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Old 03-23-2016, 06:49 PM
Ebbe Ebbe is offline
 
Join Date: Feb 2016
Location: Calhoun, GA
Posts: 26
Post Homebuilt Avionics

Hi all --

I'm new to this forum, even though I have flown my 7A for a few years, after buying it from the builder. I have seen some talk about homebuilt avionics, so I thought I'd throw my hat in the ring...

Over the last few years I have been designing the different components. It all really started out with an idea of building an autopilot controller and using MGL's servos. As I didn't really have much panel space available on my original steam gauge panel, I realized I had to design a completely new panel from scratch. Steinair cut the panel after I provided an AutoCad drawing.

Boxes are simply mounted by four screws through the panel flange. The EFIS weighs just under 3lbs and has a modest depth of 3" behind the panel, so this way of mounting is good for many more g's than the airframe.

A short description of the different components:

EFIS
A wide angle, high brightness LCD plus an industrial grade single board computer was used, together with controller board that handles front panel controls, brightness, temperature sensor, data bus communication etc. So far, it shows terrain, obstacles, all US airport and runway data, Highway-In-The-Sky to any runway, MFD with airspace, airports, NEXRAD, traffic, sectionals, TAC's.

AHRS
Invensense based IMU, uBlox7 GPS, sensors for static and dynamic air pressure, inputs from a wingtip mounted Honeywell magnetometer and an OAT probe. All processed to produce 60Hz attitude and position, plus 10Hz airdata. A second AHRS will be added later, which will have different components and architecture just in case.

Engine Monitor
A box mounted on the subpanel collects data from EGT, CHT, OP, OT, FP, MP and RPM sensors. I reused Van's existing sensors and had to do a bit research to find out the characteristics on some of them. I had to stoop to some reverse engineering too. Also, a Flowscan provides the fuel flow.

Engine Display + Control Head for Comm & Transp
Displays data from the engine monitor, and also controls a Becker CM-4201 VHF and a Sandia STX-165R transponder. These are mounted behind the baggage bulkhead, together with the AHRS. A COMM2 will be added later. The EFIS sends regularly a frequency list to this display, so that a descriptive name can be displayed next to the frequency ("STP TWR").


Audio Panel
This is a simple two-place intercom, with common functionality for two COMMs and isolation, etc. Automatic squelch level. It also has a voice annunciator for common alerts, like traffic, engine and nav related. I used audio transformers on inputs and outputs to avoid those illusive ground loops. When powered off, pilot's headset and PTT is directly connected to COMM1.
It also records all COMM and intercom audio, engine and AHRS data onto a SD card, all synchronized into one file. A future Windows program will play it all back for post flight analysis....


Autopilot
Two-axis, using MGL's stepper motor servos. This is the part that is not fully completed yet. The plan is to have the standard functionality, such as approaches, holding etc. The AHRS is used as reference. The EFIS will provide guidance for NAV and VNAV features, using the data bus. Without data from the EFIS, the autopilot will fly a heading and maintain or climb/descend to an altitude. It also has outputs for elevator trim, since these servos report back the torque being used.


Other
  • A Pathfinder ADS-B receiver with serial output is mounted on the subpanel, and feeds the four EFIS. It outputs GDL90 format data. So far, the EFIS displays TAF, METAR and NEXRAD. Traffic, winds aloft and NOTAM's will be added when I have time...

  • Two simple LED ramps display trim and flap position. I used the existing Ray Allen potentiometer for trim pos, and installed a conventional linear position sensor for the flaps, right at the flap motor. They are connected to the databus, reporting back position.

  • The databus is CAN, and keeps things in sync between all units -- such as brightness (common for all components), heading and altitude bug, altimeter setting, waypoint data etc.

  • An RC Allen 2600 horizon is used as a backup, with a switch that powers it from a small backup battery. A future miniature airspeed/altimeter/gps box will also use this emergency bus.

  • I have strived for sound designs from the start. All boxes are either steel (0.04), or alodine Al for radiation and RFI shielding. All components are rated for industrial temp range, down to -40C, with exception of the EFIS LCD that only functions down to -25C. Oh, well. So far, no vibration related issues. I guess over 30 years of electronics tinkering has paid off...

- Ebbe

__________________
RV-7A, purchased from builder
IO-360, ~450 hrs
Avionics: own design
KCZL - Calhoun, GA
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