STRUT DESIGN -95.10

[howe]

1) Checking a existing strut on my plane,has 25 x3 shs (grade unknown) inserted and bolted into oval tube 6061.

 

Would like to replace it with 25 sq solid bar.

 

Can't seem find agreement for allowable stress for -AlUMINIUM 6060 -T5 - 25 sq bar extrusion.

 

Fty- 100 mpa , Ftu- 140 Mpa, Fsy-???

 

Thanks - Howe

 

 

[M61A1]

A quick search tells me that they use 6063-t5 data for 6060-t5 as it must meet the same standard.

 

Yield- minimum 16000 psi up to 0.5" thick.

 

ultimate- 22000 psi up to 0.5" thick.

 

There was some data for thicker sections, but it wasn't as strong for thicker sections.

 

 

[howe]

Yeah, thanks for that, found info on the A.A.C. web site exactly as you confirmed above.

 

What would be the G loading a connection of this type should be designed for?

 

Thanks ... Howe

 

 

[M61A1]

I don't think that question can be answered without knowledge of the structure and weight/wing loading of the aircraft in question.

 

 

[Head in the clouds]

Yeah, thanks for that, found info on the A.A.C. web site exactly as you confirmed above.What would be the G loading a connection of this type should be designed for?

Thanks ... Howe

Howe, because of it's very low inertia and low wing loading a 95:10 aircraft's structure is subject to quite high instantaneous loads in turbulence. Therefore it is prudent, and usual, to design a SWL for accelerations of at least +6G and -4G. For SWL that would mean your calcs should be for yield rather than ultimate.

 

 

[howe]

Wow !!!! +6G & -4G on yield, hope the rest of the beast will handle that.

 

The thing I find interesting with some A/C when ever I glance at some of the combined tension&compression members, struts etc, they don't look like they would satisfy the minimum slenderness ratios (l/r <180) as per the steel structures code.

 

How do they get in?

 

 

[Head in the clouds]

Wow !!!! +6G & -4G on yield, hope the rest of the beast will handle that.The thing I find interesting with some A/C when ever I glance at some of the combined tension&compression members, struts etc, they don't look like they would satisfy the minimum slenderness ratios (l/r <180) as per the steel structures code.

How do they get in?

Yes, well you would hope so! 6G is not that hard to achieve with strut bracing, I've seen some rather woesome structures that have load tested OK. One for example, with modified struts where they are only attached to 2.5mm low grade aly sheet wrapped around the l/e tubular spar and held in place with 4-3/16" steel pop-rivets.

 

The SR can be a bit misleading in some applications since it is intended as a guide for single elements within a structure, like a column supporting a rafter in a building for example. It has little or no relevance in tension and assumes no midspan support where referenced for members in compression. In general the SR is about 25% lower for aly members than for steel ones but it is much modified according to Section where the members are tubular, and then the form (shape) and wall thickness comes into play as well.

 

I don't have a link to hand at present but there are comprehensive tables (and software) for determining optimal SRs/Sections for compression applications such as for an aircraft wing-strut. Also keep in mind that a valuable tool, often neglected, is the jury strut which allows the use of a significantly thinner strut section by providing the strut with anti-buckling support at the strut midspan during negative G loading and also provides valuable anti-buckling support to the wing spar(s) during positive G loading.

 

 

[facthunter]

I'm glad the question of compressive loading has come up when people are thinking of using solid sections. The easiest is probably cables but they are draggy. and the swages worry me.

 

+6 -4 is a minimum for me . I would prefer to go higher, particularly in the positive. The calcs are easy in tension if you know the material spec. If you are manoeuvering and hit a gust the load goes pretty high. A strut hitting a bird is a consideration perhaps. Nev

 

 

[M61A1]

Bruhn's "Analysis & design of flight vehicle structures", has some tables for compressive strengths of different materials and lengths.They may be able to be found on the net somewhere.

 

 

[howe]

Thanks for the feedback Guys.

 

I was mainly concerned with, tensile,tearing and bearing failure of the connection . see attached photo.

 

As far as the strut overall goes AS/NZS 1664 seems to cover allowable stress for compression members with given S/L.