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Effectivity of Bulbs on AND extrusions

Effectivity of Bulbs on AND extrusions

Effectivity of Bulbs on AND extrusions

(OP)
I need to find accurate information on the effectivity of the bulbed ends on AND extrusion.  The method in Bruhn is to conservative and the analysis sthe bulbs as ineffective.  I find it hard to belive they would be there if the didn't help.

Does anyone have any info that will help?

Nigel Waterhouse
n_a_waterhouse@hotmail.com

A licensed aircraft mechanic and graduate engineer. Attended university in England and graduated in 1996. Currenty,living in British Columbia,Canada, working as a design engineer responsible for aircraft mods and STC's.

RE: Effectivity of Bulbs on AND extrusions

As far as I can find out, two reasons for bulbed extrusions are:
1.  Altering the moment of inertia of the section.
2.  The bulbed end provides resistence to crippling or crimping deformations.

RE: Effectivity of Bulbs on AND extrusions

I recall those bulbs were on very old extrusions. I have seen them on aircraft dating from the 1930s (a Boeing Model 247 if you are curious, I engineered some of the restoration mods). They are no longer used because it is more weight efficient to use the material in the bulb to extend the flange, or so the stress guy told me.

As you might imagine, the mathematical tools to evaluate the effectiveness of those bulbs were rather crude in the '30s.

Terry Drinkard
terrydrinkard@yahoo.com

RE: Effectivity of Bulbs on AND extrusions

(OP)
Terry,

Thank you for your reply.  The stress guy was correct, the bulbed flanges are there to provide resistance against crippling and buckeling.  However, they are still used today on modern extrusions.

My problem is when I use the method of analysis described in Bruhn (the aircraft stress bible) the bulb on the flange is not effective.  This is due to the high degree of conservativeness built into the analysis.  What I need is some other method of analysis that gives a more accurate result.

Nigel.

Nigel Waterhouse
n_a_waterhouse@hotmail.com

A licensed aircraft mechanic and graduate engineer. Attended university in England and graduated in 1996. Currenty,living in British Columbia,Canada, working as a design engineer responsible for aircraft mods and STC's.

RE: Effectivity of Bulbs on AND extrusions

You will need to refer to design charts from one of the major aircraft companies, or possibly an old structures text book (they are the best ones) to answer this question.
ESDU may have design charts, or look around for the Royal Aeronautical Society charts that preceded ESDU.

The bulb increases stringer moment of inertia, making them very effective in redistributing beam column loads by through greater striffnes for a given flange height, by developing lower stresses than a constant thickness flange, and by having a higher crippling strength.

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