Grumman F4F Wildcat: Wing Ribs:
Recently I received an email asking if I had done any work with the Grumman F4F Wildcat. As I do have an archive for this aircraft it was indeed on my to-do list. This inquiry prompted me to have a closer look at the archive to see what information was available to derive a working ordinate dataset.
Similar to the P-39 the archive does not contain tabulated data but the part drawings do have the ordinate dimensions. Working to derive an ordinate dataset from part drawings as you can imagine is quite intensive work as you first have to collate the drawings and then develop the profiles in CAD and then extract the point data to a spreadsheet. A complete reversal of the normal process.
The work I was doing for the F6F Hellcat previously was not a priority task so I decided to do some development on the F4F Wildcat, starting with the wings. This threw up a few surprises as the wing rib dimensions were not relative to the wing chord as you would normally expect, instead, they were from a Base datum line. I had not seen this before and it transpires that the reason for this is because the wing ribs are actually not perpendicular to the wing chord. They are in fact perpendicular to wing datum line.
This next image shows a simplified sketch of how the dimensions are shown on the Grumman drawings. Note also that the vertical divisions are dimensions fore and aft of the “0” line (which I take to be the vertical datum) and not percentage breakdown of the cord length as expected.
This raises all sorts of questions as to why Grumman designed the wing structure in this manner. I cannot think of any performance or manufacturing benefit in doing so. You can also see in this scrap view from an actual Grumman drawing how the dimensions are to the baseline and not the chord line.
I posted a similar question on the WW2 Aircraft forum, so hopefully, someone will enlighten me on this unusual design feature.
Update: Solved!
The wing ribs are perpendicular to the Wing Datum line, which is 1.6 degrees from the Thrust Line (essentially the design horizontal axis) that aligns with the fuselage Thrust Line. It transpires that the various wing components are dimensioned relative to any one of 5 different datums depending on their function.
Update Sept 2018: Work in Progress:
Aviation CAD TechNotes 