Where to Put the Static Port?

Discussion in 'Instruments / Avionics / Electrical System' started by wsimpso1, Jun 30, 2019.

Help Support HomeBuiltAirplanes Forum by donating:

  1. Jun 30, 2019 #1

    wsimpso1

    wsimpso1

    wsimpso1

    Super Moderator Staff Member

    Joined:
    Oct 19, 2003
    Messages:
    5,911
    Likes Received:
    3,143
    Location:
    Saline Michigan
    I thought that I had a system. Use the Piper blade for pitot and static under one wing, Alpha Systems AOA probe under the other wing. Run heaters in both when it is cold, and I thought I had my instrument capable altimetry and an aural warning for high AOA licked. Then I find out that Garmin and Dynon both use a pitot-AOA probe with two ports and then a separate static port someplace else... Only need one device out in the wind, only need one electric power draw to keep it ice free in clouds...but then I have to find a place to put the static port that works well.

    So I start searching on here. The advice is to put it on the fuselage side where the fuselage walls are straight and parallel. Nice plan, if you have it. My fuselage walls are straight and parallel through the wing and NOWHERE else. Forward of the wing root, even two inches, and it is still transitioning to straight and parallel. Even two inches aft of the root, it is already tapering into the pressure recovery area. And through the wing? Well, the air is sped up like crazy above the wing, and I suspect that will give some crazy low pressure that varies with q and AOA... So where does it work OK for Instrument altimetry with such a fuselage?

    And I really hate the idea of poking holes in my painted composite fuselage until a find a place that works pretty well...

    So, would you go with two probes or one plus a static port? Your rationale is as important to me as your choice...
    If you go with one probe for Pitot-AOA and a separate static port? Why would you make that choice? Where would you put the static port? And why there?
    If you would go with a two probe system, why would you do that?

    Thanks for looking!

    Billski
     
  2. Jun 30, 2019 #2

    BJC

    BJC

    BJC

    Well-Known Member

    Joined:
    Oct 7, 2013
    Messages:
    9,575
    Likes Received:
    6,342
    Location:
    97FL, Florida, USA
    The Glasair series use dual static ports on the aft fuselage, one on each side, plumbed together internally. The ports have a single sensing hole in a 1 1/2 inch diameter aluminum disc, 1/8 th inch thick, that protrudes from the fuselage side. The ports are located 2/3 aft on a line from the trailing edge of the wing root to the leading edge of horizontal stabilizer root. The fuselage sides on the Glasairs are flat for most of that distance.


    BJC
     
    Jeffd and wsimpso1 like this.
  3. Jun 30, 2019 #3

    wsimpso1

    wsimpso1

    wsimpso1

    Super Moderator Staff Member

    Joined:
    Oct 19, 2003
    Messages:
    5,911
    Likes Received:
    3,143
    Location:
    Saline Michigan
    Good reference Byron. I hope that you are doing well.
     
  4. Jun 30, 2019 #4

    BJC

    BJC

    BJC

    Well-Known Member

    Joined:
    Oct 7, 2013
    Messages:
    9,575
    Likes Received:
    6,342
    Location:
    97FL, Florida, USA
    Thanks. See you at Oshkosh.


    BJC
     
  5. Jul 1, 2019 #5

    proppastie

    proppastie

    proppastie

    Well-Known Member

    Joined:
    Feb 19, 2012
    Messages:
    3,808
    Likes Received:
    1,103
    Location:
    NJ
    Mooney one heated pitot under the wing. Two static ports 1/2 aprox. Between TE wing and LE horizontal stab. Plumbed together with a T and drain out the bottom with another T for the drain. My drain is a Curtis valve. ...because I installed it as a service bulletin after flying through the rain once.
     
    wsimpso1 likes this.
  6. Jul 1, 2019 #6

    pictsidhe

    pictsidhe

    pictsidhe

    Well-Known Member

    Joined:
    Jul 15, 2014
    Messages:
    6,873
    Likes Received:
    1,853
    Location:
    North Carolina
    RVs have a pair of static ports, one each side of the fuselage. the standard type is a hollow pop rivet. Location is apparently important as well as shape. Fortunately, few will skimp on a pop rivet. Some insist on fitting 'proper' static ports, though... I will try the static location on the full size version of my plane. If that doesn't work, it's lotsa holes to find the right spot... At least with composites, they aren't as hard to fill as coroplast.
     
  7. Jul 1, 2019 #7

    Dan Thomas

    Dan Thomas

    Dan Thomas

    Well-Known Member

    Joined:
    Sep 18, 2008
    Messages:
    4,860
    Likes Received:
    2,071
    Static ports. One of the biggest and most difficult of all the stuff in building a homebuilt is getting an accurate static pressure. There are many ways it's been done. Almost all have their drawbacks. In my Jodel, the pitot head aft of the leading edge under the wing had static ports on it. The positive pressure zone there made the ASI read low. Tried vennting the static to the cabin, and the airflow around the fuselage sucked air out of the cabin and made the ASI read high. I machined an aluminum fitting that had the protruding flat disc like a Cessna port, and installed it in the fuselage side ahead of the wing, and it wasn't too bad until the AOA got higher, as in slow flight or climb, where it gave some stupid numbers as the air flowed upward around the square bottom corners of the fuselage. I machined a step across it and tried rotating it to see if I could find a sweet spot: nope. I finally reconnected the static line to the pitot head and cut it off inside the wing; Bingo. Non-moving air inside the wing was at ambient pressure, and the instruments were accurate.

    Works fine for a fabric-covered, slowpoke airplane. Not so sure it would work on a composite high-speed ship. Probably not.
     
    wsimpso1 likes this.
  8. Jul 1, 2019 #8

    Hephaestus

    Hephaestus

    Hephaestus

    Well-Known Member

    Joined:
    Jun 25, 2014
    Messages:
    1,183
    Likes Received:
    258
    Location:
    YMM
    What's your tail look like?

    Top of vertical stab with enough length to be well ahead of the LE... Should be a good spot for A static reference other than the distance.

    I think it was discussed here before for a similar issue wasn't it?
     
  9. Jul 1, 2019 #9

    wsimpso1

    wsimpso1

    wsimpso1

    Super Moderator Staff Member

    Joined:
    Oct 19, 2003
    Messages:
    5,911
    Likes Received:
    3,143
    Location:
    Saline Michigan
    Fuselage is straight vertical walls through the wing, then a gentle pressure recovery shape all the way to the rudder post.

    Empennage is about a foot deep and 3" wide at the rudder post. Rudder goes to the bottom of the fuselage with a dead vertical hinge line, about 5 feet from bottom to tip. The empennage is reducing in width all the way to the rudder post.

    Horizontal tail is 10" above bottom of the fuselage, trailing edge is barely forward of rudder hinge line. The horizontal tail 1/4c is 3.4MAC aft of wing 1/4c.

    Both surfaces have a 3/2 taper and the hinges are at 0.6c.

    What impact does the tail look have on static port placement?

    I recall some discussion of lag in the B-707 with that location. Many times more line for that bird than mine. Lots of sailplanes put a total energy sensor there, so it can not be horrible.

    We talked about other issues with pitot-static, but I had pretty well settled on the tried and proven Piper blade for both sources. Now I am tempted by being able to only have one blade dragging through the air and only one set of heaters to draw power in flight with the tail of needing a different static source - fuss, fuss, fuss.

    Billski
     
  10. Jul 1, 2019 #10

    Hephaestus

    Hephaestus

    Hephaestus

    Well-Known Member

    Joined:
    Jun 25, 2014
    Messages:
    1,183
    Likes Received:
    258
    Location:
    YMM
    Nothing wrong with KISS. Or OCD perfectionism. :)

    I was asking about the tail 'look' more as a way to give you thought/pause.

    Which is better simplified or dual redundant?
     
  11. Jul 1, 2019 #11

    wsimpso1

    wsimpso1

    wsimpso1

    Super Moderator Staff Member

    Joined:
    Oct 19, 2003
    Messages:
    5,911
    Likes Received:
    3,143
    Location:
    Saline Michigan
    Doing stuff well is required, and I hope that I am not treading the line between that and OCD or perfectionism. LOL.

    Oh, I am thinking that the space a couple wing chords aft of the wing and at least a tail chord ahead of the tail might be a really good spot. I have been thinking about the top of the tail, but I can not help thinking that it will go into and out of the prop wash with pitch changes...

    Thread drift alert. Simple is great, but redundancy is the powerful tool for reducing catastrophic failure frequencies and improving FMEA scores. For instance, redundant pressure relief valves are great for preventing boiler explosions.

    Two ports on the fuselage are not for redundancy, they give the same airspeed reading no matter which way you are slipping the ship.

    Billski
     
  12. Jul 2, 2019 #12

    BBerson

    BBerson

    BBerson

    Well-Known Member HBA Supporter

    Joined:
    Dec 16, 2007
    Messages:
    12,063
    Likes Received:
    2,355
    Location:
    Port Townsend WA
    I think they trail a funnel 100 feet behind to get a calibrated static reference. Then use that to find a similar spot in flight.
    I would just use the cockpit for my stuff :)
     
  13. Jul 2, 2019 #13

    pictsidhe

    pictsidhe

    pictsidhe

    Well-Known Member

    Joined:
    Jul 15, 2014
    Messages:
    6,873
    Likes Received:
    1,853
    Location:
    North Carolina
    Hmmm, if you don't want to drill a whole bunch of holes. Could you build a clamp on girdle? Adjustable clamps on the top and bottom at the centre line. The strap would obviously need to be as thin as you can make it and still pipe the pressure out. Bond up a mylar sandwich? If you have several ports, you can test several vertical locations each flights.

    Here's a simple NASA guide to calibration.

    And a 292 page tome from the FAA
     
    Last edited: Jul 2, 2019
  14. Jul 2, 2019 #14

    TFF

    TFF

    TFF

    Well-Known Member

    Joined:
    Apr 28, 2010
    Messages:
    11,647
    Likes Received:
    3,276
    Location:
    Memphis, TN
    The Cirrus models have the static a couple of feet behind the cabin door. After the start of the reduced fuselage cross sections. The port sits behind a step built into the part. I’m sure it’s to get a break in the airflow. Before I cut a hole. I would look at similar performance planes at Oshkosh and the local airport like Cirrus, Diamond. I would also talk the the Glasair, Lancair, guys on their accuracy. It maybe the poll method not straight scientific. It is gathering data.
     
  15. Jul 2, 2019 #15

    Toobuilder

    Toobuilder

    Toobuilder

    Well-Known Member

    Joined:
    Jan 20, 2010
    Messages:
    4,449
    Likes Received:
    3,247
    Location:
    Mojave, Ca
    The static port can be turned in situ. The RV guys will bond washers of varying thickness to get the desired calibration. This action mimics the more sophisticated milled "step" seen on some factory aircraft. I suspect this technique is not effective if it's located in an area subject to gross changes in local pressure due to aircraft configuration change (flap deployed, etc) or flight profile.
     
  16. Jul 2, 2019 #16

    Dan Thomas

    Dan Thomas

    Dan Thomas

    Well-Known Member

    Joined:
    Sep 18, 2008
    Messages:
    4,860
    Likes Received:
    2,071
    That's what I found when I milled the step on my port. The airflow changes around the fuselage as pitch changed were making a bigger mess than the step could accomodate.

    The old Champs had two forward-facing tubes on their pitot-static heads. One was a simple open-ended affair for the airspeed, and the other had its forward end brazed shut and nicely rounded, then a couple of tiny holes were drilled crossways through the tube wall about 3/4" back from the forward end. A collar about 3/8" or 7/16", IIRC, was fitted to that tube a little distance behind those holes and retained with a tiny setscrew. One adjusted the collar back and forth to get the desired static pressure. The collar somehow arrested or changed the airflow so that the ports saw neither suction or pressure.
     
    wsimpso1, Topaz and pictsidhe like this.
  17. Jul 2, 2019 #17

    BBerson

    BBerson

    BBerson

    Well-Known Member HBA Supporter

    Joined:
    Dec 16, 2007
    Messages:
    12,063
    Likes Received:
    2,355
    Location:
    Port Townsend WA
  18. Jul 3, 2019 #18

    Topaz

    Topaz

    Topaz

    Super Moderator Staff Member

    Joined:
    Jul 30, 2005
    Messages:
    13,739
    Likes Received:
    5,421
    Location:
    Orange County, California
    Dad's 65-LB Chief had the same arrangement. Seemed to work well.
     
  19. Jul 3, 2019 #19

    wsimpso1

    wsimpso1

    wsimpso1

    Super Moderator Staff Member

    Joined:
    Oct 19, 2003
    Messages:
    5,911
    Likes Received:
    3,143
    Location:
    Saline Michigan
    I will be looking at that stuff hard at OSH...
     
  20. Jul 3, 2019 #20

    wsimpso1

    wsimpso1

    wsimpso1

    Super Moderator Staff Member

    Joined:
    Oct 19, 2003
    Messages:
    5,911
    Likes Received:
    3,143
    Location:
    Saline Michigan
    Something to keep in mind when playing with fences on the aft fuselage static ports.

    These arrangements worked, but they are out of character on a slick composite bird. Sort of like fabric covered control surfaces on this bird... hard to heat for IFR too.

    If I have too much trouble making the Dynon pitot-alpha probe and aft mounted static ports work, my fall back plan is the PIper blade pItot-static on one wing and Alpha probe on the other wing.

    Billski
     

Share This Page



arrow_white