Heated Pitot tube

I am planning on certifying my airplane for IFR (Instrument Flight Rules) flight, so I decided to equip it with a heated Pitot. 

For those unfamiliar, let’s just say that the Pitot tube will sit on the nose of the plane and capture the incoming air pressure. Since this dynamic pressure relates directly to the speed of the plane, a flexible hose will connect the Pitot tube to the anemometer (air speed indicator), which in turn will display how fast the airplane is flying through the air. The airspeed indicator will also connect to the static ports, but that's a topic for another time (more on it here). 

Although flying into icing condition is to be avoided at all costs  due to its many cumulative negative effects, flying in the clouds is the first ingredient for an unintentional encounter with ice. 

Ice forming on the Pitot probe, and obstructing the hole through which the air pressure is sensed, would render the anemometer useless. One very effective way to combat this undesirable situation is to heat the probe from the inside, so I set out to build myself a heated Pitot tube, borrowing generously from other designs I found online.

Brainstorming with my friends Mike and Wade was very helpful, and allowed me to explore many ideas I wouldn’t have thought of by myself. They in turn got excited about what I was trying to do, and expressed interest in having one as well, and in the end I decided to make not one, but three.

Pitot number 1 will go to Mike, since he’s at the stage in his build where he can install it. The pictures and dimensions in this post are those of the tube I designed for him.

Fully assembled (heating element omitted)

Exploded view of the Pitot main components (heating element omitted)

Pitot "business end" dimensions

Pitot back cap dimensions

I ordered the aluminum tubes and rod...

Raw material

... and the G10 phenolic heat insulating material, and went to work. For the record, I was trying to upgrade to G11, which is rated to 500℃, but I would have had to order $500 worth. So, G10 it was!

G10 insulator

I started working on the front piece first, using a tool called Ball and Radius Turning Tool.

Shaping the nose cone

Overall it turned out pretty much like I had envisioned it in CAD.

Nose cone 3D presentation

Installed on the outer tube

Next, I set to work on the back piece, and decided to mill 6 flats on it. This feature will allow me to tighten the finished Pitot using a 15 mm wrench, as well as tighten the quick release fitting using an additional wrench.

 

Tail piece 3D presentation

Mill setup for creating the bolt-like pattern on the end piece

First 2 faces milled

All 6 faces milled, and through hole drilled

Testing for size

The small hole was enlarged and tapped (NPT 1/8 - 27), and a quick release vacuum fitting installed.

Back piece ready for mounting

Installed on the outer tube

This is my first time working on a 0.75” (1.9 cm) aluminum tube of such length, as well as with the 1.25" (3.2 cm) thick G10 insulator, and I needed to update my lathe to a bigger chuck.

So, I replaced my 3" chuck with this 5" monster! 

This is a BIG upgrade to my little lathe

Chuck completely disassembled and cleaned up

Spindle hole in plain sight (old chuck removed)

New chuck in place

The new chuck, with its bigger through hole, allowed me to shove a whole foot of tubing through the spindle hole, and out the other side of the machine, enabling me to work on one end of the tube with much greater setup rigidity. 

First thing I used it for was boring the outer tube by about 10/1000" (0.025 mm), to a depth of 3.5" (8.9 cm). This created a cylindrical receptacle to house and retain the heating element in the very front of the tube, while allowing it to slide in and out easily.

Boring bar enlarging the hole

Later I chucked the inner tube, brought it down to the proper length, and cut threads on both ends (1/4-28).

Forming the threads on the inner tube

1/4 - 28 thread

Testing the threads with the nose piece

Although the G10 is there to protect the nose cone from excessive heat, keeping the heating element outside of the nose altogether seems like a better idea.

The amount of Pitot tube that will remain outside of the airplane nose structure, being cooled down by the airflow, is predetermined by the size of the Pitot nose piece, plus the heating element.

Amount of Pitot tube to keep out of the airplane's nose

The last major operation to perform on the outer tube was milling a slot for the wires.

How to align the spindle axis with the tube axis

1/4" mill ready to cut the slot

Wire slot milled

So, here is a sketch of the master plan...

Basic layout

... and the finished Pitot tube.

Finished Pitot

A view from the front of the airplane

As seen from inside the nose cone

6.14 oz (174 gr) including G10 insulator, 4.44 oz (125 gr) without.

Here are all the components of the heated probe together.

Disassembling the Pitot

Heated Pitot tube components

The only thing left to do was an operational check, to make sure it worked. 

Testing the heated Pitot tube

As you can see in the video lack of heat is certainly not an issue. On the contrary, the voltage might need to be stepped down some, in order to be able to control the temperature, which was increasing rapidly. 

I'm thinking a 3 way switch with OFF, LOW, and HIGH settings perhaps, but more testing is in order. At the very least, I have to find out how hot the probe really gets, and I will definitely need a better thermometer for that.

Testing the heating element

Using the wind chill formula published in this website, it would appear that I cannot rely on airflow for cooling the probe. Using 200 mph and an outside temperature of 32℉, the Pitot tube would "experience" an air mass of 4℉, for a 28℉ drop in temperature. This figure does not include the additional cooling due to evaporation/sublimation of moisture, but even if this added up to a 50℉ drop, I would still be an order of magnitude away from a manageable probe temperature.

Further increasing the speed is not an option, and and even if I could, it would rapidly become counterproductive since at speeds above 250 kts friction raises the surface temperature by roughly 1℃ every additional 10 kts. I take advantage of this effect often at work, to manage the fuel temperatures of Jet A when it gets close to reaching the freezing point (-41℃). 

The element is obviously putting out excessive heat. I need to find a final solution to this problem, and it is going to be an electrical one. 

Meanwhile, Mike has received his tube, and he seems really pleased of how it has turned out.

If that is not a happy face, I don't know what is!

The big picture

Nose detail

I will be working on Wade's Pitot tube next. His might be slightly different, as he is interested in making it retractable while parked, to prevent people from accidentally stepping or tripping on it.