Patent Description:
Gas turbine engines are known and typically include a fan delivering air into a bypass duct as bypass air and into a compressor. The air delivered into the compressor is mixed with fuel and ignited. Products of this combustion pass downstream over turbine rotors driving them to rotate. The turbine rotors, in turn, drive the compressor and fan rotors.

Historically, a fan rotor was driven at the same speed as a fan drive turbine rotor. This somewhat limited the design of the gas turbine engine. It was desirable that the fan rotate at a slower speed and have an increased diameter. However, it was also desirable that the turbine rotate at faster speeds. Thus, compromises had to be made.

More recently, a gear reduction has been incorporated between the fan drive turbine and the fan rotor. This has allowed the size of the fan rotor to increase. At the same time, the turbine rotor can rotate at increased speed.

The gear reductions utilized today have utilized epicyclic planetary or star gear reductions. These gear reductions have always utilized at least five star gears between a sun gear and a ring gear. This has limited the available gear ratios.

<CIT> discloses a prior art gas turbine engine according to the preamble of claim <NUM>. <CIT> discloses a differential geared engine. <CIT> discloses a geared turbofan gas turbine engine architecture. <CIT> discloses an auxiliary oil pump for gas turbine engine gear reduction.

According to a first aspect of the present invention, there is provided a gas turbine engine as set forth in claim <NUM>.

In another embodiment according to the previous embodiment, the spacing angle is <NUM> degrees.

In another embodiment according to any of the previous embodiments, a bypass ratio is defined as a volume of air delivered by the fan into a bypass duct compared to a volume of air delivered into a compressor.

In another embodiment according to any of the previous embodiments, the bypass ratio is greater than or equal to about <NUM> and less than or equal to <NUM>.

In another embodiment according to any of the previous embodiments, the gear reduction has a gear ratio between the speed of a drive input to the sun gear, and an output speed of the fan rotor and the gear ratio is greater than or equal to about <NUM> and less than or equal to about <NUM>.

In another embodiment according to the previous embodiment, the gear ratio is greater than or equal to about <NUM>.

In another embodiment according to any of the previous embodiments, the fan drive turbine has three or four stages of blades.

In another embodiment according to any of the previous embodiments, an auxiliary oil supply system provides oil to journal bearings associated with the four star gears during a windmilling condition of the fan rotor.

In another embodiment according to any of the previous embodiments, at least one blade row in the turbine rotor has blades formed of a directionally solidified material.

The geared architecture <NUM> may be an epicycle gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about <NUM>:<NUM>.

"Low corrected fan tip speed" is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tram °R) / (<NUM> °R)]<NUM>(where °R = K x <NUM>/<NUM>). <FIG> shows a gas turbine engine <NUM> having a fan drive turbine <NUM> including three stages of blades <NUM>, <NUM> and <NUM>. An optional fourth stage <NUM> is also shown. With the arrangement of this disclosure, the fan drive turbine will preferably utilize at least three and sometimes four stages. Stated another way, the fan drive turbine has greater than or equal to three stages.

A gear reduction <NUM> is included between the fan drive turbine <NUM> and a fan rotor <NUM>. The fan drive turbine <NUM> may be similar to the low pressure turbine as utilized in <FIG>, and also directly drives a low pressure compressor stage. Alternatively, the engine <NUM> may include three turbine rotors with a high pressure turbine rotor driving a high pressure compressor, an intermediate pressure turbine rotor driving an intermediate pressure compressor, and a low pressure turbine rotor being the fan drive turbine.

<FIG> shows details of the gear reduction <NUM>. A ring gear <NUM> (shown partially) surrounds four intermediate gears 116A, 116B, 116C, and 116D, which are star gears. A sun gear <NUM>, which is driven by the fan drive turbine <NUM>, is positioned within the star gears 116A-116D. Journal bearings <NUM> mount the star gears 116A-116D.

As known, the gear reduction is connected to drive the fan rotor in a manner as known in the art.

With a four star epicyclic gear reduction, gear ratios that are much higher than available with the prior art five stage (or more) gear reductions can be achieved. As an example, a five star gearbox has a limited gear ratio of about <NUM>. This limits how fast the fan drive turbine can rotate and how slow the fan rotor can rotate.

However, with the disclosed four star epicyclic gear reduction, gear ratios of greater than <NUM> and up to about <NUM> can be achieved. In one embodiment, a gear ratio of greater than or equal to about <NUM> is achieved. In another embodiment, a gear ratio of greater than or equal to about <NUM> is achieved.

Moreover, the fan drive turbine is provided with three or four stages and can turn much faster than a turbine driving the prior epicyclic gear reductions utilizing five or more star gears. As an example, the fan drive turbine <NUM> can turn <NUM> percent faster than an example fan drive turbine in a gas turbine engine having an epicyclic gear reduction with five or more star gears.

It is conventional wisdom that gear reductions having fewer star gears may raise some other undesirable characteristics. In particular, as the number of star gears decreases for a given ring gear diameter, the sun gear's diameter becomes smaller. Further, the sun gear has fewer teeth and may see higher gear stresses. Further, the space for the drive input shaft from the fan drive turbine into the sun gear becomes smaller. Thus, the benefits of a four star gear reduction are unexpected. Still, while utilizing four star gears provides desirable characteristics, even fewer star gears may not be desirable.

As the fan drive turbine <NUM> begins to turn more quickly, the temperatures it may see also increase. Thus, it may be desirable to form at least one row, and perhaps all of the rows, of the blades in the fan drive turbine <NUM> from directionally solidified blade materials.

In addition, with this arrangement, bypass ratios of greater than or equal to about <NUM> can be achieved. In addition, bypass ratios of greater than or equal to about <NUM> and even <NUM> may be achieved.

An auxiliary oil circuit is shown schematically at <NUM>. This oil circuit will provide oil to the journal bearings <NUM> whenever there is rotation of the fan rotor. Thus, during windmilling oil will be provided. Of course, there is also a primary oil supply system <NUM>. The details of circuits <NUM> and <NUM> may be as known.

As shown in <FIG>, the center point C of the top gears 116A and 116C are spaced by <NUM> degrees from a top dead center point TDC. It is known that the greatest stresses induced between the gears <NUM> and <NUM> would be at the top dead center point TDC. Thus, spacing the gears from that point reduces the challenges the gears will face. Stated otherwise, the center point of the star gears measured from a center point SC of the sun gear is spaced from the top dead center TDC by an angle of at least <NUM> degrees. As mentioned, in one embodiment, the spacing angle is <NUM> degrees.

Claim 1:
A gas turbine engine (<NUM>) comprising:
a fan rotor (<NUM>);
a low pressure compressor (<NUM>);
a turbine rotor (<NUM>) driving said fan rotor (<NUM>); and
an epicyclic gear reduction (<NUM>, <NUM>) positioned between said fan rotor (<NUM>) and said turbine rotor (<NUM>), said epicyclic gear reduction (<NUM>, <NUM>) including a ring gear (<NUM>) and a sun gear (<NUM>), wherein said turbine rotor (<NUM>) is configured to drive said sun gear (<NUM>) to, in turn, drive said ring gear (<NUM>) to, in turn, drive said fan rotor (<NUM>), wherein said sun gear (<NUM>) defines a sun gear center axis (SC) and a top dead center point (TDC) is defined at a vertically uppermost location of said ring gear (<NUM>),
wherein said epicyclic gear reduction (<NUM>, <NUM>) comprises exactly four star gears (116A, 116B, 116C, 116D) that engage said sun gear (<NUM>) and said ring gear (<NUM>);
characterised in that the turbine rotor (<NUM>) directly drives the low pressure compressor (<NUM>); and
a center point (C) of a top two of said four star gears (116A, 116C) is at a spacing angle from a line drawn between said top dead center point (TDC) and said sun gear center axis (SC), and said spacing angle is at least <NUM> degrees.