Abstract:
A system and method for providing pressurised oil during non-feathered in-flight shutdown conditions in a variable-pitch aircraft propeller engine. A secondary pump provides pressurised oil substantially independent of an engine main oil circuit.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a system and method for providing pressurized oil during non-feathered in-flight shutdown conditions in a variable-pitch aircraft propeller engine. 
   2. Description of the Prior Art 
   Propeller control systems for turboprop engines normally employ pressurized oil as a means of system actuation. The source of this pressurized oil is usually from a positive displacement or other pump which is driven from the propeller reduction gearbox (RGB) of the engine. The oil supply for this pump is typically provided by the RGB lubrication system, which is sized to ensure that the oil supply requirements of the propeller pump are satisfied during normal engine operating conditions. 
   During in-flight engine shutdown conditions, it may be required to operate the propeller in a non-feathered or rotating condition and under these conditions it is desirable to have a functioning propeller control system. 
   During non-feathered operation, it may not be possible for the RGB lubrication system to maintain an adequate oil supply to the propeller pump, because during engine shutdown conditions, the engine spool driving the propeller pump is either stationary or rotating at very low speeds and, hence, unlikely to provide the required pressurized oil flow required at the propeller pump inlet. A low oil supply obviously prejudices operation of the propeller system during in-flight shutdown, non-feathered conditions, and accordingly there is a need to provide an improved manner of supply oil to the propeller system. 
   SUMMARY OF THE INVENTION 
   It is therefore an aim of the present invention to provide an improved system for supplying oil to a propeller system. 
   Therefore, in accordance with a general aspect of the present invention, there is provided a backup system for providing pressurised oil to a propeller control system during an in-flight shutdown of an engine, the propeller control system including a propeller pump having an outlet connected to a propeller control unit and an inlet connected to a supply of pressurised oil during engine operation, the backup system comprising a secondary pump connected to the propeller pump inlet and adapted to selectively supply said propeller pump inlet with a flow of pressurized oil during said in-flight engine shutdown. 
   In accordance with a further general aspect of the present invention, there is provided a system for providing a secondary pressurized oil supply to an engine during in-flight shutdown conditions, the engine having a main pressurised oil circuit comprising a main pump operational at least during regular engine operation, the system comprising a secondary oil circuit selectively connectable to the main oil circuit in the event of an engine in-flight shutdown, the secondary circuit including a secondary pump adapted to pressurise oil in the secondary circuit to a predetermined pressure at least during said in-flight shutdown conditions, the predetermined pressure exceeding a nominal oil pressure in the main pressurised oil circuit during said in-flight shutdown conditions, wherein the secondary pump is selectively connectable to an inlet of said main pump to provide a pressurised oil supply at said inlet of said main pump during in-flight shutdown conditions. 
   In accordance with a further general aspect of the present invention, there is provided a turboprop engine comprising a propeller having a number of variable pitch blades, a pitch control system for governing the operation of said variable pitch blades, said pitch control system employing a pressurized fluid as a means of system actuation, a propeller pump having an outlet for providing a pressurized fluid supply to said pitch control system, a main pressurized fluid source for providing a pressurized fluid supply to an inlet of said propeller pump, and a pressurized fluid back-up system for enabling operation of the pitch control system during in-flight engine shutdown conditions when said main source of pressurized fluid is unavailable, said pressurized fluid back-up system comprising a secondary source of pressurized fluid available to provide a pressurized fluid supply to said propeller pump inlet during in-flight shutdown, and a switching unit having a first state in which said main source or pressurized fluid is connected to the propeller pump inlet and said secondary source of pressurized fluid is disconnected therefrom, and a second state in which the secondary source of pressurized fluid is connected to said propeller pump inlet, while said main source of pressurized fluid is disconnected therefrom. 
   In accordance with a still further general aspect of the present invention, there is provided a method for providing pressurised oil during in-flight shutdown of a variable pitch propeller engine, comprising the steps of: providing a propeller pump adapted to supply pressurized oil during engine operation from a main pressurised oil circuit to a propeller pitch actuation system, and upon in-flight shutdown of the engine, selectively connecting the propeller pump to a secondary pressurized fluid circuit, the circuit including a secondary pump operable independently of operation of the engine to supply pressurised oil during said engine shutdown. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment thereof, and in which: 
       FIG. 1  is a schematic cross-sectional view of a turboprop gas turbine engine to which a preferred embodiment of the present invention is applied; 
       FIG. 2  is a schematic view of a system for providing a pressurized oil supply to a propeller pump in the event of an in-flight shutdown of the turboprop engine shown in  FIG. 1 , the system being shown in a normal engine operation state when the propeller pump is fed from the turboprop engine lubrication system; and 
       FIG. 3  is a schematic view of the system shown in operation during an in-flight engine shutdown when the regular supply of oil from the turboprop engine lubrication system is not available. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a turboprop engine  10  of a type preferably provided for use in subsonic flight to drive a propeller  12  via a reduction gear box (RGB)  14 . The engine  10  comprises a first rotating assembly consisting of a turbine  16  and a compressor  18 , and a second rotating assembly consisting of a power turbine  20  mounted on a power turbine shaft  22 . The first and second rotating assemblies are not connected together and turns at different speed and in opposite directions. This design is referred, to as a “free turbine engine”. It is understood that the present invention could be applied to other types of propeller engines as well. 
   The compressor  18  draws air into the engine  10  via an annular plenum chamber  24 , increases its pressure and delivers it to a combustor  26  where the compressed air is mixed with fuel and ignited for generating a stream of hot combustion gases. The compressor turbine  16  extracts energy from the hot expanding gases for driving the compressor  18 . The hot gases leaving the compressor turbine  16  are accelerated again as they expand through the power turbine  20 . The power turbine  20  provides rotational energy to drive the propeller  12 . The RGB  14  reduces the power turbine  20  speed to one suitable for the propeller  12 . 
   The propeller  12  is provided with variable pitch (angle) blades  28 . A propeller control and actuator unit  30  ( FIG. 2 ) adjusts the blade angle to maintain the propeller speed that is selected by the pilot. When more power is applied, the angle of attack of the propeller blades  28  is increased to allow the propeller  12  to absorb the additional energy without increase in propeller speed. The propeller control and actuator unit  30  can be of the type including a feather return spring and/or counterweights (not shown) for biasing the propeller blades  28  to a feather position (coarse pitch) and a servo piston (not shown) fed with pressurized oil for overcoming the coarse pitch biasing forces and, thus, displace the propeller blades  28  in a finer pitch or low blade angle position. When oil pressure is decreased, the coarse pitch biasing forces push the oil out of the servo piston and change the blade pitch to a coarser position. An increase in oil pressure, controlled by the propeller control unit  30 , drives the blades  28  towards a finer pitch. 
   As shown in  FIG. 2 , a propeller pump  32  provides the pressurized flow of oil to the propeller control and actuator unit  30  in order to actuate the propeller blades  28 . The oil supply for the propeller pump  32  is typically provided by the RGB lubrication system which is, in turn, fed by the engine lubrication pump (not shown) driven by the compressor turbine  16  of the engine  10 . In contrast, the propeller pump  32  is driven by the power turbine  20  via the RGB  14  and will thus still be operable in the event of in-flight engine shutdown conditions. However, under such shutdown conditions, the compressor turbine  16  is stationary or rotating at very low speed and, hence, the engine lubrication pump (not shown), which feeds the lubrication system of the RGB  14  and the propeller pump  32 , is unable to maintain an adequate oil supply to the propeller pump  32  in order to provide good lubrication for non-feathered operation of the propeller blades  28 . 
   It is herein proposed to overcome this problem by selectively connecting the propeller pump  32  to a secondary pressurized oil circuit  31  when the above-described main source of pressurized fluid is not available. 
   To this end, a switching mechanism, such as a multi-landed spool and sleeve valve  38 , is provided to selectively connect the engine/RGB lubrication system (indicated as the engine lubrication supply in  FIG. 2 ) or the secondary pressurized fluid circuit  31  to the propeller pump  32 . As will be seen hereinafter, the valve  38  preferably has at least two states of operation, one to support normal operation of the propeller  12  with the engine running ( FIG. 2 ) and one for supporting propeller operation when the engine is shut down ( FIG. 3 ). 
   In this embodiment, the secondary circuit  31  comprises an ejector-style pump  33  having an outlet line  37  and an inlet line  35  respectively selectively connectable to the inlet line  34  and the outlet line  36  of the propeller pump  32  through valve  38 . The ejector pump  33  is located in the sump  39  of the RGB  14  where, during normal conditions, the oil is picked up and returned back to the main engine oil tank (not shown) as indicated at  41 . In  FIGS. 2 and 3 , the ejector pump  33  is shown mounted within the RGB  14  although in practice the ejector pump  33  could be located either inside or outside the RGB cavity, or other suitable location. For instance, the ejector pump  33  could function independently of the engine accessory gearbox and draw oil from a separate emergency reservoir instead of from the RGB sump  39  if desired. In the illustrated example, the RGB sump  39  is also used to provide an oil supply to a back-up feathering pump (not shown) via a feathering line  43  at the bottom of the sump  39 . In this case, the inlet to the ejector pump  33  is preferably raised off the bottom of the sump  39  above the back-up feathering line  43 , but below scavenge return line  41  to ensure that a sufficient quantity of oil is left within the sump  39  regardless of the ejector pump operation. 
   In the illustrated embodiment, the valve  38  preferably comprises three rigidly interconnected valve members  40   a ,  40   b ,  40   c  biased to the position shown in  FIG. 3  by a compression spring  42 ; In addition to the biasing force exerted by the spring  42 , a pressure corresponding to the RGB cavity pressure (or any other suitable low reference pressure source) is exerted on the right hand side of the valve member  40   c  in order to further urge the three valve members  40   a ,  40   b  and  40   c  to the position shown in  FIG. 3 . The valve member  40   c  is exposed to the RGB cavity pressure through port  46 . 
   As shown in  FIG. 2 , under normal engine operation, the propeller pump  32  is fed by the engine/RGB lubrication system via feed line  48 . A portion of the flow coming from the engine/RGB lubrication system is diverted via branch line  50  into the left hand side of the valve member  38  to exert a pushing force against the valve member  40   a . During normal engine operation, the pressure of the incoming oil is greater than the combined action of the biasing force of the spring  42  and the RGB cavity pressure on the valve member  40   c , thereby causing the three valve members  40   a ,  40   b  and  40   c  to slide to the right against the spring  42 . In this position, which corresponds to the first state of the switching system, the inlet line  34  of the propeller pump  32  is connected to the feed line  48 , as indicated by arrows  52 . 
   In this first state, when the engine  10  is running, the valve members  40   a  and  40   b  respectively disconnect the inlet line  35  and the outlet line  37  of the ejector pump  33  from the outlet line  36  and the inlet line  34  of the propeller pump  32 , thereby completely disconnecting the secondary circuit  31  from the propeller pump  32 . The inlet of the propeller pump  32  is thus solely pressurized by the engine/RGB lubrication system and there is no flow of oil through the secondary circuit  31 . The oil pumped by the propeller pump  32  is directed into the propeller control and actuation unit  30  via output line  36  as indicated by arrows  53  in  FIG. 2 . This provides for the operation of the variable pitch propeller blades  28 . 
   In the event that the oil pressure in the feed line  48  drops below a predetermined value corresponding to the pressure exerted on the right hand side of the valve member  40   c , the rigidly interconnected valve members  40   a ,  40   b  and  40   c  jointly slide to the left to the position shown in  FIG. 3  under the action of the spring  42  and the RGB cavity pressure. This will occur, for example, in the event of an in-flight shutdown of the engine. In the position shown in  FIG. 3 , the valve member  40   b  disconnects the feed line  48  from the propeller pump inlet and fluid flow connection is established between 1) the ejector pump outlet line  37  and the propeller pump inlet line  34  and 2) the ejector pump inlet line  35  and the propeller pump outlet line  36 . 
   The surplus propeller pump flow is used to operate the ejector pump  33  to pressurize the propeller pump inlet in order to support non-feathered operation of the propeller blades  28  when the regular pressurized oil supply from the engine/RGB lubrication system is not available, such as during in-flight shutdown of the engine. The flow requirements for the propeller pump  32  are generally sized for ground handling operation, particularly transients in to and out of reverse pitch. As such, during in-flight conditions, the available flow from the propeller pump  32  is considerably in excess of that required for propeller control system operation. 
   The motive flow  61  (i.e. the surplus propeller pump flow) from the propeller pump outlet line  36  and the ejector pump inlet line  35  causes oil in the sump  39  to be drawn into the ejector pump  33 , as indicated by arrows  63 . As indicated by arrows  65 , the oil drawn from the sump  39  by the ejector pump  33  flows through the ejector outlet line  37  and then through the propeller pump inlet line  34 , thereby ensuring a continuous pressurized supply of oil to the propeller pump inlet. A first portion  67  of the flow from the propeller pump  32  is used to operate the propeller blades  28  and the surplus of oil is returned to the ejector pump inlet to act as the motive flow  61  to continuously draw oil from the sump  39 . 
   To minimize the clogging of the ejector pump  33 , the components of the propeller actuation system ( 30 ,  32 ) and the secondary circuit components resulting from contaminant residing in the sump oil, it is recommended that a filter or strainer  60  be mounted in one of the line of the secondary circuit  31 , preferably in line  35  and preferably upstream of the above components. 
   As indicated at  68  and  69 , it is also contemplated to use the surplus oil flow from the ejector pump  33  and/or the propeller pump  32  (in excess to that required to satisfy the propeller control and actuation unit) for any other suitable purpose, such as to provide back-up lubrication to critical propeller reduction gearbox gears and bearings or other components which are rotating during engine shutdown/propeller windmilling mode or to provide a oil heating source during high altitude operation where outside air temperatures are very low. This is particularly useful for propeller windmilling for extended periods that may be undertaken to maximize the range of aircraft with one or more engines inoperative. 
   The above-described invention is advantageous in that it provides for a continued supply of pressurized oil which may be used, for example, in the non-feathered operation of the propeller blades  28  during in-flight shutdown or during other situations when the regular oil supply is unavailable. Non-feathered operation of the propeller blades  28  during in-flight shutdowns advantageously permits to reduce the induced drag on the aircraft or to allow the operation of accessories mounted on accessory drives of the RGB  14 . A continuing supply of pressurized oil permits continued operation of certain engine subsystems despite the inoperative status of the engine. 
   The embodiments of the invention described above are intended to be exemplary. Those skilled in the art will therefore appreciate that the forgoing description is illustrative only, and that various alternatives and modifications can be devised without departing from the spirit of the present invention. Accordingly, the present is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims. For instance, although the above-described switching mechanism changes state based on engine oil pressure, it is understood that a similar parameter indicative of the operational status of the engine, such as engine fluid pressure, fuel pressure, shaft rotation speed, interturbine temperature, etc., could also be used to automatically actuate the switching mechanism, or manual switching may also be provided. Also, the ejector pump could be replaced by any suitable passive pump or active pump or pump-like device, such as a centrifugal pump or a gear pump. The term “passive pump” is herein intended to encompass any pump which does not require to be driven off from a driving source in order to pump a fluid. The term “active pump” is herein intended to means any pump which must be connected to a driving source to be operative. A passive pump is preferred due to its ability to operate without an external supply of mechanical or electric power. If an active pump is used, it is preferably driven from the windmilling propeller shaft or another non-engine direct power source, such as an electric motor driven pump or hydraulic motor driven pump, in order to be self-sufficiently operative during the engine shutdown condition. The second pump may be located in any suitable location. Other sources of pressurized fluid (e.g. fuel) may be used to actuate control system for the propeller blades  28 . The invention is also not limited in application to a propeller engine, but may be applied to any engine which windmills in an in-flight shutdown condition. Still other modifications will be apparent to the skilled reader, and therefore within the scope of the attached claims.