Abstract:
In order to increase efficiency in aerial wind turbine vehicles, it is desirable to reduce the system drag associated with airflow across radiator in those cases where airflow exceeds the necessary flow to sufficiently cool the motor/generators. Exemplary embodiments herein include passive flow restrictors that can activate under varying flight conditions associated with known states of excessing cooling capacity.

Description:
BACKGROUND 
       [0001]    Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. 
         [0002]    Power generation systems may convert chemical and/or mechanical energy (e.g., kinetic energy) to electrical energy for various applications, such as utility systems. As one example, a wind energy system may convert kinetic wind energy to electrical energy. 
       SUMMARY 
       [0003]    Radiator ducts with passively controlled variable airflow rates for airborne wind turbines are described herein. More specifically, example embodiments generally relate to radiator ducts that include a moveable flow restrictor. Beneficially, embodiments described herein may provide a passive mechanism for providing variable amounts for airflow to a radiator within the radiator duct, depending on operating conditions of the airborne wind turbine, and thereby reducing system drag on the airborne wind turbine. 
         [0004]    In one aspect, an example aerial vehicle cooling system may comprise an aerial vehicle, a radiator duct, a radiator, and a moveable flow restrictor. The radiator duct may comprise an airflow inlet, and internal cavity, and an airflow outlet. The radiator may be located within the internal cavity of the radiator duct and be subject to an airflow through the radiator duct. The moveable flow restrictor may be configured to: (i) when the flow restrictor is subject to a g-force less than a threshold value in a triggering direction, move to an open position and allow a first rate of airflow through the duct; and (ii) when the flow restrictor is subject to a g-force greater than the threshold value in the triggering direction, move to a closed position and allow a second rate of airflow through the duct. The first amount of airflow may accordingly be different than the second amount of airflow. 
         [0005]    In another aspect, a moveable flow restrictor may be configured to: (i) when the flow restrictor is subject to an airflow velocity at the airflow inlet less than a threshold value, move to an open position and allow a first rate of airflow through the duct; and (ii) when the flow restrictor is subject to an airflow velocity at the airflow inlet is greater than a threshold value, move to a closed position and allow a second rate of airflow through the duct. The first amount of airflow may accordingly be different than the second amount of airflow. 
         [0006]    In a further aspect, an example aerial vehicle cooling system may comprise an aerial vehicle, a pylon supporting a rotor assembly, a radiator duct, a radiator, and a moveable flow restrictor. The pylon may be an airfoil with a high pressure side and an opposing low-pressure side. The radiator duct may comprise an airflow inlet, and internal cavity, and an airflow outlet and be located at least partially internal to the high pressure side of the pylon and inline with a wake created by the rotor assembly. The radiator may be located within the internal cavity of the radiator duct and be subject to an airflow through the radiator duct. The moveable flow restrictor may comprise a flexible member. A first portion of the flexible member may be fixed to aerial vehicle. A second portion of the flexible member may be configured to: (i) when the flow restrictor is subject to a g-force less than a threshold value in a triggering direction, move to an open position and allow a first rate of airflow through the duct; and (ii) when the flow restrictor is subject to a g-force greater than the threshold value in the triggering direction, move to a closed position and allow a second rate of airflow through the duct. The first amount of airflow may accordingly be different than the second amount of airflow. 
         [0007]    These as well as other aspects, advantages, and alternatives, will become apparent to those of ordinary skill in the art by reading the following detailed description, with reference where appropriate to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0008]      FIG. 1  depicts an Airborne Wind Turbine (AWT), according to an example embodiment. 
           [0009]      FIG. 2  depicts an example of an aerial vehicle transiting an illustrative flight path. 
           [0010]      FIG. 3  depicts an example of an aerial vehicle transitioning from hover flight to crosswind flight. 
           [0011]      FIG. 4  depicts a cross-section of an aerial vehicle. 
           [0012]      FIG. 5  depicts a cross-section of an aerial vehicle cooling system. 
           [0013]      FIG. 6  depicts a cross-section of an aerial vehicle cooling system. 
           [0014]      FIG. 7  depicts a cross-section of an aerial vehicle cooling system. 
           [0015]      FIG. 8  depicts a cross-section of an aerial vehicle cooling system. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Exemplary methods and systems are described herein. It should be understood that the word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment or feature described herein as “exemplary” or “illustrative” is not necessarily to be construed as preferred or advantageous over other embodiments or features. More generally, the embodiments described herein are not meant to be limiting. It will be readily understood that certain aspects of the disclosed methods systems and can be arranged and combined in a wide variety of different configurations, all of which are contemplated herein. Further, unless otherwise indicated, Figures are not drawn to scale and are used for illustrative purposes only. 
       I. OVERVIEW 
       [0017]    Airborne wind turbines may include onboard motors, generators, and/or motor/generator hybrids on their associated aerial vehicles. A motor may be used to provide thrust to an aerial vehicle, and a generator may be used to generate electricity via drag imposed on the aerial vehicle. During operation, motor/generators generally generate more waste heat than is practical or efficient for continuous operation and they must reject the excess heat through cooling apparatuses, such as remote radiators. In general, the greater the cooling capacity provided to a motor/generator, the greater the torque capacity that may be utilized for that motor/generator. 
         [0018]    An aerial vehicle may see a range of air velocities during operation. In general, air velocity across the aerial vehicle will be a determining factor in the total cooling capacity of a given cooling system on an aerial vehicle. However, the required cooling capacity of the vehicle may change depending on its operational mode. For example, an aerial vehicle in a hover mode, where the rotors are providing thrust, may require more cooling capacity than an aerial vehicle in a cross wind flight mode where the rotors are creating drag and generating electricity. Additionally, under some conditions, the aerial vehicle may experience a mean velocity of airflow greater than is required to actually cool the motor/generator. Because cooling requires drag, and drag reduces system performance, it may be desirable to limit the airflow seen by the cooling system under that condition. 
         [0019]    Some operational modes, such as hover and crosswind flight and their related cooling requirements, may be correlated with certain flight conditions. For example, an aerial vehicle in hover mode may experience relatively low air velocity and/or a relatively low g-force loading, while also requiring a relatively high cooling capacity due to work performed by the motors to maintain hover. In that case, it would be desirable to have maximum airflow across the radiators. As another example, an aerial vehicle in crosswind flight mode may experience relatively high air velocity and/or a relatively high g-force loading in one or more directions, but the mean air velocity may be greater than is required to actually cool the generators. Therefore, it would be desirable to limit the airflow across the radiators in order to reduce inefficient drag. Preferably, a passive mechanism may react to these flight conditions and alternately restrict or allow airflow across the radiators. Beneficially, by reducing the airflow across the radiator when it is not required, drag may be reduced and system performance increased. Additionally, a passive mechanism may be less costly and more robust than an active system that may require sensors, servos, and other complex parts. 
       II. ILLUSTRATIVE SYSTEMS 
     A. Airborne Wind Turbine (AWT) 
       [0020]      FIG. 1  depicts an AWT  100 , according to an example embodiment. In particular, the AWT  100  includes a ground station  110 , a tether  120 , and an aerial vehicle  130 . An aerial vehicle may include or take the form of various types of devices, such as a kite, a helicopter, a wing and/or an airplane, among other possibilities. An aerial vehicle may be formed of solid structures of metal, plastic and/or other polymers. An aerial vehicle may be formed of any material which allows for a high thrust-to-weight ratio and generation of electrical energy which may be used in utility applications. Additionally, the materials may be chosen to allow for a lightning hardened, redundant and/or fault tolerant design which may be capable of handling large and/or sudden shifts in wind speed and wind direction. Other materials may be possible as well. 
         [0021]    As shown in  FIG. 1 , the aerial vehicle  130  may be connected to the tether  120  via a bridle portion  122  of the tether  120 , and the tether  120  may be connected to the ground station  110 . In this example, the tether  120  may be attached to the ground station  110  at one location on the ground station  110 , and attached to the aerial vehicle  130  via the bridle at three locations on the aerial vehicle  130 . However, in other examples, the tether  120  may be attached via the bridle at one or more locations to any part of the ground station  110  and/or the aerial vehicle  130 . 
         [0022]    The ground station  110  may be used to hold and/or support the aerial vehicle  130  until it is in an operational mode. The ground station  110  may also be configured to allow for the repositioning of the aerial vehicle  130  such that deploying of the aerial vehicle  130  is possible. Further, the ground station  110  may be further configured to receive the aerial vehicle  130  during a landing. The ground station  110  may be formed of any material that can suitably keep the aerial vehicle  130  attached and/or anchored to the ground while in hover flight, forward flight, crosswind flight. In some implementations, a ground station  110  may be configured for use on land. However, a ground station  110  may also be implemented on a body of water, such as a lake, river, sea, or ocean. For example, a ground station could include or be arranged on a floating off-shore platform or a boat, among other possibilities. Further, a ground station  110  may be configured to remain stationary or to move relative to the ground or the surface of a body of water. 
         [0023]    The ground station  110  may additionally include one or more components, such as winch componentry  112   a ,  112   b ,  112   c  that may be used to vary the deployed length of the tether  120 . For example, when the aerial vehicle  130  is deployed, the one or more components may be configured to pay out and/or reel out the tether  120 . In some implementations, the one or more components may be configured to pay out and/or reel out the tether  120  to a predetermined length. As examples, the predetermined length could be equal to or less than a maximum length of the tether  120 . Further, when the aerial vehicle  130  lands in the ground station  110 , one or more components  114   a ,  114   b  may be configured to receive the aerial vehicle  130 . 
         [0024]    The tether  120  may transmit electrical energy generated by the aerial vehicle  130  to the ground station  110 . In addition, the tether  120  may transmit electricity to the aerial vehicle  130  in order to power the aerial vehicle  130  for takeoff, landing, hover flight, and/or forward flight. The tether  120  may be constructed in any form and using any material which may allow for the transmission, delivery, and/or harnessing of electrical energy generated by the aerial vehicle  130  and/or transmission of electricity to the aerial vehicle  130 . The tether  120  may also be configured to withstand one or more forces of the aerial vehicle  130  when the aerial vehicle  130  is in an operational mode. For example, the tether  120  may include a core configured to withstand one or more forces of the aerial vehicle  130  when the aerial vehicle  130  is in hover flight, forward flight, and/or crosswind flight. The core may be constructed of any high strength fibers. In some examples, the tether  120  may have a fixed length and/or a variable length. For instance, in at least one such example, the tether  120  may have a length of 140 meters. 
         [0025]    Referring briefly to  FIG. 2 , the aerial vehicle  130  may be configured to fly substantially along a path  150  to generate electrical energy. The term “substantially along,” as used in this disclosure, refers to exactly along and/or one or more deviations from exactly along that do not significantly impact generation of electrical energy as described herein and/or transitioning an aerial vehicle between certain flight modes as described herein. The path  150  may be various different shapes in various different embodiments. For example, the path  150  may be substantially circular. And in at least one such example, the path  150  may have a radius of up to 265 meters. The term “substantially circular,” as used in this disclosure, refers to exactly circular and/or one or more deviations from exactly circular that do not significantly impact generation of electrical energy as described herein. Other shapes for the path  150  may be an oval, such as an ellipse, the shape of a jelly bean, the shape of the number of 8, etc. 
         [0026]    Referring again to  FIG. 1 , the aerial vehicle  130  may include a main wing  131 , pylons  132   a ,  132   b , rotors  134   a ,  134   b , a tail boom  135 , and a tail wing assembly  136 . Any of these components may be shaped in any form which allows for the use of components of lift to resist gravity and/or move the aerial vehicle  130  forward. 
         [0027]    The main wing  131  may provide a primary lift force for the aerial vehicle  130 . The main wing  131  may be one or more rigid or flexible airfoils, and may include various control surfaces, such as winglets, flaps, rudders, elevators, etc. The control surfaces may be used to stabilize the aerial vehicle  130  and/or reduce drag on the aerial vehicle  130  during hover flight, forward flight, and/or crosswind flight. 
         [0028]    The main wing  131  and pylons  132   a ,  132   b  may be any suitable material for the aerial vehicle  130  to engage in hover flight, forward flight, and/or crosswind flight. For example, the main wing  131  and pylons  132   a ,  132   b  may include carbon fiber and/or e-glass, and include internal supporting spars or other structures. Moreover, the main wing  131  and pylons  132   a ,  132   b  may have a variety of dimensions. For example, the main wing  131  may have one or more dimensions that correspond with a conventional wind turbine blade. As another example, the main wing  131  may have a span of 8 meters, an area of 4 meters squared, and an aspect ratio of 15. 
         [0029]    The pylons  132   a ,  132   b  may connect the rotors  134   a ,  134   b  to the main wing  131 . In some examples, the pylons  132   a ,  132   b  may take the form of, or be similar in form to, a lifting body airfoil (e.g., a wing). In some examples, a vertical spacing between corresponding rotors (e.g., rotor  134   a  and rotor  134   b  on pylon  132   a ) may be 0.9 meters. 
         [0030]    The rotors  134   a ,  134   b  may be configured to drive one or more generators for the purpose of generating electrical energy. In this example, the rotors  134   a ,  134   b  may each include one or more blades, such as three blades or four blades. The rotor blades may rotate via interactions with the wind and be used to drive the one or more generators. In addition, the rotors  134   a ,  134   b  may also be configured to provide thrust to the aerial vehicle  130  during flight. With this arrangement, the rotors  134   a ,  134   b  may function as one or more propulsion units, such as a propeller. Although the rotors  134   a ,  134   b  are depicted as four rotors in this example, in other examples the aerial vehicle  130  may include any number of rotors, such as less than four rotors or more than four rotors. 
         [0031]    The tail boom  135  may connect the main wing  131  to the tail wing assembly  136 , which may include a tail wing and a vertical stabilizer. The tail boom  135  may have a variety of dimensions. For example, the tail boom  135  may have a length of 2 meters. Moreover, in some implementations, the tail boom  135  could take the form of a body and/or fuselage of the aerial vehicle  130 . In such implementations, the tail boom  135  may carry a payload. 
         [0032]    The tail wing and/or vertical stabilizer may be used to stabilize the aerial vehicle and/or reduce drag on the aerial vehicle  130  during hover flight, forward flight, and/or crosswind flight. For example, the tail wing and/or vertical stabilizer  136  may be used to maintain a pitch of the aerial vehicle  130  during hover flight, forward flight, and/or crosswind flight. The tail wing assembly  135  may have a variety of dimensions. For example, the tail wing assembly  135  may have a length of 2 meters. Moreover, in some examples, the tail wing assembly  135  may have a surface area of 0.45 meters squared. Further, in some examples, the tail wing assembly  135  may be located 1 meter above a center of mass of the aerial vehicle  130 . 
         [0033]    While the aerial vehicle  130  has been described above, it should be understood that the methods and systems described herein could involve any suitable aerial vehicle that is connected to an airborne wind turbine tether, such as the tether  120 . 
         [0000]    B. Transitioning an Aerial Vehicle from Hover Flight to Crosswind Flight 
         [0034]      FIG. 3  depicts an example  300  of transitioning an aerial vehicle from hover flight to crosswind flight, such as crosswind flight substantially along path  150 , according to an example embodiment. Example  300  is generally described by way of example as being carried out by the aerial vehicle  130  described above in connection with  FIG. 1 . For illustrative purposes, example  300  is described in a series of actions as shown in  FIG. 3 , though example  300  could be carried out in any number of actions and/or combination of actions. 
         [0035]    As shown in  FIG. 3 , the aerial vehicle  130  may be connected to the tether  120 , and the tether  120  may be connected to the ground station  110 . The ground station  110  may be located on ground  302 . The tether  120  may define a tether sphere  304  having a radius based on a length of the tether  120 , such as a length of the tether  120  when it is extended. Example  300  may be carried out in and/or substantially on a portion  304 A of the tether sphere  304 . The term “substantially on,” as used in this disclosure, refers to exactly on and/or one or more deviations from exactly on that do not significantly impact transitioning an aerial vehicle between certain flight modes as described herein. 
         [0036]    Example  300  begins at a point  306  with deploying the aerial vehicle  130  from the ground station  110  in a hover-flight orientation, and one or more rotors may be operating in a thrust mode. With this arrangement, the tether  120  may be paid out and/or reeled out. In some implementations, the aerial vehicle  130  may be deployed when wind speeds increase above a threshold speed (e.g., 3.5 m/s) at a threshold altitude (e.g., over 200 meters above the ground  302 ). 
         [0037]    Further, at point  306  the aerial vehicle  130  may be operated in the hover-flight orientation. When the aerial vehicle  130  is in the hover-flight orientation, the aerial vehicle  130  may engage in hover flight. For instance, when the aerial vehicle  130  engages in hover flight, the aerial vehicle  130  may ascend, descend, and/or hover over the ground  302 . When the aerial vehicle  130  is in the hover-flight orientation, a span of the main wing  131  of the aerial vehicle  130  may be oriented substantially perpendicular to the ground  302 . The term “substantially perpendicular,” as used in this disclosure, refers to exactly perpendicular and/or one or more deviations from exactly perpendicular that do not significantly impact transitioning an aerial vehicle between certain flight modes as described herein. 
         [0038]    Example  300  continues at a point  308  while the aerial vehicle  130  is in the hover-flight orientation positioning the aerial vehicle  130  at a first location  310  that is substantially on the tether sphere  304 . As shown in  FIG. 3   a , the first location  310  may be in the air and substantially downwind of the ground station  110 . 
         [0039]    The term “substantially downwind,” as used in this disclosure, refers to exactly downwind and/or one or more deviations from exactly downwind that do not significantly impact transitioning an aerial vehicle between certain flight modes as described herein. 
         [0040]    For example, the first location  310  may be at a first angle from an axis extending from the ground station  110  that is substantially parallel to the ground  302 . In some implementations, the first angle may be 30 degrees from the axis. In some situations, the first angle may be referred to as azimuth. 
         [0041]    As another example, the first location  310  may be at a second angle from the axis. In some implementations, the second angle may be 10 degrees from the axis. In some situations, the second angle may be referred to as elevation, and the second angle may be between 10 degrees in a direction above the axis and 10 degrees in a direction below the axis. The term “substantially parallel,” as used in this disclosure refers to exactly parallel and/or one or more deviations from exactly parallel that do not significantly impact transitioning an aerial vehicle between certain flight modes described herein. 
         [0042]    At point  308 , the aerial vehicle  130  may accelerate in the hover-flight orientation. For example, at point  308 , the aerial vehicle  130  may accelerate up to a few meters per second. In addition, at point  308 , the tether  120  may take various different forms in various different embodiments. With this arrangement, the tether  120  may be in a catenary configuration. Moreover, a bottom of the tether  120  may be a predetermined altitude  312  above the ground  302 . With this arrangement, at point  306  and point  308  the tether  120  may not contact the ground  302 . 
         [0043]    Example  300  continues with transitioning the aerial vehicle  130  from the forward-flight orientation to a crosswind-flight orientation. In some examples, transitioning the aerial vehicle  130  from the forward-flight orientation to the crosswind-flight orientation may involve a flight maneuver. 
         [0044]    When the aerial vehicle  130  is in the crosswind-flight orientation, the aerial vehicle  130  may engage in crosswind flight. For instance, when the aerial vehicle  130  engages in crosswind flight, the aerial vehicle  130  may fly substantially along a path, such as path  150 , to generate electrical energy. In some implementations, a natural roll and/or yaw of the aerial vehicle  130  may occur during crosswind flight. 
       III. ILLUSTRATIVE COOLING SYSTEM CONFIGURATIONS 
       [0045]    As used herein, the terms motor, generator, and motor/generator are not meant to be exclusive. For example, the use of the term “motor” does not preclude an airborne wind turbine motor from also functioning as a generator, and a motor/generator does not have to function as both a motor and a generator. 
         [0046]      FIG. 4  illustrates a cross-section of an aerial wind turbine aerial vehicle  400 , such as the aerial vehicle  130  described with respect to  FIG. 1 . Aerial vehicle  400  is shown in side view at pylon  402 , with a cross-section through main wing  404 . Main wing  404  may comprise multiple lift-generating airfoil sections, such as main airfoil  404   a  and trailing airfoil  404   b . Pylon  402  may also act as a lift generating airfoil, and may have a cross-sectional shape similar to that of main airfoil  404   a , though the generated lift may be oriented orthogonal (or at some other angle) to the generated lift of main wing  404 . As illustrated, the high-pressure surface of pylon  402  is the side shown. 
         [0047]    Pylon  402  may support multiple rotor assemblies. For example, pylon  402  is shown with upper rotor assembly  406  and lower rotor assembly  408 , though more or fewer rotor assemblies are contemplated. Rotor assemblies  406 ,  408  may be capable of producing thrust, such as when the aerial vehicle is taking off or landing, and/or drag, such as when the aerial vehicle is flying at a large forward velocity in crosswind flight. Employing rotor assembly  406  as a representative example of other rotor assemblies, rotor assembly  406  may include nacelle  410  and motor/generator  416 , which may be connected to a set of rotor blades  414 . Motor/generator  416  may be coupled to radiator  420  which may be located in radiator duct  422 , either or both of which may be external to pylon  402 , but are preferably partially or fully enclosed within pylon  402 . Radiator  420  may be coupled to motor/generator  416  via coolant lines  418 , such as flexible hoses, semi-rigid tubes, or rigid pipes. (Additionally shown are radiator  424  and radiator duct  426 , which may be similarly or identically connected to a motor/generator in rotor assembly  408 .) The disclosed radiator locations beneficially may allow the use of a very simple radiator which may be flat in planform. If radiators (and any accompanying radiator ducts) were instead placed on a rotor assemble nacelle, they must have either complex inlet geometry or complex radiator geometry in order to account for the shape of the nacelle. 
         [0048]      FIG. 5  illustrates a cross-section view A-A of  FIG. 4  and shows an exemplary radiator  420  and radiator duct  500 . Radiator  420  may reside in radiator duct  500 . Duct  500  may include an internal duct surface  518  of pylon  402 , an external duct cover  422 , a duct side panel  506 , and another duct side panel (not illustrated in cross-section view A-A), all of which may serve to form an internal cavity of duct  500 . Further illustrated are an airflow inlet  502  and an airflow outlet  504  of the duct  700 . Duct  500  may be located on the high pressure surface  508  of pylon  402 , so that the opposing low pressure lifting surface  510  (i.e., suction surface) of pylon  402  remains undisturbed, and also so inlet  502  is in lower speed but higher stagnation pressure air. This may reduce the drag created by radiator  420  and duct  500 . Radiator  420  may be set within duct  500  at an angle to the mean airflow direction. Because radiator  420  may have a high pressure loss, it need not be aligned with the flow to get roughly uniform inlet velocity. This beneficially allows a wider radiator to be fit in a smaller duct cross-section size. 
         [0049]      FIG. 5  also illustrates moveable flow restrictor  520  in an open position P 0  and a closed position P 1 . Flow restrictor  520  may be located at the airflow inlet  502  and may include elements  512  and  514 . Element  512  may be a rigid member that rotates about or along element  514  to move between open position P 0  and a closed position P 1 . Element  514  may be, for example, a flexure or radial bearing. Alternatively, element  512  may be a flexible portion and element  514  may be a fixed portion, where the fixed portion is attached to the aerial vehicle via, for example, pylon  402  or duct surface  518 , and where the flexible portion may bend to allow element  512  to move between open position P 0  and a closed position P 1 . Optionally, element  512  may include a weighted portion  512   a.    
         [0050]    As an operational example, during hover flight flow restrictor  520  may be subject to little or no g-forces in a direction normal to, for example, high pressure surface  508 . As a result, flow restrictor  520  may naturally rest in open position P 0  as a result of tension from element  514 , gravity and the orientation of aerial vehicle, and/or airflow. However, when the aerial vehicle transitions to crosswind flight, the g-forces may climb significantly and cross a predetermined threshold value in a particular triggering direction. For example, the g-forces may increase significantly in a direction normal to the high-pressure surface  508 . (The triggering direction may include a limited range of directions, for example, the triggering direction could be ±30° from a direction exactly normal to high-pressure surface  508 .) This shift in g-forces may cause element  512  to move against a hardstop  516  and thus flow restrictor  520  would move from an open position P 1  to a closed position P 1 , reducing the relative amount of airflow into the airflow inlet  502 . Optionally, by adding or subtracting weight to weighted portion  514   a , the responsiveness of flow restrictor  520  may be tuned, thus changing the threshold g-force value and/or the triggering direction. 
         [0051]    G-forces are not the only example of a flight condition that can be harnessed to cause flow restrictor  520  to move. As another example, airflow velocity can be used as a passive control mechanism. As airflow velocity increases, static pressure may build in areas of the duct cavity. When static pressure on the internal side of element  512  exceeds static pressure on the external side of element  512 , element  512  will move from open position P 0  to closed position P 1 . Accordingly, the shape of element  512  and internal surface  518  may be configured to tune the threshold value of airflow velocity that will cause flow restrictor  520  to move from open to closed and back to open. 
         [0052]      FIG. 6  illustrates another moveable flow restrictor. Whereas flow restrictor  520  was located at the airflow inlet  502 , flow restrictor  620  is located at the airflow outlet  504 . Flow restrictor  620  functions similarly to flow restrictor  520 , except that it causes a change in airflow across radiator  420  by restricting airflow through outlet  504  instead of through inlet  502 . Flow restrictor  620  includes elements  612  and  614 , which function similarly to elements  512  and  514 , as well as weighted portion  612   a  which may be used to tune flow restrictor  620  in the same manner as flow restrictor  520 , and hardstop  616  which may function similarly to hardstop  516 . 
         [0053]      FIG. 7  illustrates another exemplary cross-section view A-A of  FIG. 4  and shows an exemplary radiator  420  and radiator duct  700 . Similar to  FIG. 5 , radiator  420  may reside in radiator duct  700 . Duct  700  may include an internal duct surface  718  of pylon  402 , an external duct cover  422 , a duct side panel  706 , and another duct side panel (not illustrated in cross-section view A-A), all of which may serve to form an internal cavity of duct  700 . Further illustrated are an airflow inlet  702  and an airflow outlet  704  of duct  700 . Duct  700  may be located on the high pressure surface  708  of pylon  402 . 
         [0054]      FIG. 7  also illustrates moveable flow restrictor  720  in an open position P 1 . Flow restrictor  720  includes a blocking slat  712 , guides  714 , spring  716 , and receiving cavity  718   a . In a closed position, blocking slat  712  may be pulled into the receiving cavity  718   a  by spring  716 . In open position P 1 , blocking slat  712  may extend out from receiving cavity  718   a  and may be guided to and held in position by guides  714 . Guides  714  may be, for example, mounted radial or plain bearings. Similarly to flow restrictors  520  and  620 , flight conditions such as g-force loading in a triggering direction parallel or approximately parallel to the direction of travel of blocking slat  712  can be harnessed to actuate flow restrictor  720 . Further, the weight of blocking slat  712  and the tension of spring  716  can be used to tune the triggering threshold for activation of flow restrictor  720 . 
         [0055]      FIG. 8  illustrates another exemplary cross-section view A-A of  FIG. 4  and shows an exemplary radiator  420  and radiator duct  800 . Radiator  420  may reside in radiator duct  800 . Duct  800  may include an internal duct surface  818  of pylon  402 , an external duct cover  812 , a duct side panel  806 , and another duct side panel (not illustrated in cross-section view A-A), all of which may serve to form an internal cavity of duct  800 . Further illustrated are an airflow inlet  802  and an airflow outlet  804  of duct  800 . Duct  800  may be located on the low pressure surface  810  of pylon  402 . 
         [0056]    As illustrated in  FIG. 8 , external duct cover  812  may serve as part of moveable flow restrictor  820 . External duct cover  812  may be a flexible member and a portion may be fixed to the aerial vehicle via pylon  402  or duct side panels (including panel  806 ) or other components of the aerial vehicle. Another portion of external duct cover  812  may be allowed to move, such that it can transition between an open position P 0  and a closed position P 1 . 
         [0057]    During, for example hover flight, flow restrictor  820  may be subject to little or no g-forces in a direction normal to high pressure surface  808 . As a result, flow restrictor  820  may naturally rest in open position P 0  as a result of the shape and stiffness of external duct cover  812 , gravity and the orientation of aerial vehicle, and/or airflow. However, when the aerial vehicle transitions to crosswind flight, the g-forces may climb significantly and cross a predetermined threshold value in a particular triggering direction. For example, the g-forces may increase significantly in a direction normal to the high-pressure surface  808 . This shift in g-forces may cause the element  812  to move against a hardstop  816  and thus flow restrictor  820  would be in closed position P 1 , reducing the relative amount of airflow into the airflow inlet  502 . The weighted portion  814   a  may be used to tune the responsiveness of flow restrictor  820  by adding or subtracting weight, thus changing the threshold g-force value and/or the triggering direction. Further the triggering direction may include a limited range of directions, for example, the triggering direction could be ±30° from a direction exactly normal to high pressure surface  808 . 
       IV. CONCLUSION 
       [0058]    The particular arrangements shown in the Figures should not be viewed as limiting. For example, relative sizes of components, dimensions, and specifically illustrated locations are intended to be exemplary only and are not intended to be limiting. It should be understood that other embodiments may include more or less of each element shown in a given Figure. Further, some of the illustrated elements may be combined or omitted. Yet further, an exemplary embodiment may include elements that are not illustrated in the Figures. 
         [0059]    Additionally, while various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are contemplated herein.