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  #1  
Old 05-20-2019, 03:19 AM
chris mitchell chris mitchell is offline
 
Join Date: Jul 2006
Location: near Harrogate, England
Posts: 381
Default Help with Garmin G-5 installation

Hi All,

I'd be grateful for thoughts on my recent Garmin G-5 installation in an RV-7. AS part of this work I took the opportunity to install a heated Garmin pitot and to re-plumb the entire dynamic and static system using the SafeAir/Dynon system of flexible tubing and connectors, and a new ASI (with appropriate speed marks for an RV-7 .

We flew an East-North-West-South course to calibrate the new ASI and plumbing with the ground speed taken from the Garmin 430. The results show that the calculated TAS taken from the ASI (corrected for altitude and temp) is within 1% of the ground speed, with the ASI slightly over-reading. So far so good. At least there are no pitot leaks!

However, the ASI and G-5 comparison is much less good.

The instruments agree at 110kts IAS, but at higher or lower speeds - 50kts up to 160 - the G-5 is reading up to 4 kts too low at both the higher and lower speeds.

Which to believe? Well the ASI agrees nicely with the ground speed calculations - so the G-5 is reading low.

I have read and re-read the installation procedure. Can't see anything to help. I did recheck the plumbing, looking for kinks etc but it all looked good. The G-5 is running on its internal GPS antenna - would that make a difference? I can't see that it would. Any other good ideas?

THe original reason for installing the G-5 was as an attitude indicator. I suppose the simple pressure driven ASI and Altimeter are highly unlikely to fail so maybe the easiest thing is to disable the ASI on the G-5, but having plumbed it I'm reluctant to do that. I'd much rather find the problem and fix it.

Many thanks

Chris
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  #2  
Old 05-20-2019, 08:19 AM
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Pat Hatch Pat Hatch is offline
 
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Location: Vero Beach, FL
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Chris,

Why not set up a water manometer to verify your in-flight results?

There are plenty of resources on the web that describe the process and the math involved. Pretty simple to set up and to calibrate your instruments.

My money is on the G5! My experience installing G5s and G3X Touch together (GSU 25 ADHRS vs. G5 internal air data) is that airspeed readings are usually within a knot of each other.
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  #3  
Old 05-20-2019, 09:52 AM
chris mitchell chris mitchell is offline
 
Join Date: Jul 2006
Location: near Harrogate, England
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Pat, please just walk me through your thought processes here.

The ASI (actually calculated TAS) correlated well with the GPS ground speed over 4 legs. The G-5 is ok at 110 but otherwise is 3-4 kts slow after calculating TAS, compared to the GPS measured ground speeds.

It seems to me that, based on our flight test data, the ASI is pretty accurate and itís the G-5 at fault.

Maybe Iím just being slow!

Thanks

Chris
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  #4  
Old 05-20-2019, 10:00 AM
Ralph Ketter Ralph Ketter is offline
 
Join Date: May 2005
Location: La Luz, NM
Posts: 35
Default Static System OK

I would check the static system for leaks as well as verify the airspeed readings with a manometer test.

Static Requirement reference at:
https://www.govinfo.gov/content/pkg/...sec23-1325.pdf

Helpful EAA video at:
https://www.eaa.org/Videos/Hints-for.../5002807430001
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  #5  
Old 05-20-2019, 11:30 AM
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Pat Hatch Pat Hatch is offline
 
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Location: Vero Beach, FL
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Default

Quote:
Originally Posted by chris mitchell View Post
Pat, please just walk me through your thought processes here.

The ASI (actually calculated TAS) correlated well with the GPS ground speed over 4 legs. The G-5 is ok at 110 but otherwise is 3-4 kts slow after calculating TAS, compared to the GPS measured ground speeds.

It seems to me that, based on our flight test data, the ASI is pretty accurate and itís the G-5 at fault.

Maybe Iím just being slow!

Thanks

Chris
Chris,

Yes, I agree, it would appear that the G5 is the erroneous one considering your TAS in-flight data. The manometer test will just help you confirm that as well as verify that you don't have a leak. One of the things I like to do as a quick check for pitot leaks is to inflate a small party balloon and slip it over the pitot tube. This usually gives an indication of around 100 knots, but if you leave it on for a while, it should hold pressure. If the balloon deflates, you have a leak. Just a quick troubleshooting technique. Again, it would appear that a leak would affect both instruments equally, but then again you might have an internal leak in one of them. It's probably a calibration issue, but it's worth investigating other causes. Please let us know what you find!
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  #6  
Old 05-21-2019, 02:00 PM
n982sx n982sx is offline
 
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Location: Chicago, IL
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I've found the G5 to work well and compare well with other instruments.

Just to clarify, are you using the four leg spreadsheet available from the test pilot school or similar? Inputs of four ground speeds and tracks derived from four legs on headings 90 degrees apart?

If so I would believe your results, providing you're comfortable with your TAS conversions. I have OAT data available to my systems so my systems have been doing those calcs automatically. I did do some manual calcs to verify them.
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  #7  
Old 05-23-2019, 01:12 AM
Cumulo Cumulo is offline
 
Join Date: Apr 2015
Location: KHMT
Posts: 18
Default

Interesting. It may be a case of two opposite errors compensating and giving the right answer. I assume that your flight calibration is accurate. My money is also on the G5 being right.

My guess for the other error is that the static port is a bit off at higher speeds. The static port is pretty fussy with respect to location and surface. The pitot not so much as long as it is within a few degrees of flow, which is usually the case at higher airspeeds.

Two wrongs make a right?

Ron
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  #8  
Old 05-30-2019, 01:20 PM
chris mitchell chris mitchell is offline
 
Join Date: Jul 2006
Location: near Harrogate, England
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Default Feeling a bit grumpy!

We used a 4 leg calibration and the TAS and ground speed (from a Garmin 430) correlated well. For what its worth a transponder check was well within limits ie altitude reported by the LARS station correlated with our altimeter.

Garmin have suggested we do a static configuration which I am loathe to do - we don't have the kit and that instruction is actually in conflict with the manual - which says replace with a known accurate unit.

We are having another session tomorrow. I'll see if the local maintenance organisation has a suitable test kit, so we can at least check the altimeter, even if we don't do the static config.

Inter alia, I am pretty unimpressed with the setup for providing manuals. I don't really understand why I have to plow through the G3X install manual to discover how to install a GMU-11 magnetometer, for example, nor a pitot head. But maybe that's just my irritation showing.

I'll report after tomorrows session.

Chris
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Chris Mitchell

Repeat offender;
RV-8, QB, built, flown 150 hours, sold;
RV-4 - attempted repair, rebuild and remediation - abandoned and junked ;
RV-4 fuselage and wing kits - both at QB stage;
2015-2018 dues paid!
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  #9  
Old 06-01-2019, 08:13 AM
chris mitchell chris mitchell is offline
 
Join Date: Jul 2006
Location: near Harrogate, England
Posts: 381
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So we connected the G5 to reference ASI. Consistently across speeds from 200kts down to 60kts, it was 2-3 kts at least out of agreement except at about 110kts. We have opted to ask for a replacement unit. Itíll be interesting to see how that stacks up.

Chris
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Chris Mitchell

Repeat offender;
RV-8, QB, built, flown 150 hours, sold;
RV-4 - attempted repair, rebuild and remediation - abandoned and junked ;
RV-4 fuselage and wing kits - both at QB stage;
2015-2018 dues paid!
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  #10  
Old 06-01-2019, 10:20 AM
swjohnsey swjohnsey is offline
 
Join Date: Nov 2018
Location: Kingsville, TX
Posts: 36
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My G5 reads about 2 mph lower than ASI. Not sure if either is correct. I do know what my stall speed is on both.
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