Clarification on wording for mounting AHRS

[tracstarr]

I'm probably just over thinking this, but the installation instruction for the Garmin AHRS unit says Mount the GSU 25 with the connectors aligned within 1.0 degree of either the longitudinal orlateral axis of the aircraft.

Does this mean it has to be installed along the center of the fuselage within a degree? Or could it be offset, just aligned parallel and/or perpendicular to the center line (longitudinal) within that 1 degree?

[jiott]

Can be parallel, but closer to long/lat axes is better as I understand it. My opinion and logic only. Mine is on the long roll axis, but is several feet behind the lateral pich axis; works fine.

[efwd]

mine is mounted on the back of the G3X panel itself. There are bolt holes for that very purpose. Mine works fine. It just has to me mounted so that the pitch is sensed 100% in the correct direction rather than what it would be if the ADAHARS was mounted at say 45 degrees. In that situation you would be sensing pitch and roll simultaneously with just a pitch change. The same goes for the roll. You will configure settings later to inform the ADAHARS which orientation its mounted. Hope that makes sense.

[tracstarr]

Thanks.

I guess the logic in my head is that if it's not centered at the lon/lat axis, then when you either pitch or roll readings would be off. For example, lets consider longitudinal axis only for a minute. If i mounted it on the left side of the plane, a roll to the right would read differently (in magnitude) than one to the left. A right roll would cause the device to have an upward movement with the roll further from the centerline, where a left roll is downward movement inside of the centerline relative to the roll. This makes me think you'd have very different measurements. Maybe i'm wrong?

[efwd]

I believe that even with it mounted 10ft off the longitudinal center line your wings will be changing in angle equally, say 20 degree bank. The fact that the ADHARS is now higher or lower than the opposite side of the line it doesn't reflect a climb or decent that I have ever noticed.

[tracstarr]

I believe that even with it mounted 10ft off the longitudinal center line your wings will be changing in angle equally, say 20 degree bank. The fact that the ADHARS is now higher or lower than the opposite side of the line it doesn't reflect a climb or decent that I have ever noticed.

Right, which is logic that also makes sense to me... and I guess why i feel confused. Now, like you said, they do have provisions to mount to the back of the g3x - which would be very unlikely to be mounted dead center of the panel, which makes me more confident that it doesn't matter.

[jiott]

I tend to agree with tracstarr's logic. My understanding of these ADHRS is that they work using solid state accelerometers sensing acceleration of gravity and motion. If that is the case, then using tracstarr's example, a sensor mounted off the axis of rotation would sence more motion and therefore greater acceleration during that period of time. As soon as the motion stopped (you reach a point where your pitch or roll is now stable and not changing) the sensor would sense a stable pitch or roll angle and would read the same no matter where it was mounted. So if this logic is correct (and I am just throwing it out there, not claiming it to be absolute) there would be some inaccuracy in the pitch or roll, but only while the motion was occuring. A relative slow motion, or where the sensor is mounted not too far from the axis, would probably have a short lived error that is not even noticeable. Maybe that's why the mounting location is not considered an important factor. Bottom line, don't overthink it like I am attempting to do.

[PapuaPilot]

Here is what the Garmin installation manual says:

19.4.2 Mounting Requirements
The GSU 25 should be mounted within 13 feet (4.0 meters) longitudinally and 6.5 feet (2.0 meters)
laterally of the aircraft CG (center of gravity). In cases where the longitudinal distance from the
CG is planned to be greater than 6.5 feet (2.0 meters), it is preferable to mount the GSU 25 forward
of the aircraft CG if possible, to enable better acceleration outputs for autopilot use.

This gives you a lot of leeway in a Kitfox. You don't need to over think it, the Garmin design team already has done the thinking for us.

[tracstarr]

I've read the manual a million times (ok not really) and can't remember seeing that! That's gold! Thanks so much.

Yep. There it is. Wow so obvious now. A case of being worried about one thing and missing the answer which is right in my face.


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[PapuaPilot]

Yep, there is so much information to find in that manual. I knew it was somewhere and started looking for it when I saw the post.

Do you know where you are going to mount yours?

[tracstarr]

Do you know where you are going to mount yours?

Well, now that all is clarified, I think I am going to put it in my originally expected location, on the pilot side equipment mount tray behind the dash. I do want to move it to behind the baggage area, but that would cause me major headache as the harness is already made. If I find there is issues then I will move it. The only other things on that tray for me will be backup battery and the servo boost.

[jiott]

The reason many of us with the Dynon system put the ADHRS in the baggage area is because the Dynon ADHRS includes the magnetometer and they recommended mounting it as far away from steel structures as possible. That spot just behind the baggage compartment is about the best you can do (other than out in the wing). I personally did not care for wing mounting due to ease of access and because of the great distance from the longitudinal axis. I know some brands have separated the ADHRS and the magnetometer into two separate boxes so the magnetometer (which is sensitive to steel) can be mounted far away from steel tubing, while the ADHRS can be mounted closer to the CG. Anyway, you're good if you just follow the manufacturer's instructions. I don't know anything about Garmin, but the Dynon system has some fine tuning you can do with the calibration software after you start flying, to compensate for small errors you may have made during the installation leveling process and also for small magnetic directional errors due to magnetometer/steel interference. Garmin, Dynon, etc. all these reputable companies know what they are doing and have made it pretty hard for us to really screw up.

[aviator79]

To add to Jim's post for the benefit of anyone else reading this thread: Dynon does now have an optional remote magnetometer that they recommend for steel tube airplanes. I have it in the wing, accessible by removing the wingtip. That said, the location where Jim put his ADAHRS appears to work quite well without the remote magnetometer, so for anyone installing Dynon, do whatever makes you feel good.