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  #291  
Old 10-01-2016, 10:30 AM
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Bill_H Bill_H is offline
 
Join Date: Aug 2010
Location: Peel, AR
Posts: 1,760
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I Wouldn't do that. I think you would have too much flex. I think you could, in place, cut the slots to to the control holes using a dremel and also a hacksaw blade.
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  #292  
Old 06-14-2017, 03:37 AM
rgmwa rgmwa is offline
 
Join Date: Feb 2010
Location: Perth, Western Australia
Posts: 1,511
Default 25 gal Fuel Tank Modification

Last year ago I crossed Australia from west to east and back in the RV-12, covering nearly 4,000 miles over a few weeks. The airstrips in our remote outback areas are typically few and far between, so there are often limited options if the weather at a planned destination closes in. Consequently fuel capacity is a significant issue for long-distance flying in much of this country, and the RV-12’s standard 75 litre (19.8 gal) tank is a bit too marginal for comfort at times. On the trip I carried an additional 20 litres (5.3 gal) in a rubber fuel bladder, but had to land somewhere to top up the tank. While this was manageable it was hardly convenient, and having fuel on board that you can’t use when you need it is pretty useless.

Others have previously posted workable solutions for carrying additional fuel in the RV-12. A marine tank in the baggage compartment plumbed into the main tank with a pump and flexible lines to transfer the fuel, or a fuel bladder on top of the tank with gravity feed into the main tank are two options. Neither of these appealed to me much, mainly for safety reasons related to securing the extra tank adequately and the risk of leaks or fumes when transferring fuel around the cabin.

In the end, I decided to increase the capacity of the existing tank from 19.8 gal to 25 gal (95 litres) by simply making it taller. In principle, it’s easy – make a skirt to raise the sides of the tank, take off the tank lid and use it as a template to make a stiff internal brace to go between the top of the original tank and the skirt, put the lid back on, install a new Moeller gauge, and shorten the filler neck and vent line. In practice, it’s quite a lot of work but a fun job if you don’t mind doing a bit of home fabrication. It would be great if Vans offered a taller tank option, but I can well understand why that’s unlikely to happen any time soon.

This is not a trivial modification and there are some issues to think about, the obvious ones being the increased load on the three tank attachment points, a slight aft shift in the c.g. due to the increased tank empty weight, and a significant reduction in baggage capacity with a full fuel load. On the other hand the big advantage is that all the extra fuel is in the tank where you want it – no plumbing, pumps or extra tanks required, and it effectively takes up no additional space.

Why 25 gal and not 30 gal when all that would take is a slightly taller skirt? I was fairly comfortable with 25 gal, but not with 30 gal because of the additional load on the attachment points and c.g. issues. I did enough calculations to satisfy myself that the increased stresses were reasonable using the existing fittings, but if you decide to attempt this, make your own assessment. I also replaced some aft bulkhead attach angle rivets with Cherrymax rivets. For local flying there’s usually no need to fill the tank with more than 20 gals anyway, but it’s nice to have the extra capacity available for longer trips. That extra 70 or so minutes of flying time could be important one day.

Skirt - 85mm high:


Various parts:


Internal diaphragm to maintain tank robustness:


Close up:


Completed tank:
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Last edited by rgmwa : 06-17-2017 at 02:38 PM.
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