Question for all the static system guru's out there

Well I have to respectfully disagree with you.... You can have a higher than actual static pressure because I have this error in flight. Also, you can change the pressure quite easily by doing very insignificant changes to your static ports and their surroundings. The way I know this is because I have been doing a ton of testing in real life and in flight trying different things to reduce the error.

The other thing is that in cabin static pressure is lower than the true static pressure. The airstream over the fuse causes a negative pressure inside the aircraft. Alternate static sources in the cabin almost always cause your readings to go up not down and the only way to do that is to reduce the static pressure.

I have tested the pitot line from 30 to 230 knots using a manometer. No leaks. Minimal error.

I have tested the static lines by pluging one port and applying vacuum with a manometer to the outside of the other port. I have applied up to 35" w/c of vacuum for over 5 minutes and there are no leaks and the altimeter reads correctly at ~2500ft with 35".

Two other builders have tripple checked my work. There is definately something wrong as I have a huge error that I cannot get rid of. Finding it is the problem.

penguin said:

It is not easy to get an over pressure static by disturbing to flow over the port - the usual result would be a low static reading. I almost typed never, but thought better of it. Just think about it, the static ports are a little proud of the surface, so the air is accelerated slightly as it passes over them, so the pressure reduces - you would need some kind of dam behind the port to raise the pressure.

I think the most likely culprit is a leaky static in the cockpit, as cockpit static is usually higher than ambient. I know you have stated that the manometer testing ruled that out - but are you really sure (sorry haven't read all the posts)? If you pressuise the pitot it doesn't matter if there is a cockpit static leak - its all at the same pressure. Have you have an altimeter test guy put one of those fancy vacuum suckers on the static ports? Last time I had it done it was $90. Can you borrow a huff & puff ASI & Alt, hook them up and compare with the Dynon?

Failing that, have someone who has had nothing to do with your project come and do a pitot static check, and compare results. Perhaps there's something wrong, and because you have looked at it so often you're just not seeing it?

Pete

Click to expand...

islandmonkey said:

as cockpit static is usually higher than ambient.

Pete, I thought cockpit static is usually Lower than ambient. Caused by the airflow around the fuse pulling the the air inside the cockpit out and creating a slightly lower pressure inside the cockpit.

Click to expand...

This may or may not be true for all aircraft types, but it seems to be true for RVs. All the static system leaks I am aware of in RVs caused high IAS and altitude readings due to low pressure in the cockpit affecting the pressure in the static system. The typical effect of a static system leak on an RV is an IAS that reads about 7 to 10 kt too high, and an altimeter that reads 100 to 125 ft too high.

I flew an additional 4.2 hours on Saturday trying to nail this thing down....Before doing that, on Friday night I performed another round of manometer testing in an effort to quadruple check my system for accuracy and leaks on the ground.

The manometer testing of the pitot system and the ASI of the Dynon pass with flying colors.

The manometer testing of the static system can hold 35" of w/c of vacuum for 5 minutes or until you get tired of waiting. The ALT of the Dynon reads almost dead on 2500ft at that vacuum and it is my understanding that this is correct. I cannot go higher than that with the manometer that I have.

I also installed two layers of clear packing tape over the N numbers that are just in front of my static ports.

I also installed an in cabin alternate air source for testing in flight.

I did not do any GPS runs to confirm the error as I know I have an issue and the error is so great that I am not sure it is worth making the runs until I can get closer to reality.

On Saturday, first flight I tested the alternate air source. At around 5K feet and 120-140 knots IAS was where this test was performed.

When comparing the difference between operating on normal static and alternate cabin static, I was seeing a difference of 10 knots IAS and 100ft of ALT change with the alternate static source always indicating the higher values. While on the normal static source, I performed a series of stalls and in both clean and dirty configurations, the IAS was around 10-12knots lower than expected...way below the white line.


On the second flight, I was flying in loose formation with another RV6A that is known good. On the normal static source, I was indicating around 10 knots lower IAS and 100 to 125 ft lower ALT. When I switched to alternate static, we were indicating about the same IAS and ALT. I performed a series of stalls and with alternate static, the clean stall was breaking right at the bottom of the white arc. Dirty about 5 knots lower. This seems about right with a heavy pilot and half a load of fuel.

I flew the rest of that flight and landed with alternate static. The pattern looked more normal and approach speeds felt more like what they do in the other RV7 I have flown.

Later in the day I decided to shave the original ports flush since another comparison aircraft I have been flying along side has his shaved flush.

This did not make a significant difference in the error. I performed stalls again and got similar results. I flew the rest of the flight with alternate static. It was super smooth and while cruising around with little to no wind, my GPS GS and my TAS indications were almost dead nuts in agreement when flying 90° to the little wind we had. Upon joining the pattern with several 150's, I was flying eyeball level with them at our pattern altitude of 2300ft. On normal static, I would have been 100-125ft higher than them due to the error.

My next step is to remove the N numbers from in front of the ports. Test and see what happens...If that does not work, I am at a loss.. I suppose I could then start playing around with air dams. I noticed that several of the certified aircraft on our field have various shapes and sizes of air dams installed. I could also drill the ports and install the Van's recommended pop rivet.

One weird thing that I have noticed is that Van's drawings show the port one bulkhead back from that of the 6. Both my friend's aircraft have them installed in the normal 6 location even though one of them is a 7. He does not remember why he put them there. One friend has the pop rivets and the other has the same ports as me but shaved flush. They both indicate the same ALT and IAS when flying in loose formation.

Brantel said:

Question for the math experts, if you have a 12-14 knot low IAS that was caused by a high static pressure error, what would the expected low ALT error be?

Click to expand...

The relationship between IAS error and altitude error varies depending on the CAS and the altitude. At 150 kt CAS and sea level, a static pressure error that caused the IAS to read 13 kt too low (i.e. 137 kt IAS) would cause the altimeter to read about 170 ft too low. At 100 kt CAS and sea level, a static pressure error that caused the IAS to read 13 kt too low (i.e. 87 kt IAS) would cause the altimeter to read about 110 ft too low. At 10,000 ft, the altimeter errors would be about 35% higher than at sea level - i.e 150 ft at 100 kt CAS and 230 ft at 150 kt CAS.

consider getting an ifr system check?

Just another data point. My rv7 has SafeAir static ports and lines, location as per rv-7 plans. I've passed all the IFR equipment checks, and my airspeed and altitude indication are proving to be trustworthy and accurate.

There are at least two difference in our installs:
(1) I riveted the SafeAir fitting, and you used pro-seal. You still seem to have a good protrusion of the static port nipple, so hopefully such a small variation does not lead to your difficulties.
(2) Your port location looks like it's about an inch forward of the plan location. This is either accidental on your part, or intentional to avoid dealing with the interference of the SafeAir fitting with the bulkhead flange. In the spec'd location, you have to file down SafeAir fitting where there's interference.

These variations seem insignificant enough to me. You've done so much testing. I wonder whether it would be worth it to get your system tested by a pro? meaning by someone who can sign off your transponder and pitot static system.

There's a good reason we have this phase 1, eh? Good Luck.

While I appreciate the help... The suggestions to have a formal pitot static system test while would not hurt anything, won't help either.

A pitot static test with all the equipment in the world will not detect a static position error in flight.......

An airplane can pass a pitot static check on the ground and still be a mile off in the air.....False security!!!

More test in progress!

Last evening I did the following:

• Removed the N numbers from in front of the static ports. I replaced them, applying them down out of harms way of the ports. Not sure this ever had anything to do with my problem but I moved them to make sure.
• I drilled the now flush Safeair ports to 1/8" and JB welded the same pop rivet that Van's supplies in their static kit into the hole. Based on info found in the archives, I added a thin washer under the pop rivet to increase its stickout by .032". The the total projection from the side of the fuse is .081"-.082" rivet, washer and JB weld included.

I did not get to fly last night since all this work took up the daylight. I will be flight testing this evening and will let you know how it goes.

It is my understanding that there is a sweet spot that I am trying to find that is neither too close to the skin or too far out in the slipstream.

Yep the pro-seal takes up some of the extension....My latest mods have the ports with ~ .030" more extension than what I started with.

Yes the ports are 1/2" further fwd than the recommended position of the plans. I did this to prevent modifying the flange of the ports. My gut tells me that this 1/2" could not be that critical since on the 6 the recommendation is a whole bulkhead further fwd.

dhall_polo said:

There are at least two difference in our installs:
(1) I riveted the SafeAir fitting, and you used pro-seal. You still seem to have a good protrusion of the static port nipple, so hopefully such a small variation does not lead to your difficulties.
(2) Your port location looks like it's about an inch forward of the plan location. This is either accidental on your part, or intentional to avoid dealing with the interference of the SafeAir fitting with the bulkhead flange. In the spec'd location, you have to file down SafeAir fitting where there's interference.

Click to expand...

thanks for the update. I for one am taking notes

Your gut is right... I did the same on mine.. made more sense than messing with the port or cutting into the bulkhead.. Works great.. so that's not it. BTW, I mounted mine from the outside, so it sticks out the thickness of the port..

Brantel said:

My gut tells me that this 1/2" could not be that critical since on the 6 the recommendation is a whole bulkhead further fwd.

Click to expand...

A pitot problem?

islandmonkey said:

as cockpit static is usually higher than ambient.

Pete, I thought cockpit static is usually Lower than ambient. Caused by the airflow around the fuse pulling the the air inside the cockpit out and creating a slightly lower pressure inside the cockpit.

Click to expand...

Yeah, my error - shouldn't type late at night!

Looking at the results of the test flight, if everything is air tight, and cockpit static gives nearly accurate results (when the experience of others indicates indications should be high), maybe the problem is on the pitot side? Opening the cockpit static removes the static ports from the equation, but the error remains - one explanation is a pitot error. Brantel, can you post a picture of your pitot tube set up?

Pete

PS The only way to measure the static pressure accurately is to use a trailing static cone - which most of us don't want to mess with as the cone should be about 3xfuselage lengths behind the aircraft. I would suggest that without that data it is difficult to be sure of the accuracy of the P-S system one way or the other, as its difficult to get a good in-flight baseline.

It is just a standard Dynon Aoa pitot... Non heated attached to a safeair mast in the normal position.

I have altitude error as well which has nothing to do with pitot.

I tend to think that with the way I piped up my cabin static source, it is more of an average between the rear ports and the open tee in the line. I did not pipe it in a way that I could completely isolate
the rear ports. This might explain why the error did not go on the other side of the curve.

Here is my thought on it:

Someone who has a normally accurate IAS and ALT gets a cabin side static leak, his IAS and ALT will typically read high by 7-10knots and 100-125ft.

I was reading ~10 knots and ~125ft low with no cabin side leaks. When I make an artificial cabin side leak, the typical cabin side error cancels out my system error and reads pretty close to reality.

At least that is how I see it could work.....I may be all wet but it sounds like it would be possible.

Alternate SafeAir Ports

Brantel,
I'm following your progress closely as I too had read all the "stuff" about the SafeAir ports and decided that I really liked the tubing method but the protrusion of their port was questionable. I've tried to link some pictures of the ones I made. I bought Van's pop rivet kits and measured the rivets then machined the protrusion, accounting for the skin thickness, to match the pop rivet. I'm hoping it works okay.







I hope you find out this evening that your latest test work. I can't believe the "N" numbers in vinyl have any effect, but I would have removed them also.

Success!!!!

We have a winner!!!

Last evening, I flew some test flights after the latest round of changes to my static system. I performed a few stall series and also flew several legs while being chased by a friend and his RV6A. This RV6A has very similar IAS and ALT indications that another friend's RV7 has as they have been compared many times to each other. I have been using both aircraft in my prior flights as a reference depending on which one was available.

The new IAS at stall is 44kts which is right at Van's published stall for solo weight of 1400lbs and that is right about were I was at during these test. As best I can tell, my aircraft and the RV6A are now displaying the same or very similar IAS and ALT. We compared readings at 100kts and 150kts and the results were great. I am now eyeball level with other planes in our pattern. My final approach speed now feels normal and not too fast. My TAS at 90° to the wind (light winds aloft) is now indicating almost exactly what the GPS GS is indicating.

I broke the rules and changed more than one thing so I am not sure what resulted in the major reduction in error:

• Moving the N numbers from in front of the static ports
• Drilling the shaved flush ports to accept Van's pop rivet, installing the pop rivet with a thin washer under it.

Here is a pic of the ports now:



Now that I am sure that I am not a mile off, I will do some GPS runs and see exactly where I am at various speeds. I feel confident that I am now withing the same narrow error band that most RV's have. GPS testing will confirm!

I am sure there are many people out there flying that have no idea how much error they have in their ASI and ALT indications. Passing a pitot/static check does nothing to confirm that you do not have in flight errors. Kind of scary really...

Brantel,

Thanks for taking the time to post and document this issue with your plane. I have been following this thread closely since I will be plumbing my static system soon.

It's this kind of learned knowledge that helps everyone. Thanks again.

Brantel said:

We have a winner!!!

I broke the rules and changed more than one thing so I am not sure what resulted in the major reduction in error:

• Moving the N numbers from in front of the static ports
• Drilling the shaved flush ports to accept Van's pop rivet, installing the pop rivet with a thin washer under it.

Now that I am sure that I am not a mile off, I will do some GPS runs and see exactly where I am at various speeds.

Click to expand...

That is great news! Congrats. Keep us posted on what you learn from the GPS runs.

After you have done the GPS runs to see what the new errors are, it would be very interesting to temporarily put some vinyl strips ahead of the port to simulate where the N numbers previously were and do some more GPS runs to see what the effect is. I can't imagine that they had anything to do with the problem, but it would be nice to confirm that, as many other folks can use this knowledge.

Congratulations, Brian! It sounds as if you have crossed a major hurdle in your Phase I test period. As you say, there are probably a lot of RV's flying with errors in their pitot-static systems. Thanks for taking the time to share your experience with us.

Yep, If I had not had such a noticable error, I would have never gave it another thought....Placing faith in a Pitot/Static signoff is another place where someone might get false security that their system is accurate.

To me, the only way to know for sure is to follow Kevin's mentoring and testing proceedures or something similar. Thanks for all the work you have done on this subject Kevin!!!!

rv7boy said:

As you say, there are probably a lot of RV's flying with errors in their pitot-static systems.

Click to expand...

When you do the pitot static check on the ground that insures no leaks in the system. To check the static you need a known good source to check against. One is the GPS another is another aircraft flying next to you. I went with vans funky static port from the get go and why, it is a proven source. It does work. Glad you got it figured out.

Now this is for the other post you made, did it do anything for the flaps?

Well, hard to tell. I would have to rig a current meter up on the flap motor.....

I do know that I have not blown the 7.5 amp fuse....

I truly believe that the reason I popped the 5 amp so quickly is that I was about 10 knots over flap speed and did not know it due to my static error. I feel better with the 7.5 amp as this gives more overhead. People that pop a 5 amp might want to doublecheck their IAS accuracy.

Everything feels more normal now at the indicated flap speed. During testing once I knew I had an error, I was padding the flap speed by about 10kts before fully extending them. Safety margin as I did not want to fold up a flap.

I inspected my wing, flaps, hinges, linkages etc. after my event to make sure I did not bend anything. I used the RV9 hex rod on my flaps instead of the normal rod and I am glad I did....

slyfox said:

Now this is for the other post you made, did it do anything for the flaps?

Click to expand...

slyfox said:

...One is the GPS another is another aircraft flying next to you...

Click to expand...

I may be misunderstanding you but, to be clear, GPS altitude and indicated altitude would only be the same by coincidence while you are flying. GPS determines altitude by geometry and the altimeter determines altitude by pressure and atmospheric models that are not normally the same as reality.

Not to speak for Sly but....

I think he was referring to the way you can use your GPS and the tools that Kevin has posted here to determine your static position error and if you know that (along with a few more details), you can determine your altitude error.....correct me if I read your post wrong Mike.

Some people do make this mistake though and think that the GPS and ALT should be the same. Most of the time they will be different and in some cases way different.

n5lp said:

I may be misunderstanding you but, to be clear, GPS altitude and indicated altitude would only be the same by coincidence while you are flying. GPS determines altitude by geometry and the altimeter determines altitude by pressure and atmospheric models that are not normally the same as reality.

Click to expand...

n5lp said:

I may be misunderstanding you but, to be clear, GPS altitude and indicated altitude would only be the same by coincidence while you are flying. GPS determines altitude by geometry and the altimeter determines altitude by pressure and atmospheric models that are not normally the same as reality.

Click to expand...

Of course a person can always fly 20ft off and 80kts at a known altitude and check things

static

Brian:
Congratulations on your extensive and successful efforts and also for sharing.

Bill

Congratulations on figuring it out and letting us all know. Am I right in thinking your original ports were 0.050" proud (about the size of the pop rivet) and gave around a 10kt low IAS indication and 120' low Alt reading. The new, rivet shaped, ports are 0.080" proud and give an accurate reading for both IAS & ALT?

Pete

Yep that pretty much covers it.....

Just read through your post on the static error. Sorry that I did not see the post earlier as I went through the exact process back in 2006 with my 9a and the flush mount static ports. After a great deal of testing I ended up with a pop rivet head bonded onto the flush port with a .040 rounded edge washer between the two. After that change the error went to insignificant. Interestingly enough, on my first flight I also popped the flap breaker.

The results of this was documented on this site--should have been in the archives.

Best wishes and again, sorry I did not see your post earlier--could have saved you some agony.

Cheers,

db

Dave, I think it was your post that gave me the idea to put the washer in between the pop rivet and the skin.... Thanks!!!!

Just got my transponder, alt, & static check done today by a professional. I'm good for two years!

No issues found! Dynon dead on the money to 20K feet!

One note:

I used Dynon's serial encoder to gray code converter with my KT76A and it works perfectly...

static port errors

There are numerous threads regarding static port type and placement.

Here is an interesting static port configuration found on my 182RG for the STEC autopilot. The altitude hold is very steady, however since it is not connected to any direct reading altimeter, I can't say if there are static errors..

It does not have any problems in the rain, perhaps the "dam" ahead of the port keeps water drops from streaming back.