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  #1  
Old 04-05-2010, 07:15 AM
Brantel's Avatar
Brantel Brantel is offline
 
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Location: Newport, TN
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Default Question for all the static system guru's out there

I seem to have a large static position error... I think that is what you static system guru's call it.
  1. I have the SafeAir static ports that are of the new design.
  2. They are mounted inside the fuse by the way of Pro-seal around the flange to the inside of the fuse skin.
  3. The ports extend from the side of the fuse .051 on the left and .053 on the right.
  4. The ports are within 1/2" horizontal and dead on the vertical position as specified on the plans.
  5. The SafeAir ports are round with square corners on the OD and the ID. There is no radius or chamfers on these ports. They look like an exact cylinder sticking out from the skin of the fuse with a hole in them.
  6. I used the SafeAir tube and fittings and the static ports are cross connected with a "T" in the exact middle between them. The static line is connected to this "T" and runs up to the D100.
The indications are:
  1. On takeoff and climb out I noticed that even though I am climbing, the altitude seems to stay pretty much at field elevation until I get to around 200ft then it starts to roll up normally. ( I think as the airspeed comes up, this is increasing my error and the error is canceling out the fact that I am climbing until the higher altitude overcomes the error)
  2. When flown beside an airplane that has been proven accurate, my altitude indicates almost 200ft low and the airspeed was also low by at least 20 knots. I did not get the exact numbers but mine was low on both. This was with me running around 2350rpm @ 3500ft.
  3. I popped a flap fuse while landing which may be because I was above flap speed at some point and did not know it since my ASI is reading low.
The test:
  1. I have performed a vacuum leak test on the static system where I applied 1000ft of elevation change for at least a minute and there were absolutely no leaks. I taped over one port and used a vacuum tool to suck it down.
  2. I made a manometer and downloaded the chart from the "Fly EZ" site and performed an ASI instrument error test on the Dynon. The Dynon ASI was within 1 knot from 230 knots all the way down to 40 knots so I know the Dynon ASI sensor is accurate.
  3. I compared my Dynon's pressure altitude reading on the ground while set to 29.92 to an airplane that is known good at the same elevation. His airplane has both a mechanical Alt and a D100 like mine. I only have the D100. Both our pressure altitudes were the same.
  4. I used the manometer to apply 35" w/c vacuum to the static system after setting the Dynon to report 0ft altitude and I got exactly 2500ft.
  5. I tried a test flight where I applied a 2"x2" square piece of aluminum tape to the side of the fuse and allowing it to ramp up and fair in the front edge of the static port without covering the hole. I honestly think this made it worse instead of better.
These test seem to indicate that on the ground the Dynon is very accurate. I also know that I do not have a leak. It all seems to point to a static system error where I am getting too high of a static pressure for some reason.

One possible theory is that I have my vinyl N numbers right in front of the static ports and that these are causing some sort of turbulence creating the error?

What says the guru's???

Here are some pics of the install and ports:







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Last edited by Brantel : 04-13-2010 at 07:38 AM.
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  #2  
Old 04-05-2010, 09:13 AM
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Kevin Horton Kevin Horton is offline
 
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Default

Let's rule out one more thing - pitot leak. Have you done a leak test on the pitot system? It would take a pretty big leak to give that large an airspeed error, but you never know.

Assuming your pitot system doesn't have a leak, it certainly looks suspiciously like you might have large errors in your static system.

I doubt the edges of the vinyl registration marks would have that much effect. For what it's worth, the aft edges of one of the letters on the side of my RV-8 end about an inch before the static ports, and my system is reasonable accurate - it has 0 error at about 130 KIAS, with the error increasing to about 2 kt at 180 KIAS. In my case, the error is such that the IAS and altimeter read high. My results are here.

I don't know if Van specifies a different location for the static ports on the -7 than he does for the -8, or maybe I screwed mine up, but my ports are located 2 or 3" lower than yours. A small location change might make a small change in the error, but I can't see how a few inches change in position could cause a large change in the error.

Once you have confirmed no pitot leak and confirmed one more time there are no static system leaks, the next step is probably to do some dedicated testing to quantify the errors you have. Find some smooth air. Fly at a constant indicated altitude and IAS. Fly a box pattern with the legs roughly 90 degrees apart, and record:
  • IAS,
  • pressure altitude (i.e. with altimeter set to 29.92),
  • OAT,
  • GPS groundspeed and
  • GPS track

On each leg, use power as required to achieve exactly the same IAS, as closely as you can do it. Stabilize on a constant IAS, altitude and heading long enough for the GPS data to stabilize before recording the GPS groundspeed and track. Do this at three or four different IAS.

Send me the data (kevin01 AT kilohotel DOT com), or post it here. Also send or post the results of your tests to determine ASI instrument error, as we need to account for that when crunching the data.
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  #3  
Old 04-05-2010, 09:33 AM
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Brantel Brantel is offline
 
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Thanks for the post Kevin, I was hoping you would catch this one...

The test I did to verify the ASI instrument error was using the pitot line all the way out to the pitot. I can't really test the pitot because the Dynon unit has a calibrated leak in it and that does not work too well with a manometer. If you plug the holes in the side, it will come out the AOA port since inside the calibrated leaks on both ports share a common chamber.

I would say that Van's does give different positions for the static port. The 6 is different from the 7.

I really think it is these darn static ports and their shape. I am tempted to shave them flush and do what Scott Will did and install the pop rivet over them.

There are no static leaks as I did this test three times. Twice with a vacuum pump and once with the manometer. Rock solid!

I will do the test runs as soon as I can get some smooth air.

The ground test on the Dynon were very impressive. It shows that it is well calibrated on ASI. I can only get two data points on the altimeter side (pressure altitude and -35"w/c) but those two points are accurate.

Here is the ASI instrument error results. As accurate as you can be with a manometer:

Manometer Chart in Knots/ASI on Dynon in Knots
230/230
220/220
210/210
200/200
190/189
180/179
170/169
160/159
150/149
140/140
130/129
120/118
110/108.5
100/99
90/89
70/69.5
60/60
50/49.5
40/40
30/Dynon quit

I am really suspecting the static issue since the Alt does not start rolling up on climbout. The ground test reveal that there is no lag in the system. It responds to changes instantly on the ground.
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RV-10, #41942, N?????, Project Sold
---------------------------------------------------------------------
RV-7/TU, #72823, N159SB
Lyc. O-360 carbed, HARTZELL BA CS Prop, Dual P-MAGs, Dual Garmin G3X Touch
Track N159SB (KK4LIF)
Like EAA Chapter 1494 on Facebook

Last edited by Brantel : 04-05-2010 at 09:35 AM.
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  #4  
Old 04-05-2010, 09:58 AM
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Kevin Horton Kevin Horton is offline
 
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Quote:
Originally Posted by Brantel View Post
The test I did to verify the ASI instrument error was using the pitot line all the way out to the pitot. I can't really test the pitot because the Dynon unit has a calibrated leak in it and that does not work too well with a manometer. If you plug the holes in the side, it will come out the AOA port since inside the calibrated leaks on both ports share a common chamber.
How long ago did you do the ASI instrument error check? Is there any chance a new pitot system leak has arrived since that time? It might be worth disconnecting the pitot line at the pitot, and doing one more leak check on that side. There could still be a small leak between the pitot tube and pitot line, but the air lost through any leak there would be quickly replenished by air coming in the pitot tube, and the effect on pitot pressure would be very small.

Another approach to a pitot leak check, given the design of the Dynon pitot, might be to somehow connect the test pressure to both the pitot and AOA ports, and cover the drain holes in the side. Would that work?
Quote:
I really think it is these darn static ports and their shape. I am tempted to shave them flush and do what Scott Will did and install the pop rivet over them.
It is quite possible that this shape is the cause of the issue. I suspect that the closer you can come to the shape of Van's specified static port the less error you will have. But, there is no point to making any changes until you have a good baseline test to compare future results against.

I'm going to be on the road for the next week, so I'll have limited web access. I'll keep an eye on this thread, but my replies might be delayed.
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  #5  
Old 04-05-2010, 10:10 AM
aerhed aerhed is offline
 
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Bran, try a quick flight with the static hose off (cabin static). You'll have error still, but if your readings get more sensible you've narrowed it to your ports. I'm still having trouble visualizing why vans static location/port shape is so touchy. Lotsa ragbags don't even read external static. Is it the wide speed envelope? I dunno.
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  #6  
Old 04-05-2010, 10:29 AM
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Lycosaurus Lycosaurus is offline
 
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Default Moisture accumulation?

Brian,

I noticed that you T'd the left and right static ports to the top. Per Van's instructions, the two ports are to be T'd on the left side, close to the left port. Any moisture accumulation in the lines will have the opportunity to drain via the left port (at some point in time... steep climbs, un-coordinated turns, or just pushing the tail down when on the ground), especially if the static line run is kept under the longeron (horizontal run).

Just a wild stab in the dark .... but you may want to check if the static line has moisture in it, and needs to be drained.
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  #7  
Old 04-05-2010, 10:30 AM
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Brantel Brantel is offline
 
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Kevin,

The ASI error check was last evening.......very recent data.

I will try and get a run in before making any changes.

Thanks for all your willingness to help! The tools you have made available are awesome. I just don't know if I understand how to use all of them.

Best regards!


.
Quote:
Originally Posted by Kevin Horton View Post
How long ago did you do the ASI instrument error check? Is there any chance a new pitot system leak has arrived since that time?

It is quite possible that this shape is the cause of the issue. I suspect that the closer you can come to the shape of Van's specified static port the less error you will have. But, there is no point to making any changes until you have a good baseline test to compare future results against.

I'm going to be on the road for the next week, so I'll have limited web access. I'll keep an eye on this thread, but my replies might be delayed.
__________________
Brantel (Brian Chesteen),
Check out my RV-10 builder's BLOG
RV-10, #41942, N?????, Project Sold
---------------------------------------------------------------------
RV-7/TU, #72823, N159SB
Lyc. O-360 carbed, HARTZELL BA CS Prop, Dual P-MAGs, Dual Garmin G3X Touch
Track N159SB (KK4LIF)
Like EAA Chapter 1494 on Facebook
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  #8  
Old 04-05-2010, 10:34 AM
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Brantel Brantel is offline
 
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No moisture in the lines. Dry as a bone. Ground checks are fine. It is an in flight issue. Moisture would not be specific.

There are various different opinions on where the T should be on a dual port setup. Most agree that the most accurate place for the T is in the middle. I put mine up high to prevent any moisture from being able to get in there.

The plane has never been outside in anything but great weather.

Thanks!!

Quote:
Originally Posted by Lycosaurus View Post
Brian,

I noticed that you T'd the left and right static ports to the top. Per Van's instructions, the two ports are to be T'd on the left side, close to the left port. Any moisture accumulation in the lines will have the opportunity to drain via the left port (at some point in time... steep climbs, un-coordinated turns, or just pushing the tail down when on the ground), especially if the static line run is kept under the longeron (horizontal run).

Just a wild stab in the dark .... but you may want to check if the static line has moisture in it, and needs to be drained.
__________________
Brantel (Brian Chesteen),
Check out my RV-10 builder's BLOG
RV-10, #41942, N?????, Project Sold
---------------------------------------------------------------------
RV-7/TU, #72823, N159SB
Lyc. O-360 carbed, HARTZELL BA CS Prop, Dual P-MAGs, Dual Garmin G3X Touch
Track N159SB (KK4LIF)
Like EAA Chapter 1494 on Facebook
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  #9  
Old 04-05-2010, 10:38 AM
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Brantel Brantel is offline
 
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Location: Newport, TN
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Quote:
Originally Posted by aerhed View Post
Bran, try a quick flight with the static hose off (cabin static). You'll have error still, but if your readings get more sensible you've narrowed it to your ports. I'm still having trouble visualizing why vans static location/port shape is so touchy. Lotsa ragbags don't even read external static. Is it the wide speed envelope? I dunno.
I think I may install an alternate static port as I have the parts to do it and see what happens....I will fix it so I can plug and unplug it while in flight. I know the inside pressure is going to be lower than the correct static pressure so I would expect to see a large swing up in both my ALT and ASI. Not sure what this will get me...It is almost a guarantee that it is the ports at this point.
__________________
Brantel (Brian Chesteen),
Check out my RV-10 builder's BLOG
RV-10, #41942, N?????, Project Sold
---------------------------------------------------------------------
RV-7/TU, #72823, N159SB
Lyc. O-360 carbed, HARTZELL BA CS Prop, Dual P-MAGs, Dual Garmin G3X Touch
Track N159SB (KK4LIF)
Like EAA Chapter 1494 on Facebook
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  #10  
Old 04-05-2010, 10:51 AM
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Ironflight Ironflight is offline
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Just as a data point Brian, I have had static leaks behind the panel a couple of times, and the airspeed error is significant. the good news if you test it with the line disconnected inside the fuse, the error will be fairly linear - not the erratic behavior that you reported in your first post.

I am afraid that I have to agree with your conclusion that it is probably the shape of the ports. The truth is that one of the hardest things to do with a new airplane in flight test is finding a good, consistent static source - it can take hundreds of hours to do this in some cases.So once a designers says "make the static ports like this!", I pretty much figure I'll use his recommendation. The pop rivets look chincy, but field experience says they work.

Paul
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