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Airspeed Calibration

transientair

Active Member
I have an RV-14A and I am at the point of determining the error of my IAS. I have done several tests and input the data into a spreadsheet calculator generously published by Kit Planes. Following several tests, my IAS appears to read 8-9 knots fast at cruise speeds.

My EFIS is an Advanced 5600T with both a primary and backup Dynon ADAHRs. I have the Dynon heated pitot tube with AOA mounted under the left wing. The ADAHRs are also mounted in the left wing using the mount built into the wing as part of the RV-14 wing design. Two air lines from the pitot (pitot and AOA) run to the ADAHRs and one line for the static runs from behind the fuselage bulkhead where the static ports are mounted on each side of the rear fuselage. I used the Cleaveland static ports (revised domed version). I do not have an alternate air static port in the flight deck.

My first clue that the IAS read high was that no matter what direction I would fly, the EFIS almost always shows a headwind at cruise speeds. Interestingly, when I perform stalls, the IAS at stall is as expected. So, the error is not the same across all speed ranges. Additionally, when I open or close the fresh air vents, the VSI will momentarily show 150-foot climb or descent, depending on if I am opening or closing the vents. So, my thought is or was that there must be a static line leak.

To that, I have replaced all the static line tubing and fittings. All the tubing ends have clean flat cuts to ensure adequate seating. I have also tried both the push in and nylo-seal (with inserts) connections. I pressurized the lines from the ADAHRS (disconnected from the ADAHRS) and sprayed soapy water on all the connections to see any visual evidence of a leak. There were none. I have not tried a manometer.

Also, to ensure the ADAHRs were calibrated, I selected zero pressure offsets in the EFIS settings when I had the plane parked in an enclosed hangar.

I could send in the ADAHRs to be bench tested, but maybe there’s something else I have not tried yet?
 
Additionally, when I open or close the fresh air vents, the VSI will momentarily show 150-foot climb or descent, depending on if I am opening or closing the vents. So, my thought is or was that there must be a static line leak.

To that, I have replaced all the static line tubing and fittings. All the tubing ends have clean flat cuts to ensure adequate seating. I have also tried both the push in and nylo-seal (with inserts) connections. I pressurized the lines from the ADAHRS (disconnected from the ADAHRS) and sprayed soapy water on all the connections to see any visual evidence of a leak. There were none. I have not tried a manometer.

Pitot-Static systems are deceptively simple, but having a leak somewhere can really throw you off.
I would start by narrowing down the known good parts:
1. Get a manometer, or even just a water column tube plumbed onto your static line from the front, with the rear plugged/taped. The water level shouldn't drop at all (therefore no intermediate leaks, bubble "test" not withstanding). This will prove the plumbing back to the actual ports on the fuselage.
2. Plumb a short piece of hose into the static port on the ADAHRS, running into the cockpit. While flying, cover and uncover the end of that hose, change air vents, whatever, should see climbs and descent registered even though flying level.
3. Have a way to seal that hose, and change altitude. Shouldn't see a climb or descent registered. If you do, find a better way to seal that hose, or ADAHRS unit has a leak in it some how, either in the screw in -> quick connect fitting, or internally (unlikely since you say both ADAHRS are showing error, but maybe they share a nearby fitting?).

My guess is you still have a static line leak, kink, slit, or puncture you haven't found yet. You might have that also in the pitot side of things too, really messing up your readings.

You have to keep narrowing down where the problem could be, but from your description, there seems to be a leak inside the cabin somewhere, thus when change the cabin pressure via vent manipulation, you see a climb/descent.
 
Then I’d go forward on the assumption that the ADHARS modules are working. That leaves:
- The static system is seeing an abnormally low pressure.
- The OAT probes are seeing an abnormally high temperature.

On the static, tape a short piece of aluminum in front of each port - see what happens. On the original Cleaveland ports (20 years ago) I had to glue the head of a 1/8” round rivet (with a 1/16” hole in the center) on top of the flat port face to get them to read right. Amazing what a touch of JB weld can do. I assume you have the newer ports that have a round head that sticks out from the boundary layer - but don’t know.

Carl
 
I have an RV-14A……
. I do not have an alternate air static port in the flight deck.

Additionally, when I open or close the fresh air vents, the VSI will momentarily show 150-foot climb or descent, depending on if I am opening or closing the vents. So, my thought is or was that there must be a static line leak.

I have not tried a manometer.

?

If opening vents causes the vsi to change, it is highly likely you have a leak within the cabin. Build the water manometer out of vinyl tubing, use it (and a syringe to apply a small static pressure) to find it.

You won’t like this, but: it seems (perusing vaf) that fancy after market static ports generate more complaints than Vans’ hollow pulled rivet design. It’s not too hard to experiment with this. I presume you used the recommended location?

Finally, having an alternate static source is not a bad idea, given their low cost.Stein sells one that looks like a toggle switch.
 
- The OAT probes are seeing an abnormally high temperature.

I assume you have the newer ports that have a round head that sticks out from the boundary layer - but don’t know.

He says he has the newer dome shaped in his OP. But maybe the placement or some other seemingly insignificant installation error would account for the instrument errors?

If you run some numbers here , the OAT would have to be off by 100*F to show an error of 9-10kts TAS. What does the OAT read on the display?

But, OP says the problem is with his IAS, so I am assuming the Dynon is not taking into account the OAT to show him a IAS?

Seems like a pitot-static leak situation to me, just haven't found where yet.
 
Two air lines from the pitot (pitot and AOA) run to the ADAHRs and one line for the static runs from behind the fuselage bulkhead where the static ports are mounted on each side of the rear fuselage.

I know this might seem stupid ("it's so crazy it just might work!"), but are you double plus sure that you don't have the AOA and the pitot lines switched? or even switched and switched back somewhere along the line? I think the prepackaged kits come with 2 different color lines, but if using clear or same color for both or all pitot-static lines, double check each line is what you think it is.
 
I know this might seem stupid ("it's so crazy it just might work!"), but are you double plus sure that you don't have the AOA and the pitot lines switched? or even switched and switched back somewhere along the line? I think the prepackaged kits come with 2 different color lines, but if using clear or same color for both or all pitot-static lines, double check each line is what you think it is.

That is a brilliant idea!
Carl
 
It sounds like you concluded that you have a static leak. I would agree. My 1/16 NPT fittings leaked on the back of the various instruments. And in all isolating and checking, i failed to solidly reattach one push in connection.
 
I appreciate all the thoughtful suggestions.

It sounds like you concluded that you have a static leak. I would agree. My 1/16 NPT fittings leaked on the back of the various instruments. And in all isolating and checking, i failed to solidly reattach one push in connection.

To add complication to this, there are no instruments in the panel that any lines connect. All of the air lines (pitot, AOA, and static) run to the left wing where the ADAHRs are mounted. I'm not saying it's impossible, but it seems like the pressure differential from opening and closing the cabin fresh air vents is unlikely to have an impact on air pressure inside the wing. But, I've been wrong many times before.

Here's a picture of when I was building the tailcone and initially installing the static line. After making the connections here, the line runs continuously from this location to the left wing (10-15 feet maybe). All of these lines have been replaced and new connections used (nylo seal and push in).

I know this might seem stupid ("it's so crazy it just might work!"), but are you double plus sure that you don't have the AOA and the pitot lines switched? or even switched and switched back somewhere along the line? I think the prepackaged kits come with 2 different color lines, but if using clear or same color for both or all pitot-static lines, double check each line is what you think it is.

I'll triple check and I like the idea that it's too obvious to have not been considered. I do have three colors of lines (one for each).

I also don't think the OAT probes are the cause. Van's has this hole prepunched. Also, I think OAT only impacts TAS, not IAS.

If opening vents causes the vsi to change, it is highly likely you have a leak within the cabin. Build the water manometer out of vinyl tubing, use it (and a syringe to apply a small static pressure) to find it.

I'll try the manometer.

Keep the ideas coming!
 

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AOA port would show less dynamic pressure in cruise, because it’s angled into the relative wind.
Check the sensor zero point calibration in the AFS. Try to load the factory calibration.
A couple of days ago I did my 2yr pitot static test and one ADHRS was off by about 4 kts. Did the zero point calibration and they both got all messed up. Reverted back to the factory zero points and they were both indicating the correct airspeed again.
 
You say that when you open an air vent there is a bump is airspeed. That indicates a static leak. You say that all of the lines terminate in the wings. Your static line runs from the static ports through the fuselage. There is probably a leak in that line somewhere, maybe at the static ports themselves or where the two static ports join together or a cracked line. About the only way opening a vent could cause your problem.
 
In the interest of closing out this thread, I did find the source of the issue.

It was, in fact, a leak in the static system that originated in the Dynon ADAHRS. All of the lines and connections were tight, but the leak was somewhere inside the ADAHRS box. So, off to Dynon for repair.

Interestingly, I did not realize that opening and closing the cabin fresh air vents would change the air pressure in the wing (where the ADAHRS is mounted in the RV-14). But, in my case, changing cabin pressure impacts the ambient air pressure inside the wing.

After re-running the calibration, the IAS error is now about +1 knot at cruise speed, which I can live with. So, the Cleaveland static ports seem to work for me.
 
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