A blade for an antitorque tail rotor of a helicopter, having a leading edge and a trailing edge opposite each other and elongated along a longitudinal axis of the blade; the trailing edge, in use, interacts with the air current after the leading edge; the blade also has two opposite surfaces extending between the leading edge and the trailing edge, and a root portion extending from a radially inner first end, with respect to a rotation axis of the blade, towards a second end opposite the first end; and the root portion, when sectioned in a plane perpendicular to the leading edge and trailing edge, has a profile asymmetrical with respect to a chord joining the leading edge and trailing edge.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to European Patent Application No. 08425368.1 filed May 22, 2008. The entirety of the application is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Helicopters are known comprising a fuselage; a main rotor fitted to the top of a centre portion of the fuselage; and an antitorque tail rotor for opposing the torque generated by the main rotor on the fuselage.

Tail rotors substantially comprise a drive shaft; a hub fitted to the drive shaft; and a number of blades fixed to and projecting radially from the hub.

More specifically, each blade extends lengthwise substantially radially, and is rotated by the hub in a plane perpendicular to the drive shaft axis.

Each blade is also movable in any plane with respect to the hub to manoeuvre the helicopter.

A need is felt within the industry to improve the aerodynamic efficiency of the blades, and reduce the loads on the blades and the tail rotor control mechanisms, without increasing the radial size of the tail rotor.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a helicopter antitorque tail rotor designed to achieve the above in a straightforward, low-cost manner.

According to the present invention, there is provided a helicopter antitorque tail rotor, as claimed in the attached claims.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 9,12and13show a tail portion of a helicopter1substantially comprising a fuselage2; a main rotor (not shown) fitted to the top of fuselage2and rotating about a respective axis; and a tail rotor3projecting from a tail fin of fuselage2to oppose the torque transmitted by rotor3to fuselage2.

More specifically, rotor3substantially comprises (FIGS. 9 to 13):a drive shaft5rotating about an axis A crosswise to the rotation axis of the main rotor;a number of—in the example shown, two—blades6extending along respective axes B substantially radial with respect to axis A; anda hub7connected functionally to shaft5, and from which blades6project.

More specifically, hub7rotates blades6about axis A, allows blades6to move freely with respect to shaft5in a plane defined by axis A and respective axes B, and allows rotation of blades6, by an external control, about respective axes B to adjust the respective angles of attack with respect to the air current.

Axis A is located outside blades6.

With particular reference toFIGS. 1 to 4, each blade6is hollow and bounded by:a leading edge8defined by the foremost points of blade6with reference to the rotation direction (shown inFIG. 9) of blade6;a trailing edge9defined by the rearmost points of blade6with respect to said rotation direction, and located on the opposite side to leading edge8;a radially inner end10located on the hub7side and interposed between leading edge8and trailing edge9; anda radially outer end11opposite end10and also interposed between leading edge8and trailing edge9.

Each blade6substantially comprises a face12and a back13, which are interposed radially between ends10and11and separated by leading edge8and trailing edge9.

More specifically, back13is interposed between face12and the tail fin from which rotor3projects.

From end10to end11, blade6comprises (FIGS. 1-4and8):a root portion14a;an intermediate portion14b; andan end portion14ccurving away from the tail fin of helicopter1with respect to root portion14aand intermediate portion14b.

In other words, end portion14chas an anhedral with respect to the rest of blade6.

From end10to end11, leading edge8(FIG. 4) comprises a straight first portion extending along root portion14a; a straight second portion sloping with respect to the first portion; and a curved portion extending along end portion14c.

More specifically, the second portion extends along intermediate portion14b.

Hub7comprises (FIGS. 9 to 13):a plate15, which is rotated by shaft5about axis A, rotates with respect to shaft5about an axis C perpendicular to axis A and axes B, and is connected to blades6in angularly fixed manner with respect to axis A, and in rotary manner with respect to relative axes B;two pairs of plates20connected in fixed manner to relative blades6; anda sleeve25(FIGS. 10 and 11) which is rotated by shaft5about axis A, is slid along axis A with respect to shaft5by a control not shown, and is connected to the two pairs of plates20to rotate blades6about respective axes B.

More specifically, plate15lies in a plane crosswise to axis A, and comprises a main portion16fitted to shaft5; and two appendixes17having respective ends18opposite axis A and fitted inside respective seats19of respective blades6(FIG. 10).

Shaft5(FIGS. 10 and 11) is surrounded by a cylindrical member22engaging a seat21defined by main portion16. Member22and seat21are connected in rotary manner about axis C and in angularly fixed manner about axis A. The surfaces of member22and seat21are conjugate, and have respective coincident centres located at the intersection of axes A and C.

Seat21and member22thus define a cylindrical hinge, of axis C, allowing blades6to oscillate integrally with each other, i.e. flap, about axis C and with respect to shaft5. More specifically, such oscillation is caused by the different aerodynamic loads on blades6, as a result of the different relative speeds of blades6with respect to the air current.

Appendixes17project from main portion16on opposite sides of axis A, and extend inside respective blades6; ends18are in the form of hollow cylinders coaxial with respective axes B; and seats19are in the form of cylindrical cavities extending along respective axes B, so that insertion of ends18inside respective seats19allows blades6to rotate about respective axes B with respect to plate15, and makes blades6and plate15angularly fixed about axes A and C.

Plates20in each pair are fixed one to the face12and the other to the back13of a relative blade6, are parallel to each other, and lie in respective substantially parallel planes.

Hub7comprises, for each pair of plates20, a pair of arms24(FIGS. 9,10,11) having first ends fixed to respective plates20in the same pair of plates20. The second ends of arms24in each pair are connected to each other by a transverse member26interposed between axis A and end10of relative blade6.

Sleeve25projects from shaft5on the opposite side of the tail fin, and comprises:first radial appendixes27(FIGS. 10,12,13) diametrically opposite with respect to axis A and connected to respective members26by respective ties29; andsecond radial appendixes28diametrically opposite with respect to axis A, and each connected by two levers31,32to a plate33angularly integral with shaft5and interposed, along axis A, between shaft5and sleeve25.

Ties29extend crosswise to axis A, and have first ends connected to relative appendixes27; and second ends, opposite the first ends, connected to respective members26eccentrically with respect to relative axes B (FIG. 10).

More specifically, ties29are connected to respective members26so that, when sleeve25slides along axis A, blades6rotate in the same direction about respective axes B.

Each lever31has a first end hinged to sleeve25; and a second end, opposite the first end, hinged to a first end of a corresponding lever32.

Each lever32has a second end, opposite the first end, hinged to plate33.

Plates20in each pair are connected to each other by a pin35perpendicular to axis B of relative blade6, and which has an intermediate portion36housed inside root portion14aof relative blade6, and engaging a seat37, defined by relative appendix17, in rotary manner with respect to axis B.

More specifically, intermediate portion36has a spherical outer surface mating with a spherical surface defined by seat37. More specifically, the spherical surfaces defined by intermediate portion36and relative seat37are concentric, and have respective centres along relative axis B.

Intermediate portions36of pins35and relative seats37thus define respective hinges allowing blades6to rotate about respective axes B with respect to plate15.

Face12and back13of each blade6have respective holes38(FIGS. 1 to 3) adjacent to end10and fitted through with opposite ends of relative pin35.

When sectioned in a plane perpendicular to leading edge8and trailing edge9(FIG. 5), root portion14aadvantageously has a profile G asymmetrical with respect to a chord P joining leading edge8and trailing edge9.

By virtue of the asymmetrical design of profile G, root portion14aplays an active part in the lift generated on blades6and, therefore, in the torque transmitted by rotor3to fuselage2.

More specifically, face12and back13are blended at leading edge8, and are joined by a sharp edge at trailing edge9, along both root portion14aand intermediate and end portions14b,14c.

At root portion14a, back13is convex, whereas face12has a concave first portion41adjacent to trailing edge9, and a convex second portion42interposed between portion41and leading edge8(FIG. 5).

In each section perpendicular to leading edge8and trailing edge9, the points of profile G defining back13are further away from chord P than the corresponding points of profile G defining face12(FIGS. 3,4,5).

With reference to profile G, chord P comprises a main portion P1interposed between face12and back13; and an end portion P2at the trailing edge9end. More specifically, adjacent to trailing edge9, portion41is interposed between end portion P2and back13(FIG. 5).

More specifically, profile G is obtained at a section of root portion14aadjacent to end10.

The points at which face12is furthest from back13at root portion14aare indicated by portion43inFIGS. 1 to 4.

In theFIGS. 6 and 7sections of blade6, chord P is interposed between back13and face12.

From leading edge8to trailing edge9in each section of blade6in a plane perpendicular to trailing edge9, face12and back13first diverge and then converge (FIGS. 5 to 7).

As shown inFIG. 8, in an intermediate longitudinal plane of blade6between leading edge8and trailing edge9, face12and back13converge at root portion14a, remain a constant distance apart at intermediate portion14b, and converge at end portion14c.

The length of chord P of blade6, i.e. the distance between leading edge8and trailing edge9, measured perpendicularly to trailing edge9, is constant at intermediate portion14b.

As shown inFIGS. 5 to 7, the slope of chords P with respect to a fixed axis perpendicular to leading edge8and trailing edge9varies from end10to end11. More specifically, the fixed axis is vertical with reference toFIGS. 5 to 7, and the angle between chords P and the fixed axis decreases from root portion14a(FIG. 5) to end portion14c(FIG. 7).

In other words, the setting angle of blades6varies along respective axes B, i.e. as opposed to lying in one plane, the locus of the points of chords P has a curved profile when viewed from above.

At end10, face12and back13comprise, from leading edge8to trailing edge9, respective first portions45lying in the same plane sloping with respect to trailing edge9; respective curved second portions46surrounding respective holes38; and respective third portions47lying in the same plane sloping with respect to the plane of portions45.

Portions45extend symmetrically with respect to relative axis B (FIGS. 4 and 13, left), whereas portions47are asymmetrical with respect to relative axis B.

The distance between end10and axis A advantageously ranges between 10 and 25% of the maximum distance between the points of end11and axis A.

The distance between end10and axis A preferably ranges between 10 and 23% of the maximum distance between the points of end11and axis A.

In actual use, shaft5rotates about axis A to rotate hub7.

Plate15rotates blades6about axis A, while the connection between member22and seat21in plate15allows blades6to oscillate freely about axis C under aerodynamic loads.

By means of an external control, blades6can be rotated by the same angle and in the same direction about respective axes B to vary the angles of attack of blades6with respect to the air current flowing over blades6.

More specifically, the external control translates sleeve25along axis A, which translation is transmitted to ties29and members26.

Ties29being connected to members26eccentrically with respect to relative axes B, translation of ties29rotates plates20and, therefore, blades6about respective axes B.

As the blades rotate, seats19of blades6rotate about respective axes B with respect to the corresponding ends18of relative appendixes17of plate15, and pins35rotate about relative axes B with respect to seats37of relative appendixes17.

During normal operation of rotor3, significant lift is generated on root portions14aof blades6.

Root portions14aof blades6therefore play an active part in the force transmitted by rotor3to the tail fin, and, therefore, the torque transmitted to fuselage2.

The advantages of rotor3according to the present invention will be clear from the above description.

In particular, by virtue of its design, root portion14aof blade6plays an active part in the aerodynamic force exchanged between the air current and blade6, and therefore the torque transmitted by rotor3to fuselage2of helicopter1.

More specifically, the Applicant has observed that the design of root portion14aprovides for generating lift even at distances from axis A ranging between 10 and 20% of the overall radial size of blade6. In other words, for a given Reynolds number, the design of root portion14aimproves the lift coefficient of blade6.

Blade6therefore provides for maximum aerodynamic efficiency of rotor3for a given overall radial size of rotor3.

Moreover, because lift is also generated at sections of blade6extremely close to axis A, stress caused by bending moments on the control members of rotor3is greatly reduced for a given righting torque generated by rotor3.

In other words, for a given righting torque generated by rotor3, and therefore a given resultant of the lift forces on blades6, the design of root portions14abrings the point of application of the resultant closer to axis A.

Clearly, changes may be made to rotor3as described and illustrated herein without, however, departing from the protective scope defined in the accompanying Claims.

In particular, the means hinging hub7to shaft5and blades6to hub7may be of a different type.