Patent Application
20110158808
The present invention is directed to propeller blades and spinners for use in aircraft propulsion systems such as turboprop engines. More particularly, the present invention relates to flow of air between roots of propeller blades near inner diameter spinners.
Turboprop engines are well suited to large payload capacity cargo planes due to their high propulsive efficiency, i.e. their ability to turn power into usable thrust, which results in high fuel efficiency. In turboprop gas turbine engines, a turbine is used to drive large diameter propeller blades at speeds slower than at which the turbine rotates. In a typical configuration, a gas turbine engine comprising a compressor and a turbine is used to sustain a combustion process and produce hot gas for driving a free turbine. The free turbine, in turn, is used to drive a gearbox that rotates a hub on which the propeller blades are mounted at their roots. In order to maintain engine efficiency during flight, turboprop engines are operated at a constant, peak-efficiency speed. A spinner, which comprises an aerodynamic faring, is mounted at the inner diameter ends of the propeller blades to cover the hub and reduce drag.
As thrust and fuel efficiency demands on modern engines grow, the number of propeller blades is being increased. These increases, however, reduce the gap between adjacent blades, as there is limited circumferential area around the hub and spinner to space additional blades. The reduction in gap size, however, produces drag and the potential for choked flow conditions near the inner diameter ends of the propeller blades as the air becomes constricted. Previous attempts to control airflow near the blade roots involved contouring the surface of the spinner to manipulate the air velocity, often using expensive manufacturing processes. For example, the spinner is shaped to prevent choking by increasing the flow area to slow the velocity of airflow over the spinner and between the propeller blades to subsonic levels. However, as more propeller blades are used, there is less surface area of the spinner available to increase the flow area and induce other flow alterations. There is, therefore, a need for improving control over blade root flow.
The present invention is directed to systems and methods for controlling flow conditions between blade root portions of propeller blades used in propulsion systems. A propeller blade root airflow control system comprises a spinner for shrouding a propeller hub and propeller roots. The spinner has root holes and airflow ducts. The root holes are disposed about a spinner circumference for receiving propeller roots. The airflow ducts are disposed about a spinner circumference for drawing air into the spinner and away from the root holes. A method for preventing choked flow conditions between adjacent propeller blades in the area of a spinner in a propulsion system comprises rotating the spinner and propeller blades to produce propulsive thrust by driving an air stream past the propeller blades and spinner, and drawing a portion of the air stream into a plurality of ducts disposed within the spinner to prevent choked flow conditions.
Air traveling between propeller blades 14 can reach sonic speeds where choked conditions may occur. In particular, choking is more likely to occur at the root portions of propeller blades 14 near spinner 12. Spinner 12 comprises the innermost boundary for air flowing past propeller blades 14 and the position where the area between adjacent propeller blades 14 is at a minimum. Thus, air flowing between propeller blades 14 is most likely to become constricted in the vicinity of spinner 12. There is, therefore, potential for choked flow conditions, particularly at high aircraft speeds. During choked flow conditions, the volume of air being driven by propeller blades 14 is greater than what the space between adjacent propeller blades 14 allows to flow unrestricted. Choked flow produces drag and shock waves that reduce the efficiency of propulsion system 10.
Airflow ports 26 comprise openings within spinner 12 that provide alternate flow paths for air flowing between propeller blades 14. Airflow ports 26 comprise shallow ramps that siphon air off of spinner 12 and discharge the air into the interior of spinner 12. Airflow ports 26 are positioned on spinner 12 so as to draw in air at positions around spinner 12 where air flow has the potential to become choked or constricted. Thus, airflow ports 26 are positioned on spinner 12 near root portions of propeller blades 14 and near holes 24. In one embodiment, airflow ports 26 are disposed between adjacent holes 24 such that airflow ports 26 and root holes 24 alternate around a circumference of spinner 12, as shown in
In any embodiment, airflow ports 26 are positioned to draw in air that would otherwise become choked when attempting to flow between adjacent propeller blades 14 under choked flow conditions. The number of airflow ports 26 and the surface area of spinner 12 that they encompass are selected to allow a sufficient amount of airflow into spinner 12 to prevent choked conditions from arising at each propeller 14. In one embodiment, the number of airflow ports 26 equals the number of propeller blades 14 or holes 24. The total surface area of airflow ports 26 is not so large so as to produce unacceptable efficiency losses of propeller blades 14. For example, efficiency losses due to ports 26 are less than the efficiency losses from drag produced by choked flow at the root portions of propeller blades 14. Thus, airflow ports 26 are shaped so as to minimize drag and disruption of air flowing around spinner 12. In the embodiment shown, airflow ports 26 comprise ducts having narrow inlet portions and wider outlet portions. In one particular embodiment, airflow ports 26 comprise duct inlets developed by the National Advisory Committee for Aeronautics (NACA), as are known in the art. These NACA ducts are described fully in “An Experimental Investigation of NACA Submerged-Duct Entrances” by Charles W. Frick, Wallace F. Davis, Lauros M. Randall and Emmet A. Mossman, Ames Aeronautical Laboratory, Moffett Field, Calif., 1945 (NACA ACR 5120).
The air, once introduced to the interior of spinner 12 can be used for a variety of purposes. For example, air from airflow ports 26 can be used to cool various engine components. Such aspects of the present invention are better described with reference to
Propulsion system 10 comprises a forward oriented turboprop, or puller prop, configuration. Nacelle 22 is mounted to gas turbine engine 20 (
In one embodiment, spinner 12 comprises a parabolic nose portion in which cooling hole 38 is disposed, and a cylindrical aft portion in which holes 24 and ports 26 are disposed. Holes 24 are provided in spinner 12 to allow blade root portions 36 of propeller blades 14 to extend through spinner 12 such that airfoil portions 34 extend into the air surrounding system 10. Holes 26 and 38 are provided in spinner 12 to allow airflow into the interior of propulsion system 10. Spinner 12 may also be shaped to manipulate airflow over its surfaces. For example, the portion of spinner 12 near airfoils 34 of propeller blades 14 can be contoured to increase surface area to slow the flow of air over spinner 12 to prevent choking.
Propeller blades 14 are rotated about shaft 30 to produce thrust and drive propulsion system 10 in a forward direction. As such, air streams A flow over spinner 12. Blade pitch actuation mechanism 32 adjusts the pitch of propeller blades 14 to adjust the angle of incidence of airflow A over the surfaces of airfoil portions 34. For example, propeller blades 14 can be adjusted between a reverse thrust position and a low drag feather position. Actuation mechanism 32 is controlled by a central engine controller such as a FADEC (Full Authority Digital Engine Controller) to adjust the pitch of propeller blades 14 as desired during operation of propulsion system 10. Actuation mechanism 32 comprises any conventional system for adjusting the pitch of propeller blades in turboprop systems as are known in the art. In various embodiments, actuation mechanism 32 comprises actuation and control systems such as are described in U.S. Pat. No. 6,811,376 to Arel et al., U.S. Pat. No. 6,981,844 to Perkinson et al. and U.S. Pat. No. 7,172,391 to Carvalho, which are assigned to Hamilton Sundstrand, Windsor Locks, Conn. In typical embodiments, a pump (not shown) provides a pressurized hydraulic fluid to piston cylinder 44, which causes actuator piston 40 to undergo an axial displacement. Axial displacement of cylinder 44 causes rotation of actuator arms 42. Displacement of actuator arms 42 causes propeller stems 46 to rotate in the sockets of hub 16 such that the pitch of airfoil portions 34 is adjusted.
During operation of propulsion system 10, air streams A travel along the surface of spinner 12 toward propeller blades 14. Likewise, other air streams are generated in the space surrounding spinner 12 that travel at high velocities toward propeller blades 14. A convergence of air streams occurs near spinner 12 and airfoil portions 34 of propeller blades 14. For example, air stream B, which is incident on one of propeller blades 14, must be directed around the propeller to flow between adjacent propeller blades. As such, the potential for choked flow conditions arises near the intersection of spinner 12 with airfoils 34. In order to reduce the potential for choked flow, airflow ports 26 are positioned around spinner 12 to provide additional flow paths for air flowing around spinner 12. For example, air stream C is able to penetrate spinner 12 at airflow port 26 to enter the interior of spinner 12. Airflow port 26 comprises an inwardly sloped indentation of spinner 12 that terminates in a planar opening perpendicular to the direction of motion of propulsion system 10, as is typical in NACA inlet ducts. In other embodiments, however, airflow ports 26 may comprise simple openings, bores or holes in spinner 12 similar to that of propeller holes 24. Thus, airflow ports 26 relieve some of the air flow burden from the space between adjacent propeller blades 14.
Flow of air into airflow ports 26 is induced by the shape of ports 26, which draw or suck air into spinner 12. Airflow ports 26 can be provided with appropriate ductwork to connect ports 26 to various portions of system 10. In the embodiment shown, duct 50 connects to airflow ports 26 to route air stream D toward components inside of propulsion system 10 that benefit from cooling. For example, air stream E can be used to cool lubricating fluid used in gearbox 18 or routed aft to cool engine components within the nacelle 22, and air stream F can be used to cool hydraulic fluid used in actuation mechanism 32. In various embodiments, other types and shapes of ducts can be used to connect ports 26 to different parts of system 10, including ductwork for passing through bulkhead 15.
Additionally, spinner 12 is provided with other holes or intake inlets to allow air into propulsion system 10 to perform various functions. For example, cooling hole 38 allows air into spinner 12 to cool other system of propulsion system 10. Cooling hole 38 is positioned at the nose or tip of spinner 12 along the axis of rotation of shaft 30 to allow air stream G, which is mostly incident on the tip of spinner 12, to enter the interior of spinner 12. Air stream G can be used to cool piston cylinder 44 by direct convection, or can be routed by appropriate ductwork to cool other components. Cooling hole 38, however, adds aerodynamic drag to spinner 12, and could be replace by airflow ports 26.
Spinner 12 provides a platform into which various air inlets are provided to improve propulsion efficiency and cooling in propulsion system 10. Airflow ports 26, when operated in conjunction with the systems and methods of the present invention, prevent choked flow conditions from arising between propeller blades 14 near spinner 12, increasing the overall efficiency of propulsion system 10. Air introduced into spinner 12 from ports 26 can be used to provide cooling to propulsion system 10 or air to gas turbine engine 20. The airflow system and method of the present invention allow large numbers of propeller blades 14 to be mounted through spinner 12 and allow the speeds of the aircraft to which propulsion system 10 is attached to be increased. Furthermore, the need for expensive manufacturing of spinner 12, arising from machining and shaping of the root region of spinner 12, can be reduced or eliminated with the airflow system and method of the present invention.
While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.
