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ANC-18 plywood buckling constraints

ANC-18 plywood buckling constraints

Hi, I have a question regarding buckling of plywood panels.
I need help with interpreting ANC-18 (1951 issue),
Chapter 2.7, figures 2-24 to 2-29 and Formulas 2:72 to 2:77.

For the calculation of buckling constants "Hc" or Hs" (compression or shear),there are plots of the different buckling equations for a number of cases .
In the plots different curves are shown for different values of "K" ranging from 0 to 1.
My question is: What is the defintion of "K"?
Observe that in the formulas the small "k" has nothing to do with the big K"
I can not find that anywhere in ANC-18, not in the definitions (nomenclature) pages or elsewhere.

I suspect it some kind of factor for the stiffness or edge restraint (simply supported, clamped, number of edges restrained etc). That is how it is usually shown in plots for sheet metal buckling. But the restraint type is already defined for each different plot. The edge length ratios (1/r, a function of the edge lengths a and b and also the different Youngs moduli E1 and E2) are also defined on one of the coordinate axes. So what´s left over to define?

Observe that these plots were not included in the earlier (1944)issue of ANC-18.
The treatment is more extensive in the 1951 issue. I also noted that some stress data for Sitka spruce differ between the issues.

When I started digging into this I didn´t realize that it would be so much to define. I found some easy to apply data on the allowable shear stresses for plywood beam webs in Alex Strojniks book "Laminar Aircraft structures" referencing an old EAA Aircraft design book and also in NACA report 344. But the numbers don´t coincide very well at all and I decided to check with ANC-18-ending up a little confused.

Thankful for help.

RE: ANC-18 plywood buckling constraints

I find a definition of "capital" K on page 66, equation 2:71.
I also find a "small" k on page 68, right below Figure 2-26.
I hope your copy is better than mine - my pages are difficult to read.


RE: ANC-18 plywood buckling constraints

Hi Sparweb,
the big K and the small k have nothing to do with each other.
I found out by googling a lot that the definition of K is hidden in reference 2(50)of ANC-18. That is NACA TM 705 and also Forest Products Laboratory "mimeograph"(= report...) no 1316. I was lucky to find a download on both of them and now it makes some sense to me.

That plywood data is not really easy to digest but I´m getting closer every day. I had a copy of ANC-18 for over 35 years (I bought it after reading an advertisemnt in the EAA Magazine before internet existed) and its just only now that I´m beginning to actually understand the buckling graphs..

RE: ANC-18 plywood buckling constraints

Wood Species, veneer [ply] quality, ply orientation/stack [relative to load orientations] and adhesives play a large role in plywood panel stiffness/strength/durability.

Yeah my antique acft has plywood floors... had to learn this stuff in a hurry when domestic high quality marine plywood [required by drawing] supplies were shrinking fast... and costs were skyrocketing. Found some potential commercial and marine-grades of plywood species and 'about as strong' [and one marine grade wood-species that was actually significantly lighter]. Lots of interesting info that is very odd/curious to a metals weenie... but would be right-at-home for a composites nerd.

Companion to ANC-18 is ANC-19 Wood Aircraft Inspection and Fabrication

You also might find the following useful...






FPL-GTR-190 Wood Handbook - Wood as an Engineering Material

FPL-1300 Summary of Formulas for Flat Plates of Plywood Under Uniform or Concentrated Loads

FPL-1316 Summary of Flat Plywood Plates in Compression, Shear or Combined Compression and Shear [various supplements listed under major document number]

The Gougeon Brothers on Boat Construction

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", forum]

RE: ANC-18 plywood buckling constraints

A book which recently came on the market, if you are dealing with wooden aircraft construction and repair is an English translation of Hans Jacobs German Classic “Werkstattpraxis fur den Bau von Gleit- und Segelflugzeugen." This translates to “Workshop Practice for building and repairing wooden gliders and sailplanes." This book was first published in 1932 in Germany, and has recently been translated by Neil Pfeiffer and Simine Short into English for the Vintage Sailplane Association to assist their members in maintaining and restoring their aircraft. The library of congress number is 2016940039, and the book is available through the usual suspects. This book covers methods and techniques of fabricating and repairing wooden aircraft structures, up to and about the early 1950s. Another interesting item is a series of appendices added while the book was being translated to bring the reader from 1955 to 2016. with regard to more modern practice in wood construction and repair. The book is light on formulas, but heavy on practical knowledge.

You are judged not by what you know, but by what you can do.

RE: ANC-18 plywood buckling constraints

Hi and thanks for all the replies:
I have most of the referenced documents but was happy to hear of some new.

The "small" problem remains now to be sure that my calculations are actually correct, that I can do by comparing with test data from the various reports.
I made a check against the shear stress of the fuselage skin of my 2-seater homebuilt (Jodel D112, cont A-65 Engine). Side skin 2mm, panel size 650 x 440 mm.
The skin stress at Va is quite low, about 1 MPa or less (applying tail lods according to JAR 23-Appendix A).The critical calculated buckling stress is just a bit lower than the actual stress and the calculated ultimate stress is a bit higher (about factor 1,5). That makes sense to me.
I think the skin would anyway buckle at loads less than the critical calculated since it is already rather uneven (50 years old plywood). Ultimate calculated correct though. Calculating the fuselage as a pure wagner beam leads to much higher allowable stresses, but I Think that is being rather too optimistic.
The truth is somewhere between.

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