Fan

Abstract

A fan comprises first and second rotor stages driven through a differential ear. The first stage is driven from the sun gear of the differential while the second stage is driven from the annulus gear of the differential. The planet carrier is driven from drive means. The invention is particularly applicable to gas turbine engines in which case the first stage of the fan may comprise a hub compressor and the drive means may comprise a gas turbine.

Description

This invention relates to a fan which is particularly but not exclusively useful in a gas turbine engine.

Fans having large capacity are useful in a number of applications and particularly in high by-pass ratio gas turbine engines. To produce the high capacity it has been the practice to increase the overall diameter of the fans, while to reduce complications entailed with very large hubs it has been the practice to make the hub of a fairly small size. A problem then arises in that to avoid the production of large amounts of noise the tip speed of the fan blades must be less than the speed of sound; this therefore puts a limit on the rotational speed of the fan and consequently causes the inner sections of the fan blades to be moving so slowly that it is difficult to cause them to operate effectively. One solution which has been proposed is to provide an additional fan stage whose diameter is considerably less than that of the main fan, this hub compressor being driven with the main fan or in some cases by a separate turbine.

The hub compressor then restores the efficiency of the central portion of the fan, but entails some complication in drive.

The present invention provides a fan having such a hub compressor and having a very simple drive which divides the torque available between the main fan and the hub compressor.

According to the present invention a fan comprises a first and second rotor stage in flow series and a differential gear comprising a sun gear, a plurality of planet gears carried from a planet carrier, and an annulus gear, the first rotor stage being connected to the sun gear, the second rotor stage being drivingly connected to the annulus gear and drive means connected to rotate the planet carrier.

Said first rotor stage may comprise a hub compressor and said second stage may comprise a main rotor stage, the blades of the hub compressor extending over only the central portion of the main rotor stage.

In the case of a gas turbine engine the drive means will normally comprise a turbine of the engine.

The hub compressor is preferably shrouded; thus the shroud may be carried from the main rotor stage and extend forwardly to encompass the hub compressor.

In a preferred embodiment the fan is mounted coaxially with a gas generator section so that the fan compresses air which provides by-pass air and also supercharged inlet air to the gas generator. In our preferred case the gas generator is a two-shaft gas generator and comprises a low pressure turbine which drives the planet carrier.

The invention will now be described merely by way of example with reference to the accompanying drawings in which:

FIG. 1 is a partly broken away view of a gas turbine engine having a fan in accordance with the invention and

FIG. 2 is an enlarged view of the fan portion of the engine of FIG. 1.

FIG. 1 shows a gas turbine engine comprising a gas generator section 10 and a fan generally indicated at 11. The fan compresses air part of which supercharges the gas generator 10 while the remainder passes between the gas generator and a fan cowl 12 to provide propulsive thrust. It will be seen that the gas generator is a two-shaft gas generator; that is it comprises intermediate pressure and high pressure compressors 13 and 14 drivingly connected to intermediate and high pressure turbines 16 and 15 respectively. A third, low pressure turbine 17 is provided and drives an LP shaft 18 which extends coaxially inside the gas generator shafts to drive the fan.

The arrangement of the fan is best seen in FIG. 2. The fan comprises a main fan rotor 20 made up of a plurality of blades which carry part way along their extent a shroud ring 21. Within the shroud ring 21 and upstream of the main fan rotor 20 there is a hub compressor 22 which again comprises a plurality of rotor blades which only extend over the central part of the main fan rotor 20.

The hub compressor 22 is driven from a shaft 23 which is mounted in bearings 24 and 25. The bearing 24 supports the shaft 23 from the shaft 26 of the main fan 20, and this shaft is in turn supported by the bearing 27 from fixed structure of the engine. In addition to these bearings, the shaft 26 supports bearings 29 and 30 whose purpose will become apparent below.

To drive the shafts 23 and 26, the shaft 23 is provided with an external set of teeth at 31 which form the sun wheel of an epicyclic differential gear. The shaft 26 is attached to an annulus gear 32 whose internal teeth form the annulus gear of the same differential. A plurality of pinions 33 mesh between the sun gear 31 and annulus 32, these components being carried on a pinion carrier 34. The carrier is extended forwardly to mount within the bearing 29 mentioned above, while the rearward portion extends to form the LP shaft 18. This rearward extension is supported in the bearing 30 and itself supports the bearing 25 of the shaft 23.

Operation of the system is as follows:

Shaft 18 is driven by the LP turbine and consequently drives the planet carrier 34. Rotation of this carrier causes the pinions to rotate about the fan axis and causes equal driving loads to be exerted on the sun wheel 31 and the annulus gear 32. Because of the difference in the diameters of these wheels, a greater torque will be exerted on the annulus gear than on the sun wheel; consequently more power is passed into the main fan rotor than into the hub compressor. This is the desired splitting of work.

The aerodynamics of the fan and hub compressor are arranged so that at designed speed for the turbine the fan will rotate at a fairly slow speed, thus avoiding any danger of sonic or near sonic fan speeds which would produce noise. The hub compressor is arranged to rotate considerably faster, since with its relatively small diameter blades there is little chance of its blades going sonic. Since the hub compressor rotates at a relatively high speed it can be reasonably efficient over its radial extent; it therefore makes up for the relative inefficiency of the central section of the fan blades. In fact the inner portion of the fan blades may be made simply as struts which perform no work on the air which has been compressed by the hub compressor.

It will be appreciated that the sun gear, planet carrier and annulus gear all rotate in the same direction, the differential speed between the sun and annulus gears being taken up by rotation of the pinions. It will be understood by those skilled in the art that the arrangement in which these gears rotate in the same sense produces considerably less friction loss than a gear in which components contra-rotate. Thus our present arrangement reduces frictional losses to a sixth of a conventional epicyclic gear arrangement. Such a reduction is of the greatest importance in applications where the transmitted power is very high since frictional losses which appear as heat in the lubricating fluid cause great difficulty in rejecting heat in sufficient quantities.

It will be appreciated that this invention is particularly relevant to the fans of gas turbine engines but can be applied to other fans, particularly propulsive fans which may be used for VTO application etc. In any of these cases it is only necessary to provide the drive in the form of a single shaft, the gear arrangement of the invention providing the necessary splitting of power.

Claims

We claim:

1. A fan comprising a first and a second rotor stage in flow series and a differential gear comprising a sun gear, a plurality of planet gears carried from a planet carrier, and an annulus gear, first connection means by which the first rotor stage is connected to the sun gear, second connection means by which the second rotor stage is drivingly connected to the annulus gear and drive means connected to rotate the planet carrier.

2. A fan as claimed in claim 1 and in which said first rotor stage comprises a bladed hub compressor, and the second stage comprises a main bladed rotor stage, the blades of the hub compressor extending over only the central portion of the main rotor stage.

3. A fan as claimed in claim 2 and in which there is a shroud round the blades of the hub compressor.

4. A fan as claimed in claim 3 and in which the hub compressor shroud is carried from the main rotor stage and extends forwardly to encompass the hub compressor.

5. A fan as claimed in claim 1 and in which said drive means comprises a gas turbine.

6. A fan as claimed in claim 5 and in which said fan drive gas turbine is driven by the exhaust gas of a gas generator.

7. A fan as claimed in claim 6 and in which said gas generator comprises a gas turbine gas generator.

8. A fan as claimed in claim 7 and in which said gas generator has an air intake, the rotors being mounted coaxially ahead of the intake so that the fan compresses air which provides by-pass air and supercharged inlet air to the gas generator.

9. A fan as claimed in claim 8 and in which said gas turbine gas generator comprises a multi-shaft gas generator.