[ccsmith51]

I have an RV-6 that has a heated Gretz pitot tube from a C170. It is mounted below the front of the left leading edge, with the nose of the probe in front of the leading edge a bit. You can see it in the photo below, between the prop and the cowl. Sorry I don't have a better one.

The ASI reads up to 19 MPH high, depending on speed. At stall it inidicates about 50 mph with full flaps and 55 clean, which is about right I think. But at at higher speeds it is off, and the faster I go the more it is off. I tested it by making GPS tracks on N, E, S, W headings at 1,000' and constant indicated airspeed. At 110 mph indicated the four GPS speeds averaged 98 mph, a 12 mph error. At 190 mph indicated the four GPS speeds averaged 171 mph, a 19 mph error.

So, I'm looking for some help in determining how to find the error and correct it.

From the pitot standpoint, some people have told me that I should get rid of the Gretz and replace it with the standard Van's pitot, in the standard Van's location. Others have told me that the current location is fine as it is in front of the wing and at our low speeds there is not a compression issue.

I don't want to move it if I don't have to as there would be a hole in the wing that I would have to patch.

As far as the static system goes, the ports are from the same 172 that the engine, pitot, rudder pedals, instruments, etc., came from. They are strange looking devices with a step in them, as shown here:

They are mounted in front of F606 with the high side facing forward. I am told that the proper placement is one bay back in front of F607. I sent Van's an e-mail asking if the placement made any difference. I got a prompt reply from Ken Scott saying "We've seen the ports in both locations and it seems to make almost no detectable difference." I did not mention to him that they were not standard Van's ports.

I tried an experiment. I put a piece of tape over the hole in the port on the right side, and made a spacer the same thickness as the step on the left port, with a hole in it. I taped that on and then flew and found that the same error was there.

BTW, I just installed another ASI. The plane had two, one TAS gauge that had a bad return spring and read 7 mph higher than the other, and one a standard gauge. I pulled both and replaced the TAS gauge with another TAS gauge that came out of a -4. Readings were the same as on the non-TAS gauge I removed. I put a G-meter in the extra hole.

So, asking all the Guru's out there for any suggestion on what to do to get my ASI reading correctly. I am at a loss, and don't want to do anything rash, like moving a pitot, if I don't have to.

Thanks and Happy New Year!!

 

[DerekS]

Buzz the tower......

A diagnostic tool in one of the FAA test pilot guides to validate the accuracy of the static system, and there by figure out where your problem is to set the altimeter to minus the height of the control tower and then flyby at eyeball level at various speeds while recording indicated altitudes as accurately as possible.

Make sure you have a chat with them beforehand.....and put serious thought into how to do it safely,

Derek

 

[rv7charlie]

I'm no flight test engineer, but everything I've read indicates that ASI problems are more commonly static related than pitot related.

DerikS has a good suggestion. I'd just do it at an uncontrolled field 'out in the country' with little traffic. I had the opposite issue with my 1st -4. It had flush static ports and would indicate ~10 kts *slow* at cruise. I noticed one day while doing a 'low approach' (home field, uncontrolled) that the altimeter was reading about 50 or 75 feet *below* field elevation while I was 10-20 ft off the deck. Dan Horton (the Canadian one) told me about using a bump at the static port location.

Your situation actually sounds like you're getting a venturi effect with the bump in front of the port. Taping one port closed & changing the other configuration at the same time means you changed 2 things at once; difficult to evaluate effects.

If you do the flyby test and it indicates too high an altitude, how hard would it be to turn the static port assy's around with the bump behind the port & re-fly the test?

 

[Neal@F14]

ASI reading high and the error increasing the faster you go sounds like a leak somewhere in the static system.

I once had a static leak on my old Cherokee 140 and was amazed that I could suddenly change the airspeed +/- 10 MPH just by opening and closing the little pilot's side vent window

 

[RVDan]

The step in the static port you used is there to compensate for static position error on the airplane it came from (C-170). Obvioulsy this wouldn't apply to your airplane so the port is likely creating an error.

Pitot probes have to be grossly wrong to make a difference. Static ports can make large errors quite easily.

The easy way to check as mentioned by others is a pass down the runway low enough you can estimate your height and add on to the altitude obtained from an approach plate for a surveyed location, typically the touchdown zone. Before doing this test set your altimeter on the ground at the survey point.

 

[ccsmith51]

ASI reading high and the error increasing the faster you go sounds like a leak somewhere in the static system.

I once had a static leak on my old Cherokee 140 and was amazed that I could suddenly change the airspeed +/- 10 MPH just by opening and closing the little pilot's side vent window

I'm in the process of trying to decide what to use for new static ports that I will place in the correct position one bay back from where they are now. Any suggestions on what to use? The Van's rivets sound easy, anyone know the size?

 

[RVDan]

The step in the port got me a bit off track from normal troubleshooting but as someone suggested you should consider a leak as a possible problem. The stepped static source should probably be removed in any case.

ACS has several flush port options that provide for fittings to connect static lines to. This is far better than the kluge to get plastic line sealed to the rivet tail. Especially if you are IFR.

 

[ccsmith51]

Thanks, Dan, I think I will try a couple of these:

 

[lostpilot28]

I believe Dan is correct. The step in front of the static port will give a "low pressure" area just behind it. This would appear to be a higher altitude, so when the pitot is getting more air molecules than it should be (because it's not really at that higher altitude), it will show as higher airspeed.

You probably also have a fair amount of altimeter error once you get moving.

I think the flat pitot port shown above is the way to go.

 

[rvbuilder2002]

As far as the static system goes, the ports are from the same 172 that the engine, pitot, rudder pedals, instruments, etc., came from. They are strange looking devices with a step in them, as shown here:

As already mentioned, I am quite sure the (stepped) static ports are your problem. Particularly since your error seems to be mostly at the high speed end (at low, speed the shape of the port will have less influence).

The shape will definitely influence the static pressure. The step is usually used to induce a correction factor on an aircraft because of the location of the port. The step is usually located aft to raise the pressure slightly.

Your pitot probe is probably fine. The static ports should be changed to something known to work correctly.
Unfortunately, the ports mentioned above will not be curate either.
You should either use the protruding rivets in the kit from Van's or something that mimics them like THIS

 

[ccsmith51]

Scott, would the static air kit from Van's work? I am going to order a baffle kit today and could add that to the order.

Thanks,

 

[Bavafa]

I had a similar issue and the problem was my static port. In my case, the port was in the correct location but I had used a flushed-to-the-skin static port. One way that I knew my static port was incorrect was that after takeoff, I would see an altitude lower than the altitude that I had seen when on the ground.

I cut several nickel size plates of different thickness in addition to different rivet head, again of different size to tape it over the static port and tried it. What I found the rivet head that VANs suggest gave me the closest and best result, again based on flying over the airport to gauge my altitude correctness. That also brought my IAS closest to the correct speed.

Hope this helps.

 

[rvbuilder2002]

Scott, would the static air kit from Van's work? I am going to order a baffle kit today and could add that to the order.

Thanks,

Yes it would work and has proven to be very accurate when installed in the recommended locations.
Some people think the pop rivet is kind of cheesy.
In that case you can use the one I linked too which is intended to mimic that shape of the pop rivet port. I don't know how well they perform, but I have not heard of any problems from people using them. I do know that the older version (flat ones) like the photo you posted did not work well. You may have to use one like that anyway, depending on how large of a hole is in the skin from your original port(s).
Keep in mind that anything you do that influences the skin in that area might still induce error regardless of what ports you use.... The pop rivets (or something that imitates their shape, work well when installed on a totally flat skin, at the recommended locations.

 

[ccsmith51]

Thanks, Scott, static kit ordered from Van's. The existing ports are located one bay forward from where they are supposed to be. They are in front of F-606 instead of F-607. So, I will install the new ports in the correct location and leave the old ones where they are.

BTW, can you tell me the measurements for the proper location? It is not on my plans and I don't have the builders manual. Someone had said 2" forward of F-607 and 1" below the longeron, but I'd like to confirm that.

Thanks again to everyone for their help with this issue, and my exhaust issue.

Happy New Year to y'all!!!!

 

[rvbuilder2002]

BTW, can you tell me the measurements for the proper location? It is not on my plans and I don't have the builders manual. Someone had said 2" forward of F-607 and 1" below the longeron, but I'd like to confirm that.

No I can't because I don't have access to plans right now.
You could talk with someone in tech. support today, they should be able to give you the info you need.
From memory it is 1" fwd of the bulkhead and 2" down from the longeron.
I'm thinking it is fwd of the next bulkhead aft of the baggage bulkhead (which I think would be the 606 bulkhead).
Where some of the confusion has developed over the years is that it is different than the RV-4 and 8.... they have to be one bulkhead further back, and I think the RV-7/9 was spec'ed to be one bulkhead further back than the RV-6.
On a 6 or a 7, I think either location will work well as long as the correct port shape is used.

 

[ccsmith51]

No I can't because I don't have access to plans right now.
You could talk with someone in tech. support today, they should be able to give you the info you need.
From memory it is 1" fwd of the bulkhead and 2" down from the longeron.
I'm thinking it is fwd of the next bulkhead aft of the baggage bulkhead (which I think would be the 606 bulkhead).
Where some of the confusion has developed over the years is that it is different than the RV-4 and 8.... they have to be one bulkhead further back, and I think the RV-7/9 was spec'ed to be one bulkhead further back than the RV-6.
On a 6 or a 7, I think either location will work well as long as the correct port shape is used.

Thanks. I sent tech support an e-mail requesting the location. My parts won't be here for a bit, so I figured no need to tie up the phone when they could be helping someone that has an immediate need.

 

[SMO]

If you want to do some testing try your static ports with the dam reversed (just turn the port 180*) - this will provide a little positive pressure instead of the vacuum you currently appear to have, and you could try with the dam aligned with the airflow as well.

Just check your stall speed each time and modify your approach speed accordingly.

 

[sblack]

Try flying with the static port disconnected so the asi static is the same as the cockpit static. Close all your air vents. This will give you an idea of how much impact the steps are having. But long term they have to go. The vans pop rivet static ports are proven.

Lots of airplanes don't even bother with a static port. The asi and alt just vent to the cockpit. Not perfect but for a vfr airplane it can work fine.

 

[GalinHdz]

As far as the static system goes, the ports are from the same 172 that the engine, pitot, rudder pedals, instruments, etc., came from. They are strange looking devices with a step in them, as shown here:

They are mounted in front of F606 with the high side facing forward.

This sounds like a typical static error problem. With the high side facing forward, as your airspeed increases airflow over the static port high side (called an air dam) increases causing an ever decreasing pressure being felt at the hole. Just like the air flowing over the top part of your wing creates a "low" pressure area. This makes the ASI show an error that increases the faster you go. If you mount the "air dam" static port with the high side facing backwards then your airspeed error will be in the opposite direction. Now you are creating a "high" pressure area over the actual hole. You will show a slower airspeed than actual with the error getting worse the faster you go.

These "air dam" static ports are used on some aircraft to compensate for static locations that have pressure differential problems due to airspeed. Not common on RV aircraft. If the port is at the correct location, use a static port without the "air dam" and test again.

In a VFR only aircraft this error is mainly a nuisance. On an IFR aircraft this error can be deadly since it will also affect your altimeter which will indicate a higher altitude than what you really are. Not good when on a "hard IFR" approach.

 

[Yen]

Some here have said it could be a static leak and others seem to consider a pitot problem. You could do a leak test of the static system and also the pitot system to check these out. Then do a test of the pitot using a manometer, made of a U of flexible clear tubing to check the accuracy of the ASI. I can't remember the actual heights of water that equate to which airspeed but will attempt to find them. probably someone on these forums would have them to hand.
Doing that would narrow down the problem and save changing static ports unless really necessary.

 

[GalinHdz]

Buzz the tower......

A diagnostic tool in one of the FAA test pilot guides to validate the accuracy of the static system, and there by figure out where your problem is to set the altimeter to minus the height of the control tower and then flyby at eyeball level at various speeds while recording indicated altitudes as accurately as possible.

Make sure you have a chat with them beforehand.....and put serious thought into how to do it safely,

Derek

This is a very good test to do. It can verify a possible static system problem and eliminate a possible pitot system problem. You can also use the glideslope intercept point of an ILS which should be within 50ft of your indicated altitude at different airspeeds.

 

[swisseagle]

Do not change the Vans Rivet Static Port!

It is not the first time I read this and also have seen buddys doing it. As soon as you change the Vans blind rivet static port, all where having ASI reading offsets!

It was discussed many times in this forum. I can just tell you this:

- Do not change the Vans setup for the static port.
- You may install a static port tube adapter on the inside, but on the outside it should look exactly like the Vans blind rivet.
- If you still have wrong readings and/or you have paint edges/transitions in front of the static port, put a thin washer under the blind rivet. This to lift the rivet head more into the clean airstream. About 0.02" is fine.

Hope it helps!

 

[jjconstant]

My own personal experience reinforces the "don't change VANS static system". I had a heated pitot with built in heated static source so I just used that for static. During Phase One I had all sorts of problems trying to get the airspeed accurate. I pulled apart all the plumbing, tried different connectors between plastic lines, tried to make the static reservoir both bigger and smaller and then gave up and simply tried to accurately map the differentials from reality.

Then, still during phase one, setting up the auto pilot, I was getting pitch oscillations, but only in banks and more to one side than the other. I tried all the various settings and then resorted to the extreme measure of declaring defeat and calling the manufacturer. The VERY FIRST question they asked is "you wouldn't happen to be using a static source on a pitot tube under the wing, would you?" Why yes, yes I was, why?

The solution was to go back to use the exact static source, including the exact same rivet that VANS calls for. The dimensions and profile of the head is critical. Once I did that, my oscillations on auto pilot went away and indicated airspeed was much closer.

 

[RVDan]

I installed flush ports in a -6 and-6a over the years. I was fortunate enough to have a measured course for calibration tests, and did measured course airspeed calibration runs for both aircraft from 70 kts to 160 kts every 10 kts, with a maximum airspeed error of .61 kts. I would consider that acceptable performance for these aircraft. I can't say the what the difference to the rivet ports would be as I never got to test them, but clearly flush ports work fine on the -6/ -6A. Can't say if this is true for other RV's.

The underwing static port error problem is well known for many applications.

 

[ccsmith51]

I installed flush ports in a -6 and-6a over the years. I was fortunate enough to have a measured course for calibration tests, and did measured course airspeed calibration runs for both aircraft from 70 kts to 160 kts every 10 kts, with a maximum airspeed error of .61 kts. I would consider that acceptable performance for these aircraft. I can't say the what the difference to the rivet ports would be as I never got to test them, but clearly flush ports work fine on the -6/ -6A. Can't say if this is true for other RV's.

The underwing static port error problem is well known for many applications.

Dan,

PM sent.

 

[rvbuilder2002]

I can't say the what the difference to the rivet ports would be as I never got to test them, but clearly flush ports work fine on the -6/ -6A.

Dan,
I can't comment on why you seem to have had a different experience, but there is many years and countless experiences of installations that proved differently.
Enough so that at least one maker of flush static ports modified the design so that it mimics the shape of the standard Van's static port.

I strongly recommend people not take the chance with flush static ports.

 

[ccsmith51]

Update...

I got the Van's static port kit (rivets, tubing, etc.) and installed it exactly per instructions. The instruction sheet shows where to place the ports on the RV-6.

I did some GPS testing today. 1,600' altitude, about 62? OAT, 30.09 pressure. I flew the ordinals, stabilized airspeed, and took readings. I took the GPS speeds, totaled them, and divided them by 4 for an average.

At an IAS of 180 mph the averaged GPS speed was 181 mph.

At an IAS of 90 mph the averaged GPS speed was 89 mph.

I did multiple stalls both clean and dirty, and the clean stall was about 51 mph and the full flap stall was about 48 mph.

Those numbers are close enough for me.

So, my conclusion is that installing the Van's rivets in the Van's locations solved my incorrect ASI readings.

Thanks to all for the input.