Patent Description:
An example of a rotary wing aircraft with a main rotor and a propeller is a so-called compound helicopter. In such a compound helicopter, the main rotor accomplishes essentially lifting duties, but usually also accomplishes propulsive duties at least at low or medium forward speeds in forward flight condition of the compound helicopter. The propeller, in turn, is mainly provided to off-load the main rotor from its propulsive duties at higher forward speeds in forward flight condition and may enable the compound helicopter to travel with comparatively high forward speeds which would not be reachable by use of the main rotor alone.

However, in operation the main rotor creates torque around the yaw axis of the compound helicopter. This torque must be counteracted by a suitable anti-torque device to guarantee a required yaw stability of the compound helicopter in flight operation.

The document <CIT> describes a compound helicopter with a main rotor, a tail boom, and a tail propeller mounted to an aft region of the tail boom, wherein the tail propeller is used only for forward thrust during an airplane mode of flight and during transition from vertical helicopter flight to forward airplane mode of flight, when the main rotor may be feathered in a no-lift attitude. Required anti-torque balancing forces during hovering mode are developed by differentially controlled aileron forces when respective wings are aligned vertically with main rotor downwash. Furthermore, a vertically moveable horizontal airfoil is provided on the tail boom, with controllable means which can provide anti-torque reaction forces from the main rotor downwash during the hovering mode.

The document <CIT> describes a compound helicopter with a main rotor, a tail propeller, and a tail boom. The tail boom forms a plenum chamber to which associated linear nozzles are connected. The associated linear nozzles are fixedly coupled to the tail boom and adapted to discharge a sheet of fluid created from pressurized air in the plenum chamber in a direction substantially tangential to an outer surface of the tail boom to divert main rotor downwash and thereby produce a force that counteracts biasing torque created by the main rotor. The pressurized air is provided by a fan or by directing exhaust air from a power plant of the compound helicopter into the plenum chamber. The compound helicopter further comprises a yaw control member which is movably coupled to the tail boom and selectively positionable based on pilot input.

The document <CIT> describes a compound helicopter with a main rotor, a tail propeller, and a tail boom that is surrounded by a cycloidal rotor. The cycloidal rotor has individual blades which are essentially parallel to the longitudinal axis of the tail boom. In operation, the cycloidal rotor is driven to provide anti-torque that counteracts biasing torque created by the main rotor.

However, the above-described anti-torque devices for compound helicopters with a main rotor and a propeller are generally complex and require elaborate components, such as differentially controlled ailerons or moveable airfoils, additional fans, or cycloidal rotors. These actuatable components increase an overall system complexity and an overall weight of these anti-torque devices.

The document <CIT> describes a compound helicopter with a main rotor and a tail propeller. The tail propeller is mounted to a circular shroud which is attached to the compound helicopter's fuselage. The circular shroud is further provided with a rudder that is pivotally mounted to the circular shroud downstream of the tail propeller. In operation, the tail propeller creates an air stream which is directed through the circular shroud toward the rudder and which may be deflected by the rudder to counteract torque created by the main rotor.

The documents <CIT> and <CIT> describe a similar compound helicopter with a main rotor and a tail propeller that is accommodated in a circular shroud which is provided with a rudder. Furthermore, a pitch trim tab is provided in addition to the rudder. The pitch trim tab is provided to improve pitch control of the compound helicopter.

The documents <CIT>, <CIT>, and <CIT> describe a similar compound helicopter with a main rotor and a tail propeller that is accommodated in a circular shroud which is provided with a rudder and a pitch trim tab. Furthermore, one or more deployable calotte-shaped sectors are provided in addition to the rudder and the pitch trim tab. The deployable calotte-shaped sectors are deployable from the circular shroud and provided to enable, in deployed state, deviation of tail propeller thrust.

The document <CIT> describes a similar compound helicopter with a main rotor and a tail propeller that is accommodated in a circular shroud which is provided with a rudder and a pitch trim tab. Furthermore, pivotal sidewall flaps are provided in addition to the rudder and the pitch trim tab. The pivotal sidewall flaps are pivotally mounted to the circular shroud and provided to enable, in pivoted state, deviation of tail propeller thrust.

The documents <CIT> and <CIT> also describe a compound helicopter with a main rotor and a tail propeller that is accommodated in a circular shroud. The circular shroud is provided with a plurality of vertically positioned, direction control vanes and a horizontally positioned pitch trim tab. In operation, the tail propeller creates an air stream which is directed through the circular shroud toward the plurality of vertically positioned, direction control vanes. This air stream may be deflected by the vertically positioned, direction control vanes to counteract torque created by the main rotor.

The document <CIT> describes a rotary wing aircraft with two horizontally positioned main rotors that are spaced apart from each other in direction of the aircraft's roll axis and accommodated in associated circular shrouds which are provided with pivotable control flaps. The pivotable control flaps are arranged in parallel to the aircraft's roll axis and may be pivoted to counteract torque created by the two horizontally positioned main rotors.

Other aircrafts with circular shrouds that are provided with rudders, flaps, vanes, or tabs and so on are also known from the state of the art. For instance, the document <CIT> describes a propeller aircraft with two propellers that are accommodated in associated circular shrouds which are provided with horizontally arranged flaps. These horizontally arranged flaps are, nevertheless, not used for anti-torque control in contrast to the above described anti-torque devices. <CIT> and <CIT> are known.

However, all above-described anti-torque devices with rudders, flaps, or vanes for anti-torque control have their main effect in hover condition of a respective compound helicopter or rotary wing aircraft, but their aerodynamic efficiency in transition and forward flight condition is restricted.

It is, therefore, an object of the present invention to provide a new compound helicopter and, more generally, a new rotary wing aircraft with a main rotor and a tail propeller, which is equipped with an improved anti-torque device with minimum loss of thrust and minimum drag increase due to control inputs and provisions.

This object is solved by a rotary wing aircraft with a main rotor and a tail propeller, said rotary wing aircraft comprising the features of claim <NUM>. More specifically, a rotary wing aircraft is provided that extends along an associated roll axis between a nose region and an aft region and that comprises a fuselage with a front section and a rear section. The rotary wing aircraft comprises a main rotor that is at least configured to provide lift in hover condition of the rotary wing aircraft; a propeller that is at least configured to propel the rotary wing aircraft in forward flight condition in a forward flight direction; and a shrouded duct that is arranged in the aft region and that forms an inner air duct which accommodates at least partly the propeller. The rear section extends between the front section and the shrouded duct and comprises an asymmetrical cross-sectional profile in direction of the associated roll axis. The rear section is configured to generate sideward thrust for main rotor anti-torque from main rotor downwash. The propeller comprises a predetermined number of propeller blades which form a circular propeller disc in rotation of the propeller around an associated rotation axis. The shrouded duct comprises a yaw and pitch stability enhancement unit for improving yaw and pitch stability of the rotary wing aircraft in the forward flight condition. Moreover, the rear section comprises at least one airfoil-shaped aerodynamic device that extends from the front section to the aft region, wherein the at least one airfoil-shaped aerodynamic device is configured to generate sideward thrust for main rotor anti-torque from main rotor downwash.

Thus, the inventive rotary wing aircraft may advantageously be controlled at least in transition and forward flight condition with an increased aerodynamic efficiency, in particular with minimum loss of thrust and minimum drag increase due to control inputs and provisions. More specifically, by realizing the yaw and pitch stability enhancement unit with fixed and movable provisions in front and aft of the shrouded duct, gear (pitch) behavior of the rotary wing aircraft may be controlled and its main rotor torque may be balanced in an aerodynamically efficient way. Thus, an increased gear authority may be enabled specifically in forward flight condition.

Preferably, the shrouded duct has a particular form and shaping, e. with respect to a non-circular cross section, a variable length, different angles of attack, varying distances of leading/trailing edges to a respective propeller blades' plane, etc. Such a particular form and shaping advantageously enable the shrouded duct to generate sideward thrust for main rotor anti-torque in transition and forward flight condition of the rotary wing aircraft.

Further additional sideward thrust for main rotor anti-torque may be generated from main rotor downwash by means of at least one source of asymmetry. For instance, the at least one source of asymmetry may be located to a side of the rotary wing aircraft that is closest to an approaching rotor blade of the main rotor in the sense of rotation of the main rotor. In other words, if the main rotor rotates in counterclockwise direction, the at least one source of asymmetry is located at the starboard side, i. the right-hand side of the fuselage of the rotary wing aircraft, which is preferably formed as a rotary wing aircraft with the main rotor and the propeller.

By way of example, the at least one source of asymmetry may be formed as a protruding edge of the fuselage that may be arranged close to a lower side of the fuselage. Such a protruding edge may be formed sufficiently large in order to be usable as a step. Furthermore, it may e. be integrated into an aerodynamically shaped skid landing gear. Alternatively, such a protruding edge may be built by a cover of a retractable nose landing gear. In this case, an aerodynamic performance of the rotary wing aircraft in fast forward flight will not be affected by the protruding edge, as it will be retracted into the fuselage together with the retractable nose landing gear such that the overall configuration is aerodynamically clean again. In fact, generation of respective anti-torque using the protruding edge preferably mainly occurs in hover condition and slow forward flight.

The at least one source of asymmetry may further include an elongation, i. a convex projection on top of the fuselage of the rotary wing aircraft, i. at an upper side of the fuselage. A suitable elongation may e. be obtained by an appropriate shaping of an upper cowling that covers an upper deck of the rotary wing aircraft. A highest point of the upper cowling may be shifted to an opposite side of the fuselage such that there is a smooth transition, at least with tangent constancy in the upper area of the cross section.

The sideward thrust for main rotor anti-torque that may be generated by the shrouded duct and/or the at least one source of asymmetry is advantageously increased by sideward thrust from main rotor downwash that is generated by means of the rear section of the fuselage of the rotary wing aircraft. More particularly, the rear section preferably generates the main part of sideward thrust from main rotor downwash. Therefore, the rear section of the fuselage of the rotary wing aircraft may exhibit a shape that is similar to a profile of a so-called high lift airfoil such that comparatively high sideward thrust may already be generated at comparatively low downwash air speed. Preferably, at least the main part of the rear section of the fuselage is asymmetrically located to a side of the rotary wing aircraft that is farther away from an approaching rotor blade of the main rotor in the sense of rotation of the main rotor. In other words, if the main rotor rotates in counterclockwise direction, the main part of the rear section of the fuselage is located at the starboard side of the rotary wing aircraft.

Preferably, a vertical cut through the rear section of the fuselage at a position that is close to the aft region of the rotary wing aircraft resembles to a high lift airfoil. This high lift airfoil is orientated at that position preferably at least approximately in vertical direction providing "lift", i. sideward thrust in the same direction in which the main rotor is rotating. In other words, if the main rotor rotates in counterclockwise direction, the sideward thrust likewise points into this direction.

Thus, less power is needed in hover condition as generation of downward drag in response to main rotor downwash is reduced due to a transformation of the main rotor downwash into sideward thrust, compared to the downward drag that is created by a conventional tail boom in response to main rotor downwash. Consequently, a gain of lifting capacity and fuel savings may be realized.

Advantageously, a transition in the region of the main rotor, preferably in a region between the main rotor and the rear section of the fuselage, from a cross-sectional profile of the rotary wing aircraft's fuselage, which may be symmetrical similar to a cross-sectional profile of a conventional helicopter, to the high lift airfoil-shaped cross-sectional profile of the rear section of the fuselage may be embodied as a smooth recess. The latter is preferably shaped in order to avoid airflow separation.

In an illustrative realization, the rear section is provided in the aft region with the shrouded duct, to which the propeller is rotatably mounted. An underlying transition of the rear section of the fuselage to the shrouded duct is preferably smooth, such that the rear section of the fuselage has at least approximately a shaping that corresponds to one quarter of the shrouded duct.

Furthermore, at least one wing-type aerodynamic device, which is also referred to as the "support wing" hereinafter, may be associated with the rear section of the fuselage for additional generation of sideward thrust for main rotor anti-torque from main rotor downwash. In operation of the rotary wing aircraft, a certain amount of main rotor downwash is present mainly in hover condition. Therefore, "lift" in horizontal direction, i. sideward thrust, may advantageously be generated simultaneously by the rear section of the fuselage, the associated support wing, as well as the front section and the shrouded duct, from the main rotor downwash in order to counteract the torque created by the main rotor in the hover condition.

Preferably, the support wing is mainly oriented vertically. More specifically, the support wing may connect the rotary wing aircraft's upper deck to the shrouded duct, e. from slightly behind of the main rotor to the shrouded duct. Advantageously, the support wing also exhibits a shape that is similar to a profile of a high lift airfoil. Preferably, the support wing has its greatest width at an area located between <NUM>% of the length of the main rotor's rotor blades and an outer end of the rotor blades.

Advantageously, the support wing and the rear section of the fuselage support the shrouded duct on both sides of the rotary wing aircraft, as there is no center part of the rear section of the fuselage, compared to a conventional tail boom. Preferably, a tail propeller drive shaft is arranged between the support wing and the rear section of the fuselage. This tail propeller drive shaft may also generate sideward thrust via the so-called Magnus effect, which occurs if a cylinder or cone is rotated in an airflow that is oriented perpendicular to its rotation axis.

The tail propeller drive shaft is preferably rotatably mounted to the shrouded duct, preferentially via a bearing that is supported by a predetermined number of provided stator profiles, such as e. three stator profiles. Preferably the stator profiles are mounted to the shrouded duct close to a respective location of the support wing and upper and lower edges of the rear section of the fuselage, where it is connected to the shrouded duct.

Alternatively, instead of providing a tail propeller drive shaft, the tail propeller may be powered independent of the main rotor, e. by means of a separate engine. This separate engine may be of a different type than a respective main engine that powers the main rotor such that engine hybridization is enabled with high redundancy, as the rotary wing aircraft may be operated in forward flight with each one of the engines independent of the other one.

According to some aspects, the yaw and pitch stability enhancement unit comprises a rudder that is arranged in the forward flight direction downstream of the circular propeller disc, the rudder comprising an elongated rudder body that is twisted around a length axis of the elongated rudder body.

According to some aspects, the elongated rudder body comprises a leading edge that is provided with a plurality of spaced tubercles.

According to some aspects, the elongated rudder body comprises an airfoil-shaped profile with a straight centerline that is rotated around the length axis of the elongated rudder body between both axial ends of the elongated rudder body, or with a cambered centerline that comprises a varying camber between both axial ends of the elongated rudder body.

According to some aspects, the rudder is rotatably mounted to the shrouded duct, or provided with one or more rotatable flaps.

According to some aspects, the yaw and pitch stability enhancement unit comprises at least one strut that is arranged in the forward flight direction downstream of the circular propeller disc, the at least one strut comprising an elongated strut body that is twisted around a length axis of the elongated strut body.

According to some aspects, the elongated strut body comprises a leading edge that is provided with a plurality of spaced tubercles.

According to some aspects, the elongated strut body comprises an airfoil-shaped profile with a straight centerline that is rotated around the length axis of the elongated strut body between both axial ends of the elongated strut body, or with a cambered centerline that comprises a varying camber between both axial ends of the elongated strut body.

According to some aspects, the yaw and pitch stability enhancement unit comprises at least one calotte-shaped Fowler-type flap.

According to some aspects, the shrouded duct comprises a ring-shaped duct body, wherein the at least one calotte-shaped Fowler-type flap is at least partly retractable into the ring-shaped duct body.

According to some aspects, the at least one calotte-shaped Fowler-type flap is arranged on a trailing edge of the shrouded duct.

According to some aspects, the at least one calotte-shaped Fowler-type flap forms, in a neutral position, at least partly the trailing edge of the shrouded duct.

According to some aspects, the yaw and pitch stability enhancement unit comprises a plurality of airfoil-shaped aerodynamic devices, in particular high lift airfoils, arranged at least approximately in parallel to the associated roll axis and forming a transition from the rear section to a leading edge of the shrouded duct.

According to some aspects, the shrouded duct comprises a leading edge that is provided with a plurality of spaced tubercles.

<FIG> shows an illustrative rotary wing aircraft <NUM> with a fuselage <NUM> and a main rotor <NUM>. By way of example, the rotary wing aircraft <NUM> is shown with three mutually orthogonal axes P, R, and Y. The axis P represents a transversal axis that corresponds to the pitch axis inherent to the rotary wing aircraft <NUM>, the axis R represents a longitudinal axis that corresponds to the roll axis inherent to the rotary wing aircraft <NUM>, and the axis Y represents a vertical axis that corresponds to the yaw axis inherent to the rotary wing aircraft <NUM>.

By way of example, the rotary wing aircraft <NUM> is illustrated in forward flight condition. Thus, only components that are required in the forward flight condition are illustrated in more detail, while illustration of other components is omitted, for simplicity and clarity of the drawing. For instance, neither the fuselage <NUM> is illustrated in greater detail for showing e. respective doors and windows, nor a possible landing gear, which may be a wheel-type landing gear or a skid-type landing gear mounted to the fuselage <NUM>, is shown, and so on.

Illustratively, the fuselage <NUM> extends along the roll axis R from a nose region <NUM> to an aft region <NUM> of the rotary wing aircraft <NUM>. The fuselage <NUM> comprises a front section <NUM> and a rear section <NUM>. Illustratively, the front section <NUM> comprises a port side wall <NUM> and a starboard side wall <NUM>. Preferably, the rear section <NUM> extends in prolongation to one of the port side wall <NUM> or the starboard side wall <NUM>.

More specifically, the rear section <NUM> preferably extends in prolongation to a side of the fuselage <NUM> that is farther away from an approaching rotor blade of the main rotor <NUM> in the sense of rotation of the main rotor <NUM>. Assuming that the main rotor <NUM> rotates in counterclockwise direction, the rear section <NUM> of the fuselage <NUM> would be located at the starboard side of the rotary wing aircraft <NUM> and, thus, be arranged in prolongation to the starboard side wall <NUM> as illustrated.

By way of example, the front section <NUM> merges into the rear section <NUM> at an associated transition or recess area <NUM>. In other words, starting at the nose region <NUM> of the fuselage <NUM> and travelling along the roll axis R, the fuselage <NUM> has the front section <NUM> that merges at the transition or recess area <NUM> into the rear section <NUM> that, in turn, terminates in the aft region <NUM>.

The front section <NUM> preferably forms a cabin <NUM> for passengers and/or cargo. The cabin <NUM> and, more generally, the fuselage <NUM> illustratively extends in direction of the yaw axis Y from a lower side <NUM> to an upper limit <NUM> that separates the cabin <NUM> from an upper deck <NUM>. The upper deck <NUM> is preferably covered by a cowling <NUM>. By way of example, the cowling <NUM> may cover one or more suitable engines and a main gear box that rotates the main rotor <NUM> in operation. Accordingly, the main rotor <NUM> is rotatably mounted at the front section <NUM> of the fuselage <NUM>.

Preferably, the main rotor <NUM> is at least configured to provide lift in hover condition of the rotary wing aircraft <NUM>. By way of example, the main rotor <NUM> forms a single rotor plane <NUM> and is adapted to provide lift and forward or backward thrust during operation. Illustratively, the main rotor <NUM> is embodied as a multiblade main rotor with a plurality of rotor blades <NUM>, <NUM>, <NUM>, <NUM>, <NUM> which are coupled at an associated rotor head <NUM> to a rotor mast <NUM>, which rotates in operation of the rotary wing aircraft <NUM> around an associated rotor axis.

According to one aspect, the rotary wing aircraft <NUM> is embodied as a compound helicopter with a propeller <NUM> that is at least configured to propel the rotary wing aircraft <NUM> in forward flight condition in a forward flight direction <NUM>. Accordingly, the rotary wing aircraft <NUM> is referred to hereinafter as the "compound helicopter <NUM>", for simplicity and clarity.

Illustratively, the propeller <NUM> comprises a predetermined number of propeller blades <NUM> which form a circular propeller disc <NUM> in rotation of the propeller <NUM> around an associated rotation axis <NUM>. More specifically, the propeller blades <NUM> rotate in operation around the rotation axis <NUM> in order to generate an airstream in a direction <NUM>, which is hereinafter also referred to as the "propulsion airstream <NUM>", for simplicity and brevity. The propulsion airstream <NUM> is preferably at least generated to propel the compound helicopter <NUM> in the forward flight condition.

The propeller <NUM> and the main rotor <NUM> may preferably be driven completely independent from each other. In particular, different types of engines may be used to drive the propeller <NUM> and the main rotor <NUM>, such as e. an air breathing propulsion engine for the main rotor <NUM> and an electric motor for the propeller <NUM>.

Preferably, the propeller <NUM> is rotatably mounted to a shrouded duct <NUM> at the rear section <NUM> of the fuselage <NUM> in the aft region <NUM> of the compound helicopter <NUM>. More specifically, the shrouded duct <NUM> is arranged in the aft region <NUM> and preferably forms an inner air duct <NUM> which accommodates at least partly the propeller <NUM>. Accordingly, the propeller <NUM> forms a tail propeller and, more particularly, preferably a pusher propeller. The propeller <NUM> may be mounted by any suitable means to the shrouded duct <NUM>, such as e. suitable stator profiles or struts. The shrouded duct <NUM> may be formed to generate sideward thrust for main rotor anti-torque at least in the forward flight condition.

According to one aspect, the shrouded duct <NUM> comprises a yaw and pitch stability enhancement unit, as described below at <FIG>. The yaw and pitch stability enhancement unit is preferably provided for improving yaw and pitch stability of the compound helicopter <NUM> in the forward flight condition.

Illustratively, the rear section <NUM> of the fuselage <NUM> extends between the front section <NUM> of the fuselage <NUM> and the shrouded duct <NUM>. The rear section <NUM> preferably comprises an asymmetrical cross-sectional profile <NUM> in direction of the roll axis R of the compound helicopter <NUM>. According to one aspect, the rear section <NUM> is configured to generate sideward thrust for main rotor anti-torque from main rotor downwash. By way of example, the asymmetrical cross-sectional profile <NUM> is at least approximately C-shaped, as illustrated with a series of cross-sectional profiles <NUM>, <NUM>, <NUM>, <NUM>.

In addition, the rear section <NUM> comprises at least one airfoil-shaped aerodynamic device <NUM> that extends from the front section <NUM> to the aft region <NUM> and, more particularly, to the shrouded duct <NUM>. By way of example, only one airfoil-shaped aerodynamic device <NUM> is shown. This airfoil-shaped aerodynamic device <NUM> is illustratively arranged in prolongation to the starboard side wall <NUM>.

In an illustrative realization, the airfoil-shaped aerodynamic device <NUM> is formed as, or by, a wing. This wing is, however, not arranged transversally to the roll axis R, but instead at least approximately in parallel to the roll axis R.

The airfoil-shaped aerodynamic device <NUM> may form the rear section <NUM>, at least partly. More particularly, the airfoil-shaped aerodynamic device <NUM> may be configured to generate sideward thrust for main rotor anti-torque from main rotor downwash.

The rear section <NUM> may further comprise at least one wing-type aerodynamic device <NUM> that extends between the front section <NUM> and the shrouded duct <NUM>. By way of example, only one wing-type aerodynamic device <NUM> is shown. Illustratively, the wing-type aerodynamic device <NUM> is at least approximately arranged in prolongation to the port side wall <NUM>.

Preferably, the wing-type aerodynamic device <NUM> is mounted to the shrouded duct <NUM> and to the upper deck <NUM> of the compound helicopter <NUM>. Illustratively, the wing-type aerodynamic device <NUM> and the airfoil-shaped aerodynamic device <NUM> are connected to opposite sides of the shrouded duct <NUM>. The wing-type aerodynamic device <NUM> may also be configured to generate sideward thrust for main rotor anti-torque from main rotor downwash of the compound helicopter <NUM>.

Illustratively, the compound helicopter <NUM> further comprises an uncovered propeller drive shaft <NUM>, in particular a cylinder shaft as described below at <FIG> or a conical shaft as described below at <FIG>, that is configured to create a Magnus effect upon rotation in main rotor downwash. The uncovered propeller drive shaft <NUM>, and likewise the shrouded duct <NUM>, may be inclined and may have an offset to the roll axis R of the compound helicopter <NUM> seen from above. Preferably, the uncovered propeller drive shaft <NUM> is configured to drive the propeller <NUM> in operation and, illustratively, extends coupling- and bearing-free between the front section <NUM> of the fuselage <NUM> and the propeller <NUM>, i. without intermediate coupling(s) and bearing(s).

The front section <NUM>, in turn, may comprise one or more sources of asymmetry, as described below at <FIG>. This source of asymmetry may also be configured to generate sideward thrust for main rotor anti-torque from main rotor downwash of the compound helicopter <NUM>.

<FIG> shows the compound helicopter <NUM> of <FIG> with the fuselage <NUM>, the main rotor <NUM>, the propeller <NUM>, the shrouded duct <NUM>, and the uncovered propeller drive shaft <NUM>. According to <FIG>, the fuselage <NUM> comprises the front section <NUM> that merges at the recess area <NUM> into the rear section <NUM>, the rear section <NUM> comprises the airfoil-shaped aerodynamic device <NUM> and the wing-type aerodynamic device <NUM>, the propeller <NUM> is rotatably mounted to the shrouded duct <NUM> and driven by the uncovered propeller drive shaft <NUM>, and the wing-type aerodynamic device <NUM> connects the upper deck <NUM> to the shrouded duct <NUM>.

According to one aspect, the shrouded duct <NUM> forms a swept back structure <NUM> and, illustratively, comprises an upper side <NUM> and a lower side <NUM>. This swept back structure <NUM> is preferably provided to adapt main rotor downwash at the shrouded duct <NUM> to a preferred behavior of the compound helicopter <NUM> during transition from hover condition to forward flight.

Illustratively, a bottom line <NUM> of the rear section <NUM> of the fuselage <NUM> is shown, which extends along the rear section <NUM> up to the lower side <NUM> of the shrouded duct <NUM>. This bottom line <NUM> is preferably angled by a predetermined inclination angle <NUM> relative to a horizontal reference plane <NUM>. By way of example, the predetermined inclination angle <NUM> is a positive (dihedral) angle that may be selected dependent on a required sideward thrust that is to be generated by the rear section <NUM> of the fuselage <NUM> in operation.

It should, nevertheless, be noted that the dihedral angle is only shown and described by way of example and not for restricting the present invention accordingly. Moreover, it should be noted that instead of selecting the illustrative dihedral angle e. a negative (anhedral) angle may likewise be selected for the predetermined inclination angle <NUM>. In this case, an increase of up to <NUM>% of generated sideward thrust may be obtained, resulting in a possibly achievable increase of more than <NUM>% of a respectively generated counteracting moment.

Furthermore, according to one aspect a rear door <NUM> and/or additional equipment, such as e. a winch, may be arranged in the recess area <NUM>. The rear door <NUM> may e. be a sliding or dual cantilever door. By way of example, the rear door <NUM> may be slidable into the fuselage <NUM>, i. toward the cabin <NUM>. Thus, in forward flight with opened door, there is no additional drag generated by the rear door <NUM>.

Preferably, this rear door <NUM> is accessible from a rear side of the compound helicopter <NUM>, i. coming from the rear section <NUM>. Thus, the compound helicopter's cabin <NUM> may be loaded from the rear side. Advantageously, by positioning the rear door <NUM> in the recess area <NUM>, penalties to the overall aerodynamic performance of the compound helicopter <NUM> due to the rear door <NUM> may be avoided.

<FIG> shows the compound helicopter <NUM> of <FIG> and <FIG> with the fuselage <NUM>, the main rotor <NUM>, the propeller <NUM>, the shrouded duct <NUM>, and the uncovered propeller drive shaft <NUM>. According to <FIG> and <FIG>, the fuselage <NUM> comprises the front section <NUM> that merges at the recess area <NUM> into the rear section <NUM>, the rear section <NUM> comprises the airfoil-shaped aerodynamic device <NUM> and the wing-type aerodynamic device <NUM>, the main rotor <NUM> comprises the rotor blades <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and the rotor head <NUM> which is arranged in the front section <NUM> of the fuselage <NUM>, the propeller <NUM> is rotatably mounted to the shrouded duct <NUM> and driven by the uncovered propeller drive shaft <NUM>, and the wing-type aerodynamic device <NUM> connects the upper deck <NUM> to the shrouded duct <NUM>.

More particularly, <FIG> clarifies the arrangement of the main rotor <NUM> and its rotor head <NUM> in the front section <NUM> of the fuselage <NUM>, as well as the merging of the front section <NUM> along the recess area <NUM> into the rear section <NUM> of the fuselage <NUM> by redirecting, i. deflecting the port side wall <NUM> of the compound helicopter <NUM> in the recess area <NUM> toward the starboard side wall <NUM>. Furthermore, the connection of the airfoil-shaped aerodynamic device <NUM> and the wing-type aerodynamic device <NUM> to almost diametrically opposed sides <NUM>, <NUM> of the shrouded duct <NUM>, which correspond to the starboard side wall <NUM> side and the port side wall <NUM> side, is also clarified. Preferably, at least the connection of the airfoil-shaped aerodynamic device <NUM> to the shrouded duct <NUM> is formed as a smooth transition <NUM>, preferentially over at least <NUM>% of an overall perimeter of the shrouded duct <NUM>.

By way of example, the wing-type aerodynamic device <NUM> has a width <NUM> that increases from the upper deck <NUM> over a predetermined length of the wing-type aerodynamic device <NUM>. More specifically, the width <NUM> illustratively starts to increase at the upper deck <NUM> and increases in direction of the shrouded duct <NUM>.

The width <NUM> may increase such that a maximum width value is reached at an area located between <NUM>% of the length of the rotor blades <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and their outer ends. Preferably, the width <NUM> increases over at least <NUM>% of the length of the wing-type aerodynamic device <NUM>. Illustratively, <NUM>% of the length of the wing-type aerodynamic device <NUM> is reached at a location <NUM>.

<FIG> shows the compound helicopter <NUM> of <FIG> with the fuselage <NUM>, the main rotor <NUM>, the propeller <NUM>, the shrouded duct <NUM>, and the uncovered propeller drive shaft <NUM>. According to <FIG>, the fuselage <NUM> comprises the front section <NUM> that merges at the recess area <NUM> into the rear section <NUM>, the rear section <NUM> comprises the airfoil-shaped aerodynamic device <NUM>, and the propeller <NUM> is rotatably mounted to the shrouded duct <NUM> and driven by the uncovered propeller drive shaft <NUM>. However, illustration of the wing-type aerodynamic device <NUM> is omitted for simplicity and clarity of the drawing.

More particularly, <FIG> clarifies the connection of the airfoil-shaped aerodynamic device <NUM> to the shrouded duct <NUM>. Furthermore, the merging of the front section <NUM> along the recess area <NUM> into the rear section <NUM> of the fuselage <NUM> by redirecting, i. deflecting the port side wall <NUM> of the compound helicopter <NUM> in the recess area <NUM> toward the starboard side wall <NUM> is also further clarified.

By way of example, at least one source of asymmetry <NUM> is connected to the front section <NUM>, preferably close to the lower side <NUM> of the fuselage <NUM>, such that the front section <NUM> comprises at least in sections an asymmetrical cross-sectional profile in direction of the associated roll axis R of <FIG>. The at least one source of asymmetry <NUM> is preferably configured to generate sideward thrust for main rotor anti-torque from main rotor downwash.

The at least one source of asymmetry <NUM> may be formed as an integral part of the fuselage <NUM>. Illustratively, the at least one source of asymmetry <NUM> is embodied as an integrally formed protruding edge of the fuselage <NUM>. Alternatively, the at least one source of asymmetry <NUM> may be pivotable and/or retractable, e. retractable into the fuselage <NUM>.

By way of example, the protruding edge is formed as a plate-shaped protrusion <NUM> of the fuselage <NUM>. The plate-shaped protrusion <NUM> is illustratively integrally formed with the fuselage <NUM>. The plate-shaped protrusion <NUM> may e. form an accessible step, for instance a step that is suitable to support passenger access into the cabin <NUM> of the compound helicopter <NUM>.

<FIG> shows the uncovered propeller drive shaft <NUM> of <FIG> which, in an illustrative realization, comprises a big diameter cylinder shaft <NUM>. This big diameter cylinder shaft <NUM> has preferably a diameter comprised in a range from <NUM> to <NUM> times the diameter of a conventional tail rotor drive shaft. Preferably, the big diameter cylinder shaft <NUM> enables creation of the so-called Magnus effect for generating additional sideward thrust in operation from main rotor downwash of the main rotor <NUM> of the compound helicopter <NUM> of <FIG>.

As described above at <FIG>, the uncovered propeller drive shaft <NUM> and, thus, the big diameter cylinder shaft <NUM> preferably extends coupling- and bearing-free between the front section <NUM> of the fuselage <NUM> and the propeller <NUM>. For purposes of illustration, the big diameter cylinder shaft <NUM> is, therefore, shown with two suitable bearings <NUM>, <NUM> at its axial ends, which are provided to rotatably support the big diameter cylinder shaft <NUM>.

In order to enable the coupling- and bearing-free extension of the big diameter cylinder shaft <NUM> between the front section <NUM> of the fuselage <NUM> and the propeller <NUM>, the big diameter cylinder shaft <NUM> must be sufficiently stiff for a reliable and secure functioning. This may be achieved by forming the big diameter cylinder shaft <NUM> using carbon composites, especially high modulus fiber.

<FIG> shows the uncovered propeller drive shaft <NUM> of <FIG> which, in another illustrative realization, comprises a big diameter conical shaft <NUM>. This big diameter conical shaft <NUM> has preferably a largest diameter comprised in a range from <NUM> to <NUM> times the diameter of a conventional tail rotor drive shaft. Preferably, the big diameter conical shaft <NUM> also enables creation of the so-called Magnus effect for generating additional sideward thrust in operation from main rotor downwash of the main rotor <NUM> of the compound helicopter <NUM> of <FIG>.

As described above at <FIG>, the uncovered propeller drive shaft <NUM> and, thus, the big diameter conical shaft <NUM> preferably extends coupling- and bearing-free between the front section <NUM> of the fuselage <NUM> and the propeller <NUM>. For purposes of illustration, the big diameter conical shaft <NUM> is, therefore, shown with two suitable bearings <NUM>, <NUM> at its axial ends, which are provided to rotatably support the big diameter conical shaft <NUM>. Preferably, the bearing <NUM> supports the big diameter conical shaft <NUM> at the propeller <NUM> of <FIG>.

In order to enable the coupling- and bearing-free extension of the big diameter conical shaft <NUM> between the front section <NUM> of the fuselage <NUM> and the propeller <NUM>, the big diameter conical shaft <NUM> must be sufficiently stiff for a reliable and secure functioning. This may be achieved by forming the big diameter conical shaft <NUM> using carbon composites, especially high modulus fiber.

<FIG> shows the uncovered propeller drive shaft <NUM> of <FIG>, which either comprises the big diameter cylinder shaft <NUM> of <FIG> or the big diameter conical shaft <NUM> of <FIG>. In an illustrative operation of the main rotor <NUM> of the compound helicopter <NUM> of <FIG>, the uncovered propeller drive shaft <NUM> is rotated in main rotor downwash <NUM> in a rotation direction <NUM>. Thus, as a result of the Magnus effect, the uncovered propeller drive shaft <NUM> generates a sideward force <NUM>. This sideward force <NUM>, in turn, results in sideward thrust applied to the rear section <NUM> of the fuselage <NUM> of the compound helicopter <NUM> of <FIG>.

It should be noted that the Magnus effect is well-known to the person skilled in the art. Therefore, for brevity and conciseness the Magnus effect and its application for generation of sideward thrust by means of the uncovered propeller drive shaft <NUM> is not described in more detail.

<FIG> shows the shrouded duct <NUM> of <FIG>, seen in a direction VIII of <FIG>. The shrouded duct <NUM>, which forms the inner air duct <NUM> of <FIG>, illustratively comprises a ring-shaped duct body <NUM> that forms a trailing edge <NUM>. Furthermore, as described at <FIG>, the shrouded duct <NUM> according to the present invention comprises a yaw and pitch stability enhancement unit <NUM>. The yaw and pitch stability enhancement unit <NUM> is preferably at least provided to generate sideward thrust for main rotor anti-torque in forward flight condition of the compound helicopter <NUM> of <FIG>, at least partly by deviating the propulsion airstream <NUM> of <FIG>, for provision of an increased yaw and pitch stability of the compound helicopter <NUM> of <FIG>.

According to one aspect, the yaw and pitch stability enhancement unit <NUM> comprises a rudder <NUM>. The rudder <NUM> is preferably rotatably mounted to the shrouded duct <NUM> and illustrated in a neutral state, i. without deflection.

The rudder <NUM> preferably comprises an elongated rudder body <NUM> and is preferably vertically positioned with respect to the ring-shaped duct body <NUM> of the shrouded duct <NUM>. Illustratively, the elongated rudder body <NUM> extends along an associated length axis <NUM> from an upper axial end <NUM> toward a lower axial end <NUM>. By way of example, a central section <NUM> of the elongated rudder body <NUM> is positioned at the rotation axis <NUM> of the tail propeller <NUM> of <FIG>.

The elongated rudder body <NUM> illustratively forms a leading edge <NUM> and a trailing edge <NUM>. Preferably, the elongated rudder body <NUM> and, more generally, the rudder <NUM> is twisted around the length axis <NUM>.

According to one aspect, the yaw and pitch stability enhancement unit <NUM> comprises in addition, or alternatively, at least one strut <NUM>. The at least one strut <NUM> comprises an elongated strut body <NUM> and is preferably horizontally positioned with respect to the ring-shaped duct body <NUM> of the shrouded duct <NUM>. By way of example, the elongated strut body <NUM> is arranged at an angle of approximately <NUM>° with respect to the elongated rudder body <NUM>.

Illustratively, the elongated strut body <NUM> extends along an associated length axis <NUM> from a left-hand axial end <NUM> toward a right-hand axial end <NUM>. By way of example, a central section <NUM> of the elongated strut body <NUM> is coaxially positioned with respect to the rotation axis <NUM> of the tail propeller <NUM> of <FIG>.

The elongated strut body <NUM> illustratively forms a leading edge <NUM> and a trailing edge <NUM>. Preferably, the elongated strut body <NUM> and, more generally, the at least one strut <NUM> is twisted around the length axis <NUM>.

According to one aspect, the yaw and pitch stability enhancement unit <NUM> comprises in addition, or alternatively, at least one calotte-shaped Fowler-type flap. Illustratively, two calotte-shaped Fowler-type flaps <NUM>, <NUM> are provided, by way of example on diametrically opposed sides of the shrouded duct <NUM>. These calotte-shaped Fowler-type flaps <NUM>, <NUM> are described in more detail below at <FIG>.

<FIG> shows the shrouded duct <NUM> of <FIG> with the ring-shaped duct body <NUM> that forms the trailing edge <NUM> and a leading edge <NUM>. The ring-shaped duct body <NUM> forms the inner air duct <NUM> through which the propulsion airstream <NUM> of <FIG> is guided from the leading edge <NUM> toward the trailing edge <NUM> to propel the compound helicopter <NUM> of <FIG> in the forward flight direction <NUM>. The shrouded duct <NUM> further comprises the yaw and pitch stability enhancement unit <NUM> of <FIG>, which illustratively comprises the rudder <NUM> with the leading edge <NUM> and the trailing edge <NUM>, the at least one strut <NUM> with the leading edge <NUM> and the trailing edge <NUM>, and the calotte-shaped Fowler-type flaps <NUM>, <NUM>. The rudder <NUM> is again shown in the neutral state, i. without deflection.

By way of example, the shrouded duct <NUM> is illustrated together with the circular propeller disc <NUM> which is formed by rotation of the propeller <NUM> of <FIG> inside the inner air duct <NUM> of the shrouded duct <NUM>. According to one aspect, the rudder <NUM> is arranged in the forward flight direction <NUM> downstream of the circular propeller disc <NUM>, i. the circular propeller disc <NUM> is positioned closer to the leading edge <NUM> of the shrouded duct <NUM> than the rudder <NUM>. Likewise, the at least one strut <NUM> is preferably arranged in the forward flight direction <NUM> downstream of the circular propeller disc <NUM>, i. the circular propeller disc <NUM> is positioned closer to the leading edge <NUM> of the shrouded duct <NUM> than the at least one strut <NUM>.

As the circular propeller disc <NUM> is positioned closer to the leading edge <NUM> of the shrouded duct <NUM> than the rudder <NUM> and/or the at least one strut <NUM>, the rudder <NUM> and/or the at least one strut <NUM> are positioned in the propulsion airstream <NUM>, which propels the compound helicopter <NUM> of <FIG> in the forward flight direction <NUM>. However, the propulsion airstream <NUM> has not only a velocity in rearward direction, but also a superimposed rotation around the propeller axis <NUM> of <FIG>. This rotation is caused by the rotation of the propeller <NUM> of <FIG>. Nevertheless, by positioning the rudder <NUM> and/or the at least one strut <NUM> in the propulsion airstream <NUM> and by twisting both as described at <FIG>, the propulsion airstream <NUM> may advantageously be straightened, thus, increasing a respective efficiency of the shrouded duct <NUM> at least a few percentages.

In any case, the at least one strut <NUM> is preferably used as a stator profile to support the propeller <NUM> of <FIG> in the shrouded duct <NUM> and, more specifically, in the inner air duct <NUM>, and to transfer the thrust of the propeller <NUM> first to the shrouded duct <NUM> and then into the fuselage <NUM> of the compound helicopter <NUM> of <FIG>. Usually, at least three struts are used. If these struts are positioned as illustrated behind the propeller <NUM> and equipped e. with rudders or suitable flaps, they may be used to support trim and control around the pitch axis P of Figure <NUM>. Therefore, the struts are preferably aerodynamically formed.

The rudder <NUM>, in turn, is preferably provided at least for yaw control around the yaw axis Y of <FIG> of the compound helicopter <NUM> of <FIG> in forward flight. To this end, the rudder <NUM> may entirely be rotatable around its length axis <NUM> of <FIG> similar to a so-called pendulum rudder, as described below at <FIG>, or it may be implemented by means of a conventional discrete rudder, as described below at <FIG>.

<FIG> shows the rudder <NUM> of <FIG> with the elongated rudder body <NUM> that extends along the length axis <NUM>. The elongated rudder body <NUM> forms the leading edge <NUM> and the trailing edge <NUM>.

According to one aspect, the elongated rudder body <NUM> comprises an airfoil-shaped profile <NUM> with a straight centerline <NUM>. Illustratively, airfoil-shaped profile <NUM> and, thus, the straight centerline <NUM> is rotated around the length axis <NUM> of the elongated rudder body <NUM> between both axial ends (<NUM>, <NUM> in <FIG>) of the elongated rudder body <NUM> such that the elongated rudder body <NUM> is twisted around the length axis <NUM>.

More specifically, the rudder <NUM> is formed with a central airfoil profile <NUM> located at the central section <NUM> of <FIG>, an upper airfoil profile <NUM> located at the upper axial end <NUM> of <FIG>, and a lower airfoil profile <NUM> located at the lower axial end <NUM> of <FIG>. The central airfoil profile <NUM> is illustrated with the straight centerline <NUM> in unrotated position, the upper airfoil profile <NUM> is illustrated with a straight centerline <NUM> that corresponds to the straight centerline <NUM> which is rotated around the length axis <NUM> in counterclockwise direction, and the lower airfoil profile <NUM> is illustrated with a straight centerline <NUM> that corresponds to the straight centerline <NUM> which is rotated around the length axis <NUM> in clockwise direction.

<FIG> is likewise applicable to the at least one strut <NUM> of <FIG>, which comprises the elongated strut body <NUM> that extends along the length axis <NUM>, wherein the elongated strut body <NUM> forms the leading edge <NUM> and the trailing edge <NUM>. In this case, the elongated strut body <NUM> comprises the airfoil-shaped profile <NUM> with the straight centerline <NUM>, wherein the airfoil-shaped profile <NUM> and, thus, the straight centerline <NUM> is rotated around the length axis <NUM> of the elongated strut body <NUM> between both axial ends (<NUM>, <NUM> in <FIG>) of the elongated strut body <NUM> such that the elongated strut body <NUM> is twisted around the length axis <NUM>. Accordingly, the at least one strut <NUM> comprises the central airfoil profile <NUM> located at the central section <NUM> of <FIG>, the airfoil profile <NUM> located at the left-hand axial end <NUM> of <FIG>, and the airfoil profile <NUM> located at the right-hand axial end <NUM> of <FIG>. Consequently, the central airfoil profile <NUM> is illustrated with the straight centerline <NUM> in unrotated position, the airfoil profile <NUM> is illustrated with the straight centerline <NUM> that corresponds to the straight centerline <NUM> which is rotated around the length axis <NUM> in counterclockwise direction, and the airfoil profile <NUM> is illustrated with the straight centerline <NUM> that corresponds to the straight centerline <NUM> which is rotated around the length axis <NUM> in clockwise direction.

<FIG> shows the airfoil profiles <NUM>, <NUM>, <NUM> of the rudder <NUM> (or the at least one strut <NUM>) according of <FIG>. However, although the airfoil profile <NUM> still comprises the straight centerline <NUM> of <FIG>, the airfoil profiles <NUM>, <NUM> now comprise in contrast to <FIG> cambered centerlines <NUM>, <NUM> with a varying camber according to an alternative realization.

In other words, the camber of the centerlines <NUM>, <NUM> is varied around the length axis <NUM> (or <NUM>) of the elongated rudder body <NUM> (or the elongated strut body <NUM>) from the central section (<NUM> or <NUM> in <FIG>) toward both axial ends (<NUM>, <NUM> or <NUM>, <NUM> in <FIG>) of the elongated rudder body <NUM> (or the elongated strut body <NUM>) in order to obtain the twist of the elongated rudder body <NUM> (or the elongated strut body <NUM>). By way of example, the centerline <NUM> is illustratively cambered in counterclockwise direction and the centerline <NUM> is illustratively cambered in clockwise direction.

<FIG> shows the airfoil profiles <NUM>, <NUM>, <NUM> of the rudder <NUM> (or the at least one strut <NUM>) according of <FIG>. However, in contrast to <FIG> all airfoil profiles <NUM>, <NUM>, <NUM> now comprise a cambered centerline <NUM>.

<FIG> shows the airfoil profile <NUM> of the rudder <NUM> (or the at least one strut <NUM>) with the cambered centerline <NUM> according of <FIG>. However, in contrast to <FIG> the twist of the elongated rudder body <NUM> (or the elongated strut body <NUM>) is now obtained by providing the elongated rudder body <NUM> (or the elongated strut body <NUM>) with one or more rudder flaps. By way of example, two rudder flaps <NUM>, <NUM> are illustrated, which are deflected in opposite directions by way of example.

If the elongated rudder body <NUM> is provided with the one or more rudder flaps <NUM>, <NUM>, then the rudder <NUM> as such may be mounted fixedly to the shrouded duct <NUM> of <FIG>. In other words, it suffices if either the rudder <NUM> as a whole or the rudder flaps <NUM>, <NUM> together, or separately, are rotatable.

<FIG> shows the shrouded duct <NUM> of <FIG> with the ring-shaped duct body <NUM> that forms the trailing edge <NUM> and the leading edge <NUM>. The ring-shaped duct body <NUM> forms the inner air duct <NUM> through which the propulsion airstream <NUM> of <FIG> is guided from the leading edge <NUM> toward the trailing edge <NUM>. The shrouded duct <NUM> further comprises the yaw and pitch stability enhancement unit <NUM> of <FIG>. However, in contrast to <FIG> the yaw and pitch stability enhancement unit <NUM> now only comprises the calotte-shaped Fowler-type flaps <NUM>, <NUM> in order to illustrate that in selected realizations the rudder <NUM> and the at least one strut <NUM> according to <FIG> may also be omitted. Furthermore, it should be noted that the calotte-shaped Fowler-type flap <NUM> may also be omitted.

Illustratively, the calotte-shaped Fowler-type flaps <NUM>, <NUM> are arranged on, or close to, the trailing edge <NUM> of the shrouded duct <NUM>. Preferably, the calotte-shaped Fowler-type flaps <NUM>, <NUM> form at least partly the trailing edge <NUM> of the shrouded duct <NUM> in an associated neutral position.

According to one aspect, the calotte-shaped Fowler-type flaps <NUM>, <NUM> are at least partly deployable from the ring-shaped duct body <NUM> of the shrouded duct <NUM> and/or at least partly retractable into the ring-shaped duct body <NUM> of the shrouded duct <NUM>. By way of example, the calotte-shaped Fowler-type flap <NUM> is shown in the associated neutral position, but also in a completely deployed position. In this completely deployed position, the calotte-shaped Fowler-type flap is labelled with the reference sign <NUM>. Similarly, the calotte-shaped Fowler-type flap <NUM> is shown in the associated neutral position, but also by way of example in a completely retracted position. In this completely retracted position, the calotte-shaped Fowler-type flap is labelled with the reference sign <NUM>.

<FIG> shows the shrouded duct <NUM> of <FIG> with the ring-shaped duct body <NUM> comprising the trailing edge <NUM> and the leading edge <NUM>. The shrouded duct <NUM> further comprises the calotte-shaped Fowler-type flap <NUM> which is shown in the associated neutral position, in which the calotte-shaped Fowler-type flap <NUM> illustratively forms the trailing edge <NUM> of the shrouded duct <NUM>, and in the completely retracted position, in which the calotte-shaped Fowler-type flap is farther retracted into the shrouded duct <NUM> and labelled with the reference sign <NUM>.

More specifically, according to one aspect the ring-shaped duct body <NUM> comprises a hollow interior <NUM>. The hollow interior <NUM> is preferably formed such that the calotte-shaped Fowler-type flap <NUM>, <NUM> may at least partly be accommodated in the hollow interior <NUM> in the associated neutral position (<NUM>) as well as in the completely retracted position (<NUM>), as illustrated.

<FIG> shows the shrouded duct <NUM> of <FIG> with the ring-shaped duct body <NUM> comprising the trailing edge <NUM> and the leading edge <NUM>. The shrouded duct <NUM> further comprises the calotte-shaped Fowler-type flap <NUM> which is shown in the associated neutral position, in which the calotte-shaped Fowler-type flap <NUM> illustratively forms the trailing edge <NUM> of the shrouded duct <NUM>, and in the completely deployed position, in which the calotte-shaped Fowler-type flap is completely extracted and deployed from the shrouded duct <NUM> and labelled with the reference sign <NUM>.

More specifically, as described at <FIG> the ring-shaped duct body <NUM> preferably comprises the hollow interior <NUM>. The hollow interior <NUM> is preferably formed such that the calotte-shaped Fowler-type flap <NUM>, <NUM> may at least partly be accommodated in the hollow interior <NUM> in the associated neutral position (<NUM>) and extracted and deployed therefrom into the completely deployed position (<NUM>), as illustrated.

<FIG> shows the shrouded duct <NUM> with the trailing edge <NUM> and the leading edge <NUM> according to <FIG>, wherein the shrouded duct <NUM> has the ring-shaped duct body <NUM> that forms the hollow interior <NUM>. <FIG> further illustrates the calotte-shaped Fowler-type flap <NUM> in the associated neutral position, wherein the calotte-shaped Fowler-type flap <NUM> is at least partly accommodated in the hollow interior <NUM> and forms the trailing edge <NUM> of the shrouded duct <NUM>.

<FIG> shows the shrouded duct <NUM> with the trailing edge <NUM> and the leading edge <NUM> according to <FIG>, wherein the shrouded duct <NUM> has the ring-shaped duct body <NUM> that forms the hollow interior <NUM>. <FIG> further illustrates the calotte-shaped Fowler-type flap <NUM> in the completely deployed position, in which the calotte-shaped Fowler-type flap <NUM> is spaced apart from the ring-shaped duct body <NUM> by a predetermined gap <NUM>.

The predetermined gap <NUM> may, however, lead to aerodynamically unfavorable conditions at the trailing edge <NUM>. Such aerodynamically unfavorable conditions may be prevented by modifying the calotte-shaped Fowler-type flap <NUM> as described in detail below at <FIG>.

<FIG> shows the shrouded duct <NUM> with the trailing edge <NUM> and the leading edge <NUM> according to <FIG>, according to which the shrouded duct <NUM> has the ring-shaped duct body <NUM> that forms the hollow interior <NUM>, wherein the calotte-shaped Fowler-type flap <NUM> is at least partly accommodated in the associated neutral position such that it forms the trailing edge <NUM> of the shrouded duct <NUM>. However, in contrast to <FIG> the calotte-shaped Fowler-type flap <NUM> is now formed as a multi-part component that illustratively comprises two separate flap components <NUM>, <NUM> which are, preferably, spaced apart from each other. The two separate flap components <NUM>, <NUM> are preferably simultaneously deployable from the ring-shaped duct body <NUM>.

<FIG> shows the shrouded duct <NUM> with the trailing edge <NUM> and the leading edge <NUM> according to <FIG>, wherein the shrouded duct <NUM> has the ring-shaped duct body <NUM> that forms the hollow interior <NUM>. However, in contrast to <FIG> the calotte-shaped Fowler-type flap <NUM> with the two separate flap components <NUM>, <NUM> is now illustrated in the completely deployed position, in which the flap component <NUM> preferably forms the trailing edge <NUM> of the shrouded duct <NUM>, while the flap component <NUM> is illustratively spaced apart from the ring-shaped duct body <NUM>.

<FIG> shows the shrouded duct <NUM> with the trailing edge <NUM> and the leading edge <NUM> according to <FIG>, according to which the shrouded duct <NUM> has the ring-shaped duct body <NUM> that forms the hollow interior <NUM>, wherein the calotte-shaped Fowler-type flap <NUM> is at least partly accommodated in the associated neutral position such that it forms the trailing edge <NUM> of the shrouded duct <NUM>. However, in contrast to <FIG> the calotte-shaped Fowler-type flap <NUM> is now formed with the flap component <NUM> of <FIG> and an extension <NUM> that abuts and, thus, prolongates the flap component <NUM> in direction of the leading edge <NUM> of the shrouded duct <NUM>.

<FIG> shows the shrouded duct <NUM> with the trailing edge <NUM> and the leading edge <NUM> according to <FIG>, wherein the shrouded duct <NUM> has the ring-shaped duct body <NUM> that forms the hollow interior <NUM>. However, in contrast to <FIG> the calotte-shaped Fowler-type flap <NUM> with the flap component <NUM> and the extension <NUM> is now illustrated in the completely deployed position.

<FIG> shows the shrouded duct <NUM> with the trailing edge <NUM> and the leading edge <NUM> according to <FIG>, wherein the shrouded duct <NUM> has the ring-shaped duct body <NUM> that forms the inner air duct <NUM>. The shrouded duct <NUM> comprises the yaw and pitch stability enhancement unit <NUM>.

However, in contrast to <FIG> the yaw and pitch stability enhancement unit <NUM> now illustratively comprises a plurality of airfoil-shaped aerodynamic devices <NUM>, <NUM>, <NUM>, <NUM>, in particular high lift airfoils, instead of the calotte-shaped Fowler-type flaps <NUM>, <NUM> of <FIG>. The airfoil-shaped aerodynamic devices <NUM>, <NUM>, <NUM>, <NUM> are preferably arranged at least approximately in parallel to the associated roll axis R of the compound helicopter <NUM> of <FIG> and form a transition from the rear section <NUM> of the compound helicopter <NUM> to the leading edge <NUM> of the shrouded duct <NUM>. Illustratively, the airfoil-shaped aerodynamic devices <NUM>, <NUM>, <NUM>, <NUM> are connected to a section <NUM> of the shrouded duct <NUM> that has a shorter axial length than the remaining part of the shrouded duct <NUM>.

More specifically, the shrouded duct <NUM> is set back in the section <NUM> that is located in a lower area of the shrouded duct <NUM>. In this lower area, a respective downwash of the main rotor <NUM> of the compound helicopter <NUM> of <FIG> hits the shrouded duct <NUM> essentially vertically and, thereby, creates an aerodynamically damaging flow which, in turn, creates a strong aerodynamic drag in flow direction of the downwash. This drag is amplified when the propeller <NUM> of <FIG> in the inner air duct <NUM> is sucking in flow at the same time. In the lower area and, more specifically, in the section <NUM>, it is therefore advantageous to reduce the depth, i. the axial length of the shrouded duct <NUM>, as described above. Thus, the airfoil-shaped aerodynamic devices <NUM>, <NUM>, <NUM>, <NUM> may deflect the aerodynamically damaging flow laterally and use the latter thereby to generate an additional counter torque to the torque created by the main rotor <NUM> of the compound helicopter <NUM> of <FIG>. This may especially be helpful in a respective transition phase between pure hover and forward flight of the compound helicopter <NUM> of <FIG>.

<FIG> shows the shrouded duct <NUM> with the ring-shaped duct body <NUM> and the leading edge <NUM> according to any one of <FIG>. According to one aspect, the leading edge <NUM> is now alternatively, or in addition, provided with a plurality of spaced tubercles <NUM>. The plurality of spaced tubercles <NUM> may be provided in one or more selected sections of the perimeter of the ring-shaped duct body <NUM>, or along the entire perimeter.

<FIG> shows the shrouded duct <NUM> with the trailing edge <NUM> and the leading edge <NUM> according to <FIG>, wherein the shrouded duct <NUM> has the ring-shaped duct body <NUM> that forms the inner air duct <NUM>. The shrouded duct <NUM> further comprises the yaw and pitch stability enhancement unit <NUM> of <FIG>. However, in contrast to <FIG> the yaw and pitch stability enhancement unit <NUM> now only comprises the rudder <NUM> with the elongated rudder body <NUM> and the at least one strut <NUM> with the elongated strut body <NUM> in order to illustrate that in selected realizations the calotte-shaped Fowler-type flaps <NUM>, <NUM> according to <FIG> may be omitted.

According to one aspect, the leading edge <NUM> of the elongated rudder body <NUM> of the rudder <NUM> is now alternatively, or in addition, provided with a plurality of spaced tubercles <NUM>. In addition, or alternatively, the leading edge <NUM> of the elongated strut body <NUM> may be provided with the plurality of spaced tubercles <NUM>. In both cases, the leading edges <NUM>, <NUM> may be provided in one or more sections, or over their entire lengths, with the spaced tubercles <NUM>.

<FIG> shows the elongated rudder body <NUM> of the rudder <NUM> according to <FIG> with the spaced tubercles <NUM> on the leading edge <NUM>, which are magnified for purposes of clarification. <FIG> may analogously be applied to the at least one strut <NUM> of <FIG> and, thus, also shows the elongated strut body <NUM> of the at least one strut <NUM> according to <FIG> with the spaced tubercles <NUM> on the leading edge <NUM>.

As described above at <FIG>, the leading edge <NUM> of the shrouded duct <NUM> may be provided with the plurality of spaced tubercles <NUM> and/or the leading edge <NUM> of the elongated rudder body <NUM> of the rudder <NUM> may be provided with the plurality of spaced tubercles <NUM> and/or the leading edge <NUM> of the elongated strut body <NUM> of the at least one strut <NUM> may be provided with the plurality of spaced tubercles <NUM>. Thus, a respective airflow separation that occurs at the leading edges <NUM>, <NUM>, and/or <NUM> can be shifted to higher angles of attack. Accordingly, such an airflow separation that would cause higher drag and an increased disturbance of the airflow inside the shrouded duct <NUM> may advantageously be avoided or, at least, be substantially reduced. Thus, an underlying efficiency of the shrouded duct <NUM>, the rudder <NUM>, and/or the at least one strut <NUM> is at least less reduced for high deflection angles of those parts, as the airflow does not separate.

<FIG> shows the shrouded duct <NUM> with the trailing edge <NUM> according to <FIG>, wherein the shrouded duct <NUM> has the ring-shaped duct body <NUM> that forms the inner air duct <NUM>. The shrouded duct <NUM> further comprises the yaw and pitch stability enhancement unit <NUM> of <FIG>. However, in contrast to <FIG> the yaw and pitch stability enhancement unit <NUM> now only comprises the rudder <NUM> with the elongated rudder body <NUM> and the at least one strut <NUM> with the elongated strut body <NUM> in order to illustrate that in selected realizations the calotte-shaped Fowler-type flaps <NUM>, <NUM> according to <FIG> may be omitted.

According to one aspect, the rudder <NUM> and, more specifically, the elongated rudder body <NUM> that forms the leading edge <NUM> and the trailing edge <NUM> now exhibits an undulated shaping <NUM>. Illustratively, the undulated shaping <NUM> corresponds at least essentially to an S-shape that is formed with two turning points <NUM>, <NUM> between both axial ends <NUM>, <NUM> of the elongated rudder body <NUM>. However, other undulation shapes are likewise contemplated, such as e. simple C-shapes with a single turning point, double S-shapes with four turning points, and so on.

Preferably, the undulated shaping <NUM> is formed as an out-of-plane undulation. More specifically, a respective rudder body plane <NUM> is illustratively formed by virtually connecting the leading edge <NUM> at the axial ends <NUM>, <NUM> and the trailing edge <NUM> at the axial ends <NUM>, <NUM>. Accordingly, the axial ends <NUM>, <NUM> as well as the central section <NUM> of the elongated rudder body <NUM> are lying in the rudder body plane <NUM>, while the elongated rudder body <NUM> as such is essentially lying outside of the rudder body plane <NUM> and, therefore, exhibits an out-of-plane undulation.

<FIG> shows the shrouded duct <NUM> with the yaw and pitch stability enhancement unit <NUM> that comprises the rudder <NUM> with the elongated rudder body <NUM>, as well as the at least one strut <NUM> according to <FIG> further illustrates the undulated shaping <NUM> of the elongated rudder body <NUM> in the form of an out-of-plane undulation relative to the rudder body plane <NUM> of <FIG>. By providing the elongated rudder body <NUM> with the undulated shaping <NUM>, vortices that are generated by the propeller blades (<NUM> in <FIG>) upstream of the rudder <NUM> do not hit the rudder all at once, compared to a straight rudder as illustrated in <FIG>. Thus, an important source of noise may be eliminated and, accordingly, noise generation may be reduced significantly.

However, any deflection of the rudder <NUM> will move the elongated rudder body <NUM> at least in sections closer to the propeller blades (<NUM> in <FIG>). To avoid this, an in-plane undulation may be advantageous, either instead of the out-of-plane undulation or in combination therewith, as described hereinafter.

<FIG> show the shrouded duct <NUM> with the trailing edge <NUM> according to <FIG>, wherein the shrouded duct <NUM> has the ring-shaped duct body <NUM> that forms the inner air duct <NUM>. The shrouded duct <NUM> further comprises the yaw and pitch stability enhancement unit <NUM> of <FIG>. However, in contrast to <FIG> the yaw and pitch stability enhancement unit <NUM> now only comprises the rudder <NUM> with the elongated rudder body <NUM> and the at least one strut <NUM> with the elongated strut body <NUM> in order to illustrate that in selected realizations the calotte-shaped Fowler-type flaps <NUM>, <NUM> according to <FIG> may be omitted.

In analogy to <FIG>, the rudder <NUM> and, more specifically, the elongated rudder body <NUM> that forms the leading edge <NUM> and the trailing edge <NUM> exhibits an undulated shaping that is now labelled with the reference sign <NUM>. Illustratively, the undulated shaping <NUM> corresponds at least essentially to a double C-shape that is formed with three turning points <NUM>, <NUM>, <NUM> between both axial ends <NUM>, <NUM> of the elongated rudder body <NUM>. However, other undulation shapes are likewise contemplated, such as e. simple C-shapes with a single turning point, quadruple C-shapes with five turning points, and so on.

Preferably, the undulated shaping <NUM> is formed as an in-plane undulation. More specifically, a respective rudder body plane <NUM> is illustratively formed by virtually connecting the leading edge <NUM> at the axial ends <NUM>, <NUM> and the trailing edge <NUM> at the axial ends <NUM>, <NUM>. Accordingly, the rudder body plane <NUM> illustratively corresponds to the drawing layer in <FIG> and the elongated rudder body <NUM> is completely lying in the drawing layer, i. the rudder body plane <NUM>, and, therefore, exhibits an in-plane undulation.

According to one aspect, the rudder <NUM> and, more specifically, the elongated rudder body <NUM> now exhibits a combination of the undulated shaping <NUM> of <FIG> and the undulated shaping <NUM> of <FIG>. In other words, the elongated rudder body <NUM> now exhibits both an out-of-plane undulation and an in-plane undulation.

<FIG> show the shrouded duct <NUM> with the leading edge <NUM> and the trailing edge <NUM> according to <FIG>, wherein the shrouded duct <NUM> has the ring-shaped duct body <NUM> that forms the inner air duct <NUM>. The shrouded duct <NUM> further comprises the yaw and pitch stability enhancement unit <NUM> of <FIG>. However, in contrast to <FIG> the yaw and pitch stability enhancement unit <NUM> now only comprises the rudder <NUM> with the elongated rudder body <NUM> and the at least one strut <NUM> with the elongated strut body <NUM>.

According to one aspect, the trailing edge <NUM> of the shrouded air duct <NUM> is now formed as an undulated edge <NUM>. The undulated edge <NUM> is preferably at least provided to reduce noise generation at the shrouded duct <NUM>.

By way of example, the undulated edge <NUM> is formed in <FIG> by a plurality of chevrons <NUM>, i. by tooth-shaped protrusions formed in axial direction of the shrouded duct <NUM>. In <FIG>, the undulated edge <NUM> is illustratively formed by means of a wave-shaped border with protrusions formed in radial direction of the shrouded duct <NUM>.

At this point, it should be noted that the shrouded duct <NUM> is described above as being provided with the yaw and pitch stability enhancement unit <NUM>, which in turn is described with different components according to different embodiments. In other words, the yaw and pitch stability enhancement unit <NUM> is described to comprise one or more of the rudder <NUM>, the at least one strut <NUM>, the calotte-shaped Fowler-type flaps <NUM>, <NUM>, and the airfoil-shaped aerodynamic devices <NUM>, <NUM>, <NUM>, <NUM>. Furthermore, the rudder <NUM> and/or the at least one strut <NUM> may be provided with the spaced tubercles <NUM>. Furthermore, the rudder <NUM> may have the undulated shaping <NUM>, <NUM>. Moreover, the leading edge <NUM> of the shrouded duct <NUM> may be provided with the spaced tubercles <NUM> and/or its trailing edge <NUM> may be provided with the undulated edge <NUM>. However, any suitable combination of these characteristics other than the ones described above is likewise contemplated and may be determined in an application-specific manner dependent on an underlying configuration of the compound helicopter <NUM> of <FIG>. Determination of such a suitable combination is, nevertheless, considered to fall into the common knowledge of the person skilled in the art.

Furthermore, it should be noted that modifications to the above-described realizations are also within the common knowledge of the person skilled in the art and, thus, also considered as being part of the present invention. For instance, the airfoil-shaped aerodynamic device <NUM> described above may be realized with one, two or more separate airfoil-shaped aerodynamic devices. Similarly, instead of being provided with the single wing-type aerodynamic device <NUM>, two or more wing-type aerodynamic devices may be provided.

Claim 1:
A rotary wing aircraft (<NUM>) that extends along an associated roll axis (R) between a nose region (<NUM>) and an aft region (<NUM>) and that comprises a fuselage (<NUM>) with a front section (<NUM>) and a rear section (<NUM>), the rotary wing aircraft (<NUM>) comprising:
a main rotor (<NUM>) that is at least configured to provide lift in hover condition of the rotary wing aircraft (<NUM>),
a propeller (<NUM>) that is at least configured to propel the rotary wing aircraft (<NUM>) in forward flight condition in a forward flight direction (<NUM>), the propeller (<NUM>) comprising a predetermined number of propeller blades (<NUM>) which form a circular propeller disc (<NUM>) in rotation of the propeller (<NUM>) around an associated rotation axis (<NUM>),
a shrouded duct (<NUM>) that is arranged in the aft region (<NUM>) and that forms an inner air duct (<NUM>) which accommodates at least partly the propeller (<NUM>),
the shrouded duct (<NUM>) comprising a yaw and pitch stability enhancement unit (<NUM>) for improving yaw and pitch stability of the rotary wing aircraft (<NUM>) in the forward flight condition,
the rear section (<NUM>) extending between the front section (<NUM>) and the shrouded duct (<NUM>) and comprises an asymmetrical cross-sectional profile (<NUM>) in direction of the associated roll axis (R), the rear section (<NUM>) being configured to generate sideward thrust for main rotor anti-torque from main rotor downwash (<NUM>),
characterized in that
the rear section (<NUM>) comprises at least one airfoil-shaped aerodynamic device (<NUM>) that extends from the front section (<NUM>) to the aft region (<NUM>), and wherein the at least one airfoil-shaped aerodynamic device (<NUM>) is configured to generate sideward thrust for main rotor anti-torque from main rotor downwash (<NUM>).