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A puzzle for the collective

cdeggz

Well Known Member
I’ve been having a lot of trouble with airspeed and AoA measurement on my new-to-me RV-10. After a day of hard landings where I just could not get a good flare before hitting the tarmac, we decided to recalibrate the airspeed to the GPS, flying with and against the wind. After doing this, we noted that the AFS 5600 with dual Dynon ADAHRS required an adjustment of -9 KTS on each ADAHRS in order to get the calculated true airspeed and GPS speed to agree (the plane was flying slower than the instruments were suggesting).

Interestingly, the G5 that is on the same pitot static system needed a -6 KTS adjustment as well, although I can’t figure out how to do this. Garmin manuals say it is factory calibrated and never wrong.

I lived with this for awhile but later noticed as I accelerated down the runway I also saw a rise in altitude (500 fpm). This led me to believe that airflow was creating a pressure decrease on the static side, which would explain the climb rate and airspeed inaccuracies.

Today in order to find the static system “leak” I separated the static system into two halves. One Adahrs was connected to the right static port. The other adhrs and the G5 were connected to the left static port. I set up each screen to read a different ADAHRS. I then flew some patterns in the air and found that no adjustment was needed from the -9kts that was already in the system for each Adahrs.

So now I sit dumbfounded. I have a pitot static test/cert coming up in a week or two, but would like to understand the issue first if possible.

-any other ideas as to what might be going on?
-anybody know of a way to calibrate the g5 airspeed indication?
 
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I’ve been having a lot of trouble with airspeed and AoA measurement on my new-to-me RV-10. After a day of hard landings where I just could not get a good flare before hitting the tarmac, we decided to recalibrate the airspeed to the GPS, flying with and against the wind. After doing this, we noted that the AFS 5600 with dual Dynon ADAHRS required an adjustment of -9 KTS on each ADAHRS in order to get the calculated true airspeed and GPS speed to agree (the plane was flying slower than the instruments were suggesting).

Interestingly, the G5 that is on the same pitot static system needed a -6 KTS adjustment as well, although I can’t figure out how to do this. Garmin manuals say it is factory calibrated and never wrong.

I lived with this for awhile but later noticed as I accelerated down the runway I also saw a rise in altitude (500 fpm). This led me to believe that airflow was creating a pressure decrease on the static side, which would explain the climb rate and airspeed inaccuracies.

Today in order to find the static system “leak” I separated the static system into two halves. One Adahrs was connected to the right static port. The other adhrs and the G5 were connected to the left static port. I set up each screen to read a different ADAHRS. I then flew some patterns in the air and found that no adjustment was needed from the -9kts that was already in the system for each Adahrs.

So now I sit dumbfounded. I have a pitot static test/cert coming up in a week or two, but would like to understand the issue first if possible.

-any other ideas as to what might be going on?
-anybody know of a way to calibrate the g5 airspeed indication?

I would first get the system tested and visually check as much of the plumbing as possible. The method you are using is not the appropriate way to solve the issue. Something is wrong given a 500 FPM altitude rise. The first thing I do with a new aircraft before pattern work is to do some stall tests in the landing configuration to establish approach speeds. This assumes a good system. If you have a leak in the system somewhere you don’t want to be adjusting the airspeed readouts. Work on only one variable at a time. I am not quite sure what you meant on making adjustments via GPS to get the correct TAS in the pattern. Many aircraft have airspeed errors at approach speeds. I think most on here check TAS at cruise speeds. Minor airspeed errors at slow speeds are somewhat irrelevant as long as you are flying proper IAS speeds relative to stall speeds. 9 knots however I would agree is not minor. Here is a article ftom Vic Syracuse that might be of assistance.
https://inspire.eaa.org/2021/04/29/whats-wrong-with-my-airspeed/
 
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I agree with George and Mickey.

The recommend method to determine airspeed is to fly a box, not just a headwind and tailwind. The process is well documented so I won’t repeat it here.

On Tim Olson’s site (myrv10.com). There is a good page on the the static ports and the impact caused by the various shapes, sizes and location. Are you using the plans based rivet or a third party static port?

I’m not sure why you are linking airspeed to hard landings. That sounds more like a technique issue with when to flair. You should be able to grease it in at any airspeed.

It seems like you’re chasing several issues here. I would recommend focusing on one issue at a time.
 
You need to go find your static leak. Start off with a simple static test rig (made from HF parts):
127-F1-F53-0-C7-D-4677-BBF5-027-F788-E7-DCE.jpg

There are a lot of videos on this - for now you are most interested in static leak rate. Take it up to 1000’ by drawing a slight vacuum. See if you can hold that 1000’ for a minute without loosing more than 100’. Watch your airspeed indication as you do this (it goes up). Do not draw more vacuum as with this not connected to your pitot your airspeed DP cell can be damaged.

If you want to get fancy, make a Manometer to verify airspeed readings. Connect this to your pitot tube, temporarily taping off the drain holes if you have any:
FBD938-D0-CD31-4996-A63-A-53-DC30-CA40-FE.jpg

This rig is simple but amazingly accurate. I used it to verify the SkyView ADAHRS airspeed reading to be dead on, and to calibrate the not so accurate out of the box Van’s analog airspeed instrument.

Carl
 
READ THE MANUALS regarding pressure tests

It seems like you’re chasing several issues here. I would recommend focusing on one issue at a time.

You need to go find your static leak.
Carl



You need to be extremely careful in doing a static or pressure test. The AOA sensors are very sensitive and easily damaged in a normal test pressure/vacuum.

Read the manuals. Carefully and repeat until you understand.

I have the Garmin system and have to remove the tubes from the pitot to do the testing. I am on site and do this with my two hands for the official pitot/static testing.
 
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Can you post a picture of your static ports, and their location?

I think you are on to something here. I took a look at the dynon static port manual and it looks like these were installed incorrectly. I believe they should be installed with the flange on the inside of the aircraft. I wonder if the step in the flange is enough of a disturbance to cause the 9 kt difference?
 

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I agree with George and Mickey.

The recommend method to determine airspeed is to fly a box, not just a headwind and tailwind. The process is well documented so I won’t repeat it here.

I did a box the next day to calibrate. -9 was still the correct correction factor.

I’m not sure why you are linking airspeed to hard landings. That sounds more like a technique issue with when to flair. You should be able to grease it in at any airspeed..

Try to grease a landing in a 10 with an approach speed of 30kts. :)
 
You need to go find your static leak. Start off with a simple static test rig (made from HF parts):
127-F1-F53-0-C7-D-4677-BBF5-027-F788-E7-DCE.jpg

There are a lot of videos on this - for now you are most interested in static leak rate. Take it up to 1000’ by drawing a slight vacuum. See if you can hold that 1000’ for a minute without loosing more than 100’. Watch your airspeed indication as you do this (it goes up). Do not draw more vacuum as with this not connected to your pitot your airspeed DP cell can be damaged.

If you want to get fancy, make a Manometer to verify airspeed readings. Connect this to your pitot tube, temporarily taping off the drain holes if you have any:
FBD938-D0-CD31-4996-A63-A-53-DC30-CA40-FE.jpg

This rig is simple but amazingly accurate. I used it to verify the SkyView ADAHRS airspeed reading to be dead on, and to calibrate the not so accurate out of the box Van’s analog airspeed instrument.

Carl

Carl I’m not convinced there is a leak since I split the system in half and had the same results on both halves.
 
You found the cause.

Most definitely this could be the root cause. That flange is supposed to be on the inside as you state. Definitely repair these ports. The Vans are quite sensitive to any shape of the port except the original, so choose carefully when repairing tis. It appears to be glued, pro-sealed to the skin so should come off ok. Some repainting will be required.

Do a little homework on what you want to install as final.

I think you are on to something here. I took a look at the dynon static port manual and it looks like these were installed incorrectly. I believe they should be installed with the flange on the inside of the aircraft. I wonder if the step in the flange is enough of a disturbance to cause the 9 kt difference?
 
I think you are on to something here. I took a look at the dynon static port manual and it looks like these were installed incorrectly. I believe they should be installed with the flange on the inside of the aircraft. I wonder if the step in the flange is enough of a disturbance to cause the 9 kt difference?

Many people on the forums have noted large errors due to static ports that are different from what Van's tested. These look like they are sticking out a lot, and that can cause a difference in static pressure from the factory designed shape of the static port.
 
An alternative and simple solution is to use a pitot mast that also has the static ports coaxial like the Gretz Aero company used to sell (and perhaps still does)

This could be mounted on an underwing inspection port cover for trial install and calibration / cross check of your existing system.

You might find it to be more accurate and trouble free than the hull sides static ports.
 
Just for reference...

...this slightly raised edge of paint around the static ports is worth 5KTS and 500FT @ 6500ft / 150KTS indicated...
 

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...for the future VAF searchers

To close this out . . .

I worked at the JB Weld that held the static port in with a knife for 2 hours or so and hardly made progress. Of course I slipped a few times too and put in some scratches. I eventually realized I was cutting more into the aluminum than the epoxy and gave up. I also tried a vinegar soak as recommended a few places with no luck. I decided I would just deal with whatever damage the heat gun would do to the paint. I put the heat gun on the port and it came off in about 45 seconds. No yellowing or bubbling on the paint at all. Lesson learned!

I smoothed it out with the dremel and then re-installed from the inside. I flew it and it seems a calibration of -1 or -2kts on the Dynon AHARS is about right (matches the GPS and G5 now).

Thanks for the input everyone.
 
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