Patent Publication Number: US-2016229544-A1

Title: Ice accretion prevention

Description:
FIELD OF THE INVENTION 
     The present invention relates to a device and method for preventing the accretion of large sheets of ice on airstream-facing exterior surfaces of an aircraft, particularly the ice accretion caused by super-cooled large droplets (SLD) of water. 
     BACKGROUND OF THE INVENTION 
     Ice formations on aircraft external surfaces are of great interest in the aerospace industry. Ice accretion can distort the aerodynamic surface profile, modifying the aircraft performance and handling characteristics. De-icing systems are commonly found on aircraft forward-facing edges, such as fixed wing leading edges, rotary wings, fixed horizontal and vertical tailplane leading edges, nose cones, etc. These typically comprise a flexible element and/or a heat source to dislodge the ice accretion. 
     Aircraft certification, e.g. as defined by FAR/CS 25 Appendix C, has previously only accounted for an icing envelope characterised by water droplets with mean diameters of up to 50 microns, sometimes referred to as “cloud droplets”. Recent aircraft accidents, in particular the loss of American Eagle Flight 4184, have highlighted the dangers of super-cooled large droplets (SLD) which may be defined as having a droplet spectrum where a significant part of the distribution, typically about 30%, has a diameter greater than 50 microns. SLD are considered to include “freezing drizzle” (a significant part of the distribution having droplet diameters of 100-500 microns) and “freezing rain” (a significant part of the distribution having droplet diameters greater than 500 microns, up to around 2500 microns). Whilst rare, SLD icing tends to create ice accretion over a wider area of the aircraft&#39;s surface, often beyond that commonly protected by de-icing systems. In the American Eagle Flight 4184 accident a ridge of ice behind the de-icing boot caused a region of separated flow resulting in an extreme uncontrolled aileron deflection. 
     In view of the issues discussed above, forthcoming aircraft certification changes will extend the icing envelope to account for SLD icing conditions. It is thought that certification will be based largely on simulated, i.e. predicted, ice formation as the rare SLD conditions are currently believed to be too difficult to incorporate into a test-based certification program. 
     The new regulations will bring many challenges for airframe design to meet the new certification and operational criteria. For example, exterior surfaces of the aircraft not previously requiring anti-icing or de-icing systems for certification may need ice protection for SLD icing. Extending the effective area of existing anti-icing or de-icing systems to the wider area to accommodate SLD icing may not be technically feasible or cost-effective. Alternative solutions to the problems of SLD icing are therefore required. 
     SUMMARY OF THE INVENTION 
     The present inventors propose a device and method for controlling the size of a sheet of ice which can build up on an aircraft surface as a result of icing, particularly SLD icing. In some circumstances it is possible to tolerate a build up of some ice without significant aerodynamic penalties. However, the size of such ice sheets must be controlled to ensure that any ice sheet detaching from the aircraft surface has a mass that is within the acceptable margins for projectiles which may impact aircraft structure downstream. 
     Thus, a first aspect of the invention provides an aircraft having an exterior surface arranged to face upstream in the airflow direction during flight and a plurality of anti-ice accretion projections extending away from the exterior surface, each anti-ice accretion projection having a leading edge facing upstream in the airflow direction and a trailing edge facing downstream of the airflow direction, wherein the trailing edge provides an aerodynamic step extending substantially perpendicular to the airflow over the exterior surface, the step being arranged to: create a shadow region immediately downstream of the step where water droplets carried in the airflow cannot impinge on the exterior surface; and/or create a region of separated flow over the exterior surface immediately downstream of the step. 
     By creating a shadow region in which water droplets are prevented from impinging on the exterior surface, ice cannot accumulate in that shadow region. Thus, the shadow region can define the maximum extent of an ice sheet and/or create a break between neighbouring ice sheets. 
     The term impinge is used in the sense of its normal technical usage, to mean the initial contact between a water droplet carried by an airflow and the exterior surface over which that air flow is travelling. That is, impingement is direct contact between airborne water droplet and exterior surface, not indirect contact following a ricochet of the water droplet off other structure, or water runoff, for example. Thus, the shadow region is a region in which such contact is prevented because this region is shielded from the air flow by the step. 
     By creating a region of separated flow over the exterior surface, water run-back caused by e.g. SLD icing in warm atmospheric conditions can be dispersed into the air flow. That is, the separated flow causes any water running over the exterior surface in the downstream direction towards the step of the projection to be lifted away from the exterior surface and thereby dispersed into the fast-flowing air travelling over the exterior surface. 
     The aircraft includes a plurality of anti-ice accretion projections. Thus, any accreted ice may be divided by the plurality of projections into a plurality of acceptably small ice sheets. 
     It may be desirable for the step to be arranged to create both the shadow region and the region of separated flow. In this way, the step can provide a break in accreted ice caused by both direct impingement of water droplets and by water run-back in mild conditions. 
     In all of the described aspects of the invention each step extends substantially perpendicular to the airflow over the exterior surface, i.e. transverse to the local airflow. This arrangement provides an effective shadow region and/or region of separated flow, and also enables the extent of any accumulated ice to be limited to a particular position within the airflow. 
     Each projection may be integrated into a boundary between exterior panels of the aircraft. That is, of a pair of exterior panels having a boundary between them that runs transverse to the air flow, the trailing face of the forward-most panel may have a greater height than the leading face of the aft-most panel, the region of the trailing face of the forward-most panel which projects beyond the leading face of the aft-most panel providing the aerodynamic step. 
     The exterior surface is typically a surface of a fixed airframe structure, rather than a movable surface. 
     A second aspect of the present invention provides an aircraft having an exterior surface arranged to face upstream in the airflow direction during flight, the exterior surface having a super-cooled large droplet (SLD) impingement region within which super-cooled large droplets (SLD) of water can (or are predicted to) impinge on the exterior surface, and a plurality of anti-ice accretion projections extending away from the exterior surface from within the SLD impingement region, each anti-ice accretion projection having a leading edge facing upstream in the airflow direction and a trailing edge facing downstream in the airflow direction, wherein the trailing edge provides an aerodynamic step extending substantially perpendicular to the airflow direction and the step is arranged to create a shadow region immediately downstream of the step where water droplets cannot impinge on the exterior surface. 
     The term super-cooled large droplets (SLD) is used herein in the sense of its normal technical meaning, as is well known in the art. In prior art aircraft SLD can cause unacceptably large accumulations of ice, in part because the large droplets travel via relatively straight trajectories rather than following the streamlines of the air flow, thus creating a large impingement zone. The shadow region of the second aspect provides a region in which ice cannot accumulate because there are no impinging water droplets to freeze, and thus serves to define the maximum extent of an ice sheet and/or create a break between neighbouring ice sheets. 
     In certain conditions, particularly when the total temperature is in the region of 0 degrees centigrade, SLD can result in water run-back from the SLD impingement site. The run-back water travels downstream from the impingement site before it freezes, thus causing large ice sheets. The region of separated air flow of the third aspect causes such run-back water to be dispersed into the air flow. That is, the separated flow causes any water running over the exterior surface in the downstream direction towards the step of the projection to be lifted away from the exterior surface and thereby dispersed into the fast-flowing air travelling over the exterior surface. 
     Total temperature is the temperature that the air reaches at the stagnation point on an aircraft, i.e. the position on the exterior surface at which the air velocity is zero. Mathematically, the total temperature is the static temperature (also called the ambient temperature or outside air temperature) plus V 2 /2010, where V is the true air speed in metres per second. For example, at an air speed of 100 m/s the total temperature is approximately 5 degrees centigrade warmer than the static temperature. Total temperature is typically used to determine the conditions for ice formation because the icing threshold depends not only on the static temperature, but also on aircraft speed and the kinetic heading that is generated through that speed. Total temperature incorporates this kinetic heading effect. 
     The step of the second aspect is preferably arranged to also create a region of separated flow over the exterior surface immediately downstream of the step. 
     A third aspect of the invention provides an aircraft having an exterior surface arranged to face upstream in the airflow direction during flight, the exterior surface having a water run-back region within which impinged water droplets can flow over the exterior surface, and a plurality of anti-ice accretion projections extending away from the exterior surface from within the water run-back region, each anti-ice accretion projection having a leading edge facing upstream in the airflow direction and a trailing edge facing downstream in the airflow direction, wherein the trailing edge provides an aerodynamic step extending substantially perpendicular to the airflow direction and the step is arranged to create a region of separated air flow over the exterior surface immediately downstream of the step for dispersing water droplets flowing over the exterior surface. 
     The step of the third aspect is preferably arranged to also create a shadow region immediately downstream of the step within which water droplets (e.g. SLD) cannot impinge on the exterior surface. 
     The exterior surface preferably has a water droplet impingement region within which droplets of water carried in the airflow having a mean diameter of  50  microns or less are predicted to impinge on the exterior surface, and the anti-ice accretion projection is preferably outside of, i.e. not within, the water droplet impingement region. That is, the projection may be provided outside of sites where normal ice accumulation occurs, and where alternative de-icing protection is typically provided. 
     Each step preferably has a height of 3 mm or more above the exterior surface, and preferably 5 mm or more. The maximum height of the step will probably be determined by the calculated drag penalty, but a typical maximum threshold may be 100 mm or less, preferably 20 mm or less. 
     Preferably each projection has a pair of sides which extend from the trailing edge to the leading edge, and the pair of sides are spaced apart from each other at the trailing edge by a distance of 25 mm or more. This provides a sufficiently long trailing edge to have a significant effect. 
     A fourth aspect provides an aircraft having an exterior surface arranged to face upstream in the airflow direction during flight and a plurality of anti-ice accretion projections extending away from the exterior surface, each anti-ice accretion projection having a leading edge facing upstream in the airflow direction and a trailing edge facing downstream of the airflow direction, wherein the trailing edge provides an aerodynamic step extending substantially perpendicular to the airflow over the exterior surface, the step having a height of 3 mm or more above the exterior surface. 
     Thus, the step is significantly larger than a typical panel to panel step caused by manufacturing tolerances. The step height is preferably 5 mm or more. 
     The exterior surface is preferably not protected by an ice protection system arranged to dislodge ice accumulated within an ice protection zone of the exterior surface. 
     Thus, the present invention may be used in areas of the aircraft where typical de-icing measures are not necessary, but where protection against SLD icing is required. 
     Where the present invention is deployed on an exterior surface that does have an ice protection system, the projection is preferably located downstream of the ice protection zone. In this way, the projection can prevent excessive ice accumulation in downstream areas not protected by the ice protection system, especially during SLD icing events. 
     Preferably, an intersection between the trailing edge step of the anti-ice accretion projection and the exterior surface downstream of the projection forms an angle of 150 degrees or less, and preferably 135 degrees or less. The steepness of this angle may be critical for ensuring that the shadow region and/or flow separation region is created. 
     The anti-ice accretion projection is preferably not heated, e.g. by a heater mat or similar. Thus, the projection is a passive device which is simple to maintain and is unlikely to fail. 
     The anti-ice accretion projection may be not moveable with respect to the exterior surface. In some embodiments each anti-ice accretion projection is movable between an extended position in which the trailing edge provides the aerodynamic step and a retracted position in which the trailing edge is substantially flush with the exterior surface. The movement may be in response to a change in flight conditions. For example, the projection may be deployed during take-off, holding conditions, diversions and/or landing (i.e. flight below about 31,000 feet or 9,450 metres), when ice accumulation is possible, and retracted during high altitude cruise (i.e. flight above about 31,000 feet or 9,450 metres), when ice accumulation is unlikely and parasitic drag from the projection is particularly undesirable. 
     Each anti-ice accretion projection preferably has a ramp configuration. The anti-ice accretion projection may have an aerodynamic surface extending between the leading edge and the trailing edge, the distance between the aerodynamic surface and the exterior surface increasing continuously from the leading edge to the trailing edge. This shape may reduce parasitic drag caused by the projection. 
     The plurality of projections may be spaced apart from one another in the airflow direction, or in a direction substantially perpendicular to the airflow direction. 
     In some embodiments there may be one or more projections within the SLD impingement zone to create one or more shadow regions, and one or more other projections downstream of the SLD impingement zone to create one or more regions of separated flow. 
     Optionally the projections have a rectangular planform shape, each projection having a pair of sides which run parallel with each other as they extend from the trailing edge to the leading edge. Alternatively each projection has a pair of sides which become progressively closer to each other as they extend from the trailing edge to the leading edge. This tapered shape is preferred because it enables the sides to divert water droplets away from the shadow region or the region of separated flow. 
     Preferably there are channels between the projections, and the projections are arranged so that water droplets are driven by the airflow through the channels between the projections during flight of the aircraft. Preferably each projection is arranged to divert the water droplets into the channels between the projections. 
     Each projection may have any suitable planform shape or aspect ratio, preferable examples including a rectangular or triangular planform shape. 
     The exterior surface preferably comprises an exterior surface of a nose cone, fuselage, wing, vertical tail plane, or horizontal tail plane of the aircraft. In the case of a wing, vertical tail plane or horizontal tail plane or other aircraft structure with a leading edge and a trailing edge, then typically the projections are located closer to the leading edge than the trailing edge. 
     A fifth aspect of the invention provides a method of preventing ice accretion on an exterior surface of an aircraft facing upstream in the airflow direction during flight, the method including the steps of: providing a plurality of anti-ice accretion projection extending away from the exterior surface; and: creating a shadow region immediately downstream of each projection where water droplets carried in the airflow cannot impinge on the exterior surface; and/or creating a region of separated flow over the exterior surface immediately downstream of each projection where water droplets flowing over the exterior surface are dispersed into the airflow. 
     The projections may have any of the features of the anti-ice accretion projections discussed above in relation to the first, second, third or fourth aspects. 
     Optionally, in step (a) the anti-ice accretion projections may be provided within a super-cooled large droplet (SLD) impingement region within which super-cooled large droplets (SLD) of water can impinge on the exterior surface, and step (b) may be carried out. 
     Alternatively or in addition, in step (a) the anti-ice accretion projections may be provided within a water run-back region within which impinged water droplets (preferably SLD) can flow over the exterior surface, and step (c) may be carried out. 
     The method may include the step of retracting the anti-ice accretion projections during cruise conditions so that they are substantially flush with the exterior surface. 
     Step (a) may include providing an anti-ice accretion projection having a leading edge facing upstream in the airflow direction and a trailing edge facing downstream of the airflow direction, the trailing edge providing an aerodynamic step extending substantially perpendicular to the airflow over the exterior surface, and the step creating the shadow region and/or region of separated flow. 
     Any of the optional, or preferred, features described above may be applied to any of the aspects of the invention, either alone or in any combination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described with reference to the accompanying drawings, in which: 
         FIGS. 1 ( a ) and ( b )  illustrate the possible ice accretion on a prior art aircraft nose cone as a result of SLD icing,  FIG. 1( a )  showing the nose cone before ice accretion and  FIG. 1( b )  showing it after an SLD icing event; 
         FIG. 2  is a schematic drawing of an ice accretion prevention device according to a comparative example, including an expanded detail view; 
         FIG. 3  is a schematic drawing showing the ice accretion prevention device of  FIG. 2  after an SLD icing encounter, including an expanded detail view; 
         FIG. 4  is a cross-sectional viewing showing an alternative ice accretion prevention device; 
         FIG. 5 a    shows an aircraft according to a first embodiment of the invention; 
         FIG. 5 b    is a schematic drawing of the nose cone of the aircraft of  FIG. 5   a;    
         FIG. 6  is a side view of one of the projections; 
         FIG. 7  is a plan view of part of a wing of an aircraft according to a second embodiment of the invention; and 
         FIG. 8  is a plan view of part of a wing of an aircraft according to a third embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENT(S) 
       FIGS. 1 ( a ) and ( b )  illustrate the possible build up of ice  100  on an exterior surface  200  of the nose cone of a prior art aircraft as a result of an SLD icing event. In this embodiment the exterior surface  200  comprises a radome of the nose cone. 
     During normal flight conditions, when the mean diameter of water droplets carried in the air flow is less than 50 microns, the size of the accumulated ice sheet may be tolerable. That is, the parasitic drag (and lift loss in the case of an aerodynamic lifting surface such as a wing) associated with the ice may be within acceptable margins, and the ice sheet may have a sufficiently low mass that no critical damage will be sustained to downstream aircraft structure if it were to detach in one piece from the nose cone. However, in conditions where super-cooled large droplets (SLD) are present, the ice sheet may be much larger. The present embodiment is concerned with ensuring that such an ice sheet is not able to detach in one piece, since its mass would be outside of acceptable margins for projectiles which may impact the aircraft structure (wing leading edge, empennage etc) downstream of the nose cone. It is also concerned with limiting the size of any accumulated ice sheet during SLD impingement. 
       FIGS. 2 and 3  illustrate a comparative example, not according to the invention. A ramped projection  300  extends around a circumference of the nose cone exterior surface  200  so that it is generally ring-shaped. The projection  300  has a leading edge  310 , a trailing edge  320 , and an aerodynamic surface  330  extending therebetween. The projection  300  is arranged with respect to the exterior surface  200  so that the trailing edge  320  is substantially perpendicular to the direction of airflow over the exterior surface  200 . 
     The leading edge  310  is substantially flush with the exterior surface  200 , while the trailing edge  320  is offset from the exterior surface  200  so that there is an aerodynamic step  340  between the trailing edge and the exterior surface. The step  340  has a height of approximately 5 mm, although steps of between 3 mm and 100 mm, preferably 20 mm or less, may be acceptable. A typical acceptable tolerance for a step on an aircraft surface, e.g. between neighbouring skin panels, is 2 mm or less, usually substantially less than this in regions where parasitic drag must be carefully controlled. Thus, the step  340  represents a departure from the typical design of an aircraft fixed surface. 
     An SLD impingement region is defined as the region within which SLD are predicted to impinge (i.e. directly impact) on the exterior surface. The SLD impingement region is larger than a water droplet impingement region defined as the region within which water droplets having a mean diameter of less than 50 microns will impinge on the exterior surface. This is because small water droplets typically follow a trajectory that follows the streamlines, while larger water droplets are less influenced by the airflow and tend to have straighter trajectories. Ice may accumulate within or downstream of the SLD impingement region because of a phenomenon known as water run-back, which occurs when there is SLD impingement in relatively warm atmospheric conditions (typically where the total temperature is around 0 degrees centigrade). In such conditions water droplets run along the exterior surface before freezing further downstream. Thus, the ice sheet that accumulates in the absence of any countermeasures (as shown in  FIG. 1 ( b ) ) typically covers a larger expanse of the exterior surface  200  than the SLD impingement region. 
     In this embodiment the projection  300  is located within the SLD impingement region so that the step  340  has the effect of providing a “shadow region”  342  in the exterior surface immediately downstream of the projection  300 . That is, the step  340  provides a barrier preventing water droplets carried in the air flow from impinging on the exterior surface in the shadow region  342 . Since water droplets are prevented from impinging on the shadow region  342 , ice cannot accumulate in this region, and the shadow region  342  thus provides a break between sheets of ice upstream of the projection and downstream of the projection. 
     The step  340  also provides a localised flow separation region  344  immediately downstream of the step  340  which causes the air flow in this region to become separated so that run-back water flowing over the exterior surface  200  is drawn away from the exterior surface  200  and carried away by the air flow. Thus, in warm conditions (where the total temperature is around 0 degrees centigrade) the run-back water caused by SLD impingement is dispersed so that it cannot freeze on the exterior surface. 
     The projection  300  thus prevents the accumulation of large ice sheets as a result of both SLD impingement and water runback caused by SLD impingement in warm conditions. 
     The step  340  is at an angle of approximately 90 degrees to the exterior surface  200 . It is important that this angle is sufficiently steep (a maximum of about 150 degrees, preferably 135 degrees or less, is considered acceptable, with a minimum of about 10 degrees) to create the shadow region  342  or localised flow separation region  344 . 
       FIG. 4  shows an alternative ramped projection  400  formed by an elongate sheet of material which is bent longitudinally to form an attachment portion  402  and a ramp portion  404 . The projection  400  extends around the nose cone of an aircraft in a direction substantially perpendicular to the direction of air flow, in the same way as the arrangement of  FIGS. 2 and 3 . The attachment portion  402  is seated on the aircraft exterior surface  200  and attached thereto by fasteners  210 . The ramp portion  204  projects away from the exterior surface  200 , so that an aerodynamic surface  430  thereof extends from a leading edge  410  at the intersection with the attachment portion to a trailing edge  420  which extends substantially perpendicular to the direction of airflow (indicated by arrow  220 ) over the exterior surface  200 . The trailing edge  420  is thus suspended above the exterior surface  200  to provide an aerodynamic step  440 . 
     Although the step  440  is at an acute angle to the exterior surface  200 , unlike the closed step  320  of  FIGS. 2 and 3  which is approximately perpendicular to the surface  200 , its effect is the same. That is, it provides a shadow region within which SLD droplets cannot impinge on the exterior surface  200 , and it provides a localised flow separation region which causes run-back water to detach from the exterior surface  200  and be carried away by the air flow. The shadow region provided by the projection  400  can be considerably wider than that of the projection  500 , since it can include the region of the exterior surface which is sheltered directly beneath the ramp portion  204 , i.e. upstream of the trailing edge  420 . 
       FIG. 5 a    shows an aircraft according to an embodiment of the invention. The aircraft has a nose cone with an exterior surface  200  shown in detail in  FIG. 5 b   . As illustrated in  FIGS. 5 b    and  6 , a plurality of ramped projections  500  are spaced apart around a circumference of the nose cone exterior surface  200 . Each projection  500  has a leading edge  510 , a trailing edge  520 , and an aerodynamic surface  530  extending therebetween. Each projection  500  is arranged with respect to the exterior surface  200  so that the trailing edge  520  is substantially perpendicular to the direction of local airflow over the exterior surface  200 . 
     The leading edge  510  of each projection is substantially flush with the exterior surface  200 , while the trailing edge  520  is offset from the exterior surface  200  so that there is an aerodynamic step  540  between the trailing edge and the exterior surface. The step  540  has a height of approximately 5 mm, although steps of between 3 mm and 100 mm, preferably 20 mm or less, may be acceptable. 
     Each projection  500  is located within the SLD impingement region so that each projection  500  provides a similar effect to the projections  300 ,  400  described above. 
     That is, each step  540  has the effect of providing a “shadow region”  542  in the exterior surface immediately downstream of the projection  500 , providing a barrier preventing water droplets carried in the air flow from impinging on the exterior surface in the shadow region  542 . Since water droplets are prevented from impinging on the shadow regions  542 , ice cannot accumulate in these regions. 
     Each step  540  also provides a localised flow separation region  544  immediately downstream of the step  540  which causes the air flow in this region to become separated so that run-back water flowing over the exterior surface  200  is drawn away from the exterior surface  200  and carried away by the air flow. Thus, in warm conditions (where the total temperature is around 0 degrees centigrade) the run-back water caused by SLD impingement is dispersed so that it cannot freeze on the exterior surface. 
     Each projection  500  thus prevents the accumulation of large ice sheets as a result of both SLD impingement and water runback caused by SLD impingement in warm conditions. 
     The projections  500  are spaced apart to enable the airflow over the exterior surface to flow through channels  560  between them. Thus accreted ice may be divided by the plurality of projections into a plurality of acceptably small ice sheets, each ice sheet occupying a respective one of the channels  560 . 
     The step  540  is at an angle of approximately 90 degrees to the exterior surface  200 . It is important that this angle is sufficiently steep (a maximum of about 150 degrees, preferably 135 degrees or less, is considered acceptable, with a minimum of about 10 degrees) to create the shadow region  542  or localised flow separation region  544 . 
     Each projection  500  has a triangular planform shape as shown in  FIG. 5 b    with a pair of sides  550  which become progressively closer to each other as they extend from the trailing edge  520  (where the distance between them is at a maximum) to the leading edge  510  where they meet at a point. This tapered shape causes water droplets driven by the surface airflow to be diverted by the sides  550  of the projections as indicated by arrows  570  into the channels  560  between the projections  500  and away from the shadow region  542 . 
     The trailing edge  520  has a spanwise width of 25 mm or more. In other words, the pair of sides  550  are spaced apart from each other at the trailing edge  520  by a distance of 25 mm or more. This provides a sufficiently long trailing edge  520  to have a significant effect. Note that the number of projections  500  shown in  FIG. 5 b    is indicative only, and optionally a larger number of projections may be provided. 
     Optionally a wing of the aircraft of  FIG. 5 a    may also be provided with projections as shown in  FIG. 7 . The wing has a leading edge  610  and trailing edge  620 . A plurality of projections  500  are arranged on the upper exterior surface of the wing, spaced apart in a span-wise direction. Each projection  500  in  FIG. 7  is identical to the projections  500  shown in  FIGS. 5 b    and  6  so will not be described again. The local airflow over the upper exterior surface of the wing is approximately perpendicular to the leading edge  610 , so the trailing edges  520  of the projections are arranged substantially parallel with the leading edge  610  of the wing. 
     The projections  500  in  FIGS. 5 b    and  7  are spaced apart in a direction parallel to the leading edge  610  and substantially perpendicular to the airflow direction, but optionally they may be arranged in an offset manner as shown in  FIG. 8 , i.e. spaced apart from one another in the airflow direction as well as in a direction substantially perpendicular to the airflow direction. In both cases the projections  500  are arranged so that water droplets are driven by the local airflow through the channels  560  between the projections. 
     The nose cone of  FIG. 5 b    and the wing of  FIGS. 7 and 8  do not have any alternative ice protection systems, but in other embodiments in which the exterior surface does incorporate an ice protection system such as an electro-thermal heater mat or flexible element to dislodge any accumulated ice, the projections  500  will preferably be located downstream of the zone protected by the ice protection system. 
     The projections  500  in the embodiments of  FIGS. 5-8  are all in the form of thickened skin regions (like the projection  300  of  FIG. 2 ) but optionally some or all of the projections  500  may be replaced by projections  400  of the kind shown in  FIG. 4 . 
     Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.