Aviation CAD TechNotes

F4F/FM2 Wildcat Wing Questions 2

I have given this article the title of “Questions 2” as moving on from my previous post on the FM2 wing, I have identified another anomaly that I cannot explain…another chapter in the development of the FM2.

This relates to the wing trailing-edge ribs that cover the main flaps. Normally, when you set out a win,g the wing chord is divided into percentages which would then provide a straight line segment on the surface when lofted between root and wing tip profiles.

For the Wing Trailing Edge rib profiles, this is not the case. At the wing 80% chord line, the top surface calculates with minimal deviation, as one would expect, similarly at the extreme wing tip. However, between 85% and 90% rib chords the deviation is not as expected. The contours at 85% and 90% trace a curve where you would expect a straight line.

What appears to be happening is that the wing TE ribs are dimensioned at various stations from the main Rib Sta 0. At stations 48, 52, 56, and 60, the offset dimension from the Baseline is the same for each rib at each of those locations. The end result is that the wing top surface is actually perpendicular to the wing root chord and does not follow the transition lines you would expect on a conventional wing loft. The transition lines at chords 85% and 90% are curved as you can see from the calculated offset tables below, which would normally be expected as a straight line.

At 80% and 95% chords respectively the wing top surface is for all intents and purposes a straight line as you would expect. The residuals column in the above tables shows the necessary correction offset for the selected point to align with the calculated Best Fit Line in millimeters. It could be argued that the offsets are no greater than +/- 1mm, which is not very much, but the flap ordinates are as shown and could have easily been dimensioned to a 1/64th inch had the draughtsman intended to show something other than they did. This alone demonstrates deliberate intent. So far I have identified alignment issues with the Flaps and Ailerons in my previous post, and this anomaly just adds more questions.

I know that this plane was originally conceived as a BiPlane, which explains the 5 datum lines we have for the wings and I am curious whether that design decision introduced a number of key aspects from which these questions have arisen. The truth is at this stage, I do not know, though the Flaps can possibly be explained; everything else is a mystery.

I have searched and read many forum discussions on the FM,2 and as far as I kno,w none of these issues have been identified or discussed… even the fact that the wing tip NACA profile is not a typical 23009 I suspect should have raised some red flags. Identifying and finding answers for design issues like this is part of the reason why I do what I do.