and an aerodynamic tip 29 of the OGV 26.
A distance Y is defined as the axial distance between the air intake 12 to the bypass duct cowl 21 and a mid-chord of the outlet guide vanes 24 at the tip. A ratio between the distance Y and the outer radius ROGV of the outlet guide vanes 24 is typically between 1 and 1.7. This ratio is relatively
A distance Xa is defined as the axial distance between the inlet centre of pressure 12 and the centre of the OGVs 26. The location of the inlet centre of pressure can be determined by summing the internal inlet pressures and external cowl pressures across the inlet. During manoeuvres (such as take-off from a runway), a force Fi is generated at the forward end of the engine 10, effectively acting at the inlet 12. The intake upload may be calculated by summing the inlet internal and external surface pressures at a typical sizing case, e.g. high incidence take off rotation.
Certain relations between these geometric properties can also be determined. In particular, a ratio of the outlet guide vane span SPANOGV to the bypass fan radius RFAN is between 0.45 and 0.55. This parameter is believed to be unique in a high bypass turbofan having a bypass ratio between 8 and 12.
In the present disclosure, as discussed above, the OGVs provide structural support for the front of the engine 10. On the other hand, they must also have high aerodynamic performance (i.e. provide low drag in use) and low weight. Consequently, it has been found that by providing a short span OGV relative to the bypass fan radius, a low weight, high strength OGV can be provided, which can support the engine in structurally and aerodynamically efficient manner. In particular, this has been found to result in a front structural support which is sufficiently stiff to prevent flexing of the fan case relative to the core casing, which could otherwise cause fatigue, and/or fan tip rubs.
In order to provide the desired bypass ratio with a short span OGV 26, the OGV is provided at a large radial extent, i.e. the OGV root 27 is provided relatively far from the longitudinal axis. Consequently, a large flow area can be provided at the OGVs, in spite of their short span. Consequently, in the present disclosure a second ratio is defined. The second ratio is defined by a ratio of the OGV inner radial distance Rogv root to the OGV outer radial distance Rogv. The second ratio is between 0.5 and 0.6. The radial distances may be measured at the mid-chord position.
Further changes relative to conventional engine arrangements can also be made to maintain a short span OGV in combination with a large bypass ratio. The outer radius of the OGVs ROGV is generally equal to or greater than the radius of the fan Rfan. Typically, the ratio is between 1 and 1.2. In contrast, in most conventional engine arrangements, the OGVs have a smaller diameter than the fan. This greater outer diameter of the OGVs, in conjunction with the large inner diameter Rogv root helps to enable a high bypass ratio with a short OGV, without necessitating a significant restriction at the OGVs, which would accelerate and possibly choke the flow, leading to a reduced pressure ratio across the fan and possible stalling, as well as increased fan noise due to the high velocity fan air.
A third ratio is also defined. The third ratio is defied by the axial distance Xa, divided by the outer radius of the outlet guide vanes Rogv and is typically between 1 and 2. By providing a low axial distance relative to the outer radius of the outlet guide vanes, the deflection of the outlet guide vane tips is minimised for a given inlet upload. Consequently, the fan OGVs can again be made less stiff for a given core inlet upload.
In keeping with the above constraints the engine geometry is arranged such that the condition
is satisfied. It has been found that, for an aerodynamically optimised structural OGV (i.e. ones that act as a forward mount, while providing minimum drag), the constant x should be between −20 mm and +20 mm. This ensures that deflections of the tip of the OGVs 24 are kept to within limits, to prevent the fan cowl 21 from being excessively deflected, which may cause fatigue, and “out of round” conditions, which may in turn result in tip rubs of the fan 13. OGV tip deflection may be in one or more planes. For example, the OGV tip may be deflected radially inwardly or outwardly, though tip deflection is to some extent constrained by the fan cowl 21. The OGV tip may alternatively or in addition be deflected axially (forward or backward), or circumferentially.
OGV tip deflection may be determined by measuring the net deflection in all axes. Alternatively, tip deflection may be determined by modelling the engine (for example, in a Finite Element model), and taking measurements from the model.
Consequently, a highly efficient, light weight aircraft engine is provided.
The inventors have modelled a number of engines to determine optimum values of these parameters for various engine sizes and thrust ranges.
A first engine is designed to provide between 75000 and 85000 pounds of thrust at maximum static flat rated takeoff thrust. The engine has an overall pressure ratio of approximately 50:1, and a bypass ratio of 15:1. The engine is of a two-spool, geared design, having a low pressure compressor coupled to a fan via a reduction gearbox, a high pressure compressor, and high and low pressure turbines).
In this engine, the ratio of the outlet guide vane span (OGVSPAN) to the bypass fan radius (RFAN) is approximately 0.54. Consequently, a high bypass ratio is provided, while a relatively short OGV is also provided, thereby resulting in a relatively strong, lightweight engine.
The ratio of the inner radius of the outlet guide vanes and an outer radius of the outlet guide vanes is 0.5 for this engine.
The ratio of the axial distance between the inlet to the bypass duct cowl and the mid chord of the outlet guide vanes at the tip, and an outer radius of the outlet guide vanes is 1.5 for this engine. Where a longer fan inlet is used, this ratio may increase to approximately 1.7.
A second engine is designed to provide approximately 50,000 pounds of thrust at maximum static flat rated takeoff thrust. The engine has an overall pressure ratio of approximately 42:1, and a bypass ratio of 11. The engine is of a two-spool, geared design, having a low pressure compressor coupled to a fan via a reduction gearbox, a high pressure compressor, and high and low pressure turbines).
In this engine, the ratio of the outlet guide vane span (OGVSPAN) to the bypass fan radius (RFAN), is approximately 0.46. Again, a high bypass ratio is provided, while a relatively short OGV is also provided, thereby resulting in a relatively strong, lightweight engine. As can be seen, the disclosed ratio varies somewhat for engines having different thrust and bypass ratios, but remains within the disclosed range.
The ratio of the inner radius of the outlet guide vanes and an outer radius of the outlet guide vanes is 0.53 for this engine.
The ratio of the axial distance between the inlet to the bypass duct cowl and the mid chord of the outlet guide vanes at the tip, and an outer radius of the outlet guide vanes is 1.49 for this engine.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1812875.1 | Aug 2018 | GB | national |
