So, work on the tank progresses. I’ve got the top part mostly clad in sheet CSM, but now am focusing on the lower part – I need to do this in order to be sure the top tank mates well to the lower tank – they’re going to be made as two tanks then bonded. I can’t make them as two totally separate but connected tanks because the fuel level sensor wasn’t bought to work like that.
Here is how it looks with some of the sheet clad into place. Note that I’m not stepping it as the cut kingspan in above shows. If I were to try and follow that, there would be two consequences: Firstly, less capacity. Secondly getting cloth to follow tight right-angles won’t work and will lead to bridging and a weaker part. I will take the orbital sander to all the edges where the places meet to smooth them out. They’ll then be filled with bog and coated with resin so I can make a mould from it. Needless to say, I will have to be sure the bog is slightly recessed so after being painted with resin (to seal it) I don’t create an unwanted positive profile that gives me mechanical lock in the mould. Even half a millimeter may make it so hard to get the part out without destroying the mould. I’m not intending to reuse the mould but it would be a massive shame to destroy it in case I ever need to modify the part.
Here we have the trial holder for the ATL Fuel Sender Probe. The housing is a cut down mixing cup and it’s set at an angle to point the sender down into the tank. Once the angle is right and it sits just where I want it in the lower tank I will glass over this so it becomes part of the main tank (that is, part of the part of the main tank that becomes the mould.
Here is the tank screwed into place on temporary brackets using self tapping screws. I know the fuel-sender will sit a little proud of the old boot floor, but that doesn’t bother me. I’d sooner make a demountable hat of some description for it rather than have the tank lower and lose capacity. I had to screw the tank in early to be sure I had reliable datum points when locating and attaching the lower tank.
Here you can see the tank avoiding the diff-attaching plate and the diagonal uprights that are part of the diff-carrier. The angle isn’t great but it I have at least an 25mm between the tank and all uprights. On reflection, the clearance is a little too great and I may be losing a little capacity. However, if I’m in a sideways shunt I have a lot of room for things to deform. The layup will also be very tough – aramid and e-glass. This will have massive impact resistance, and using layers of e-glass rather than carbon will add a little flexibility. This is not load-bearing or structural.
I had to build in two recesses, one on either side – the assembly order is specific here. The tank goes in first and the diff carrier second. This means I need to leave enough room around the tank to get a bolt in to secure the diff carrier. And I did – I’m quite pleased with myself.
Here’s the top view of the top tank. Note the massive void behind it, which is where the old tank sat, well high and well outside of the axle line. You appreciate how bad this is for polar moment of inertia, right? I was tempted to add some extra bulges on the back of the tank under the roll-bar, but this will be putting mass outside/behind the axle line. What’s the point of taking such a purist view if I then compromise? If I find myself really struggling for range I can always add a tank in this void where the old tank was. If it’s empty it will only add a kilo or two. My future plan is to remove the tank-mounting frame at the back and replace it all with a single CF crush-cell. This follow’s Warren’s law of composites. The new part will be lighter, and stronger than the rotting steel it replaces.
Here’s the lower part of the tank in place. The wooden bits show there the original guestimate cuts of kingspan need extending out to in order to maximise volume available. This angle doesn’t show it well, but there’s an inch minimum between the tank bottom and the prop. Again, it’s an IRS so the prop isn’t moving anywhere unless it lets go. The tank also swells out at the sides at the back (where the two sticks cross). There’s about half a litre or so to be gained by swelling out the sides like this.
All these little additions to capacity may not seem a lot, but if every half-litre can be baked in then I make serious gains in capacity. Bearing in mind the old tank weighed 6KG and carried 18L of fuel in the wrong place, I feel this will be a step forward.