A question for Atlimeter/atmo pressure experts

I have been struggling with static port issues. I have finally settled on a fix that still gives a bit of lower pressure than atmospheric at cruise speeds. At 170 MPH TAS, I am having to remove 6 MPH via the EFIS to get an accurate TAS and remove 8 MPH at 190 MPH. This is bench marked against 4 point GPS runs.

Clearly this lower pressure at the port is also causing my altimeter to read higher than it should. However, I don't have the tools to figure out how how far off it is. Can anyone help me figure out my altitude error, based upon the known airspeed error?

Thanks for any assistance you can provide.

Larry

lr172 said:

Can anyone help me figure out my altitude error, based upon the known airspeed error?

Click to expand...

I don't think a simple "fudge factor" will work in this case. You can see the delta pressure (in inches of water) corresponding to various airspeeds here. Note that the difference between 160 and 170 Kts is 2.22" H20, but the difference between 80 Kts and 70 Kts is only ~1" H20. It's not a linear relationship.

What you want to do first is calibrate your static port(s). You do that by making passes over the runway (or other known elevation) at the same elevation but different airspeeds and note the indicated altitude. It should read the same at all airspeeds. (You can use your GPS altitude to make sure you're really at the same altitude.)

After that you can "tweak" your static ports to make the airspeed read correctly at different airspeeds by flying wind triangles (or squares).

On my plane I had to install little "dams" in front of the static ports to get them to read right.

I ran the following cases through the Python standard atmosphere and airspeed calculation routines I created many years ago:

• pressure altitude = 8000 ft
• TAS = 170 mph
• indicated TAS on EFIS = 176 mph

This condition implies the pressure at the static port is equivalent to an altitude of 8071 ft, or an error of 71 ft


I also ran:

• pressure altitude = 8000 ft
• TAS = 190 mph
• indicated TAS on EFIS = 198 mph

This condition implies the pressure at the static port is equivalent to an altitude of 8108 ft, or an error of 108 ft

snopercod said:

I don't think a simple "fudge factor" will work in this case. You can see the delta pressure (in inches of water) corresponding to various airspeeds here. Note that the difference between 160 and 170 Kts is 2.22" H20, but the difference between 80 Kts and 70 Kts is only ~1" H20. It's not a linear relationship.

What you want to do first is calibrate your static port(s). You do that by making passes over the runway (or other known elevation) at the same elevation but different airspeeds and note the indicated altitude. It should read the same at all airspeeds. (You can use your GPS altitude to make sure you're really at the same altitude.)

After that you can "tweak" your static ports to make the airspeed read correctly at different airspeeds by flying wind triangles (or squares).

On my plane I had to install little "dams" in front of the static ports to get them to read right.

Click to expand...

I made a series of dams. If I got the right airspeed at cruise, my pattern and stall airspeeds were way off. What I settled on was the best compromise across the envelop. I could not produce an air dam that was stable across the range of speeds in my case.

Larry

Kevin Horton said:

I ran the following cases through the Python standard atmosphere and airspeed calculation routines I created many years ago:

• pressure altitude = 8000 ft
• TAS = 170 mph
• indicated TAS on EFIS = 176 mph

This condition implies the pressure at the static port is equivalent to an altitude of 8071 ft, or an error of 71 ft


I also ran:

• pressure altitude = 8000 ft
• TAS = 190 mph
• indicated TAS on EFIS = 198 mph

This condition implies the pressure at the static port is equivalent to an altitude of 8108 ft, or an error of 108 ft

Click to expand...

Thanks for running these numbers Dan. I really appreciate it. It is comforting to know that I am not dealing with 100's of feet of error.

A quick follow up question if you don't mind. I did most of my testing of air dams and speeds at 4000' Should I expect the same TAS offset (6, 9, etc.) to work at all altitudes or will I find that I need different error correction at different altitudes?

Larry

I'm Kevin, Dan's long lost brother he didn't know he had

I ran those cases at 4000 ft, and came up with similar results:

• 71 ft error for the 6 mph TAS error at 170 mph TAS.
• 107 ft error for the 8 mph TAS error at 190 mph TAS.

The errors in the static pressure are likely a function of angle of attack more than anything else, as that is what drives the way the air flows around the aircraft. If you increase altitude at the same TAS, the CAS will decrease, and you need a higher angle of attack to continue to develop lift = weight.

If you determine error in CAS vs CAS, that is likely roughly constant with altitude (ignoring large changes in aircraft gross weight).

Kevin - Thanks for that perspective. It is basic, but the light bulb just went off in my head when you said it differently. Airspeed error is one thing but the actual cause of static error results in error of actual altitude. So - what is the FAA certificated error allowance for a static port?

Isn't this particularly important for instrument operations? Even more than airspeed?

Larry, are you using the standard Vans rivet button, in the print location, for the 6 and don't have vinyl numbers forward of the port?

BillL said:

Kevin - Thanks for that perspective. It is basic, but the light bulb just went off in my head when you said it differently. Airspeed error is one thing but the actual cause of static error results in error of actual altitude. So - what is the FAA certificated error allowance for a static port?

Isn't this particularly important for instrument operations? Even more than airspeed?

Larry, are you using the standard Vans rivet button, in the print location, for the 6 and don't have vinyl numbers forward of the port?

Click to expand...

Bill - I certainly agree the static system position errors are important for IFR operations due to the potential effect on indicated altitude. But, from a regulatory point of view, there is nothing that says that this must be addressed on amateur-built aircraft. It is up to the individual aircraft owner's sense of self preservation to motivate him to investigate the static system position error on his aircraft, and either correct it at the source (relocate or modify static ports), or at least apply a correction to IFR approach minima to ensure safety.

For type-certificated aircraft, FAR 23.1325 requires that the static system position error be no more than 30 ft for every 100 kt, i.e. up to 30 ft at 100 kt CAS and 60 ft at 200 kt CAS. This requirement applies to airspeed range from 1.3Vs with flaps extended and 1.8 Vs with flaps retracted (i.e. in the range of speeds likely during approach). The error is not requried to be less than 30 ft.

On my RV-8, I investigated static system errors with flaps retracted and found them to be less than the FAR 23.1325 limits. I didn't investigate with flaps extended, as I fly instrument approaches with flaps retracted until I am visual with the runway, so altitude error with flaps extended has no effect on safety.

Kevin Horton said:

I investigated static system errors...

Click to expand...

I'm inpressed. Nice work!

Thanks Kevin and my apologies. Unfortunately the GRT doesn't let me create offsets to the CAS, only the TAS. However, it should be close enough. I was most concerned with my altitude. I don't want to eat too much into the vertical spacing margins during IFR enroute.

Kevin Horton said:

I'm Kevin, Dan's long lost brother he didn't know he had

I ran those cases at 4000 ft, and came up with similar results:

• 71 ft error for the 6 mph TAS error at 170 mph TAS.
• 107 ft error for the 8 mph TAS error at 190 mph TAS.

The errors in the static pressure are likely a function of angle of attack more than anything else, as that is what drives the way the air flows around the aircraft. If you increase altitude at the same TAS, the CAS will decrease, and you need a higher angle of attack to continue to develop lift = weight.

If you determine error in CAS vs CAS, that is likely roughly constant with altitude (ignoring large changes in aircraft gross weight).

Click to expand...

BillL said:

Kevin - Thanks for that perspective. It is basic, but the light bulb just went off in my head when you said it differently. Airspeed error is one thing but the actual cause of static error results in error of actual altitude. So - what is the FAA certificated error allowance for a static port?

Isn't this particularly important for instrument operations? Even more than airspeed?

Larry, are you using the standard Vans rivet button, in the print location, for the 6 and don't have vinyl numbers forward of the port?

Click to expand...

Yes, I am using the specified rivet in the specified location for the 6 and it is not due to a static system leak. My numbers are above the horizontal port line and a couple feet behind.

Clearly there are things that can disturb airflow and therefore the pressure imparted at that location for each plane. I don't know if it is macro, like from my cowl or micro, like a unique undulation of the skin next to the rivet or the way paint is laying on the CS rivets right next to the port. Clearly something is unique on my ship, as my error was quite a bit larger than the standard that I see reported in my research here. Though it is clear I am not the only one to have a problem.

Larry

Kevin Horton said:

It is up to the individual aircraft owner's sense of self preservation to motivate him to investigate the static system position error on his aircraft, and either correct it at the source (relocate or modify static ports), or at least apply a correction to IFR approach minima to ensure safety.

Click to expand...

I also had a cross over point where the error reverses. While I am reading high at cruise (lower pressure), the pressure becomes higher than ambient at approach speeds, therefore mitigating concerns over decision altitudes as I am reading lower than actual. In fact, my static error is pretty small at approach speeds as that is near the cross over point for me.

Larry