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Static Source Variation

avrojockey

Well Known Member
Patron
I posted my static port issue in another thread but it has branched out into a problem I can't seem to solve on my VS indication.

I was able to change my static ports to the Van's pop-rivet type and it corrected my static source error to <1 KTAS, which is great! However, I now get these spikes in static pressure which mainly cause variation in VS indication. See this video...https://www.youtube.com/watch?v=7iFLKqXmxAw

I played around with modeling clay and creating a smooth transition around the static port "rivet" but it didn't help with the static pressure variations/spikes that cause these uncommanded variations in the VS (see video). I tried to isolate it to a specific port but it was occurring on both ports, with the left port slightly worse. Opening the alternate static source to the cabin fixed it, so this tells me it's something outside. I checked for static leaks and there is zero leak rate in my static system.

I also tried using a .004" and .008' gauge snubber on the GSU25s and went for a test flight. Both seemed to do the job with .004" doing a little bit better, but there does seem to be slight lag VS indication. I'm not sure this is a good solution, and may have some unintended risk associated with it.

I'm wondering if anyone else has seen this behavior or have any recommendations.

My next steps will be to tuff test the airflow on the fuselage and record the results. Also, I replaced the old vinyl 1/2 and 1/4 tubing that was installed with new tubing that was all 1/4" poly...wondering if going to all 1/4" would make VS indication more sensitive and thus the reason some 1/2" was installed, acting as a shock absorber in pressure pulses.

Any thoughts?
 
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It looks like the AP is engaged?

No...I actually had to hand fly because the AP (at the current gain setting) was chasing the VS and couldn't hold it smooth. Thats why the video isn't stable...had to film one-handed.
 
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g3expert

Seems like one for the g3xperts! How much lag do you have with the snubbers?
 
Seems like one for the g3xperts! How much lag do you have with the snubbers?

I't didn't seem like much...I'm going to re-shoot with my kid's gimbal next time so its stable and take the snubber off one of the GSU25s and switch back and forth to compare.

I'm going to call the EXP folks to see if they have any insight, but the fact it was stable with the alternate static source tells me it's not an issue with the LRU.
 
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...I'm going to call the EXP folks to see if they have any insight, but the fact it was stable with the alternate static source tells me it's not an issue with the LRU.
Anything strange on your aircraft that might cause turbulence near the static?

What I might try if the Garmin boys don't have any hints is to disconnect the static at the back and see if there is any change - should be the same as when you use alternate air source. (I know getting back there is not easy for anyone older than 13 years old and greater than 100 lbs.)
 
You might try putting an accumulator bottle in the static system. I don't have much of a feel for the size but I'd start with something smallish. Doubt if you'd need to go bigger than a pint.

Perhaps even a fuel filter would work, if it has low enough flow restriction, although that also adds some damping. It might matter where in the system it's installed, and I can't help there. I know that when they are used on certified planes they are sometimes under the panel, though.

If you do use a fuel filter and decide to keep it installed, make a note in your plane's maintenance manual as to why it's there, or else in the future you might be under some pressure to replace it at the condition inspections.

Dave
 
You might try putting an accumulator bottle in the static system. I don't have much of a feel for the size but I'd start with something smallish. Doubt if you'd need to go bigger than a pint.

Perhaps even a fuel filter would work, if it has low enough flow restriction, although that also adds some damping. It might matter where in the system it's installed, and I can't help there. I know that when they are used on certified planes they are sometimes under the panel, though.

If you do use a fuel filter and decide to keep it installed, make a note in your plane's maintenance manual as to why it's there, or else in the future you might be under some pressure to replace it at the condition inspections.

Dave

I was thinking the same thing...I'm going to whip up an "accumulator" and put it inline with one of the GSUs and compare.
 
I . Opening the alternate static source to the cabin fixed it, so this tells me it's something outside. I checked for static leaks and there is zero leak rate in my static system.
Any thoughts?

I believe this is faulty logic. Leaks/restrictions inside the cabin won’t show up when the alternate source is used, because the static line is at exactly the same pressure as the cabin. In essence, the alt source is the same as a giant leak.

No real vacuum system has zero leak rate. They all leak some, maybe too small to detect. Here’s something I think you should be looking for, just one example: Somewhere the 1/4” tubing is kinked, cutting off any flow. Downstream is a very small leak. As air flows out the leak, the pressure difference at the kink slightly unbends the tubing, allowing air to flow. Once pressure is equalized, the kink closes again, and the cycle starts again.

I cannot think of any reason the external port pressure would vary on a 3-5 sec time scale. I’d be looking inside. Don’t overlook the possibility that the problem is inside the instrument itself.
 
I believe this is faulty logic. Leaks/restrictions inside the cabin won’t show up when the alternate source is used, because the static line is at exactly the same pressure as the cabin. In essence, the alt source is the same as a giant leak.

No real vacuum system has zero leak rate. They all leak some, maybe too small to detect. Here’s something I think you should be looking for, just one example: Somewhere the 1/4” tubing is kinked, cutting off any flow. Downstream is a very small leak. As air flows out the leak, the pressure difference at the kink slightly unbends the tubing, allowing air to flow. Once pressure is equalized, the kink closes again, and the cycle starts again.

I guess I should have used another paragraph, because those two statements aren't related to each other...I didn't leak check it with the alternate air source. I put a vacuum on the static system to 1000' AGL and the ALT and airspeed remained constant for over 5 minutes. Allowable leak rate for IFR is 100'/min at 1000' AGL and mine didn't move for 5 minutes so I think I'm good on the leak front.

Since opening the alternate air fixes the problem, then the problem is between the alternate air switch and the outside near the static port.

I cannot think of any reason the external port pressure would vary on a 3-5 sec time scale. I’d be looking inside. Don’t overlook the possibility that the problem is inside the instrument itself.

I have 2 GSU25s installed and can switch between them in flight. They both indicate the same thing. Also, if it was a failure of the GSU25 then the gauge snubbers or alternate static source wouldn't have fixed the problem. The G3X is known to indicate variations of this type because the instrument is so sensitive (I had to install pressure snubbers for manifold pressure and fuel pressure), and Garmin support indicated they've seen this behavior and it's normally a result of static port location.

I replaced all the static line tubing as it was old vinyl and brittle...I'm going recheck the run of new tubing again.
 
Update...

I did a couple more tests.

1. I taped some yarn tuffs to see how turbulent the flow was preceding the static port. Here's the video...https://www.youtube.com/watch?v=A9cJ27XFfng You can tell it's more turbulent towards the top half of the fuselage...I'm guessing the targa strip (over and 0.125" thick) is causing some rough air. Not sure if this amount of turbulence could cause enough pressure variation to be picked up by the static port and indicated on the G3X.

2. Instead of using a gauge snubber to even out the indication I used an "accumulator" courtesy of a Vitamin Water bottle (see pic) This size bottle removed the VS variations just like the .004" snubber did, also with a slight lag in VS indication. The same lag one would encounter with the alternate static air selected.


I'm starting to think switching the static line from 1/2" vinyl tube to 1/4" poly has made my VS indication more sensitive to the pressure pulses caused by the turbulence around the static port.
 

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One thing I didn't consider...normal VSIs have an inherent lag in them and this is how my VS is working with the pressure snubber or accumulator. Not sure how the VS indication is suppose to normally act on the G3X...like a traditional VSI or IVSI
 
Not related in anyway to your static issues, but you might want to check the flap/aileron rigging -- it the recent video, it looks like the left flap is riding up a bit more than the aileron (1/8"+).

Can you share some pictures of the static system tubing/interface-to-static-ports?

Cheers!
 
Increased capacity in a static system usually means lag on the VS. Some years ago TruTrak advised to feed their autopilots with a static pipe direct from the static port to the AP computer. If the main static system was fed from 2 ports Teed together with a pipe down the left side then add a pipe on the right side just for the AP. The symptom was the AP driving the aircraft up and down 50' while turning. An alternative solution is to feed the AP cockpit static, but then accurate capturing of a specific altitude is not possible.

If too many instruments are attached to the static system, or if the tubing is too large, the large volume of air in the system has to move through a relatively small hole to change the pressure and register as a change in altitude. If you basic static system is accurate can you reduce the overall capacity of the system?

Pete
 
speed of pressure change of air in a tube

It's been about 30 years since I took fluid mechanics but I would have thought that the pressure change of air in a tube would be almost instantaneous unless there is a pretty small restriction somewhere. Perhaps due to the simplified formulas we had I learned it wrong (ignore gravity, ignore friction, assume ideal gas, assume standard temperature, ...).
 
Not related in anyway to your static issues, but you might want to check the flap/aileron rigging -- it the recent video, it looks like the left flap is riding up a bit more than the aileron (1/8"+).

Can you share some pictures of the static system tubing/interface-to-static-ports?

Cheers!

Yea rigging has been an issue and checked during condition insp in Sep...left wing incidence is a little off from builder so flap is rigged about half-way between aileron alignment and bottom flap skin touching the fuselage. Aileron is properly rigged and more important IMHO, although that was another deal as the builder decided to use the rigging reference hole in the left wing for wire support.

I find if I rig per plans it requires a little aileron trim...even when solo in the left seat. It's unsightly for us pedantic folks but I'm living with it.

I'll get some picts posted of static system soon...I paused this issue to install a backup battery (IBBS) for the G3X because the weather is sucking for the next few days
 
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It's been about 30 years since I took fluid mechanics but I would have thought that the pressure change of air in a tube would be almost instantaneous unless there is a pretty small restriction somewhere. Perhaps due to the simplified formulas we had I learned it wrong (ignore gravity, ignore friction, assume ideal gas, assume standard temperature, ...).

See...this is wear my head is better served as a hat rack. I don't remember any of this stuff. I think there may be a difference between gases and fluid react to volume because air can be compressed...someone smarter than me can chime in about this. I think of a syringe with a plunger on both ends - if its a liquid the plunger opposite the one moved will be immediate, but if it's air there will be lag in the movement.

I do know I want get rid of these pressure spikes causing the VS to jump slightly. My AP does not like them unless I turn the pitch gain down, and then it doesn't react quick enough to actual VS variation...such as in a turn.

I can fix the root cause, which I believe is turbulent air around static port, or put a bandaid on it. I don't like the idea of moving the static port or removing the targa strip. So far, the .004" snubber and the bottle accumulator do the trick. I don't like the idea of the snubber because a .004" hole could be plugged pretty easily. I really like Walts idea of the Cessna sump, would also provide some safety margin against water blockage even though the lowest point of the system currently is the static ports.

Thanks for all the input folks...keep the idea coming!
 
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Increased capacity in a static system usually means lag on the VS. Some years ago TruTrak advised to feed their autopilots with a static pipe direct from the static port to the AP computer. If the main static system was fed from 2 ports Teed together with a pipe down the left side then add a pipe on the right side just for the AP. The symptom was the AP driving the aircraft up and down 50' while turning. An alternative solution is to feed the AP cockpit static, but then accurate capturing of a specific altitude is not possible.

If too many instruments are attached to the static system, or if the tubing is too large, the large volume of air in the system has to move through a relatively small hole to change the pressure and register as a change in altitude. If you basic static system is accurate can you reduce the overall capacity of the system?

Pete

It seems that when I decreased the capacity of the system somewhat from 1/2" line to 1/4" line it has had an effect, but this is the problem now...it's sensitive enough to pick up the pressure spikes from turbulence around the static port (I believe this is the cause).

If I increase the system volume using an accumulator or sump it fixes the issue, then there's lag, but it seems like lag an aneroid VSI normally has. I called Garmin Support to see if the G3X VSI is an IVSI (instantaneous), and they didn't know(?) other than to say there is no programed lag in the measurement.
 
Static system question…

Hey Tim, I think I'm having the same problems as you with the stock Van's static system and G3X Touch. What was the outcome of your research? Did you use a snubber (which is what I was considering), the static sump bottle that Walt mentioned, or something else?

Thanks in advance -

Dave

ps - nice job tufting your fuse and recording the video!
 
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I’m not sure what Tim ended up doing, but I had the exact same issue as he described and replaced the port with the Cleveland machined copy of the vans rivet and it fixed it entirely.
The seating of the rivet to the surface of the skin appears to be important.
If you monkey with it with glue/sealant/washers/paint this could be part of your issue.
Check out his other thread for more details.
Cheers
 
I’m not sure what Tim ended up doing, but I had the exact same issue as he described and replaced the port with the Cleveland machined copy of the vans rivet and it fixed it entirely.
Thanks Richard, I checked out Tim's other thread as you suggested - very helpful! Cleaveland doesn't sell the machined static port any more, but SteinAir does...

https://www.steinair.com/product/p-port/

Does this look like the one you have?

Many thanks -

Dave
 
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Looks like it is a Paul Bickle design published in "Soaring" magazine. Bickle was a well-known sailplane pilot who held the altitude record set in 1961 of 46,267 ft feet (with a total altitude gain of 42,300 ft) for many decades. He was flying his Schweizer SGS 1-23E sailplane at the time. He had a distinguished career with the Air Force and NASA at Edwards AFB.

Trailing static cones (also known as Mach cones for high speed aircraft) are used frequently for airspeed calibration. Boeing Commercial Airplanes routinely use them during initial Experimental flight testing.
 
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