AIRCRAFT WITH THE SAME

BACKGROUND Technical Field

The present disclosure generally relates to stabilization systems and methods of modifying aircraft with the stabilization systems, and more specifically to modifying the tail booms of helicopters with stabilization systems that alter the aerodynamics of the helicopter.

Description of the Related Art

Traditional single rotor helicopters have a main lifting rotor ("main rotor") that provides a lifting force and a tail rotor that provides a laterally directed force used to counter reaction torque of the main rotor and adjust yaw alignment. As the main rotor passes over the tail boom of the helicopter, the main rotor generates downwash that flows around the tail boom of the helicopter. It is known to include strakes on the approaching side of the tail boom to alter the flow of downwash from the rotating main rotor so as to generate a compensation force that at least partially counteracts the reaction torque produced by rotation of the main rotor. The "approaching side" of the tail boom is the side of the tail boom the main rotor blade approaches during rotation.

For example, U.S. Patent No. 4,708,305 describes a system for controlling main rotor torque which reduces the power and size requirements of conventional anti-torque means (such as a tail rotor). Torque countering forces are generated by disrupting the main rotor downwash flowing around the fuselage. In particular, the downward flow is separated from the fuselage surface by strakes positioned at specified locations on the approaching side of the tail boom. U.S. Patent No. 8,210,468 describes a stabilizer system for a helicopter that includes strakes installed on the approaching side of the tail boom and a modified vertical stabilizer. The components of the stabilizer system cooperate to improve handling of the helicopter (e.g., increased cross wind tolerance), reduce fatigue (e.g., tail boom fatigue, fuselage fatigue, and the like), improve climb performance, improve cruise performance, increase control safety margins, combinations thereof, and the like.

BRIEF SUMMARY

In each of the systems noted above, strakes are installed on an approaching side of the tail boom to promote air flow separation. However, none of the systems discussed above include alterations to the side of the tail boom opposite to the approaching side to further improve flow characteristics of downwash from the rotating main lifting rotor.

It has been recognized that altering the profile of the tail boom of a helicopter to make the opposite side of the tail boom more aerodynamic in a way that promotes flow attachment on the opposite side of the tail boom can improve the stability of the helicopter. In addition, adding a plurality of vortex generators to the opposite side of the modified tail boom can further improve the performance and stability of the tail boom in a manner that was not possible for a tail boom that was not modified.

In some aspects, a method of modifying a helicopter, includes providing a tail boom of the helicopter. The tail boom includes a first side located on an opposite side of the tail boom from a second side of the tail boom. A main rotor blade of the helicopter approaches the second side of the tail boom during rotation. The first side and the second side of the tail boom include asymmetrical surface profiles about a vertical plane of the tail boom. The method further includes securing a plurality of vortex generators to the first side of the tail boom.

Securing the plurality of vortex generators on the first side of the tail boom can include positioning each of the plurality of vortex generators on a single, continuous curve that extends from a vertical apex of the modified tail boom to a horizontal apex of the modified tail boom. Securing the plurality of vortex generators on the first side of the tail boom can include positioning the vortex generators vertically at about at 5% to 20% of a chord length of the boom. Securing the plurality of vortex generators on the first side of the tail boom can include placing the vortex generators at an angle a of about 10° to about 30° off a vertical axis.

Securing the plurality of vortex generators on the first side of the tail boom can include positioning the vortex generators so to all be angled the same relative to a vertical axis. Securing the plurality of vortex generators on the first side of the tail boom can include placing the vortex generators to alternate back and forth along the tail boom.

In some embodiments, a method of modifying a tail boom of a helicopter is provided. The tail boom includes an approaching boom side and exiting boom side that respectively extend vertically on opposite sides of a vertical plane of the tail boom. The helicopter includes a main rotor arranged to pass over the approaching boom side of the tail boom before passing over the exiting boom side of the tail boom in each of a plurality of rotations of the main rotor. The method includes providing the tail boom of the helicopter. The approaching boom side and the exiting boom side of the tail boom have substantially symmetrical surface profiles about the vertical plane of the tail boom. The method further includes modifying at least the exiting surface profile of the tail boom so that an exiting surface profile of the modified tail boom is not symmetrical to an approaching surface profile of the modified tail boom. The method further includes affixing a plurality of vortex generators to the exiting surface of the modified tail boom.

Modifying the tail boom can include removing a first rotor drive shaft cover installed on the tail boom, in which the first rotor drive shaft cover includes a first profile; and coupling a second rotor drive shaft cover to the tail boom, in which the second rotor drive shaft includes a second profile that is more aerodynamic than the first profile. Modifying the tail boom can include adding a structural member to the tail boom on at least the exiting boom side of the tail boom.

In another aspect, the method of modifying the tail boom can include providing at least one strake on the approaching side of the tail boom.

A method of modifying a tail boom of a helicopter may be summarized as including removing a first rotor drive shaft cover from the tail boom, in which the first rotor drive shaft cover including a first external surface that is substantially symmetrical about a vertical plane of the tail boom when installed on the tail boom; and installing a second rotor drive shaft cover on the tail boom, in which the second drive shaft cover including a second external surface that is asymmetrical about the vertical plane of the tail boom when installed on the tail boom. The second external surface includes a plurality of vortex generators.

The second external surface can be shaped to promote flow attachment on a first side of the tail boom when the modified tail boom is subjected to a downdraft from rotation of a main rotor of the helicopter. The first side of the tail boom is opposite to a second side of the tail boom towards which the main rotor approaches the tail boom during rotation. The second external surface can be shaped to promote flow separation at a fixed separation point on the second side of the modified tail boom when the modified tail boom is subjected to the downdraft from rotation of the main rotor of the helicopter. The second rotor drive shaft cover can include at least one strake positioned and shaped to promote flow separation at the fixed separation point on the second side of the modified tail boom when the modified tail boom is subjected to the downdraft from rotation of the main rotor of the helicopter. The method can further include coupling at least one strake to the second side of the tail boom in a location that will promote flow separation at a fixed separation point on the second side of the tail boom when the modified tail boom is subjected to the downdraft from rotation of the main rotor of the helicopter.

The second external surface can include a first portion that extends in a single, continuous curve from a vertical apex of the second rotor drive shaft cover to a horizontal apex on the first side of the tail boom. The second external surface can include a second portion that extends in a single, continuous curve from the vertical apex of the second rotor drive shaft cover to a shelf that extends substantially vertically on the second side of the modified tail boom. The method can further include coupling a first strake to the second side of the tail boom at a transition point between the single, continuous curve on the second external surface and the shelf. The method can further include coupling a second strake to the second side of the modified tail boom at a position vertically below the first strake. The transition point can be located above a vertical center of the modified tail boom.

The first external surface can include shoulder portions that are symmetrical about the vertical plane, and the second external surface includes asymmetrical shoulder portions about the vertical plane. In another aspect, the first external surface includes shoulder portions that are symmetrical about the vertical plane, and the second external surface does not include any shoulder on at least one side of the modified tail boom. The second external surface may not include any shoulder on a first side of the tail boom that is opposite to a second side of the tail boom towards which a main rotor blade of the helicopter approaches during rotation. The second external surface can include an airfoil shape that promotes a lower air pressure on the first side of the modified tail boom when subjected to the downdraft from the main rotor.

In some embodiments, a method of modifying a tail boom of a helicopter includes coupling an airflow modification element to an external surface of a fuselage of the tail boom to modify an external profile of the tail boom on at least a first side of the tail boom and thereby promote a continuous flow pattern on the first side of the modified tail boom when the modified tail boom is subjected to a downdraft from a main rotor of the helicopter. The first side of the tail boom is opposite to a second side of the tail boom towards which the main rotor approaches the tail boom during rotation. The method further includes coupling a plurality of vortex generators to the airflow modification element. The modified external profile can include a first portion that extends in a single, continuous curve from a vertical apex of the modified tail boom to a horizontal apex on a first side of the modified tail boom. Coupling the airflow modification element to the external surface of the fuselage of the tail boom can modify the external profile of the tail boom on the second side of the tail boom such that the modified tail boom includes a single, continuous curve from the vertical apex of the modified tail boom to a shelf that extends substantially vertically on the second side of the modified tail boom. The method can further include coupling a first strake to the second side of the tail boom at a transition point between the single, continuous curve on the second external surface and the shelf. The method can also include coupling a second strake to the second side of the tail modified boom at a position vertically below the first strake. The transition point can be located above a vertical center of the modified tail boom. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elements or acts.

FIG. 1 is a side elevation view of a helicopter prior to modification. FIG. 2A is a cross-sectional view of the tail boom of the helicopter of FIG. 1 that illustrates airflow around the tail boom of the helicopter due to downdrafts from the main rotor.

FIG. 2B is a cross-sectional view of the tail boom of the helicopter of FIG. 1 modified to include strakes on the tail boom that illustrates airflow around the tail boom of the helicopter due to downdrafts from the main rotor.

FIG. 3 is a flow chart of a method for modifying a tail boom according to one embodiment.

FIG. 4A is a cross-sectional view of a tail boom, looking forward from the tail, prior to modification.

FIG. 4B is a cross-sectional view of the tail boom of Figure 4A with the drive shaft cover removed. FIG. 4C is a cross-sectional view of the tail boom of Figure 4B with a more aerodynamic drive shaft cover installed.

FIG. 4D is a cross-sectional view of the tail boom of Figure 4C with strakes installed on the tail boom.

FIG. 5 is a side elevation view of another helicopter prior to modification.

FIG. 6A is a cross-sectional view of a tail boom, looking forward from the tail, prior to modification.

FIG. 6B is a cross-sectional view of the tail boom of Figure 6A with the drive shaft cover removed.

FIG. 6C is a cross-sectional view of the tail boom of Figure 6B with a more aerodynamic drive shaft cover installed.

FIG. 6D is a cross-sectional view of the tail boom of Figure 6C with strakes installed on the tail boom.

FIG. 7 is a side elevation view of another helicopter prior to modification.

FIG. 8A is a cross-sectional view of the tail boom of the helicopter of FIG. 7 that illustrates airspeeds of air flowing around the tail boom of the helicopter due to downdrafts from the main rotor.

FIG. 8B is a cross-sectional view of the tail boom of the helicopter of FIG. 7 modified to include strakes on the tail boom that illustrates airspeeds of air flowing around the tail boom of the helicopter due to downdrafts from the main rotor.

FIG. 9 is a cross-sectional view of the tail boom of the helicopter of FIG. 7, looking forward from the tail, prior to modification.

FIG. 10 is a cross-sectional view of a modified tail boom according to one embodiment.

FIG. 1 1 is a cross-sectional view of a modified tail boom according to another embodiment.

FIG. 12 is an isometric view of a portion of a tail boom prior to modification. FIG. 13 is an isometric view the portion of the tail boom in Figure 12 following modification.

FIG. 14 is a side elevation view of the portion of the tail boom in Figure 13 further modified to include vortex generators.

FIG. 15 is a detail view of the modified tail boom in Figure 14.

FIG. 16 is a cross-sectional view of the modified tail boom in Figure 15, looking aft.

FIG. 17 is a non-limiting example of a vortex generator.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. However, one skilled in the art will understand that the invention may be practiced without these details. Tail boom modification systems are disclosed in the context of tail sections of helicopters because they have particular utility in this context. However, these modifications can be

incorporated into other types of aircraft in which aerodynamics is a significant consideration. Terms, such as "rear," "front," "rearward," "forward," "counter clockwise," "clockwise," "upward," and "downward," and variations thereof are used to describe the illustrated embodiments and are used consistently with the description of non-limiting exemplary applications. It will be appreciated, however, that the illustrated embodiments can be located or oriented in a variety of desired positions.

Unless the context requires otherwise, throughout the specification and claims which follow, the word "comprise" and variations thereof, such as, "comprises" and "comprising" are to be construed in an open, inclusive sense, that is as "including, but not limited to."

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

As used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the content clearly dictates otherwise. It should also be noted that the term "or" is generally employed in its broadest sense, that is as meaning "and/or" unless the content clearly dictates otherwise.

The Abstract of the Disclosure provided herein is for convenience only and does not interpret the scope or meaning of the embodiments.

Figure 1 shows a helicopter 100 including a cabin fuselage section 104 and a tail section 108 connected to and extending outwardly from the cabin fuselage section 104. A vertical stabilizer 1 18 is fixedly coupled to a free end 1 10 of the tail section 108. A tail rotor 1 16 is rotatably coupled to the vertical stabilizer 1 18. A main rotor 1 12 provides a lifting force.

Figure 2A is a cross-sectional view of the tail boom 150 of the helicopter 100 of FIG. 1 , taken looking forward from the tail 1 18 of the

helicopter 100. The tail boom 150 includes a tail rotor drive shaft cover 155 that covers the drive shaft for the tail rotor 1 16. The tail boom 150 includes distinct shoulder portions 152a and 152b on either side of the tail rotor drive shaft cover 155. Several structural elements are left out of the representation in Figure 2A for ease of representation. For example, the tail rotor drive shaft is not illustrated in Figure 2A.

Figure 2A illustrates airflow around the tail boom 150 of the helicopter 100 due to downwash from the main rotor 1 12. In this example, the main rotor 1 12 has a counter-clockwise rotation that sends the downwash over the tail boom 150 at an approximately 7-10° angle of attack. The shape of the profile of the tail boom 150 combined with the angle of attack of the downwash creates a low pressure zone on the side of the tail boom the main rotor approaches during rotation ("the approaching side" of the tail boom) and a high pressure zone on the opposite side of the tail boom. As illustrated in Figure 2A, this pressure differential generates a surge in lift force that opposes the tail rotor thrust with the passage of each blade of the main rotor 1 12.

As noted above, it is known to include strakes on the approaching side of the tail boom to alter the flow of downwash from the rotating main rotor so as to generate a compensation force. For example, as shown in Figure 2B, strakes 140 and 142 are installed on the approaching side of the tail boom to promote a flow separation. By fixing the flow separation points with the strakes 140 and 142, a region of stalled flow with a relatively high pressure is created, which can improve the stability of the tail boom 150 during, for example, takeoff and hovering operations.

It has been further recognized that altering the profile of the tail boom of a helicopter to make the opposite side of the tail boom (the side opposite the approaching side) more aerodynamic in a way that promotes flow attachment on the opposite side of the tail boom has many benefits. For example, altering the profile of the opposite side of the tail boom can, among other benefits: (i) remove downforce penalties resulting from a

disadvantageously shape tail boom profile; (ii) result in horsepower savings by at least partially counteracting the reaction torque produced by rotation of the main rotor; and (iii) and can improve directional control by promoting a reduction in what is known as "loss of tail rotor effect" (in helicopters that include a main rotor that rotates counterclockwise) or "uncommanded left yaw" (in helicopters that include a main rotor that rotates clockwise).

In one embodiment, the profile of the tail boom is altered to slow flow on the approaching side of the tail boom and speed up air flow on the opposite side of the tail boom. The alteration can be achieved by removing an existing tail rotor drive shaft cover from the tail boom and replacing it with a tail rotor drive shaft cover that has a more aerodynamic profile. The alteration can also be achieved by adding structure to the tail boom. The resulting tail boom can have a profile that resembles an airfoil (e.g. having a continuous cambered surface). The alteration may, or may not include the addition of strakes to the approaching side of the tail boom. Although the examples discussed below relate to modifications of tail booms that include a separate tail rotor drive shaft cover, the principles of the present disclosure are also applicable to

modifications of tail booms in which the tail rotor drive shaft is housed entirely within the fuselage of the tail boom. In such cases, the modification may include, for example, the addition of at least one structural element to at least the opposite side of the tail boom to improve the aerodynamic profile of the tail boom.

As shown in Figure 2B, airflow over the opposite side of the tail boom 150 takes a sharp turn at the shoulder 152b, which is adjacent the tail rotor drive shaft cover 155. This sharp turn not only detrimentally slows the speed of flow passing on the opposite side of the tail boom 150, but also results in a downforce that counteracts the lift generated main rotor 1 12.

Figure 3 is a flow chart of a method for modifying a tail boom to according to one embodiment. Figures 4A-4D are cross-sectional views of a tail boom undergoing the modification described in Figure 3. At 300, a tail boom that includes a rotor shaft cover is provided. For example, the tail boom 150, which includes the rotor shaft cover 155 is provided. At 310, the tail rotor drive shaft cover is removed from the tail boom. At 320, an aerodynamic tail rotor drive shaft cover is coupled to the tail boom. For example, Figure 4C illustrates an aerodynamic tail rotor drive shaft cover 1055 coupled to the tail boom resulting in a modified tail boom 150'. The outline of the prior location of the rotor shaft cover 155 is shown in broken lines. As can be seen in Figure 4C, the aerodynamic tail rotor drive shaft cover 1055 eliminates the shoulder 152b on the opposite side of the tail boom. At 330, one or more strakes are coupled to the approaching side of the tail boom. For example, Figure 4D illustrates strakes 140 and 142 coupled to the approaching side of the modified tail boom 150'.

Thus, the tail boom 150 is modified from having symmetrical shoulders 152a and 152b on either side of a vertical plane of the tail boom, to eliminating the shoulder 152b on the opposite side of the tail boom. The tail boom 150 also has a substantially symmetrical profile about the vertical plane of the tail boom, whereas the modified tail boom 150' has an asymmetrical profile. The profile of the modified tail boom 150' includes a continuous surface from a vertical apex of the aerodynamic drive shaft cover 1055 to a horizontal apex on the opposite side of the modified tail boom 150'. As a result, the profile on the opposite side of the modified tail boom 150' promotes flow attachment from main rotor downwash.

The strakes on the approaching side of the tail boom promote flow separation at fixed separation points. Although this embodiment illustrates one or more strakes coupled to the tail boom following modification, the strakes can be attached prior to modification. Further, the aerodynamic drive shaft cover may include structure that function as a strake. In other examples, no strakes are added to the approaching side of the tail boom. In addition, the flow separation on the approaching side can be achieved by strakes, shape modification of the tail boom, or a combination of both.

Unlike the tail boom 150 in Figure 2A, the modified tail boom 150' of Figure 4D has a high pressure zone on the approaching side of the tail boom and a low pressure zone on the opposite side of the tail boom. This improved pressure distribution results in horsepower savings by at least partially counteracting the reaction torque produced by rotation of the main rotor. It also improves directional control by reducing loss of tail rotor effect.

Figure 5 shows a helicopter 200 including a cabin fuselage section 204 and a tail section 208 connected to and extending outwardly from the cabin fuselage section 204. A vertical stabilizer 218 is fixedly coupled to a free end 210 of the tail section 208. A tail rotor 216 is rotatably coupled to the vertical stabilizer 218. A main rotor 212 provides a lifting force.

Figures 6A-6D are cross-sectional views of a tail boom undergoing a modification according to another embodiment. Figure 6A shows a tail boom 250, which includes a tail rotor drive shaft cover 255 covering the rotor drive shaft 256. The tail rotor shaft cover 255 has a profile that is substantially symmetrical about a vertical plane that passes through a vertical apex of the cover 255. Figure 6B shows the tail boom 250 with the tail rotor drive shaft cover 255 removed. Figure 6C illustrates an aerodynamic tail rotor drive shaft cover 2055 coupled to the tail boom resulting in a modified tail boom 250'. Unlike the cover 255, the cover 2055 is asymmetrical about a vertical plane that passes through a vertical apex of the cover 2055. The cover 2055 can include a hinge at point 2055c and be coupled to the tail boom 250 at points 2055a and 2055b. Figure 6D illustrates strakes 204b and 204c coupled to the approaching side of the modified tail boom 750'. A strake 204a may also optionally be coupled to the approaching side of the tail boom 250'.

As can be seen in Figures 6C and 6D, the aerodynamic tail rotor drive shaft cover 2055 defines a continuous curve between a vertical apex of the cover 2055 and a horizontal apex on the opposite side of the modified tail boom 150'. The cover 2055 defines a shelf on the approaching side of the tail boom 250'. The resulting asymmetrical profile promotes flow attachment on the opposite side and flow separation on the approaching side. In this example, the strake 2044 is positioned at a transition point 2055c between a continuous curve and a vertical surface of the shelf on the approaching side. The strake 2044 generates a fixed point of flow separation on the approaching side.

The modified tail boom 250' in Figure 6D generates a more favorable pressure distribution than the tail boom 250 in Figure 6A. For example, the continuous curve on the opposite side of the modified tail boom 250' and the shelf and strakes on the approaching side of the modified tail boom 250' create a lift force in the same direction as the tail rotor force.

Figure 7 shows a helicopter 300 including a cabin fuselage section 304 and a tail section 308 connected to and extending rearwardly from the cabin fuselage section 304. A vertical stabilizer 318 is fixedly coupled to a free end 310 of the tail section 308. A tail rotor 316 is rotatably coupled to the vertical stabilizer 318. A main rotor 312 provides a lifting force.

Figures 8A and 8B illustrate the beneficial flow distribution that can be achieved by adding strakes to the approaching side of the tail boom 350. Figure 9 illustrates a cross-sectional view of the tail boom 350 prior to modification, and Figures 10 and 1 1 illustrate two different tail boom

modifications according to the present disclosure. In Figure 10, the tail boom cover 355 is replaced with an aerodynamic tail boom cover 3055 that eliminates the shoulder 352b on the opposite side of the tail boom 350. The modified tail boom 350' in Figure 10 also includes strakes 344 and 342 on the approaching side of the tail boom.

The modified tail boom 350" in Figure 1 1 includes an aerodynamic drive shaft cover 3065 that eliminates both of the shoulders 352a and 352b. The approaching side of the cover 3065 includes a structure 3046 that functions as a strake to separate flow at a fixed location. A strake 342 is also affixed to the approaching side of the modified tail boom 350".

As with the previous examples, the modified tail booms 350' and 350" provide for a more advantageous flow pattern around the tail boom than the symmetrical profile of the tail boom prior to modification.

In another aspect, adding vortex generators to the opposite side of a modified tail boom provides a significant improvement in performance.

Figure 12 is an isometric view of a tail boom 450 with an external rotor shaft 455. This "symmetrical" tail boom geometry was analyzed using computational fluid dynamics (CFD) to determine whether attaching vortex generators on the opposite side of the tail boom would improve performance when paired with the modifications described in U.S. Patent No. 8,210,468, i.e. installing strakes on the approaching side of the tail boom and modifying the vertical stabilizer. Just as the addition of strakes on the approaching side is intended to promote flow separation, and thereby increase pressure on the approaching side of the tail boom, adding vortex generators to the opposite side of the tail boom is intended to promote flow attachment from main rotor downwash, and thereby decrease pressure on the opposite side of the tail boom.

The CFD analysis revealed that the most effective place to locate the vortex generators on the opposite side of a "symmetrical" tail boom that includes an external shaft cover is between 40% and 60% of chord length of the tail boom. The chord length of the tail boom is a vertical length with 0% of chord length located at the apex of the shaft cover and 100% of chord length at the bottom of the boom. Locating the vortex generators between 40% and 60% of chord length of the tail boom avoids the "aerodynamic chaos" higher up on the boom that is attributable to the discontinuity at the tail boom - drive shaft cover interface. Further, the analysis revealed that locating the vortex generators between 40% and 60% of chord length of the tail boom was the analytically preferred location for preventing flow separation in a region of organized flow.

The CFD analysis further revealed that locating the vortex generators between 40% and 60%of chord length of the tail boom resulted in a very small difference in the pressure distribution on the opposite side of the boom when compared to "symmetrical" tail booms without vortex generators.

Modifying the profile of the opposite side of the tail boom can eliminate the discontinuity at the tail boom - drive shaft cover interface. This allows the vortex generators to be placed at a far more advantageous location on the tail boom. Figure 13 illustrates a tail boom modification that includes adding structure 460 to the tail boom. The resulting, modified tail boom has a profile that resembles an airfoil (e.g. having a continuous cambered surface) on the opposite side, as shown in Figure 16.

The placement of vortex generators on the modified tail boom in Figure 13 will now be discussed with reference to Figures 14-17. Figure 17 illustrates an example vortex generator 500, that includes a leading edge 502, a trailing edge 504, a top edge 506, and a bottom 508. In this example, the vortex generator 500 ranges in length I from about 1 inch to about 2 inches and in height h from 0.25 inches to 0.50 inches. Although the leading edge 502 is illustrated as a tapered leading edge, a blunt leading edge is also within the scope of the present disclosure.

Figure 14 illustrates a plurality of vortex generators 500 installed on the modified tail boom 450. The vortex generators can be bonded to the modified tail boom individually; installed as a single assembly, such as a long strip with the vortex generator pattern integral to it, attached to the boom: or as an integral part of the redesigned shaft cover or other aerodynamic structural shape, such as the structure 460, that is attached to an unmodified tail boom.

In the examples in Figure 15, the vortex generators are placed in a pattern along the boom in the area under the shadow of the main rotor disk. In this example, the vortex generators 500b and 500c are placed a distance d approximately 3 inches to 6 inches apart at an angle a of about 10° to about 30° off a vertical axis. By way of reference, a 0° placement would have the long axis of the vortex generator 500a perpendicular to the centerline axis of the boom . The vortex generator placement off the vertical axis of 10° to 30° can result in the vortex generators being canted either forward or aft. The vortex generators can all be angled the same relative to a vertical axis or can alternate back and forth along the tail boom, as shown in Figures 14 and 15.

Figure 16 is a cross-sectional view of the modified tail boom in Figure 15. The vortex generators are placed on the side of the boom opposite to the "approaching side,"as referred to as the "asymmetrical" aerodynamic side. The vortex generators 500 are positioned vertically at 5% to 20% of a chord length C in which 0% is the top of the boom and 100% is the bottom of the boom. As shown in Figure 16, the vortex generators 500 are positioned on a single, continuous curve that extends from a vertical apex of the modified tail boom to a horizontal apex of the modified tail boom on the opposite side of the modified tail boom.

As noted above, applying vortex generators to the only practical location on the opposite side of a symmetrical tail boom resulted in very small changes. By contrast, it has been found that adding vortex generators to a tail boom that was modified to include an asymmetrical profile resulted in doubling the benefits of the asymmetrical modification.

Although the example above relates to adding vortex generators to a tail boom that has been modified to include an asymmetrical profile, the present disclosure also encompasses applying vortex generators to a tail boom that includes an asymmetrical shape without additional modification. For example, applying vortex generators to the opposite side of helicopters with asymmetrical tail booms, such as the Augusta Westland AW169 and the Bell 525 Relentless, can improve the performance of these aircraft.

Further, the tail boom modifications disclosed herein can be incorporated into a wide range of helicopters. As used herein, the term

"helicopter" includes, without limitation, rotorcraft aircraft, rotary-wing aircraft, or other heavier-than-air aircraft that are lifted and sustained in the air horizontally by rotating wings or blades turning about a vertical axes using power supplied by an engine. For example, helicopters including the Bell UH-1 , Bell Huey II, Sikorsky UH-60, and Eurocopter HH-65A Dolphin helicopters are well suited for retrofitting with the tail boom systems disclosed herein. The various

embodiments described above can be combined to provide further

embodiments.

All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. The embodiments, features, systems, devices, materials, methods and techniques described herein may, in some embodiments, be similar to any one or more of the embodiments, features (e.g., strakes), systems, devices, materials, methods and techniques described in U.S. Provisional Patent Application No. 60/930,233 and U.S. Patent Nos. 4,708,305; 6,869,045; and 7,063,289. In addition, the embodiments, features, systems, devices, materials, methods and techniques described herein may, in certain embodiments, be applied to or used in connection with any one or more of the embodiments, features, systems, devices, materials, methods and techniques disclosed in the above-mentioned U.S. Provisional Patent Application No. 60/930,233, and U.S. Patent Nos.

4,708,305; 6,869,045; 7,063,289; and 8,210,468. U.S. Provisional Patent Application Nos. 60/930,233; 61/816,507; U.S. Patent Application No.

14/228,120 and U.S. Patent Nos. 4,708,305; 6,869,045; 7,063,289; and

8,210,468 are hereby incorporated by reference herein. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments.

These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.