Smoke System: Tank Selection / Mounting

The Tank

There are a number of options for smoke tanks for an RV - with a popular model being the 4.7 gallon (17 litre) hidden tank. I see these in a lot of RV's. However, there is the obvious cost issue here ($550 AUD plus at least $100 shipping). Instead, i had a friend install a 'race car fuel cell' as a smoke tank and i liked this idea. 

These can be had on eBay for less than $150 AUD. I paid $125 AUD for mine (delivered). These have a couple of drawbacks:

• They take up a little more floor space than the hidden tank - I plan to get around this by making the tank removeable. There is plenty of space left in the -14 for baggage on the way to an airshow etc. 
• They have a 'sump' at the bottom meaning they need to be raised off the floor for mounting - a couple of aluminium tube standoffs will solve this one. 
• The cap is vented - the may leak a tiny bit of fluid during aerobatics. I will have to see how this goes in practice. No issues from the other tank i have seen in service. 
• The fittings are large - 10AN. Will need reducing to -6AN. 

The angled portion is the 'sump' - meaning the tank can't be mounted flat on the floor. 

The small 'knob' is the cap vent. We will see if we get any oil coming out of this over time. 

Tank Modifications / Mounting

The tank obviously needs to be mounted flat to the baggage floor. There would be a number of ways to do this, including just strapping it down. Since i am up to the section of working on the baggage floors, i have elected to install it using AN4 bolts and corresponding nutplates. 

Similar to the baggage tie down rings, these nutplates will need doublers. These were manufactured in a similar manner to the baggage tie down doublers and will be back riveted to the floors. They were left with #40 holes for all rivet locations, and the bolt holes were also left at #40 at this stage to allow match drilling later. 

The initial doubler template - the circled hole will be the bolt hole. This nutplate holes were drilled using the baggage tie down doubler template to accurately space the hole locations. The rest of the holes are just for additional support and are based on 2D edge distance. 

They doublers were designed to allow the tank to mount as close to the edge of the ribs as possible. The blue lines on the baggage skins represent where the flanges of the underlying rib structure is located.

The template was used to drill off 4 identical doublers.

This is the rough location where they will go - all 4 of them in the corner of a rib bay for additional strength. 

To place the tank flat on the floor, i will need to raise it off the baggage floor surface by using some 40x40mm Aluminium tube as a spacer. This was cut to length then marked for the bolt holes. These were initially drilled to #40, being careful to keep the holes normal (square) to the top/bottom surface of the tubes. 

My cheapo eBay shop saw comes in very handy for rough cutting stock like this. 

A centre line was drawn in the part and some extra marker placed roughly where the tank mounting holes were located.

The centre of the tank mounting holes was then marked on the centreline of the tubes.

I then centre punched on the middle of these holes, removed the tank and carefully drilled through the tube from either side ensure the #40 hole remained square.

The tubes were then glued to the bottom of the tank using epoxy putty. The epoxy putty was there to attach the tubes to the tank to make it easier to mount, but mostly to even out the slightly uneven tank mounting tab flanges. Since this is a cheap eBay tank, the 4 flanges were not planar to each other - but by weighting the tank down on top of the putty, this took out all the unevenness. 

Epoxy putty was used to glue the tubes on, and took out the slight unevenness of the tank mounting flanges. I also filled the tank mounting holes with the putty.

It was weighted down on my flat bench to ensure the 2 tubes were in the same plane to each other.

The mounting holes in the tank are way to large for an AN4 bolt (the dark grey circles in the photo below), so i filled the holes in the mounting flanges with the epoxy putty when the tubes were glued on. I was then able to file the top surface flat on the mounting flanges, and back drill #40 the mounting locations from the tubes. 

The known square holes in the lower tubes were back drilled through the epoxy putty which was filling the oversized tank mounting holes.

The tank will sit flat on the floor. Since the floor is attached with pull rivets, the tank will have a rubber matting attached to the underside of the tubes and just sit over the top of the rivets heads. 

A rest fit shows the rough location of where the tank will be mounted.

Once this had been done, i was able to match drill the tanks to the baggage skins. I then had an accurate #40 hole for the location of all the bolts, so was able to match drill the doublers to the skins. 

The holes all ended up nicely perpendicular to the skins.

The tank was placed up on some pieces of timber - i matched drilled the mounting holes, and clecod as i went along. 

Match drilling was done with the long 12" #40 drill bit.

The doublers were then match drilled to the skins, using the holes match drilled from the tank to accurately locate them. The centre bolt hole was upsized to 1/4"

Once this was all done, all the holes for the bolts were upsized on all parts to 1/4".  

I had to get creative in upsizing the upper tube / tank mount hole. I drilled from the top to 3/16" using a deburring extension and a drill bit from my angle drill kit. 

Then reamed to 1/4" from below to ensure the holes stayed in alignment.

I was pleasantly surprised when all 4 bolt slipped in with no problems. 

I clecod the doubler to the skins to check that there was no interference with the underlying structure. 

I then used contact adhesive to attach some rubber matting to the bottom of the tank mount tubes. I sharpened a piece of copper water pipe to cut some nice clearance holes around the bolt locations. 

I was then able to work out that a AN3-23 bolt was the correct length for mounting the tank, including a large area washer under the bolt head to make sure the load is bearing on the tank mounting flanges, and not mostly on the epoxy putty. 

Once the baggage floors and the doublers are primed, they will be dimpled and riveted to the skins, along with the AN3 nutplates, before the skins are installed.