View Full Version : lower longeron?

09-16-2004, 09:06 PM
Here's a bit of a technical question.

I was pouring over my new Northland fuselage print and on the lower longeron about halfway back it shows a splice with a tube liner installed. The material thickness and diameter is the same on both sides fore and aft, then rosette welded to the insert.

Why cut a perfectly good piece of tubing just to splice it back together again? Is it stronger with the insert there and if so then why is that area induced to greater structural loads?

Off topic. My first shipment of tubing should be arriving in the next few days. All preparations done and tools have been aquired. Soon I will finally be starting on what I have been dreaming of doing for about 10 years now. Can't wait!!!!


09-16-2004, 09:26 PM
I think it was because you couldnt buy the tubing long enough to go the entire length of the fuselage.

09-16-2004, 09:35 PM
I thought that at first too but then realized that the top longeron runs the entire length of the fuselage seamless.

Steve Pierce
09-16-2004, 10:13 PM
The Short Wing Pipers are the same way and their fuselages are a lot shorter.

09-16-2004, 10:19 PM
storm_pilot, better look at the CD again, I'm not 100% certain but as I recall the top longerons break at the door posts on both sides. This is far short of the entire fuselage...

As to the original question, here's a fairly un-educated guess.... as a framer I order lumber based on lengths but not neccasarily total lenghts. Cut-off ( when in mass production ) can be a huge factor as well. The sleeve in question occurs almost precisely 10' off the tail. My guess would be that this tube was purchased in 20' sticks making the cut-off almost nil.... hence minimum waste. The front half ( to the gear leg cluster ) is approx. 4' .....5 more of these can be cut from another stick with almost no waste again...

Of course If I were an engineer rather than a Framer I would go with the strength / load factor theory :o :drinking:

09-16-2004, 10:22 PM
Steve, you posted while I was in "slow type"....and your post kinda shoots the sh*t out of my theory...lol...

09-16-2004, 10:23 PM
all the cubs i can recall had the splice you are refering to also.the present pa-18 i just uncovered dose also,i have been told in the past it is a factory splice.univair and airframes inc. build their long tail sections to be spliced at that point i belive.
clyde smith is holding one his work shops at univair @ denver on sept. 24-26,try to remember to ask him about it.


09-16-2004, 10:49 PM

I spent a lot extra to have the tubes shipped in 20ft lenghts. Ratts!!!

09-17-2004, 12:53 AM
I was told a long time ago that the splice was to transition from 4130 to 1020 steel in an effort to reduce costs.

09-17-2004, 08:33 AM
ksecub is correct. The splice is to transition from 4130 to 1020 mild steel. If you are building from scratch, then you can make that 1 piece. there is no need for that splice in that case.

09-17-2004, 08:57 AM
I am building from scratch and since I already have the lengths of tubing I will probably omit the splice unless I hear of a good reason not too.

09-17-2004, 09:09 AM
Stormpilot, Go for it. there is no good reason for that splice other than what ksecub said.

09-17-2004, 10:08 AM
I made my lower longeron out of one piece too and omitted that insert splice tube.
Do not forget the inserts that go all the way in the end near the tailpost though! those are structural.

09-17-2004, 11:17 AM
roger that!

Thanks everyone. I'll keep you posted as to my progress.