GB2187796A

GB2187796A - Ducted fan and ducted propeller gas turbine engines

Info

Publication number: GB2187796A
Application number: GB08605896A
Authority: GB
Inventors: Jeremy Benjamin Wykeham Howard
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1986-03-11
Filing date: 1986-03-11
Publication date: 1987-09-16
Also published as: GB8605896D0

Abstract

In a large (nominally 20:1) bypass ratio ducted fan or ducted propeller gas turbine engine, the accessory drive means and accessories are removed from within the fan or propeller cowl and re- positioned on a bulkhead 32 between the core gas generator 14 and its casing 18. The space taken up by the accessories is generated when defining the fan or propeller duct and exit throat by the casing 18 and the fan or propeller cowl 12. A further bulkhead 34 and drive gears 44 may be provided. <IMAGE>

Description

SPECIFICATION Ducted fan and ducted propeller gas turbine engines This invention relates to gas turbine engines which include a stage of fan blades or propeller blades which project radially beyond the gas turbine generator and are rotated by a turbine stage of the gas generator. A cowl is provided and with the core gas generator provides a duct through which ambient air is pumped by the stage of fan blades or propeller blades.

It is common practice to utilise such engines as those described hereinbefore, for the purpose of powering aircraft. Since the aircraft fly at considerably altitudes, it is necessary to provide inter alia cabin pressurisation. Such services need e.g. pumps and pump drive means and all are embraced in the turbine art by the phrases "accessories" and "accessory drives". More specifically, the accessories and drives comprise such things as oil tank filler, drive generator, alternator, starter, hydraulic pump, oil pumps, de-oiler and combined fuel pumps and control.

Again, it is common practice to cluster the aforementioned accessories in space between the walls which define the fan cowl. Normally such space would not be available if the cowl were manufactured such that it was truly symetrical about the axis of the duct which it defines. Consequently, that portion of the cowl which contained the accessories had to be bulged in order to accommodate them.

Frontal areas was thus increased, which is a source of drag. Moreover, the weight of the accessories in the cowl greatly exacerbate the effect of the already large operating loads on the cowl during the flight of an aircraft which is powered by an engine or engines of the kind described hereinbefore. Such excessive loads contribute towards "nodding" of the cowl relative to the core gas generator and has been known to generate friction between the tips of the fan blades and the cowl inner wall.

The present invention seeks to provide an improved ducted fan or ducted propeller gas turbine engine.

According to the present invention a ducted fan or ducted propeller gas turbine engine comprises a core gas generator, a cowl of substantially symetrical axial profile surrounding at least the upstream end of the core gas generator in coaxial, spaced relationship and supported therefrom by radial struts, a stage of fan or propeller blades supported within the cowl by shafting which projects beyond the upstream end of the core gas generator, a casing surrounding the core gas generator and defining with the cowl, a fan or propeller duct and an airflow exit throat and wherein the bypass ratio of the ducted fan or propeller gas turbine engine is in the order of 20:1, and wherein in one plane normal to the axis of the core gas generator, the ratio of the diameter of the casing thereof and the core gas generator is in the order of 2.5::1 and in that plane an annular bulkhead is provided which supports accessories as defined herein.

Preferably the bulkhead defines the downstream end of a core gas generator low pressure compressor air intake structure.

The bulkhead, in conjunction with a further bulkhead upstream thereof, may provide an oil tank.

Preferably the oil tank comprises a segment of a annular space defined by the bulkheads and casing.

Preferably a further segment is partitioned off so as to provide an enclosed space, in which accessory drive gears are housed and are connected for driving by a shaft of the core gas generator.

Preferably the fan or propeller blades are driven by a turbine of the core gas generator via a stepdown gearbox.

Further, the fan or propeller blades may include a mechanism with which to vary their pitch.

The invention will now be described, by way of example and with reference to the accompanying drawing which is a diagrammatic, axial cross sectional view of a ducted fan gas turbine engine in accordance with an aspect of the present invention.

In the drawing a ducted fan, gas turbine engine 10 has a fan cowl 12, the downstream end of which surrounds the upstream portion of a core gas generator 14. The cowl 12 is coaxial with the core gas generator 14 and is spaced therefrom by rigid struts 16, several of which are equi-angularly spaced about a casing 18 which surrounds the core gas generator 14.

A stage of fan blades 20 is connected to a turbine (not shown) for driven rotation thereby. In the present example, the connection is made via a shafting 21 and a step down gear box 22. This however is not an essential feature of the present invention.

A fan blade pitch varying mechanism 24 is also illustrated but again, this is not an essential feature of the invention.

A fan air flow duct 26 and a fan air flow propulsion nozzle 28 are defined by the cowl 12 and the upstream portion of the casing 18 and the design of the profile and cross sectional area of the duct 26 is such that in operation of the engine 10, the fan stage 20 pumps therethrough, about ten to twenty times more ambient air than the amount of air which enters the low pressure compressor 30 of the core gas generator 14, i.e. the bypass ratio of the engine 10 is in the order of 20:1.

The casing 18 consists of a sheet metal or plastic composite tubular portion 18a and a relatively short metal structure 18b which is of rigid construction which may be cast or fabricated and which provides the air intake structure and outer wall for the low pressure compressor 30. The downstream end of the structure 1 8b terminates in a rigid annular bulkhead 32. A further rigid annular bulkhead 34 is spaced upstream therefrom. An annular space 36 is thus defined by the bulkheads 32 and 34.

The diameter of the casing 18 in the plane of the bulkhead 32, is approximately two and a half times the diameter of the core gas generator 14 in the same plane. There is thus provided a considerable space volume 48 between the core gas generator 14 and the casing 18, along with a large surface area on the bulkhead 32, to which accessories can be easily fitted as is described hereinafter. If it is desired to raise the pressure ratio of the core gas generator 14, its diameter in the relevant area will be reduced and will thus enhance the space volume.

The shaft 21 includes a bevel gear 38 and this is connected via a further shaft 40 and 42 and gearing 44, to drive an alternator 46.

The alternator 46 is mounted on the bulkhead 32. Other accessories such as the starter, hydraulic pumps, combined oil pumps, de-oilers, fuel pumps and oil tank filters, none of which are shown, may also be mounted on the bulkhead 32, around the core gas generator 14.

Moreover, the space 36 between the bulkheads 32 and 34 may be sub-divided to provide e.g. one or more oil tanks and a gearbox for the support of the aforementioned gears.

This obviates the necessity to provide a separate gear box or oil tank and thus saves considerable weight.

Claims

1. A ducted fan or ducted propeller gas turbine engine comprising a core gas generator, a cowl of substantially symmetrical axial profile surrounding at least the upstream end of the core gas generator in coaxial, radially spaced relationship and supported therefrom by radial struts, a stage of fan or propeller blades supported within the cowl by shafting which projects beyond the upstream end of the core gas generator, a casing surrounding the core gas generator and defining with the cowl a fan or propeller duct and an airflow exit throat and wherein the bypass ratio of the ducted fan or propeller gas turbine engine is in the order of 10 to 20:1 and wherein in one plane normal to the axis of the core gas generator, the ratio of the diameter of the casing thereof and the core gas generator is in the order of 2.5::1 and in that plane an annular bulkhead is provided in the casing which supports accessories of the kind which are defined herein.

2. A ducted fan or ducted propeller gas turbine engine as claimed in claim 1 wherein the bulkhead defines the downstream end of a core gas generator low pressure compressor air intake structure.

3. A ducted fan or ducted propeller gas turbine engine as claimed in claim 1 or claim 2 wherein the bulkhead, in conjunction with another bulkhead upstream thereof, defines an oil tank.

4. A ducted fan or ducted propeller gas turbine engine as claimed in claim 3 wherein the oil tank comprises a segment of an annular space which is defined by the bulkheads and the casing.

5. A ducted fan or ducted propeller gas turbine engine as claimed in claim 4 and including a further partitioned off segment in which accessory drive gears are housed and connected for driving by a shaft of the core gas generator.

6. A ducted fan or ducted propeller gas turbine engine as claimed in any previous claim wherein the fan or propeller blades are driven by a turbine of the core gas generator via a step down gearbox.

7. A ducted fan or ducted propeller gas turbine engine as claimed in any previous claim and including a mechanism with which to vary the pitch of the blades thereof.

8. A ducted fan or ducted propeller gas turbine engine substantially as described in this specification and with reference to the drawing.

9. A ducted fan or ducted propeller gas turbine engine including a core gas generator surrounded by a casing which is spaced from the core gas generator by a pair of annular bulkheads which are adapted to support a plurality of gears which comprise accessory drive gears.

10. A ducted fan or ducted propeller gas turbine engine as claimed in claim 9 wherein at least a portion of the space defined by the casing, the bulkheads and the core gas generator is partioned off to form an oil tank.