Patent Publication Number: US-11662728-B2

Title: Methods for maintaining difficult-to-access structures using unmanned aerial vehicles

Description:
RELATED PATENT APPLICATION 
     This application is a divisional of and claims priority from U.S. patent application Ser. No. 16/202,347 filed on Nov. 28, 2018. 
    
    
     BACKGROUND 
     This disclosure generally relates to automated systems for carrying maintenance tools across limited-access surfaces of large structures, such maintenance tools including (but not limited to) sensors used in nondestructive inspection (NDI). In particular, this disclosure relates to apparatus for performing automated maintenance operations on airfoil-shaped bodies, such as wind turbine blades and rotors. 
     As used herein, the term “maintenance” includes, but is not limited to, operations such as NDI, target or decal attachment, surface treatment, drilling, damage marking, placement of materials, coating removal, cleaning, sanding and painting. For the sake of illustration, the bulk of the following disclosure will focus on nondestructive inspection of limited-access areas on large structures. However, it should be appreciated that at least some of the concepts disclosed below have application when performing other types of maintenance. 
     In-person human-based inspections of large structures and various types of large objects can be time consuming, expensive and difficult for an individual to perform. Examples of large structures that pose significant inspection challenges include such as wind turbine blades, aircraft fuselages, storage tanks, bridges, dams, levees, power plants, power lines or electrical power grids, water treatment facilities; oil refineries, chemical processing plants, high-rise buildings, and infrastructure associated with electric trains and monorail support structures. 
     Nondestructive inspection of structures involves thoroughly examining a structure without harming the structure or requiring significant disassembly of the structure. Nondestructive inspection is advantageous for many applications in which a thorough inspection of the exterior and/or interior of a structure is required. Various types of sensors may be utilized to perform NDI. One or more sensors may scan the structure to be examined, acquiring NDI sensor data from the structure from which internal anomalies can be identified. The data acquired by the sensors is typically processed by a processing element, and the processed data may be presented to a user via a display. 
     With current approaches for automated NDI of large or difficult-to-access structure, an NDI scanner (e.g., a self-propelled crawler vehicle equipped with an NDI sensor unit) may drive across the surface to be inspected. But such drive motion is achieved via friction, which requires sufficient normal forces to enable shear forces between the wheels of the scanner and the structure being inspected (or wheels and a track). Lifting carts with cables holding the NDI scanner may be placed on and moved along a surface of the structure to be inspected, but this approach can be complicated, costly, and time-consuming to set up and run. Portability and packaging all necessary systems onto the structure can be a challenging issue as well, if that structure is far off the ground, like a wind turbine blade. 
     Utilizing an unmanned aerial vehicle (UAV), an operator can safely acquire images of structures without being placed in harm&#39;s way and without requiring cumbersome and expensive equipment, such as cranes or platforms. A typical UAV, however, does not have the ability to provide any NDI beyond visual imaging because the typical UAV is not designed to place an NDI sensor unit in contact with or in proximity to a surface of the structure being inspected and then scan the NDI sensor unit across that surface. It would be desirable to provide an improved method for using a UAV to place a maintenance tool (such as an NDI sensor unit) in contact with or in proximity to a limited-access area of a large structure or object. 
     SUMMARY 
     The subject matter disclosed in some detail below is directed to methods for performing maintenance operations using unmanned aerial vehicles (UAVs). The methods are enabled by equipping a UAV with a maintenance tool capable of performing a desired maintenance operation (e.g., nondestructive inspection) on a limited-access surface of a large structure or object (e.g., a wind turbine blade) while the UAV is hovering adjacent to and in contact with that surface. (As used herein, the term “hover” should be construed broadly to include each of the following scenarios: (a) the vertical rotors are rotating, the UAV is not in contact with any structure and the UAV is not moving; or (b) the vertical rotors are rotating, the UAV is in contact with a structure and the UAV is not moving.) In a particular embodiment, the UAV uses re-orientation of lifting means (e.g., vertical rotors) to move the maintenance tool continuously or intermittently across the surface of the structure while maintaining contact with the surface of the structure undergoing maintenance. Such flight of a UAV while in contact with a surface will be referred to herein as “skimming”. 
     The UAVs disclosed herein include a controller which preferably takes the form of a plurality of rotor motor controllers that communicate with an onboard computer system configured to coordinate the respective rotations of the rotors. The controller is configured (e.g., programmed) to control the rotors to cause the UAV to fly along a flight path to a first location whereat a plurality of standoff contact elements (e.g., ball rollers, wheels or low-friction sliding surfaces) of the UAV contact the surface of the structure being maintained (e.g., inspected). Once the standoff contact elements are in contact with the surface of the structure, the controller controls the rotors to produce a net thrust that maintains the UAV stationary at the first location with the standoff contact elements in contact with the surface of the structure. Then the maintenance tool is activated to perform a maintenance operation on the surface. Thereafter one or more rotors may be reoriented to produce a net thrust that causes the UAV to skim from the first location to a second location while the remaining rotors ensure flight and sufficient pressure against the surface for smooth scanning during skimming. The maintenance tool may be activated to perform another maintenance operation while the UAV hovers at the second location with the standoff contact elements in contact with the surface of the structure or the maintenance operation may be performed continuously or intermittently as the UAV skims from the first location to the second location. During skimming, the position of the UAV may be determined by encoders, or for higher fidelity, encoders supplemented with an off-board positioning method, such as tracking using a local positioning system or motion capture using cameras. Once the scanning has been completed, the UAV lifts offs from the surface, again using reorientation and speed changes on a subset of the rotors. 
     The tool-equipped UAVs disclosed herein do not rely on traction on the surface like the traditional crawling robot, so the UAV may traverse dirty or wet surfaces without slippage or danger of falling. Scanning can be done at a low cost relative to many other approaches, and still be automated and rapid. The tool-equipped UAV is light in weight and does not require that a complicated support system be placed on the structure. 
     In accordance with some embodiments, the UAV is equipped with an NDI sensor unit for enabling full UAV-based scanning inspection of structures and eliminating on-structure drive approaches for NDI. The UAV is configured to place the NDI sensor unit in contact with or in proximity to a surface of a structure being inspected and then scanning the NDI sensor unit across that surface while maintaining contact or proximity. 
     In cases where the maintenance operation is NDI, NDI sensor data is collected during the scanning while being simultaneously tied to (correlated with) the measured position, and stored in a non-transitory tangible computer-readable storage medium onboard the UAV or transferred wirelessly to a separate computer on the ground. Multiple maintenance tool-equipped UAVs may be used at the same time, as long as their relative positions are checked and controlled to avoid collision. 
     In a particular application of the method for UAV-based NDI, the NDI sensor unit may be scanned across a surface of a limited-access airfoil-shaped body such as a wind turbine blade. As used herein, the term “airfoil-shaped body” means an elongated body having two surfaces connecting a leading edge having a curved (e.g., rounded) profile to a sharp (e.g., angled) trailing edge. The method and UAV proposed herein enables rapid, large-area NDI of wind turbine blades, which capability may provide manifold benefits to the wind generation industry. The technology disclosed in some detail below provides a simplified and potentially lower cost solution for scanning a sensor or sensor array across the surfaces of a wind turbine blade (or other structure or object) to collect sensor data representing characteristics of the structure inspected. 
     Although various embodiments of methods for performing a maintenance operation in a limited-access area of a large structure using unmanned aerial vehicles are described in some detail later herein, one or more of those embodiments may be characterized by one or more of the following aspects. 
     One aspect of the subject matter disclosed in detail below is an unmanned aerial vehicle comprising: a frame comprising a plurality of standoff support members and a plurality of tool support members; a maintenance tool supported by the plurality of tool support members; a plurality of rotor motors coupled to the frame; a plurality of rotors coupled to respective rotor motors of the plurality of rotor motors; a plurality of motor controllers for controlling operation of the respective rotor motors of the plurality of rotor motors; a plurality of standoff contact elements coupled to distal ends of respective standoff support members of the plurality of standoff support members; and a maintenance tool supported by the plurality of tool support members in a fixed position relative to the plurality of standoff contact elements. 
     Another aspect of the subject matter disclosed in detail below is a method for performing a maintenance operation using an unmanned aerial vehicle, comprising: (a) the unmanned aerial vehicle flies to a first location whereat a plurality of standoff contact elements of the unmanned aerial vehicle contact respective areas on a surface of the structure; (b) the unmanned aerial vehicle hovers at the first location with the standoff contact elements in contact with the surface of the structure; and (c) a maintenance tool on-board the unmanned aerial vehicle performs a first maintenance operation while the unmanned aerial vehicle is hovering at the first location with the standoff contact elements in contact with the surface of the structure. 
     In accordance with some embodiments of the method described in the immediately preceding paragraph, the method further comprises: (d) the unmanned aerial vehicle moves from the first location to a second location whereat the plurality of standoff contact elements of the unmanned aerial vehicle contact respective areas of the surface of the structure; (e) the unmanned aerial vehicle hovers at the second location with the standoff contact elements in contact with the surface of the structure; and (f) the maintenance tool performs a second maintenance operation while the unmanned aerial vehicle is hovering at the second location with the standoff contact elements in contact with the surface of the structure. 
     In accordance with other embodiments, the method further comprises: (d) the unmanned aerial vehicle moves away from the first location while maintaining the plurality of standoff contact elements in contact with the surface of the structure; and (e) the maintenance tool performs a second maintenance operation during movement of the unmanned aerial vehicle away from the first location. 
     A further aspect of the subject matter disclosed in detail below is a method for performing a maintenance operation on an airfoil-shaped body using an unmanned aerial vehicle, the method comprising: (a) equipping the unmanned aerial vehicle with a maintenance tool and a plurality of standoff contact elements, the plurality of standoff contact elements being arranged to simultaneously contact a surface of the airfoil-shaped body, and the maintenance tool being arranged to confront an area on the surface of the airfoil-shaped body while the plurality of standoff contact elements are in contact with the surface; (b) flying the unmanned aerial vehicle to a first location whereat the plurality of standoff contact elements of the unmanned aerial vehicle contact respective areas on a surface of the airfoil-shaped body; and (c) while the unmanned aerial vehicle is at the first location with the plurality of standoff contact elements in contact with the surface of the airfoil-shaped body, activating the maintenance tool to perform a first maintenance operation on the surface of the airfoil-shaped body. 
     In accordance with some embodiments of the method described in the immediately preceding paragraph, the method further comprises: (d) upon completion of step (c), flying the unmanned aerial vehicle to a second location while maintaining the plurality of standoff contact elements of the unmanned aerial vehicle in contact with the surface of the airfoil-shaped body; and (e) while the unmanned aerial vehicle is at the second location with the plurality of standoff contact elements in contact with the surface of the airfoil-shaped body, activating the maintenance tool to perform a second maintenance operation on the surface of the airfoil-shaped body. 
     In accordance with other embodiments, the method further comprises: (d) upon completion of step (c), flying the unmanned aerial vehicle away from the first location while maintaining the plurality of standoff contact elements in contact with the surface of the airfoil-shaped body; and (e) while the unmanned aerial vehicle is flying away from the first location with the plurality of standoff contact elements in contact with the surface of the airfoil-shaped body, activating the maintenance tool to perform a second maintenance operation on the surface of the airfoil-shaped body. 
     In accordance with one proposed implementation of the method for performing a maintenance operation on an airfoil-shaped body, the maintenance tool is a sensor array, step (d) comprises moving the sensor array along a scan path that follows the surface of the airfoil-shaped body, and step (e) comprises activating the sensor array to acquire nondestructive inspection sensor data representing characteristics of the airfoil-shaped body during movement of the sensor array along the scan path. For example, such method is especially useful for inspecting a wind turbine blade. 
     Other aspects of methods for performing a maintenance operation in a limited-access area using an unmanned aerial vehicle are disclosed below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features, functions and advantages discussed in the preceding section may be achieved independently in various embodiments or may be combined in yet other embodiments. Various embodiments will be hereinafter described with reference to drawings for the purpose of illustrating the above-described and other aspects. None of the diagrams briefly described in this section are drawn to scale. 
         FIG.  1    is a diagram showing a system for inspecting an airfoil-shaped body using an airborne UAV. 
         FIG.  2    is a diagram representing a top view of a UAV equipped with a sensor array in accordance with one embodiment. 
         FIGS.  3  and  4    are diagrams representing rear and side views respectively of the UAV depicted in  FIG.  2   . 
         FIG.  5    is a diagram representing a three-dimensional view of the frame of the UAV depicted in  FIGS.  2 - 4   . 
         FIG.  6    is a block diagram identifying some components of a system for performing nondestructive inspection of a structure using a remote-controlled UAV in accordance with one embodiment. 
         FIG.  7    is a diagram representing a top view of a UAV equipped with a sensor array and gimbal-mounted vertical rotors in accordance with another embodiment. 
         FIG.  7 A  is a diagram representing a top view with magnified scale of one of the gimbal-mounted vertical rotors depicted in  FIG.  7   . 
         FIG.  8    is a block diagram identifying some components of the UAV depicted in  FIG.  7   . 
         FIG.  9    is a diagram representing a top view of a UAV equipped with a sensor array in accordance with another embodiment. 
         FIGS.  10  and  11    are diagrams representing rear and side views respectively of the UAV depicted in  FIG.  9   . 
         FIGS.  12 A through  12 E  are diagrams representing views of a UAV equipped with a sensor array during respective stages of a remotely controlled procedure for NDI of an airfoil-shaped body. 
         FIG.  13    is a flowchart identifying steps of a method for performing a maintenance operation on an airfoil-shaped body using a UAV equipped with a maintenance tool. 
     
    
    
     Reference will hereinafter be made to the drawings in which similar elements in different drawings bear the same reference numerals. 
     DETAILED DESCRIPTION 
     For the purpose of illustration, methods for performing a maintenance operation on a limited-access surface of a structure or object using a UAV will now be described in detail. However, not all features of an actual implementation are described in this specification. A person skilled in the art will appreciate that in the development of any such embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     In accordance with the embodiments disclosed in some detail below, the UAV takes the form of a rotorcraft having at least three rotors. In accordance with the implementation disclosed herein, each rotor has two mutually diametrally opposed rotor blades. However, in alternative implementations, UAVs having rotors with more than two rotor blades may be used. As used herein, the term “rotor” refers to a rotating device that includes a rotor mast, a rotor hub mounted to one end of the rotor mast, and two or more rotor blades extending radially outward from the rotor hub. In the embodiments disclosed herein, the rotor mast is mechanically coupled to an output shaft of a drive motor, referred to hereinafter as a “rotor motor”. The rotor motor drives rotation of the rotor. As used herein, the term “rotor system” means a combination of components, including at least a plurality of rotors and a controller configured to control rate of rotor rotation to generate sufficient aerodynamic lift force to support the weight of the UAV and sufficient thrust to counteract aerodynamic drag in forward flight. 
       FIG.  1    is a diagram showing a system for performing a maintenance operation on an airfoil-shaped body  100  using an unmanned aerial vehicle  20  (hereinafter “UAV  20 ”).  FIG.  1    shows the UAV  20  during flight. In the scenario depicted, the UAV  20  is a rotorcraft comprising four vertical rotors  8   a - 8   d  and one normal rotor  4 . As used herein, the term “vertical rotor” means a rotor having an axis of rotation that is vertical when the UAV  20  is level (e.g., the pitch, yaw and roll angles each equal zero degrees). As used herein, the term “normal rotor” means a rotor having an axis of rotation that is normal to a vertical plane intersecting the axis of rotation of the vertical rotor defined in the immediately preceding sentence.  10 . In accordance with one proposed implementation, the plurality of rotors comprise first through fourth rotors (e.g., vertical rotors  8   a - 8   d ) having axes of rotation which are parallel and a fifth rotor (e.g., normal rotor  4 ) having an axis of rotation perpendicular to the axes of rotation of the first through fourth rotors. 
     The UAV  20  depicted in  FIG.  1    may be equipped to perform a maintenance function. For the purpose of illustration, the UAV  20  is shown in  FIG.  1    equipped with a sensor array  72  for use in NDI. As will be described in more detail below, the frame of the UAV  20  includes a standoff system (only standoff support members  64   a  and  64   b  and standoff contact elements  68   a  and  68   b  of the standoff system are shown in  FIG.  1   ) for maintaining the sensor array  72  in a standoff position relative to the surface being inspected. Various embodiments of such a sensor array-equipped UAV will be described in some detail below with reference to  FIGS.  2 - 5   . 
     Although the structure being inspected is illustrated as an airfoil-shaped body  100  having two side surfaces  104  and  106  connected by a leading edge  102 , the system is equally well adapted for use in inspecting a wide range of other structures including, but not limited to, power lines, power-generating facilities, power grids, dams, levees, stadiums, large buildings, bridges, large antennas and telescopes, water treatment facilities, oil refineries, chemical processing plants, high-rise buildings, and infrastructure associated with electric trains and monorail support structures. The system is also particularly well suited for use inside large buildings such as manufacturing facilities and warehouses. Virtually any structure that would be difficult, costly, or too hazardous to inspect by a human controlling the inspection device or the platform carrying the inspection device may potentially be inspected using the system depicted in  FIG.  1   . 
     For inspection applications, a rotorcraft is preferred due to its ability to hover and move at very slow speeds. The vertical take-off and landing capability of remote-controlled unmanned rotorcraft also may be highly advantageous in many applications, especially when operating inside of structures or facilities such as manufacturing plants, warehouses, etc., or when inspecting complex facilities such as oil refineries or chemical processing that may have many tall structures (e.g., smoke stacks) clustered closely together. The ability to hover and/or move only vertically enables remote-controlled unmanned rotorcraft to fly close to and inspect large vertical structures such as vertical support posts of bridges, antennas or vertical surfaces of dams. 
     The system depicted in  FIG.  1    further comprises a remote control station  10  for sending and receiving wireless communications to and from the UAV  20 . In accordance with one embodiment, the remote control station  10  comprises a laptop computer  12 , a transceiver  14  and an antenna  16 . The transceiver  14  is in communication with the antenna  16  for enabling communication between the laptop computer  12  and the UAV  20 . 
     The on-board system of the UAV  20  may further comprise a guidance and control hardware and software system (not shown in  FIG.  1   ) that is able to implement one or more different, stored flight plans digitally represented by flight plan data stored in a non-transitory tangible computer-readable storage medium (not shown in  FIG.  1   ). The on-board system may further comprise a global positioning system/inertial navigation system (GPS/INS) for controlling the orientation of UAV  20  and assisting in carrying out the preprogrammed flight plan stored in memory. A wireless transceiver and an on-board antenna (not shown in  FIG.  1   ) enable bidirectional, wireless electromagnetic wave communications with the remote control station  10 . 
       FIG.  2    is a diagram representing a top view of a UAV  20  equipped with a sensor array  72  in accordance with one embodiment.  FIGS.  3  and  4    show rear and side views respectively of the UAV  20  depicted in  FIG.  2   .  FIG.  5    is a three-dimensional view of the frame  60  of the UAV  20  depicted in  FIGS.  2 - 4   . In alternative embodiments, the UAV  20  may be equipped with a maintenance tool other than a sensor array. 
     In addition to the sensor array  72 , the UAV  20  depicted in  FIGS.  2 - 5    includes a rotor system consisting of rotors, rotor motors and a controller  70 . In the depicted example, each rotor has two rotor blades  58   a  and  58   b . However, each rotor may have more than two rotor blades. As best seen in  FIG.  2   , the UAV  20  includes four vertical rotors  8   a - 8   d  and four vertical rotor motors  6   a - 6   d  which respectively drive rotation of vertical rotors  8   a - 8   d . As best seen in  FIG.  3   , the UAV  20  further includes a normal rotor  4  and a normal rotor motor  2  which drives rotation of the normal rotor  4 . The UAV  20  depicted in  FIGS.  2 - 5    further includes a frame  60  designed to support the aforementioned rotors, motors, controller and sensor array. Frame  60  may comprise integrally formed sections or fastened or joined parts. The frame components named hereinafter may be integrally formed or separately assembled regardless of any implication in the component name that the component is a separate (not integral) part. 
     As best seen in  FIG.  2   , frame  60  includes four vertical rotor deflector rings  62   a - 62   d  and a normal rotor deflector ring  62   e  having fixed positions relative to each other. The rotor masts of vertical rotors  8   a - 8   d  are rotatable inside bearings (not shown in the drawings), which bearings are supported by radial struts  82  which connect to the vertical rotor deflector rings  62   a - 62   d  respectively. The vertical rotor motors  6   a - 6   d  (see  FIG.  3   ) are also mechanically coupled to the vertical rotor deflector rings  62   a - 62   d  respectively by means of radial struts  82 . Likewise the rotor mast of normal rotor  4  is rotatable inside a bearing (not shown in the drawings) supported by radial struts  82  (only one of which is visible in  FIG.  3   ) which connect to the normal rotor deflector ring  62   e . The normal rotor motor  2  is also mechanically coupled to the normal rotor deflector ring  62   e  by means of radial struts  82 . Each rotor motor may be mechanically coupled to the outer ring of a respective bearing, with a respective rotor mast mechanically coupled to the output shaft of a respective rotor motor and mechanically coupled to the inner ring of the respective bearing. 
     As best seen in  FIG.  3   , the frame  60  further includes four standoff support members  64   a - 64   d  which extend from the rotor deflector rings in the manner of cantilever beams. Four standoff contact elements  68   a - 68   d  are coupled to distal ends of respective standoff support members  64   a - 64   d . In alternative embodiments, three or more standoff support members and standoff contact elements may be employed. Regardless of the number of standoff contact elements, the standoff contact elements are preferably positioned relative to each other and relative to the sensor array  72  such that the sensor array  72  is in a stable position and nearly parallel to the confronting portion of the surface to be inspected when the standoff contact elements are all in contact with the surface. 
     In the example embodiment depicted in  FIGS.  2 - 5   , the standoff contact elements  68   a - 68   d  are respective ball rollers (a.k.a. ball-and-socket bearings). In one alternative embodiment, the standoff contact elements  68   a - 68   d  are wheels having mutually parallel axes of rotation. In another alternative embodiment, the standoff contact elements  68   a - 68   d  are pivotably coupled sliding blocks capable of adjusting their angular position to lie flat on a non-planar surface when the standoff contact elements  68   a - 68   d  are placed in contact with that non-planar surface. The sliding blocks have low-friction surfaces suitable for sliding along a surface of a structure. Preferably the frame  60  of UAV  20  is configured so that the standoff contact elements  68   a - 68   d  may all contact a surface (e.g., a planar surface) at the same time. More specifically, the UAV  20  may be flown to a location adjacent the surface of a structure scheduled to undergo a maintenance operation, whereat the standoff contact elements  68   a - 68   d  all contact the surface of the structure (as seen in  FIG.  12 B , described in more detail below). 
     As best seen in  FIG.  5   , the frame  60  further includes a sensor support plank  76  and two tool support members  66   a  and  66   b  which support opposite ends of the sensor support plank  76 . The sensor support plank  76  supports a sensor array  72  that includes a plurality of sensors  74  (e.g., ultrasonic transducers or eddy current probes). One end of the sensor support plank  76  is connected or attached to tool support member  66   a  and the other end of the sensor support plank  76  is connected or attached to tool support member  66   b . The plurality of sensors  74  may be arranged in one or more rows, the sensors in each row being arranged with equal spacing between adjacent sensors. For example, the sensor support plank  76  may be formed with openings in which the sensors  74  are respectively installed. 
     The tool support members  66   a  and  66   b  and standoff support members  64   a - 64   d  of frame  60  are configured such that the sensor array  72  (or other maintenance tool) is supported by the plurality of tool support members in a fixed position relative to the plurality of standoff contact elements  68   a - 68   d . Thus when the standoff contact elements  68   a - 68   d  all contact a surface of a structure, the sensor array  72  will have a specified position with respect to the confronting area of the surface. Depending on the type of sensor being used, the frame  60  may be designed such that the sensors  74  of the sensor array  72  will be in contact with or at a standoff distance from the surface being contacted by standoff contact elements  68   a - 68   d . In the case wherein the sensor array  72  is separated from the confronting surface by a standoff distance, the sensors  74  are preferably separated from the surface by equal standoff distances.  FIG.  4    shows a scenario in which standoff contact elements  68   a - 68   d  are in contact with a surface  110  while the sensors  74  of the sensor array  72  are separated from the surface  110  by a standoff distance. 
     As best seen in  FIGS.  3  and  4   , the UAV  20  in accordance with one embodiment further includes a controller  70 . The controller  70  controls the operation of the normal rotor motor  2  and vertical rotor motors  8   a - 8   d . The controller  70  receives electrical power from a power source on the ground via an electrical cable  78 . The electrical cable  78  may optionally also include wires for conducting electrical control signals from a ground station to the controller  70  and conducting electrical sensor data signals from an onboard NDI sensor unit to the ground station. 
     More specifically, the controller  70  may include respective motor controllers (a.k.a. electronic speed control circuits) for controlling the rotational speeds of the rotor motors. In one embodiment shown in  FIG.  6   , the controller  70  includes a computer system  44  and a plurality of motor controllers  46 . In one proposed implementation, the computer system  44  is configured with various software modules, including a software module that controls UAV flight and a software module that controls an NDI sensor unit  34 . (The sensor array  72  shown in  FIGS.  2 - 5    is a component of the NDI sensor unit  34  shown in  FIG.  6   .) The computer system  44  sends information to the motor controllers  46  for controlling the revolutions per minute and direction of each rotor motor. The UAV  20  may also be equipped with a video camera  30  that operates under the control of the computer system  44 . More specifically, the video camera  30  may be activated by the computer system  44  to acquire an image and then send the image data back to the computer system  44  for storage and later transmission to the ground. 
     In the embodiment partly depicted in  FIG.  6   , the UAV  20 , the video camera  30  and the NDI sensor unit  34  are controlled by the computer system  44  on-board the UAV  20  as a function of radiofrequency commands transmitted by a control station  10 . Those radiofrequency commands are transmitted by a transceiver  14  on the ground; received by a transceiver  38  on-board the UAV  20 ; converted by the transceiver  38  into the proper digital format; and then forwarded to the computer system  44 . The control station  10  may comprise a general-purpose computer system configured with programming for controlling operation of the UAV  20  and the NDI sensor unit  34  on-board the UAV  20 . For example, the flight of the UAV  20  can be controlled using a joystick, keyboard, mouse, touchpad, or touchscreen of a computer system at the control station  10  or other user interface hardware (e.g., a gamepad or a pendant). In addition, the computer system at the control station  10  is configured with programming for processing data received from the UAV  20  during an inspection operation. In particular, the computer system of the control station  10  may comprise a display processor configured with software for controlling a display monitor (not shown in  FIG.  6   ) to display images acquired by the video camera  30 . The optical image field, as sighted by a video camera  30  onboard the UAV  20 , can be displayed on the display monitor. 
     Referring again to  FIG.  6   , the equipment on-board the UAV  20  further comprises an inertial measurement unit  36  (hereinafter “IMU  36 ”). An inertial measurement unit works by detecting linear acceleration using one or more accelerometers and rotational rate using one or more gyroscopes. In a typical configuration, an inertial measurement unit comprises one accelerometer and one gyroscope per axis for each of the three vehicle axes: pitch, roll and yaw. The computer system  44  may further comprise a separate processor configured with inertial navigation software that utilizes the raw IMU measurements to calculate attitude, angular rates, linear velocity and position relative to a global reference frame. The data collected from the IMU  36  enables the computer system  44  to track the UAV&#39;s position using a method known as dead reckoning. 
     As previously described, the maintenance tool and video camera on-board the UAV  20  may be activated by control signals (e.g., via electrical cables) generated by the computer system  44 . The computer system  44  also controls the flight of the UAV  20  by sending commands to the motor controllers  46  which respectively control the rotation of respective rotor motors  2  and  6   a - 6   d  that drive rotation of rotors  4  and  8   a - 8   d  respectively. 
     When the UAV operator manipulates the remote control joysticks, flight control signals are sent to the computer system  44 . The computer system  44  then controls the respective speeds of the rotor motors. The computer system  44  also receives information from the IMU  36  and from proximity sensors (not shown) and calculates the location of the UAV  20  using programmed flight parameters and algorithms. The motor controllers  46  may take the form of electronic speed control circuits configured to vary an electric motor&#39;s speed, direction and braking. Such electronic speed controllers provide high-frequency, high-resolution three-phase AC power to the motors, which are preferably brushless electric motors. 
     In order for a UAV  20  with four vertical rotors  8   a - 8   d  to rise into the air, a lifting force must be created which exceeds the force of gravity. The faster the rotors spin, the greater the lift and vice versa. The UAV  20  is capable of hovering, ascending or descending in a vertical plane. To hover, the net upward thrust of the four vertical rotors  8   a - 8   d  must be exactly equal to the gravitational force being exerted on the UAV  20 . The UAV  20  may ascend by increasing the thrust (speed) of the four vertical rotors  8   a - 8   d  so that the upward force is greater than the weight of the UAV  20 . The UAV  20  may descend by decreasing the rotor thrust (speed) so the net force is downward. 
     The tool-equipped UAVs disclosed herein are also capable of flying forward, backward, or sideways or rotate while hovering during the performance of a maintenance function. For example, in the case of a UAV having four fixed vertical rotors  8   a - 8   d  as shown in  FIGS.  2 - 5   , increasing the thrust produced by the rear pair of vertical rotors  8   b  and  8   c  causes a hovering UAV  20  to pitch forward and fly forward. Similarly, increasing the thrust produced by the left pair of vertical rotors  8   a  and  8   b  causes a hovering UAV  20  to roll to the starboard side and fly laterally rightward. 
     The maintenance methodology disclosed herein takes advantage of the UAV&#39;s mobility to “land” onto a surface of a structure, hover while contacting that surface, and then skim along the surface. Before, during and after skimming of the UAV across the surface, the maintenance tool onboard the UAV is able to scan that surface (e.g., for the purpose of NDI). In accordance with the embodiment depicted in  FIGS.  2 - 5   , in which the axes of rotation of the rotors are fixed relative to the UAV frame, the orientation of the UAV may be changed by generating unequal individual rotor thrusts. In accordance with alternative embodiments, the axes of rotation of the rotors are variable relative to the UAV frame.  FIG.  7    is a diagram representing a top view of a UAV  20  equipped with a sensor array  72  and gimbal-mounted vertical rotors  8   a - 8   d.    
     The UAV  20  depicted in  FIG.  7    differs from the UAV depicted in  FIG.  2    only in that the four vertical rotors  8   a - 8   d  are mounted on respective two-axis gimbals  40   a - 40   d  which enable the rotor masts (not visible in  FIG.  7   ) of the vertical rotors  8   a - 8   d  to be tilted. The two-axis gimbals  40   a - 40   d  rotatably couple respective rotor motors  6   a - 6   d  to the frame  60 . Each of the two-axis gimbals  40   a - 40   d  includes respective gimbal rings  84   a - 84   d  rotatably coupled to respective vertical rotor deflector rings  62   a - 62   d , respective A-axis gimbal motors  42   a - 42   d  mounted to respective vertical rotor deflector rings  62   a - 62   d  for driving rotation of the respective gimbal rings  84   a - 84   d  relative to vertical rotor deflector rings  62   a - 62   d , and respective B-axis gimbal motors  43   a - 43   d  mounted to respective gimbal rings  84   a - 84   d  for driving rotation of respective vertical rotors  8   a - 8   d  relative to gimbal rings  84   a - 84   d . The two-axis gimbals  40   a - 40   d  further include respective pairs of A-axis axles  88   a - 88   d  (which enable the gimbal rings  84   a - 84   d  to rotate relative to vertical rotor deflector rings  62   a - 62   d ) and respective pairs of B-axis axles  90   a - 90   d  (which enable the vertical rotors  8   a - 8   d  to rotate relative to gimbal rings  84   a - 84   d ). The B axis is perpendicular to the A axis. The mounting of the vertical rotors  8   a - 8   d  on gimbals enables the UAV  20  to fly forward, backward or sideways or rotate while hovering. 
       FIG.  7 A  is a diagram representing a top view with magnified scale of gimbal-mounted vertical rotor  8   a  depicted in  FIG.  7   . In this instance, the vertical rotor  8   a  is mounted on a two-axis gimbal  40   a . The gimbal ring  84   a  is rotatably coupled to and disposed within the perimeter of the vertical rotor deflector ring  62   a , and the vertical rotor  8   a  along with the vertical rotor motor  6   a  are rotatably coupled to and disposed within the perimeter of the gimbal ring  84 . More specifically, one pair of A-axis axles  88   a  are rotatably coupled to the vertical rotor deflector ring  62   a  and affixed to the gimbal ring  84   a , enabling the gimbal ring  84   a  to rotate relative to the vertical rotor deflector ring  62   a . In addition, one pair of B-axis axles  90   a  are rotatably coupled to gimbal ring  84   a  and affixed to the vertical rotor motor  6   a , enabling the vertical rotor motor  6   a  to rotate relative to the gimbal ring  84   a . The A and B axes are mutually perpendicular. Thus the vertical rotor motor  6   a  and the vertical rotor  8   a  may be tilted about the A-axis when gimbal ring  84   a  rotates relative to vertical rotor deflector ring  62   a , and tilted about the B-axis when vertical rotor motor  6   a  rotates relative to gimbal ring  84   a . In the same manner, the other vertical rotors  8   b - 8   d  are also tiltable about two axes. This construction enables the UAV  20  to be selectively rotated about pitch and roll axes as needed to help maintain the standoff contact elements  68   a - 68   d  in contact with the surface during scanning and help maintain the normal rotor  4  (which is fixed to the frame  60 , not tiltable) so that its axis of rotation is generally normal to the confronting surface area being inspected. In addition, the orientation of the UAV  20  and sensor array  72  can be adjusted using the gimbaled rotors. The sensor array  72  may be held with a horizontal orientation while the gimbaled rotors are thrusting the UAV  20  sideways. Also, the sensor array  72  may be held at an angle while the UAV  20  is maneuvering in a different orientation. 
       FIG.  8    is a block diagram identifying some components of the UAV  20  depicted in  FIG.  7   . The UAV  20  depicted in  FIG.  7    may also include the video camera  30 , encoders  32 , NDI sensor unit  34 , IMU  36  and transceiver  38  identified in  FIG.  6   , but not included in  FIG.  8     
     In accordance with the embodiment partly depicted in  FIG.  8   , the controller  70  includes a computer system  44  and a plurality of motor controllers  46  (one motor controller for each motor). One motor controller controls the operation of the normal rotor motor  2  that drives rotation of the normal rotor  4 ; a first set of four motor controllers respectively control the operation of the vertical rotor motors  6   a - 6   d  that respectively drive rotation of the vertical rotors  8   a - 8   d ; a second set of four motor controllers respectively control the operation of the A-axis gimbal motors  42   a - 42   d  that drive rotation of the gimbal rings  84   a - 84   d ; and a third set of four motor controllers respectively control the operation of the B-axis gimbal motors  43   a - 43   d  that drive rotation of the vertical rotors  6   a - 6   d  relative to the gimbal rings  84   a - 84   d . The computer system  44  is programmed to coordinate the operation of all motors so that the UAV  20  follows a prescribed scanning path along the surface of the structure being inspected. 
       FIG.  9    is a diagram representing a top view of a UAV  20  equipped with a sensor array  72  in accordance with another embodiment.  FIGS.  10  and  11    are diagrams representing rear and side views respectively of the UAV  20  depicted in  FIG.  9   . In alternative embodiments, the UAV  20  may be equipped with a maintenance tool other than a sensor array. 
     In addition to the sensor array  72 , the UAV  20  depicted in  FIGS.  9 - 11    includes a rotor system consisting of three rotors having axes of rotation which are generally mutually perpendicular, three rotor motors for driving rotation of the three rotors, and a controller  70 . In this example, each rotor has two rotor blades  58   a  and  58   b . In other examples, the rotors may have more than two rotor blades. As best seen in  FIG.  9   , the UAV  20  includes a vertical rotor  8  and a vertical rotor motor  6  which drives rotation of vertical rotor  8 . As best seen in  FIGS.  10  and  11   , the vertical rotor  8  is coupled to the vertical rotor motor  6  by way of a swashplate  56 . The swashplate  56  is mounted to the frame  60   a  and coupled to the vertical rotor  8  to enable control of a pitch of vertical rotor  8 . The swashplate  56  is controlled by the controller  70 . As best seen in  FIG.  10   , the UAV  20  also includes a normal rotor  4  and a normal rotor motor  2  which drives rotation of normal rotor  4 . As best seen in  FIG.  11   , the UAV  20  also includes a tail rotor  54  and a tail rotor motor  52  which drives rotation of tail rotor  54 . The tail rotor  54  is a smaller rotor mounted so that its axis of rotation is generally horizontal when the UAV  20  is flying level. The position and distance of the tail rotor  54  from the center of gravity of the UAV  20  allow it to develop thrust to counter the torque effect created by the vertical rotor  8 . 
     The UAV  20  depicted in  FIGS.  9 - 11    includes a frame  60  designed to support the aforementioned rotors, motors, controller and sensor array. As best seen in  FIG.  9   , frame  60  includes a vertical rotor deflector ring  62  and a tail rotor support beam  50 . As best seen in  FIGS.  9  and  10   , the frame  60  further includes a normal rotor deflector ring  62   e  supported by four struts  61   a - 61   d.    
     As best seen in  FIG.  9   , the frame  60  further includes a sensor support plank  76  attached to the normal rotor deflector ring  62   e . The sensor support plank  76  supports a sensor array  72  that includes a plurality of sensors  74  (e.g., ultrasonic transducers or eddy current probes). The plurality of sensors  74  may be arranged in one or more rows, the sensors in each row being arranged with equal spacing between adjacent sensors. For example, the sensor support plank  76  may be formed with openings in which the sensors  74  are respectively installed. 
     As best seen in  FIG.  10   , the frame  60  further includes four standoff support members  64   a - 64   d  which extend from the sensor support plank  76  in the manner of cantilever beams. Four standoff contact elements  68   a - 68   d  are coupled to distal ends of respective standoff support members  64   a - 64   d . In the example embodiment depicted in  FIGS.  2 - 5   , the standoff contact elements  68   a - 68   d  are respective ball rollers. In one alternative embodiment, the standoff contact elements  68   a - 68   d  are wheels having mutually parallel axes of rotation. In another alternative embodiment, the standoff contact elements  68   a - 68   d  are pivotably coupled sliding blocks capable of adjusting their angular position to lie flat on a non-planar surface when the standoff contact elements  68   a - 68   d  are placed in contact with that non-planar surface. The sliding blocks have low-friction surfaces suitable for sliding along a surface of a structure. Preferably the frame  60   a  of UAV  20  is configured so that the standoff contact elements  68   a - 68   d  may all contact a surface (e.g., a planar surface) at the same time. 
     The frame  60  is configured such that the sensor array  72  (or other maintenance tool) is supported in a fixed position relative to the plurality of standoff contact elements  68   a - 68   d . Thus when the standoff contact elements  68   a - 68   d  all contact a surface of a structure, the sensor array  72  will have a specified position with respect to the confronting area of the surface. Depending on the type of sensor being used, the frame  60  may be designed such that the sensors  74  of the sensor array  72  will be in contact with or at a standoff distance from the surface being contacted by standoff contact elements  68   a - 68   d . In the case wherein the sensor array  72  is separated from the confronting surface by a standoff distance, the sensors  74  are preferably separated from the surface by equal standoff distances. 
     The UAV  20  depicted in  FIGS.  9 - 11    further includes a controller  70 . The controller  70  controls the operation of the normal rotor motor  2 , vertical rotor motor  6  and tail rotor motor  52 . The motors may be electric or internal combustion engines. For electrical variants, the power may be supplied by an onboard battery or by a power source on the ground via an electrical cable  78  as shown in  FIGS.  10  and  11   . The electrical cable  78  may optionally also include wires for conducting electrical control signals from a ground station to the controller  70  and conducting electrical sensor data signals from an onboard NDI sensor unit to the ground station. 
     A UAV  20  in accordance with any one of the above-described embodiments may be used to perform a maintenance operation in a limited-access surface area on a structure. The UAV  20  may be moved intermittently to successive locations whereat a respective maintenance operation is performed. Or the UAV  20  may be moved continuously to cause the maintenance tool (e.g., an NDI sensor unit) to scan the surface. 
     For the purpose of illustration, one example maintenance operation will now be described with reference to  FIGS.  12 A- 12 E , which diagrams represent views of a UAV  20  at respective stages of a process for inspecting an airfoil-shaped body  100 . In this example, the airfoil-shaped body  100  is a wind turbine blade having two side surfaces  104  and  106  which are connected by a curved leading edge  102  and which intersect at an angled trailing edge (not shown in  FIGS.  12 A- 12 E ). The arrows in  FIGS.  12 A through  12 E  indicate the directions in which air is being propelled by the rotating rotors. The thrust produced, being a reaction force, will be in the opposite direction. 
       FIG.  12 A  shows a stage in the NDI operation wherein an airborne UAV  20 , equipped with a sensor array  72  (and other components of an onboard NDI sensor unit not shown), is maneuvering toward the airfoil-shaped body  100 . During such maneuvering, the vertical rotors are selectively operated to propel air (in directions indicated by arrows in  FIG.  12 A ) in a manner that produces a net thrust that causes the UAV  20  to fly toward the airfoil-shaped body  100 . As the UAV  20  approaches the airfoil-shaped body  100 , proximity sensors (not shown in the drawings) may be operated to measure respective distances to the airfoil-shaped body  100 , which measurements may then be used to align the standoff contact elements (only standoff contact elements  68   a  and  68   b  are shown in  FIG.  12 A ) with respect to the side surface  106  of airfoil-shaped body  100 . 
     The UAV  20  then flies to the location depicted in  FIG.  12 B  (hereinafter “the first location”). To accomplish this movement, in addition to lift forces, a normal force thrust in the forward direction (hereinafter “forward thrust”) is produced by rotating the normal rotor  4  to propel air in the direction indicated by the horizontal arrow in  FIG.  12 B . If the UAV  20  is level, this will cause the airborne UAV  20  to move horizontally. The forward thrust may be adjusted to ensure that the plurality of standoff contact elements  68   a - 68   d  (see  FIG.  3   ) are not damaged when they come into contact with side surface  106 . While the UAV  20  is hovering at the location depicted in  FIG.  12 B  with the plurality of standoff contact elements  68   a - 68   d  in contact with respective areas on side surface  106 , the normal rotor  4  continues to produce a forward thrust that presses the standoff contact elements  68   a - 68   d  against the side surface  106 , thereby setting the standoff distance for the sensor array  72 . 
     While the UAV  20  is hovering adjacent to and in contact with side surface  106  at the first location, the sensor array  72  is activated to acquire NDI sensor data. The UAV  20  then moves from the first location shown in  FIG.  12 B  to a second location shown in  FIG.  12 C  while maintaining standoff contact elements  68   a - 68   d  in contact with respective areas of the side surface  106 . While the UAV  20  is hovering at the second location and in contact with side surface  106 , the sensor array  72  is activated to acquire additional NDI sensor data. Optionally, the sensor array  72  may also be activated to continuously acquire additional NDI sensor data from side surface  106  as the UAV  20  moves from the first location to the second location. During the upward movement from the first location to the second location, the normal rotor produces a forward thrust that presses the standoff contact elements  68   a - 68   d  against the side surface  106 , while the vertical rotors produce upward thrusts that lift the UAV  20  to a higher elevation. The respective directions in which the rotating rotors propel air are indicated by arrows in  FIG.  12 C . 
     In the case of the airfoil-shaped body  100  depicted in  FIGS.  12 A- 12 E , the scanning process during an NDI procedure may start on one side surface  106 , continue as the UAV  20  follows the profile of the leading edge  102  (shown in  FIG.  12 D ), and then continues on the other side surface  104  (shown in  FIG.  12 E ). At the stage depicted in  FIG.  12 D , the UAV is standing on the curved surface of the leading edge  102  of the airfoil-shaped body  100  with the sensor array  72  facing downward. More specifically, the UAV  20  depicted in  FIG.  12 D  is oriented vertically with the standoff contact elements  68   a - 68   d  arranged to contact the curved surface of the leading edge  102  and the sensor array  72  either in contact with or setoff from the same surface when the standoff contact elements  68   a - 68   d  all contact the surface. The UAV  20  may be held at this third location while the sensor array  72  is activated to acquire NDI sensor data from the leading edge  102 . This may be accomplished by stopping rotation of the vertical rotors  8  and reversing their rotation as the momentum from the UAV  20  carries to the side surface  104 . The normal force rotor  4  continues to provide forward thrust that keeps the standoff contact elements  68   a - 68   d  in contact with the airfoil-shaped body  100 . 
     Thereafter, the vertical rotors may be activated to produce the thrusts required to move the UAV  20  from the third location depicted in  FIG.  12 D  to a fourth location adjacent to and in contact with the other side surface  104  of the airfoil-shaped body  100 . For the sake of illustration, UAV  20  is visible in  FIG.  12 E  even though it is behind the airfoil-shaped body  100  and would ordinarily be hidden when viewed from the vantage point of  FIG.  12 E . At the fourth location, the UAV  20  is now upside-down relative to its location as depicted in  FIG.  12 B  with the standoff contact elements  68   a - 68   d  now in contact with (and pressed against) the other side surface  104  of the airfoil-shaped body  100 . The UAV  20  may be held at this fourth location while the sensor array  72  is activated to acquire NDI sensor data from the leading edge  102 . Holding the UAV  20  at the fourth location may be accomplished by rotating the upside-down vertical rotors  8  in the opposite direction from the direction in which they were rotating during the stage depicted in  FIG.  2    while rotating the normal rotor  4  to produce a forward thrust in a direction opposite to the sideways-pointing arrow in  FIG.  12 E ). 
     In addition to ultrasonic and eddy current inspection techniques, optical imaging, infrared thermography, laser shearography, and digital radiography are other inspection methods that could be applied using the apparatus and methodology disclosed herein. Such image-based sensing methods require some standoff with the structure being inspected. For example, an imager or two-dimensional detector array may be supported by the UAV frame at a small distance away from the structure. 
     As previously mentioned, as the UAV  20  scans across the surface of a structure, the position of the maintenance tool may be tracked using encoders. For example, the standoff contact elements may be rotary encoders. For higher fidelity, encoders supplemented with an off-board positioning method, such as tracking using a local positioning system or motion capture using cameras mounted 
     The apparatus disclosed herein can be adapted for use in the automation of various maintenance functions, including but not limited to: nondestructive inspection, painting, light sanding, cleaning, drilling (with a suction cup attachment to react the drill forces), target attachment (motion capture targets, NDI targets, visual survey targets), decal attachment, damage marking (to denote the outer extent of visible or inspected damage), placement of materials (repair adhesive, repair composite plies, release film, breather material, vacuum bag), and application of repair adhesive tape. Additional maintenance functions which could be performed using a UAV include coating removal using abrasive pellets, laser ablation, chemical treatment, etc. as well as surface treatments for corrosion prevention, abrasion resistance or application of specialized coatings. 
       FIG.  13    is a flowchart identifying steps of a method  200  for performing a maintenance operation on an airfoil-shaped body using a UAV in accordance with one embodiment. First, a UAV is equipped with a maintenance tool (e.g., an NDI sensor unit having a sensor array) and a plurality of standoff contact elements (step  202 ). The plurality of standoff contact elements are arranged to simultaneously contact a surface of the airfoil-shaped body. The maintenance tool is arranged to confront an area on the surface of the airfoil-shaped body while the plurality of standoff contact elements are in contact with the surface. Then the UAV operator flies the UAV to a first location whereat the plurality of standoff contact elements contact respective areas on a surface of the airfoil-shaped body (step  204 ). While the UAV is at the first location with the plurality of standoff contact elements in contact with the surface of the airfoil-shaped body, the maintenance tool is activated to perform a first maintenance operation on the surface of the airfoil-shaped body (step  206 ). 
     The computer system onboard the UAV may be configured to control the movements of the UAV and the operations of the maintenance tool such that the maintenance operation is performed intermittently at successive spaced-apart locations of the UAV or continuously along a scan path followed by the UAV. If the maintenance operations are performed continuously along a scan path, then upon completion of step  206 , the UAV is flown away from the first location while maintaining the plurality of standoff contact elements in contact with the surface of the airfoil-shaped body (step  208 ). Then while the UAV is moving away from the first location with the plurality of standoff contact elements in contact with the surface of the airfoil-shaped body, the maintenance tool is activated to perform a second maintenance operation on the surface of the airfoil-shaped body (step  210 ). 
     In contrast, if the maintenance operations are performed intermittently at successive spaced-apart locations, then upon completion of step  206 , the UAV is flown from the first location to a second location. Optionally the UAV may be “flown” along a path that maintains the plurality of standoff contact elements in contact with the surface of the airfoil-shaped body (step  212 ). Then while the UAV is at the second location with the plurality of standoff contact elements in contact with the surface of the airfoil-shaped body, the maintenance tool is activated to perform a second maintenance operation on the surface of the airfoil-shaped body (step  214 ). 
     In the case where the maintenance tool is an NDI sensor unit, step  208  comprises moving the NDI sensor unit along a scan path that follows the surface of the airfoil-shaped body, and step  210  comprises activating the NDI sensor unit to acquire NDI sensor data representing characteristics of the airfoil-shaped body during movement of the NDI sensor unit along the scan path. This technique may be used to inspect a wind turbine blade in which the NDI sensor unit-equipped UAV circumnavigates the wind turbine blade except at the angled trailing edge. For example, the UAV may move from a location adjacent to the trailing edge and in contact with one side surface of a wind turbine blade, and then fly (while maintaining contact along the way) to a location on the curved leading edge of the wind turbine blade, and thereafter fly (while maintaining contact along the way) to a location adjacent to the trailing edge and in contact with the other side surface of the wind turbine blade, acquiring a swath of NDI sensor data continuously as the UAV travels around the wind turbine blade. 
     Multiple UAVs of the types described above can be used at the same time during maintenance operations at large structures. For example, a plurality of tool-equipped UAVs (e.g., rotorcraft) may be moved around a structure requiring periodic inspection. Such a system includes a computer system for controlling the flight of the UAVs, the operations of the maintenance tools and the acquisition of data. The system may be adapted for use in inspecting a wide range of structures including, but not limited to, wind turbine blades, storage tanks, aircraft, power plants, dams, levees, stadiums, high-rise buildings, large antennas and telescopes, water treatment facilities, oil refineries, chemical processing plants, and infrastructure associated with electric trains and monorail support structures. The system is also particularly well suited for use inside large buildings such as manufacturing facilities and warehouses. Virtually any structure that would be difficult, costly, or hazardous to inspect by a human-piloted vehicle or a human lifted by a crane may potentially be inspected using a swarm of tool-equipped UAVs. 
     In accordance with one embodiment, each UAV includes an onboard system that is able to navigate the UAV in accordance with a preprogrammed flight plan and control the NDI sensor unit to acquire NDI sensor data while the UAV is hovering adjacent to or skimming along a surface of the structure being inspected. The preprogrammed flight plan carried by each UAV enables each UAV to follow a respective unique flight path around a portion of the structure being inspected. Thus, it will be appreciated that the preprogrammed flight plan (and therefore flight path) for each UAV is unique and formed with respect to a designated portion of the structure to be inspected. Generally, the greater the number of UAVs employed in any given inspection task, the shorter the duration of time to complete the inspection task. 
     The system further may include a control station  10  (see  FIG.  6   ) for receiving wireless communications from each of the UAVs  20 . The control station  10  may include a computer control system and a display monitor for viewing by an inspection technician or operator. A transceiver  14  is in wireless communication with the transceivers  38  for enabling wireless communication between the computer control system and the onboard computer system  44  of each UAV  20 . The computer control system may be configured to send commands to each UAV  20 , to receive NDI sensor data from the NDI sensor unit  34  carried by each UAV  20 , or to monitor various operating performance parameters of each UAV  20  such as fuel remaining. The computer control system may also be used generate commands to alter the flight plan of any one of the UAVs  20 . 
     The onboard computer system  44  may include guidance and control software configured to implement a pre-stored flight plan. The onboard system may include a global positioning system (GPS)/inertial navigation system for controlling the orientation of its associated UAV  20  and assisting in carrying out the pre-stored flight plan. A wireless transceiver  38  and an onboard antenna (not shown in  FIG.  6   ) enable bidirectional, wireless electromagnetic wave communications with the control station  10 . 
     A multiplicity of NDI sensor unit-equipped UAVs may be deployed to form an inspection “swarm”. When the UAVs reach the structure to be inspected, each UAV begins acquiring NDI sensor data for the portion of the structure which that UAV has been designated to inspect. In one proposed implementation, the UAVs transmit their acquired NDI sensor data to the control station  10  via their transceivers  38  and antennas. Alternatively the UAVs could each store their acquired NDI sensor data in a non-transitory tangible computer-readable storage medium onboard the UAV for future downloading once the UAV lands. 
     While methods for performing a maintenance operation on a limited-access surface of a structure or object using remotely controlled unmanned aerial vehicles have been described with reference to various embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the teachings herein. In addition, many modifications may be made to adapt the teachings herein to a particular situation without departing from the scope thereof. Therefore it is intended that the claims not be limited to the particular embodiments disclosed herein. 
     As used in the claims, the term “location” comprises position in a three-dimensional coordinate system and orientation relative to that coordinate system. 
     As used herein, the term “computer system” should be construed broadly to encompass a system having at least one computer or processor, and which may have multiple computers or processors that communicate through a network or bus. As used in the preceding sentence, the terms “computer” and “processor” both refer to devices comprising a processing unit (e.g., a central processing unit) and some form of memory (i.e., computer-readable medium) for storing a program which is readable by the processing unit. 
     The methods described herein may be encoded as executable instructions embodied in a non-transitory tangible computer-readable storage medium, including, without limitation, a storage device and/or a memory device. Such instructions, when executed by a computer system, cause the tool-equipped unmanned aerial vehicle to perform at least a portion of the methods described herein.