Static Port Conundrum

This entry is part 60 of 67 in the series 13 - Electrical / Instruments

The static port serves two primary functions: 1) It provides the ambient air pressure which the altimeter uses to determine the altitude. 2) It provides a reference pressure for the pitot tube to determine the airspeed.

The static port called out in the manual is built entirely by the builder.  I elected to purchase a pair of static ports from Aircraft Spruce similar to the static ports on my Cessna 182-RG.  The reason for having two is to correct the static pressure when the airplane is in a slip or skid. If the airplane is in a slip or skid and you have only one static port, then it could be in either a high pressure area or a low pressure area.

When I flew my plane (solo) for the first time, I was elated at the indicated and true airspeed. Indicated (IAS) is what the airspeed indicator shows and true airspeed (TAS) is your actual speed through the air.  To determine true airspeed, you take the indicated airspeed and compensate for temperature and altitude. The bottom line is that I was hauling ass!

But then I noticed my groundspeed. Normally when you’re flying, there’s either a headwind or a tailwind. If your true airspeed is 190 knots and there’s a 10 knot headwind, then you’ll only be traveling 180 knots over the ground.  But when I looked at my groundspeed while traveling south, it was showing about 20 knots slower than my TAS. Okay, I’ve seen 20 knot headwinds more times than I can count. After I turned around, my groundspeed was still about 20 knots slower than my TAS.

Now that’s peculiar.

The next couple of flights, I did a tests of the airspeed. This is accomplished by flying at least three different headings (usually greater than 90 degrees), noting the groundspeed on each heading and by using a formula you can determine your actual TAS along with the current wind speed and direction. What I discovered is that at cruise speed, the airspeed was reading about 25 knots faster than I was actually traveling.

But the slower I got, the closer the TAS got to the actual airspeed.

Now if it were reading slower than actual, that would point to a leak in the pitot system. But faster?!?!

After a lot of thought, I came to the conclusion that the static port must be drawing a vacuum. Since airspeed indication is a result of the pressure of the air being forced into the pitot tube compared against the static air pressure, it seemed likely that if the static pressure were less, the airspeed would read high.

I talked this over with Scott and Rick at the builders center. Scott said that he’s had to put “trip strips” in front of the static port to disrupt the air because it was pressurizing the port but he’s never had a static port effectively de-pressurizing before. He suggested putting a small piece of stir stick behind the port. This should stop the vacuum by pressurizing the port. Once it’s determined that was the problem, then the thickness could be adjusted.

So I taped over one of the ports (easier to test with one port), hot-glued a short length of stir-stick behind the static port and went for a flight.

Now the airspeed is reading about 15 knots low at cruise. This would seem to prove the concept.

But… if the static port were drawing a vacuum, then altitude would be showing higher than actual altitude (less pressure the higher you fly). And if the static port were being pressurized, then the altimeter would be indicating lower than actual.

This caused the hair to stand up on the back of my neck.  Because if I’m flying along at what I think is 6,500′ (westbound VFR altitude) and the static port is being pressurized, then I could actually be flying at 7,000′ (eastbound IFR altitude). But air traffic control would keep me from bumping into someone else, right?  Nope.  ATC thinks I’m at the altitude that my transponder is sending… which is what my static port pressure says it is… which is wrong.

So I got back on the ground to figure this out.

After talking with Rick and Dale (sounds like a 60’s surfer rock duo, doesn’t it?), I decided to build a manometer. AKA, pressure sensitive water level. So I removed the stir stick and hooked up my home made manometer.

Here you go. Tell me that doesn’t scream “Experimental”.


One end is connected to the static port (right tube), the other is open (if you already see the problem, you’re way ahead of me. But I figured it out at Sandy’s Grill over a cider that night).

Sitting on the ground, I made marks to show level and a few other reference marks. When air moves over the static port, if the water level in the left tube goes down, then the port is in a vacuum. If the level goes up, it’s being pressurized.

So here I am at 196 knots and 160 knots over the ground.



So I decided to change the shape of the static port from a flat disc to a dome (I’m obviously getting tired at this point) and went up again.


Even more of a vacuum!?!?

That’s when I decided to call it quits.

Because it was Thursday night, I went to Sandy’s Grill (Thursday is steak night there). While I was there thinking about things over a steak ka-bob and an apple cider, I had a couple epiphanies. The problem with the manometer is that it was measuring the static pressure relative to the pressure in the cabin. Which is typically lower than the outside ambient pressure to begin with and at this point is a completely unknown variable. Which means that was a pretty useless experiment to begin with.  The only way a manometer reading could be valid is if I could locate a true undisturbed ambient pressure location for the other end of  the tube.

Second was the modification to the static port. But making it domed shape, I effectively created an airfoil. Like the top of the wing. Which is a low pressure area. Which means it would be creating a vacuum. That is why it showed an even greater vacuum.

So I decided to approach it as simply as possible. There are two primary sensing instruments based on the pitot/static system: altitude and airspeed. If there are no leaks and the instruments are calibrated, then the only variables are the static port location and the pitot tube location. I was careful to locate the pitot tube where the manual specifies which obviously leaves the static port position as the problem.

So all I need to do is get one of those two instruments reading correctly and the other will have to be correct. Because I can’t determine my altitude with any accuracy (without something like a radar altimeter), then I’ll just have to tweak the static port until the airspeed reads correctly. Once the airspeed is showing the correct speed, the altimeter would have to be accurate as well.

Now this makes some pretty big assumptions. I didn’t skimp when I purchased the pitot tube (it cost me about $500) so I’m comfortable it should be creating the correct ram air pressure. I checked the system for leaks so I know that’s good. The last variable is whether the GRT Air Data Computer/Attitude/Heading Reference System (AHRS) is correctly computing the airspeed. The AHRS is a rather complicated piece of electronics and at this point I have to trust it’s doing it’s job.

And finally, since the changes I’ve made to the static port created variations that are expected predictable, then I’m confident that’s where the adjustments need to be made.

So I removed the static port, got a file and started filing down the leading part of the port. I only took able 1/16″ of an inch off (right is facing the front of the airplane).


Then I hooked it up and went flying.

When I crunched the numbers, the indicated airspeed was about 15 knots high.  I made a couple more passes with the file and did another flight. Now it was reading about 10 knots high.

So next I’ll make a couple more passes with the file and try again… and again… and again.  Basically sneaking up on it because I don’t want to go to far. When I get it to where the TAS equals the groundspeed (compensated for winds aloft), then the altimeter should be reading correctly as well. The only way that I can think of to test that is to make a high-speed low pass at the airport. If I do that at a visual height of 50 – 100 feet, then the altimeter should show that I’m flying at the field elevation plus 50-100 feet.

But for now, that will be one of the first things I do when I get back to Panama City.


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