Pitot (Prandtl) Tube

Let me start off by saying, it's probably much easier to just buy a pitot tube. But one of my major motivations for a wide variety of my hobbies is the voice in my head that says, "I can do that, and I can probably do it better*." So in the spirit of not accepting the status quo, I decided to see if I could make my own without buying any new tools.

Let me also explain that a wide variety of homebuilt aircraft builders find inexpensive ways around all sorts of problems, and this is one I've seen a lot of creativity with. From just soft aluminum tubing to VW pushrods to all manner of "I had this laying around the house" DIY, it's been proven that with a little trial and error, you can make almost anything work if you try hard enough. However, due to the speed at which this plane travels, and the amount of trouble I would be in if my airspeed indicator decided to stop working due to a bent/broken pitot tube, I felt it a worthwhile pursuit to build a very sturdy, very likely to be accurate structure. I didn't design anything new, just copied other designs that made sense to me, focusing first on strength and second on aerodynamics. With that out of the way, here's what I did.

I determined that 2024 aluminum was a good material to use, as it's quite strong and still quite light. Also, because it's "just aluminum," I was willing to try turning it on my wood lathe. I've turned wood for about 7 years, which certainly doesn't make me an expert, but I've had the pleasure of playing with a lot of different species. Aluminum is much easier to turn than many of them. It takes much longer to shape, but there's no grain to it, so I had no troubles turning it with even my cheap lathe chisels. I started with a 3/8" rod, cut it to length, trued up the ends in the lathe, then drilled a 3/16" hole through it, and finally turned one end to a point. This would have worked fine, but...

One of the difficulties with an airplane is, you're moving through the air. The difficulty comes in figuring out just how far away the ground is. We know that, as a general rule, air is less dense the higher we are. The altimeter in an airplane counts on that fact to display its altitude. An altimeter measures air density by measuring air pressure, but where do you take that measurement? Inside the airplane isn't a safe bet, as air currents can vary wildly and cause massive pressure shifts depending on airspeed, air vents, and where you put your flight bag. Outside the airplane, the air is moving very rapidly, and if the static air source is tipped slightly into *or* slightly away from the direction of airflow, your reading will be very very wrong. One of the more common methods is to place your static source, or port, on the side of the fuselage, where the port will be perfectly perpendicular to the airflow. If you fly the airplane in a slip or skid, however, your measurement will be off again, so you need another port on the other side of the plane so the measurements can even out. One of the more elegant solutions to this is called a Prandtl tube.

A Prandtl tube combines the functions of a pitot tube and a static port, and does so in a way that, theoretically, is less prone to error under normal conditions. Do other methods work? Just fine, as a matter of fact. However, for this airframe, a number of people have used a number of methods, and it seems that what works for this guy causes major problems for the next guy. A Prandtl tube should work on any airframe, as long as it's placed in a free airstream. I stole a diagram from Wikipedia:

Pitot and static, measured from the same spot. How to accomplish this with minimal effort?

First, I got a 5/8" 2024 aluminum rod and turned the shape. Next, I drilled it down the center with three drill bits: 1/8" bit from the front straight back about an inch, 3/16" bit from the back all the way to the start of the 1/8" hole, and 3/8" from the back about an inch shallower than the 3/16" bit.

Next, I drilled 4 1/16" holes about 2 inches back from the tip, each 90* from each other, so I ended up with a hole in the top, bottom, and each side of the tube. I then took some 3/16" OD aluminum 3003 soft tubing I had laying around (after quite accidentally ordering 25 feet of the wrong size) and floxed a piece all the way down into the 3/16" hole, and then floxed another piece about an inch inside the large hole, then used more flox to seal the large chamber. What I ended up with looks about like this:

The red is the tubing, the green is the flox. Only two of the 4 holes are shown. Once that all cured, I hooked it up to my airspeed indicator and altimeter and took a drive, holding the Prandtl tube out the window. It seemed to work poorly, until i disconnected the altimeter. Turns out the altimeter leaks like a sieve, but the Prandtl tube works like a charm. I was getting very consistent readings, even angling the tube 15 degrees or so from straight on into the wind.

Ok, so it works. Now all I had to do was build a strut to hold it ~4" below the leading edge of the wing. To accomplish that, and because I was feeling picky, I printed off a low drag NACA airfoil (NACA 0021, I believe) and cut out two wood templates, glued them onto a piece of foam, and sanded that foam to shape. Then I glassed the foam, drilled a few holes to attach the Prandtl tube and to allow the two 3/16" tubes to run up the center of the foam, and glassed that all together.

After it had all cured, I put on a couple more layers of micro, building up the trailing edge, fairing in the transition between strut and Prandtl, and then sanded it nice and smooth. It'll stay that way until I'm ready to attach it to the plane. (Obviously the last picture is not sanded smooth. I'll update this with another picture once it's finished and attached.)

*Better, more often than not, means good enough for less money, or better for my specific application. If I really thought everything I did was better, I'd be a businessman (and probably a very, very broke one at that.)