The present invention relates generally to ducted fan air-vehicles, and more particularly, relates to the design and orientation of stators or exit-guide vanes (EGVs) of ducted fan air-vehicles.
Ducted fan air-vehicles, such as an Unmanned Aerial Vehicle (UAV) or a vertical-take-off-and-landing (VTOL) vehicle, may have at least one ducted fan and a fan engine to drive the fan blades. Ducted fan air-vehicles are well-known for performance capability in multiple flight conditions. For instance, ducted fan air-vehicles have the ability of forward flight and are well known for stationary hovering aerodynamic performance.
The ducted fan air-vehicle 100 may also include a stator assembly 108 and a plurality of fixed and/or movable vanes 110 for providing thrust vectoring for the air-vehicle 100. The stator assembly 108 may be located just under the fan 104 within the air duct 102 to reduce or eliminate the swirl and torque produced by the fan 104. Further downstream of the stators, the thrust vectoring vanes 110 may be located within or outside the air duct 102. For instance, the vanes 110 may be placed slightly below an exit section of the air duct 102.
The ducted fan-air vehicle 100 may further include struts 202 which support the center body 106. Struts 202 also provide a connection for the landing gear 203 of the UAV.
In order to be effective and controllable in multiple flight conditions, ducted fan air-vehicles such as air-vehicle 100 preferably have clean and attached air flow around the duct lip in the multiple flight conditions. Further, ducted fan air-vehicles preferably have a favorable center of gravity in order to be effective and controllable. A uniform inflow velocity profile into the fan is also desirable to minimize the acoustic signature of the duct-fan interaction.
Additionally, ducted fan air-vehicles may need to carry a variety of components when in operation. For instance, in operation ducted fan air-vehicles may need to carry, without limitation, visual sensors, infrared sensors, cameras, radio communication devices, inertial sensor units, ground level sensor units, and/or payload. Due to the limited size of the ducted fan air-vehicle, in order to store the variety of units in the ducted fan, the units may be placed in external pods that are attached to the ducted fan air-vehicle. These pods may (i) cause a shift in the center of gravity, (ii) create negative interference with airflow characteristics inside the duct by blocking air intake and exhaust, and (iii) create additional drag on the UAV when the UAV is in forward flight. Additionally, the added weight of the equipment may require additional engine capacity and fuel storage capacity. It may be beneficial to increase the volume within the duct lip in order to decrease or eliminate the need for external pods while maintaining the aerodynamic requirements of a ducted fan air-vehicle.
Unmanned aerial vehicles (“UAVs”) are remotely piloted or self-piloted aircraft that can carry cameras, sensors, communications equipment, or other payloads. A UAV is capable of controlled, sustained, level flight and is powered by either a jet or an engine. The UAVs may be remotely controlled or may fly autonomously based on pre-programmed flight plans or more complex dynamic automation systems.
UAVs have become increasingly used for various applications where the use of manned flight vehicles is not appropriate or is not feasible. Such applications may include military situations, such as surveillance, reconnaissance, target acquisition, data acquisition, communications relay, decoy, harassment, or supply flights. These vehicles are also used in a growing number of civilian applications, such as firefighting when a human observer would be at risk, police observation of civil disturbances or crime scenes, reconnaissance support in natural disasters, and scientific research, such as collecting data from within a hurricane.
Currently, a wide variety of UAV shapes, sizes, and configurations exist. Typically it is the payload of the aircraft that is the desired product, not the aircraft itself. A payload is what the aircraft is carrying. UAVs are the delivery system for a payload and are developed to fill a particular application and a set of requirements. As previously mentioned, there are numerous applications for which a UAV may be used. For each new application, a different type of payload may be used. Because different payloads may require different processing capabilities, or may comprise different sizes, a variation of the UAV typically must be developed for each type of payload, or a completely new aircraft typically must be designed. Designing a new aircraft or developing a variation of the current UAV in use is time-consuming and costly.
In ducted fans, the stators are used to remove the swirl (or flow rotation) introduced by the fan as it pushes the air through the duct. Removing the swirl improves the fan aerodynamic performance by converting the flow swirl energy into thrust. For reasons of compactness, the stators are generally positioned in close proximity downstream of the fan and within the duct (see
The present disclosure describes a ducted fan air-vehicle having a lean and swept stator design for optimum noise reduction. The ducted fan air-vehicle comprises an air duct including an outer rim and an inner rim, a fan located within the air duct having a plurality of rotor blades and rotating in a direction. The fan further includes a hub located in the fan, a plurality of struts extending from the hub and positioned adjacent the outer rim of the fan, a plurality of stators extending from the hub and positioned adjacent the inner rim of the fan, and the plurality of rotor blades extending from the hub and positioned between the plurality of struts and the plurality of stators. The plurality of stators are lean in a plane of fan rotation and swept in a plane normal to fan rotation.
A method for arranging stators in a ducted fan air-vehicle is also described. The method includes providing an air duct including an outer rim and an inner rim, providing a fan located within the air duct having a plurality of rotor blades and rotating in a direction, providing a hub located in the fan, providing a plurality of struts extending from the hub and positioned adjacent the outer rim of the fan, providing a plurality of stators extending from the hub and positioned adjacent the inner rim of the fan, and providing the plurality of rotor blades extending from the hub and positioned between the plurality of struts and the plurality of stators. The method further includes leaning the plurality of stators in a plane of fan rotation and sweeping the plurality of stators in a plane normal to fan rotation to reduce noise generated by the ducted fan air-vehicle.
These as well as other aspects and advantages will become apparent to those of ordinary skill in the art by reading the following detailed description, with reference where appropriate to the accompanying drawings. Further, it is understood that this summary is merely an example and is not intended to limit the scope of the invention as claimed.
Presently preferred embodiments are described below in conjunction with the appended drawing figures, wherein like reference numerals refer to like elements in the various figures, and wherein:
Ducted fan air-vehicles are known for their superior stationary aerodynamic hovering performance, three-dimensional precision position hold, low speed flights, precision vertical take-off and landing (“VTOL”) and safe close-range operations. Ducted fan air-vehicles may be preprogrammed to perform operations autonomously, or they may be controlled by a human operator. Therefore, ducted fan air-vehicles may be unmanned aerial vehicles (“UAV”).
UAVs may have avionics equipment on board to control the flight and operation of the UAV. For instance, the avionics may control the direction, flight, stability compensation, and other aspects of flight control. Additionally, UAVs may carry a variety of equipment on board tailored to the mission the UAVs are assigned to accomplish. UAVs may carry sensors on board to obtain information about surroundings, or the UAVs may carry a payload to be deposited at a target site. The UAV engine to drive the UAV requires that fuel be carried on board the UAV. The avionics equipment, sensors, payload, and fuel may be stored on the UAV.
In order to be effective and controllable in multiple flight conditions, ducted fan air-vehicles preferably have clean and attached air flow around the duct lip in the multiple flight conditions. Further, ducted fan air-vehicles preferably have a favorable center of gravity in order to be effective and controllable. A uniform inflow velocity profile into the fan is also desirable to minimize the acoustic signature of the duct-fan interaction.
The invention in practice is implemented by both leaning and sweeping the stators and/or the struts, located within the fan of the UAV, to get the maximum noise reduction. Sweep is defined as an axial displacement of the stator leading edge that varies along the span. Similarly, lean is defined as circumferential displacement of the stator. The reason for the effectiveness of sweep and lean is that it introduces a phase variation in the upwash velocity, which causes the noise, resulting in strong cancellation between contributions to the noise field from different locations along the stator span.
There is a particular combination of lean and sweep that leads to optimum noise reduction. This optimum configuration is system dependant upon the characteristics of each particular vehicle, such as fan design, fan speed, diameter, number of fan blades, and number of stator vanes.
Referring to
The struts 202 are generally positioned in close proximity upstream of the fan 104, near the outer rim 105 of the air duct 102. The struts may also be attached to the outer rim 105 of the air duct. The struts function to support the center body 106, and also provide a connection to landing gear 203. The struts 202 also interconnect the center body 106 and the air duct 102.
For reasons of compactness, the stators 204 are generally positioned in close proximity downstream of the fan 104, near the inner rim 10 of the air duct 102. The stators 204 function to remove the swirl (or flow rotation) introduced by the fan as it pushes air through the duct. Removing the swirl improves the fan aerodynamic performance by converting the flow swirl energy into thrust. The stators 204 also provide additional structural integrity to the system by, e.g., structurally interconnecting the center body 106 and the air duct 102.
The fan 104 contains the rotor blades 206 that need to be very well centered in relation to the air duct 102. The rotor blades 206 are generally positioned between the struts 202 and the stators 204. The rotor functions to provide thrust so the vehicle can lift, hover, cruise, etc. A power plant or engine (not shown) is needed to spin and force air through the duct.
In a conventional UAV, as shown in
In another embodiment, lean of the stators 204 may be implemented by curving the stators 204. Due to simplicity in the fabrication process, stators are generally straight as shown in the
The stators 204 are also swept in a plane normal to the fan rotation, as shown in
In another embodiment, the stators 204 may be swept by moving the end of the stator connected to the hub downstream on the hub 200, as shown in
Furthermore, the plurality of stators 204, the hub 200, and the air duct 102 may be manufactured as a single component to be incorporated into the UAV for easy assembly of the vehicle and provide a stiff structure, as compared to having a set of individual stators that need to be assembled.
While certain features and embodiments of the present application have been described in detail herein, it is to be understood that the application encompasses all modifications and enhancements within the scope and spirit of the following claims.
The United States Government has acquired certain rights in this invention pursuant to Contract No. MDA972-01-0018, awarded by the United States Army.