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20-03 & 05-06 ADAHRS Mounting & Lower Skin Preparation

The main thing to consider when deciding a location for the magnetometer is unwanted magnetic interference, both from electrical sources and the airframe.

Options include the right wingtip, which keeps the unit away from the pitot tube heat wiring (actually I did consider putting the Pitot Tube on the right wing, but thought it might be best to keep it away from the aileron servo.) Also it means the recommended “L-shaped” CanBus layout is more easily preserved. But everything is a compromise, & since I plan to fit an Archer Antenna in the right wingtip, I decided to stick with the Vans suggested location in the left wing nearer the root. Other builders have had good results with this location.

Next decision … Garmin GMU 11 or 22 Magnetometer?

The possible advantage of the GMU 11 is that it is smaller, lighter & transmits data via the CanBus, which means the G5 Standby instrument gets independent magnetic heading information since it is also on the CanBus.

But of course it will require wiring changes from the Vans supplied harnesses?

Anyway, I’m fitting a Garmin GMU 11 Magnetometer, and plan to use the Vans confusingly named ADAHRS mount which I’d originally purchased for the larger GMU 22 Magnetometer. (I think they call it an ADHARS mount because this is where they suggest fitting a Dynon ADHARS unit)?? This will provide a rigid, removable platform on which to site the GMU 11.

Magnetometer Mount


As usual I deburred all the components first.

Happily I discovered that I’d already prepared the Lower Wing Box J-Stiffeners when I made the upper ones ages ago!

The W-00012A ADAHRS Mounting Plate is match drilled into the W-00009B Wing Box J-Stiffener – Short.


As with lots of cases, it sounds easy but needs a bit of thought.

I used a 12″ drill bit and a sheet of hardboard to gain access and protect the skin.

I clamped the plate in place until the two outer holes were drilled and clecoed. After that it was easy!


Fig 3 on page 20-03 isn’t that explicit about which holes get dimpled, but I guess we are big boy’s & girls now, and it’s not too difficult to work it all out.


The GMU 11 can be mounted upright on any of the orthogonal axes, on a horizontal platform parallel to within 3.0° of the in-flight level cruise attitude.

That’s another advantage of using the vans mount!

I have chosen to mount mine along the left/right axis, and drilled the mount accordingly. I’m thinking it’ll probably be easier to keep the wiring tidier doing it this way?

Match Drilling Inboard Wing Skins

IMG_4722 IMG_4723

The #40 holes are match drilled at the wing root …


… and then the nut plate holes are final drilled #19.


After the usual deburring …


… the holes are dimpled.

Final Lower Skin Preparation


The corners where the inner/outer lower skins overlap are tapered to make this area less unsightly.

Those of you lucky enough to have built the wings : ) will have already have done this on the top skins. So I refer quickbuilders to this section of my blog.


Out with the DRDT2 for some more satisfying dimpling.


I also dimpled the inspection access panels ready for priming.

OAT Probe


There is an undocumented hole near the inboard access panel on the left lower inboard wing skin.

Being very close to the Dynon ADHARS mounting postion, it’s pretty obvious it was for the OAT Probe.

I’ve decided to fit my probe on the right wing, so I’ve made a slightly smaller hole in exactly the same location but on the other skin.

Why? Well it maybe unwarranted, but I reckon there is a better chance on that side, that the probe will not be so affected by warm exhaust gases, due to the airflow swirl around the fuselage.


Another marathon priming session resulted in more parts ready for assembly.

But before that, I need to spend some time planning the wing wiring and connectors.


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