BRUHN C10.18 and EAA Spar Design

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proppastie

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Looking at Bruhn section C10.18 Fig. C10.20 for Web =.025, h/t=388. Willford Spar Design Spread sheet cell E72. Willford has this number at -4654 and the Bruhn extrapolation should be lets say +4654. That sign makes a difference in the result.

Any comments from the Stress Engineers on site?

If the correct calculation for example shows "Collapsing Shear Flow Allowable" = 65 lb/in what does that mean?
 
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Matt G.

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There are some errors in the examples in Bruhn. There is at least one, if not multiple lists of errata that correct them. Google 'Bruhn errata pdf' or something along those lines.
 

proppastie

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Well the Bruhn calculation and the graph both give about 50-60 lb per inch, so I thought Willford might be wrong. I am not quite understanding that result. I am working through the example, but still missing it. In which direction is the "per inch" .... Y or Z inch of what,... inches of length of spar under load or inches of height of web in shear. Is the allowable ultimate, or yield. Pretty basic stuff I guess, but not easy on your own. Thanks for your help.
 

skyscooter

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Looking at Bruhn section C10.18 Fig. C10.20 for Web =.025, h/t=388. Willford Spar Design Spread sheet cell E72. Willford has this number at -4654 and the Bruhn extrapolation should be lets say +4654. That sign makes a difference in the result.
Note what the spreadsheet says in row E63 "** calculations good for h/t and c/t between 60 and 300. Values for 2024-T3 (24ST)." Your h/t is 388. The cell has a curve fit of the Bruhn figure. Calculating a value based on a curve fit beyond a defined range can give incorrect values, as in your case. Also note the disclaimer in row 4.
 

proppastie

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Note what the spreadsheet says in row E63 "** calculations good for h/t and c/t between 60 and 300. Values for 2024-T3 (24ST)." Your h/t is 388. The cell has a curve fit of the Bruhn figure. Calculating a value based on a curve fit beyond a defined range can give incorrect values, as in your case. Also note the disclaimer in row 4.
That explains the Willford and is what I figured.

Also "This spreadsheet is for educational purposes only" which is what I am using it for. One extends the curve to 388 and looks at Fig. c10.21 also and runs through the extensive calculation for my numbers and I get correlation at approx. 65 lb in. I say approx because the curves are somewhat small. Its an Ultra Lite glider gross of 330#. Is that 65 LB IN in the Z or Y direction and are we measuring the whole 1/2 span if in Y direction. And is it yield or ultimate. 1/2x330lbx8G/256.5=5.146 lb/in... Y direction for my paper airplane. Is this what I am comparing to? Thank you for your help.
 

proppastie

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shear flow.jpg












1320/9.714=136 lb/in is the shear flow. he=9.714 in 1320= 8g x 330lb gross x 1/2 (one wing half)

so if the 65 lb/in is correct allowable then it fails.

MS=(65/136)-1 = -.52
 
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