Radio Stack

(15-1-2007) I am writing this after having spent the best part of a week in the sweltering workshop (35-42 degC), designing and building the radio stack. RVs with sliding canopys do not allow easy access to the front fuse superstructure, and hence avionics wiring. The problem is that the front skin and windshield are rivetted, clamped and fiberglassed in place, and so it is virtually impossible to get to the space under the front skin without crawling underneath the forward cabin. This is in contrast to the tip-ip canopy design, where the wiring is accessible with the canopy tipped up. Just don't tip the canopy in the rain, unless you want to bathe your avionics in rainwater! I had been warned of this, and so I set out to design and implement a system to get over this limitation - because I want to be able to update my avionics easily if at all possible.

All stackable radios and instruments are provided with an aluminiun sheath which has at least four mounting holes at the front and one at the back. The instrument is designed to slide into this sheath, and usually is secured by means of a 3/32" hex Allen key which fits into a small hole in the face of the instrument. At the back of this sheath, there are guide pins and connectors. Older radios have large "molex" connectors which are screwed to the sheath, and into which the circuit board connectors slides securely. Later instruments have miniature "D" connectors which serve the same purpose, but probably need a little more care. My garmin 340 audio panel has TWO such 44-pin connectors. Care will be required.

Normally, these aluminium instrument rack are placed one above the other and held securely by -6 csk screws and slide-on anchor nuts to two pieces of alumionium angle which are fastened to the instrument panel. Normally, once these structures are in place, you cannot remove them without pulling everything apart.

My strategy is to make the entire radio stack into a removeable sub-assembly which can be unbolted and dropped out of the front superstructure with a minimum of fuss. It seems to have worked, but we shall see how well this system works in a few years when I save the funds to add a Garmin 430 GPS and CDI.

To escape the 38deg C heat, my wife and I drove up nearby Mt Borah, a world-famous site for hang-gliding and paragliding. Some guy was setting up to fly and he took off while we were there. Very peaceful provided the wind is not gusting (images 1 and 2). That night, back at the workshop (image 3) I re-installed the panel and front skin and looked at various options.

When I started this project I had absolutely no idea how a "radio stack" was constructed or installed. I give a lot of detail below to help out any new builders who might be in a similar position. It is actually very simple, once you know how it all goes together. Each unit has a rectangular aluminium support rack. These need to be fived, one above the other, two two lengths of angle which in turn are fixed to the inside of the instrument panel. There are four holes up front for each rack, two on each side, plus there are two holes in back which need to be supported to stop the instruments from "sagging" down with time.

Image 4 shows the four avionic units in their racks just staked one on top of the other. My LAME TC suggested having a space of about 1/8" between units, so I inserted pieces of 1/8" aluminium scrap to do this (images 5-7). Image 6 shows the large front mounting holes on two of the racks. These each take -6 size countersunk screws which attach each rack to a piece of aluminium support angle. Image 7 shows all the front rack supportholes, with the instruments positioned so their front faces are all vertically aligned ( as we need them to be). Image 8 shows two pieces of scrap aluminium channel from a single largepiece which came with teh QB fuselarge (it held the front fuse shape intact, widthwise). I cut this channel into 2 suitably sized lengths, and removed one flange from each to form the front rack support angle brackets (see below)

 

On top of Mt Borah..

Panel skeleton

Radios, stacked

separated by 1/8"

Mounting holes

Measure..measure

Spare angle

Once the instruments were all aligned vertically and horizontally (images 1 and 2), I carefully measured the vertical and horizontal distrances between ALL the front mounting holes, and marked up the aluminium angle to drill #40 holes in the right places (image 4). There is obviously no "standard' for these mounting holes, the dimensions were differnt for each instrument. If you want a nice flat panel you MUST do it this way. Had I drilled all the mounting holes along a single vertical line, the instruments would not have lined up. Image 3 shows that, at the same time, I prepared the two pieces of cap channel which cover the main spar bulkhead channel on each side, beside the pilot and copilot seats. As suggesed by Dan Checkoway (http://www.rvproject.com/) I added a few platenuts to each cap strip, so as to me able to mount stuff in front of the pilots in future. Image 5 shows how I added the final platenuts to secure the Affordable Panels RV-7 XL panel frame to teh fure decks on each side. I also had to drill out (image 6) two platenuts I had already installed in this frame - I had not realisd that these holes would need to secure BOTH the radio stack support angles and the left and right instrument panels. Image 7 shows the raw panel frame with the original subpanel behind, all primed.

Note dimensions

trimmed

skeleton mounted

had to remove nuts

ready to roll

note dimensions

Images 1-5 below show the processes of drilling the fron mounting angles. One was drilled carefully, the the other was positioned next to it (image 2) and match-drilled (image 4), all to #40 pilot holes. Image 5 shows the two drilled angle pieces.

Most avionics seem tohave the instrument itself attached to its mounting rack by a mechanism which is activated by a small internal Allen screw, access provided by a small hole in the fromnt of the instrument. I think 3/32" is the right sixed Allen key. Images 6 and 7 show an Allen key being used to extract the GMA 340 audio panel from its mount. The mounting holes in the al support angle were expanded o #27, and i aquired a few slip-on -6 sixe anchor nuts (image 8) which sat over the anlge mounting holes and recived the threads of -6 size countersunk screws which attached each rack to this mounting angle.

cut off one flange

and mark

hole positions

match-drill

mounting angle

3/32" Allen key

to unmount instrument

-6 anchor nut

I then fitted the front mounting angles to the panel frame with clamps, and trimmed them so everything lined up. The GMA 340 rack was screwed in place (image 1) and the instrument inserted an d secured (image 2). This was just to check that everything was working! With that success, I added all the other racks (mages 3 & 4), having to cut a ather large rectangula hole in the subpanel to take the three long lower frames. Image 5 show a rear view with the old wires (these are 2nd hand) sticking out. Image 6 shows the end-result. Nice! On the way back to the house that night I photographed my little pet spider which, at this time of year, spins a lovely web JUST off to one side of the pathway. I call her 'Shelob", with apologies the J.R.R.Tolkein. The next day I began by positioning the angle which attached to the lower subpanel and supports the throttle, mixture and carby heat cables (image 8).

one rackmounted

with is instrument

now a whole stack

of racks

with wires at back

and instruments

SHELOB

Engine cable mount

I decided that the only sensible course of action was to extend the hole in the subpanel down to the edge and to remove a section of the subpanel completely (image 1, upside down). With this piece removable, I will be able to "drop" the entire radio stack down and out of the front superstructure. the alternative is to have everything buried in there and inacessible without rippiong out the entire front panel frame, which I want to have rivetted in place eventually. The removed piece of subpanel was then match-drilled to a couple of pieces of angle (images 2 and 3) and clecoed back in position. These were rivetted to the excise piece of subpanel angle (image 4), and the throttle/mixture/carby heat support panel was positioned in front (image 5). The rear supports for the instrument racks are provided by four pieces of .032 alcald, bent into a right angle shape, and screwed to the rear mounting holes of the three lowere long racks. The front piece is visible, in position, in image 6. These were rivetted together to form a single sub assembly on each side, and attahced to the subpanel by platenuts, one on each side (see image 3, next row of images). Everything was then frilled and fitted with -8 size platenuts and screws (images 7,8).

 

Big hole in sub-pnl

removable angle

and more angle

ready to rivet

with cable mount

all is removeable

by platenuts

excellent

Image 1 below shows the details of platenuts that hold the subpanel together (horiz), and the rear support brackets to the subpanel (vert orientation). Image 2 has the subpanelm repainted a nice grey enamel and repositioned in place. Image 3 has the rack assembly out on the bench. Note rear bracket shapes and details, note mounting screw/anchor nut positions, front and rear. Image 4 is the same, but back in position (four -8 size screws hold it all in place - easy. Image 5 is a local storm I captured at the end of the day. Images 6-8 show the end result with side panels in place. I finished of (not shown) by cutting the plate which will be positioned above and below the instrument faces.

subpanel platenuts

painted and in posn

back on the bench note screws positions

back in position - easy!

a summer storm

with instruments

and side panels

all good

This is probably an overkill, but i am happy that the whole rack assembly is removable. I will make up one, or a few, good quality connectors for lines which go from the rack to the aircraft (e.g. lines to headphone connectors, power, ground. This means I can go ahead and rivet on the top skin without fear of access problems to the radios.

Radio stack mechanicals finished January 2007