Patent Publication Number: US-11046442-B2

Title: Weeping ferrofluid anti-ice system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application is a divisional of U.S. patent application Ser. No. 14/688,614, which was filed on Apr. 16, 2015 and issued on Dec. 4, 2018 as U.S. Pat. No. 10,144,522, and is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The present invention relates to anti-ice systems, and more specifically, to anti-ice systems that recirculate a ferrofluid. 
     Ice buildup on aerodynamic surfaces of aircraft can be problematic. For example, ice can build up on the leading edges of wings and/or engine nacelles. The ice can add weight to the aircraft and affect the center of gravity of the aircraft. The ice can also disrupt the intended airflow over the aerodynamic surfaces, causing a loss of lift generated by the aerodynamic surface. A combination of design considerations of modern airfoils and modern certification requirements result in less ice tolerance, meaning that modern aircraft need to have more anti-ice capability than some conventional anti-icing technologies can provide. However, existing anti-ice technologies are complicated and/or expensive. 
     Generally, aircraft with on-board anti-ice or de-ice capability use one of three systems: bleed air systems, Tecalemit-Kilfrost-Sheepbridge (TKS) systems, and pneumatic/mechanical boots. Bleed air systems extract hot air from a compressor section of a gas turbine engine and direct the hot air to the leading edges of the wing and the engine inlet. Such bleed air systems require extensive ductwork and valves to direct the hot air and significant shielding to protect aircraft components in the event of a leak. TKS systems use a glycol-based fluid that is wept onto the leading edge of an airfoil, an engine nacelle, and/or a spinner for a propeller or fan. The glycol-based fluid mixes with water droplets, lowering the freezing point of the water droplets so that the water droplets cannot freeze. The mixture of glycol-based fluid and water droplets then flow off the aircraft together. The glycol-based fluid used by TKS systems can be very expensive. For example, one particular brand is currently available for over one hundred United States dollars for five gallons. Pneumatic/mechanical de-icing boots generally include a flexible and resilient material (e.g., rubber) that covers the leading edge of an airfoil and/or an engine nacelle. When a certain amount of ice accumulates, a pneumatic bladder behind the resilient material can be inflated and/or a mechanical actuator can be actuated, causing the resilient material to deform. The deformation causes any accumulated ice to break off and be shed into the airstream. The de-icing boots can be very effective for ice that has built up. However, as stated above, modern airfoil designs and/or certification requirements may only allow for less ice formation than a boot system can effectively remove. 
     SUMMARY 
     According to one aspect, a method for preventing ice from forming on a surface includes flowing a ferrofluid from a reservoir onto a first region of the surface, wherein ferrofluid on the first region flows to a second region. The method also includes generating a first magnetic field, wherein the first magnetic field is oriented toward an aperture in a second region of the surface, and wherein the magnetic field attracts the ferrofluid on the second region of the surface into the aperture. The method also includes flowing the ferrofluid from the aperture to the reservoir. 
     According to one aspect, an ice prevention system includes a reservoir containing a ferrofluid. The system also includes at least one orifice in fluid communication with the reservoir and arranged on a first region of an aerodynamic surface through which the ferrofluid can be flowed onto the first region of the aerodynamic surface, wherein ferrofluid on the first region flows to a second region. The system also includes at least one aperture arranged on the second region of the aerodynamic surface, wherein the aperture is in fluid communication with the reservoir. The system also includes a first magnetic field source arranged relative to the at least one aperture, wherein the magnetic field is oriented to attract ferrofluid that is proximate to the aperture on the second region of the aerodynamic surface into the aperture. The system also includes a pump in fluid communication with the reservoir, wherein the pump is configured to flow the ferrofluid from the at least one aperture to the reservoir and from the reservoir to the at least one orifice. 
     According to one aspect, an aircraft includes a reservoir containing a ferrofluid. The aircraft also includes a wing, wherein a leading edge surface of the wing includes a first plurality of orifices through which the ferrofluid can be flowed. The wing includes a first at least one aperture arranged on a downstream surface of the wing. The ferrofluid on the leading edge surface flows to the downstream region. The aircraft also includes at least one first magnetic field source arranged in the wing, wherein the first at least one magnetic field source is oriented to attract ferrofluid that is proximate to the first at least one aperture on the wing into the first at least one aperture. The aircraft also includes a pump in fluid communication with the reservoir. The pump is configured to flow the ferrofluid from the at least one aperture to the reservoir and from the reservoir to the plurality of orifices. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a cross-sectional schematic side view of a forward portion of an airfoil of an aircraft with an anti-ice system according to various aspects; 
         FIG. 2A  is a schematic cross-sectional side view of a magnetic field generator arranged under an aerodynamic surface such that magnetic field lines generated by the magnetic field generator are parallel to the aerodynamic surface; 
         FIG. 2B  is a schematic cross-sectional side view of a magnetic field generator arranged under an aerodynamic surface such that magnetic field lines generated by the magnetic field generator are perpendicular to the aerodynamic surface; 
         FIG. 2C  is a cross-sectional side view of the magnetic field generator of  FIG. 2B  with a ferrofluid on the aerodynamic surface that is reacting to the magnetic field generated by the magnetic field generator; 
         FIG. 2D  is a cross-sectional side view of a magnetic field generator arranged relative to an aperture in an aerodynamic surface, wherein the magnetic field generator is oriented such that magnetic field lines of a magnetic field lines are positioned in the aperture; 
         FIG. 3A  is a top view of an airfoil for an aircraft with an anti-ice system according to various aspects; 
         FIG. 3B  is a top view of an airfoil for an aircraft with an anti-ice system according to various aspects; 
         FIG. 4  is a front perspective view of a gas turbine engine and an engine nacelle with an anti-ice system according to various aspects; 
         FIG. 5  is a front perspective view of a propeller and propeller spinner with an anti-ice system according to various aspects; 
         FIG. 6  is a top schematic view of an aircraft with an anti-ice system according to various aspects; and 
         FIG. 7  is a flow chart for a method for using an anti-ice system according to various aspects. 
     
    
    
     DETAILED DESCRIPTION 
     In the following, reference is made to particular aspects presented in this disclosure. However, the scope of the present disclosure is not limited to specific described aspects. Instead, any combination of the following features and elements, whether related to different aspects or not, is contemplated to implement and practice contemplated aspects. Furthermore, although aspects disclosed herein may achieve advantages over other possible solutions or over the prior art, whether or not a particular advantage is achieved by a given aspect is not limiting of the scope of the present disclosure. Thus, the following aspects, features, and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s). Likewise, reference to “the invention” shall not be construed as a generalization of any inventive subject matter disclosed herein and shall not be considered to be an element or limitation of the appended claims except where explicitly recited in a claim(s). 
     In aspects described herein, a ferrofluid is wept onto a first region (e.g., an upstream region) of a surface (e.g., a leading edge of an airfoil, a leading edge of an engine nacelle inlet, or a forward portion of a propeller spinner). The ferrofluid is a fluid that becomes magnetized in the presence of a magnetic field. In various aspects, the ferrofluid is a colloidal liquid that includes ferromagnetic particles suspended in a fluid. In various instances, the ferromagnetic particles could be nanoscale in size (i.e., between one and one hundred nanometers). In various other instances, the ferromagnetic particles could be micrometer scale in size (e.g., one to one hundred micrometers). In the various aspects, the ferrofluid has a sufficiently low freezing point such that it does not freeze when it is dispersed over the surface. As the ferrofluid travels aft along the surface (due to moving air pushing the fluid and/or magnetic forces, discussed in greater detail below), the ferrofluid can mix with the water droplets in the air, preventing the water from freezing and/or carrying any freezing water droplets away from the leading edges. In various aspects, the ferrofluid can be hydrophobic. In such aspects, the hydrophobic ferrofluid can act as barrier, preventing water droplets from reaching the surface. Rather, the water droplets would float on top of the hydrophobic fluid. The ferrofluid (and any water droplets carried with the ferrofluid) eventually reaches one or more apertures in a downstream region of the surface. Magnetic field generators (i.e., magnetic field sources) arranged relative to the apertures generate magnetic fields that attract the ferrofluid into the apertures while the water droplets continue to be carried downstream. The ferrofluid can be recovered from the apertures and recirculated to the leading edge. 
       FIG. 1  is a cross-sectional schematic side view of the forward portion of a wing  100  of an aircraft. The wing  100  includes an aerodynamic surface  102  that generates lift for the aircraft. The wing  100  includes an upstream region  104  and a downstream region  106  aft of the upstream region  104 . A leading edge panel  110  of the upstream region  104  can include an array of orifices  112  therethrough. In various aspects, the leading edge panel  110  could include eight hundred orifices  112  per square inch, wherein each orifice  112  has a diameter of approximately 0.0025 inches. In various other aspects, the sizes of the orifices and/or the number of orifices per given area could differ. The orifices  112  are in communication with a plenum  150  within the wing  100 . The plenum  150  is in fluid communication with a reservoir  144  that contains a ferrofluid  146 . The reservoir  144  is in communication with a pump  136  that can pump the ferrofluid  146  into the reservoir  144 . In various aspects, the pump  136  could be a mechanical pump that mechanically moves the ferrofluid  146 . In various other aspects, the pump  136  could be could be a ferrofluid pump that has no moving parts and uses electromagnetic fields to pump the ferrofluid  146  to the reservoir  144 . The pumping by the pump  136  can also push the ferrofluid  146  from the reservoir  144  to the plenum  150 . The ferrofluid  146  in the plenum  150  is then pumped out through the orifices  112  in the leading edge panel  110  of the wing  100  in the directions of arrows A and B. The direction in which the ferrofluid travels upon exiting a particular orifice  112  could depend on the orientation of the orifice  112  relative to a stagnation point on the wing  100  (i.e., the location on the wing  100  where impinging air splits to go above or below the wing  100 ). The ferrofluid  146  continues to travel aft along the wing  100  in the direction of arrows C and D. The ferrofluid  146  could be urged along the wing  100  in the directions of arrows C and D by air flowing over and under the wing  100 . The ferrofluid  146  could also be urged along the wing  100  by magnetic field generators  152  and  156  arranged behind the aerodynamic surface  102  of the wing  100 . The magnetic field generators  152  and  156  could be one or more permanent magnets and/or electromagnets. Magnetic fields generated by the magnetic field generators  152  and  156  could exert magnetic forces on the ferrofluid  146  moving along the aerodynamic surface  102  to urge the ferrofluid  146  in an aft direction and/or in a lateral direction. 
     The wing  100  includes a first aperture  120  along the top of a downstream region of the aerodynamic surface  102 . The wing  100  also includes a second aperture  122  along the bottom of a downstream region of the aerodynamic surface  102 . The first aperture  120  could include a baffle  124  that partially covers the first aperture  120 . The baffle  124  could be continuous with the aerodynamic surface  102  of the wing  100  to provide a smooth transition for air traveling over the aerodynamic surface  102  and the baffle  124 . The baffle  124  includes openings  126  therethrough. The second aperture  122  could also include a baffle  128  that partially covers the second aperture  122 . The baffle  128  could be continuous with the aerodynamic surface  102  of the wing  100  to provide a smooth transition for air traveling over the aerodynamic surface  102  and the baffle  128 . The baffle  128  includes openings  130  therethrough. The wing  100  includes magnetic field generators  132  and  134  arranged relative to the apertures  120  and  122 , respectively. The magnetic field generators  132  and  134  could be permanent magnets or could be electromagnets. The magnetic field generators  132  and  134  can be oriented so that generated magnetic fields extend through the apertures  120  and  122 , respectively. As indicated by arrows E and F, ferrofluid passing over the baffles  124  and  128  will be attracted by the generated magnetic fields and drawn into the apertures  120  and  122  through the openings  126  and  130  in the baffles  124  and  128 , respectively. Water droplets may continue past the apertures  120  and  122  in the directions of arrows G and H. Ferrofluid that collects in the first aperture  120  can be drawn through a fluid conduit  138  to the pump  136 . Ferrofluid  146  that collects in the second aperture  122  can be drawn through a fluid conduit  140  to the pump  136 . Ferrofluid can then be pumped through fluid conduits  141  and  142  to the reservoir  144 . As discussed above, the ferrofluid can be pumped from the reservoir  144  to the plenum  150  through a fluid conduit  148 . When the ferrofluid  146  is not flowing, shutters could cover the openings  126  and  130  in the baffles  124  and  128 , respectively. 
     In various aspects, a filter  162  can be arranged between the pump  136  and the reservoir  144 . For example, the filter  162  could be arranged between fluid conduits  141  and  142 . The filter  162  could remove any contaminants, such as water, from the ferrofluid  146 . The filter could be connected to a drain line that can dump accumulated water and other contaminants overboard. In various aspects, the wing is provisioned with one or more heating elements operable to heat the ferrofluid  146 . Heating the ferrofluid may assist in preventing water droplets from freezing on the aerodynamic surface  102  of the wing. In addition, thermally controlling the ferrofluid may allow the viscosity of the ferrofluid to be adjusted to a desired level. In one aspect, a heater  160  is arranged between the reservoir  144  and the plenum  150  so that the heater  160  can heat the ferrofluid  146  flowing to the plenum  150 . In addition or in the alternative, the wing  100  includes induction coils  154  and  158  that can heat the ferrofluid  146  traveling along the aerodynamic surface  102  of the wing  100  in the directions of arrows D and C. 
     As discussed above, magnetic field generators  152  and  156  could be arranged relative to the aerodynamic surface  102  of the wing  100  to urge the ferrofluid  146  in a particular direction. Referring now to  FIG. 2A , a magnetic field generator  204  (e.g., a permanent magnet or an electromagnet) is arranged behind an aerodynamic surface  202  and is oriented with its poles  206  and  208  substantially parallel to the aerodynamic surface  202 . The magnetic field generator  204  generates a magnetic field  210  with magnetic field lines  212  that are substantially parallel to the aerodynamic surface  202 . As a result, ferrofluid traveling along the aerodynamic surface  202  is urged by the magnetic field  210  in the direction arrow U.  FIG. 2B  illustrates a magnetic field generator  224  (e.g., a permanent magnet or an electromagnet) that is arranged behind an aerodynamic surface  222  and that is oriented with its poles  226  and  228  substantially perpendicular to the aerodynamic surface  222 . The magnetic field generator  224  generates a magnetic field  230  with magnetic field lines  232  that are substantially perpendicular to the aerodynamic surface  222 . As a result, ferrofluid traveling along the aerodynamic surface  202  proximate to the magnetic field generator  224  can be urged away from the aerodynamic surface in the direction of arrow V.  FIG. 2C  illustrates ferrofluid  240  being urged away from the aerodynamic surface  222 . Peaks  242  can form in the ferrofluid  240  as the ferrofluid  240  is urged by the magnetic field lines  232 . Such peaks  242  in the ferrofluid  240  could disrupt any ice formation on a surface of the ferrofluid  240 , causing the ice to break apart. 
       FIG. 2D  illustrates a detail view of an aperture  260  in an aerodynamic surface  252 . Similar to the apertures  120  and  122  shown in  FIG. 1 , the aperture  260  includes a baffle  262  with openings  264  therethrough. The baffle  262  can provide a surface that is nearly continuous with the aerodynamic surface  252  so that air flowing over the aerodynamic surface  252  (in the direction of arrow U) is disturbed as little as possible. A magnetic field generator  266  (e.g., a permanent magnet and/or an electromagnet) is arranged under the aerodynamic surface and relative to the aperture  260  such that at least a portion of the magnetic field  268  generated by the magnetic field generator  266  is oriented toward the aperture  260 .  FIG. 2D  show magnetic field lines  270  of the magnetic field  268  passing into the aperture  260 . In various aspects, as shown in  FIG. 2D , some of the magnetic field lines  270  (e.g., magnetic field line  274 ) extend past the aerodynamic surface  252 . As shown in  FIG. 2D , a dense magnetic field region  272  of the magnetic field lines  270  (i.e., where the magnetic field  268  is the strongest) may be positioned in the aperture  260 . 
       FIG. 2D  also illustrates a ferrofluid  254  on the aerodynamic surface  252 . The ferrofluid  254  can be urged toward the aperture  260  by the air flowing over the aerodynamic surface  252  and/or by magnetic fields generated by other magnetic field generators (e.g., magnetic field generators  152  and  156  shown in  FIG. 1 ). The ferrofluid  254  can carry water droplets  256  that would otherwise impinge on the aerodynamic surface  252  (e.g., super cooled water droplets in the atmosphere that could freeze upon contact with the aerodynamic surface  252 ). As discussed above, in various aspects, the water droplets  256  may be intermixed with the ferrofluid  254 . In various other aspects, the ferrofluid  254  can comprise a hydrophobic fluid, which repels the water droplets. In such aspects, the water droplets may sit on top of the ferrofluid  254  (rather than intermix with the ferrofluid  254 ). The ferrofluid  254  and water droplets  256  move toward the aperture  260  in the direction of arrow U. When the ferrofluid  254  and the water droplets  256  reach the baffle  262  over the aperture  260 , the ferrofluid  254  is urged toward the aperture  260  by the magnetic field  268 . The ferrofluid  254  can pass through the openings  264  in the baffle  262  to enter the aperture  260  (as indicated by droplets  258  entering the aperture  260 ). The water droplets  256 , which are not attracted by the magnetic field  268 , can pass along the baffle  262  and continue downstream of the aperture  260 . 
     A certain amount of water  256  may enter the aperture  260 . As discussed above, various aspects of an anti-ice system could include a filter (e.g., filter  162  shown in  FIG. 1 ) to remove water and/or other contaminants that could enter the aperture  260 . As shown in  FIG. 2D , a certain amount of ferrofluid  254  may not be pulled into the apertures  260  by the magnetic field  268 . Instead, such ferrofluid  254  may continue downstream along the aerodynamic surface  252  and be lost. In various circumstances, the ferrofluid  254  can be treated as a consumable, wherein a certain amount of loss of ferrofluid  254  could be acceptable. When such circumstances occur, ferrofluid  254  can be added to a reservoir (e.g., the reservoir  144  shown in  FIG. 1 ) to replace the consumed ferrofluid  254 . 
       FIGS. 3A and 3B  illustrate two configurations for apertures on wings according to various aspects.  FIG. 3A  illustrates a portion of an aircraft  300  that includes a fuselage  302  and a wing  304  extending from the fuselage  302 . A leading edge  306  of the wing  304  includes orifices, such as orifices  112  shown in  FIG. 1 , through which a ferrofluid can weep out. The wing  304  also includes an aperture  308  running along almost the entire span of the wing  304 . During flight, ferrofluid weeping from the leading edge  306  of the wing  304  is urged rearward toward the aperture  308  by air passing by the wing  304 . The region  310  denoted by dashed lines indicates an area of the wing  304  covered by ferrofluid as the ferrofluid travels from the leading edge  306  to the aperture  308 . 
       FIG. 3B  illustrates a portion of an aircraft  320  that includes a fuselage  322  and a wing  324  extending from the fuselage  322 . A leading edge  326  of the wing  324  includes orifices, such as orifices  112  shown in  FIG. 1 , through which a ferrofluid can weep out. The wing  324  includes a plurality of apertures  328  spaced apart along span of the wing  324 . The wing  324  also includes a plurality of first magnetic field generators  330  and a plurality of second magnetic field generators  332  arranged inside of the wing  324  (i.e., beneath an aerodynamic surface of the wing  324 ). The plurality of first magnetic field generators  330  are spaced between the apertures  328  and are oriented to repel the ferrofluid. The plurality of second magnetic field generators  332  are substantially aligned with the apertures  328  and are oriented to attract the ferrofluid. During flight, ferrofluid weeping from the leading edge  326  of the wing  325  is urged rearward by air passing over the wing  324 . The ferrofluid is also urged laterally by the magnetic fields of the magnetic field generators  330  and  332 . Specifically, the ferrofluid is urged toward the plurality of second magnetic field generators  332  and away from the plurality of first magnetic field generators  330 . The regions  340  denoted by dashed lines indicate areas of the wing  324  covered by ferrofluid. As the ferrofluid travels from the leading edge  326 , the ferrofluid is laterally steered toward the apertures  328  by the attractive magnetic fields of the plurality of second magnetic field generators  332  and the repulsive magnetic fields of the plurality of first magnetic field generators  330 . 
       FIG. 4  illustrates an anti-ice system according to various aspects on a gas turbine engine  400 . The gas turbine engine includes a nacelle  402  mounted on a pylon  404 . The pylon  404  could connect the nacelle  402  to a wing or fuselage of an aircraft, for example. The nacelle  402  includes a leading edge  406 . The leading edge  406  includes a plurality of orifices  410 , such as orifices  112  shown in  FIG. 1 , through which a ferrofluid can weep out. The ferrofluid weeping out of the orifices  410  can travel in the direction of arrow I toward an inward-facing downstream surface  408  of the nacelle  402  or in the direction of arrow J toward an outward-facing downstream surface  411  of the nacelle  402 . The inward-facing downstream surface  408  of the nacelle  402  includes an aperture  412 , similar to aperture  120  or aperture  122  shown in  FIG. 1 . The aperture  412  could be arranged as a continuous aperture (similar to the aperture  308  shown in  FIG. 3A ) or as a series of spaced-apart apertures (similar to the apertures  328  shown in  FIG. 3B ). Ferrofluid traveling toward the aperture  412  can be drawn into the aperture  412  in the direction of arrow K and water carried by the ferrofluid can continue into the engine in the direction of arrow M. The outward-facing downstream surface  411  of the nacelle  402  includes an aperture  414 , similar to aperture  120  or aperture  122  shown in  FIG. 1 . The aperture  414  could be arranged as a continuous aperture (similar to the aperture  308  shown in  FIG. 3A ) or as a series of spaced-apart apertures (similar to the apertures  328  shown in  FIG. 3B ). Ferrofluid traveling toward the aperture  414  can be drawn into the aperture  414  in the direction of arrow L and water carried by the ferrofluid can continue aft in the direction of arrow N. 
     A spinner  420  for the gas turbine engine  400  can also include an anti-ice system. An array of orifices  426 , such as orifices  112  shown in  FIG. 1 , can be arranged on a first region  422  (e.g., an upstream region) of the spinner  420 . An aperture  428 , similar to aperture  120  or aperture  122  shown in  FIG. 1 , can be arranged on a second region  424  (e.g., a downstream region) of the spinner  420 . The aperture  428  could be arranged as a continuous aperture (similar to the aperture  308  shown in  FIG. 3A ) or as a series of spaced-apart apertures (similar to the apertures  328  shown in  FIG. 3B ). Ferrofluid traveling from the orifices  426  (in the direction of arrow O) toward the aperture  428  can be drawn into the aperture  428  in the direction of arrow P and water carried by the ferrofluid can continue into the engine in the direction of arrow Q. 
       FIG. 5  illustrates an anti-ice system according to various aspects on an aircraft propeller  500 . The propeller includes four propeller blades  504  extending from a spinner  502 . An anti-ice system can be arranged for the spinner  502 . An array of orifices  510 , such as orifices  112  shown in  FIG. 1 , can be arranged on a first region  506  (e.g., an upstream region) of the spinner  502 . An aperture  512 , similar to aperture  120  or aperture  122  shown in  FIG. 1 , can be arranged on a second region  508  (e.g., a downstream region) of the spinner  502 . The aperture  512  could be arranged as a continuous aperture (similar to the aperture  308  shown in  FIG. 3A ) or as a series of spaced-apart apertures (similar to the apertures  328  shown in  FIG. 3B ). Ferrofluid traveling from the orifices  510  (in the direction of arrow R) toward the aperture  512  can be drawn into the aperture  512  in the direction of arrow S and water carried by the ferrofluid can continue toward the blades  504  the direction of arrow T. 
     In various aspects, the ferrofluid used for anti-ice could also be used by other aircraft systems. For example, the ferrofluid could be used by a hydraulic system onboard the aircraft and/or by a cooling system onboard the aircraft. For example, a hydraulic system may be used to power aircraft control surfaces (e.g., ailerons, elevator, rudder, spoilers, and flaps) or landing gear actuation. The ferrofluid could be used as hydraulic fluid. As another example, cooling systems are used to extract heat from avionics and/or to provide refrigeration to galley refrigerators. The ferrofluid could be used to carry heat away from such avionics or refrigerators.  FIG. 6  is a schematic view of an aircraft  600 , illustrating a portion of the fuselage  602  and the wings  604  of the aircraft  600 . The aircraft includes a reservoir  606  of ferrofluid. The ferrofluid can be provided to an anti-ice system  610  in the wings  604  of the aircraft  600  and to an anti-ice system  612  in engines  614  of the aircraft  600 . The anti-ice system  610  in the wings  604  could draw ferrofluid from the reservoir  606 , pass the ferrofluid through orifices in a leading edge of the wing, recover at least some of the ferrofluid through apertures in a downstream region of the wing, and return the ferrofluid to the reservoir  606 . Similarly, the anti-ice system  612  in the engines  615  could draw ferrofluid from the reservoir  606 , pass the ferrofluid through orifices in a leading edge of the engine nacelle (and a spinner), recover at least some of the ferrofluid through apertures in a downstream region of the nacelle (and the spinner), and return the ferrofluid to the reservoir  606 . The reservoir  606  can also be in fluid communication with other systems  608  onboard the aircraft, such as a hydraulic system or cooling system, described above. The other systems  608  could circulate ferrofluid to and from the reservoir  606  for use. 
     In various aspects, the aircraft  600  could include an icing detector  620 . The icing detector  620  could be a sensor that detects the buildup of ice thereon and/or that detects atmospheric conditions (e.g., temperature and humidity levels) in which icing conditions are possible. The anti-ice systems described herein according to various aspects have a minimal impact on aircraft performance (e.g., they do not detract from engine power). Thus, in various aspects, computer systems onboard the aircraft  600  could monitor the icing detector  620  and automatically activate the anti-icing systems  610  and  612  if icing and/or icing conditions are detected. 
       FIG. 7  is a flow chart that illustrates a method  700  for operating an anti-ice system. In block  702 , ferrofluid is pumped from a reservoir. Optionally, in block  704 , the ferrofluid can be heated. In block  706 , the ferrofluid is pumped onto a first region (e.g., an upstream region) of an aerodynamic surface. For example, the ferrofluid could be pumped through orifices onto a leading edge surface of a wing and/or an engine nacelle. As another example, the ferrofluid could be pumped onto an upstream surface of a spinner for a gas turbine engine or a propeller. Optionally, in block  708 , a magnetic field can be generated relative to the aerodynamic surface to direct flow of the ferrofluid along the surface or relative to the surface. For example, magnetic field generators (e.g., permanent magnets and/or electromagnets) could be arranged beneath the aerodynamic surface to provide the magnetic field. The magnetic field generator could be oriented such that its magnetic field is oriented parallel or perpendicular to the aerodynamic surface to direct the ferrofluid along the aerodynamic surface, away from the aerodynamic surface, or toward the aerodynamic surface. In block  710 , a magnetic field is generated in an aperture in a second region (e.g., a downstream region) of the aerodynamic surface. The magnetic field is oriented to attract the ferrofluid into the aperture. In block  712 , the ferrofluid is pumped from the reservoir. In block  714 , the ferrofluid is optionally filtered to remove impurities, such as water, dust, or the like. In block  716 , the ferrofluid is returned to the reservoir. When the system is turned off, shutters or the like could cover the apertures. 
     Various aspects described herein could be arranged on other surfaces, such as antenna or instruments. For example, certain aircraft include various antennas that extend away from the fuselage. A ferrofluid could be flowed through one or more orifices at a base of an antenna and collected through one or more apertures at or near the tip of the antenna. Alternatively, a ferrofluid could be flowed through one or more orifices at a tip of an antenna and collected through one or more apertures at or near the base of the antenna. 
     Other aspects may also be used to prevent ice accumulation on cold, condensing surfaces where atmospheric air cause ice formation, such as on terrestrial auxiliary heat exchangers used in nitrogen recovery systems and cryogenic fuel tanks such as those used on launch vehicles. 
     In various instances, the ferrofluid systems described above could be activated in non-icing conditions. For example, aircraft sometimes impact flying insects when flying at low altitudes (e.g., during the takeoff, climb, descent, and landing phases of a flight). Cumulative insect impacts could result in a buildup of insect debris on the leading edge surface of the wings, engines, etc. In addition to being unsightly, a significant accumulation of insect debris could have an impact on aerodynamic performance. Thus, aircraft are periodically cleaned to remove such debris. In certain circumstances, a coating of ferrofluid on the aerodynamic surfaces could prevent insects from reaching the underlying aerodynamic surface and/or could prevent any insect debris from sticking to the aerodynamic surface. As an example, the pilots of an aircraft departing from an airport in a region with a lot of mosquitos could activate a ferrofluid system before takeoff to reduce the amount of insect debris that would accumulate on the wings, engines, etc. due to impacts with mosquitos. Other possible uses for aspects of a ferrofluid would be dusty conditions, smoky conditions, or the like. 
     In the above-described aspects, anti-ice systems operate with a small engine performance penalty (e.g., compared to an engine bleed anti-ice system). The above-described anti-ice systems are also capable of operating continuously (e.g., compared to the inflatable boot system). Furthermore, the above-described systems operate with the small engine performance penalty and continuous operation without consuming significant quantities of sometimes-expensive anti-ice fluid. 
     The descriptions of the various aspects have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the aspects disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described aspects. The terminology used herein was chosen to best explain the principles of the aspects, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the aspects disclosed herein. 
     While the foregoing is directed to aspects of the present invention, other and further aspects of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.