Chapter 21 - What should I do before I put the bottom skins on?

I'm getting ready to put the bottom skins on the fuse and seem to recall reading somewhere about some stuff that is easier done before the skins go on. I can't seem to find the links/topic/suggestions.

Thanks,

Bob

Fuel system. Return line bulkhead fitting. Just do it!
Regards,
Dave Hastwell
120485

+1 for the return line bulkhead fitting .. do it prior to installing the bottom skin.

I was able to easily (almost) install the fuel lines, pump, shutoff valve, etc while the bottom skin was in place.

daveyator said:

Fuel system. Return line bulkhead fitting. Just do it!
Regards,
Dave Hastwell
120485

Click to expand...

That's the one thing I've read about as well. Unfortunately I read about it right after I riveted the bottom skin on!

Jim

Is that the bulkhead fitting that attaches to the F-1204D Center Section Bulkhead (page 28-06, step 1)?

Thanks,

Bob

And do it up tight...

I had no idea how tight to do up fuel line fittings. As a result my plane leaked like a sieve after I had completed building it.

This fitting is a bugbear. I managed to tighten the fitting but only after I had got someone to heat and bend the appropriate end of a spanner (wrench) through 100 degrees, so that I could attack the fitting through an inspection panel. It did work, however. So there's always an answer somewhere...

Cheers..Keith

Tubing wrenches

I bought a set of tubing wrenches and cut the appropriate sizes in half. Worked great for putting the fuel lines in with the belly skin on.

Bulkhead fitting

bhassel said:

Is that the bulkhead fitting that attaches to the F-1204D Center Section Bulkhead (page 28-06, step 1)?

Thanks,

Bob

Click to expand...

This is the one:

It won't make it any easier to actually tighten the fuel lines, but installing that 45 degree bulkhead fitting now will be one less headache later.it will give you a fixed point to aim for when you fabricate your fuel return line. If you are having a good day at that point, you should be able to finger tighten your flare fittings till everything is nicely snugged up. After they you only need to do the final tightening. Access is tight, and you may need to be 'inventive' with your tools, but it is doable.
Have fun
DaveH
120485

lpeeds said:

I bought a set of tubing wrenches and cut the appropriate sizes in half. Worked great for putting the fuel lines in with the belly skin on.

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Thanks for the tip Paul. I've put a set on my "to buy" list.

Jim

Do you use any kind of sealant on the fittings to the tube or is it just tight? Is there a specific torque?

Thanks,

Bob

Would you recommend doing the return line prior to the bottom skin or is it a problem not being able to mount something?

Bob

Jim T said:

Thanks for the tip Paul. I've put a set on my "to buy" list.

Jim

Click to expand...

Tubing wrenches???? Where does one find those?

Thanks,

Bob

bhassel said:

Tubing wrenches???? Where does one find those?

Thanks,

Bob

Click to expand...

Here's what I've come up with so far: http://www.amazon.com/s/?ie=UTF8&ke...=aps&hvadid=3484463147&ref=pd_sl_98w4obg1io_e

Jim

If you're going to cut them up anyway - -

http://www.harborfreight.com/catalogsearch/result?q=flare+nut+wrenches

These are cheap for low usage.

Requested torque values

Bob - to answer your questions regarding thread sealant ? NO thread sealant on the threads of the flare fittings because the seal is actually made at the flare and not at the threads like a water pipe thread. Plus, the sealant will mess with the running torque of the fitting. I did put a very thin (and I mean very thin) coating of fuel lube on the mating face of the fittings just in case the line twisted a bit during torquing so hopefully there will not be any gulling if it does. Probably a superfluous step but I had it so I used it. Early section 5 plans mentions using fuel lube (mine does) but I was told newer versions of the plans make no mention of it.

As for the torque values for the flared fittings used with the soft aluminum tubing as in the RV-12:

Torque 1/4" flare fittings to 50-65 inch pounds.
Torque 3/8" flare fittings to 110-130 inch pounds.

The torques vary slightly depending on who?s chart you are looking at but you should be in good shape if you aim for 58 inch pounds for the 1/4" tubing?s flair fitting nut and 120 inch pounds for the 3/8" tubing?s flare fitting nut.

I was able to use a crowfoot wrench on the end of a torque wrench to do the final torque of the return line bulkhead fitting flare nut through the inspection port with the bottom skin on. Had the fuselage laying on its side with a second helper on the other side of the bulkhead through fitting holding it with a wrench to prevent twisting.

A little math is involved when using the crowfoot wrench on the end of a torque wrench because it will change the length of the torque wrench so the torque setting on the wrench needs to be adjusted down accordingly. I made calculations for straight and 45 degrees ? using the crows foot at 90 degrees basically requires no correction (should be within the margin of error of the torque wrench). Here is how I calculated the torques when using the crowfoot wrenches.
http://www.dogaviation.com/2014/01/calculating-torque-values-for-fuel.html

Although as the project moves forward, knowing a lot more tubing fittings will be installed in the not too distant future, suspect owing a set of inexpensive flare nut wrenches would not be a bad idea. For what it is worth, noticed Harbor Freight also has a set of crowfoot wrenches that are for flair nuts as well.

Happy building,

Thanks John!

I hope to get to Harbor Freight tomorrow and pick up the tubing wrenches. I'll add the crowfeet as well. Did you use metric or SAE?

Bob

SAE

Bob - All SAE so far. However, that said, I'm sure the firewall forward will require Metric since the Rotax engine is an overseas product.

As a point of reference. There are a lot of people, including myself and many other A&P's, that use standard wrenches on AN fittings - they work just fine. Tube wrenches can be used but they are not necessary.

I do have many different stubby wrenches that I use in places with limited access. I love buying tools, so if I thought tube wrenches were necessary I'd own a set.

Using a torque wrench with a crow's foot will definitely provide accurate results and I would not discourage using a torque wrench when you have good access to the fitting.

On the other hand, many people use the "flats" method to torque AN fitting. For the AN3 & AN4 fittings that we use, 1 1/2 Flats past hand tight gives the proper torque on flare fittings.

http://www.vansaircraft.com/pdf/Torque_Spec_Aluminum_Fittings.pdf

Thanks Rick for the update and link.

Bob