Ongoing ASI errors

My airspeed has been reading 8-9 MPH high (compared to 4 leg GPS testing) at cruise speeds for the last several months. I found a static leak and fixed it, but that made no improvement. Last week I had my pitot static certification done and the tester indicated that that static system was very tight with no leaks. We also confirmed that the GRT ASI reads within 1 MPH of his test set.

This leaves me with a static port shape / location or pitot problem (a Dynon Pitot). I have the standard Van's recommended rivet, though I am not sure it is in the exact recommended location. Does anyone have details for the RV-6 location? I struggled to find it on the plans.

Beyond this, I am scratching my head on how to address this. The odd part is that I tested and confirmed fairly accurate airspeed's last year. I was certain it is was the leak, but it wasn't. Then I was certain the Dynon was the problem, but it had the same error as the new GRT that has been confirmed to be accurate.

Thanks for any guidance you can provide.

Larry

Do you know for sure it is the recommended rivet?

Post a photo.....

Could your GPS testing method be inaccurate, maybe a miscalculation somewhere in the spreadsheet?

I called Van's and asked when I replaced what the original builder put on when sorting this out last year. It read accurately after the replacement. I used the rivets from the engine baffle kit. The large headed pop rivets used to attach the rubber baffle seal. I would have to look up the actual part number.

Larry

blueflyer said:

Could your GPS testing method be inaccurate, maybe a miscalculation somewhere in the spreadsheet?

Click to expand...

I don't think so. I've done it too many times and simple averaging reaches a similar number. Maybe the experts can help me find an error. Here is the data from today's test:

cardinal headings are based on track
airspeed in MPH
True airspeed = 174
Indicated airspeed = 168
Forecasted Temp @ altitude = 1 C

GPS readings

N 160
W 159
S 169
E 170

Winds aloft forecasted as 310 @ 10

lr172 said:

I called Van's and asked when I replaced what the original builder put on when sorting this out last year. It read accurately after the replacement. I used the rivets from the engine baffle kit. The large headed pop rivets used to attach the rubber baffle seal. I would have to look up the actual part number.

Larry

Click to expand...

The baffle seal attach rivet is the correct one.
If installing the rivet previously corrected an error problem then I don't see how the current problem could be caused by the static port shape.

It takes a very stabil air mass to get accurate speed data. Flying a test box compensates for horizontal movement but not vertical. If there is vertical movement it will influence the data, though it is not likely that the data would account for an error of 8-9 MPH.

rvbuilder2002 said:

The baffle seal attach rivet is the correct one.
If installing the rivet previously corrected an error problem then I don't see how the current problem could be caused by the static port shape.

It takes a very stabil air mass to get accurate speed data. Flying a test box compensates for horizontal movement but not vertical. If there is vertical movement it will influence the data, though it is not likely that the data would account for an error of 8-9 MPH.

Click to expand...

I have always done these tests with the dual axis A/P on and wait until stabilized before capturing a reading. Today's test was in very calm air.

Larry

lr172 said:

My airspeed has been reading 8-9 MPH high (compared to 4 leg GPS testing) at cruise speeds for the last several months.

Click to expand...

You mention "for the last several months." What has changed? Was it reading correctly until recently?

Use this website to calculate your indicated and tas.

http://www.dauntless-soft.com/products/freebies/trueairspeedcalculator/

My 160 hp, -6 with 3 blade catto will do the following:

WOT 2660 rpm. Slightly overpitched.
7500ft indicated altitude
8000ft DA confirmed by calculator
Altimeter 30.03
Oat 5deg C
Indicated asi 163 mph
Tas calculated 184 mph
Tas from 4 way gps 182.5 mph

My pitot/static system has a 1.5 mph error somehwere. I have standard flush holes in the sides for static with no rivet heads.

If you use this website and input your data, it will tell you what your tas should be based on your indicated asi. You can subtract the difference between what you should be doing and the 4 way gps run to come up with a true error number.

lr172 said:

I don't think so. I've done it too many times and simple averaging reaches a similar number. Maybe the experts can help me find an error. Here is the data from today's test:

cardinal headings are based on track
airspeed in MPH
True airspeed = 174
Indicated airspeed = 168
Forecasted Temp @ altitude = 1 C

GPS readings

N 160
W 159
S 169
E 170

Winds aloft forecasted as 310 @ 10

Click to expand...

You say forecasted temp at altitude - did you measure the actual temp?

1001001 said:

You mention "for the last several months." What has changed? Was it reading correctly until recently?

Click to expand...

^^^^^^^^^^^^
This is important

Also, does the error increase the faster you go? If so, and it has always been incorrect, then more than likely your static port location is causing the error.

lr172 said:

I don't think so. I've done it too many times and simple averaging reaches a similar number. Maybe the experts can help me find an error. Here is the data from today's test:

cardinal headings are based on track
airspeed in MPH
True airspeed = 174
Indicated airspeed = 168
Forecasted Temp @ altitude = 1 C

GPS readings

N 160
W 159
S 169
E 170

Winds aloft forecasted as 310 @ 10

Click to expand...

This is obviously wrong. If you fly in different directions (as you have) then at some point you must have a tail wind. Your ground speed (GPS) cannot always be less than your TAS

Ironflight said:

You say forecasted temp at altitude - did you measure the actual temp?

Click to expand...

No. I have the GRT HX as a loaner and it doesn't show OAT on the PFD. I forgot to check. I have confirmed that my OAT is accurately reflecting ambient temps on the ground though. That said, it doesn't seem to be a temp issue, as even the IAS is a few MPH over the GPS speed.

Larry

BobTurner said:

This is obviously wrong. If you fly in different directions (as you have) then at some point you must have a tail wind. Your ground speed (GPS) cannot always be less than your TAS

Click to expand...

I can't see how the gps could have an error that would be off by 10 mph (would seem it either works or doesn't), so I believe that the indicated airspeed is inaccurate/has an error and therefore the TAS is inaccurate as it's electronically derived from the IAS.

EDIT: As I think about this, maybe I shouldn't blindly trust the 396. Next flight, I'll compare ground speed against the KLN-90

GalinHdz said:

^^^^^^^^^^^^
This is important

Also, does the error increase the faster you go? If so, and it has always been incorrect, then more than likely your static port location is causing the error.

Click to expand...

I have wracked my brain on this one and can't think of anything that changed. I had a leak in the static, but that is resolved without improving this situation. I don't know exactly when it happened. I did several tests over a year ago and was within 2 MPH after replacing the rivets. Last fall I remember seeing airspeeds that seemed to high for the RPM and started testing. It's possible it's always been off and I made the errors when testing after the rivet swap.

The error is at 8-9 at cruise and 1-2 at pattern airspeeds.

Larry

lr172 said:

I have always done these tests with the dual axis A/P on and wait until stabilized before capturing a reading. Today's test was in very calm air.

Larry

Click to expand...

Using the AP is good for testing accuracy, but it will also induce the biggest influence on the data if there is a vertical component to the air.
If you are flying through air that is rising, the AP will perfectly keep the airplane level by pitching nose down (making the airplane go faster)
If the air is descending, it will do the opposite and an attempt to hold altitude, making you go slower.

Smoothness of the air is usually a good indicator of stability of the air, but not always.

The fact that you have no tail wind component as part of your data could be an indicator that there are other influences involved.

lr172 said:

I have wracked my brain on this one and can't think of anything that changed

The error is at 8-9 at cruise and 1-2 at pattern airspeeds.

Larry

Click to expand...

Just guessing now but is your pitot tube inboard enough to 'see' prop wash?

lr172 said:

I have always done these tests with the dual axis A/P on and wait until stabilized before capturing a reading. Today's test was in very calm air.

Larry

Click to expand...

What do you have for pitot tube and is that in the correct location or orintation?

lr172 said:

The error is at 8-9 at cruise and 1-2 at pattern airspeeds.

Larry

Click to expand...

Do another test at fast cruise speed and see if the error gets even bigger.

Another test you can do (if you have an ILS) is fly towards the final approach fix at fast cruise while holding FAF altitude. When you are at the FAF, if your Glide Slope indication shows you are below the Glide Slope at the FAF you have just verified the static port is causing the problem. As you go faster, the pressure felt at the static port drops making the IAS as well as the indicated altitude error greater and greater.

lr172 said:

I have always done these tests with the dual axis A/P on and wait until stabilized before capturing a reading. Today's test was in very calm air.

Larry

Click to expand...

Do NOT do these test with the autopilot on track/course hold mode of any kind. Your AP will compensate for winds aloft and give you false readings. If you use an autopilot, make sure it is only holding a bearing and not following a ground track of any kind. IOW let the winds aloft do their thing to your airplane.

BobTurner said:

This is obviously wrong. If you fly in different directions (as you have) then at some point you must have a tail wind. Your ground speed (GPS) cannot always be less than your TAS

Click to expand...

Not necessarily. Consider this, the error in the TAS is wrong by 15MPH and wind a loft is only 5 MPH. In this situation, you will see a slower ground speed regardless of headwind or tailwind.

BobTurner said:

Just guessing now but is your pitot tube inboard enough to 'see' prop wash?

Click to expand...

I have no real idea how far out the prop wash goes, but the pitot is past the half way point on the wing. Outboard of the access panel on the RV-6.

Larry

Is the GPS reading in knots instead of mph? You didn't put units on the GPS ground speeds, but said your airspeed is in mph.

Bavafa said:

What do you have for pitot tube and is that in the correct location or orintation?

Click to expand...

It is the dynon heated pitot, installed with the safeair stalk. It is within a degree or two of in-line with the longitudinal axis and near the spar laterally. I have kinda of written off the pitot as a cause. I can't see how it would get excess pressure to the sensor, only ways it could get less.

Ironflight said:

Is the GPS reading in knots instead of mph? You didn't put units on the GPS ground speeds, but said your airspeed is in mph.

Click to expand...

Yes GPS is in MPH. I had considered a configuration error here, but I was getting a 165 average and that would be 190 MPH. I was only turning 2440 RPM (fixed pitch) and burning 6.7 GPH, at 3500'. Based upon my experience and my research, it is not possible to achieve 190 MPH at that fuel burn in an RV-6A.

Larry

Left field

I have no idea what i am talking about, but what about water in the pitot or static lines?

Static Source Pressure Error

Larry,

Lot's of good advice so far...

It's a bit of a multi-step process to sort out your airspeed error. Sounds like you've started your homework and checked static system integrity.

The next, simple flight test step is to properly set the altimeter to read field elevation, then take off, fly a "closed" pattern, and make a high speed pass (approximately Vno) at very low altitude above the runway and note the altimeter reading. If it's not accurate or very close to what you noted before starting the takeoff roll, then you need to further investigate static source pressure error. Obviously, there is some risk associated with this flight test technique, and if you aren't comfortable operating in this manner, you can simply omit this step or, perhaps, get another pilot to assist.

The next step is to build a simple manometer. This will allow you to check instrument accuracy and system integrity if you have a conventional ASI and pitot system integrity if you have an EFIS. If you have electronic instruments, be very careful how you apply pressure. For example, Dynon publishes a white paper detailing pitot/static test techniques. By and large, the transducers in electronic instruments are highly accurate, thus any error will be associated with plumbing (installation). Conventional ASI's do have some instrument error in addition to the plumbing error. The manometer will help you sort this out at zero G's and no knots. Here are some links that might help:

http://www.rst-engr.com/rst/articles/KP89JUL.pdf

http://www.eaa1000.av.org/technicl/instcal/instcal.htm

http://a.moirier.free.fr/Instruments/Badin/Building%20a%20manometer.pdf

A google search will turn up some more helpful information on the topic. Also, there is good discussion in 43-13 on how to use rubber surgical tubing to test if you decide to forgo the manometer.

The next step is to flight test doing GPS runs, and, as Paul pointed out, it's necessary to have OAT information to do that. If not equipped, you can approximate using standard lapse rates (2 deg C/1000 feet) from a nearby station (ASOS, ATIS, etc.). Not as desirable as an on-board sensor, but better than omitting OAT from consideration. Best to hand-fly in smooth air (early morning, night or late afternoon); so some days will simply be unsuitable for air work. If you have electronic flight displays, the trend indicators are remarkably helpful when hand-flying to make very fine, accurate corrections.

Then you'll have to plug the numbers into a spreadsheet that calculates static errors. Kevin Horton has some outstanding discussion over on his site worth reading if you haven't already been there: http://www.kilohotel.com/rv8/index.php?option=com_jdownloads&view=category&catid=2&Itemid=218. If you need a spreadsheet that calculates static source pressure error based on ground testing and GPS runs, drop me a PM and I'll be happy to share one with you. Even though you have the Van's recommended static port, you may still need to bias the airflow around the port (although that's not too likely) after you've determined both the pitot and static systems are leak free, no water is present and have a handle on ASI accuracy.

Good luck!

Vac

I will do some further altitude testing to confirm the static error. Yesterday I noted the difference between my baro alt and GPS alt at 175 MPH. I then slowed to 120 MPH and noticed that the difference changed by about 50 feet. It was not a well structured test, just gave me a data point.

Later today I will go collect some better data and determine the actual altitude error vs airspeed and report the data here. That should give us better info to confirm it's that static port error.

Larry

lr172 said:

I have no real idea how far out the prop wash goes, but the pitot is past the half way point on the wing. Outboard of the access panel on the RV-6.

Larry

Click to expand...

Prop wash could cause some issues if the pitot tube is not installed in the correct location, but it looks like you have already checked it.

EDIT: Disregard. I think that I figured it out. I put the paint on a bit too heavy in the area of the rivet and got a very small run on the forward side of the rivet, hugging the rivet shape. This is creating an effect similar to what people have done with o-rings in front or behind the rivet. Because they are on the forward side, it is creating a low pressure. I will pull the rivets and sand/scrape the paint down flat. I may need a spacer to lift the rivet up to the paint level as well, as the rivet doesn't have paint. That should resolve my issue. I'll report back with results.

Ok, ready to start playing with the rivets to force out the apparent low pressure I am getting at the static rivets. I have seen several posts with folks making a small dam behind the rivet to increase the pressure. I have also heard of folks filling off part of the rivet head. I prefer the latter approach and wonder if anyone has done this successfully and could share their approach.

Larry

Well, I re-did the rivets so that it was a smooth transition and absolutely no change. I am really pulling my hair out on this. No leaks. Standard Van's rivet in standard location. What am I missing here? EFIS tests within 1 MPH of the calibrated test equipment. Could it be an issue with the Dynon pitot?

Larry

Do the high speed pass suggested by others. If the altimeter reads high then you know the issue is the static system.

BTW, the simplest way to make a static port dam is with layers of electrical tape. This is how I dialed in my ASI. Adjustments to the dam thickness is a piece of cake, and the tape sticks just fine.

I used Gorilla tape since it is thicker than electrical tape. Once I figured out how thick the dam needed to be then I made a permanent one out of aluminum. Fairly easy to do, just a bit time consuming.

I'll try the dam. Are you using some kind of crescent shape or just putting squares of tape with a cut out of the riivet?

I am guessing this is like a washer cut in half. Is that what typically works?

Larry

I used squares of gorilla tape. Then made a 1/2 moon wedge and bonded it (in my case behind) as close to the static hole as the tape was. I will get a pic of the finished dam on Saturday.

I started with a bunch (5 or 6) of layers of ~1" strips of electrical tape in front of the static port. The goal was to see a significant change in the ASI as determined by running a 3 leg GPS and plugging the result into Kevin Horton's spreadsheet. From there I would add or take away as required until I got it nailed. Shape didn't seem to make a significant difference.

BTW, each time I made a change, during the subsequent test flight I would also do a stall to check that I could still use the same approach speed.

I also made an aluminum piece to replace the tape when I was satisfied. I have been able to repeatedly confirm my ASI is within 1 knot at 190+ knots.

It is not physically possible for a pitot tube to read high. (except if it is in propwash or other source of increased flow energy).

You have not said anything about the ASI instrument yet - or did I miss it? Make a manometer and check the calibration of your ASI. Mine reads 7 kts high at 60 kts.

Very first post, he says ground check showed ASI within 1 knot.

lr172 said:

I have wracked my brain on this one and can't think of anything that changed. I had a leak in the static, but that is resolved without improving this situation. I don't know exactly when it happened. I did several tests over a year ago and was within 2 MPH after replacing the rivets. Last fall I remember seeing airspeeds that seemed to high for the RPM and started testing. It's possible it's always been off and I made the errors when testing after the rivet swap.

The error is at 8-9 at cruise and 1-2 at pattern airspeeds.

Larry

Click to expand...

HI Larry,

If you were previously getting a smaller error with the same static port, it doesn't seem very likely that the static port is the problem. You can try to tweak the local pressures at the port with the placement of tape etc., but this might just be masking another problem (no pun intended). I know you had the static system checked out but I'd suggest checking this again. You mentioned you had a leak previously but that it was resolved without improving the situation. This sounds rather suspicious; if there was a leak previously then fixing it should have made some kind of difference.

Just for reference, when I vent my static system to cabin air at cruise speed the indicated altitude jumps up 100+'. 100' is ~0.1" of Hg, which equals 1.36" of water. According to the test data (AN 05-10-24) presented in Jim Weir's article that was referenced in an earlier post, a static pressure decrease of 1.36" water translates into an increase of ~6 knots in airspeed (interpolating between 160 and 170 knots). This assumes I'm doing the calculation correctly!

Good luck.

His error changes with airspeed (increases the faster he goes) which is a tale tell sign of a static port position error if he already corrected the leaks.

Alan Carroll said:

HI Larry,

If you were previously getting a smaller error with the same static port, it doesn't seem very likely that the static port is the problem. You can try to tweak the local pressures at the port with the placement of tape etc., but this might just be masking another problem (no pun intended). I know you had the static system checked out but I'd suggest checking this again. You mentioned you had a leak previously but that it was resolved without improving the situation. This sounds rather suspicious; if there was a leak previously then fixing it should have made some kind of difference.

Just for reference, when I vent my static system to cabin air at cruise speed the indicated altitude jumps up 100+'. 100' is ~0.1" of Hg, which equals 1.36" of water. According to the test data (AN 05-10-24) presented in Jim Weir's article that was referenced in an earlier post, a static pressure decrease of 1.36" water translates into an increase of ~6 knots in airspeed (interpolating between 160 and 170 knots). This assumes I'm doing the calculation correctly!

Good luck.

Click to expand...

I have checked a lot and the IFR static check in January showed the system as very tight. I was on a flight yesterday at 9500' and for testing, I opened my alt. static input and the VSI immediately read a 1000 FPM climb. I did not check the actual altitiude change though. I am pretty sure this is not a leak.

Larry

Thanks for details on your approaches to resolution here. I will try some square pieces of tape this weekend and see if I can get it dialed in.

Larry

SMO said:

I started with a bunch (5 or 6) of layers of ~1" strips of electrical tape in front of the static port. The goal was to see a significant change in the ASI as determined by running a 3 leg GPS and plugging the result into Kevin Horton's spreadsheet. From there I would add or take away as required until I got it nailed. Shape didn't seem to make a significant difference

BTW, each time I made a change, during the subsequent test flight I would also do a stall to check that I could still use the same approach speed.

I also made an aluminum piece to replace the tape when I was satisfied. I have been able to repeatedly confirm my ASI is within 1 knot at 190+ knots.

Click to expand...

Except that I used gorilla tape instead of electrical tape, this is exactly how I did mine and got the same results SMO got. ASI is within 1kt across my entire range.

Thanks for the additional clarification. I noticed SMO put tape in front of the rivet. I thought I read that approach would raise the airspeed and material aft of the rivet would reduce the airspeed. Is that how it worked for you guys?

Larry

lr172 said:

Thanks for the additional clarification. I noticed SMO put tape in front of the rivet. I thought I read that approach would raise the airspeed and material aft of the rivet would reduce the airspeed. Is that how it worked for you guys?

Larry

Click to expand...

Yes, you are correct. I have mine aft since my airspeed error increased as my airspeed increased, ASI was reading high. Apparently SMO's error was opposite of mine so his is in front.

I put mine in front because my ASI was reading low (there was too much pressure at the static port).

Here is how my simple mind thinks about it:

The ASI effectively subtracts static pressure from pitot pressure and displays the result as airspeed. If airspeed is reading high, assuming pitot pressure is correct (you have checked the pitot system for leaks?), then static pressure must be low (the difference is greater than it should be).

How does static pressure get to be too low? Because the passing airflow is causing a low pressure area at the static port(s) effectively sucking air out of the static system. So in this case you need to put a dam aft of the static port(s) to create a little higher pressure at the static port(s).

With respect to the pitot system, I checked mine my making a simple manometer, connected it to the pitot tube and pressurized the system (by blowing in the tube, takes very little pressure) to 200 knots. Confirmed it would hold pressure. I recall you have confirmed the ASI reads correctly - if the test was done through the full pitot system then this has been covered off.

Here is a close up photo of the aluminum "wedge" I made that corrected the ASI error. The "wedge" looks big in the photo but it is less than 1/16" thick. They don't have to be real pretty to work.



Since I have two static ports I wound up making and installing two wedges, one on each port. Make sure you take that into account when you are doing your test flights or your "wedge(s)" won't work correctly.

Thanks for posting the pics. Today I did some experimenting. 10 squares of electrical tape got me within a mph or 2. It was about .055" I will try making an aluminum piece in .061" and play with that.

Thanks again for the assistance.

Larry

I ended up making the air dams out of .063" AL and my ASI at cruise seems to be about dead on. However, since adding the air dam, I have felt that my approaches have been fast, with floating. I did some stalls and my stall speed is about 7-8 MPH faster than during my Phase I. I did some GPS boxes and at 80 MPH, My Indicated is about 4 MPH high. I suspect it is even more at 60 MPH, but haven't tested there yet.

For those of you that have added these dams to increase static pressure, have you found similar results? Maybe the shape of my dam is part of the cause. Here is a picture of mine. It is just taped on until I complete testing.



Larry

lr172 said:

I ended up making the air dams out of .063" AL and my ASI at cruise seems to be about dead on. However, since adding the air dam, I have felt that my approaches have been fast, with floating. I did some stalls and my stall speed is about 7-8 MPH faster than during my Phase I.
Larry

Click to expand...

Yes, this was my experience too. Without air dams, my airspeed calibrations showed I am right on at slow speeds but that I read about 6 knots slow at high speed. I could add dams to get the cruise TAS accurate but it would then throw off the stall speeds. I ended up deciding I could live with a TAS that indicated about 5-6 kts too slow at cruise and so I removed the dams altogether. It would be nice for it to read perfect at all speeds, but does it really matter? I don't think so.