Patent Publication Number: US-2010116806-A1

Title: Automated heating system for ports susceptible to icing

Description:
BACKGROUND OF THE INVENTION 
     Aircraft and various missiles have pitot static systems that collect the RAM air pressure from the forward air speed and feed the pneumatic pressure to the airspeed and altitude measuring devices. These systems are used in air data (instruments) and engine performance systems. When flying in icing conditions the pitot tubes can ice over and block the RAM air pressure. This causes the aircraft to lose the airspeed measuring capability, to lose accurate altitude measuring ability, or to get inaccurate engine performance indications. 
     Current aircraft use a coil of resistive wire that is wrapped around the pitot tube and warmed by the aircraft electrical power to keep the pitot vents from icing over—see  FIG. 1 . In some applications the pilot is responsible for turning the pitot tube heating system on and off When outside air temperature is 90° F. the pitot will get hot enough to burn one&#39;s hand. It will also consume 10+amps of current creating an unnecessary power drain. Also the heater has no redundancy. If the coil of wire opens anywhere the heater will stop working. 
     There are other pitot tube heating systems that use temperature sensors and microprocessors for determining when to activate/deactivate the pitot tube heating element. However, these systems are overly complex, thus making them expensive. They are also prone to the failure described above. 
     Therefore, there exists a need for an improved, low cost pitot tube heating system. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved heating system air pressure sensing port. An example aircraft pitot tube heater uses Positive Temperature Coefficient (PTC) switching thermistors arranged to automatically sense the outside air temperature and heat to a design set point temperature. The pitot tube heater only turns on below the design set point and consumes less current than the standard aircraft pitot tube heaters when operating at very cold temperatures. Virtually no current is drawn when the pitot tube temperature increases past the set point, thus reducing the power drain on the aircraft system. This heater incorporates redundancy in the form of an array of thermistors. If any one thermistor fails, there will be no overall effect on the operation of the pitot tube heater. The pilot can just leave this heater on all the time and reduce the possibility that under a high work load the pilot forgets to turn on the pitot heater. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
         FIG. 1  illustrates a pitot tube heating system formed in accordance with the prior art; 
         FIG. 2  illustrates static port and pitot tube heating element system formed in accordance with embodiments of the present invention; 
         FIG. 3  illustrates a partial x-ray view of a pitot tube formed in accordance with an embodiment of the present invention; and 
         FIG. 4  illustrates temperature versus resistance graph for example thermistors used in the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 2  illustrates a pitot system  20  formed in accordance with an embodiment of the present invention. The system  20  includes a pitot tube  24  and one or more static ports  26  that are directly connected to air pressure indicators, such as an air speed indicator  30 , a vertical speed indicator (VSI)  32 , and an altimeter  34 . The tube  24 , ports  26  and indicators may also be connected to an Air Data Computer (not shown). 
     In one embodiment, the pitot tube  24  includes an automatic heating element component  38  that is connected up to a power supply (not shown). The heating element component  38  includes one or more thermistors that go to high resistance (essentially shut off) above a preselected set-point temperature. Therefore, when the thermistors are at low resistance they are conducting electricity from the power supply, thereby generating heat and keeping the pitot tube  24  from reaching the freezing point. 
     In another embodiment, or in conjunction with the thermistors located in the pitot tube  24 , similar thermistors are used in the static ports  26  in order to keep them from freezing. 
       FIG. 3  illustrates a partial x-ray view of an example pitot tube  24  formed in accordance with an embodiment of the present invention. The pitot tube  24  includes an outer hull  36  and an air receiving tube  40 . The outer hull  36  and/or the tube  40  are formed of an electrically conductive material, such as brass or copper. A plurality of thermistors  42  are attached to an outer wall of the tube  40 . The thermistors  42  may be attached in any of a number of different ways, such as with a thermally conductive epoxy/adhesive. In one embodiment, twelve thermistors  42  (three annular sets of four) surround the tube  40  along the length of the tube  40 . Each thermistor  42  includes two electrical leads. One of the leads is attached to either the tube  40  or the outer hull  36 . The tube  40  and/or the outer hull  36  are electrically conductive and are connected to aircraft ground. The other leads of each of the thermistors  42  are connected to an aircraft power supply  52 , such as a 12 volt source, via a switch  50 . The switch  50  is operable by the flight crew or is the master power-on switch for the aircraft. 
     In one embodiment, the thermistors  42  are connected in parallel. The parallel connection allows for robust operation, because if one of thermistors  42  should fail the other thermistors  42  continue to operate. Other circuit configurations may be used. 
     In one embodiment, the thermistors  42  are selected to have a set point of 25° C. Therefore, as the thermistors  42  experience temperatures at or below 25° C., their resistance is low, for example roughly 17 ohms, thereby increasing current flow and causing the thermistors  42  and the tube  24  to heat up. If the temperature experienced by the thermistors  42  is above 25° C., the resistance of the thermistors  42  becomes high, for example roughly 2000 ohms, thereby stopping current from flowing through the thermistors  42 . See  FIG. 4  where T 1  is 25 degrees C. 
     The pitot tube  24  also includes a thermal insulator sleeve  60  that surrounds the thermistors  42 . Heat produced by the thermistors  42  is maintained close to the inner tube  40  by the thermal insulator sleeve  60 . The thermal insulator sleeve  60  is formed of any of a number of insulating materials. In one embodiment, the thermal insulator sleeve  60  is a Teflon outer covering that is molded to the outside of the pitot tube  24 . Ice will shed off easily of the Teflon outer covering because of low surface tension and the Teflon can easily handle the temperature produced by the thermistors. 
     In other embodiments, the PTC thermistors  42  may be used at other locations where blockage of ports might affect operational capabilities. For example, the PCT thermistors may be used in conjunction with the static ports  26  as well as with pitot tubes located at various other locations, such as jet engine intake. 
     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.