pitot position

Discussion in 'Instruments / Avionics / Electrical System' started by jgnunn, Nov 18, 2006.

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  1. Nov 18, 2006 #1

    jgnunn

    jgnunn

    jgnunn

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    hi guys, I am getting close to being done on my second wing [Skybolt biplane]. I am wanting to fit a under-wing type pitot tube, and wondered how far back this should be mounted so it is in the optimum position? I need to know so i can fit a spruce cleat ready to accept it...
     
    Last edited: Nov 18, 2006
  2. Nov 18, 2006 #2

    Captain_John

    Captain_John

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    The way I understand this is it can be most anywhere, so long as it gets a nice, steady and smooth flow in incedental air!

    Grumman places them WAY outboard and the Citabria has it on the strut only a few feet out.

    Considerations are clear of the tie downs so the lines won't interfere and not affected by airflow around the wing. Soooo a sufficiently long mast. My RV has about a 6" mast.

    The placement fore and aft shouldn't be a concern. Some planes have them extending out beyond the nose of the ship!

    :D CJ
     
  3. Nov 20, 2006 #3

    jgnunn

    jgnunn

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  4. Nov 21, 2006 #4

    BD5builder

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    +1

    I suggest going out to the local airport and taking a walkaround the ramp (just make sure you let them know about it) and seeing the positions of the pitot tube on other aircraft (pictures help to keep the memory fresh).

    on my project the pitot tube is about 12" long and sticks out of the nose of the airplane kinda like the ones you see on military jets.., but its a pusher.
     
  5. Nov 21, 2006 #5

    Falco Rob

    Falco Rob

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    For an underwing pitot the general concensus around here is to place it as far outboard as possible, extending below the wing a minimum of 150mm (6") and with the tip of the pitot tube no further aft than about 20% of the chord (measured at the location of the pitot).

    For biplanes, which usually have the top wing forward of the lower, it's normal to fit it under the top wing, although the amount of interference from the lower wing is probably zip.

    They will work in other locations but if you follow the general rule of keeping it as clear as possible from any source of turbulent airflow your chances of erratic readings are lessened.

    Usually more problematic is the location of the static ports, unless you have one of those all-in-one pitots which also incorporates a static port.

    Rob
     
  6. Nov 21, 2006 #6

    Craig

    Craig

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    Pitot/static

    Beej, on the Acrosport, I have it under the lower wing, third rib from the tip. Front of the pitot is 5" below the wing, 3" aft of the front spar. I used a scrap of streamline tube to make the mount for it; welded on an "L" bracket to bolt to the rib there. It is one of the plated ribs, so I just slipped in some 5/8" spruce where the bolts are.

    On the Duce, it is mounted on the jury strut, 1' below the wing.

    On these open-cockpit airplanes, I terminate the static behind the instrument panel, but with a filter on the end of it. Has worked well so far.

    I believe that your choice of fitting is OK. Do you have a selector switch for alternate pitot?
     
  7. Nov 21, 2006 #7

    jgnunn

    jgnunn

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    Re: Pitot/static


    Phew, after hunting around for this consensus, that's just about where I decided to fit the cleat last night....

    Lesson time....explain a bit more please...

    No redundent plan as yet. Tho I was toying with the idea of adding another on the other wing? Is that a common strategy?
     
  8. Nov 21, 2006 #8

    Craig

    Craig

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    More on pitot/static

    Beej -

    Only one pitot system for our size airplanes. When you get into the multi engine, hundred passenger type, put in a second complete system.

    On closed-cockpit airplanes, it is common to have the static port on both sides of the fuselage - you've seen them, usually a little stainless disc with a #60 drill hole in the center. And it is on both sides so that when you slip or skid, static pressure will remain the same.

    In open-cockpit airplanes, we have static pressure all around us - no cabin inlet/outlet to raise or lower the pressure in the instrument area. So, a simple 2 or 3" tube out the back of the vertical speed indicator, WITH a filter, takes care of our needs. Just be sure that it is not exposed to the slipstream at any time, as that will affect the readings.

    Thus, you have one tube - preferably aluminum - from the pitot head to the airspeed, then a static connection from the airspeed to the altimeter to the VSI. Pretty simple, really, but then we have simple airplanes. Light, simple, durable - name of the game.

    If you are equipping it for flight into known IFR, it will pay to have a heated pitot, otherwise not. Mine is a model for heating, but I do not plan to connect the wires, as I stay strictly VFR, away from ice, clouds, etc. Just that a friend gave me this nice $1200 pitot head, and I hate to let it go to waste. Well, I did have to paint his airplane for him . . . .
     
  9. Nov 21, 2006 #9

    wally

    wally

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    On my Pitts, I have a probe on the lower left wing outboard about as far as the I strut.

    I made an aluminum block that attaches to the wing front spar and the probe extends straight out about 18 inches through the leading edge.

    I made it the pitot and static tubes to fit it. I used concentric stainless tubes and machined a pointy thing for the end. The mount block has concentric holes for the tubes to sit in and out the side of the block are 1/8 pipe hose barbs for the pitot and static lines. Holes for the static port are back from the pointy end a couple inches and are 4 small radially spaced holes.

    For testing, I connected it to the indicator and held it out the window of a speeding pickup and it seemed to match the speedometer quite well.

    I plan on flight testing it too - comparing it with GPS ground speeds will tell me how close it is. I hope to be flying sometime soon.

    Wally
     
  10. Nov 22, 2006 #10

    Falco Rob

    Falco Rob

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    Oops ! Looks like I screwed up with my comments about installing on the upper wing for biplanes.

    I must confess my exposure to aeroplanes with 2 wings is limited to a Culp Special built by one of the guys in our Chapter who has his pitot on the top wing, but from what I've read here it's not all that critical.

    As long as it's in an area of uninterrupted airflow it should be OK.
     

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