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Static Port Lesson Learned

lluciani

Active Member
This might be obvious to most folks here but it also might be useful to some. I have been flying my RV14A for almost 300 hours now and one thing that has bugged me the whole time was that the wind vector shown in my G3X display was always wrong. No matter what heading I flew, it almost always indicated a headwind. I even flew a rectangular course several times and proved this wasn’t just my negative thinking.

And on the ground, every time I started the engine, airspeed would jump up to over 20kts. Once for a test, I started the plane with the pitot cover in place and still had the same results. So the problem was not on the pitot tube side. I then disconnected the static tubing from the AHRS, went flying and there was no change. This seemed to indicate a leak in the static system but I checked it out and there were none.

So I started to suspect the port itself. The epiphany moment (happens a lot with me over little things) happened when I blew compressed air down the side of the fuselage tangent to the static port (with tape over the one on the other side). The airspeed display on the G3X registered 20kts.

I replaced the static port with one that was completely flat (from Spruce - intended for a Cessna) and the problem went away. The same test this time registered 0 kts. Yes!

The static port I had originally installed was really nice looking but had a slightly rounded dome in the middle. This is apparently enough for Bernoulli action to create a vacuum causing a greater delta-P between the static port and pitot tube, and showing an airspeed that was too high.

Here’s a picture of one of the two static ports that was causing problems. Notice the slight dome shape. It makes a big difference.

IMG_3829.jpg

Maybe I should have just used rivets as static ports as Vans calls out and saved myself the aggravation. I hope this is useful to someone seeing the same problem.
 
This might be obvious to most folks here but it also might be useful to some. I have been flying my RV14A for almost 300 hours now and one thing that has bugged me the whole time was that the wind vector shown in my G3X display was always wrong. No matter what heading I flew, it almost always indicated a headwind. I even flew a rectangular course several times and proved this wasn’t just my negative thinking.

And on the ground, every time I started the engine, airspeed would jump up to over 20kts. Once for a test, I started the plane with the pitot cover in place and still had the same results. So the problem was not on the pitot tube side. I then disconnected the static tubing from the AHRS, went flying and there was no change. This seemed to indicate a leak in the static system but I checked it out and there were none.

So I started to suspect the port itself. The epiphany moment (happens a lot with me over little things) happened when I blew compressed air down the side of the fuselage tangent to the static port (with tape over the one on the other side). The airspeed display on the G3X registered 20kts.

I replaced the static port with one that was completely flat (from Spruce - intended for a Cessna) and the problem went away. The same test this time registered 0 kts. Yes!

The static port I had originally installed was really nice looking but had a slightly rounded dome in the middle. This is apparently enough for Bernoulli action to create a vacuum causing a greater delta-P between the static port and pitot tube, and showing an airspeed that was too high.

Here’s a picture of one of the two static ports that was causing problems. Notice the slight dome shape. It makes a big difference.

View attachment 35153

Maybe I should have just used rivets as static ports as Vans calls out and saved myself the aggravation. I hope this is useful to someone seeing the same problem.

Hard to tell from that pic but it looks like the Cleveland copy of the vans pop rivet.
I can attest to those being almost perfect using the NTPS PEC method on both RV7s and RV10s.
Have you done any static error testing other than looking at wind aloft?
The reason I ask is that in practice most people find the flat “Cessna style” ports are much more likely to cause significant Position error.
I would be very surprised if your position error was low now with flat ports.
Cheers
 
It's hard to tell from the picture, but it looks like you had the Cleaveland version of the static ports. It's also hard to tell, but it looks like the entire piece was mounted outside the airframe (with only the connecting tube inside the airframe) rather than just the domed part sitting proud of the outside skin (and the flat area on the inside of the skin). I say this because the flat part looks like it has finish paint on it.

I also have the Cleaveland ports and I nearly drove myself mad finding a static leak. I did have a static leak and once fixed, the real world tests now show a one knot error.

I believe you when you say that you had no static leaks. I'm just wondering if how you had the ports mounted changed the airflow performance versus having only the domed part extending above the skin.
 
It's hard to tell from the picture, but it looks like you had the Cleaveland version of the static ports. It's also hard to tell, but it looks like the entire piece was mounted outside the airframe (with only the connecting tube inside the airframe) rather than just the domed part sitting proud of the outside skin (and the flat area on the inside of the skin). I say this because the flat part looks like it has finish paint on it.

It sure does look that way, like they were installed wrong, which would surely give erroneous readings.

The dome head "rivet" port has proven to be pretty accurate.
 
Just as others have said, that static port has been the most accurate that I have used. I have done the 3 way GPS run and my TAS is within a fraction of a knot in all speed envelops (pattern, cruise and top speed)
I used to have a flat face static port in my RV7 and my speed was not accurate at all.

P.S. In fact, if you are sure you are not going to use them, I will be happy to buy them from you.
 
It's hard to tell from the picture, but it looks like you had the Cleaveland version of the static ports. It's also hard to tell, but it looks like the entire piece was mounted outside the airframe (with only the connecting tube inside the airframe) rather than just the domed part sitting proud of the outside skin (and the flat area on the inside of the skin). I say this because the flat part looks like it has finish paint on it.

I also have the Cleaveland ports and I nearly drove myself mad finding a static leak. I did have a static leak and once fixed, the real world tests now show a one knot error.

I believe you when you say that you had no static leaks. I'm just wondering if how you had the ports mounted changed the airflow performance versus having only the domed part extending above the skin.

I agree.
It looks like they were installed incorrectly which means that what you changed to made it slightly better but it is likely still a long way from providing an accurate static source.
 
9 Knot issue

This install created a 9-knot error. After correcting 1 knot. Cleveland recommends painting yet I've seen very few done this way.
 

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For these aftermarket static ports, my understanding is that only the small (1/4" or so) nub is supposed to stick out through the skin. The large flange attaches onto the inside of the skin. I used proseal to attach mine.
 
I have the Dynon static port and it looks similar to the Cleveland port in the picture. The Dynon instruction has the static port mounted from the inside and only the small dome head is exposed to the outside. The large circular flange is attached inside the skin using proseal or equivalent.
 
Boundary layer

I think the static port needs to run in the boundary layer. If it is pushed out, it will be in free stream flow and probably act more like a suction sensor.
 
Static port

That's what shown in the instruction video for the Cleaveland static ports: https://www.youtube.com/watch?v=6duzMKyRkMw

That's how I did it but mine is the SafeAir fitting.
Fancy fitting is inside both prosealed and riveted to the side skin. It ends up flush to the outside skin. Vans rivet port is installed from the outside with proseal. Static lines route from each side up to the top then "T" and one goes back down toward the ADAHRS. Fitting is NPT.
 
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This might be obvious to most folks here but it also might be useful to some. I have been flying my RV14A for almost 300 hours now and one thing that has bugged me the whole time was that the wind vector shown in my G3X display was always wrong. No matter what heading I flew, it almost always indicated a headwind. I even flew a rectangular course several times and proved this wasn’t just my negative thinking.

And on the ground, every time I started the engine, airspeed would jump up to over 20kts. Once for a test, I started the plane with the pitot cover in place and still had the same results. So the problem was not on the pitot tube side. I then disconnected the static tubing from the AHRS, went flying and there was no change. This seemed to indicate a leak in the static system but I checked it out and there were none.

So I started to suspect the port itself. The epiphany moment (happens a lot with me over little things) happened when I blew compressed air down the side of the fuselage tangent to the static port (with tape over the one on the other side). The airspeed display on the G3X registered 20kts.

I replaced the static port with one that was completely flat (from Spruce - intended for a Cessna) and the problem went away. The same test this time registered 0 kts. Yes!

The static port I had originally installed was really nice looking but had a slightly rounded dome in the middle. This is apparently enough for Bernoulli action to create a vacuum causing a greater delta-P between the static port and pitot tube, and showing an airspeed that was too high.

Here’s a picture of one of the two static ports that was causing problems. Notice the slight dome shape. It makes a big difference.

View attachment 35153

Maybe I should have just used rivets as static ports as Vans calls out and saved myself the aggravation. I hope this is useful to someone seeing the same problem.

That Cleaveland port should be installed like this (see pic) - if the large flange was on the outside of the fuselage, sure, there would be a huge error.

_Alex
 

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Yes, without a doubt, the Cleveland port needs to be mounted from the inside and flush outside. And they work very well. But, airspeed and altitude accuracy also depend greatly on location, specifically they need to be located at a point of neutral pressure, i.e. there is a pressure gradient from above ambient to below ambient along the fuselage, and the easiest way to get accuracy is locating the ports at the neutral point (actually a line, on which the vertical position matters also for accuracy across varying airspeed due to angle of attack variation.

I spent a bunch of time on this when I located my static ports and for what it's worth, wrote a Kitplanes article on it "Static Port Location and Altitude Calibration":
https://www.kitplanes.com/static-port-location-and-altitude-calibration/

Reinhard Metz
 
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Yes

This static port is installed incorrectly, it should be on the inside...

Just showing an example how it should not be done with pictures. (It was not my airframe) The adhesive needs to go on the outer ring and not be allowed to extend into the inner ring. No primer on the inside. They do recommend painting the outside dome region and placing a toothpick in the hole, but I see few done this way. (Including mine)

Screenshot 2022-12-14 205853.jpg
 
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Thanks. That makes total sense. Yes, I did mount them on the outside of the skin.

I am considering cleaning them up and remounting them correctly on the inside if the Cessna style ones don’t work out. But I went on a test flight today with Cessna style and they worked like a dream. The indicated wind vector was the same regardless of heading. Stall speeds were a couple of knots slower in all configurations and consistent with Vans’ numbers. I need to update my POH.

It was windy day today. Seeing that much of a tail wind filled me with joy.

537698DF-D500-4040-94D1-14E30DCAD74A.jpg
 
Thanks. That makes total sense. Yes, I did mount them on the outside of the skin.

I am considering cleaning them up and remounting them correctly on the inside if the Cessna style ones don’t work out. But I went on a test flight today with Cessna style and they worked like a dream. The indicated wind vector was the same regardless of heading. Stall speeds were a couple of knots slower in all configurations and consistent with Vans’ numbers. I need to update my POH.

It was windy day today. Seeing that much of a tail wind filled me with joy.

Luis,
Years ago when it first became popular to use after market static ports, the ones that were being produced were flat. Just like a Cessna port.
It didn't take long for people to figure out that the shape of the blind rivet used in the simple static port kit sold by vans was important. Everyone using the flat ones had a static error.
That is why the ones you just removed have a bump produced in the shape, so that when they are installed correctly, it protrudes beyond the surface of the skin and then mimics the head of the previously mentioned blind rivet.

You may think that you now have a great static source, but as I mentioned in my other post, that is only because your other one was so very bad.
Unless you are the only person on earth to have the flat port produce a great static pressure on the sides of the aft tail cone on an RV (who knows, maybe the discontinuity you probably have now because of the paint ridge around the perimeter of your just removed static port is compensating) it is highly likely that with some simple testing you will find that you still have a noticeable amount of error.... just not nearly as much as you had before.
 
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My original static ports from SafeAir were flat on the boss that extended beyond the skin. I had a large static error. I shaved them flush with the skin and then drilled and installed the Van's pop rivet with the domed head and the issue went away. I think they later changed the design to the domed head.
 
So If I can introduce another aspect of static air into this discussion. What is everyone doing for and alternate air source?
 
I did one of these in the panel with a red flip cover over it:

https://www.steinair.com/product/pitot-static-toggle-switch/

One tip about these valves, if you grind or mill a channel in the threaded portion you can use a key washer rather than a star washer to prevent it from rotating:

20081109_static_valve2.jpg


Don't forget to follow best practice by labeling it:

20201121_placards3.jpg
 
Static ports: what design and where to put them....?

Airspeed and altitude accuracy also depend greatly on location, specifically they need to be located at a point of neutral pressure, i.e. there is a pressure gradient from above ambient to below ambient along the fuselage...

It might be that the location is more important than what design is used, although it seems like both are important. I have the (probably: they have been there for 25 years) Van's old flush ports and all of my static testing was spot-on. It might be a combination of luck and placement. Different aircraft are going to have different pressure areas. I think (that was a LONG time ago) I put them where Van's recommended.

I spent a bunch of time on this when I located my static ports and for what it's worth, wrote a Kitplanes article on it "Static Port Location and Altitude Calibration":
https://www.kitplanes.com/static-port-location-and-altitude-calibration/

Great article and good reference for testing the system...https://www.kitplanes.com/issues/26_10/builder_spotlight/8959-1.html
 
It might be that the location is more important than what design is used, although it seems like both are important. I have the (probably: they have been there for 25 years) Van's old flush ports and all of my static testing was spot-on.

Van's has never sold a static port or kit with ports that were flush.

They have always been either the low profile blind rivet that comes in the basic kit, or something that mimics the shape of the blind rivet such as this one

https://store.vansaircraft.com/threaded-static-port-kit-pair-static-port-kit.html
 
It might be that the location is more important than what design is used, although it seems like both are important. I have the (probably: they have been there for 25 years) Van's old flush ports and all of my static testing was spot-on. It might be a combination of luck and placement. Different aircraft are going to have different pressure areas. I think (that was a LONG time ago) I put them where Van's recommended.



Great article and good reference for testing the system...https://www.kitplanes.com/issues/26_10/builder_spotlight/8959-1.html

The gold standard for gathering flight data to determine AS and Altitude accuracy is Kevin Horton's spreadsheet, which you can download with this link: tinyurl.com/jwp9r5a

Yes, it doesn't look like a URL, but it will download a zip with the files
 
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