Rigid Fuel Lines

[Freemasm]

Without doing any real analysis or calculations, I?m going to SWAG that this is not a good idea. There is relative motion between the engine (on flexible mounts) and the relatively stiff airframe. Yes, there should be plenty of length to not over stress the tubing but the metal will fatigue and subsequently fail at some point. You could ?steam loop? it to spread out the related stresses or install a short run of flexible tubing to isolate the vibs and related cycles. Yes. I?m aware that there are certified aircraft that utilize hard tubing between the engine and fixed connections but they are usually small gage (=less stiff) sense lines and still utilize such stress management loop configurations. As someone has mentioned, designs are full of compromise. Choose yours wisely.

[efitzgerald]

- Freemasm

I?m not saying what I proposed is a good idea but you misread or missed something.

The hard stainless tubing would be run between two fixed points on the engine and connected to the firewall with flexible hose.

[Freemasm]

Let me say that I?m wicked guilty of not reading for detail like I should. The plethora of electronic messaging/media has made me scan to try and get through everything. Sounds like you?ve thought it through. My bad. Build safe.

[hgerhardt]

Originally Posted by Ed_Wischmeyer

Hard fuel lines, firewall forward? Ask the man who?s had one fatigue crack in flight... like me.

Do you happen to have a photo of that installation? Would be interesting from an educational standpoint.

[Dart11]

I know this is a little bit of apples vs oranges, but the stakes of your decision may be high.

In 1973 I had an engine failure in a Globe Swift when the factory installed aluminum line between the engine-mounted fuel pump and the carburetor failed due to fatigue. I was about 100 feet in the air on takeoff (for what was supposed to be my first trip to OSH) and the engine quit immediately. The fuel spray also caught fire. Although I got it down ok the airplane burned to the ground. Had I been higher when it failed, I would probably not have made it.

Yes you are talking about stainless, but rigid lines are still a potential fatigue issue, and I cannot really see a benefit worth the risk trade.

[Toobuilder]

I've said this a million times and probably have a million more in me, but rigid lines are the aerospace STANDARD. Flex lines are the EXCEPTION, used only when there is significant relative motion between components or maintenance access requires it. This is not an opinion of some guy on the internet - it's the engineering standard for aerospace.

Properly fabricated and installed hard lines will last the life of the airframe many times over. There is NO hose assembly that can make that claim.

To the OP - properly fabricate, install and support your fuel lines and you are essentially done forever. Compared to hose, your weight will be down, MTBF higher, cost lower, and maintenance non existent. Other than that, hose is fine.

[rongawer]

Originally Posted by efitzgerald

- Freemasm

I’m not saying what I proposed is a good idea but you misread or missed something.

The hard stainless tubing would be run between two fixed points on the engine and connected to the firewall with flexible hose.

For fixed runs of line that aren't exposed to relative motion - hard line makes sense. As mentioned previously, you should not mix your fitting and line metals. Are you using SS tubing because of either a perceived need for greater strength or fire resistance? if so, why aluminum fittings then? (BTW, I don't consider either drivers for SS pipe on such low pressure systems...different topic)

If it's a cost saving measure, reconsider saving money somewhere else...maybe skip that new lawn troll.

[efitzgerald]

- Michael,

I have read many of your posts on hard lines and they were one of the reasons I felt it was an acceptable choice. As well as the prop oil tube being made from stainless with only one support along it entire length.

In the end I'm just going to go with the standard fire sleeved hose. It is less work and probably will be cheaper.

I was planning on steel bulkhead fittings on the firewall. What about the rest of the fittings? Is aluminum acceptable with flexible hose?

Thanks for the input.

[Toobuilder]

There is no way hose is going to be less expensive, not by a huge margin. A quality, conductive lined firesleved hose from someone who knows what they are doing (like Tom at TS Flightlines) is going to cost north of $100 bucks (and well worth it, if you need flex hose). That same length of SS Hardline and fittings will cost about $10 bucks at most. And don't even get me started on the difference in weight...

Also, it's perfectly acceptable to mix aluminum nuts and sleeves on SS tube. There are aerospace specifications that describe that exact combination. There are reasons you don't often see aluminum nuts on SS, but it has nothing to do with dissimilar materials. The general reason is because the operating pressure of the system that drives you to use SS tube is so high that it also drives you to steel fittings. Under normal circumstances, our fuel system pressure is so low that aluminum is more than adequate. It's the fire scenario that drives me to use SS tube, not the pressure.

[efitzgerald]

- Michael

I bent tubing out of aluminum and was going to have Tom copy them in SS. I never got pricing from him but he made it sound expensive, or at least time consuming. It is my understanding that flaring and bending tools for aluminum are not appropriate for SS tubing.

I am aware that the firesleeved tubing is not cheap.