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proppastie

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distance between closest ribs or vertical stiffeners on the spar? !

Using Douglas Page 257 sample B5.1.2.7 I come up with spacing of 10" on stiffeners for .025 web. If I had ribs on that spacing you seem to say above that that counts as a stiffener.

With 6" spacing I might be able to get away with .016 web leastwise that calculation shows that.

any old stress engineers want to comment?
 

Matt G.

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Well, I'm not an 'old' stress engineer, but my gut feel is that you'll have a hard time showing your fastened joints good on a web that thin at the root. 10" rib spacing is going to result in a lot of parts and be very weight-inefficient. I haven't had a chance to look at the manual you are referencing, but I would caution you to fully understand the math and assumptions behind the checks you are doing. 'Cookbook' solutions can make this difficult, which is why myself and several others have been recommending Bruhn, which will allow you to more fully understand what you are doing.

If your goal is the lightest possible wing, you need to do a trade study to find the optimum combination of web thickness,skin thickness, and rib spacing, and this can be done with (you guessed it) Excel.
 

dcstrng

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With 6" spacing I might be able to get away with .016 web leastwise that calculation shows that.
I haven’t read through the book, and I’m certainly no engineer (I tend to look at proven designs and then figure that’ll work if they’ve worked for 20-30 years or so…).

For instance the elderly Zodiac CH601HD (not the XL with the supposed spar issues, this is the old-standby…); had 0.025 spar web (about 10.7 in deep) with 0.025 ribs on 23”+ centers, with wing skins of 0.016 in the outer panels – inboard section to take bending/landing loads was heavier duty, … but a spar 10” deep can carry quite a bit with 1.50 x 1.0 0.125 caps… center section had 0.040 web… and closer spacing especially in the wing-walk area, etc., et…

All 6061T6...
 

proppastie

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Well, I'm not an 'old' stress engineer, but my gut feel is that you'll have a hard time showing your fastened joints good on a web that thin at the root. 10" rib spacing is going to result in a lot of parts and be very weight-inefficient. I haven't had a chance to look at the manual you are referencing, but I would caution you to fully understand the math and assumptions behind the checks you are doing. 'Cookbook' solutions can make this difficult, which is why myself and several others have been recommending Bruhn, which will allow you to more fully understand what you are doing.

If your goal is the lightest possible wing, you need to do a trade study to find the optimum combination of web thickness,skin thickness, and rib spacing, and this can be done with (you guessed it) Excel.
I certainly am warming to Excel, I also am looking at the Willford xls and am seeing the same formula as in the design guide. I think a full understanding to the theory can not hurt, but a good example of exactly what you need to calculate will get the job done lots faster and probably more accurately.

They used this to actually design there, when I took a stress course at McDonnell (not much stuck) they would hand out pages from MAC 339, and this is what MAC 339 turned into.

Willford has some rivet inputs, but I have not got to that part in Douglas yet. I was extremely happy to do the shear web calculation and then compare it with Willford.


I set up my spread sheet and solve the example then copy and change the inputs to use my numbers. The practical aspects or some confusion of terminology can be a problem and this site has been extremely helpful in that regard. Thank you for all your help.
 
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