AOA Probe on Piper Pitot Mast

[rvsxer]

Like many of you, I used a Piper pitot/static mast because it was a cheap route to a heated pitot. I soon found the static source was no good (see related threads) and added the Van's recommended static system. Recently I got the idea of using the static port on the mast for an AOA differential sensor for my Dynon D10A similar to what Mel Asberry is using. These pictures show the result. I used .125 OD 5052 aluminum tube, inserted 1" or so into the drilled out static port. JB Weld to secure it. It is bent to approximate the relative angle used by Dynon on their AOA/Pitot. A shorter tube may have been okay but I wanted to make sure it got clean air. The old static tubing was then connected to the AOA port on the D10A. It seems to work after the recommended cal procedure. I will need to fly more to confirm it. By the way, the probe is no longer heated so it won't affect the adhesive.

[rocketbob]

Those Piper pitots are infinitely tunable to make them work well as a static source. If the airspeed reads high you just have to cut the bottom steeper to increase static pressure, which will lower indicated airspeed. And vise-versa. The tube you have here approximates doing exactly the same thing.

[rvsxer]

Originally Posted by rocketbob

Those Piper pitots are infinitely tunable to make them work well as a static source.

Sounds like I gave up too easily...

[rocketbob]

Yep

Just think driving along on a summer day, hand out the window...what happens when you change the AOA of your hand...

[rv7boy]

Originally Posted by rocketbob

... cut the bottom steeper to increase static pressure...

There is a difference between "pressure measured by the static port" and "static pressure." Rocketbob, I think in your rush you meant to say "by cutting the bottom at a steeper angle, the pressure measured by the static port increases."

The Pitot port measures TOTAL Pressure and the STATIC port measures STATIC Pressure. Subtract Static Pressure from Total Pressure and you get Velocity Pressure. (An Air Speed Indicator converts this Velocity Pressure measurement into a number (knots or MPH) which our brain understands.)

By cutting the bottom of the Piper mast at a steeper angle, the static port will measure not only the Static Pressure but also some fraction of the Velocity Pressure. The Static Pressure reading is higher, but the Static Pressure surrounding the aircraft is the same. The measurement of Static Pressure should be the same all around the aircraft as long as the Static port is not measuring any of the pressure caused by the airflow (velocity pressure). That's one way measurement errors occur.

As Mike, the original poster, indicated, another port for Static Pressure for other instruments such as Altimeter should be used, as the pressure measured by his new tube in the Static Port is now measuring more than just Static Pressure.

The AOA probe shown by Mike, Mel and others works by measuring the angle at which the Pitot port measures more Static Pressure than Total Pressure AND the "Static" port measures more Total Pressure than "Static" Pressure, not necessarily "absolute" but certainly "relatively." The two ports reverse roles as AOA is increased. By bending the probe slightly into the airflow, the tubing probe Mike made measures more Total Pressure when the aircraft is near a stalled airflow condition. (In the above, "Static" implies Static plus some fraction of Total pressure, as decribed in the paragraph above.)

[rv7boy]

One question for Mike (rvsxer):

Will you be able to bend the tubing without removing the mast from the wing? I'm thinking you might have to make some fine adjustments, and it may be hard to do while it's on the airplane. I do note your tubing is aluminum so it may be easier than I first thought. The Lift Reserve Indicator mast uses an adjustable mount which appears to be easy to adjust. Its ports are at 90 degrees to each other; that means both ports will be moved when it is adjusted.

Keep us posted on how well this works. I'm interested because I bought a used Piper mast and hadn't thought about using it for an AOA probe until I saw yours.

[rmartingt]

I hope thread necromancy isn't frowned upon...

I'm at work so I can't view the pictures right now, but has anyone tried doing this (making AOA work) without adding additional tubing? Now that my wings are here I probably need to make a decision soon on a pitot system, and using an old heated Piper blade seems like a wonderful solution--at least, much better than shelling out $550 for the Dynon probe and mount.