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Old 02-20-2018, 09:09 PM
Thunda Down Under Thunda Down Under is offline
Join Date: Feb 2018
Location: Mildura.
Posts: 17
Default Auxiliary fuel tank.

I have an auxiliary fuel tank of 55 litres that sits on the rear seat, (in my RV4)even with the rear control column removed and the tank held in place with the rear harness it still likes to move around a bit.
I intend taking out the rear seat, harness and and column permanently and using the whole area for stowage, camp gear and supplies.
I would like to have an aluminum seat made up as a fuel tank, (about 100 liters) gusseted and pressure tested, a mate has suggested that l could get fried if l bagged in, l could also fry without it.
Or get hit by a meteorite.
So , to get the required am mount of fuel and keep the C of G in place, carry my stores and camp gear it sounds like a good option.
Apart from wing tip tanks at $6000 AU.
Or something like permanently fixed drop tanks.
So if there is any opinions I'd like to hear em for or against.
One more thing, l have seen a photo on the Internet on a yellow RV4 converted to a jet with retractable gear, l would like to know if this is the real deal or has someone photo shopped it.
Thanks, Thunda.

4's rule, thanks Mr Van, ya built the perfect plane fer me.
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Old 02-20-2018, 10:02 PM
Kyle Boatright Kyle Boatright is online now
Join Date: Aug 2005
Location: Atlanta, GA
Posts: 3,404

There are several RV's out there with the outboard leading edges converted to fuel tanks. Depending on how many bays you use, 100 liters is doable.

You can convert the wingtips to tanks, maybe 7 US gallons each, net >40 liters.

You could also add roughly 30 liters using these tanks:

Here's an interesting link to someone who has added fuel capacity:

There are many options, you just need to choose the one(s) which are most appropriate for your installation and appetite for work.
Kyle Boatright
Atlanta, GA
2001 RV-6 N46KB
2019(?) RV-10
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Old 02-20-2018, 11:12 PM
Thunda Down Under Thunda Down Under is offline
Join Date: Feb 2018
Location: Mildura.
Posts: 17

Thanks Kyle.

I am already aware of tip tanks and wing tanks, and l have had long discussions on the subject with Jon Johanson here in Aus, this is where l got the $6,000 price for wing tip tanks, and yes it's a lot more than you would pay there.

I was hoping to get some feedback on the seat tank, as the cost of importing anything from the states is out of the question.

I have a quote for building an aluminum seat tank locally at a good price, and was looking for comments on a seat tank.

Thanks for your reply anyway.
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Old 02-21-2018, 07:39 AM
YellerDaisy's Avatar
YellerDaisy YellerDaisy is offline
Join Date: May 2012
Location: Mountain Southwest
Posts: 138

I've been halfheartedly reviewing the same options for my -4. I'm not sure if any of this helps but maybe triggers some thoughts...

No doubt that an aluminum rear seat tank will work. It is probably the most affordable option (at least for a simple box shaped version).

A custom fit fiberglass tank might achieve the most capacity in the least amount of space. I say this as you can fit it into the nooks and crannies much easier than a welded aluminum tank. Line the area with plastic and then pour in expanding foam. Trim that up (so removable and no interference) and glass to form the tank. I would incorporate some means to secure the tank to the fuselage (other than the seat belt). Having made a couple of glass tanks, I can say with certainty that this will require lots of time/effort.

Since the -4 is easy to get into aft CG condition (especially with loads of camping gear), some sort of fuselage header tank might be the way to go. It depends on how much space you have available forward of the panel. Not sure you could get 50 liters up there though, maybe.

Of course, aux fuel in the wings is the neatest option but....
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Old 02-21-2018, 08:47 AM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is offline
Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 1,769

Originally Posted by YellerDaisy View Post

A custom fit fiberglass tank might achieve the most capacity in the least amount of space. I say this as you can fit it into the nooks and crannies much easier than a welded aluminum tank. SNIP.
One of my neighbors at Dogwood (VA42) is Bill Harrelson - the guy that holds the world record for longest flight (Guam to Jacksonville Florida) and flying over both the North and South poles in his highly modified Lancair IV. His approach to adding fuel tanks (he had five tanks in addition to two expanded wing tanks):
- For the odd shaped areas like passenger side and behind the seats he used premade carbon fiber board (carbon fiber on honeycomb) cut as needed to conform to the space. The tank edges get a couple of bids of carbon to seal. These tanks are amazingly strong, fit like a glove so they donít bounce around, light and can be removed when not in use.
- On top of the two carbon board tanks behind the pilot seat he added two aviation fuel bladders. He always burned the gas in these bladders off first to move the CG forward from the gross aft postion at takeoff.

For the new RV-8 project I plan on a removable 20 gallon or so carbon board tank in place of the rear seat. I did plumb the right tank with an extra fitting as I plan on transferring fuel from this tank to the right tank instead of feeding the engine directly. Tank vent will be routed to the right wing root area.

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Old 02-21-2018, 09:04 AM
rv7charlie rv7charlie is offline
Join Date: May 2006
Location: Pocahontas MS
Posts: 3,188

100 liters, plus the weight of the tank itself, would be about the same weight as an FAA 'standard' human. 180 lbs in the back seat location will leave very little for baggage, if you want to keep cg in a manageable location. (With that much volume, it won't be much of a seat, either.)

It's obviously the easiest way to add fuel because you could keep it a more or less simple box shape, but not the best option if you want reasonable extra carrying capacity in the baggage compartment/above the tank. I flew my 1st -4 with a 235 lb guy in the back. Once. Wouldn't have done it then, but he was short in stature & didn't look that dense, so I neglected to ask his weight. It was not a pleasant experience, for either of us.

You could get a fair quantity of fuel between your legs and up between the instrument panel & firewall, keeping cg manageable, but you'd still have the gross weight issue with all the extra load in the fuselage.

If you can get the fuel out in the wing (leading edges or tips), the only issues are landing loads on the gear and spin vulnerability. DIY tip tanks might well be your best compromise for difficulty/usability.
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Old 02-21-2018, 09:20 AM
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ColoRv ColoRv is offline
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Location: Tampa (BKV)
Posts: 894

Summit racing has many shapes and sizes of pre-made race car fuel tanks. Could save a lot of time and effort just remove when not using and put the seat back in.
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Old 02-21-2018, 03:41 PM
Thunda Down Under Thunda Down Under is offline
Join Date: Feb 2018
Location: Mildura.
Posts: 17

To Yeller daisy.
Good to hear someone else is thinking along the same lines.
But like should have been more specific.

I want an aluminum PILOT seat full width with arm rests, sort of wrap around, then the C of G is good for gear stowage aft of the pilots seat.

Sorry if I've put a spanner in the works, the main reason for the post was to see if there were any good reason NOT to sit on a 100 liters of fuel.

Apart for, as a cobbler said last could get fried if l bagged in, but that could happen with standard wing tanks.

My greater concern is ending up on the roof on an out landing and getting out (if I'm alive) so l carry a canopy cracker.

So, any reason why I shouldn't sit on a 100 liters of fuel in a robust and pressure tested fuel tank???????
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Old 02-21-2018, 07:51 PM
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n82rb n82rb is offline
Join Date: Jan 2007
Location: fort myers fl
Posts: 736

the problem i see is the sitting height with the seat in, im 6-1 and had to have oregon aero reduce the stock cushion to about 1 3/4 thick so my head does not hit the canopy. the problem i see is getting enough capacity and not getting the tank too high to sit on it. interesting idea to work on. if you go that route please keep us informed about the mod.

bob burns
RV-4 N82RB
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Old 02-21-2018, 10:47 PM
Thunda Down Under Thunda Down Under is offline
Join Date: Feb 2018
Location: Mildura.
Posts: 17

G'day Bob.

Thanks for your input, I'm the same height.

I have worked out basic dimensions, base is 100 mm don't know what it is in feet and inches any more, we moved into the 21st century years ago.
100 gives room for a bit of padding and still has enough head room.

I made up my own helmet out of foam rubber 10mm thick with ear phones, Mike, on demand oxygen tube, full face visor fan forced air (stops the plexiglass full face visor fogging up) covered it over with fiberglass and finished it off with gel coat.

Soft rubber neck skirt totally quiet, hear every thing on the radio, and everyone says that my transmissions are 10 over10 also very very comfortable.

Looks like a star wars get up.

But have plenty of head room.

The back of the seat comes up to (and attached to) the seat pylon, with wrap around sides to the arm rests, this gives 100 liters of fuel.

Filler cap (o ring sealed) is inside at the top of the tank seat with breather vent and tube is vented outside just below the bottom of the canopy no fumes.

I have made a plastic mock up and it fits in quite easy, built to my body shape, the arm rests are a bonus and slightly shaped downwards so the left hand is right on the throttle quadrant.

The right side has provision for drink container and drinking tube attached inside helmet within easy reach, helps keep the ears clear.

I think it will be a good thing, but let came onto the site to see if anyone had a valid reason not to go this way.

Pulling the wings apart to add other fuel tanks is not in my opinion a logical way to get better endurance.

With this way it can be a gravity feed to left main tank via an in line valve and site tube inside the cockpit.

But l still welcome any further comments and ideas.

Jim Thunda.
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