Method of optimizing a ducted anti-torque rotor of a rotorcraft, in particular a helicopter, to minimize acoustic annoyance, and a ducted anti-torque rotor obtained thereby
View Patent ↗The present invention relates to a method of minimizing the noise emitted by a ducted rail rotor anti-torque device at a frequency FE and perceived at a frequency FP, in which method the diameter D, the number of blades b, and the tangential speed U of the rotor are determined so that the perceived frequency FP is less than or equal to the bottom limit frequency FL of a predetermined one-third frequency octave centered on a frequency FC.
1. A method of optimizing an anti-torque device ( 2 ) for a rotorcraft that is noise certified at a forward velocity V, the rotorcraft comprising a multi-bladed ducted tail rotor ( 7 ) of diameter D with a given number b of variable pitch blades ( 10 ), mounted to rotate at a speed of rotation Ω substantially coaxially in an airflow duct ( 6 ) of axis extending substantially transversely relative to the rotorcraft and passing through a fairing ( 5 ) incorporated in the tail portion of the rotorcraft, wherein the following steps are performed:
a bottom limit frequency FL is selected for a predetermined one-third octave centered on a center frequency FC;
the perceived frequency FP of the tone produced by said ducted tail rotor anti-torque device is determined by multiplying the emission frequency FE of that tone by a multiplicative coefficient C to take account of the Doppler effect, i.e.:
FP=C×FE
the parameters relating to the diameter D of the rotor, the given number b of blades, and to the tangential speed U at the ends of the blades are selected so that the perceived frequency FP of the tone as given by:
C×[U× ( n×b ) ±m ]/( π×D)
is less than the bottom limit frequency FL of a predetermined one-third octave centered on a frequency FC, n and m being positive integers.
2. A method according to claim 1 , wherein said coefficient C is such that:
C
=
1
1
-
(
M
×
cos
θe
)
in which expression M designates the Mach number of the noise-emitting source, i.e. the anti-torque device, traveling at a forward velocity V, the Mach number being equal to the ratio V/a where “a” represents the speed of sound, and θe designates the angle formed between the forward velocity V and the axis between the noise source and the observer.
3. A method according to claim 2 , wherein a temperature range of 15° C. to 25° C. is used for determining the speed of sound.
4. A method according to claim 2 , wherein said angle θe is selected to lie in the range 30 degrees to 45 degrees.
5. A method according to claim 2 , wherein the forward velocity V expressed in knots is equal to:
V=(0.45 ×VH )+65kts
where VH is the maximum velocity of the rotorcraft.
6. A method according to claim 1 , wherein said center frequency FC is equal to 500 Hz.
7. A method according to claim 1 , wherein the integer numbers n and m are respectively equal to 1 and 0.
8. An acoustically optimized anti-torque device ( 2 ) for a rotorcraft that is noise certified at a forward velocity V, the device comprising a multi-bladed ducted tail rotor ( 7 ) of diameter D having a given number b of variable pitch blades ( 10 ), mounted to rotate a speed of rotation Ω substantially coaxially in an airflow duct ( 6 ) of axis extending substantially transversely relative to the rotorcraft and passing through a fairing ( 5 ) incorporated in the rear portion of the rotorcraft, wherein the parameters relating to the diameter D of the rotor, to the given number b of blades, and to the tangential speed U, equal to Ω×(D×2), at the ends of the blades are such that:
U
×
[
(
n
×
b
)
±
m
]
D
<
π
×
FC
2
6
[
1
-
(
V
a
×
cos
θe
)
]
where n and m are positive integers, FC is the center frequency of a predetermined one-third octave, and a and θe represent respectively the speed of sound and the angle between the forward velocity V and the axis between the noise source, i.e. the anti-torque device and the observer.
9. A device according to claim 8 , wherein the speed of sound a lies in a range appropriate for an outside temperature lying in the range 15° C. to 25° C.
10. A device according to claim 8 , wherein said angle θe lies in a range 30 degrees to 45 degrees.
11. A device according to claim 8 , wherein the forward velocity V expressed is knots is equal to:
[(0.45 ×VH )+65 kts]
VH designating the maximum forward velocity of the rotorcraft.
12. A device according to claim 8 , wherein the center frequency FC is equal to 500 Hz.
13. A device according to claim 8 , wherein said ducted tail rotor anti-torque device ( 2 ) includes a deflector stator ( 8 ) stationary in the duct ( 6 ), downstream from the rotor ( 7 ) relative to the flow direction of the air flowing through the duct ( 6 ) and comprising stationary vanes ( 9 ) disposed substantially in a star configuration about the axis x-x of the duct ( 6 ).