阅读说明：本技术 具有限定转子上方的腔的风扇外壳的涵道风扇 (Ducted fan having a fan case defining a cavity above a rotor ) 是由 D·L·崔德特 S·A·卡立德 于 2020-10-12 设计创作，主要内容包括：提供了一种涵道风扇,该涵道风扇包括风扇外壳,风扇外壳包绕安装到旋转驱动轴的多个风扇叶片。多个叶片限定大于68度的末梢交错角,并且,风扇外壳限定由风扇外壳的内壁限定的环形凹陷部,环形凹陷部接近多个叶片中的每个的叶片末梢而围绕周向方向延伸。环形凹陷部可限定大于末梢弦长的10%的平均凹陷部深度。环形凹陷部还可限定等于凹陷部长度相比于末梢轴向弦长的长度比,该长度比大于1.5。(A ducted fan is provided that includes a fan casing that surrounds a plurality of fan blades mounted to a rotating drive shaft. The plurality of blades define a tip stagger angle greater than 68 degrees, and the fan housing defines an annular recess defined by an inner wall of the fan housing, the annular recess extending about the circumferential direction proximate the blade tip of each of the plurality of blades. The annular recess may define an average recess depth that is greater than 10% of the tip chord length. The annular recess may also define a length ratio equal to a recess length to a tip axial chord length, the length ratio being greater than 1.5.)

1. A ducted fan defining an axial direction, a radial direction, and a circumferential direction, the ducted fan comprising:

a fan housing extending around the circumferential direction and defining a flow passage;

a drive shaft positioned within the fan housing and rotatable about the axial direction;

a plurality of blades operably coupled to the drive shaft and extending substantially along the radial direction toward the fan housing, the plurality of blades defining a tip stagger angle greater than 68 degrees; and

a recess defined by an inner wall of the fan casing, the recess extending about the circumferential direction proximate to a blade tip of each of the plurality of blades.

2. The ducted fan according to claim 1, wherein the scoop defines an average scoop depth measured along the radial direction, the average scoop depth being greater than 0.5% of a tip radius of the plurality of blades.

3. The ducted fan according to claim 2, wherein the scoop defines a scoop length measured along the axial direction, wherein the scoop length is greater than a tip axial chord length plus 1% of the tip radius of the plurality of blades.

4. The ducted fan according to claim 3, wherein the tip stagger angle of the plurality of blades is greater than about 74 degrees.

5. The ducted fan according to claim 4, wherein the average scoop depth is greater than 1% of the tip radius of the plurality of blades.

6. The ducted fan according to claim 5, wherein the recessed portion length is greater than a tip axial chord length plus 2% of the tip radius of the plurality of blades.

7. The ducted fan according to claim 6, wherein the tip stagger angle of the plurality of blades is greater than about 80 degrees.

8. The ducted fan according to claim 7, wherein the average scoop depth is greater than 1.5% of the tip radius of the plurality of blades.

9. The ducted fan according to claim 8, wherein the recessed portion length is greater than the tip axial chord length plus 3% of the tip radius of the plurality of blades.

10. The ducted fan according to claim 3, wherein the blade tip of each of the plurality of blades is substantially aligned with a reference surface extending between the inner wall at a front of the recess and the inner wall at a rear of the recess.

Technical Field

The present subject matter relates generally to ducted fans having improved performance and efficiency, and more particularly, the present subject matter relates to an improved fan casing having ducted fans for vertical flight and hover modes of operation.

Background

Ducted fans with low flow coefficients (e.g., less than 0.36) typically have very high blade tip stagger angles (e.g., greater than 70 degrees). At such high stagger angles, the blade tip sections are nearly aligned with the circumferential direction or fan rotational direction, and thus are nearly perpendicular to the primary air flow through the ducted fan. Thus, there is a strong reverse air flow or void flow through the tip void defined between the blade tip and the adjacent ducted surface. In addition, the trailing blade tip vortex causes a reverse airflow at the ducted surface. These void flows and blade tip vortices can result in significant operational losses and inefficiencies.

For example, performance losses may be associated with substantial flow recirculation at the rotor tip, which may have a strong adverse effect on the ducted surface boundary layer. Specifically, such reverse airflow and blade tip vortices effectively block the near-tip primary flow, causing the duct-wall boundary layer downstream of the fan to be separated or to remain with little or no flow momentum. As such, the boundary layer has a substantial flow area blockage even when generally still attached, and the boundary layer lacks sufficient momentum to avoid later separation in the presence of adverse pressure gradients (e.g., associated with increased ducted regions).

For some conventional ducted fans with low flow coefficients, a larger tip clearance may be desirable. For example, fans with large tip clearances are geometrically simple, easy to implement/manufacture, and are expected to have little or no weight penalty. In addition, blade tip rub and the risk of rub related problems or failures is less. Furthermore, such a design has large tolerances for manufacturing and fabrication, and therefore, parts and assemblies are cheaper, and there is greater flexibility in material selection. However, large tip clearances between the blade tips and the inner surface of the duct exacerbate adverse interactions, resulting in serious disadvantages in both performance and operational stability. Smaller tip clearances may reduce problems associated with boundary layer disruption near the duct, at least in the absence of inlet flow distortion, but may increase the likelihood of blade impingement and may complicate the mechanical design of the fan.

Because reverse flow and boundary layer blockage in the downstream ductwork greatly reduces fan flow and thrust, insufficient tolerance to this condition may result in abrupt changes in flow and thrust, for example, during hover operation or other operational scenarios that may result in varying inlet flow distortion. This phenomenon is exacerbated by fan ducts having diverging downstream sections, which are also desirable for many applications.

Therefore, ducted fans with improved hover performance and operational stability would be useful. More specifically, ducted fans having features for reducing boundary layer disruption, void flow, and adverse effects of blade tip vortices would be particularly beneficial.

Disclosure of Invention

Aspects and advantages of the invention will be set forth in part in the description which follows, or may be obvious from the description, or may be learned by practice of the invention.

In one exemplary embodiment, a ducted fan is provided that defines an axial direction, a radial direction, and a circumferential direction, the ducted fan comprising: a fan housing extending around a circumferential direction and defining a flow passage; a drive shaft positioned within the fan housing and rotatable about an axial direction; a plurality of blades operably coupled to the drive shaft and extending substantially in a radial direction toward the fan casing, the plurality of blades defining a tip stagger angle greater than 68 degrees; and a recess defined by an inner wall of the fan casing, the recess extending around the circumferential direction proximate to the blade tip of each of the plurality of blades.

The ducted fan may include a scoop defining an average scoop depth measured along a radial direction, the average scoop depth being greater than 0.5% of a tip radius of the plurality of blades. The recess may define a recess length measured along the axial direction, wherein the recess length may be greater than the tip axial chord plus 1% of the tip radius of the plurality of blades. The tip stagger angle of the plurality of blades may be greater than about 74 degrees. The average dimple depth may be greater than 1% of the tip radius of the plurality of blades. The recessed portion length may be greater than the tip axial chord plus 2% of the tip radius of the plurality of blades. The tip stagger angle of the plurality of blades may be greater than about 80 degrees. The average dimple depth may be greater than 1.5% of the tip radius of the plurality of blades. The recessed portion length may be greater than the tip axial chord plus 3% of the tip radius of the plurality of blades. The blade tip of each of the plurality of blades may be substantially aligned with a reference surface extending between an inner wall located at a front of the recess and an inner wall located at a rear of the recess. The blade tip of each of the plurality of blades may be positioned at least partially within the recess. The annular recess may be axisymmetric.

In another exemplary embodiment, a ducted fan is provided that defines an axial direction, a radial direction, and a circumferential direction, the ducted fan comprising: a fan housing extending around a circumferential direction and defining a flow passage; a drive shaft positioned within the fan housing and rotatable about an axial direction; a plurality of blades operably coupled to the drive shaft and extending substantially in a radial direction toward the fan casing, the plurality of blades of the ducted fan operating with a flow coefficient based on a blade tip speed of less than 0.4; and a recess defined by an inner wall of the fan casing, the recess extending around the circumferential direction proximate to the blade tip of each of the plurality of blades.

The squealer fan squealer may define an average squealer depth measured along a radial direction that is greater than 0.5% of a tip radius of the plurality of blades. The recess may further define a recess length measured along the axial direction, wherein the recess length may be greater than the tip axial chord plus 2% of the tip radius of the plurality of blades. The bypass fan may have a flow coefficient less than 0.25. The average dimple depth may be greater than 1.5% of the tip radius of the plurality of blades. The blade tip of each of the plurality of blades may be positioned at least partially within the recess. The annular recess may be axisymmetric.

In another exemplary embodiment, a ducted fan is provided that defines an axial direction, a radial direction, and a circumferential direction, the ducted fan comprising: a fan housing extending around a circumferential direction and defining a flow passage; a drive shaft positioned within the fan housing and rotatable about an axial direction; a plurality of blades operably coupled to the drive shaft and extending substantially in a radial direction toward the fan housing; and a recess defined by an inner wall of the fan casing proximate to the blade tip of each of the plurality of blades, wherein the recess defines an average recess depth measured along the radial direction, the average recess depth being greater than 1.0% of a tip radius of the plurality of blades.

The scoop of the ducted fan may define a scoop length measured along the axial direction, wherein the scoop length may be greater than a tip axial chord plus 2% of a tip radius of the plurality of blades. The plurality of blades may define a tip stagger angle of greater than 68 degrees. An average tip clearance may be defined between the blade tip of each of the plurality of blades and the inner wall of the fan casing, the average tip clearance being greater than 1% of a radius defined by the blade tip of each of the plurality of blades. The annular recess may be axisymmetric. The blade tip of each of the plurality of blades may be positioned at least partially within the recess. The blade tip of each of the plurality of blades may be positioned at least partially within the recess.

Technical solution 1. a ducted fan defining an axial direction, a radial direction, and a circumferential direction, the ducted fan comprising:

a fan housing extending around the circumferential direction and defining a flow passage;

a drive shaft positioned within the fan housing and rotatable about the axial direction;

a plurality of blades operably coupled to the drive shaft and extending substantially along the radial direction toward the fan housing, the plurality of blades defining a tip stagger angle greater than 68 degrees; and

a recess defined by an inner wall of the fan casing, the recess extending about the circumferential direction proximate to a blade tip of each of the plurality of blades.

The ducted fan according to any preceding claim, wherein the dimples define an average dimple depth measured along the radial direction, the average dimple depth being greater than 0.5% of a tip radius of the plurality of blades.

The ducted fan according to any preceding claim, wherein the scoop defines a scoop length measured along the axial direction, wherein the scoop length is greater than a tip axial chord plus 1% of the tip radius of the plurality of blades.

The ducted fan according to any preceding claim, wherein the tip stagger angle of the plurality of blades is greater than about 74 degrees.

The ducted fan according to any preceding claim, wherein the average dimple depth is greater than 1% of the tip radius of the plurality of blades.

The ducted fan according to any preceding claim, wherein the recessed portion length is greater than a tip axial chord length plus 2% of the tip radius of the plurality of blades.

The ducted fan according to any preceding claim, wherein the tip stagger angle of the plurality of blades is greater than about 80 degrees.

The ducted fan according to any preceding claim, wherein the average dimple depth is greater than 1.5% of the tip radius of the plurality of blades.

The ducted fan according to any preceding claim, wherein the recessed portion length is greater than the tip axial chord length plus 3% of the tip radius of the plurality of blades.

The ducted fan according to any preceding claim, wherein the blade tip of each of the plurality of blades is substantially aligned with a reference surface extending between the inner wall at a front of the recess and the inner wall at a rear of the recess.

The ducted fan according to any preceding claim, wherein the blade tip of each of the plurality of blades is at least partially positioned within the recess.

The ducted fan according to any one of the preceding claims, wherein the annular recess is axisymmetric.

The invention of claim 13, a ducted fan defining an axial direction, a radial direction, and a circumferential direction, the ducted fan comprising:

a fan housing extending around the circumferential direction and defining a flow passage;

a drive shaft positioned within the fan housing and rotatable about the axial direction;

a plurality of blades operably coupled to the drive shaft and extending substantially along the radial direction toward the fan casing, the plurality of blades of the ducted fan operating with a flow coefficient based on a blade tip speed of less than 0.4; and

a recess defined by an inner wall of the fan casing, the recess extending about the circumferential direction proximate to a blade tip of each of the plurality of blades.

The ducted fan according to any preceding claim, wherein the dimples define an average dimple depth measured along the radial direction, the average dimple depth being greater than 0.5% of a tip radius of the plurality of blades.

The ducted fan according to any preceding claim, wherein the scoop defines a scoop length measured along the axial direction, wherein the scoop length is greater than a tip axial chord plus 2% of the tip radius of the plurality of blades.

The ducted fan according to any preceding claim, characterized in that the flow coefficient of the ducted fan is less than 0.25.

The ducted fan according to any preceding claim, wherein the average dimple depth is greater than 1.5% of the tip radius of the plurality of blades.

The ducted fan according to any preceding claim, wherein the blade tip of each of the plurality of blades is at least partially positioned within the recess.

The ducted fan according to any preceding claim 19, wherein the annular recess is axisymmetric.

A ducted fan defining an axial direction, a radial direction, and a circumferential direction, the ducted fan comprising:

a fan housing extending around the circumferential direction and defining a flow passage;

a drive shaft positioned within the fan housing and rotatable about the axial direction;

a plurality of blades operably coupled to the drive shaft and extending substantially along the radial direction toward the fan housing; and

a recess defined by an inner wall of the fan casing proximate a blade tip of each of the plurality of blades, wherein the recess defines an average recess depth measured along the radial direction that is greater than 1.0% of a tip radius of the plurality of blades.

The ducted fan according to any preceding claim, wherein the scoop defines a scoop length measured along the axial direction, wherein the scoop length is greater than a tip axial chord plus 2% of a tip radius of the plurality of blades.

The ducted fan according to any preceding claim, wherein the plurality of blades define a tip stagger angle of greater than 68 degrees.

The ducted fan of any preceding claim, wherein an average tip clearance is defined between the blade tip of each of the plurality of blades and the inner wall of the fan casing, the average tip clearance being greater than 1% of a radius defined by the blade tip of each of the plurality of blades.

The ducted fan according to any preceding claim, wherein the annular recess is axisymmetric.

The ducted fan according to any preceding claim, wherein the blade tip of each of the plurality of blades is at least partially positioned within the recess.

The ducted fan according to any preceding claim, wherein the blade tip of each of the plurality of blades is at least partially positioned within the recess.

These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

Drawings

A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures.

Fig. 1 provides a partial perspective view of an exemplary ducted fan in accordance with exemplary embodiments of the present subject matter.

FIG. 2 provides a schematic illustration of a row of fan blades of the exemplary ducted fan of FIG. 1, in accordance with an exemplary embodiment of the present subject matter.

FIG. 3 provides a schematic illustration of the void flow over a conventional blade tip.

FIG. 4 provides a schematic side view of a blade tip of a blade of the exemplary ducted fan of FIG. 1, in accordance with exemplary embodiments of the present subject matter.

Fig. 5 provides a schematic elevation view of a ducted fan having an alternative scoop shape in accordance with an exemplary embodiment of the present subject matter.

Fig. 6 provides a schematic elevation view of a ducted fan having an alternative scoop shape in accordance with an exemplary embodiment of the present subject matter.

Fig. 7 provides a schematic elevation view of a ducted fan having an alternative scoop shape in accordance with an exemplary embodiment of the present subject matter.

Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention.

Detailed Description

Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope of the invention. For example, features illustrated or described as part of one embodiment can be used with another embodiment to yield still a further embodiment. Thus, it is intended that the present invention cover such modifications and variations as come within the scope of the appended claims and their equivalents.

As used herein, the terms "first," "second," and "third" may be used interchangeably to distinguish one element from another, and are not intended to indicate the position or importance of an individual element. The terms "upstream" and "downstream" refer to relative directions with respect to fluid flow in a fluid pathway. For example, "upstream" refers to the direction from which the fluid flows, and "downstream" refers to the direction to which the fluid flows. In addition, approximate terms such as "approximately," "substantially," or "approximately" mean within a ten percent error margin.

Aspects of the present disclosure relate to a ducted fan including a fan case surrounding a plurality of fan blades mounted to a rotating drive shaft. The plurality of blades define a tip stagger angle greater than 68 degrees, and the fan housing defines an annular recess defined by an inner wall of the fan housing, the annular recess extending about the circumferential direction proximate the blade tip of each of the plurality of blades. The annular recess may define an average recess depth that is greater than 10% of the tip chord length. The annular recess may also define a length ratio equal to a recess length to a tip axial chord length, the length ratio being greater than 1.5.

Fig. 1 provides a partial perspective view of a ducted fan 100 according to an exemplary embodiment of the present disclosure. As shown, the ducted fan 100 defines an axial direction a, a radial direction R, and a circumferential direction C. The ducted fan 100 generally includes a substantially tubular fan casing 102, the fan casing 102 extending around a circumferential direction C and defining an annular inlet 104 for receiving an air flow 106. More specifically, the fan casing 102 may define a flow passage 108, and the ducted fan 100 may include a fan assembly 110 positioned within the fan casing 102, the fan assembly 110 for drawing and propelling the airflow 106 through the flow passage 108.

Still referring to the exemplary embodiment of FIG. 1, fan assembly 110 includes a drive shaft 112, drive shaft 112 rotatably positioned within flow passage 108 and rotatable about axial direction A. The drive shaft 112 may be operably coupled to a drive motor 114, the drive motor 114 configured to selectively rotate the drive shaft 112. While the drive shaft 112 is illustrated herein as being operably coupled to the drive motor 114, it should be appreciated that, according to alternative embodiments, the drive shaft 112 may be mechanically coupled to any other suitable drive mechanism, such as one or more shafts or another suitable rotating component of a gas turbine engine.

The fan assembly 110 may further include a plurality of blades 120, the blades 120 being operably coupled to the drive shaft 112 and extending substantially along the radial direction R toward the fan casing 102. According to the illustrated embodiment, the blades 120 are fan blades, but it should be appreciated that the ducted fan 100 may include any other suitable blades or airfoils, such as turbine blades, compressor blades, and the like, according to alternative embodiments. The blades 120 may be coupled to the drive shaft 112 in any suitable manner. For example, according to the illustrated embodiment, the blades 120 are coupled to a disk (not shown) in a spaced-apart manner along the circumferential direction C. The disk may be covered by a rotatable forward nacelle 122, with the forward nacelle 122 being aerodynamically contoured to promote airflow 106 through the plurality of blades 120 and through the flow path 108.

For the depicted embodiment, the plurality of blades 120 are fixed-pitch blades that may be mounted to the disk or drive shaft 112 in any suitable manner (e.g., via a dovetail, press fit, welding, mechanical fasteners, etc.). However, according to an alternative embodiment, fan assembly 110 may be a variable pitch fan assembly that includes a blade pitch mechanism 123 (see, e.g., fig. 5-7), blade pitch mechanism 123 for selectively rotating each of the plurality of fan blades 120 about a pitch axis P. Further, in accordance with the illustrated embodiment, the fan assembly 110 may include a plurality of circumferentially spaced vanes or struts 124, the vanes or struts 124 being stationary and extending between the fan casing 102 and a stationary portion of the ducted fan 100 surrounding the drive shaft 112.

Ducted fan 100 may generally be used in any suitable application, and may include changes and modifications while remaining within the scope of the present subject matter. For example, for reasons described in greater detail below, the ducted fan 100 may include blades 120, the blades 120 having a high stagger angle, such that the ducted fan 100 may be particularly suitable for operating at low flow coefficients. In this regard, the ducted fan 100 may be particularly suitable for hovering applications, personal mobile device applications, or for vertical takeoff and landing (VTOL) aircraft. However, it should be appreciated that, according to alternative embodiments, the ducted fan 100 may house any other suitable type of rotating blades for performing any other suitable function. In this regard, for example, aspects of the present subject matter may extend to a compressor 126 or a turbine 128 (both shown schematically in FIG. 1) of a gas turbine engine or to any other suitable ducted fan. According to such embodiments, the ducted fan 100 may be a fan of a gas turbine engine as follows: may include a core engine including a compressor 126 and a turbine 128. Thus, it should be appreciated that the exemplary ducted fan 100 illustrated in FIG. 1 is provided by way of example only, and that in other exemplary embodiments, the ducted fan 100 may have any other suitable configuration and the present subject matter may be applicable to other types of turbomachines.

Referring now also to FIG. 2, a schematic view of the rows of blades 120 of the exemplary ducted fan 100 will be described. As shown, each blade 120 includes an airfoil 130, the airfoil 130 having a pressure side 132 opposite a suction side 134. Opposite pressure and suction sides 132, 134 of each airfoil 130 extend radially from a blade root 138 along a span 136 to a blade tip 140 (see FIG. 1). As depicted, the blade root 138 is the radially innermost portion of the blade 120, and the blade tip 140 is the radially outermost portion of the blade 120. Thus, the blade root 138 is positioned at or near the inner wall of the nacelle 122 proximate the drive shaft 112, and the blade tip 140 terminates at or near the fan casing 102. Moreover, it will be readily appreciated that the blade root 138 may define a tab having a dovetail or other shape for receipt in a complementary shaped slot on the disk to couple the blade 120 to the disk or drive shaft 112, as generally well known in the art.

As further shown in FIG. 2, the pressure side 132 and the suction side 134 of the airfoil 130 extend between a leading edge 142 and an opposite trailing edge 144. Airfoil 130 defines a chord line 146, chord line 146 extending between opposite leading edge 142 and trailing edge 144. As will be readily appreciated, a chord line 146 may be defined at each spanwise location. Thus, the length of chord 146 may vary along span 136. As used herein, the term "tip chord" 148 or simply "chord" is intended to refer to the average length of chord line 146 at blade tip 140 of airfoil 130. Additionally, the term "average," when used to refer to a characteristic of the plurality of fan blades 120, is intended to refer to a mathematical average of the characteristic for each of the plurality of fan blades 120. In this regard, the chord length may be referred to as the average chord length to compensate for minor variations between the fan blades 120. Additionally, the term "tip axial chord" (see reference numeral 149 in FIG. 2) may be used herein to refer to the tip chord length multiplied by the cosine of the tip stagger angle, taken within the outer 5% of the blade span 136, and averaged over a plurality of blades 120. Thus, the tip axial chord length is the length of the tip chord as projected on the X-R plane, i.e., the length of the tip chord in the mainstream (or axial) direction a.

As shown in FIG. 2, a blade pitch 150 is defined between adjacent blades 120 within a row of blades 120. Blade pitch 150 is the circumferential spacing of blades 120 at a given span 136. In other words, blade pitch 150 is the circumferential length at a given span 136 location divided by the number of blades 120, and accordingly may vary along span 136. For example, span 136 may include a plurality of span locations, and each location may correspond to a fraction or percentage of span 136.

Still referring to fig. 2, the plurality of blades 120 may define a stagger angle 152, and the stagger angle 152 may be optimized to improve the performance and/or operability of the ducted fan 100. The stagger angle 152 may be defined as the angle between the chord line 146 and the axial direction a (e.g., the primary direction of the air flow 106) at a particular cross-section or region of the span 136. Specifically, as used herein, "tip stagger angle" or sometimes just "stagger angle" or "stagger" is intended to refer to the minimum stagger angle 152 within the outer 5% of the span 136 along the radial direction R. In other words, the tip stagger angle is defined by the distal region of the span 136 of each blade 120 (e.g., the region closest to the inner wall 164 of the fan casing 102).

As explained above, the ducted fan 100 may be configured for operation at low flow coefficients (e.g., less than 0.4, less than 0.3, less than 0.2, or lower). In general, the fan flow coefficient is a dimensionless parameter that is calculated as the volumetric flow through the fan normalized by the product of the fan frontal (front) area and the rotational speed of the blade tip. To achieve such a low flow coefficient, according to an exemplary embodiment, each blade 120 may have a tip stagger angle 152 that is greater than about 68 degrees. According to still other embodiments, the tip stagger angle 152 may be greater than about 74 degrees, greater than about 80 degrees, or even greater up to 90 degrees. Other tip stagger angles 152 may also be used, but in general, for low flow coefficient fans, higher stagger angles are often desirable for optimal performance and operability.

Notably, as mentioned above, high stagger angles may be associated with reduced efficiency and performance due to conditions related to boundary layer flow at the fan casing, blade vortices, and reverse flow near the fan casing (see fig. 3). In this regard, high stagger angles and low flow coefficients are associated with increased endwall separation and higher endwall losses due to secondary flow. Accordingly, aspects of the present subject matter are directed to minimizing the adverse effects of these fluid dynamics problems on high stagger, low flow coefficient ducted fans. While an exemplary solution will be described below, it should be appreciated that changes and modifications may be made to such a solution while remaining within the scope of the present subject matter.

Referring now specifically to fig. 4, the fan casing 102 may define an annular recess 160, and the annular recess 160 may be configured for receiving a void flow (e.g., indicated by reference numeral 162) to prevent negative interaction with the primary air flow 106. More specifically, an inner wall 164 defining a radially innermost surface of fan casing 102 and defining flow passage 108 may define annular recess 160 as a circumferential groove or slot positioned between blade tip 140 and fan casing 102 along radial direction R.

As shown, the reference surface 170 may be defined as a surface extending between the inner wall 164 located at the front of the annular recess 160 and the inner wall 164 located at the rear of the annular recess 160. In this regard, the reference surface 170 may generally follow the contour of the inner wall 164 as if the annular recess 160 were not present (e.g., similar to a conventional fan casing). According to the illustrated embodiment, the blade tip 140 is substantially aligned with the reference surface 170. In this manner, the void flow 162 is generally pushed through the annular recess 160, rather than flowing against the primary air flow 106 within the flow passage 108. Notably, such positioning of the blade tips 140 would not be possible in conventional ducted fan designs due to problems associated with blade rubbing or hitting against the fan casing 102.

Although the illustrated embodiment shows the blade tip 140 extending along the reference surface 170, it should be appreciated that the blade 120 may extend any suitable length into the annular recess 160 according to alternative embodiments. For example, the blade tip 140 may be positioned at least partially within the annular recess 160. According to still other embodiments, the blade tip 140 may be positioned only outside of the annular recess 160. Additionally, a tip clearance 172 may be defined between each blade tip 140 and the fan casing 102 along the radial direction R. For example, the average tip clearance 172 of all blades may be greater than 10% of the tip chord length. In alternative embodiments, the average tip clearance 172 may be greater than 15%, greater than 20%, greater than 30%, or greater than the tip chord length. According to an exemplary embodiment, the average tip clearance 172 of all blades 120 may be less than 1 inch. In alternative embodiments, the average tip clearance 172 may be less than about 0.25 inches, less than about 0.1 inches, or less.

According to an exemplary embodiment, the tip clearance may be a clearance ratio defined relative to a blade tip radius 174 (see FIG. 5). As used herein, the tip radius 174 may generally refer to an average tip radius 174 for a plurality of blades 120. For example, the clearance ratio may be defined as the average tip clearance (as measured above) compared to the radius defined by the blade tip 140 of each of the blades 120. Additionally or alternatively, the void ratio may be defined as the average tip void compared to the span 136. According to an exemplary embodiment, the clearance ratio with respect to the blade tip radius 174 may be greater than 1%, greater than 2%, greater than 3%, or greater. Other void ratios are also possible while remaining within the scope of the present subject matter.

It is noted that the dimensions of the annular recess 160 may vary while remaining within the scope of the present subject matter. In particular, computational fluid dynamics or other suitable flow analysis may be used to determine the ideal profile and geometry of the annular recess 160 for a particular application. The exemplary configurations and geometries described herein are not intended to limit the scope of the present subject matter.

As shown, the annular recess 160 is generally defined by a front wall 180, a rear wall 182, and an end wall 184, the front wall 180, the rear wall 182, and the end wall 184 all extending circumferentially around the fan casing 102. As shown, the annular recess 160 defines a substantially rectangular cross-section such that the inner wall 164 turns approximately 90 degrees at the transition between the forward portion of the inner wall 164 and the front wall 180, at the transition between the front wall 180 and the end wall 184, at the transition between the end wall 184 and the rear wall 182, and at the transition between the rear wall 182 and the downstream portion of the inner wall 164. However, it should be appreciated that according to alternative embodiments, the front wall 180 and the rear wall 182 may extend from the inner wall 164 at an angle that is more or less than 90 degrees. In addition, according to an exemplary embodiment, the walls 180-184 may be curved or provided with a gradual radius. Thus, the geometry of the annular recess 160 may vary while remaining within the scope of the present subject matter.

Still referring to fig. 4, the annular recess 160 may define an average recess depth 190 measured along the radial direction R between the reference surface 170 and the fan casing 102 (e.g., the end wall 184). Specifically, for example, the average recess depth is the average of the recess depths as measured between the reference surface 170 and the fan casing 102 along the entire chord line 146 of the blade 120. The average recess depth 190 may have any suitable size for accommodating the blade tip vortex and/or the void flow 162 for a particular application. For example, the average recess depth 190 may be defined as a percentage of the average tip chord 148 of the blade 120. According to an exemplary embodiment, the average recess depth is greater than 10% of the tip chord length. However, other suitable depths are possible and within the scope of the present subject matter. For example, the recess depth may be greater than 15%, greater than 20%, greater than 30%, or greater than the tip chord length.

According to an exemplary embodiment, the recess depth 190 may also be defined relative to the blade tip radius 174. As explained above, the tip radius 174 may generally refer to the average tip radius 174 for the plurality of blades 120. According to an exemplary embodiment, the recess depth 190 may be greater than 0.5% of the blade tip radius 174. According to still other embodiments, the recess depth 190 may be greater than 1%, greater than 1.5%, greater than 2%, greater than 5%, or greater than the blade tip radius 174. Additionally or alternatively, the recess depth 190 may be less than 10% of the blade tip radius 174, less than 5% of the blade tip radius 174, or less than 3% of the blade tip radius 174. It should be appreciated that the recess depth 190 may vary while remaining within the scope of the present subject matter.

Additionally, the annular recess may define a recess length 192 measured along the axial direction a between the front wall 180 and the rear wall 182. The recess length 192 may have any suitable size for a particular application, for example, to accommodate blade tip vortices and/or void flow 162. For example, the length ratio may be defined as the recess length 192 compared to the tip axial chord 149 of the blade 120. According to an exemplary embodiment, the length ratio is greater than 1.5. However, other suitable length ratios are possible and within the scope of the present subject matter. For example, the length ratio may be greater than 2, greater than 2.5, greater than 3, or greater.

Ducted fan 100 is described herein in accordance with various exemplary embodiments of the present subject matter. Each of the embodiments described may have one or more features, characteristics, or dimensions that may be interchangeably implemented on a ducted fan in accordance with further exemplary embodiments while remaining within the scope of the present subject matter. For example, the ducted fan 100 is described herein as having particular stagger angles 152, scoop depths 190, scoop lengths 192, flow coefficients, and other features or characteristics within various ranges. It should be appreciated that the exemplary ducted fan may include such features or characteristics within any specified range and in any suitable combination while remaining within the scope of the present subject matter.

For example, the ducted fan 100 may have a stagger angle 152 of greater than 68 degrees, greater than 74 degrees, greater than 80 degrees, or greater. Additionally or alternatively, the annular recess 160 may have a recess depth 190 that is greater than 0.5%, greater than 1%, greater than 1.5%, or greater than the blade tip radius 174. Additionally or alternatively, the annular recess 160 may have a recess length 192 that is greater than the tip axial chord 149 plus a percentage of the blade tip radius 174 (such as 1%, 2%, 3%, or more of the blade tip radius 174). Additionally or alternatively, the ducted fan may have a flow coefficient of less than 0.4, less than 0.3, less than 0.2, or lower. Other ranges, combinations of features, and additional exemplary ducted fans are possible and within the scope of the present subject matter.

Referring now to fig. 5-7, a ducted fan 100 will be described in accordance with various alternative embodiments of the present subject matter. Due to the similarity between the described embodiments, like reference numerals will be used to refer to the same or similar features. As shown, the annular recess 160 can be defined by a wall (e.g., such as the wall 180-184) having a non-uniform, varying, or otherwise non-axisymmetric profile. Exemplary profiles and features are described below as being incorporated into the ducted fan 100. It should be appreciated, however, that changes and modifications may be made to the described embodiments while remaining within the scope of the present subject matter. In this regard, for example, the scope of the present invention is not intended to be limited to the particular geometries, features or configurations described.

Specifically, as shown in fig. 5, the end wall 184 may define a wavy profile (generally indicated by reference numeral 200). Specifically, as shown, the end wall 184 may define an undulating profile 200 in the form of a circumferential wave, however, any undulating shape, serpentine shape, irregular shape, or other suitable shape may be used. Additionally, it should be appreciated that such a wavy profile 200 may be incorporated into or defined in any one or more of the front wall 180, the rear wall 182, or the end wall 184. Other suitable shapes may include, for example, a saw-tooth shape (e.g., as indicated by reference numeral 202 in fig. 6), a curved shape, and so forth.

Referring now specifically to fig. 7, the ducted fan 100 can further include a plurality of slots 210 extending from the end wall 184. One or more of the slots 210 may be in fluid communication with the relief cavity 212. In this manner, the slots 210 may be used to bleed the void flow 162 from the annular recess 160 at a desired circumferential location through the slots 210. According to still other exemplary embodiments, the slot may be completely removed, and a simple port may be defined in the end wall 184 and placed in fluid communication with the relief cavity 212 or the ambient environment.

Locally recessing the duct inner surface above the rotor tip to create an open, relatively large, axisymmetric cavity and placing the rotor tip at or near the radius of the original (non-recessed) surface allows for the use of much larger clearances while achieving the performance and operability of a tight clearance configuration (e.g., under vertical flight and hover conditions). In this regard, by providing an annular recess, the blade tip may be nearly radially aligned with the duct surface or inner wall 164 located just upstream and downstream of the cavity. As such, the aerodynamically effective gap between the blade tip and the adjacent ducted surface or inner wall 164 approaches zero, thereby improving performance and operability. Additionally, it is contemplated that aspects of the present subject matter may improve inlet flow distortion tolerance.

According to one theoretical and exemplary embodiment, aspects of the present subject matter are directed to relocating tip clearance flow and swirl to regions outside of the main flow stream, i.e., to spaces in the annular recess. To be effective, the cavity may be large enough so that the tip vortex stays in the cavity and does not eventually exit or roll out (roll out) onto the downstream culvert wall. Thus, the effective cavity may be relatively large in both axial extent and radial depth. Moreover, because the tip clearances are all located in the cavity above the primary flowpath, the blade tips may be radially aligned with the duct walls located upstream and downstream of the cavity. This allows the blade tip to energize the flow near the duct wall, increasing momentum, delivering a stronger flow to the downstream boundary layer.

Ducted fans with low flow coefficients generally require relatively tight blade tip clearances in order to achieve good aerodynamic performance, but this may not be possible or practical. In addition, better aerodynamic performance with respect to a much larger physical gap may be achieved because the gap of the blade tip from the inner surface of the annular recess may be large, while the blade tip section is active more near the outer shell flow (energize). For example, the large axial length and radial depth of the annular depression are particularly suitable for accommodating blade tip vortices generated by high stagger angle fan blades as described herein. It may also provide greater performance/stability margins for fan operation with inlet ducted flow distortion.

Typically, large over-rotor cavities have conventionally been avoided because they are expected to significantly degrade aerodynamic performance. However, ducted hover fans with low flow coefficients with high blade tip stagger angles may produce tip flow and vortex effects directly against the ducted wall flow. Such a fan design may benefit from a large cavity, at least under vertical flight and hover operating conditions, since then a relatively large clearance between the blade tips and the duct is desirable.

Further aspects of the invention are provided by the subject matter of clauses below:

1. a ducted fan defining an axial direction, a radial direction, and a circumferential direction, the ducted fan comprising: a fan housing extending around a circumferential direction and defining a flow passage; a drive shaft positioned within the fan housing and rotatable about an axial direction; a plurality of blades operably coupled to the drive shaft and extending substantially in a radial direction toward the fan casing, the plurality of blades defining a tip stagger angle greater than 68 degrees; and a recess defined by an inner wall of the fan casing, the recess extending around the circumferential direction proximate to the blade tip of each of the plurality of blades.

2. The ducted fan of any preceding clause, wherein the dimples define an average dimple depth measured along the radial direction, the average dimple depth being greater than 0.5% of the tip radius of the plurality of blades.

3. The ducted fan of any preceding clause, wherein the scoop defines a scoop length measured along the axial direction, wherein the scoop length is greater than the tip axial chord plus 1% of the tip radius of the plurality of blades.

4. The ducted fan of any preceding clause, wherein a tip stagger angle of the plurality of blades is greater than about 74 degrees.

5. The ducted fan of any preceding clause, wherein the average dimple depth is greater than 1% of the tip radius of the plurality of blades.

6. The ducted fan of any preceding clause, wherein the scoop length is greater than the tip axial chord plus 2% of the tip radius of the plurality of blades.

7. The ducted fan of any preceding clause, wherein a tip stagger angle of the plurality of blades is greater than about 80 degrees.

8. The ducted fan of any preceding clause, wherein the average scoop depth is greater than 1.5% of the tip radius of the plurality of blades.

9. The ducted fan of any preceding clause, wherein the recessed portion length is greater than the tip axial chord plus 3% of the tip radius of the plurality of blades.

10. The ducted fan of any preceding clause, wherein a blade tip of each of the plurality of blades is substantially aligned with a reference surface extending between an inner wall located at a front of the recess and an inner wall located at a rear of the recess.

11. The ducted fan of any preceding clause, wherein a blade tip of each of the plurality of blades is at least partially positioned within the recess.

12. The ducted fan of any preceding clause, wherein the annular recess is axisymmetric.

13. A ducted fan defining an axial direction, a radial direction, and a circumferential direction, the ducted fan comprising: a fan housing extending around a circumferential direction and defining a flow passage; a drive shaft positioned within the fan housing and rotatable about an axial direction; a plurality of blades operably coupled to the drive shaft and extending substantially in a radial direction toward the fan casing, the plurality of blades of the ducted fan operating with a flow coefficient based on a blade tip speed of less than 0.4; and a recess defined by an inner wall of the fan casing, the recess extending around the circumferential direction proximate to the blade tip of each of the plurality of blades.

14. The ducted fan of any preceding clause, wherein the dimples define an average dimple depth measured along the radial direction, the average dimple depth being greater than 0.5% of the tip radius of the plurality of blades.

15. The ducted fan of any preceding clause, wherein the scoop defines a scoop length measured along the axial direction, wherein the scoop length is greater than the tip axial chord plus 2% of the tip radius of the plurality of blades.

16. The ducted fan of any preceding clause, wherein the ducted fan has a flow coefficient less than 0.25.

17. The ducted fan of any preceding clause, wherein the average scoop depth is greater than 1.5% of the tip radius of the plurality of blades.

18. The ducted fan of any preceding clause, wherein a blade tip of each of the plurality of blades is at least partially positioned within the recess.

19. The ducted fan of any preceding clause, wherein the annular recess is axisymmetric.

20. A ducted fan defining an axial direction, a radial direction, and a circumferential direction, the ducted fan comprising: a fan housing extending around a circumferential direction and defining a flow passage; a drive shaft positioned within the fan housing and rotatable about an axial direction; a plurality of blades operably coupled to the drive shaft and extending substantially in a radial direction toward the fan housing; and a recess defined by an inner wall of the fan casing proximate to the blade tip of each of the plurality of blades, wherein the recess defines an average recess depth measured along the radial direction, the average recess depth being greater than 1.0% of a tip radius of the plurality of blades.

21. The ducted fan of any preceding clause, wherein the scoop defines a scoop length measured along the axial direction, wherein the scoop length is greater than the tip axial chord plus 2% of the tip radius of the plurality of blades.

22. The ducted fan of any preceding clause, wherein the plurality of blades define a tip stagger angle greater than 68 degrees.

23. The ducted fan of any preceding clause, wherein an average tip clearance is defined between the blade tip of each of the plurality of blades and the inner wall of the fan casing, the average tip clearance being greater than 1% of a radius defined by the blade tip of each of the plurality of blades.

24. The ducted fan of any preceding clause, wherein the annular recess is axisymmetric.

25. The ducted fan of any preceding clause, wherein a blade tip of each of the plurality of blades is at least partially positioned within the recess.

26. The ducted fan of any preceding clause, wherein a blade tip of each of the plurality of blades is at least partially positioned within the recess.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.