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IBob

Savannah S Build Notes - Fuel Tanks

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First time through of any build there are points where you spend a lot of time scratching your head. For me, fuel tanks was one, although it's actually not hard...on the second wing.

 

My kit was manufactured Dec 2014, has 4 tanks (thanks, Australia!) and the SS fuel lines.

 

Comments and suggestions welcomed. Always.

 

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Some prep:

 

The fuel delivery lines will run along the back of the main ribs. There is already a hole in the ribs for 1 line, but if you have long rang tanks, you will need 2 holes in Ribs 1 & 2. These are easier to drill before wing assembly or skinning. Build pics online show this second hole to the rear of the first, and slightly higher. The pic shows how I located them.

 

The manual calls for the skin over the fuel tanks bays to be riveted shut once the tanks are fitted. Some builders prefer to fit rivnuts and screws. If you are doing this, then the 2 rivnuts that will support the very outboard end of the tank/s centre support are best put in before the wing skins go on. See RH side of second pic.

 

I spent a lot of time sorting out and supplementing the grommets for this job. It pays to sort this early, so you know what size holes to drill. What I used is listed further on in this strand.

 

MOST IMPORTANT: DO NOT BEGIN BY DRILLING THE TANKS.

 

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The wing is upside down on blocks, on the bench.

 

DO NOT DRILL THE TANKS.

 

First, temporarily fit the tank, as follows:

 

Ensure tank bay is clear of debris. Put tank into bay.

 

Fit tank front supports, they sit on the mainspar under the nose and bottom skins. Bolt in place and tighten, holding supports down flush to tank bottom.

 

Fit tank centre support and cleko/screw in place.

 

Move tank to centre tank filler in hole in upper, skin. I used a mirror to check this.

 

With tank now located correctly, mark for rivet holes in the front supports.

 

Disassemble all the above, and drill the front supports.

 

The tanks vary in shape. I also got mounting brackets from 2 different batches: if mounting multiple tanks, mark the matched components.

 

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Sort out the grommets to be used, so correct sized holes can be drilled.

 

I spent a lot of time scratching my head over the grommets supplied, and ended up supplementing them by buying more. I only used grommets from the kit on the root rib, as detailed below. Sizes are approximate:

 

Rib3:

 

If long range tanks are fitted, this has one 9mm fuel feed line (at back of rib). The SS fuel line is 9mm.: 1 off 9 x 20 grommet

 

Rib2:

 

If long range tanks are fitted, this has two 9mm fuel feed line (at back of rib). The SS fuel line is 9mm.: 2 off 9 x 20 grommet.

 

Plus one breather or fuel return pipe. This is also 9mm, but uses 16mm fuel hose coupling: 1 off 17 x 23 grommet.

 

Plus two sight glass connections. These use 14mm(?) fuel hose couplings: 2 off 15 x 20 grommets.

 

Rib1:

 

If long range tanks are fitted, this has two 9mm fuel feed line (at back of rib). I used the big grommets from the kit. 2 off

 

Plus one breather or fuel return pipe. This is also 9mm: 1 off 9 x 20 (or smaller grommet from kit?)

 

Plus two sight glass connections. These are Rislan pipe. I used the smaller grommets from the kit. 2 off

 

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Temporarily re-install tank, as above, this time with clekos through front tank mounts.

 

Use felt pen or similar to mark the tank, through the lower hole in the rib. This gives the position, on the tank, for the upper sight glass fitting.

 

Looking through the upper hole in the rib, you will see this does not align with the tank, and so cannot be used.

 

Remove the tank. Measure and mark the tank side for the other two fittings:

 

For the fuel return/breather line, I used the measurements in the manual, but lowered the hole in the tank side by 2mm to be well clear of the tank edge.

 

For the lower sight glass fitting, I chose to move back to be clear of the big lightening hole, 120mm from front of tank and 22mm from bottom. See pic.

 

Measure and mark tank rear for fuel delivery pipe,as per manual.

 

I found the square shown very useful for marking out on the tanks

 

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Drill required holes in tank:

 

For the fittings to be leak-free, it is important that these holes are 'clean', with no rags or swarf, particularly on the inner edge.

 

I ran some drilling tests on polyprop offcuts, and found that putting a large drill through a small pilot hole tended to give ragged inner edge to the hole. Based on that, I started with a small hole and worked up carefully through my drill sizes. The final drill I turned through by hand, which seemed to be the best way of preventing this soft material from dragging the drill in.

 

I then 'felt' gently round the inside of each hole with a small wire hook, to be sure there was not a hanging rag of swarf attached to the hole. Apparently this is a principal cause of fuel leaks.

 

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Insert fittings:

 

Coat flange and thread of fitting with Loctite 577 and slide down feed wire.

 

The smaller fittings will drop fully through the tank wall, the larger ones will have to be wound through the wall (see end of this post for more on this). For winding through, I used a piece of spare hose over the fitting, and multi-grips, watching as the thread emerged the width of the nut and feeling when the fitting was fully home. For these fittings, I also applied more Loctite to the nut thread and face.

 

Tighten nut on fitting:

 

I chose to use multigrips to carefully hold the fitting next to the nut, while tightening. This does mark the fitting, but the seal will be provided by the unmarked end. This allows very positive tightening.

 

Note that for the big fittings, one of the wrenches supplied with the rivet gun is a good fit.

 

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The manual calls for an 8mm hole for the 8mm fittings, but an 11mm hole for the 12mm fittings, so the latter have to be wound through.

 

I cannot see the benefit of doing this through soft material (and I can see several possible drawbacks around the likelihood of raising fresh rags or swarf round the hole).

 

I have been unable to get any answer as to why we are doing this. If anyone can tell me I would greatly appreciate it.

 

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With all tank fittings in place, again temporarily fit and tighten the tank.

 

Mark the drilling position of the side fittings on Rib2. I used tightly rolled paper round the fittings to extend them to the rib for marking.

 

Remove tank, drill holes for selected grommet sizes. Step drills do a good job here. Debur and fit grommets.

 

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Check that the hose pieces used for coupling to the fittings will go through these grommets.

 

I also took the opportunity to trim the Rislan sight-tubes so that they will sit neatly against Rib1.

 

Finally, install the tank, using Loctite on front mounting nuts as per the manual. I also used washers there, although they were only supplied for the long range tanks.

 

For the Rislan sight tubes, both clips on the coupling hose sit between Rib2 and the tank:

 

Fit Rislan tube through grommets in Rib1, then push on pieces of coupling hose. Position 2 hose clips on each tank fitting, push the coupling hose through rib 2 and onto the fitting, threading the clips onto the hose. Tighten clips to hold Rislan and tank fittings.

 

For the fuel return/breather line, one coupling hose clip sits either side of Rib2:

 

Clip coupling hose to line, position 1 hose clip at the tank fitting. Push the coupling hose through rib 2 and onto the fitting, threading the clip onto the hose, and tighten. Pass outer end through the grommet at Rib1 (at front of rib for Right wing/fuel return, at back of rib for Left wing/breather).

 

The fuel delivery lines are easily fitted to the rear of the tanks and out along the rear of the ribs.

 

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Important bits I left out of the above:

 

All lines and pipes were pre-marked at coupling to hose for correct and adequate insertion.

 

All clips were positioned to ensure correct grip and seal at tank fittings.

 

And tanks were leak tested before final fitting.

 

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And....it seems to me you can fit the tank fittings then drill the rib, or drill the rib then fit the tank fittings.

 

I tried this second way for my second wing: it worked okay too, and required less in and out of the tanks. But it also required some fairly sneaky measuring, and I think the first way, as outlined in this thread, is the more reliable way of ensuring things line up.

 

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This is useful information ...and it is keeping me inspired to build my own Sav.

 

 

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I found it wise to put some strips of soft foam tape on top of the tanks. It stops chafing fron the top wing skins.

Thanks for that, John. I did wonder about that, but my tanks are all pretty hard up against the top skin. Not that that will stop them fretting, but I think tape at this stage would have raised lumps or ridges in the skin.

 

I notice the tanks vary a little, no doubt due to small changes in the moulding process. One tank in particular had noticeably more 'bulge' to it.

 

 

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Yeah. Theres not much space.

 

I used the door stop foam tape from the junk shop. It's very soft and doesnt push hard. I noticed some minor chafing on the skins at 800 hrs, while replacing the fuel lines. My tanks were both the same so I was lucky.

 

 

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Hi IBob

 

I have a question. Given you have 2 tanks per wing, are they permanently joined, in which case when filling either tank it will fill the other, or are they independently isolated from each other? If so how is this done?

 

Thanks for this well detailed explanation. 

 

Barry 

 

 

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Hi Barry,

 

there has been a fair bit of conversation and debate over here regarding the valving of the Savannah with the 2 + 2 tanks.

 

My decision...and it is only my opinion...has been to copy what Mark Kyle did on his original build here: that is, have a manifold on the wall, behind the passenger where the pilot can see it, with four valves, one for each tank.

 

The thinking is this:

 

Most of the time I would only be using the 2 inner tanks, so they would be valved on, and the outer tanks valved off (as if I had only 2 tanks).

 

When a larger fuel load is required, I would still take off and climb on the inner tanks.

 

I would then switch to the outer tanks (and inner tanks off), with an eye on the low fuel light, and burn off the outer tanks. If flying a distance this way, I would set a timer (as suggested by JG here). And I would also have routinely tested my low fuel light by turning off all tanks at altitude and expecting a low fuel light within 5min(?) (as suggested by Mark Kyle here.) I would be aiming to empty or near empty my outer tanks before switching back to the inner.

 

The benefits of this arrangement, as I see it, are that it allows the outer tanks to be burnt off. And it does not spread remaining fuel across all the tanks, which is what will happen with the standard setup: the arrangement there is that only the outer tanks are valved, so once you open those valves, the fuel level will equalise across all 4 tanks.

 

(There is also a secondary issue with parking: unless you have a park brake you will have to park across any slope, whereupon the fuel will all run to the low tank, and if your tanks are fairly full, will overflow on the low side. This is an issue in NZ, where we have many sloping farm strips.)

 

The principal argument against this arrangement is that it is possible to take off with no tanks selected. And that fuel handling errors have resulted in a lot of accidents and incidents. This is absolutely true, and not to be underestimated. However I am building this aircraft for my own use,  I always dip tanks before takeoff, I am confident of adhering to a rigorous fuel handling process, and there is a low fuel switch/light on the reserve tank behind the seat

 

which gives many minutes of flight after activation.

 

None of this will convince those pilots who have been trained never to turn off the fuel. And they are absolutely entitled to their view.

 

It should be noted also that longer distances are flown in Australia. In NZ, the more likely use of the extra tanks is to avoid the need to find mogas during a there-and-back sortie.

 

 

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Thanks IBob. What you have explained and what I read in Marks blog, is is exactly as I thought. I agree it's not ideal to have free flow between tanks for the reasons you have outlined above, and an additional reason is it would play havoc on your visual Fuel Level display which will change as fuel flows between tanks as you fly and taxi etc.

 

Look forward to more of your progress.

 

Barry

 

 

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Thank you, Barry.

 

I had some time out from the build, am back on it now, but not posting here as I did for the first part.

 

Right now I have the engine out of the box, but still swinging on the hoist.

 

I guess any valving arrangement for the tanks has potential drawbacks. At one stage I came across ICP drawings (online?) which showed pumps for shifting the fuel from outer to inner tanks. Which would be one way of ensuring you got most of the fuel to the inner tanks, but added further complication and expense.

 

Traditionally, some aircraft with inner and outer tanks had a 3 position fuel selector that went OFF/INNER/OUTER. I see Mark's setup as an economical version of that. It seems both simple and intuitive to me, and particularly as the valves are clearly visibly to the pilot.

 

 

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