[Courtesy of Joe Wurts, JoeWurts "at" csi.com, June 1999]
The spreadsheet linked below is very useful for calculating an appropriate fiberglass and carbon layup for sailplane wings. It will show the predicted loading of the wing panels, and compare the predicted loading to the strength of a proposed layup. Here is some background information that may be useful in interpreting the spreadsheet and the results. The links to download the spreadsheet and instructions sheet are placed below the following Q&A section.
If you're curious, here's a graphic of the spreadsheet data entry area and results charts (click the small thumbnail image below for a full-size picture):
There are several assumptions embedded as part of the spreadsheet calculations, with answers presented in Q&A format (answers by Joe Wurts, June 1999):
Q: What length is assumed for the wing joiner(s), and are there any other wing joiner construction assumptions?
A: The joiner(s) are assumed to be long enough so that the kick loads in the joiner do not introduce local compression failure, and that the method of embedding it does not introduce large local stress concentrations. That is, no hard and fast rule, but bad construction methods will invalidate the results.
Q: What units are used for the joiner stress value, and what material is assumed?
A: The joiner stress is in lb/sq in, and the calculations assume a uni-directional carbon joiner, at 100,000 psi yield strength. (I always run the joiner tube well past the end of the joiner, to reduce stress concentrations)
Q: Any thoughts on whether the model is still reasonable for a single-piece wing?
A: The model is quite reasonable for a single piece wing, you just have to make sure that the large shear loads introduced by the method of attaching to the fuselage will transmit the shear without stress concentrations. Also, beware of kick loads due to any dihedral.
Q: Does the spreadsheet assumes that the entire air load will be handled by the wing skin, and that the wing does not have any internal spars other than the joiner?
A: This spreadsheet is assuming a stressed skin construction. The current version also calculates a tube strength, or inversely calculates the required tube wall thickness to get the strength. Many people are now inserting spars to minimize the requirement for "good" construction (a little imperfection in a stressed skin, and poof, delamination).
Q: Is there some lower limit to wing size that this spreadsheet models reasonably? (In other words would you guess that the model is still reasonable for 2m or even HLG size wing planforms?
A: I've used it for F3B, thermal duration, cross-country, and HLG. Seems to give good numbers across the gamut.
Q: The text file suggests a donation to the team, but it doesn't say where to send the donation!
A: Nowadays, the donations should go to the F3B team manager, Scott Condon. It might be a good idea to put a link to the F3B team site, or put my address in there (in four months you would have to change the address again to get the new TM).
My address is: Joe Wurts 25327 Avenida Ronada Valencia CA 91355
Wing Layups Instructions (4 kB text file)
Wing Layups Excel Spreadsheet (40 kB Excel spreadsheet)