REFLECTOR: Static port calibration

Scott Baker sbakr at comcast.net
Thu Mar 5 16:45:01 CST 2009


hi jack
i don't think dual static ports are guaranteed to help neutralize static system pressure.  it may even multiply errors!
sb
  ----- Original Message ----- 
  From: Jack Prock 
  To: Velocity Aircraft Owners and Builders list 
  Sent: Wednesday, March 04, 2009 6:24 PM
  Subject: Re: REFLECTOR: Static port calibration


  Scott, 

  Do you see any benefit to having two static ports, one on each side?

  Jack
    ----- Original Message ----- 
    From: Scott Baker 
    To: Velocity Aircraft Owners and Builders list 
    Sent: Wednesday, March 04, 2009 3:20 PM
    Subject: Re: REFLECTOR: Static port calibration


    A typical static system installation includes the connection of the altimeter; air speed; VSI; and altitude encoder.  Sometimes autopilots with altitude/vertical speed control functions are connected to the same static source; and sometimes they have their own static source.

    The placement of the static source determines what pressures influence the static system.  A static port located in a high pressure area, for example, will cause air speed indicator to erroneously report lower than actual numbers.  The degree of error differs with the aircraft air speed and altitude, since both affect 'pressure' on the static port.  It's possible to get correct readings at one altitude and air speed - and large errors at different altitudes and air speeds.  We obviously want to locate the static pressure port in a neutral air pressure area.  In the Velocity design, the best location for the static port is halfway down the fuselage (just behind the rear door beams).  Even then, air pressures may be present to the degree that they cause errors in the static instruments.  When this occurs we can install a fence (a small air dam made of wood, plastic, or whatever) near the static port.  To reduce the pressure on the static system, install the fence in front of the static port.  To increase pressure, install the fence behind the static port.  The height of the fence determines the amount of pressure change.  Correcting static errors often turns into a trial and error process of flying, measuring, and then altering the height of the fence.

    Some EFIS systems allow field calibration of the altimeter and air speed.  Correcting static errors becomes a simple process - go fly with keyboard in hand, and enter the correct values on the calibration scale.  Those who want to impress their friends and neighbors are able to display 300 knots on the top end simply by changing the calibration scale.  We at the factory would never do that, of course!

    Regarding the question about the altitude reported by the transponder.  The transponder receives its altitude information from the altitude encoder.  Encoders must be inspected and calibrated by a licensed shop every 24-months.  The inspection and calibration is done with the aircraft standing still (static pressure is neutral).  Static pressure differences during flight (see above) affect the accuracy of the altitude being reported by the encoder.  One would assume that the same error would appear on the stand alone altimeter.  Pilot and ATC controller should be seeing the same altitude - but the reported altitude may not be accurate because of static errors!  It is a good idea to test the accuracy of the system - comparing side by side aircraft performance with a friend - or maybe with WAAS GPS altitude.

    Scott B.





      ----- Original Message ----- 
      From: aminetech at bluefrog.com 
      To: Velocity Aircraft Owners and Builders list 
      Sent: Wednesday, March 04, 2009 1:16 PM
      Subject: Re: REFLECTOR: Static port calibration


      If ATC is seeing the altitude transmitted by your transponder, naturally it will be identical, assuming you are reading from the same instrument that feeds the transponder.  Am I missing something?

      John

      --- ALVentures at cox.net wrote:






      I’ve always found my altitudes consistent with where ATC says I am – so I don’t think I need to change anything. 


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