Abstract:
A ducted fan air-vehicle capable of generating control moments. The ducted fan air-vehicle includes an air duct, a fan, a center body, a plurality of control vanes. The vanes are independently controlled and are deflected in the same direction but at different angles, thereby providing an increased control moments to the vehicle compared to the prior art. The increased pitching moment allows for additional control authority. Additional control authority is useful in forward flight and is especially desirable when the ducted fan air-vehicle is maneuvering in unsteady or turbulent winds or with various types of cargo that may effect the vehicle center of gravity location.

Description:
GOVERNMENT INTEREST 
     The invention described herein was made in the performance of work under U.S. Government Contract No. HR0011-05-C-0043, awarded by DARPA (Defense Advanced Research Projects Agency). The Government may have rights to portions of this invention. 
    
    
     RELATED APPLICATION 
     The application is related to U.S. patent application Ser. No. 12/359,407, filed Jan. 26, 2009, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     Ducted fan air-vehicles typically have at least one ducted fan and a fan engine to drive the fan blades. These vehicles may be preprogrammed to perform operations autonomously, or they may be controlled by a human operator. Ducted fan air-vehicles are well known for excellent stationary hovering aerodynamic performance and have the ability of forward flight. 
     Ducted fan air-vehicles have become increasingly used in a variety of applications. Such applications 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 fire fighting, police observation, reconnaissance support in natural disasters, and scientific research. 
     Many of these applications require that the ducted fan air-vehicles carry a variety of payloads or cargo. For example, ducted fan air-vehicles may need to carry visual sensors, infrared sensors, cameras, radio communication devices, inertial sensor units, ground level sensor units, and/or other payloads. This cargo may cause a shift in the center of gravity, which can create negative interference with airflow characteristics inside the duct by blocking air intake and exhaust, and create additional drag on the vehicle when the vehicle is in forward flight. When the vehicle CG changes, the center of lift needs to change in order have the aerodynamic forces balance the forces due to gravity acting on the vehicle. 
     Ducted fan air-vehicles are designed to have a specific center of gravity in order to be effective and controllable. These vehicles are sensitive to even slight weight redistributions, i.e. any change to the weight distribution can impact the airflow within the duct. Accordingly, even slight modifications to the cargo/payload can negatively impact performance specifically stability. Differing weight distributions are typically dealt with by either developing a new variation of the vehicle or developing an entirely new aircraft for each type of cargo. Both designing a new aircraft and developing a variation of the vehicle are time-consuming and costly. 
     Furthermore, some flight conditions and mere forward flight can interfere with the desired airflow characteristics and, by implication, flight control and performance. It is important to design the vehicle with the proper CG location (or range of locations), just as for a fixed wing aircraft. 
     SUMMARY OF THE INVENTION 
     The present invention provides an example ducted fan air-vehicle that provides differential control of one or more vane pairs to generate a pitching moment. The vanes generate aerodynamic control forces and moments. The primary purpose of the vanes is to generate pitch, roll, and yaw moments to enable control of the vehicle. The pitching moment facilitates control of the vehicle. The control is beneficial in forward flight as well as in extreme conditions, such wind, and gusts. The control could also be used to balance out different types of cargo or counteract shifting cargo. 
     The vehicle includes an air duct, a fan, a center body, a plurality of control vanes located within or downstream from the air duct, and a separate servo for each control vane for independently controlling each vane. Two adjacent control vanes are deflected in the same direction. The first vane is deflected to a first angle and the second vane is deflected to a second angle different from the first angle. The first angle of the first vane maintains desirable airflow around the second vane. The differential between the two angles allows the first vane to deflect at a greater angle than if the vanes were deflected at the same angle. The increased angle of the first vane allows for an increased pitching moment. 
     In accordance with further aspects of the invention, a flight control system determines the orientations of the vanes to generate a desired pitching moment and control the movement of the vanes accordingly. 
     As will be readily appreciated from the foregoing summary, the invention provides a ducted fan air-vehicle capable of generating improved pitching moments through differential vane control. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
         FIG. 1  is a side, partial cross sectional view of a ducted fan air-vehicle; 
         FIG. 2  is a perspective view of a ducted fan air-vehicle with the control vanes in a first position; 
         FIG. 3-1  is a side view of a servo mounted on or internal to a vane; 
         FIG. 3-2  is a side view of a servo mounted external to the vane; 
         FIG. 4  is a bottom view of an air duct in a standard position; 
         FIG. 5  is an end view of vanes deflected in the same direction but at different angles in accordance with the present invention; 
         FIG. 6  illustrates an alternate embodiment for the vanes; 
         FIG. 7  illustrates an embodiment having a mechanical linkage between the vanes; and 
         FIG. 8  illustrates an embodiment having more than two vanes in a section. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 and 2  show a ducted fan air-vehicle  100  that provides differential control of one or more vane pairs to generate a pitching moment. The ducted fan air-vehicle  100  includes an air duct  102  having a fan  104  located within the air duct  102 . The ducted fan air-vehicle may have a center body  106 . In this embodiment, the center body  106  is a housing that contains other components of the air-vehicle  100 , including an engine  107 , a payload or cargo  109 , a duct pod  113  and engine mounts  111 . The engine  107  powers the air-vehicle  100 . The engine mounts  111  support the center body  106 . Landing gear  108  is connected to the center body  106  with engine mounts  111 . The ducted fan air-vehicle  100  is stabilized when it is on the ground by landing gear  108 . 
     The ducted fan air-vehicle  100  also includes a stator assembly  110 . In this embodiment, the stator assembly  110  is located just under the fan  104  in the air duct  102  to reduce or eliminate the swirl and torque produced by the fan  104  by providing the correct amount of anti-torque to counteract engine/fan torque. In this embodiment, the stator assembly  110  adds to the vehicle&#39;s structural integrity. 
     The ducted fan air-vehicle  100  also includes a plurality of fixed or moveable control vanes  112  for providing the necessary forces and moments for vehicle control. The vanes include leading edges  116  and trailing edges  115 . The vanes  112  may be located under the fan  104  within the air duct  102 . In this embodiment, the vanes  112  are connected to the air duct  102  by control vane supports  117 . The vanes  112  may be placed below the exit section of the air duct  102 . The vanes  112  are placed in the fan airflow and away from the vehicle center of gravity (CG) location. The farther away the vanes  112  are placed from the CG, the better they are at generating moments for vehicle altitude control. The vanes  112  may also include moveable flap surfaces  114  at a trailing edge  115 . The flap surfaces  114  deflect as the vanes  112  are deflected. The moveable flap surfaces  114  enable the control vanes to produce more lift than a single rigid surface. 
     A servo converts electrical signals to mechanical energy in order to move the vanes  112  to desired orientations. In one embodiment of the present invention, the surface of each vane  112  includes its own servo  118  or method of independent actuation as shown in  FIG. 3-1 . In an alternative embodiment, an externally mounted servo  119  moves the vanes via a system of linkages as shown in  FIG. 3-2 . With each vane  112  having its own servo  118 , the vanes  112  are free to move independently. In this embodiment, a flight control system which is part of an avionics system, controls the deflection of the vanes  112  by sending command signals to the servos  118 . The flight control system is a collection of on-board electronics (sensors, computer, etc.), and is located wherever there is suitable space. 
     If the vehicle  100  encounters a strong unsteady wind or gust during flight, the wind could cause tilt the vehicle  100  in a different direction than it was originally travelling. In response to the tilt induced by the unsteady conditions, it is desirable for the vehicle  100  to quickly be tilted in response to stabilize its flight and maintain control. To achieve the desired nose-down tilt into the wind, the vehicle  100  must overcome the inherent nose-up pitch moment present on the windward side of the duct lip. Therefore, tilting the vehicle  100  into the wind requires overcoming its natural tendency to pitch away from the oncoming wind. 
       FIG. 5  shows a vane pair  202  in an orientation capable of generating a pitching moment in the ducted fan air-vehicle  100 . The vane pair  202  includes a first vane  204  and a second vane  206  deflected in the same direction. The first vane  204  includes a flap surface  209 . The second vane  206  includes a flap surface  208 . The first vane  204  and the second vane  206  are separated by a distance  224 . An arrow  201  represents a general direction of airflow originating from the fan  104 . The orientation of the first vane  204  and the arrow  201  define a first angle of attack (AOA)  210 , and the orientation of the second vane  206  and the arrow  201  define a second AOA  212 . 
     Traditional ducted fan air-vehicles generate a pitching moment by deflecting the vane pair  202  in the same direction at equal AOAs, i.e. the first AOA  210  and the second AOA  212  are substantially identical. The magnitude of the pitching moment generated depends on the amount of deflection, i.e. greater AOAs  210  and  212  lead to a generated pitching moment with greater magnitude. However, the magnitude of the generated pitching moment is limited because the vane pair  202  is limited in how far it deflects. Specifically, if the vane pair  202  is adjusted beyond a threshold angle, a stall condition is triggered resulting in undesirable air flow over the control vane and a reduced pitching moment is experienced. 
     The vehicle  100  generates an improved pitching moment by deflecting the vane pair  202  in one direction, but deflecting the individual vanes  204  and  206  at different angles. The orientations of the first vane  204  and the second vane  206  are adjusted so that the second AOA  212  is greater than the first AOA  210 . The different orientations of the vane AOAs  210 ,  212  allow the second vane  206  to extend beyond the tandem threshold angle. The airflow generated by the first vane  204  reduces the stall characteristics of the second vane  206 . More specifically, the air flow generated by the first vane  204  allows airflow on a top surface of the second vane  206  to maintain contact an AOA greater than the tandem threshold angle without generating a stall condition. The increased deflection of the second vane  206  beyond the tandem threshold angle allows for the generation of an increased pitching moment, with reduced risk of stalling the control vanes. 
     Although  FIG. 5  shows the vanes  204 ,  206  being rotated clockwise, they are capable of being deflected in the counterclockwise direction. 
     In operation, the differential between AOAs  210  and  212  can vary from 0° to max°. The AOAs  210  and  212  range from a first AOA and end at a max AOA. Deflecting beyond these maximum values can lead to a stall condition. In one embodiment, the differential defined by the first AOA  210  and the second AOA  212  is 10 degrees. In another embodiment, the differential defined by the first AOA  210  and the second AOA  212  is 20 degrees. The specific differential defined by AOAs  210  and  212  that generates the desired balance between generated pitching moment/reduced thrust depends on the properties of the specific ducted fan air-vehicle such as the distance  224  between the first vane  204  and the second vane  206 . Thus, the differential defined by the first AOA  210  and the second AOA  212  may smaller than 10 degrees, larger than 20 degrees or any angle in-between. Other properties that influence the desired angular difference between the two vanes include: chord length of vanes, vane airfoil shape, vane flap design, Reynolds number). In one embodiment, the optimal vane deflection values for generating various pitching moments for various vehicle configurations are stored in a look-up table included in the avionics system. 
     In one embodiment, more than one pair of adjacent vanes are deflected in the same direction at different AOAs. In other words, multiple pairs of adjacent vanes ( FIG. 2 ) are deflected at different AOAs to generate a desired combination of control moments for executing desired movements. 
     Compared to traditional methods where the vanes  112  are deflected in tandem, the inventive differential control of the vane pairs generates an improved pitching moment and thereby facilitates improved control of the vehicle  100 . The improved control could be used in forward flight as well as in extreme conditions, such wind, and gusts. The improved control could also be used to balance out different types of cargo or counteract shifting cargo. 
       FIG. 6  is an embodiment for the vanes where the vanes do not include flap surfaces such as that shown in  FIG. 5 . The motion of the vanes relative to each other is similar to that described above, thereby providing the benefit of differential deflection. 
       FIG. 7  illustrates an embodiment that includes a mechanical linkage  250  connected between the vanes. The mechanical linkage  250  is configured to apply differential deflection of the vanes as described above. For example, as one of the vanes moves the mechanical linkage  250  moves the other vane in accordance with the desired motion described above. 
       FIG. 8  shows an embodiment with more than two vanes working in cooperation to provide controlling forces. Each of the vanes may be driven to different angles as they near a stall position thereby improving the airflow over the adjacent vane. 
     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.