Blade for a helicopter anti-torque device
View Patent ↗A blade ( 1 ) for a tail anti-torque device of a helicopter, said device comprising a ducted rotor ( 13 ). The blade ( 1 ) has an assembly of sections, a leading edge ( 2 ) and a trailing edge ( 3 ); a stacking line ( 4 ) of the profiles extending at a distance lying in the range 25% to 50% of the chord (C). The stacking line ( 4 ) presents a shape that is curved having successively from the root ( 5 ) to the end ( 6 ) of the blade a back sweep, a forward sweep, and a last back sweep; and a chord (C) that becomes larger going towards the end ( 6 ) of the blade ( 1 ) over at least the end profile ( 1 a ).
1. A blade for a tail anti-torque device of a rotorcraft, the device comprising a ducted rotor, the blade having a three-dimensional geometrical shape defined by different airfoil profile sections extending between a leading edge and a trailing edge of the blade and defined radially by a root section and by an end section of the blade; at least two of the airfoil profile sections having different chord values along a stacking line extending from the root section to the end section, the blade being twisted, the stacking line, prior to twisting, defining a plane extension surface defined radially from the root section to the end section, and a twisting line of the plane extension surface that is defined by a predetermined twisting relationship, the stacking line extending at a distance lying in a range 25% to 50% of a respective chord (C) for each of the airfoil profile sections from the leading edge and presenting a curve on the plane extension surface comprising in succession from the root section: a first back sweep; a forward sweep; and finally a last back sweep going to the end section; the end section having an end chord (C max ) and the root section having a root chord (C ref ) less than the end chord (C max ) the end chord (C max ) being the largest of the respective chords, and the end chord (C max ) being equal to or less than 1.6 times the root chord (C ref ) of the root section,
wherein the respective chords (C) are defined by a variable relationship such that a chord value C(r) for a radius r of a respective section of the airfoil profile sections, is given by:
C ( r )= C ref , if k≦r≦b c ×R max
and
C
(
r
)
=
C
ref
+
[
C
max
-
C
ref
]
×
[
r
-
(
b
c
×
R
max
)
]
n
[
R
max
-
(
b
c
×
R
max
)
]
n
if
r
≥
b
c
×
R
max
where k is a radius of the root section, R max designates a maximum radius of the end section, C max designates the chord of the end section and is equal to a c ×C ref , where a c is a factor, where x designates the multiplication sign and b c and n designate first and second predetermined constants.
2. The blade according to claim 1 , wherein the respective airfoil profile sections present relative thickness that decreases progressively in a radial direction from the root section towards the end section to present a relative thickness at the end section that lies in a range 9% to 6%.
3. The blade according to claim 1 , wherein the root section presents a relative thickness that lies in a range 9% to 14%.
4. The blade according to claim 1 , wherein the factor a c is equal to 1.38.
5. The blade according to claim 1 , wherein the twisting line corresponds to the stacking line.
6. The blade according to claim 1 , wherein the stacking line presents a shape having a curve given by a polynomial of third degree, the stacking line extending at a distance of 30% of the respective chords (C) starting from the leading edge of the respective airfoil profile sections.
7. The blade according to claim 1 , wherein the blade has an assembly of six distinct airfoil profile sections along the stacking line.
8. The blade according to claim 1 , wherein the leading edge presents in continuity from the root section to the end section a shape that is concave and then convex.
9. The blade according to claim 1 , wherein the first predetermined constant b c lies in a range 0 to 1.
10. The blade according to claim 9 , wherein the first predetermined constant b c is 0.68.
11. The blade according to claim 1 , wherein the second predetermined constant n lies in a range 1 to 5.
12. The blade according to claim 11 , wherein the second predetermined constant n is 1.7.
13. The blade according to claim 1 , wherein the stacking line is a curve traced relative to a radial direction of the tail anti-torque device and given by a polynomial such that:
Y AC /C ref =a e ×Q 3 +b e ×Q 2 +c e ×Q with 0≦ Q≦ 1
where:
Q
=
r
-
k
R
max
-
k
is a non-dimensional number;
Y AC is a position of the stacking line of the respective airfoil profile sections for a radius r; k is the radius of the root section, and a e , b e and c e are first, second, and third predetermined values.
14. The blade according to claim 13 , wherein the first, second, and third predetermined values a e , b e and c e are selected to obtain the stacking line that extends in a distribution that is substantially identical on either side of a geometrical pitch axis of the blade, the distribution balancing static pitch control moments acting on the blade.
15. The blade according to claim 13 , wherein the first predetermined value a e is −2.5681, the second predetermined value b e is +3.9238, and the third predetermined value c e is +1.3558.
16. An anti-torque device comprising a fairing defining an airflow passage having a rotor placed therein together with members for driving the rotor and members for adjusting a pitch of a blade, wherein the rotor is provided with blades in accordance with claim 1 .
17. The anti-torque device according to claim 16 , wherein the device includes an airflow guiding stator downstream from a blade path of the blades of the rotor, the guide stator being provided with profiled vanes.