Patent Publication Number: US-9849968-B2

Title: Propeller

Description:
TECHNICAL FIELD 
     This disclosure relates to an apparatus and method for use of a propeller and, more particularly, to a ring propeller for attenuating volume dependent thickness noise amplitude. 
     BACKGROUND 
     Many manned and unmanned (“UAVs”) aircraft driven by propeller are susceptible to ground threats such as small arms fire and manned-portable air defense systems (“MANPADS”). Given the nature of typical missions and operations, it may be desirable to reduce audible detectability. The ability to cancel or substantially reduce critical tones of a propeller system&#39;s acoustic signature may be important in reducing these vehicles&#39; acoustic signatures and enhancing mission effectiveness. Small UAVs typically use fixed pitch propellers which are neither subject to the complexities nor the stresses of variable pitch propellers which are used for manned vehicles or large UAVs. Hence, innovative propeller concepts which are subject to structural constraints may be better implemented on these less-complex systems, compared to manned vehicles or large UAVs. 
     SUMMARY 
     In an embodiment, a propeller is described. A hub coaxially surrounds a longitudinal axis. A ring shroud coaxially surrounds the longitudinal axis and is spaced radially from the hub. At least one propeller blade is fixedly attached to both the hub and ring shroud and extends radially therebetween for mutual rotation therewith. At least one stub blade has a first stub end radially spaced from a second stub end. The first stub end is fixedly attached to a selected one of the hub and ring shroud. The second stub end is cantilevered from the first stub end and is radially interposed between the first stub end and the selected one of the hub and ring shroud. 
     In an embodiment, a propeller is described. A hub coaxially surrounds a longitudinal axis. A ring shroud coaxially surrounds the longitudinal axis and is spaced radially from the hub. A plurality of motive blades extends radially across at least a portion of the distance between the hub and the ring shroud. Each motive blade has a blade root directly attached to a chosen one of the hub and the ring shroud, for rotation about the longitudinal axis due to the attachment to the chosen one of the hub and the ring shroud, and a blade tip extending toward the other one of the hub and the ring shroud. At least one selected blade tip is directly attached to the other one of the hub and the ring shroud. At least one other blade tip is cantilevered from the blade root and is radially spaced apart from the other one of the hub and the ring shroud. 
     In an embodiment, an aircraft is described. The aircraft includes a body, at least one fixed wing and at least one propeller mount extending from the body, and at least one drive shaft positioned within a corresponding at least one propeller mount and drivable by a motor or gear/clutch system. At least one propeller is operationally attached to the at least one drive shaft to obtain motive power therefrom. The propeller includes a hub coaxially surrounding a longitudinal axis. A ring shroud coaxially surrounds the longitudinal axis and is spaced radially from the hub. A plurality of motive blades extends radially across at least a portion of the distance between the hub and the ring shroud. Each motive blade has a blade root directly attached to a chosen one of the hub and the ring shroud, for rotation about the longitudinal axis, under motive power from the drive shaft, due to the attachment to the chosen one of the hub and the ring shroud, and a blade tip extending toward the other one of the hub and the ring shroud. At least one selected blade tip is directly attached to the other one of the hub and the ring shroud. At least one other blade tip is cantilevered from the blade root and is radially spaced apart from the other one of the hub and the ring shroud. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding, reference may be made to the accompanying drawings, in which: 
         FIG. 1  is a schematic front view of one embodiment; 
         FIG. 2  is a perspective view of the embodiment of  FIG. 1 ; and 
         FIG. 3  depicts the embodiment of  FIG. 1  in an example use environment. 
     
    
    
     DESCRIPTION OF ASPECTS OF THE DISCLOSURE 
     The invention comprises, consists of, or consists essentially of the following features, in any combination. 
     The Figures depict an example of a High Attenuation, Low Observable (“HALO”) ring propeller for delaying the onset of aural detection by a human observer. Propulsion mechanisms are generally the primary offending source mechanism for all modern day aircraft, with the exception of some ultralights and alternative energy designs. Other contributing sources include, but are not limited to, airframe, exhaust, and fan noise. Propellers are among the greatest acoustic challenges in developing propulsion noise reduction technologies for attenuating volume dependent thickness noise amplitude. The HALO propeller system responds to this challenge by utilizing an inverted blade, segmented annulus design with incremental spacing to attenuate noise via increased blade passage frequency, and distributed blade loading. This design may assist with both high solidity and asymmetric spacing for temporal phase mismatching. The annulus minimizes thickness noise by reducing transverse forces across the blades. An “in-unison” rotation may help to enhance aerodynamic performance and propulsive efficiency by reducing vortex shedding at low tip speeds produced by the inverted blades. In many cases, high efficiency has positive correlation to low noise. Potential aerodynamic benefits indicate HALO to be a viable noise reduction technology. 
       FIG. 1  depicts an example propeller  100  which uses a ducted HALO concept with incremental blade spacing to increase blade passage frequency and reduce blade loading. The propeller includes a hub  102  coaxially surrounding a longitudinal axis  104  (seen end-on in  FIG. 1 ). The term “coaxial” is used herein to indicate that the objects described as such have coincident axes (here, both the hub  102  and the propeller  100  share the longitudinal axis  104 ). 
     A ring shroud  106  coaxially surrounds the longitudinal axis  104  and is spaced radially from the hub  102 . A “radial” direction, as used here, is a direction toward and away from the longitudinal axis  104 , in the plane of the page of  FIG. 1 . A plurality of motive blades  108 , of any mix of types as will be described below, each extend radially across at least a portion of the distance R between the hub  102  and the ring shroud  106 . Each motive blade  108  has a blade root  110  attached (e.g., directly attached) to a chosen one of the hub  102  and the ring shroud  106 , for rotation of that motive blade  108  about the longitudinal axis  104  due to such attachment. Each motive blade  108  also has a blade tip  112  extending radially away from the blade root  110 , toward the other one (i.e., the one to which the blade root  110  is not directly attached) of the hub  102  and the ring shroud  106 . 
     As can be seen in  FIG. 1 , certain of the motive blades  108  are propeller blades  108 A, having both the blade root  110  and the blade tip  112  for that propeller blade  108 A directly attached to respective ones of the hub  102  and the ring shroud  106 , with the body of the propeller blade spanning the distance R. (It should be noted that the identification of a particular end of a motive blade  108  as a blade root  110  or blade tip  112  is done herein for orientation purposes only, and no indication or significance of a particular structural feature is implied or intended by this orienting terminology.) 
     Other ones of the motive blades  108  are stub blades  108 B, having the blade root  110  directly attached to a chosen one of the hub  102  and the ring shroud  106  (the blade roots  110  of the stub blades  108 B are shown here as being attached to the ring shroud  106 , but could be instead attached to the hub  102  by one of ordinary skill in the art, as desired for a particular use environment). In contrast to the propeller blades  108 A, however, stub blades  108 B each have a blade tip  112  that is cantilevered from the blade root  110  and is radially spaced apart from the other one of the hub  102  and the ring shroud  106  (here, the stub blade  108 B blade tips  112  are radially spaced from the hub  102 ). The term “cantilevered” is used herein to indicate a projecting beam or other horizontal member supported at one or more points (e.g., the blade root  110 ) but not at both ends. 
     In other words, the stub blades  108 B are each directly attached to a chosen one of the hub  102  and the ring shroud  106  but only span a portion of the distance R to the other one of the hub  102  and the ring shroud  106 . Stated differently, at least one stub blade  108 B may have a first stub end  114  radially spaced from a second stub end  116 , the first stub end  114  being fixedly attached to a selected one of the hub  102  and ring shroud  106 . The second stub end  116  is then cantilevered from the first stub end  114  and is radially interposed between the first stub end  114  and the selected one of the hub  102  and ring shroud  106 . 
     At least one propeller blade  108 A is fixedly attached to both the hub  102  and the ring shroud  106  and extends radially therebetween for mutual rotation therewith. That is, the hub  102 , propeller blades  108 A (four shown in  FIG. 1 ), and ring shroud  106  are attached together and rotate about the longitudinal axis  104  as a unit, under motive force. Generally, the motive force will be provided to the hub  102  via a drive shaft (not shown) extending along the longitudinal axis  104 , but it is contemplated that other drive means, of any desired type, may exert motive force upon any structure (e.g., the ring shroud  106 ) of the described propeller  100 . 
     The motive blades  108  of the propeller  100  may be arranged in any desired circumferential sequence(s) or grouping(s) about a perimeter  118  of the hub  102 . For example, the propeller blades  108 A shown in  FIG. 1  are circumferentially spaced about the perimeter  118  of the hub  102 . The term “circumferentially” is used herein to indicate a circular direction which is centered on the longitudinal axis  104 , such as the counterclockwise direction indicated by arrow CCW in  FIG. 1 . Each circumferentially adjacent pair of propeller blades  108 A in  FIG. 1  is shown as having at least one stub blade  108 B interposed circumferentially therebetween, though such is not required. Optionally, two or more stub blades  108 B could be interposed circumferentially between a circumferentially adjacent pair of propeller blades  108 A, or vice versa. 
     The arrangement of propeller blades  108 A and stub blades  108 B may be optionally, though not necessarily, done in a rotationally symmetrical manner. That is, the propeller  100  is “rotationally symmetrical” if it can be rotated less than 360° around the longitudinal axis  104  and still match its appearance before the rotation occurred. Other, nonlimiting options for potentially rotationally symmetrical arrangements include a pair of circumferentially adjacent propeller blades  108 A with no circumferentially interposed stub blades  108 B, a pair of circumferentially adjacent propeller blades  108 A with three or more circumferentially interposed stub blades  108 B, and a pair of circumferentially adjacent propeller blades  108 A with one circumferentially interposed stub blade  108 B. If the arrangement of propeller blades  108 A and stub blades  108 B is done in a rotationally asymmetrical manner, it may be desirable to balance the system for better temporal phase matching of thickness and loading noise sources, such as by locating lighter and/or smaller blades in areas of more concentrated spacing. 
     As is known to one of ordinary skill in the propeller arts, one or more of the motive blades  108  may be angled in a selected “twist direction”, as can be seen in the perspective view of  FIG. 2 . The cross-sectional shape of the motive blade  108  changes over the length of the motive blade  108 , resulting in a twist, as shown. Optionally, the blade root  110  and/or blade tip  112  of a single blade  108  may be attached to a respective hub  102  or ring shroud  106  at an angle to aid with creating, maintaining, and/or carrying out a particular twist configuration. The twist helps the propeller  100  produce thrust, and a twist design considers factors including lift, relative speed of the motive blade  108  at various points along its radial length (e.g., along distance R), angle of attack, the weight of the aircraft, the speed of the propeller  100  (RPM), the power of the engine, and the final thrust required to maintain flight. 
     Optionally, selected motive blades  108  of the propeller  100  could be angled in the same or different twist directions from other motive blades  108  of the same propeller. For example, some or all of the propeller blades  108 A could be angled in a first twist direction, while some or all of the stub blades  108 B could be angled in a second twist direction which is substantially opposite the first twist direction. As another example, it is generally contemplated that at least one motive blade  108  (of any type) angled in the first twist direction may be directly circumferentially adjacent to at least one motive blade  108  (of any type, whether or not the same type as the first motive blade in this example) angled in the second twist direction. The twist direction(s) for a particular propeller  100  may be chosen and assigned as desired to various one(s) of the motive blades  108  (e.g., the propeller blades  108 A and/or stub blades  108 B) by one of ordinary skill in the art based on any desired factors, such as, but not limited to, achieving particular vortex properties during use of the propeller  100  and controlling tip speeds of the propeller blades  108 A and/or stub blades  108 B toward the hub  102 . 
     In one example configuration, the stub blades  108 B could be at a minimum twist angle at the ring shroud  106  and approach a maximum twist angle at the end of the stub blade  108 B located radially nearest the hub  102 , while the propeller blades  108 A could simultaneously exhibit the inverse variation, having a relatively steep pitch at the hub  102  and achieving a shallower twist angle as the propeller blades  108 A extend towards the ring shroud  106 . 
     The propeller  100  shown in  FIG. 1  can be used for attenuating volume dependent thickness noise amplitude, particularly for small UAVs, over that which is currently available. A propeller  100 , such as that depicted, is rotated in a first direction at a first rotational speed such as, for example, by motive power supplied by a drive shaft (not shown) extending along the longitudinal axis  104  and operatively connected to the hub  102 . In other words, the hub  102 , ring shroud  106 , and motive blades  108  (including any propeller blade(s)  108 A and stub blade(s)  108 B provided to the propeller  100 ) are rotated in the first rotational direction at the first rotational speed. The propeller  100  should be configured to provide a blade passage frequency configured to absorb into an atmosphere surrounding the propeller  100 , to substantially reduce audible detection range from an art-recognized value (e.g., a value currently achieved by commercially available small UAVs and/or toward a mission, immersed-background, and altitude-dependent parameter). 
     Enhanced aerodynamic performance and propulsive efficiency may be attained by minimizing the vortex shedding from the stub blades  108 A via decreased tip speeds. The increase in blade passage frequency will reduce detection by taking advantage of atmospheric absorption. The propeller  100  uses inverted blades, such as the stub blades  108 B, to take advantage of increased frequencies generated by high solidity. The inverted, or stub, blades also serve to reduce vortices, thus reducing drag due to thrust via decreased tip speeds. Moreover, the ring shroud  106  is used at least partially to increase circumferential spacing for the addition of inverted/stub blades to reduce loading noise while also reducing thickness noise via mitigating transverse forces across the blades. 
     It is contemplated that a propeller  100 , such as that shown in  FIG. 1 , may remain a relevant technology in the event aeroacoustic performance is substantially enhanced at the expense of aerodynamic operating efficiencies. 
       FIG. 3  depicts an example use environment for the propeller  100 . An aircraft  120  is shown in  FIG. 3  as a small UAV, but suitable use environments for the propeller  100  include, as nonlimiting examples, fixed-wing aircraft, helicopters or other rotor-driven aircraft, small UAVs, large UAVs, jet turbines, gas turbines, hydroelectric turbines, or any other desired use environments. Any number of propellers  100  can be provided to an aircraft  120 , as desired, though a single propeller is shown in the Figures. The propeller(s)  100  could be in any suitable position or physical relationship to the other structures making up the aircraft  120 . The aircraft  120  shown in  FIG. 3  includes a body  122 , at least one fixed wing  124  (two shown), and at least one propeller mount  126  (one shown) extending from the body  122 . At least one drive shaft indicated schematically at  128 ) is positioned within a corresponding at least one propeller mount  126  and is drivable by a motor or gear/clutch system (indicated schematically at  129 ) to provide a source of rotationally oriented motive power. The propeller  100  is operationally attached to the drive shaft  128 , optionally indirectly such as via a gearbox (not shown), to obtain motive power therefrom. 
     The stub blades  108 B are depicted in  FIG. 1  as all being directly attached to the ring shroud  106  and radially spaced from the hub  102 . Alternatively, though not shown, the stub blades  108 B could instead be directly attached to the hub  102  and radially spaced from the ring shroud  106 . It is additionally contemplated that, for a particular propeller  100 , some of the stub blades  108 B could be directly attached to the hub  102  and others of the stub blades  108 B could be directly attached to the ring shroud  106 . One of ordinary skill in the art could determine a desired number, orientation, spacing, length(s), configuration, arrangement, or other physical properties of the motive blades  108  for a particular use environment. 
     While aspects of this disclosure have been particularly shown and described with reference to the example embodiments above, it will be understood by those of ordinary skill in the art that various additional embodiments may be contemplated. For example, the specific methods described above for using the apparatus are merely illustrative; one of ordinary skill in the art could readily determine any number of tools, sequences of steps, or other means/options for placing the above-described apparatus, or components thereof, into positions substantively similar to those shown and described herein. In an effort to maintain clarity in the Figures, certain ones of duplicative components shown have not been specifically numbered, but one of ordinary skill in the art will realize, based upon the components that were numbered, the element numbers which should be associated with the unnumbered components; no differentiation between similar components is intended or implied solely by the presence or absence of an element number in the Figures. The propeller  100  could be used in any application or use environment wherein a fluid (e.g., liquid, gas, or any other material behaving in a fluid-like manner) interacts with a rotating structure (i.e., the propeller) to exchange (e.g., remove and/or provide) energy and/or motive power between the two. Any of the described structures and components could be integrally formed as a single unitary or monolithic piece or made up of separate sub-components, with either of these formations involving any suitable stock or bespoke components and/or any suitable material or combinations of materials. Any of the described structures and components could be disposable or reusable as desired for a particular use environment. Any component could be provided with a user-perceptible marking to indicate a material, configuration, at least one dimension, or the like pertaining to that component, the user-perceptible marking aiding a user in selecting one component from an array of similar components for a particular use environment. A “predetermined” status may be determined at any time before the structures being manipulated actually reach that status, the “predetermination” being made as late as immediately before the structure achieves the predetermined status. The term “substantially” is used herein to indicate a quality that is largely, but not necessarily wholly, that which is specified—a “substantial” quality admits of the potential for some relatively minor inclusion of a non-quality item. Though certain components described herein are shown as having specific geometric shapes, all structures of this disclosure may have any suitable shapes, sizes, configurations, relative relationships, cross-sectional areas, or any other physical characteristics as desirable for a particular application—e.g., certain of the stub blades  108 B could be longer or shorter than others of the stub blades  108 B. Any structures or features described with reference to one embodiment or configuration could be provided, singly or in combination with other structures or features, to any other embodiment or configuration, as it would be impractical to describe each of the embodiments and configurations discussed herein as having all of the options discussed with respect to all of the other embodiments and configurations. A device or method incorporating any of these features should be understood to fall under the scope of this disclosure as determined based upon the claims below and any equivalents thereof. 
     Other aspects, objects, and advantages can be obtained from a study of the drawings, the disclosure, and the appended claims.