WO2019080442A1 - Rotorcraft - Google Patents

Abstract

A rotorcraft comprising: a ducted fan (2) comprising a main duct (3) having a central axis; at least two first arms (10) and at least one second arm, respectively perpendicular to the central axis Provided on the outer wall (8) of the main duct (3); at least two first rotors (4) having a first rotating shaft are provided in at least two of the first rotating shaft perpendicular to the first arm (10) a free end of the first arm (10); and at least one second rotor (6) having a second axis of rotation, the second axis of rotation being perpendicular to the second arm and the second axis being at an angle to the central axis The free end of at least one second arm. The rotorcraft can increase the load capacity, extend the battery life, and has better steering stability.

Description

Rotorcraft Technical field

The present invention relates to the field of drones, and more particularly to a rotorcraft.

Background technique

In recent years, the drone industry has developed in a blowout manner, especially in the civilian consumer entertainment multi-rotor UAVs. The demand for users is large, the industry profits are extremely high, and the value-added potential is huge. However, the more prominent problem of multi-rotor drones is that their load and life time are greatly limited by the development of battery technology, and battery technology is difficult to break in a short period of time. The long-haul heavy-duty drone is an important trend in the development of future drones. Therefore, it is imperative to develop a drone that can realize long-haul heavy loads based on market prospects.

The prior art UAVs mainly include: pure oil moving multi-rotor, traditional oil-electric hybrid multi-rotor, pure oil moving ducted fan drone and traditional oil-electric hybrid ducted fan drone.

Pure oil-powered multi-rotor, that is, powered by multiple engines, directly drives the rotor to rotate, the rotor pitch is fixed, and the rotor speed is changed by adjusting the engine speed to change the lift. The rotor is mounted directly on the engine shaft, eliminating the need for a transmission. The drawback is that the number of engines is too large, the weight is high, the engine itself has a delay in response, and the power efficiency is low. At the same time, due to the large vibration level of the engine itself, the rotor is directly driven, and the vibration of the engine is transmitted to the rotor, causing the vibration response of the blade. The response is coupled with the aerodynamic force of the blade to the response of the blade, which easily leads to rotor power. Unstable problems can cause fatigue problems in the blades and the engine, reducing the service life, and the blades may even fail quickly in severe cases.

The conventional oil-electric hybrid multi-rotor, that is, the main lift unit is driven by an engine, the auxiliary lift or attitude control unit is driven by a motor, and the configuration of the drone has good stability, but has a defect, and the aerodynamic efficiency is relatively low, which is also A ubiquitous defect in multi-rotating unmanned wings.

Pure oil moving ducted fan unmanned aerial vehicle, that is, a fan with a fan inside. The engine is installed inside the hub or above the hub, and the static vane is used to balance the counter torque and act as an air flow guide. The rudder surface is used to control the attitude. The defect of this type of oil-moving duct is that the aerodynamic performance of the ducted state is better for the hovering state, but in the forward flight, the aerodynamic center is located above the duct, and the pitching moment characteristic is more prominent, and the position of the center of gravity of the whole machine is prominent. The limitation is relatively large, and the control is realized by the control of the rudder surface. The control is very difficult, and the overall airflow of the rudder facing the duct has a certain blockage, which reduces the aerodynamic efficiency.

technical problem

The traditional oil-electric hybrid ducted fan unmanned aerial vehicle is based on the above-mentioned pure oil moving ducted fan, and an auxiliary electric rotor is added. This improves the attitude stability control, but the control difficulty of the control surface is adopted. Still very big. At present, the oil-electric hybrid ducted fan drones mainly include four-rotor aircraft, fixed-wing aircraft, helicopter aircraft and single-ducted aircraft. The four-rotor aircraft is currently mainly an electric four-rotor, which is limited by the battery, has a short battery life and a small load; although the fixed-wing aircraft can achieve a long-haul heavy load, it requires a long runway for take-off and landing, and cannot achieve hovering. The flight control of the helicopter is very complicated and difficult to control. The flight control of the single-ducted aircraft is mainly realized by adjusting the deflection of the moving blades below the duct. The structure is complex, the weight is large, and the flight control is complicated.

Technical solution

In view of the problems in the related art, an object of the present invention is to provide a rotorcraft capable of improving load capacity and endurance time and having better steering stability.

To achieve the above object, the present invention provides a rotorcraft comprising: a ducted fan including a main duct having a central axis; at least two first arms and at least one second arm, respectively perpendicular to the central axis Provided on an outer wall of the main duct; at least two first rotors having a first rotating shaft are disposed at a free end of the at least two first arms in a manner that the first rotating shaft is perpendicular to the first arm; and at least one has The second rotor of the second rotating shaft is disposed at a free end of the at least one second arm with the second rotating shaft perpendicular to the second arm and the second rotating shaft being at an angle to the central axis.

According to an embodiment of the present invention, the method further includes: providing an auxiliary duct at a free end of the at least one second arm, the second rotor being disposed in the auxiliary duct, and the second rotating shaft and the axis of the auxiliary duct being in the same straight line.

According to an embodiment of the invention, the number of the first arms is set to four, the number of the second arms is two, and the four first arms and the two second arms are arranged around the central axis in the main duct. The circumferential direction is equiangularly arranged, the two second arms are disposed on opposite sides of the main duct in the same radial direction, the number of the first rotors is four, and the number of the second rotors is two, two The second rotors are respectively disposed on the two second arms, and the four first rotors are respectively disposed on the four first arms.

According to an embodiment of the invention, the four first rotating shafts are parallel to the central axis, and the two second rotating shafts form an angle opposite to the central axis and of equal magnitude.

According to an embodiment of the invention, the first arm and the second arm are each provided in two numbers, and the two first arms and the two second arms are equiangularly equi-angled about the central axis in the circumferential direction of the main duct Arranging that two first arms are disposed on opposite sides of a main duct in a radial direction, and two second arms are disposed on opposite sides of the main duct in another radial direction, the first rotor The number of the two rotors is two, the two first rotors are respectively disposed at the free ends of the two first arms, and the two second rotors are respectively disposed at the free ends of the two second arms. .

According to an embodiment of the invention, the two first rotating shafts form an angle opposite to the central axis and are equal in magnitude, and the two second rotating shafts form an angle opposite to the central axis and are equal in magnitude.

According to an embodiment of the invention, the rotorcraft further comprises a driving device, the ducted fan comprises a moving blade disposed in the main duct and a stationary blade located below the moving blade, and the moving blade, the first rotor and the second rotor are respectively connected with the driving device .

According to an embodiment of the invention, the driving device comprises an engine, and the main culvert is provided with a mounting frame, the mounting frame is located above the moving blade or below the stationary blade, the engine is arranged on the mounting frame, and the engine and the moving blade are connected by a transmission mechanism.

According to an embodiment of the invention, the driving device further includes a first motor and a second motor, the first rotor is coaxially disposed with the first motor at a free end of the first arm, and the second rotor is coaxial with the second motor. The free end of the two arms.

According to an embodiment of the invention, the first arm and the second arm are each connected to the main duct by a folding mechanism such that the first arm and the second arm have an open position in a radial direction of the main duct And a storage position parallel to the tangential direction of the main duct.

Beneficial effect

Advantageous technical effects of the present invention are:

The invention can better balance the counter torque generated by the fan by providing the second rotor at an angle with the central axis on the arm on the main duct, and at the same time providing the first rotor to control the flight attitude of the drone. The stable flight of the drone improves the working efficiency and handling stability of the ducted fan, and realizes the long flight time and large load of the drone.

DRAWINGS

1 is a top plan view of a rotorcraft in accordance with one embodiment.

2 is a front elevational view of a rotorcraft in accordance with another embodiment.

Figure 3 is a top plan view of the rotorcraft of Figure 2.

4 is a perspective view of the rotorcraft of FIG. 1.

Figure 5 is a perspective view of the rotorcraft of Figure 1 with the main duct removed.

Figure 6 is a perspective view of a rotorcraft in accordance with still another embodiment.

Figure 7 is a front elevational view of the rotorcraft of Figure 6.

Figure 8 is a top plan view of the rotorcraft of Figure 6.

Figure 9 is a side elevational view of the rotorcraft of Figure 6.

Figure 10 is a perspective view of the ducted fan of the rotorcraft of Figure 1.

Figure 11 is a perspective view of the ducted fan of the rotorcraft of Figure 1.

Embodiments of the invention

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

Referring to Figure 1, the present invention provides a rotorcraft comprising a ducted fan 2, at least two first arms 10, at least one second arm, at least two first rotors 4, and at least one second rotor 6 . Among them, the ducted fan 2 includes a main duct 3 having a central axis. At least two first arms 10 and at least one second arm are perpendicular to the central axis and are disposed on the outer wall 8 of the main duct 3. The first rotor 4 has a first axis of rotation, and at least two first rotors 4 are disposed at the free ends of the at least two first arms 10 in a manner that the first axis of rotation is perpendicular to the first arm 10. The second rotor 6 has a second rotating shaft, and the at least one second rotor 6 is disposed at a free end of the at least one second arm with the second rotating shaft perpendicular to the second arm and the second rotating shaft at an angle to the central axis.

That is, the first arm 10 and the second arm extend outwardly spaced around the outer circumference of the main duct 3, and the first arm 10 corresponds to the number of the first rotor 4, and the second arm and The number of second rotors 6 corresponds.

In the above embodiment, the first rotor 4 is used to control the pitch and roll attitude while providing a certain auxiliary lift when necessary. It should be understood that the first rotor 4 is capable of variable speed control of the drone, making flight simple and reliable. The second rotor 6 is used for heading maneuvering, and at the same time, to counteract the whole machine counter torque, so that the composite configuration drone can maintain the flight attitude. It should be understood that the second rotor 6 is mainly used to compensate the counter torque of the ducted fan 2.

Referring to Figures 2 and 3, in accordance with a first embodiment of the present invention, the rotorcraft further includes a secondary duct 12 disposed at a free end of the at least one second arm, the second rotor 6 being disposed within the secondary duct 12, and second The axis of rotation is on the same line as the axis of the secondary duct 12 . It should be understood that the main duct 3 of the rotorcraft is provided with at least two first rotors 4 in the circumferential direction, wherein the main ducts 3 and the propellers in the ducted fan 2 work together to provide most of the lift for the aircraft, and the rest of the lift is provided by the vice. The duct 12 and the second rotor 6 are compensated. In addition, the pitch and roll attitude control of the aircraft is accomplished by controlling the first rotor 4, which is achieved by the action of the second rotor 6 and the secondary ducts 12. Preferably, the number of the first arms 10 is four, and the number of the second arms is one, wherein the angle between the second arm and the adjacent first arm is 60 degrees, four One arm 10 is symmetrically disposed with the second arm as a line of symmetry.

Referring to FIGS. 4 and 5, according to a second embodiment of the present invention, the number of the first arms 10 is four, the number of the second arms is two, four first arms 10 and two The two arms are equiangularly arranged around the central axis in the circumferential direction of the main duct 3, and the two second arms are disposed on opposite sides of the main duct in the same radial direction, and the number of the first rotors 4 is four. The number of the second rotors 6 is two, and the two second rotors 6 are respectively disposed on the two second arms, and the four first rotors 4 are respectively disposed on the first arm 10.

That is, the rotorcraft includes four first rotors 4, and two second rotors 6, which are disposed on the outer wall 8 spaced apart from each other. The four first rotors 4 are symmetrically disposed on both sides of the second rotating shaft connection of the two second rotors 6. That is to say, two first rotors 4 are respectively disposed on two sides of the second rotating shaft of the two second rotors 6, and the first rotors 4 on both sides are connected to each other with respect to the two second rotors 6 symmetry. The pitch and roll attitude control of the aircraft is mainly performed by the first rotor 4, and the yaw control of the aircraft is mainly implemented by the second rotor 6.

Referring to Figures 4 and 5, in accordance with an embodiment of the present invention, the four first axes of rotation are parallel to the central axis, and the two second axes of rotation form an angle of opposite and equal magnitude with respect to the central axis. In an alternative implementation, the second axis of rotation of the second rotor 6 is perpendicular to the central axis.

According to an alternative embodiment, the second axis of rotation of the second rotor 6 is perpendicular to the central axis of the ducted fan 2, and the second axes of rotation of the two second rotors 6 are parallel to each other, the blades of the two second rotors 6 facing oppositely That is, the rotation of the two second rotors 6 produces an opposite direction of the airflow, which is advantageous for canceling the counter torque generated by the ducted fan.

Referring to Figures 6, 7, 8, and 9, in accordance with a third embodiment of the present invention, the first arm 10 and the second arm are disposed in two numbers, two first arms 10 and two The second arm is equiangularly arranged around the central axis in the circumferential direction of the main duct 3, the two first arms 10 are disposed on opposite sides of a main duct in a radial direction, and the two second arms are disposed On the opposite sides in the other radial direction of the main duct, the number of the first rotors 4 is two, the number of the second rotors 6 is two, and the two first rotors 4 are respectively disposed at the two first branches. At the free end of the arm 10, two second rotors 6 are respectively disposed at the free ends of the two second arms, and the first rotor and the second rotor are disposed at equal angles to make the attitude control of the aircraft more stable.

According to the third embodiment of the present invention, the two first rotating shafts form an angle opposite to the central axis and are equal in size, and the two second rotating shafts form an angle opposite to the central axis and are equal in size, which is more advantageous. Counteracting the counter torque generated by the ducted fan.

Referring to FIG. 6, according to an alternative embodiment, the two first rotating shafts are a first front rotating shaft a and a second rear rotating shaft c, respectively, and the two second rotating shafts are a second front rotating shaft b and a second rear rotating shaft d, respectively. Wherein, the first front rotating shaft a is installed at a clockwise inclination β angle around the first arm, and the second rear rotating shaft c is installed at an angle β inclined in the opposite direction around the first arm; the second front rotating shaft b is inclined counterclockwise around the second arm The β-angle is mounted, and the second rear shaft d is mounted at an angle β about the second arm in the opposite direction.

Wherein, the two first rotors and the two second rotors realize the attitude control of the aircraft by the difference in the rotational speed. When the first front rotation axis a and the second rear rotation axis c simultaneously increase (or decrease) the same amount of change, the second front rotation axis b and the second rear rotation axis d simultaneously decrease (or increase) the same amount of change, Realizing the roll control of the aircraft; when the second front axle b and the second rear axle c simultaneously increase (or decrease) the same amount of change, the first front axle a and the second rear axle d simultaneously decrease (or increase) When the same amount of change is made, the rollback control of the aircraft can be achieved. The first front rotating shaft a and the second front rotating shaft b simultaneously increase (or decrease) the same amount of change, and when the second rear rotating shaft c and the second rear rotating shaft d simultaneously decrease (or increase) the same amount of change, Realize the pitch control of the aircraft. The length of the first arm 10 and the second arm, the angle of inclination of the first rotor and the second rotor are related to the maneuverability required by the aircraft.

Referring to FIGS. 10 and 11, according to an embodiment of the present invention, the rotorcraft further includes a driving device, and the ducted fan 2 includes a moving blade 18 disposed in the main duct 2 and a stationary blade 20 located below the moving blade 18, the moving blade 18. The first rotor 4 and the second rotor 6 are respectively connected to the drive device.

Referring to Figures 5, 10 and 11, in accordance with an embodiment of the present invention, the drive unit includes an engine 16 having a mounting bracket 22 disposed thereon, the mounting bracket 22 being positioned above the rotor blades 18 or below the stationary blades 20, the engine 16 Provided on the mounting frame 22, the engine 16 and the rotor blades 18 are connected by a transmission mechanism. In an alternative embodiment, the transmission mechanism is the main drive shaft 24, and the rotor blades 18 are mounted to the main drive shaft 24 via the hub 26. One end of the stationary vane 20 is fixedly coupled to the ducted wall 8 and the other end is connected to the stationary vane mounting disc 28, wherein the stationary vane mounting disc 28 is provided with a bearing 30 and the bearing 30 is coupled to the main drive shaft 24.

In the above embodiment, the outer wall 8 is mounted on the main bearing structure of the ducted fan 2. The function of the outer wall 8 is to generate additional lift due to the low pressure zone at the culvert lip, while the outer wall 8 inhibits the formation of the tip vortex of the rotor blade 18, increasing the efficiency and reducing noise. In addition, the outer wall 8 also serves as a security protection.

Further, the rotor blade 18 is powered by an oil-powered engine, and the outer wall 8 and the rotor blade 18 constitute the main lift unit of the drone, and have the capability of large load and long flight time. The stationary vane 20 is mounted on the stationary vane mounting plate 28 for balancing most of the counter torque generated by the moving vane 18 or the like.

Further, the engine 16 is mounted on the engine mount 22 above the lip of the duct wall 8 and is connected to the main bearing structure of the body for stably mounting the engine 16 while moving the center of gravity upward to bring the center of gravity close to the pneumatic center. Reduce the pitching moment of the drone before flying, and reduce the difficulty of maneuvering. In addition, a reducer 32 is connected to the main drive shaft 24 of the engine 16, and the reducer 32 is mounted on the engine 16 through a joint matched with the engine 16, and is mounted on the main structure of the main body through a link for the lateral joint for The rotational speed of the output of the engine 16 is reduced.

The aircraft also includes a plurality of landing gears 34 spaced apart from each other disposed at the bottom of the ducted fan 2. Each of the landing gears 34 is a cushioned landing gear that is extended by the bottom of the ducted fan 2 and that is outwardly curved away from the rotational axis of the ducted fan 2. The cushioned landing gear is mounted on the main bearing structure of the body to enable the composite configuration drone to make a soft landing.

According to an embodiment of the invention, the driving device further comprises a first motor and a second motor, the first rotor 4 is coaxially disposed with the first motor at a free end of the first arm 10, and the second rotor 6 is coaxial with the second motor Set at the free end of the second arm.

Referring to Figure 1, in accordance with an embodiment of the present invention, both the first arm 10 and the second arm are coupled to the main duct 3 by a folding mechanism 14 such that the first arm 10 and the second arm have a main duct The open position in the radial direction of 3 and the storage position parallel to the tangential direction of the main duct. The arm 10 is retracted according to the working state. When the drone is not working, the folding arm 14 is used to rotate the mounting arm 10 in the horizontal plane, and the mounting arm 10 is folded to reduce the occupied space.

It should be understood that the aircraft of the present invention can be used for oil and gas pipeline inspection, agriculture and forestry plant protection, power inspection, customs border inspection, fire rescue and the like.

Take the aircraft with four first rotors and two second rotors as an example:

Embodiment 1 The drone provided by the embodiment of the present invention is in a vertical flight state.

In order to make the hybrid electric-powered ducted fan 2 composite configuration drone provided by the embodiment of the present invention in a vertical take-off state, the specific operation method and steps may be as follows:

The first step: inspecting, debugging, and conducting an interview with the aircraft, and proceeding to the next step without any abnormality;

The second step: starting the engine 16, performing single-stage deceleration through the reducer 32, driving the main drive shaft 24, thereby driving the hub 26, and the rotor blades 18 rotate together. The throttle is adjusted from small to the required stroke of the work, and the rotor 18 rotates normally during the process. The torque generated by the moving blades 18 is offset by the counter torque generated by the underlying air flow acting on the stationary blades 20. The whole machine is smoothly off the ground and there will be slight self-rotation;

The third step: controlling the second rotor 6 to generate an auxiliary counter torque to counteract the part of the reverse torque that the static blade 20 cannot cancel in real time, so that the aircraft does not rotate and is in a stable hover state;

Step 4: If the wind of level 6 or lower is encountered, adjust the speed of the horizontal motor to control the first rotor 4, and the tension changes to generate the corresponding pitching moment and rolling torque, so that the attitude of the aircraft remains hovering smoothly. .

Embodiment 2: The drone provided by the embodiment of the present invention is in a forward flying state

Step 1: On the basis of the hovering state, adjust the rotation speed of the tilting motor, control the tension of the second rotor 6, and generate a correspondingly varying counter torque, also known as the yaw moment, to rotate the aircraft to achieve yaw Manipulate

The second step: adjusting the rotation speed of the horizontal motor, thereby controlling the tension change of the first rotor 4, so as to generate the corresponding pitching moment and rolling torque, realizing the pitching and rolling manipulation, so that the drone can be stabilized before flying;

The third step: If the wind is below 6th level, adjust the rotation speed of the horizontal motor, thereby controlling the tension of the first rotor 4 to make the corresponding pitching moment and rolling torque, so that the attitude of the whole machine is kept stable before flying.

Embodiment 3: The drone provided by the embodiment of the present invention is in an upcoming landing state

The first step: controlling the throttle stroke of the engine 16, reducing the speed of the rotor blade 18, and causing the drone to fall vertically;

The second step: controlling the first rotor 4 and the second rotor 6 to make the drone smoothly land;

The third step: when landing to the ground, at an altitude of 0.5m, the engine is turned off, the purpose is to prevent the airflow flowing out of the outer wall 8 from interacting with the ground, forming a large ground effect, generating a very large vector thrust, which may result in huge The overturning moment causes the drone to instantaneously flip and break. At this time, the attitude control of the first rotor 4 and the action of the additional lift can make the drone land relatively smoothly. When landing, the cushioned landing gear acts as a load for a long time, unloading, and reduces the damage caused by the impact load;

The fourth step: operating the folding mechanism 14 to rotate the electric rotor mounting arm 10 horizontally, thereby retracting the first rotor 4 and the second rotor 6, and the operation example is completed.

The above are only the preferred embodiments of the present invention, and are not intended to limit the present invention, and various modifications and changes can be made to the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and scope of the present invention are intended to be included within the scope of the present invention.

Claims (10)

A rotorcraft characterized in that it comprises: a ducted fan (2) comprising the main duct (3) having a central axis; At least two first arms (10) and at least one second arm, respectively perpendicular to the central axis and disposed on the outer wall (8) of the main duct (3); At least two first rotors (4) having a first axis of rotation, the freedom of the at least two first arms (10) being provided in a manner that the first axis of rotation is perpendicular to the first arm (10) End; At least one second rotor (6) having a second rotating shaft disposed at the at least one of the second rotating shaft perpendicular to the second arm and the second rotating shaft at an angle to the central axis The free end of the second arm. The rotorcraft according to claim 1, further comprising: an auxiliary duct (12) disposed at a free end of said at least one second arm, said second rotor (6) being disposed on said pair In the duct (12), the second rotating shaft is on the same straight line as the axis of the auxiliary duct (12). The rotorcraft according to claim 1, characterized in that the number of the first arms (10) is four, the number of the second arms is two, and the four first branches An arm (10) and the two second arms are disposed equiangularly about the central axis in a circumferential direction of the main duct (3), and the two second arms are disposed on the main duct The opposite sides of the same radial direction, the number of the first rotors (4) is four, the number of the second rotors (6) is two, and the two second rotors (6) ) respectively disposed on the two second arms, the four first rotors (4) being respectively disposed on the four first arms (10). The rotorcraft according to claim 3, wherein said four first rotating shafts are parallel to said central axis, said two second rotating shafts forming an angle opposite to said central axis and having an equal angle . The rotorcraft according to claim 1, wherein said first arm (10) and said second arm are disposed in two numbers, said two first arms (10) and The two second arms are disposed equiangularly about the central axis in a circumferential direction of the main duct (3), and the two first arms (10) are disposed at one of the main ducts On opposite sides in the radial direction, the two second arms are disposed on opposite sides of the other radial direction of the main duct, and the number of the first rotors (4) is two. The number of the second rotors (6) is two, and the two first rotors (4) are respectively disposed at the free ends of the two first arms (10), and the two second rotors ( 6) are respectively disposed at the free ends of the two second arms. The rotorcraft according to claim 5, wherein the two first rotating shafts form an angle opposite to the central axis and are equal in magnitude, and the two second rotating shafts form an axis with the central axis An angle that is opposite in direction and equal in size. The rotorcraft according to claim 1, wherein said rotorcraft further comprises a driving device, said ducted fan (2) comprising a moving blade (18) disposed in said main duct (2) and A stationary blade (20) located below the moving blade (18), the moving blade (18), the first rotor (4) and the second rotor (6) are respectively connected to the driving device. The rotorcraft according to claim 7, wherein said driving device comprises an engine (16), said main duct (3) is provided with a mounting bracket (22), said mounting bracket (22) being located Above the moving blade (18) or below the stationary blade (20), the engine (16) is disposed on the mounting frame (22), and the engine (16) and the moving blade (18) pass through a transmission mechanism connection. The rotorcraft according to claim 8, wherein said driving device further comprises a first motor and a second motor, said first rotor (4) being coaxially disposed with said first motor at said first A free end of the arm (10), the second rotor (6) being disposed coaxially with the second motor at a free end of the second arm. The rotorcraft according to claim 9, wherein said first arm (10) and said second arm are each coupled to said main duct (3) by a folding mechanism (14) such that The first arm (10) and the second arm have an open position in a radial direction of the main duct (3) and a storage position parallel to a tangential direction of the main duct (3) .