Abstract:
Pitot probes are described that are designed to prevent water blockage of the probe&#39;s air pressure orifice. The pitot probes are small, inexpensive, light weight and do not require power for heat generation. This makes the pitot probes particularly useful for use on UAV&#39;s and other small aircraft where size, cost, weight and power constraints are a major concern. The pitot probes utilize passive methods in the form of geometrical and material configurations to prevent blockage of the pitot probes by water droplets. The probes can also include water reservoirs that store collected water.

Description:
FIELD 
   This disclosure relates to pitot probes, in particular pitot probes that are useful on aircraft that fly in rain, sleet or snow. 
   BACKGROUND 
   Pitot probes are commonly used on aircraft and other applications for measuring airspeed. In certain weather conditions, a water droplet(s) can plug up the air pressure orifice at the front of the pitot probe, causing the pitot probe to incorrectly read the airspeed. In the past, this problem has been solved by using a heated pitot probe which uses heat to vaporize the water droplet(s). The heat is either generated electrically, or by diverting heat from the aircraft&#39;s engine(s). Other ways this problem has been solved is by using a specially calibrated pitot probe with a drain hole to remove water droplet(s) from the probe while installed, or by creating a pitot probe with a large diameter to prevent water droplet(s) from plugging up the probe. 
   Many small aircraft, including unmanned aerial vehicles (UAV&#39;s), typically cannot afford to carry the weight and size of previous pitot probes such as the probes listed above, and may not have sufficient energy resources to spare energy for use by a heated probe. 
   SUMMARY 
   Improved pitot probes are described that prevent water blockage of the pitot probe air pressure orifice. The pitot probes are small, inexpensive, light weight and do not require power for heat generation. This makes the pitot probes particularly useful for use on UAV&#39;s and other small aircraft where size, cost, weight and power constraints are a major concern. However, these pitot probes could also be used on aircraft that have previously used traditional pitot probes. 
   The pitot probes utilize passive methods in the form of geometrical and/or material configurations to prevent blockage of the pitot probes by water droplets. The geometrical configurations include the geometry of the internal air pressure passage of the probe body and/or the geometry of the inlet orifice of the probe. 
   In one embodiment, a pitot probe includes a probe body having an inlet air pressure sensing orifice at an inlet end thereof and an internal air pressure passage extending from the orifice through the probe body. The probe also includes a water reservoir. There is no water exit port connected to the water reservoir so that water collected in the water reservoir does not drain from the water reservoir. Instead, water collects in the water reservoir, and if the reservoir becomes full, the pitot probe can be drained or removed and discarded for a new pitot probe. 
   The water reservoir can have a number of different forms. Examples include, but are not limited to, a hydrophilic material inside the probe body, a chamber connected to the probe body and connected to the internal air pressure passage, and a shroud connected to the probe body and surrounding the orifice. 
   In another embodiment, a pitot probe includes a probe body having an inlet air pressure sensing orifice at an inlet end thereof and an internal air pressure passage extending from the orifice through the probe body. The orifice has a geometry that is configured to break up water droplets. Examples of suitable geometry include, but are not limited to, the orifice being serrated, the orifice being divided into a plurality of smaller tubes, and a screen in the orifice. 
   Further, there is no through-flow of air from the internal air pressure passage to an exterior of the probe body. In previous pitot probe designs, a through flow of air is provided which assists in preventing water droplet blockage of the orifice. Therefore, in the designs described herein, alternative means are provided for preventing water droplet blockage. 

   
     DRAWINGS 
       FIG. 1  is a cross-sectional view of a portion of a pitot probe according to one embodiment. 
       FIG. 2  is a cross-sectional view of a portion of a pitot probe according to another embodiment. 
       FIG. 3  is a cross-sectional view of a portion of a pitot probe according to another embodiment. 
       FIG. 4  is a side view of one embodiment of an inlet end of a pitot probe. 
       FIG. 5  is an end view of another embodiment of an inlet orifice of a pitot probe. 
       FIG. 6  is an end view of another embodiment of an inlet orifice of a pitot probe. 
   

   DETAILED DESCRIPTION 
   With reference to  FIG. 1 , a portion of a pitot probe  10  according to one embodiment is illustrated. The probe  10  includes a generally cylindrical probe body  12  having an inlet air pressure sensing orifice  14  at an inlet end  16  thereof. An internal air pressure passage  18  extends from the orifice  14  through the probe body  12  to a rear end of the probe body. The passage  18  leads to an air pressure sensor  20  which senses the air pressure. The air pressure reading is then used to calculate the air speed in a known manner. 
   In use, the probe  10  is connected to an aircraft, for example a UAV, and projects forwardly from the aircraft so that the orifice  14  faces forwardly toward the oncoming air flow. When the aircraft is flying in rain, sleet or snow, there is a chance that a water droplet will strike the tip of the probe  10  and will plug the orifice  14  through surface tension. If plugged, the probe will not correctly read the air speed. 
   To prevent plugging of the orifice  14 , the probe  10  is provided with means to prevent plugging. In  FIG. 1 , the means to prevent plugging comprises a hydrophilic material  22  disposed in the internal passage  18 . The hydrophilic material  22  is circumferentially disposed within the interior of the probe body around the flow passage  18  and extends from the orifice  14  towards the rear of the passage  18 . The hydrophilic material  22  draws water away from the orifice  14  and acts as a water reservoir by collecting and holding the water. The extent of the hydrophilic material  22  along the passage  18  can be chosen based on how much water one anticipates needs to be collected and retained. 
   As used herein, a hydrophilic material is one that performs the functions of drawing water away from the orifice  14  and collecting and holding water. Any material or combinations of materials that performs these functions can be used and are intended to be encompassed by the term hydrophilic material. The hydrophilic material can be in any suitable form including, but not limited to, fibers, gels, particulates, combinations thereof, and combinations of hydrophilic and non-hydrophilic materials. An example of a suitable hydrophilic material for use in the pitot probe  10  is a superabsorbent polymer (SAP), such as a cross-linked sodium polyacrylate. 
   Because hydrophilic materials have a tendency to swell when water is absorbed, a retaining means, such as a mesh screen, can be provided at the interior diameter of the hydrophilic material  22  to hold the material in place and prevent inward swelling of the material that could cause a reduction in the diameter of the flow passage  18 . 
   There is no water exit port connected to the hydrophilic material  22  or any special provision for removing the water from the hydrophilic material  22 . Therefore, the hydrophilic material  22  can be said to permanently retain the collected water, except for any amounts of water that may evaporate from the material  22  or drain from the material  22  out the orifice  14  when the aircraft is not in use. 
   If it is determined that the hydrophilic material  22  is saturated, or whenever desired, the pitot probe  10  is replaced with a new probe. As a result, the probe  10  can be characterized as being disposable. The probe body  12  can be made of any materials suitable for forming a pitot probe body, for example metal or plastic. Because the probe  10  is intended to be replaceable, the probe body  12  is preferably made of a material, for example plastic, that is relatively inexpensive. 
     FIG. 2  illustrates another embodiment of a pitot probe  30 . The probe  30  includes a generally cylindrical probe body  32  having an inlet air pressure sensing orifice  34  at an inlet end thereof, and an internal air pressure passage  38  that extends from the orifice  34  through the probe body  32  to a rear end of the probe body. The passage  38  leads to an air pressure sensor (not shown) which senses the air pressure as in the embodiment of  FIG. 1 . 
   In  FIG. 2 , the means to prevent plugging of the orifice  34  comprises an increase in the diameter of the passage  38  behind the orifice  34 . In the illustrated embodiment, the increase in diameter is caused by a water reservoir  40  formed by a chamber  42  that is integral with the probe body  32 . The chamber  42  can be integrally formed with the probe body  32  or formed separately and then attached to the probe body. Two holes  44 ,  46  connect the passage  38  to the reservoir  40 . It is not necessary for the diameter of the two holes  44 ,  46  to be bigger or smaller than the orifice  34 . However, the two holes  44 ,  46  must be big enough to prevent water from plugging them up. In some embodiments, the water reservoir  40  may contain a hydrophilic material to pull water through the two holes  44 ,  46 . Also, in some embodiments, only a single hole may be used and in other embodiments three or more holes may be used to connect the passage  38  to the reservoir  40 . 
   In use of the probe  30 , the increase in the diameter of the passage  38  allows water droplets to be pulled into the passage  38  away from the orifice  34  through the forces of surface tension and the pressure force in front of the water droplets from the moving air. The water droplets then flow through the holes  44 ,  46  and into the reservoir  40  which collects water droplets as the droplets are pulled away from the orifice  34 . As in the embodiment of  FIG. 1 , there is no water exit port from the reservoir  40  that would allow water to drain. 
     FIG. 3  illustrates another embodiment of a pitot probe  50 . The probe  50  includes a generally cylindrical probe body  52  having an inlet air pressure sensing orifice  54 , and an internal air pressure passage  58  that extends from the orifice  54  through the probe body  52  to a rear end of the probe body. The passage  58  leads to an air pressure sensor (not shown) which senses the air pressure as in the embodiment of  FIG. 1 . The probe body  52  is curved upwardly at the front end so that the axis A 1 -A 1  of the orifice  54  is displaced from the axis of the rear end of the probe body. 
   A shroud  60  is connected to the probe body  52  and surrounds the orifice  54 . The shroud  60  includes an inlet opening  62  at a leading end thereof with a diameter greater than a diameter of the orifice  54  of the probe body  52 . The larger diameter of the opening  62  prevents plugging of the opening  62  by water. The opening  62  can also include a surfactant to reduce the surface tension with water. 
   In the illustrated embodiment, the shroud  60  includes first conical section  64  that flares outwardly, a constant diameter section  66  extending forwardly from the first conical section  64 , and a second conical section  68  connected to the front end of the section  66  and which tapers inwardly to the inlet opening  62 . The rear end of the section  64  opposite the inlet opening  62  is connected to the probe body  52 . It is to be realized that the shroud can have other geometries as well. 
   The shape of the shroud  60  is sufficient to form a water reservoir to collect water droplets and other moisture that enters the shroud  60 . In the illustrated embodiment, the axis A 1 -A 1  of the orifice  54  is offset from the axis A 2 -A 2  of the inlet opening  62 . This helps prevent water droplets, sleet or snow that enters the inlet opening  62  from directly impinging on the orifice  54 , thereby preventing plugging. The mass of any moisture particles that enter the inlet  62  will generally prevent those particles from then flowing upward to the orifice  54 . Instead, the moisture particles will likely impact on the probe body  52  or on the conical section  64 . The moisture particles will then collect at the bottom of the shroud  60 . As in the embodiments of  FIGS. 1 and 2 , there is no water exit port from the shroud  60  that would allow collected water to drain. This also prevents any loss of pressure in the probe  30 . 
   Although the axis A 1 -A 1  of the orifice  54  is described as being offset from the axis A 2 -A 2  of the inlet opening  62 , the axes A 1 -A 1  and A 2 -A 2  could be coaxial as long as means are provided to prevent plugging of the orifice  54 . For example, the measures described in  FIGS. 1 and 2  could be used. 
   In each of the embodiments of  FIGS. 1-3 , there is no through-flow of air from the internal air pressure passage to the exterior of the probe body. Therefore, the pitot probes do not need to be calibrated. In some previous pitot probe designs, a through-flow of air is provided which not only aids in preventing moisture plugging of the inlet orifice, but also requires that the pitot probes be calibrated. 
   The geometry of the leading end of the probe body and/or the geometry of the orifice can also be designed to prevent plugging by liquid droplets. 
   For example, with reference to  FIG. 4 , an inlet end  70  of a probe body  72  is illustrated. The inlet end  70  is provided with serrations  74  which help break up liquid droplets when they impinge upon the inlet end  70 . 
     FIG. 5  illustrates another example which is an end view of an inlet orifice  80  of a pitot probe  82  where the orifice  80  is divided by a plurality of small tubes  84 . The tubes  84  can extend a short distance into the probe body. Due to surface tension, when a water droplet impinges on the orifice  80 , some of the tubes  84 , spaces between the tubes, and spaces between the tubes and the interior of the passage will plug up, while typically at least one space will remain open allowing an accurate air pressure reading. 
     FIG. 6  is another example where an inlet orifice  90  of a pitot probe  92  is provided with a grate-like structure  94 . Due to surface tension, the grate-like structure  94  helps break up water droplets that impinge on the orifice  90 , thereby preventing plugging of the orifice  90 . 
   The embodiments in  FIGS. 4-6  can be used separately or together with one or more of the embodiments in  FIGS. 1-3 . In addition, the embodiment in  FIG. 4  can be used together with the embodiments in  FIGS. 5-6 . 
   The embodiments disclosed in this application are to be considered in all respects as illustrative and not limitative. The scope of the invention is indicated by the appended claims rather than by the foregoing description; and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.