Abstract:
A gas turbine engine including a gas generator module and a reduction gearbox module. The gas generator module has an axis which extends longitudinally along a centreline of the gas generator module, and the reduction gearbox module has an axis which extending longitudinally along the centreline of the reduction gearbox and is offset from said generator axis. The reduction gearbox is driven by an intermediate shaft angled relative to the turbine output shaft.

Description:
TECHNICAL FIELD 
     The present invention relates to gas turbine engine design, and in particular, to a gas turbine engine having an offset driven output shaft. 
     BACKGROUND OF THE INVENTION 
     Light general aviation aircraft typically employ engine driven propellers to provide forward thrust. Many of these aircraft have been designed to employ internal combustion piston engines, but there is a relatively recent trend toward retrofitting such aircraft with gas turbine engines. However, a seemingly simple obstacle has so far stifled a more universal replacement of piston engines with gas turbines, and the difficulty is purely a matter of space. Piston engines are typically “short and fat”, whereas gas turbine engines tend to be relatively “long and thin”. Accordingly, most light aircraft designed to employ a piston engine simply do not have the space to readily accommodate a retrofitted gas turbine engine. 
     A somewhat unrelated matter which occupies turboprop designers is keeping the installation inlet and exhaust losses to a minimum. The rather large gearbox required to reduce the rotational output speed to drive the propeller poses an obstacle which must be negotiated by designers in getting inlet air to the engine and extracting exhaust gas therefrom. A gas path which is long and is not straight suffers significant pressure losses. Long air inlet paths also typically require increased anti-icing protection. 
     Accordingly, there is a general need for improvements in the design of gas turbine engines, and in particular, to an engine better adapted for retrofitting a piston-powered aircraft. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved gas turbine engine. 
     It is another object of the present invention to provide a gas turbine engine which is better adapted to retrofitting a piston-powered aircraft. 
     It is another object of the present invention to shorten and straighten the gas path of a turboprop or turboshaft engine. 
     Therefore, in accordance with the present invention, there is provided a gas turbine engine comprising a gas turbine engine, comprising a gas generator module having an turbine shaft for providing rotating output power, and a reduction gearbox module having a gearbox input shaft and a main output power shaft, wherein the gearbox input shaft is drivingly connected to an intermediate drive shaft, the intermediate drive shaft being drivingly connected to the turbine shaft, and wherein the intermediate drive shaft is disposed at an angle to the turbine shaft. 
     There is also provided, in accordance with the present invention, a gas turbine engine comprising a gas turbine engine, comprising a gas generator module, the gas generator module including a compressor portion, a combustor portion, and a turbine portion, and having an turbine shaft for providing rotating output power, and a reduction gearbox module adapted to drivingly connect the gas generator module to an output power shaft, the reduction gearbox module being disposed substantially laterally beside the gas generator module. 
     There is also provided, in accordance with the present invention, a gas turbine engine comprising a gas generator module having an turbine shaft for providing rotating output power, and a reduction gearbox module adapted to drivingly connect the gas generator module to an output shaft, wherein the reduction gearbox module is drivingly connected to the turbine shaft through a bevel gear on the turbine shaft. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the present invention will become apparent from the following detailed description, in combination with the appended drawings in which: 
     FIG. 1 is an isometric view of a gas turbine engine in accordance with the present invention. 
     FIG. 2 is a front view of the gas turbine engine of FIG.  1 . 
     FIG. 3 is a partial cross-sectional view of the gas turbine engine of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, a turboprop gas turbine engine according to the present invention is shown generally at  10 . Engine  10  includes a gas generator module  12 , a reduction gearbox module  14  and an accessory gearbox module  16 . The gas generator module  12  generally has a compressor portion  18 , a turbine portion  20 , and a combustor portion  22 . These components are all generally symmetrically placed about the centreline CL of gas generator module  12 . Referring to FIG. 3, both the reduction gearbox module  14  and the accessory gearbox module  16  are offset from the main engine centreline CL. The offset centreline of the reduction gearbox module  14  is denoted by OCL. 
     In this embodiment, the compressor portion  18  includes an air inlet  24 , a booster stage or boosted rotor type low pressure (LP) compressor  26  (which may be of the type described in co-pending application Ser. No. 09/680281, incorporated herein by reference), and a centrifugal impeller  28  type high pressure (HP) compressor at the outlet end of a compressor air flow duct  30 . The air inlet configuration is relatively straight and generally parallel and concentric to each of the centreline axis CL, the compressor portion  18  and the turbine portion  20 , as will be discussed further below. 
     The turbine portion  20  of the gas generator module  12  is typical and generally includes turbine discs (not shown) connected to a set of drive shafts, in this case an inner LP turbine shaft  36  and an outer HP turbine shaft  38 . The HP shaft  38  drives the impeller  28 , while the LP shaft  36  drives the rotor  26 , the reduction gearbox module  14  and the accessory gearbox module  16 . One will appreciate, however, that these components may be driven by different shafts. 
     The reduction gearbox module  14  receives input power from an RGB tower shaft  40  drivingly connected, via bevel gear  42  and bevel gear  44 , to the LP shaft  36 . The tower shaft  40  extends at an angle to the main centreline CL and LP turbine shaft  36 , and in this case is roughly perpendicular thereto. The tower shaft extends through the inlet gas path  30  through a fairing  46 . A bevel gear set  48  transfers rotational power to an RGB input shaft  50  which, in turn, drives an RGB output shaft  52  through an epicyclic reduction gear train  54 . The output shaft  52  terminates (in this example) in a propeller flange  56  for connection of a suitable propeller (not shown). 
     The epicyclic reduction gear train  54  is typical and generally includes a central sun gear  60 , a plurality of planet gears  62  on a carrier  64 , mounted for rotation within a fixed outer ring gear  66 . The sun gear  60  is driven by the input shaft  50  and the planet gear carrier  64  drives the output shaft  52 . 
     The accessory gearbox module  16  is driven from the LP shaft  36  via an AGB tower shaft  70 . The AGB output shaft  72  is used to drive accessory devices, such as fuel pumps, starter generators, mechanical fuel controls, air/oil separators, and oil pumps, etc. 
     All rotating shafts are journalled by suitable bearings. Generally, the bearings of this embodiment include LP turbine shaft bearings  80 , HP turbine shaft bearings  82 , an roller bearing  84  and an ball and roller bearing combination  86  supporting the RGB tower shaft  40 , a ball bearing  88  and a roller bearing  90  journalling shaft  50 , and a ball bearing  92  and a roller bearing  94  journalling shaft  52 . A ball bearing  96  and roller bearing  98  support the AGB tower shaft  70 . 
     In use, the operation of the gas generator  12  causes output rotational power to be delivered by the LP turbine shaft  36 . As the LP shaft rotates, which can be at speeds upward of 25,000 to 30,000 RPM, torque is transferred via bevel gear  42  and bevel gear  44 , to RGB tower shaft  40 , then through bevel gear set  48  to RGB input shaft  50 , and through reduction gear train  54  to the RGB output shaft  52 . The reduced speed of the output shaft  52  is typically around 2000 RPM, but depends on the application. 
     While it is known to have turbine engines with reduction gearboxes and output drive shafts-which are offset from the main turbine shaft, such devices typically utilize spur gear trains to drive the output shaft. An example of such a configuration is shown in U.S. Pat. No. 4,825,645. The spur gear drive train, however, poses a large obstacle which must be negotiated by the gas path. In contrast, the shaft  40  of the present invention crosses the gas path relatively unobtrusively, housed in a fairing or other housing. Thus, a relatively simple means of locating the main or reduction gear box laterally beside the gas generator is provided. Also novel in the present inventions is the use of a bevel gear set (i.e. gear  42  and  44 ) to take power directly from the LP shaft to drive the reduction gear box. 
     Many gas turbine engines have accessory gearboxes which are offset from the engine centreline. However, the present invention has a reduction gearbox module which is offset from the main centreline, driven by a drive shaft which is angled relative to the main turbine output shaft. This offset permits a substantially more compact design to be achieved, with the overall shape approximating the “short and fat” engine envelopes in aircraft designed to be powered by piston engines. It allows the reduction gearbox to be placed more or less laterally beside the gas generator module, significantly shortening the length of the overall unit. 
     When the offset centreline OCL of the reduction gearbox is sufficiently offset from the main centreline CL, the gas path is relatively unobstructed by the main gearbox and thus a straighter, ‘line-of-sight’ inlet air flow is possible. Similarly, the present invention permits a parallel (rather than serial) arrangement of main gearbox and gas generator, which permits the overall length of the gas path to be substantially shortened. The benefits of a shorter, straighter gas path are well known. The straight inlet also allows ‘ram’ air pressure effect increase the inlet air pressure in the turboprop when in flight, which improves engine output power and performance. 
     Also, the shorter inlet duct length reduces the area where de-icing is required, and the use of the boosted rotor multiplies the benefit in this respect. 
     The shaft  40  of the present invention extends at an angle to the main centreline CL (i.e. is not parallel to it), and in this case is almost perpendicular thereto. In fact, in this case, the shaft  40  is canted slightly aft to permit a placement for propeller flange  40  which is as close as possible to inlet duct  24 . The relative positioning of the gas generator module  12  and the reduction gearbox module  14  is a matter of design choice, and the amount of offset and the relative angles between the modules may vary, depending on the parameters of the intended application. 
     Advantageously, the present invention also permits the placement of the engine on the wing to be optimized. Typically, it is desirable to keep the engine relatively low on the wing to reduce losses, however the propeller of course cannot be permitted to touch the ground. The present invention can permit the prop to be positioned higher, relative to the wing, while the engine is kept lower, which is particularly advantageous in low-wing applications. 
     The offset output drive also permits the propeller in a turboprop application to include a double acting propeller pitch control, which offers an additional weight savings. 
     The placement of the reduction gearbox module  14  also permits the boosted rotor and high turbine rotor to be removed without disturbing the oil system, which reduces the potential for oil contamination. 
     The embodiment of the invention described above is intended to be exemplary only. Modifications may be made which do not depart from the spirit and intent of the invention disclosed herein. While a turboprop configuration is described, the design has application to other gas turbine configurations such as turboshafts, for example. The scope of the invention is, therefore, intended to be limited solely by the scope of the appended claims.