Pitot Tube

August 29, 2010

Now that I am working on the left wing, the first order of business is to run the line for the Pitot tube for the airspeed indicator.  The following is a very simple and light way to install the Pitot tube system.

Here are the parts needed for the system:

Shown above are 20' of 1/4" Poly-Flow tubing, a 2' length of 3/8" x .058" aluminum tube (6061), a Brass Tee fitting threaded for 1/8" pipe thread, the end fitting for the Poly-Flow tubing with 1/8" pipe thread, and some nylon bushings.

The 3/8" x .058" aluminum tube will become the Pitot tube mast. It will protrude from the wing's leading edge near the wing lift strut location. The Poly-Flow tubing will run through holes and bushings in the nose ribs, running from the wing root to the inspection hole that is forward of the wing spar. To drill the holes in the nose ribs, I used a Unibit on my air drill to cut the proper sized holes for the Nylon bushings:

The air drill was needed as it was the only thing I had that was small enough to fit between the nose ribs of the wing. To cut the holes, I had to reach in through each lightening hole in the spar:

This is fairly tight fit and you have to reach inside and just feel your way around.  

Each hole needs to be located about 1/2" from the lightening hole in the nose rib and clear of the spar attach angles. Here is a shot inside the nose rib area showing the holes with bushings installed:

Next, the Poly-Flow tubing was threaded through each bushing, until it reached the inspection hole at the wing strut attachment location.  Here is at the wing root rib:

 Now, with the Poly-Flow tubing in place, its time to install the Pitot Tube mast.  After I took that first picture on this page, I visited the local Lowes department store was able to find an "L" brass fitting that was more suited to the job and lighter than the "Tee" fitting.  The Pitot Tube will thread into one side of the "L" fitting and the Poly-Flow tubing will attach to the other side.

The "L" fitting must somehow be attached to the wing spar web to hold the Pitot Tube Mast at the proper angle. I made an attachment strip out of some .032" thick 4130 steel. The attachment strip was 3/8" wide by 1 3/4" long. It was cleaned thoroughly and then brazed to the "L" fitting as shown here:

After it was brazed, I took a file to the fitting removed a whole bunch of excess weight.  It was a bit thick and bulky as-is, so I rounded off the corners and removed a lot of material.  After-all, this is not a high pressure application and we are trying to make a light airplane. The "L" fitting was then primed and painted to prevent corrosion of the steel attachment strip.

The 3/8" tube that we are using for the Pitot tube mast was threaded on one end with 1/8" pipe threads.  

Using a Unibit, a 3/8" hole was cut into the leading edge of the wing at the correct location and the threaded end of the Pitot tube was inserted.  The brass "L" fitting was then threaded onto the Pitot tube mast and pushed back against the wing spar web.

To make sure the Pitot tube mast was perpendicular to the wing's leading edge and at the correct angle, a 6' long metal straight edge was laid along the rivet line of the rib and a weight was placed on it to keep it tight to the bottom skin of the wing (note: the wing is laying on the table bottom-side-up in the picture):

Note that the pitot tube is aligned with the straight edge (which is on a rivet line) and is parallel with the bottom of the wing.  Slight adjustments can be made later by bending the tube as needed, but I figure this is a good starting point.

With the pitot tube mast properly located and aligned, holes were drilled through the mounting strip on the brass "L" fitting and through the wing spar web.  It was then attached to the spar web using some #8-32 screws and nylon lock nuts. Here is a picture looking through the inspection hole showing the brass "L" attached to the wing spar web, and with the Poly-Flow tubing attached to one side of the "L" and the Pitot Tube attached to the other:

I used some teflon tape on the pipe thread portion of Poly-Flow fitting but not on the Pitot tube yet as it will be removed and stored until the wing is finished and installed on the airplane.

Here is the leading edge of the wing with the Pitot tubing sticking out:

That's it for now.  This will be removed and taped inside the inspection hole so I won't lose it.  It won't be attached again until the wings are completed, painted and installed on the fuselage for the final time.

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