Wing leading edge pitot tube length?

Discussion in 'Instruments / Avionics / Electrical System' started by cluttonfred, Mar 3, 2017.

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  1. Mar 3, 2017 #1

    cluttonfred

    cluttonfred

    cluttonfred

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    Does anyone know a rule of thumb for the distance that a wing leading edge pitot-static tube tube should project to get a clear reading? Perhaps as a function of maximum speed and possiblly leading edge radius? Thanks!
     
  2. Mar 3, 2017 #2

    BJC

    BJC

    BJC

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    Matthew:

    Good question. Scroll down about half way here http://cafe.foundation/v2/aboutcafe_orderfromchaos.php for some info on the high-accuracy CAFE pitot tube.

    The Pitts uses a pitot tube that is, in effect, a 16" extension of the 36" chord line on a NACA 0012 airfoil. I don't know the accuracy, but Curtis was pretty thorough, and got input from highly experienced engineers on his designs.


    BJC
     
  3. Mar 3, 2017 #3

    cluttonfred

    cluttonfred

    cluttonfred

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    Thanks, BJC. I am just thinking about the pitot-static for a Volksplane so hardly need CAFE-level accuracy. The reason for the question is that I was looking at the Blue Fly Vario and it turns out that Alistair uses a pitot-static probe intended for drones and big model aircraft. It's just 5" long, so only about 3-1/2" would actually project beyond the wing leading edge and I was wondering if that would be enough.
     
  4. Mar 3, 2017 #4

    TFF

    TFF

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    3-4" is too close; 9-12" at least. Not a fan of the snag factor of the leading edge probes. I would go under the wing.
     
  5. Mar 3, 2017 #5

    BJC

    BJC

    BJC

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    The CAFE link was just to show an example of what it takes to get the inlet out close to the free air stream.

    I agree with TFF that 3 1/2 inch is too short. I have seen homebuilts with removable pitots to avoid the problem of people bumping into them. I would GUESS that you would get decent measurements with the pitot 8 or 10 inches ahead of the LE, but is accuracy really important if you have repeatibility? I would be very easy to have a small aluminum or plastic block on the front of the spar that the pitot screws into, a la the Pitts, and start with a long pitot, test it, unscrew it, shorten it, and repeat until it has been shortened to the length that satisfies your needs.


    BJC
     
  6. Mar 3, 2017 #6

    cluttonfred

    cluttonfred

    cluttonfred

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    Thanks, the removable pitot (only) with separate static source is already in the plans, just a removable aluminum tube press fit into a pine block (see below). But I was thinking of being lazy and using a single combined pitot-static instead. And now that I look at it more closely, Bud Evans says 6" so there you go. ;-)

    59.jpg

    PS--The static vents on the plans are installed left and right on the rear fuselage forward of the rudder. They are just as simple, tubes press fit into pine blocks glued in place. There is an addendum to the plans that points out the problem with wasps plugging the holes with mud. The recommended solution is to cut two small circles out of beer can aluminum, punch a few small holes in each, and glue them over the vents.
     
    Last edited: Mar 3, 2017
  7. Mar 3, 2017 #7

    FritzW

    FritzW

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    I thought about mounting one of the "L" shaped pitot/static tubes on the front left spar attach inspection hole cover...

    15144s.jpg ...putting one of these

    VP-2 36 crop.jpg ...here

    The "per the plans" static ports are easy enough but they're just more things you have to mess with (gluing the beer can material between the block and the fuselage skin and still keep all the holes lined up, making sure there are no low spots where condensation can pool up, running the static line through the cockpit without it looking hokey, etc...)

    If you go "per the plans" keep a few extra tubes handy, they do get bent. (FYI a soda straw with some paper napkin tucked in around it works in an emergency) :gig:
     
    cluttonfred likes this.

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