My latest projects on my Bede BD-4C airplane were mounting a pair of MGL Avionics sensors: the SP-6 Magnetometer (electronic compass) and the SP-7 Attitude Heading Reference System (AHRS). These two little boxes, just six ounces apiece, can sense the airplane’s magnetic heading and attitude, all with no moving parts. Quoting from MGL Avionics’ website,
The SP-6 CAN is a magnetic compass sensor (magnetometer) for use with all MGL Avionics products that show heading. It contains a three axis magnetic field sensor and a 3 axis accelerometer. The system uses a Cortex M3 32 bit processor. If an SP-7 CAN AHRS is used on the same communications link, tilt compensation can be performed using gyro based attitude data.
The SP-7 CAN AHRS is MGL Avionics’s seventh generation AHRS system (attitude sensor) for use with all MGL Avionics products that display attitude. The core building blocks of the system include a three axis ST MEMS rate gyroscope, a three axis ST MEMS accelerometer and an ARM Cortex 32-bit microprocessor system. The SP-7 CAN will allow the display of attitude (horizon), G-force, turn rate, and slip/skid on MGL Avionics products that allow these features.
There are challenges, of course. 🙂 The SP-6 Magnetometer cannot be mounted near anything magnetic, which includes electric motors and steel nuts and bolts. The SP-7 AHRS needs to be mounted near the center of rotation of the airplane, which means near the center of the main spar (the strong tube that runs the length of the wing and supports the entire weight of the airplane in flight).
I started with the compass. The back of the fuselage is largely open space. I picked a likely position and used my trusty ol’ Boy Scout compass to confirm that there was very little magnetic interference in that spot. I tried with the airplane oriented roughly north, east, south, and west. The compass pointed in the same direction when “in” the airplane and held a few feet away from the side of the airplane. There was just one tiny little problem with that spot: it did not have anything to which the SP-6 could be attached.
OK, I lied; there was a single diagonal brace one inch wide. All I needed to do was add a second brace and then construct a shelf. Long story short, it ended up looking like this:
Click any photo to see a larger version.
You can see the brass bolts and nuts that I used to assemble the shelf. Since these are not lock nuts, I added some Loctite to assure that nothing comes loose. A Velcro strap holds the SP-6 onto the shelf. I slit a couple of extra pieces of brake line and added that to the edges of the shelf so that the metal will not abrade the Velcro. I do still need to come up with something to keep the box from sliding forward and backwards but have a couple of ideas to try.
The outside diagonal piece is part of the BD-4C fuselage. I added the inside piece. It runs from an horizontal piece of aluminum behind the top of the baggage compartment down the a spot where several of the fuselage braces meet. At the top, I added a small angle of aluminum like this:
At the bottom/back end, I fabricated an oddly shaped piece and was able to piggy-back on a pair of existing bolts. No new holes required 🙂 In the second photo, you can get see that the bottom of the brace is right near one of the autopilot servos.
The magnetometer shelf was fairly complex. It took seven pieces of aluminum, none of them rectangular. Pretty much every cut and bend was at an odd angle. By comparison, the AHRS shelf was a piece of cake.
I decided to mount the MGL SP-7 AHRS to the front of the spar, above the passenger’s seat and above the headliner. Here is the finished installation in my Bede BD-4c.
This second photo gives you a better view of the shelf’s construction. Just three pieces of metal. Sweet.
The SP-7 is supposed to be protected from vibration so I plan to put a piece of dense foam rubber (sound insulation, actually) underneath it. As with the SP-6, I will use a Velcro strap to hold the SP-7 in place.
