Bending of a pipe slotted within a pipe

[jedstress]

Hello all,
I am looking at a setup which includes a simply supported pipe with another pipe of same length slotted inside it.Assuming no friction exists between the two pipes, Am I correct to assume that the two pipe sections do not act as one whole section with a combined second moment of area? Instead they act as two individual pipes that share the applied load based upon their EI values?
Now what would change if the pipe ends were welded?
Many thanks in advance.

 

[Compositepro]

thread507-420430

 

[jedstress]

Many many thanks.
Its great to see engineers debating various theories and possibilities for what is a "seemingly" simple problem.

 

[MOHR1951]

This problem,or question may seem very theoretical to many.In fact it's been around in aircraft tubular structural repairs for decades,I for one ,like perhaps many others on this blog maybe never thought of it's importance or ramifications.FAA 43.13 is filled with circular aircraft fuselage repairs using this exact,tube in tube,with lightening holes/slots,tack welded,to restain it in place.This was and I guess still is an acceptable method of repair ,for a splice,do to bent.out of round,or corroded circular tubing.Im these applications the splice or doubler acts as an internal patch,and the sections work in unison,they are not free to translate within one another.I'm wondering,after all these years,if any consideration was even made,of composite action,as all design calculations were based solely on the geometric properties of the sections.

 

[Sparweb]

Mohr,
The AC43.13 repairs are based on assumptions that adequate load transfer is possible through the welded joint from small original tube to outer repair tube.
That's why the repair designs require "fishmouth" cuts and such details. When the weld joint is correctly designed and implemented, 100% of the load can transfer through the joints.
I believe these repairs were also substantiated by tests. That opinion is based on most of the other things I've seen in AC 43.13 - it is substantiated somewhere but they don't say where.
Giving all those cross-references would bog the text down with dozens of footnotes per page.

STF

 

[MOHR1951]

SparWeb,would really like to see or know of the footnotes for 43.13,just to see what the federal agency considered right or relevant at the time.Where do you think I should search for them ?

 

[Sparweb]

The NACA tech reports server. Most of it.

STF

 

[Sparweb]

Sorry Mohr, sometimes a trite answer is just too funny (in my head) to resist!


But it is true:

https://ntrs.nasa.gov/search.jsp?N=17&Ntk=All&Ntt=welded steel tubing joint&Ntx=mode matchallpartial

STF

 

[WKTaylor]

I personally encountered this odd design in my Dad's homebuilt aircraft Thorp T-18, years ago [1960s--1970s].

The engine mount and main landing gear were integrated for simplicity and weight savings into a 3-piece Assy, that attached at [3] places to the firewall [tail dragger design].

Simplistically...

The engine-mount truss picked-up the [4] Lycoming mounts on the accessory case and then transitioned with steel tubes to a single upper-central fitting [focal point] on the firewall... and to the lower LH and RH corners of the firewall [with crisscross stiffening tubes added as necessary]. Spanning between the 3 locations on the firewall was a triangular steel-tube welded structure which grossly stiffened the installation it was held to the firewall with bolts and rubber pads for some shock absorption. The outer 2-tubes of the triangle were deliberately left 'hollow' so the canted landing gear legs [tubes] were inserted and welded to the upper ends of the triangular frame quite close to the upper-center mount-attach point.

NOTE.
Come-to-think-about-it, the side triangle frame tubes were cut short of the peek and the LDG tubes were inserted to the upper-center mount fitting... and these were all welded together at that location. Essentially the LDG leg tubes floated within the triangle frame side tubes and this allowed for independent strain and shock absorption.

I'll see if I can find a cutaway drawing of the Thorp T-18... a picture is worth a thousand words in this regard.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[WKTaylor]

For grins, previous post... Cutaway view of Thorp T-18 showing engine mount landing gear and firewall attachment as described above.

http://files.engineering.com/getfil...-af867748d1ae&file=Thorp_T-18_Cutaway_ISO.jpg

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[Sparweb]

Wil,
Ever look at a VAN's RV-6? Some common heritage, there, with the T-18. There's a lot more of those at the EAA fly-ins, these days!


Mohr,
Sorry - don't mean to hijack the thread!
I thought I'd contribute a few pictures to illustrate. Seems the internet is determined to supply me with horrible examples of weld jobs gone wrong (all merrily posted to blogs by proud homebuilders... sigh).
Hopefully by now you've just downloaded AC43.13 from the FAA and looked up the appropriate section.
If not, but you're still interested in seeing examples of how this should be done, just ask.

STF

 

[WKTaylor]

SparWeb... Yep... RV-6 Does reflect 'certain' T-18 design features for the 2-seat SBS seat all-metal low-wing aircraft.

Obviously, Richard VanGrunsven [RV] developed a wider following since his acft had more benign flying characteristics [longer/larger wing, lower span-loading] while maintaining very good performance numbers... and had overall more crew comfort than the cramped-cockpit T-18 [John Thorp had a very small stature]... while being simpler to build, was durable and adaptable to various purposes. Not to mention that as 'production' scaled-up, costs and kit-built times went down. As my Dad used to say during the building phase: he was 50% done... only 80% more-to-go!

I flew with my dad in his Thorp for several years and eventually soloed it a few times [my dad was very possessive of his T-18**]. Lesson: Never get complacent in it! The short-span/small-area Hershey-bar wing [high span loading for the heavier versions] combined with the stabilator, a very light [sensitive/balanced] control system [all axis] and high power/weight ratio made for some wonderful flying and some very sporty takeoffs and landings [we had a sod field in SoCal, semi-desert]!

NOTES.

A few years ago I heard that VAF claimed to originate matched-hole-tooling... which was in-use with the T-18 in the late 1960s. The T-18 association had to politely educate the VAF about history OF MHT.

I think RV was a Boeing engineer for awhile... so he had truly professional engineering background.
------

PS: SparWeb
Several years ago, in my last city, I saw two cars running around with personalized license plates: SPARGUY and SPARGAL were these Yours... or know who they were?



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[Sparweb]

Nope.
My wife has a vanity license plate, but not me.

STF