Abstract:
A system is described that is configured to determine a plurality of air data parameters for an air vehicle. The system includes a mass air flow sensor, a pressure sensor, and a controller. The mass air flow sensor is mounted to sense an air flow caused by movement of the air vehicle. The pressure sensor is mounted to sense a static pressure at the air vehicle. The controller is configured to receive signals from the mass air flow sensor and the pressure sensor and determine an air velocity and a static pressure using the received signals.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to U.S. Provisional Application Ser. No. 60/703,991, filed Jul. 29, 2005, which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates generally to air data properties of air vehicles, and more specifically, to methods and systems for providing air data parameters using mass flow and pressure sensors.  
         [0003]     Air data properties are important in maintaining optimum performance of air vehicles. Three air data parameters include altitude, velocity, and mach number. Pressure altitude is a function of static pressure and is used to determine a quantity of flight conditions. For example, a decrease in pressure is typically indicative of an increase in altitude. Also, as pressure decreases, air density decreases. Air density is also a function of static air temperature. For example, if one plane is flying at sea level and one is flying at 10000 feet they both could be indicating 300 knots but the higher vehicle is actually flying faster. This phenomenon becomes more pronounced at higher altitudes.  
         [0004]     Velocity is a function of impact pressure (i.e. the total pressure minus the static pressure). Velocity is the most common parameter used to control air traffic and other aircraft maneuvers. Velocity, as well as air density, is used to control fuel consumption and required power needed to fly at cruise conditions.  
         [0005]     The mach number is the ratio of air vehicle speed and the speed of sound. As the air vehicle moves through the air, the air molecules near the air vehicle are disturbed and move around the air vehicle. If the air vehicle is moving at a relatively low speed, for example, less than 250 mph, the density of the air flow remains relatively constant.  
         [0006]     At higher air vehicle speeds, some of the energy from the air vehicle compresses the air and locally changes a density of the air. This compressibility effect alters the amount of the resulting force on the air and becomes more important as speed increases. Near and beyond the speed of sound, about 330 meters per second or 760 mph at sea level, small disturbances in the flow are transmitted to other locations. Such disturbances have a constant entropy. For example, a sharp disturbance may generate a shock wave that could affect both the lift and drag of the air vehicle. As a result, the mach number is an important air data parameter that is used to control the performance of the air vehicle. The mach number also changes as a function of altitude.  
         [0007]     As such, the air data parameters are very important in large aircraft and it may be considered flight critical. These parameters provide precision inputs for flight control. The air data computers that monitor these parameters tend to be high cost items because of accuracy requirements and complexity. For example, some air data systems are configured to be dual and triple redundant and incorporate very high precision and accurate pressure sensors. Extensive characterization procedures are also required to assure the air data computer system meets the desired performance levels.  
         [0008]     In smaller air vehicles such as targets, missiles, and unmanned air vehicles (UAVs), the accuracy of the air data parameters is typically less critical. However, they are still important if air vehicle optimum performance is to be achieved. These air vehicle types are typically relatively low in cost and as such, any air data system incorporated therein must also be low cost.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     In one aspect, a system configured to determine a plurality of air data parameters for an air vehicle is provided. The system comprises a mass air flow sensor mounted to sense an air flow caused by movement of the air vehicle, a pressure sensor mounted to sense a static pressure at the air vehicle, and a controller. The controller is configured to receive signals from the mass air flow sensor and the pressure sensor. The controller is further configured to determine an air velocity and a static pressure using the received signals.  
         [0010]     In another aspect, a method for determining an air velocity for an air vehicle is provided. The method comprises mounting a mass air flow sensor to sense an air flow caused by movement of the air vehicle, mounting a pressure sensor to sense a static pressure at the air vehicle, and configuring a controller to receive signals from the mass air flow sensor and the pressure sensor and determine the air velocity using the received signals.  
         [0011]     In still another aspect, an air data system is provided. The air data system comprises a mass air flow sensor, a pressure sensor, a probe configured to provide an airflow across the air flow mass sensor and allow an ambient pressure to contact the pressure sensor, and a controller. The controller is configured to determine an air velocity based on signals received from the mass air flow sensor and the pressure sensor. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a block diagram of a system for determining air data parameters for an air vehicle.  
         [0013]      FIG. 2  is an end cross-sectional view of a probe incorporated into the system of  FIG. 1 .  
         [0014]      FIG. 3  is a diagram of molecules at a lower temperature and high pressure.  
         [0015]      FIG. 4  is a diagram of molecules at a higher temperature and a low pressure.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     A low cost air data solution can be achieved by using a combination of a low cost mass flow sensor and a cost effective static pressure sensor. More specifically,  FIG. 1  is a block diagram of an air data system  10  for determining air data parameters. System  10  includes a microcomputer  12 , memory  14  and input/output (I/O) circuits  16  which are sometimes collectively referred to as a controller.  
         [0017]     A mass air flow sensor  20  provides air mass flow data (e.g., a velocity) to a velocity analog-to-digital converter (ADC)  22 , which is interfaced to microcomputer  12 . A pressure sensor  30  provides pressure data to a pressure ADC  32 , which is interfaced to microcomputer  12 . System  10  includes a probe  40  which includes a number of ports formed therein. A velocity port  42  is located along a longitudinal axis of probe  40  and allows an airflow across mass air flow sensor  20  which is placed adjacent velocity port  42 . Pressure ports  44 ,  46 ,  48 , and  50  are formed substantially perpendicularly to velocity port  42 . Pressure ports  44 ,  46 ,  48 , and  50  are interconnected, for example, by a hollow ring  52  formed within a perimeter of probe  40 . Hollow ring  52  is configured to allow static pressure to contact pressure sensor  30 .  
         [0018]      FIG. 2  is a cross-sectional end view of probe  40  illustrating velocity port  42  and the pressure ports  44 ,  46 ,  48 , and  50 , along with hollow ring  52 .  FIG. 2  further illustrates that a secondary port  54  may be located within hollow ring  52  of probe  40  that measures the average pressure at the other static pressure (Ps) ports, allowing the pressure to be applied to pressure sensor  30 .  
         [0019]     An air velocity, Vf, is measured using mass flow sensor  20  and static pressure, Ps, from pressure sensor  30 . Additional air data parameters can be determined from these two measurements. More specifically, the static pressure, Ps, is utilized to correct a pressure dependency of air velocity, Vf, to produce an accurate determination of air flow, Vc, which is referred to as a calibrated air velocity, which is also sometimes referred to as CAS.  
         [0020]     From air flow and static pressure, a pressure altitude (Hp), a total pressure (Pt), and a mach number can be derived as shown by the following formulas:  
         CAS   =     Vc   =           a   °     ⁡     [     5   ⁢     {         (       Qc     p   °       +   1     )       2   /   7       -   1     }       ]       .5     ⁢   where         ,       a   °     =       661.4786   ⁢           ⁢   kts   ⁢           ⁢   and   ⁢           ⁢     p   °       =     29.92126   ⁢           ⁢     inHg   .               
 
         [0021]     Therefore, knowing Vc, then Qc, the difference between total pressure and static pressure can be derived:  
       Qc   =       f   ⁡     (   Vc   )       =       p   o     ⁡     [         {         1   5     ⁢       (     Vc     a   °       )     2       +   1     }     3.5     -   1     ]             
 
         [0022]     Qc=Pt−Ps; therefore Pt=Qc+Ps and the mach number, M, can then be determined:  
       M   =       2.236068   *       [         (     Pt   Ps     )     0.2857142     -   1     ]     0.5       =     2.236068   *       [         (       Qc   Ps     +   1     )     0.2857142     -   1     ]     0.5             
 
         [0023]     Pressure altitude, Hp, can be calculated knowing Ps:  
         Hp   =           1   -       (     Ps   Po     )     0.19026         6.875586   ×     10     -   6           ⁢   forHp     ≤     36   ⁢           ⁢   Kft         ;           ⁢     Po   =     29.92126   ⁢           ⁢   inHg           
         Hp   =         -   20805     ⁢           ⁢   .85   ×   1   ⁢     n   ⁡     (     Ps   6.68322     )         +     36089   ⁢           ⁢   .23         ;           ⁢       for   ⁢           ⁢   36   ⁢   K   ⁢           ⁢   …   ⁢           ⁢   ft     &lt;   Hp   &gt;     65.6   ⁢           ⁢   Kft           
           Hp   =       65616   ⁢           ⁢   .7979     +     710793   ⁢           ⁢     .9632   ⁡     [         (     Ps   1.61673394     )       -   .0292712672       -   1     ]             ;           ⁢       for   ⁢           ⁢   65616   ⁢           ⁢   .79   ⁢           ⁢   ft     &lt;   Hp   &lt;     104986   ⁢           ⁢   .87   ⁢           ⁢     ft   .           ⁢               
 
         [0024]     In one embodiment, flow sensor  22  incorporates microbridge technology, such that flow sensor  22  is configured to operate in harsh environments. In addition, flow sensor  22  is capable of sensing extended air mass flow rates of more than 30 grams per square centimeter per second. In one embodiment, flow sensors have a range of 1000:1 (i.e. the sensors can sense mass flow rates from 30 gm/cm 2 /sec to 0.030 gm/cm 2 /sec). Flow sensor  22  is also bi-directional, that is, it can measure air mass flow in both directions.  
         [0025]     Microbridge mass air flow sensors, for example, sensor  22 , measures actual mass flow of a gas media. Actual flow is driven by a pressure gradient flowing from a higher pressure region to a lower pressure region. Sensor  22  measures air mass flow which is a function of the gas density, the gas density being a function of pressure and temperature. Sensor  22 , in one embodiment, is compensated for temperature and therefore does not vary as a function of temperature due to altitude changes.  
         [0026]      FIG. 3  is a diagram illustrating the flow of molecules  70  at low temperature and higher pressure, such as would be found at lower altitudes. To keep mass flow constant, volumetric flow, in cubic centimeters/second has to be decreased.  FIG. 4  is a diagram illustrating the flow of molecules  80  at a higher temperature and/or at a lower pressure, such as would be found at higher altitudes. To keep mass flow constant, volumetric flow, in cubic centimeters/second has to be increased.  
         [0027]     Mass air flow sensor  22  translates to an air flow sensor according to the following gas law equation:  
         Q   =       m   _     ⁢     nRT   mP         ,       
 
 where the mass flow rate of air,  m , is measured. The other variables are: n=1, R=82.1 cm 3  atm/mole ° K, m=mass of air in grams per one mole, P=Pressure in atmospheres (e.g., a function of altitude), and T=temperature in Kelvin (e.g., a function of altitude and impact temperature). 
 
         [0028]     For air, the mass can be calculated based on its composite makeup, in mass/mole, e.g., about 78.8% nitrogen (N 2 ) which is about 28.01 grams per mole, and about 21.2% oxygen (O 2 ) which is about 32.00 grams per mole. For this calculation the approximate 0.9% of argon, and 0.03% of carbon dioxide in the air can be ignored, and the result is that air is about 28.854 grams per mole.  
         [0029]     As can be seen by referring to  FIGS. 3 and 4  and the above gas law equation, the volumetric flow of air changes as a function of both temperature and pressure.  
         [0030]     In one embodiment, mass air flow sensor  22  is also a thermal anemometer, that is, it contains a temperature sensor. As a result, any air mass flow is compensated for temperature during calibration of the sensor  22 . Therefore, the air mass flow is only a function of pressure:  
         Q   =       m   _     ⁢     nRT   mP         ;       
 
 Therefore, Q is a function of 1/P. Since static pressure, Ps, is measured in system  10  by pressure sensor  30 , then Q=Vc can be readily compensated as Ps varies with altitude. 
 
         [0031]     The above described methods and systems provide a low cost, viable air data system that provides calibrated air velocity (Vc), static pressure (Ps), total pressure (Pt), pressure altitude (Hp), and mach number (M) to, for example, a flight control system of an air vehicle.  
         [0032]     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.