Static port placement

Discussion in 'Instruments / Avionics / Electrical System' started by WurlyBird, Mar 3, 2009.

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  1. Mar 3, 2009 #1

    WurlyBird

    WurlyBird

    WurlyBird

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    Quick question on static port placement; I have read up on them and realize that the opening MUST be parallel to the airflow. I want to install a dual port setup (one on each side) on my Kitfox because I do not like the inherent inaccuracy of the ASI. The simplest place I can find is on the sides of the nose cowling which comes all the way back to the door opening. At this point the fuselage is still increasing in width so the port opening would not be parallel with the direction of flight, probably no more then 5 degrees off. Can it be assumed that the airflow here will follow the contour of the aircraft and so the opening will parallel the airflow? Is it good enough? Thanks for the help.

    James
     
  2. Mar 3, 2009 #2

    orion

    orion

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    Not the best idea since the flow will tend to change with power setting and airspeed - could result in some inconsistent readings. If parallel to the airstream, aft of the door might be better or possibly using a blade type pickup, out on the wing.
     
  3. Mar 4, 2009 #3

    dirtstrip

    dirtstrip

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    My experience. I had placed the Cessna style static ports on each side of cabin behind the cowling and NACA vents but ahead of the doors on the cabin. The Tundra cabin at that point is still increasing slightly in width and is not parallel to the air flow.
    Initial test flight. At full throttle the prop blast interfered with the exhausting static air and the back pressure caused low readings. I did not catch this until airborne in climb out. The numbers for airspeed were way too low. Add to this that the electronic readouts of the Dynon EFIS system for most functions depend on proper pitot/ static operation so attitude, rates of turn, nearly everything was a bogus reading. I timed the cruise for two miles at intersections. It was nearly 60 seconds, or 120 mph. Airspeed indicated only 98. Needing a safe approach speed for landing, I checked for consistency of readings at low power stalls and noted the airspeed. Indicated stall speeds seemed consistent so I used that consistently wrong number and added 30 percent plus for my approach. The landing speed turned out good. As it turned out, with power back the back pressure created from the prop blast was reduced and readings for airspeed were closer to accurate.

    Contacting Dynon, they suggested I disconnected the static tubes and exhaust them inside the cabin. The next flight everything registered correctly and agreed with the timed 60 second two miles or 120 mph. The location of the static port was the culprit. The second recommendation from them was to relocate the ports rear of the doors where the fuselage is parallel to the air flow. First I wanted to try another solution. I fashioned a small aluminum cover for the static outlet that protected it from the prop blast, closed at the front and open at the rear. It worked perfectly and so far has been accurate. It would have been difficult to reroute the static lines between the double floor to the rear of the door as suggested without causing a low point or droop in the static line and possibly trapping condensation and moisture and allowing it to freeze or block the line when parked. So far I am happy with this solution and also do not have to fill an unused hole on each side of the cabin left from changing the static port location.
    If someone else has experienced this or can see what other potential problem this solution could create I would be interested in hearing it. So far so good at 21 hrs. and now I have shared my $ .02.
     
  4. Mar 4, 2009 #4

    Dana

    Dana

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    For my plane, I simply built a concentric tube for pitot/static... a 1/4" OD tube for pitot and a 3/8" OD thin wall brass tube around it with four small holes drilled 90° apart in the middle of it.

    -Dana

    The citizens of the United States are getting the government they deserve. The problem is that I'm also getting the government they deserve.
     
  5. Mar 7, 2009 #5

    WurlyBird

    WurlyBird

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    I keep reading about the concentric static ports and that is the way I would like to go on future projects, however this plane is built and I can not get inside the wing.


    The static port is currently vented inside the cabin and it leads to a whole lot of erroneous readings. I can decrease ASI by 5+ kts by slipping, with the doors closed.

    I am not sure there is a place on the airframe that is parallel to the airflow except maybe the doors themselves and those are about to be omitted. Also the whole plane is fabric except for the cowling. Is it possible to mount to fabric?

    I am thinking about using the vane idea, this may prove to be the easiest. Any other ideas? Thanks.
     
  6. Mar 7, 2009 #6

    Joe Fisher

    Joe Fisher

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    The BC12d Tcrafts vent inside of one wing and use the drain holes. It would be impossible to do a calibrated static test but from a piratical stand point it works with no quirks. For instance TriPacers have two vents on the bottom of the fuselage. On take off when you rotate the altimeter will show a 200' loss.
    Joe Fisher
     
  7. Mar 7, 2009 #7

    Dana

    Dana

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    I always wondered about that... my T-Craft had the static line simply ending somewhere inside the wing root. Never seemed right but seemed to work OK, too, so I never messed with it.

    If you can't get inside the wing... what size tube is the pitot line? If it's big enough, and you can get a smaller tubing to go loosely inside of it, you could suck a string through it, then use the string to draw the smaller tube through (I've used a similar method to add a wire to an existing conduit on my PPG).

    -Dana

    Puritanism: the haunting fear that someone, somewhere, may be happy.
     

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