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Static port - pain in the rear!

KiloWhiskey1

Well Known Member
I have an RV-7 and I am having issues with my airspeed indicating about 5-10knots high. I just had an IFR certification done and the pitot and static system checked out fine. I have heard that some static ports can cause a low pressure area around the port that causes inaccurate readings.

This picture is of my static ports with a half washer glued just in front of the port hole. I had heard that this would disrupt the air and allow for a more accurate reading. I flew it and did not see a difference.

Please let me know what ports are working well for you.

If someone is using the same ports that I have, did you mount them differently?

Any other advice would be much appreciated.

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Van knows what he is doing

Standard Super duper Van's Static system aka prosealed pop rivet.
Dynon D180, smackdaddy on speed.

I have noticed with Van's (barring one maybe two exceptions) fancier is not always better, just more expensive
 
Usually placing a dam/riser in front of the port will lower the pressure across the port (think flute). This RAISES indicated speed. If you are indicating high you already have low static or bad gaging.
 
Do a search for "Static" Keith - you'll find lots of experiences and things folks have tried - some work, some don't.

It seems to be a common misconception among many folks that the pitot/static test can ensure a goo static source - it doesn't! What it DOES do is tell you that your static system doesn't leak - because the accuracy of the ports are dependent on the aerodynamics of the airframe at speed, the only way to test that is in the air. It can take hundreds of hours for a manufacturer to find a good location and configuration for static ports - that's why I just stick with Van's solution - saves me a lot of Avgas!

Paul
 
Leaks?

Are you absolutely sure that you don't have any pitot static system leaks?

I use the basic pop rivet static ports and corrected initial airspeed errors by fixing leaking pipe fittings.

Jim Sharkey
RV-6
 
Normally, the best static port is a very small hole in the skin on both sides (to equalize differences in pressure due to side slips) of the aircraft in an area of the fuselage where there is very little normal disruption of airflow. From past posts on VAF it appears that the use of the Van's specified pop rivet in the specified fuselage position(s) works as advertised and even a slight deviation may cause problems. It appears from your picture that you have installed static port(s) that cause severe disruption of airflow past the static port(s). The main problem appears to be the protrusion of the ports from the fuselage skin. I think they were intended to be installed inside the fuselage skin.
 
Standard Super duper Van's Static system aka prosealed pop rivet.
Dynon D180, smackdaddy on speed.

I have noticed with Van's (barring one maybe two exceptions) fancier is not always better, just more expensive

I agree, after a number of test with different size "dam" and other shapes, I found the size and shape of a pop rivet head gave me the best result. I less concern about IAS then altitude, specially for IFR so I believe I have mine that the altitude shows very close and IAS has narrowed to about 1-2 knots accuracy.
 
Maybe only a pain because maybe the plans were not followed? This is not really directed towards you but a message to builders not yet to that point of building. It seems like most everyone that doesn't use the standard static ports from Vans has a problem. Location and SHAPE are important. After market ports seem to cause about a 10 MPH error. Larry
 
I drilled a small hole down the centre of a #8 stainless machine screw, same as that used to screw the floor down on the RV7. Then grind the head flat on your scotch bright wheel until the head is about the thickness of a rivet head. You can now use a nut to hold the #8 screw firmly into the fuselage, the thread diameter is perfect size for a quarter inch plastic tube. Works perfectly

peter
 
Thanks

Thanks for all the replies. I went back via the search engine and found some additional info and I have some good ideas to try from there as well. I tried the pop-rivets from Vans first and switched to the ones pictured in my original post after leaks became an issue. I chose these because of their outside mounting configuration. They are similar to many certified ports found on Cessnas and Beachcrafts. Ultimately they didn't work well either.

Cleveland Tools has inside mounted ports for $24/pr that look like they would fit the holes that will be left by the current ports. Does anyone have the Cleveland ports installed in their RV-7. If so, how and where did you install them? Are they working well?

Thanks.

Keith
 
Static

Maybe it is your airspeed indicator that's the problem. My first airspeed indicator would read high compared to my EFIS. With the two of them being different, I wasn't sure which one was wrong? A few flights beside different aircraft confirmed my EFIS was correct and the airspeed indicator was the culprit. Installed a new one and it now reads the same. Your static may be fine, and I agree with everyone ....keep to the plans on the static setup.
 
The Cleavland ports work perfectly. They mount inside the skin and look more or less like the pop rivet head from the outside. They are more robust and easier to worknwith than the pop rivet.
 
The 24-month pitot/static check performed by an avionics shop doesn't ensure an accurate IAS under dynamic conditions. Depending on how it's performed, it really only ensures that the altimeter performs with 43-13 tolerance and that your transponder encoder is providing accurate output. Flight testing a pitot static system is a challenge in any airplane, and every RV is different so accurate displays require individual testing of each plane. If you look in the flight test section, Kevin Horton has a great thread that will have links taking you to his web site. As has been pointed out, the Van's approved solution of the modified pop rivet has proven to work well; but it sounds like you've been down that road.

Here's a summary of the basic process: build or borrow a manometer and check the accuracy of your airspeed indicator. Then obtain GPS data and enter that along with your manometer data and the altimeter error from your 24- month check (or altimeter data card if it is a certified type), and then determine the magnitude of your "static source pressure error," i.e., how well your static ports are working. Mr. Horton wrote a helpful spreadsheet that does the math for you and will show the amount of airspeed and altimeter error present in your system under dynamic conditions. After you have this data, then you can start making changes to the port configuration. You can even approach this incrementally by using tape in front (if your IAS reads LOW) or behind the port (if your IAS reads HIGH). A variation is to cut adesive backed paper hole re-inforcements in half (the kind you get an office supply store) and affix them around your port--this will look like a 1/2 washer. If your current port is flush, this is fairly straight forward. On our plane, each layer produced a nominal .45 MPH bias in IAS. You can approximate a magnitude by looking at the difference between CAS and IAS you determined in your initial test, add layers incrementally and re-test. This technique allows you to "tune" the static system to mitigate error. Since you've already got a half washer in front, it may just be a matter of adjusting the thickness or removing it all together.

Now, having said all that, keep in mind it's not all that important (from an IAS standpoint), since your cross country performance is mearly a function of ground speed and fuel burn, and the airspeed indicator is just an approximation of angle of attack ("lift reserve" if you will). Until we developed GPS and advanced cockpit displays, I would wager most error went undetected (at least for non-certified airplanes).

On the other hand, if you have advanced cockpit displays, TAS or winds aloft data are only as good as the input; so if there are air data errors (e.g., static source pressure error), there may be errors in the displayed output. Depending on the system it may be possible to enter a correction factor vs. changing the configuration of your static source.

Hope this is helpful,

Vac
 
Last edited:
Thanks

Vac,

Thanks for the excellent response regarding fine tuning the static system. I am switching to Cleveland ports this weekend. They are designed to mount on the inside and they mimic the Van's pop-rivet setup, only with a more robust way to connect to the plumbing. I hope the new ports will get me close. I can then use your method to tweek it.

Keith
 
Follow up to pitot/static airspeed solution

I just wanted to follow up regarding my indicated airspeed problem.

Ultimately I determined that the issue was not the static ports that I was using. After trying several different ports (rivets, Cleaveland, modified originals, etc.), I decided to chase the issue from a different angle and found that I had a leak in the static system. The culprit was a stripped fitting at my analog ASI. This was causing my EFIS and other ASI instruments to read the same bad data. When I built the airplane I decided to use cheap fittings that looked like the SafeAir and Stein stuff, but were plastic and not of good quality. Bad mistake on my part!!! I decided to replace all pitot and static lines and fittings with the good stuff from Stein. After a lot of painful crawling around under the panel and back in the fuselage I finally came out with a good product.

I have test flown it now on multiple days at various altitudes and average a 1-2 knot low reading (using Kevin Horton's published method and borrowed spreadsheet...Thanks Kevin!!). I've decided to live with this variance for now. After flying many speed runs using the TAS spreadsheet, I would guess that most airplanes have minor variances and fine tuning small errors is probably more trouble than it is worth. Having said that, I started with a major problem (10-17 knots too high per the spreadsheet) and am glad that I went through the process to fix it. I also learned a great deal along the way.

Thanks again for all the suggestions! They spurred me to keep at!

Keith
 
I have an RV-7 and I am having issues with my airspeed indicating about 5-10knots high. I just had an IFR certification done and the pitot and static system checked out fine. I have heard that some static ports can cause a low pressure area around the port that causes inaccurate readings.

This picture is of my static ports with a half washer glued just in front of the port hole. I had heard that this would disrupt the air and allow for a more accurate reading. I flew it and did not see a difference.

Please let me know what ports are working well for you.

If someone is using the same ports that I have, did you mount them differently?

Any other advice would be much appreciated.

Don't go back to this shop. They caused you a lot of grief...........
 
My Piper Archer II has a correction table in the POH showing 8 kts high at 130 kts. It feels good to look down and see 136-138 kts TAS, but it's not real!
 
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