What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

IAS error 7 MPH a lot?

Iluke

Well Known Member
Hi everyone,

I'm doing my test cards as part of phase 1 and have determined my IAS is reading about 7 MPH low at cruise. Haven't finished all the tests yet, but it seems more accurate at lower speeds.

It's an RV-7 with a Dynon Skyview system including their pitot tube and stock Vans static ports.

7 MPH error at cruise sounds like a lot to me- Is it?

I previously tested my pitot system for leaks and found none. Any suggestions for where else to start looking fora problem?

thanks
Ivan
 
Manometer

Search the post for building a Manometer. It will tell you if your indication is good or not. If good then it is probably your static port.
 
A 7 MPH error will make the Dynon wind indications unusable. As mentioned it’s probably the static ports. A jewelers file and several test flights fixed my issue.
 
Thanks everyone-

Sailvi767- What did you do with the jewelers file that helped?

BillL- I will shoot and post some pictures as soon as I can get back to the airport.

Cdeggz- It is reading slower that actual. IAS indicating 150 mph when calculations based on GOS ground speed, density altitude and flying a three leg route to average out wind says it should have been 157 MPH.


-Ivan
 
Thanks everyone-

Sailvi767- What did you do with the jewelers file that helped?

BillL- I will shoot and post some pictures as soon as I can get back to the airport.

Cdeggz- It is reading slower that actual. IAS indicating 150 mph when calculations based on GOS ground speed, density altitude and flying a three leg route to average out wind says it should have been 157 MPH.



-Ivan

I have the rivet head style static ports. They needed to be filed down to provide the best airspeed reading. I would test fly using the below TAS calculator, land and file then repeat. Took 3 flights to get it right.
http://www.csgnetwork.com/tasgpscalc.html
 
Last edited:
Adjust the indicated airspeed for temperature and altitude to determine "true airspeed": you may very well find it is indicating closer to what it should than you would first think.
 
Static port problem?

Please post a picture of your static port as installed. This will help.

Thanks all for the replies. I collected and analyzed more data and confirmed that my IAS is reading low across the speed range, suggesting high pressure in the static system.

I am posting pictures of my static ports and would welcome any thoughts.

How do I know IAS is low? I ran the three-leg test recording GPS ground speed, averaging it to adjust for wind, corrected to TAS using an electronic E6B for temperature, altitude and altimeter setting. I consistently found IAS 4-5 MPH low at pattern and approach speeds and 7-9 MPH low at cruise.

This suggests that the pressure differential between the pitot and static lines is less than it ought to be, meaning either the pitot side is too low or the static side is too high. I did a leak-test on the pitot side and found no significant leak-down. So my suspicion is that the static ports are somehow resulting in high pressure.

Is my logic sound?

Thanks, -Ivan
 

Attachments

  • Static port 1.jpg
    Static port 1.jpg
    88.6 KB · Views: 297
  • Static port 2.jpg
    Static port 2.jpg
    98.3 KB · Views: 296
I had the opposite happen where an incorrectly installed static port caused a low pressure burble and a higher than actual airspeed. This magnified at higher speed.

Also skydivers when positioned just right on the rear step of a king air, sometimes caused altitude to jump a few hundred feet.
 
To troubleshoot the static port airflow you could take a piece of vinyl wrap lay it over a 12x12 area and then poke the static hole with a pin. Go fly and see if it corrects any airflow issues. Easy to remove and minimal effort.
 
Thanks all for the replies. I collected and analyzed more data and confirmed that my IAS is reading low across the speed range, suggesting high pressure in the static system.

I am posting pictures of my static ports and would welcome any thoughts.

How do I know IAS is low? I ran the three-leg test recording GPS ground speed, averaging it to adjust for wind, corrected to TAS using an electronic E6B for temperature, altitude and altimeter setting. I consistently found IAS 4-5 MPH low at pattern and approach speeds and 7-9 MPH low at cruise.

This suggests that the pressure differential between the pitot and static lines is less than it ought to be, meaning either the pitot side is too low or the static side is too high. I did a leak-test on the pitot side and found no significant leak-down. So my suspicion is that the static ports are somehow resulting in high pressure.

Is my logic sound?

Thanks, -Ivan

How much is "no significant leak down"? (what is the leakage rate)

It is hard to tell from your photos with nothing to judge scale by, but it looks like the static ports are a shape / size different from the ones supplied by us.
 
I have the rivet head style static ports. They needed to be filed down to provide the best airspeed reading. I would test fly using the below TAS calculator, land and file then repeat. Took 3 flights to get it right.
http://www.csgnetwork.com/tasgpscalc.html

Before I started taking a file to anything, I'd probably look at cleaning up all that paint bleed around the static ports the OP has...maybe it will help, maybe not, but it couldn't hurt to get a neater, crisper paint edgeline around those ports.

Pulling a new rivet if you file away too much is one thing, but the OP has a different style that would require considerably more work to repair if it gets F'ed up.
 
Before you do any of the above, check if your displayed altitude is correct while flying at "high" cruise speed. DO NOT USE GPS ALTITUDE FOR THIS! If you also have an altitude error then your problem is the static source. If your altitiude is correct, then you have a pitot problem. With this info you actually know where the problem is and can fix it.

:cool:
 
Last edited:
Ivan, Scott has the most experience in the ports and effects. Here is a Kitplanes article on static ports that might be helpful background.
 
DO NOT USE GPS ALTITUDE FOR THIS!

Is this due to inconsistencies in the geoid datum that the GPS uses to extrapolate MSL?

For what it's worth, I've found the WAAS GPS to be pretty accurate. I figure if it's good enough to do coupled LPV approaches, it good enough to determine accuracy of static system...
 
Is this due to inconsistencies in the geoid datum that the GPS uses to extrapolate MSL?

For what it's worth, I've found the WAAS GPS to be pretty accurate. I figure if it's good enough to do coupled LPV approaches, it good enough to determine accuracy of static system...
Nope. It's because Baro altitude is different than GPS altitide so you will be comparing apples to oranges. Remember barometric altitude changes based on the air pressure felt at the static port while GPS altitude does not. LPV approach plates already take this difference into consideration when they are designed.

BTW, this is a common mistake many people make when doing aviation related calibrations. Especially with modern EFIS systems.

;)
 
Last edited:
Nope. It's because Baro altitude is different than GPS altitide so you will be comparing apples to oranges. LPV approach plates already take this into consideration.

BTW, this is a common mistake many people make when doing aviation related calibrations, especially with modern EFIS sydtems.

Thanks Galin, However, if I adjust the altimeter for local barometer it should be close, no? I know the barometer at field elevation is going to be different at altitude because of lapse rate, but....
 
Thanks Galin, However, if I adjust the altimeter for local barometer it should be close, no?
Yes, in a properly installed static system is should be close. But with the info on this thread I am not convinced the OP has a good static setup hence the recommendation. Especially if the altitude error changes with airspeed.

Ivan, Scott has the most experience in the ports and effects. Here is a Kitplanes article on static ports that might be helpful background.
THIS is a superb article on this issue. One I highly recommend reading before doing changes.
 
Last edited:
One way to do Galin’s test is to land at an airport, set the altimeter to field elevation, then take off and do a high speed pass. You might be able to use something of known height - top of a control tower, or light poles, etc., to visually establish your elevation. Compare to altimeter. As long as you are close to the runway the altimetry error should be low.
 
Last edited:
Just a further comment about barometric altimetry. When you shoot an approach into an airport at anything other than sea level, you get an altimeter setting that is not ‘barometric pressure converted to sea level’, despite what handbooks may say. You get an altimeter setting that makes a properly calibrated standard altimeter read field elevation when you’re on the runway.
 
One way to do Galin’s test is to land at an airport, set the altimeter to field elevation, then take off and do a high speed pass. You might be able to use something of known height - top of a control tower, or light poles, etc., to visually establish your elevation. Compare to altimeter.
Yes, this is a good way of doing the test. But make sure you are at a "high speed" during the pass.

FWIW you can read how I fixed this issue on my plane by downloading the SkyView TAS Calibration file from the Maintenance section of my webpage www.puertoricoflyer.com/Maintenance

:cool:
 
Last edited:
Yes, in a properly installed static system is should be close. But with the info on this thread I am not convinced the OP has a good static setup hence the recommendation. Especially if the altitude error changes with airspeed.


THIS is a superb article on this issue. One I highly recommend reading before doing changes.

I am chasing a similar issue -- TAS in my airplane (RV-7) displays about 9 knots lower than previous, similarly equipped RV-7. The results from the NTPS spreadsheets show a TAS (Calculated) of 173KTAS while the G3X shows 164KTAS (148KIAS).

I've leak checked pitot system, plugging the drain holes on the Pitot tube and verifying that there was no leak down.

I've leak checked static system as well -- no leaks.

My static ports are in the location specified in RV-7 DWG 28 Detail E, but they are NOT SD-42-BSLF rivet bodies, rather they are machined parts made to match the profile of the pop-rivets, but have a threaded after-body and barb to allow for direct, gas tight connection to .170"ID poly tubing.

My altimeter (adjusted for barometric pressure) shows TDZE at the runway threshold and matches GPS Altitude at this point. As the aircraft accelerates and climbs to test altitude, I can see a divergence between the two altitudes, reaching a 300ft difference at the test altitude, power, and IAS, e.g. 6500 (indicated) FT, GPS shows 6800ft, 147Kias, 2500RPM, Full Throttle, 9.7gps, 50°LOP. This is repeatable, despite different OATs and pressures, but same (similiar) IAS and test altitude (6500), altitude delta is 300ft (GPS: 6800' vs G3X Altimeter @6500').

Based upon the observed behavior my inference is there is a high pressure area at the static ports forming and scaling with airspeed and altitude causing the instruments to register a lower IAS and lower altitude than actual. And, of course, throwing everything else off -- Winds aloft, TAS, etc.

....but this is just me....
 
Last edited:
This is repeatable, despite different OATs and pressures, but same (similiar) IAS and test altitude (6500), altitude delta is 300ft more on the GPS.
..

As just discussed, this should not be repeatable. For the same gps altitude you should see different barometric altitudes, if the lapse rate/OAT varies flight to flight. As also discussed, the lower you go the less the difference should be. If you see a 300’ difference at 1000’ agl you for sure have a problem.
 
As just discussed, this should not be repeatable. For the same gps altitude you should see different barometric altitudes, if the lapse rate/OAT varies flight to flight. As also discussed, the lower you go the less the difference should be. If you see a 300’ difference at 1000’ agl you for sure have a problem.

See https://apps.savvyaviation.com/flights/5317030/33486f51-0ee3-4cd0-a2c8-ed7cdd22e085

Slight correction -- OAT was almost the same (+/- 4°) and Baro was similiar between test series...

As I mentioned the difference between runway elevation and GPS is essentially nothing -- maybe < 10 ft. As the flight progresses, the two diverge. Of note, I climb at a constant (or as best as the AP can hold) IAS ~ 115 - see data at 14:40 into the flight.
 
As just discussed, this should not be repeatable. For the same gps altitude you should see different barometric altitudes, if the lapse rate/OAT varies flight to flight. As also discussed, the lower you go the less the difference should be. If you see a 300’ difference at 1000’ agl you for sure have a problem.

Yup, hence a big reason not to use GPS altitude for these checks. Always compare apples to apples and oranges to oranges. :cool:
 
Did you try disconnecting a static line at the instrument panel. I believe cabin pressure is slightly lower than outside static pressure due to airflow around the airplane. You could have an inaccurate airspeed indicator!
 
A loooong time ago when I was in Aero Engineering school, we did flight testing in a Cessna 206. To validate static pressure we had a 200’ or so plastic tube that was deployed in-flight through the tail cone (don’t recall details of the exit) that had a funnel attached to the end for stability, and a hole in the tube about 2 feet fwd of the funnel. The opening in that tube was considered far enough from the plane to give an accurate static reading. We compared the tube pressure to the aircraft static pressure across a range of airspeeds.
So if you really want to calibrate your static source…
 
How much is "no significant leak down"? (what is the leakage rate)

It is hard to tell from your photos with nothing to judge scale by, but it looks like the static ports are a shape / size different from the ones supplied by us.

Hi Scott,
Thanks for the reply. No, the static ports are billet pieces I got from Cleveland, mostly because they looked way more cool than your pop rivets. now I wish I had used your pop rivets!

Pitot leak-down was less that that allowed by the AC as I recall. I believe it sets a limit of 2 kts per minute o something similar. Mine was less than that.

-Ivan
 
One way to do Galin’s test is to land at an airport, set the altimeter to field elevation, then take off and do a high speed pass. You might be able to use something of known height - top of a control tower, or light poles, etc., to visually establish your elevation. Compare to altimeter. As long as you are close to the runway the altimetry error should be low.

Thanks, Bob,

I hope to find a way to test my altitude reading at high speed, but frankly I am not entirely comfortable making a high-speed low-altitude pass, at least not until I have more hours in the airplane. I'll look for a tall radio tower with a know altitude to fly-by.

-Ivan
 
Thanks, Bob,
I'll look for a tall radio tower with a know altitude to fly-by.

-Ivan

Ack! Watch out for nearly invisible guy wires. Just remember, the higher you go, the more indicated altitude might differ from true altitude. With Fall here, you may find a day with or close to a standard lapse rate (15 C at sea level: 0 C at 7500’). Or, alternatively, if you measure the OAT at altitude and ground, there are approximate methods to correct barometric altitude to true altitude.
 
Don't complicate things and use the ILS Glideslope method for verifying altitude. Safe, acurate and easy to do. :cool:
 
Last edited:
Thanks all for the replies. I collected and analyzed more data and confirmed that my IAS is reading low across the speed range, suggesting high pressure in the static system.

I am posting pictures of my static ports and would welcome any thoughts.

How do I know IAS is low? I ran the three-leg test recording GPS ground speed, averaging it to adjust for wind, corrected to TAS using an electronic E6B for temperature, altitude and altimeter setting. I consistently found IAS 4-5 MPH low at pattern and approach speeds and 7-9 MPH low at cruise.

This suggests that the pressure differential between the pitot and static lines is less than it ought to be, meaning either the pitot side is too low or the static side is too high. I did a leak-test on the pitot side and found no significant leak-down. So my suspicion is that the static ports are somehow resulting in high pressure.

Is my logic sound?

Thanks, -Ivan
Yes. I like your method and logic. &#55357;&#56397;

You checked for leaks but said no significant leak? There shouldn't be any leak. You should be able to pull a vacuum and it hold.
https://youtu.be/DN-74HluxC0

As far as your GPS averaging of course you have to do three legs 120 degrees apart. Record each leg separate and divide by 3. Smooth air is very helpful.

I agree static side is the biggest culprit typically. Tape on a bump forward of static port to break up laminar flow. You have 2 ports opposite side of fuselage righr.

Your static ports look different. Also one looks rounded. The surface finish and paint around then and at edge of ports is rough and uneven. If port is round that is a problem. If they are different L & R that is an issue. Paint should be even and clean. The port has a lip to break up laminar flow.

You can hook up a static cone that hangs out past the plane. This is undisturbed are and should be ambient.
https://youtu.be/nahalMXlLTA
https://youtu.be/kZsi4A5PcTw
https://youtu.be/IgCiiSXwyNk

You have Dynon? If you can get analog airspeed gage to try.

A really good avionics shop and Tech can test your pitot, static system, altimeter tracking, and while at it get transponder sign-off.
 
Last edited:
...

I agree static side is the biggest culprit typically. Tape on a bump forward of static port to break up laminar flow. You have 2 ports opposite side of fuselage righr.

Your static ports look different. Also one looks rounded. The surface finish and paint around then and at edge of ports is rough and uneven. If port is round that is a problem. If they are different L & R that is an issue. Paint should be even and clean. The port has a lip to break up laminar flow.

You can hook up a static cone that hangs out past the plane. This is undisturbed are and should be ambient.
...

Anyone know why we don't get static pressure from inside a wing or other area where the air is not moving that much and is at the external air pressure?
 
Anyone know why we don't get static pressure from inside a wing or other area where the air is not moving that much and is at the external air pressure?

What external pressure? Pressure on top is lower than pressure on bottom. Exactly how does air leak into the wing? You have the same issues as when using internal cabin pressure as a static source. Vents open, heat on,… all make a difference.
 
As far as your GPS averaging of course you have to do three legs 120 degrees apart. Record each leg separate and divide by 3. .

This is bad math, and untrue. The advantage of three legs is that there is sufficient data to calculate both true airspeed and the speed of the wind. But a simple average is incorrect.
 
I suggest you make the simple, but very accurate manometer and verify if your airspeed instrument is working properly or not.

If you cannot find the speed template on line, PM your email to me and I’ll provide.

Carl
625-C6532-1-C32-45-F0-B1-DE-5-EEAA725-FFC7.jpg
 
+ 1 on water manometer

Physics doesn’t lie. Make sure the device is vertical in two planes when recording measurements.
 
This is bad math, and untrue. The advantage of three legs is that there is sufficient data to calculate both true airspeed and the speed of the wind. But a simple average is incorrect.

Tell me your method. The math is simple. The three legs and GPS SPEED is averaged. (V1 + V2 + V3)/3 = Average TAS Speed. The idea is to average out WINDS. It is called the wind triangle. You can do FOUR LEGS but it does not result in significant better numbers. There are errors and there are many methods (about 4). :D (I forgot to say you need to calculate IAS) .

My main Flt test method is not to use the GPS (only as back up). I fly low (500 ft AGL) over a N-S freeway (sparsely area). The freeway has fixed distance markers on ground. I fly both ways and used a stop watch. The winds were sometimes calm, sometimes out of the North. AWOS at nearby airport gave me the winds as well. The main point is calm winds or winds aligned with freeway. The down side is flying low to the ground. This is legal as long as you are 500 ft from people and objects.

EDIT: My 3 leg triangle simple math method works but is not as accurate as the 3-leg horseshoe method making 90 degree heading change and using a spreadsheet to crunch the numbers. https://www.kitplanes.com/flight-testing-finding-tas-from-gps-data/

I have posted this before but this website is very easy to use and will give you a exact speed. It actually works off 3 90 degree turns.
http://www.csgnetwork.com/tasgpscalc.html
I now recall the three leg horseshoe 90 degree heading method. https://www.kitplanes.com/flight-testing-finding-tas-from-gps-data/


NOTE: All GPS methods have errors in part because winds are variable and GPS error rounding to only one significant digit.

There is a two leg METHOD with GPS. Fly making shallow turns and check GPS for slowest ground speed. That is straight into the wind. With fancy EFIS it gives you wind speed and direction. ONCE you know wind direction you can make TWO RUNS, one into the wind and do a 180 and second leg downwind. Average the two ground speeds by simple math That is your TAS. Calculate IAS using altitude and temp.
 
Last edited:
Tell me your method. The math is simple. The three legs and GPS SPEED is averaged. (V1 + V2 + V3)/3 = Average TAS Speed..

Just a calculated example: I used TAS=150 knots, wind = 30 knots from 180, fly GPS ground tracks of 030, 150, 270. If everything is perfect the gps ground speeds will be 175.2, 123.3, 147.0 knots, respectively. Note the average is 148.5, not 150. It just isn't that simple. Is 1.5 knots important? Probably only for bragging rights.
Curiously, for the same numbers as above, but this time just flying N then south and averaging gps ground speeds, of course you get 150 knots as you should. But what if you're wrong about the wind direction? If you're off by 20 degrees it only makes an error of 0.4 knots. Off by 40 deg, error just 1.3 knots. This is why I advocate the "into the wind, with the wind, average GPS ground speed" method. It's the quickest, and that, I think, minimizes other uncontrollable errors like the winds changing with time or physical location, etc.
 
Bob,
You are wrong! The runs are not "ground tracks" they should be headings: 90 or 120 degrees apart. This difference lets the ground track drift with the air mass, effectively canceling out the effects of the wind.
 
Bob,
You are wrong! The runs are not "ground tracks" they should be headings: 90 or 120 degrees apart. This difference lets the ground track drift with the air mass, effectively canceling out the effects of the wind.

I’m sorry, Boyd, but I’m not wrong. You can easily verify this yourself without doing any calculations, for the case where the wind is out of the south, and you fly N, S, E, and W headings. The average gps ground speed of the N and S heading runs will equal true airspeed, as previously discussed. For the E and W runs, you will drift north. But the gps sees this, as it measures ground track and ground speed. For both legs, the gps ground speed will be the square root of the sum of true speed squared plus wind speed squared. e.g., the average of 4 legs will be higher than true airspeed if there’s any wind. It doesn’t magically cancel out.
 
I’m sorry, Boyd, but I’m not wrong. You can easily verify this yourself without doing any calculations, for the case where the wind is out of the south, and you fly N, S, E, and W headings. The average gps ground speed of the N and S heading runs will equal true airspeed, as previously discussed. For the E and W runs, you will drift north. But the gps sees this, as it measures ground track and ground speed. For both legs, the gps ground speed will be the square root of the sum of true speed squared plus wind speed squared. e.g., the average of 4 legs will be higher than true airspeed if there’s any wind. It doesn’t magically cancel out.
^^^^^ THIS ^^^^^ is correct. You must use ground track and ground speed (GPS derived is the easiest way) for the calculations.

FWIW every spreadsheet I have ever used, including the ones available from the National Test Pilot School, specifically state this.

:cool:
 
Last edited:
Actually, you have 3 unknowns (TAS, wind speed, wind direction) so taking data from 3 different legs is needed to get an unambiguous answer. In theory you can write equations for headings or ground track (slightly different equations) but most think gps track is more accurate than a magnetic heading.
 
Last edited:
Bob, GPS direction is a track, right? I think that is what you meant.

I have used the method you describe for decades. Simple. As a practical matter, other factors will determine accuracy rather than the smidgen of wind direction error.
ron
 
Last edited:
Thanks, fixed bad grammar.
I agree: I favor the ‘with the wind, against the wind’ method because I think it minimizes the chances of other errors (up/downdrafts, changing winds, etc) creeping in.
BTW, doing 4 legs results in the TAS being ‘over-determined’. A good program will use 3 legs at a time, and calculate 4 answers. If there’s a significant spread in those answers, it suggests something went wrong (wind changed, etc). And if they’re all close, it gives you a warm and fuzzy feeling!
 
Back
Top