Abstract:
For retrieval of a hovering aircraft, a cable, bar, or similar fixture is suspended in an approximately horizontal orientation across the retrieval area between two well-separated supports. The aircraft slowly flies into this fixture, which then slides along the aircraft in a direction approximately parallel with the aircraft&#39;s thrust line. This leads to the aircraft becoming fastened to the fixture by an interceptor or aircraft capturer, which in alternative embodiments are respectively on the aircraft or the fixture or both. Thrust is then reduced, and the aircraft comes to rest hanging from the fixture for subsequent removal. Retrieval is thus accomplished with simple and economical apparatus, light and unobtrusive elements on the aircraft, low risk of damage, and only moderate piloting accuracy.

Description:
PRIORITY CLAIM 
     This application is a continuation of, and claims priority to and the benefit of, U.S. patent application Ser. No. 13/024,843, filed on Feb. 10, 2011, which is a divisional of, and claims priority to and the benefit of, U.S. patent application Ser. No. 11/837,878, filed on Aug. 13, 2007, which issued as U.S. Pat. No. 7,954,758 on Jun. 7, 2011, which is a non-provisional of, and claims priority to and the benefit of, U.S. Provisional Patent Application Ser. No. 60/823,442, filed on Aug. 24, 2006, now expired, the entire contents of each of which are incorporated herein by reference. 
     CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application relates to the following commonly-owned co-pending patent applications: U.S. patent application Ser. No. 12/702,935, filed on Feb. 9, 2010; U.S. patent application Ser. No. 13/037,436, filed on Mar. 1, 2011; and U.S. patent application Ser. No. 13/527,177, filed on Jun. 19, 2012. 
    
    
     NOTICE OF GOVERNMENT RIGHTS 
     This invention was made with U.S. Government support under Contract No. W31P4Q-06-C-0043, effective Nov. 23, 2005 (“the contract”), issued by U.S. Army Aviation and Missile Command. The U.S. Government has certain rights in the invention. More specifically, the U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of the contract. 
    
    
     BACKGROUND 
     1. Field 
     The present disclosure addresses retrieval of a hovering aircraft, especially in turbulent winds or onto a rough or irregularly-moving surface, such as the deck of a ship in a rough sea. The present disclosure is especially suited to unmanned aircraft of small size, and requires only modest accuracy in automatic or manual piloting. 
     2. Description of Prior Art 
     Hovering aircraft, be they helicopters, thrust-vectoring jets, “tail-sitters,” or other types, usually land by gently descending in free thrust-borne flight onto a landing surface, coming to rest on an undercarriage of wheels, skids, or legs. This elementary technique can be problematic in certain situations, for example when targeting a small, windswept landing pad on a ship moving in a rough sea. Decades ago, the Beartrap or RAST system was developed to permit retrieval with acceptable safety in such conditions. Retrieval with this system involves securing a line between a helicopter and landing deck, and then winching the helicopter down onto a trolley. The helicopter is fastened to the trolley. After retrieval, the trolley is used to move the helicopter along the deck. The system is effective and widely used, but requires an expensive and substantial plant in the landing area, and coordination between aircraft and ground crew. Furthermore, the helicopter must carry a complete undercarriage in addition to the necessary Beartrap components. 
     By comparison, simple methods for retrieving aircraft from wing-borne flight into a small space have been described in U.S. Pat. No. 6,264,140 and U.S. Pat. No. 6,874,729. These involve flying the aircraft into a cable suspended in an essentially vertical orientation. Typically, the cable strikes a wing of the aircraft and slides spanwise along the wing into a hook; the hook snags the cable; the cable decelerates the aircraft briskly but smoothly; and the aircraft comes to rest hanging by its hook. Advantages of this technique include: simplicity of the apparatus; relatively easy targeting, since the aircraft can make contact anywhere within its wingspan and almost anywhere along the cable; elimination of undercarriage from the aircraft; and safety, since the aircraft simply continues in wing-borne flight if it misses the cable, and since all components, apart from the cable itself, are kept well clear of the flight path. 
     SUMMARY 
     One embodiment of the present disclosure provides for snag-cable retrieval of thrust-borne or hovering aircraft, and particularly those with large rotors. The present disclosure offers the same advantages as does snag-cable retrieval for wing-borne aircraft; namely, simplicity, relatively easy targeting, elimination of undercarriage, and safety. 
     Furthermore, since loads can be low during retrieval from hover, the apparatus can be light, inexpensive, and easy to deploy. Easy targeting makes the technique well-suited for both manual control and economical automation. 
     Additional features and advantages are described herein, and will be apparent from the following Detailed Description and the Figures. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIGS. 1A ,  1 B,  1 C, and  1 D are a series of diagrammatic rear-quarter perspective views of an embodiment of the present disclosure for a helicopter, showing the helicopter sequentially approaching above, sliding along, decelerating against, and hanging from a retrieval cable. 
         FIG. 2  is a perspective view of a representative hook installation on a pole deployed by a helicopter or fixed wing aircraft, as constructed according to one embodiment of the present disclosure. 
         FIG. 3  is plan view of an embodiment of a clamping hook constructed according to one embodiment of the present disclosure. 
         FIGS. 4A ,  4 B,  4 C, and  4 D are a series of diagrammatic rear-quarter perspective views of an embodiment of the present disclosure for a hovering aircraft, showing the aircraft sequentially approaching, sliding along, decelerating against, and hanging from a retrieval cable. 
         FIG. 5  is a perspective view of a hook installation on a cruciform empennage, according to one embodiment of the disclosure 
         FIGS. 6A ,  6 B,  6 C, and  6 D are a series of diagrammatic rear-quarter perspective views of an embodiments of the present disclosure for a hovering aircraft, showing the aircraft sequentially approaching, sliding along, and decelerating against a retrieval cable, and corning to rest on an adjacent support cable. 
         FIG. 7  is a side view of a hovering aircraft on a horizontal approach to a retrieval cable, with the approach made from upwind of the cable, and the wing aligned at knife-edge to the relative wind in order to minimize drag. 
         FIG. 8  is a side view of a hovering aircraft on a descending approach to a retrieval cable, with the approach made from downwind of the cable, and the wing aligned at knife-edge to the relative wind in order to minimize drag. 
         FIG. 9  is a side view of a hovering aircraft on a descending approach to a retrieval cable, with the approach made from downwind of the cable, and the wing generating lift. 
         FIGS. 10A ,  10 B, and  10 C are a series of diagrammatic rear-quarter perspective views of an embodiment of the present disclosure for a hovering aircraft, in which the aircraft uses a trailing string to capture a retrieval boom. 
         FIGS. 11A ,  11 B,  11 C, and  11 D are a series of diagrammatic rear-quarter perspective views of an embodiment of the present disclosure for a hovering aircraft, showing the aircraft sequentially approaching, engaging, and hanging from a cantilever retrieval fixture that has a set of cleats for engaging the aircraft. 
         FIGS. 12A ,  12 B,  12 C, and  12 D are a series of diagrammatic rear-quarter perspective views of an embodiment of the present disclosure for a hovering aircraft, showing the aircraft sequentially approaching, engaging, and hanging from a cantilever retrieval fixture which has a set of latches for engaging a detent in the aircraft fuselage. 
         FIGS. 13A ,  13 B,  13 C, and  13 D are a series of diagrammatic rear-quarter perspective views of an embodiment of the present disclosure for a hovering aircraft, showing the aircraft sequentially engaging and translating along a retrieval cable into a parking fixture, parking in the fixture, and being stored or released for another flight. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1A ,  1 B,  1 C, and  1 D show an illustrative embodiment of the present disclosure, as used with a helicopter of conventional layout. This embodiment includes a retrieval fixture in the form of a cable  12  that is suspended by supports  13  across the intended landing area. The supports  13  are sufficiently separated to allow the helicopter  1  to comfortably pass between them. In preparation for retrieval, the helicopter  1  extends an interceptor, which in one embodiment includes a pole  5 . The interceptor also includes one or more hooks  10  attached to the end of the pole  5  as, for example, shown in  FIG. 2 . The helicopter  1  approaches the cable  12  in slow, nearly-horizontal flight at speed VG along a path  42  at a suitably large angle relative to the line  14  between the supports  13 . In one embodiment, the approach is flown automatically, with three-dimensional position and velocity of the helicopter  1  relative to the cable  12  being measured, for example, by differencing satellite-navigation solutions between an antenna  15  on the helicopter and on a reference point  16  near the cable. Approach brings the pole  5  into contact with the cable  12 , which then applies a force as indicated by arrow  17  to the surface  8  of the pole  5 . The cable  12  then slides along the pole  5 . This can be arranged by: (a) deploying the pole  5  with a suitable sweepback angle relative to the line of approach; or (b) by making the pole attachment compliant under the cable load  17 ; or (c) by attaching the pole  5  rigidly along the spin axis  4  of the rotor  2 , leaving the cable load  17  to rotate the helicopter  1  bodily about the cable axis  14 ; or (d) by a suitable combination of these arrangements. Sliding directs the cable  12  through a gate  22  into a hook  10  as shown in  FIG. 2 , and the gate  22  then closes to ensure that the cable  12  will not be released until desired. Closing of the gate  22  may be sensed directly, or inferred from deflection of the pole  5  under the cable load  17 , or from deceleration or rotation of the helicopter  1 . When capture of the cable  12  is recognized, drive power is reduced and the rotor  2  gradually slows to a stop. The helicopter  1  comes to rest hanging upside-down from the cable  12 . A winch  18  or other suitable device for adjusting the height of the cable  12  can then be used to lower the helicopter  1  onto a handling platform. The hook  10  can then be released. Alternatively, a small helicopter  1  can be removed from the cable  12  by hand. 
     If the approach speed of the helicopter  1  is sufficiently high, then the cable  12  may have to comply in order to make deceleration loads acceptably small. This may be done by: (a) incorporating elastic segments into the cable  12 ; or (b) by paying-out slack from a winch  18  in order to control tension in the cable  12 ; or (c) by a combination thereof. In either case, provision may be made quickly to take up the slack during the latter part of deceleration in order to limit sag of the helicopter  1  as it comes to rest. 
     It should be noted that instead of deploying the retrieval-fixture interceptor downward as in  FIG. 1 , the helicopter  1  in an alternative embodiment could deploy the interceptor upward from its rotor hub  3 . It would then approach so that its rotor  2  passes below rather than above the cable  12 , and it would come to rest hanging right-side-up rather than upside-down. While coming to rest right-side-up would be desirable, especially for a manned helicopter, passing above the cable  12  as in  FIG. 1  offers two safety advantages over passing below. First, it increases the clearance between the cable  12  and the rotor  2  during approach. Second, it permits the helicopter  1  to attempt a climb to test for capture (much as a fixed-wing aircraft landing on an aircraft carrier increases power immediately at touchdown). Thus, shortly after passing the cable axis  14 , or upon detecting an indication of contact with or capture of the cable  12 , power to the rotor  2  can be increased. If capture has not occurred, then the helicopter  1  will climb away from the retrieval area and can return for another approach. If the helicopter  1  fails to climb, then this can be taken as confirmation that capture has occurred, and power can be reduced. The helicopter  1  will then descend, and be left hanging upside-down from the cable  12 . Swinging motion, including rotations about the approach axis  42  caused by rotor gyroscopic effect, can be damped by appropriate management of rotor thrust and in-plane moments during deceleration. 
       FIG. 2  shows a detailed view of an installation of carabiner-type hooks  10  in one embodiment of the present disclosure. A cable  12  slides along a surface  8  onto a one-way gate  22 , which then opens about a hinge  23 . The cable  12  is captured when the gate  22  doses behind it. Meanwhile, the cable  12  remains free to slide along its axis through the hook  10 . In one embodiment, the hook  10  includes a sloped deflector surface  25 . If the cable  12  misses the capture aperture and strikes the deflector surface  25 , then it will be directed to slide clear of the aircraft with low applied force. 
       FIG. 3  shows an alternative embodiment of a hook of the present disclosure, which includes a slot  24  to clamp the cable  12  in the manner of a jam cleat. This prevents the captured cable  12  from sliding along its axis relative to the hook  9 . It should be appreciated that other suitable forms of the hook or hook installation may be employed in accordance with the present disclosure. 
       FIGS. 4A ,  4 B,  4 C, and  4 D show another embodiment applied to an aircraft  28  having a configuration suited to efficient wing-borne flight. The aircraft  28  has a fixed wing  29  and a propeller  2  installed at its nose. The propeller&#39;s spin axis  4  is aligned with the fuselage. The retrieval pole of the interceptor as in  FIGS. 1A to 1D  is unnecessary, since the aft fuselage  8  provides a suitable surface for intercepting the cable  12 , and hooks  10  can be mounted on the rear of the fuselage  8  as shown in  FIG. 5 . It should be appreciated that in an alternative embodiment, an interceptor having a retrieval pole may be employed. To prepare for retrieval, the aircraft  28  pitches up from forward flight, with its thrust line near horizontal, into hovering flight, with its thrust line near vertical. Rotor thrust  20  is adjusted to balance aircraft weight. The thrust vector  20  is tilted along the approach path  42 , and the aircraft  28  slowly draws the rear surface of its fuselage  8  across the retrieval cable  12 . The cable load indicated by arrow  17  causes the aircraft  28  to tilt further along the approach path  42  as indicated by arrow  19 . The cable  12  slides along the fuselage  8  (as shown in  FIG. 5 ) and through a gate  22  into a hook  9 . Retrieval is then completed in the same or similar manner as for the helicopter  1  in  FIG. 1 . The aircraft  28  finishes hanging nose-down on the cable  12 . Again, this would be impractical for a manned aircraft, but quite acceptable for an aircraft that is small and unmanned. 
       FIGS. 6A ,  6 B,  6 C, and  6 D show another embodiment in which a second cable  46  is attached to the cable supports  13  adjacent and essentially or substantially parallel to the snag cable  12 . The position of this second cable  46  is such that the aircraft is intercepted as it rotates around the snag cable  12 , and so comes to rest in a more nearly horizontal orientation than that shown in  FIG. 4 . The height of the snag cable  12  can therefore be reduced, and the final nose-down orientation avoided. It should be appreciated that more than one additional cables can be employed in alternative embodiments, and supported in any suitable manner. In other embodiments, a net, mattress, boom or similar support could perform the same function as the second cable  46 . Of these choices, a second cable  46  has the advantage that it can exchange roles with the snag cable  12  depending upon the approach direction. In any case, the aircraft support  46  must be positioned so that it remains clear of the propeller as the aircraft comes to rest. It must also comply as necessary to arrest the aircraft without damage. 
     In any of these example embodiments, should the cable  12  not be captured because of incorrect altitude, failure to capture can be recognized as the cable axis  14  is passed. The aircraft can then climb away from the retrieval area and return for another approach. 
       FIG. 7 ,  FIG. 8 , and  FIG. 9  illustrate possible paths for a fixed-wing aircraft to approach a retrieval cable  12  in a wind V W . In general, the rotor thrust vector T opposes the sum of the weight vector W and the drag vector D. Thus, to maintain nonzero airspeed V A , the thrust vector T must be tilted to balance drag D. Required thrust T and thrust-vector tilt θ are minimized by minimizing drag, which can be done by orienting the wing  29  at knife-edge to the wind V W  as shown in  FIG. 7  and  FIG. 8 . 
     For successful capture, the aircraft  28  must contact the cable  12  in an aperture between the wing  29  and the hook  9 . When the airspeed vector V A  is into-wind V W , the thrust-vector tilt θ makes the aperture on the downwind side of the aircraft h d  broader than the aperture on the upwind side h u . Hence, guidance for a horizontal approach can be less precise if the aircraft approaches the cable  12  while moving downwind rather than upwind. In a sufficiently strong wind, tilt of the thrust vector could be so large that the upwind aperture hu would vanish, and a horizontal approach would have to be made downwind in order to engage the cable  12 . 
     The approach, however, need not be horizontal.  FIG. 8  shows an alternative in which the aircraft  28  approaches while descending into-wind with knife-edge wing orientation. If the slope γ of the approach path is selected to be approximately equal to the thrust-vector tilt θ, then the aperture h u  for successful capture of the cable  12  is kept large. For a given wind speed V W  this form of upwind approach requires more thrust (but not necessarily more power) than a downwind approach since it calls for higher airspeed. 
     A further possibility, as shown in  FIG. 9 , is to approach with the wing  29  in a lifting rather than knife-edge orientation. In this case, the vector sum of thrust T and lift L balances drag D and weight W. Again, the aircraft  28  presents maximum capture aperture h u  to the cable  12  by approaching into-wind while descending on a slope γ which is approximately equal to the thrust-vector tilt θ. If the wind speed exceeds the stall airspeed in wing-borne flight, then descent can be vertical. 
     Of these approach methods, downwind drift in knife-edge orientation as in  FIG. 7  requires the least thrust in a light wind. Wing-borne upwind descent as in  FIG. 9  requires the least thrust in a strong wind. Hence, the best choice of approach path and aircraft orientation will depend at least in part on wind speed. 
     In an automatic approach, thrust-vector tilt θ and rotor power are adjusted to regulate the approach velocity vector V G . Upon encountering the cable  12 , progress is retarded, and the automatic-control logic calls for the thrust vector T to be tilted toward the approach path  42 . This causes the aircraft  28  to rotate around the cable  12  in the desired direction indicated by arrow  19  in  FIG. 4B , so that sliding of the cable  12  into the hook  10  is promoted. 
       FIGS. 10A ,  10 B, and  10 C show an embodiment of the present disclosure in which the retrieval fixture is a boom  48  cantilevered from a mast  13 . A large aperture for capturing the retrieval fixture is created by trailing an interceptor having a string  44  with a grappling hook  9 , or alternatively with a small trailing mass  43  as shown in  FIGS. 11A ,  11 B,  11 C, and  11 D. Contact may excite waves in the trailing string and so make sliding over the boom  48  intermittent. Steady sliding can be promoted by including a string tail  45  below the hook  9  or trailing mass  43 . Sliding of the string  44  along the boom  48  leads to capture by the grappling hook  9 , or, alternatively, if the string  44  contacts the boom  48  at sufficient speed V G , then the inertia of the trailing mass  43  will cause the string  44  to wrap around the boom  48 . The aircraft comes to rest hanging by the string  44 . The longer the string  44 , the larger the aperture for capture, and so the more relaxed are requirements for accuracy in approach. However, this advantage is balanced by the need to elevate the cable boom  48  to allow sufficient room for the aircraft to hang on the string. In one embodiment, the necessary clearance could be reduced by retracting the string  44  after capture. This embodiment would require a suitable retraction mechanism. 
       FIGS. 11A ,  11 B,  11 C, and  11 D show an alternative embodiment in which the aircraft  28  need not have a hook. Instead, the retrieval fixture includes a boom  48  to which multiple cleats  24  are attached. The aircraft  28  trails an interceptor including a string  44  with a mass  43  and a tail  45 . The axis  14  of the boom  48  includes a component parallel to the aircraft&#39;s direction of approach  42 . Consequently, as the aircraft  28  draws the string  44  across the boom  48 , the string slides along the boom into a cleat  24 , which in one embodiment captures the string as discussed in connection with  FIG. 3 . Retrieval is completed as discussed in connection with  FIGS. 10A ,  10 B, and  10 C. 
       FIGS. 12A ,  12 B,  12 C, and  12 D show another embodiment in which one of a set of latches  56  on a cantilever boom  48  engages one or more detents  54  in the aircraft  28 . The aircraft  28  approaches on a path  42  controlled so that a detent  54  is directed into a latch  56  on the boom  48 . Retrieval is completed as discussed in connection with  FIGS. 10A ,  10 B, and  10 C. 
     In the embodiment of  FIGS. 12A ,  12 B,  12 C, and  12 D, the cantilever boom  48  is rotatable on a hinge  50  about a vertical axis  49  as shown by arrow  58 . An aerodynamic surface  52  orients the boom  48  passively relative to the wind. Similarly, the boom  48  is rotatable about a horizontal axis  14 , and is rigidly connected to an aerodynamic surface  53 . The weight of this surface  53 , and its attachment  57  to the boom  48 , are chosen so that the latches  56  are oriented appropriately for a horizontal approach in calm wind. The area of the surface  53  is chosen so that as the wind speed increases, the latches orient appropriately for a descending approach as shown in  FIG. 8  and  FIG. 9 . 
     In the embodiments illustrated above, the aircraft&#39;s thrust axis rotates substantially out of the vertical during the course of retrieval.  FIGS. 13A ,  13 B,  13 C, and  13 D show an alternative embodiment in which the thrust axis remains near vertical until the aircraft “parks.” The aircraft approaches and captures a retrieval cable  12  as in  FIGS. 4A and 4B . Then, upon detecting contact, it applies pitch and yaw torques, for example by appropriate adjustment of rotor cyclic pitch, so that rotation about the cable is arrested and near-vertical orientation is maintained. By further application of control torques, the aircraft slides along the cable such that it is guided by the cable into a docking fixture  5   a  near a cable support as shown in  FIGS. 13B and 13C . The docking fixture may include devices suitable for orienting and securing the aircraft in a desired position, which is provided so that secure docking can be detected, after which the aircraft&#39;s motor can be shut down. In one example, the docking fixture includes an arm, such as the arm illustrated in  FIGS. 13A ,  13 B,  13 C, and  13 D, configured to engage and secure the aircraft. The docking station may further include suitable devices for conveniently servicing the aircraft, stowing the aircraft, or launching the aircraft for another flight as shown in  FIG. 13D . 
     It should be understood that various changes and modifications to our illustrative embodiments will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.