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Is there a reason for all of the different dimensions of fuel hoses in the Savannah?


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Hi everyone,

 

It´s time for me to exchange all the rubber fuel tubings in my SavannahS, Rotax 912 (eight years since built). Four dimensions (6, 7, 8, 10 and 11 mm) were delivered with the kit and as some dimensions are difficult to find (particularly if you want a quality brand) I wonder if all dimensions are really needed. For instance,  the 7 mm tube is used to connect the mechanic pump with the electric pump while the 6 mm tube connects the mechanic pump with the "spider". In other words, fuel to the mechanic pump is delivered via 7 mm tube and fuel from the mechanic pump via  6 mm tube. Is there a reason for this based on flow, pressure or what or can I use the same dimension to and from the mechanic pump? If there is a reason, would it be possible to keep the 6 mm and exchange the 7 mm to a 8 mm tube which is easier to find than the 7 mm tube? 

 

Also, the 11 mm tube is used for a few connections in the wings while all connections from the wings to the fuel reservoir are 10 mm (3/8") as well as the tube from the reservoir to the fuel valve. Perhaps I could use 3/8" tubes for the wing connctions instead of the 11 mm?

Do you Think it would be possible to skip the 7 and 11 mm tubes and stick to the 6, 8 and 10 mm (1/4", 5/16" and 3/8")? 

 

Many thanks

 

Hans

 

 

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  • 2 weeks later...

It is common for pump suction to be larger diameter than delivery but the difference usually greater than 1mm. When Aerokits here in Australia were supplying replacement hose with low vapour loss, there were only two sizes. Reg supplied the same to me when I bought my kit and I binned the supplied hoses and used the low loss hose from the start. I am sorry I can't recall what size they were but am nearly certain they were imperial sizing.

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I don't see why so many hose sizes are needed - particularly if short runs are involved. You can substitute metric rubber hoses for imperial sizes and vice versa, provided the hose fits onto the hose fitting satisfactorily,

Many hoses are marked with dual (metric and imperial) sizes. The only area a problem may arise is if you substitute a smaller metric size for an imperial size on a long run, such as a fuel line from, say a wing tank, to the engine.

In that case, the internal flow resistance of the smaller diameter hose may be excessive, and the pump suction may cause the hose to collapse, restricting flow even further.

It's not likely you would ever have problems substituting a larger diameter hose, to replace a smaller diameter hose.

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Seems a bit odd having so many sizes. Suction hoses have always been larger than the pressure hoses, usually about 1.5 times the diameter. That is due to the need to keep velocity down in them.

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I think your issue of multiple sizes probably comes from an engineer trying to match up imperial fittings with metric hose and just making a mess of it..

My aircraft with Rotax 912 uses 8mm all the way from wings to Rotax mechanical pump, then 6mm to the carbs.

Larger (suction side) hose reduces potential vapour lock but can allow more "dwell" time if near a heat source.

The smaller 6mm under pressure from the fuel pump generally won't get vapour lock due to it being under pressure.

The smaller diameter allows fast transfer from pump to carbs reducing heat absorption....

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I think your issue of multiple sizes probably comes from an engineer trying to match up imperial fittings with metric hose and just making a mess of it..

My aircraft with Rotax 912 uses 8mm all the way from wings to Rotax mechanical pump, then 6mm to the carbs.

Larger (suction side) hose reduces potential vapour lock but can allow more "dwell" time if near a heat source.

The smaller 6mm under pressure from the fuel pump generally won't get vapour lock due to it being under pressure.

The smaller diameter allows fast transfer from pump to carbs reducing heat absorption....

I think Downunder is largely "on the money" although I am not so sure of his reasoning.

 

I use Gates fuel injection hose & clamps in 8 & 6 mm.

Why fuel injection;

It's only slightly more expensive than carbi (low pressure) hose

Has slightly better heat/fire resistance

Is less fuel vapour permeable

After 5 years it is only slightly stiffer/less flexible than when new

Its readily available through your local Repco (dont forget to bargain hard)

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  • 2 months later...

Thanks for the comments! I will use the Gates fuel tubings as recommended and will stick to three dimensions (1/4", 5/16" and 3/8")(6, 8 and 10mm).

 

I am not looking forward to the job exchanging all those fuel tubes. Removing all the sheets, all rivets, awkward positions, emptying all tanks from fuel and explosive gas. Would it be a good idea to ventilate the tanks with N2 or carbon dioxide, perhaps? Anybody who has done this and could give some rocommendation?

Thanks

Hans

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Removing all the sheets, all rivets, awkward positions,

 

This is why we fit Rivnut nutserts and bolts on three sides of the tank covers as we build them. The only side not bolted on is along the main spar because the rivnut requires a larger hole to be drilled and this would threaten the integrity of the wing spar.

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And with that one mention of rivnuts in this private forum I am now getting sponsored ads for rivnuts tools in my Facebook feed. How does that work? Was it the fact that I thought about the word or the fact that I typed the word?

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Well all 4 sides of my tank covers are screwed on...no rivnuts in the spar :) only on the 3 other sides...I have a strip plate that is tapped and then sikaflexed under the lip of the angle for the spar. Then I just use 4mm stainless screws

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And the kits now come with inspection hatches for all tank connections: same shape as the inspection hatch at the main spar/strut junction, but set in flush rather than planted on....

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  • 1 month later...

Hi all,

Just an update. I changed the fuel hoses from the main fuel switch and forward (under the cowling) and used 5/16 (suction) and 1/4 (pressure) - no problems.

However, I got worried when I saw the condition of the original hoses (that came with the kit seven years ago). In particular the bit that runs outside beneath the floor from the fuel switch to the electric pump was in a very poor condition with cracks and was actually leaking. I suppose that the hoses in the new Savannah kits are better than what I got seven years ago but if not - don`t use them!

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Hi all,

Just an update. I changed the fuel hoses from the main fuel switch and forward (under the cowling) and used 5/16 (suction) and 1/4 (pressure) - no problems.

However, I got worried when I saw the condition of the original hoses (that came with the kit seven years ago). In particular the bit that runs outside beneath the floor from the fuel switch to the electric pump was in a very poor condition with cracks and was actually leaking. I suppose that the hoses in the new Savannah kits are better than what I got seven years ago but if not - don`t use them!

 

That's why there is a recommended 5 year rubber replacement (you will even find something similar in the fine print of vehicle servicing)

 

What brand ? & why imperial ?

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Many hoses produced today have interchangeable metric and imperial dimensions listed on their sizing, as the differences in dimensions are minimal, when it comes to something like flexible rubber hose.

It's not like thread specifications, or engineering fit of metal components, where dimensions are critical.

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Many hoses produced today have interchangeable metric and imperial dimensions listed on their sizing, as the differences in dimensions are minimal, when it comes to something like flexible rubber hose.

It's not like thread specifications, or engineering fit of metal components, where dimensions are critical.

 

Critical ?? If a 6mm hose is specified why convert to its nearest imperial ?

 

Critical ? if it leaks fuel/oil/coolant at an inappropriate time due splitting because it was too tight or leaking because you tried to prevent it doing so with the clamp.

 

Hoses may not be quit like threaded components but you should be aiming for a secure fit , without excessive effort to push it on, and not so loose you are using the clamp to try & prevent a leak/hose detachment. Go with the recommended sizes.

 

True, many hoses will have both metric & imperial sizing but Rotax is European (metric) - why would you confuse the situation by attempting an imperial (obsolete) conversion ?.

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