Verification of the Model Geometry: the Wing

In this post I will continue verification of my model by matching it against the photos. This time I will check the wing geometry.

In the first photo from the Pacific Aviation Museum (in my model it is marked as PAM-1) I identified several differences (Figure 31‑1):

Figure 31-1 First differences that I found in the outer wing panel

First I noticed that the hinge of the upper flap in my model is in the wrong location (I had to shift it forward by 0.7 inch). The upper edge of the aileron bay had slightly different shape on this photo. In this picture the tip of the aileron (the point lying on the wing tip outer edge) is located in the front of the corresponding point in my model. (The difference is less than 1 inch). Surprisingly, the inner (root) rib of the aileron seems to be a little bit higher in my model than on the photo. I can see also similar difference in the root rib of the outer wing panel. Location of the aileron bay upper edge on this photo can also be interpreted as located below the corresponding edge in my model. Does it mean that I made an error in forming this wing? The last visible difference are the outlets of the fixed slats. According the photo they were smaller and set at slightly different angle.

To check the differences in the wing thickness and the details around its trailing edge, I matched my model to another photo (Figure 31‑2):

Figure 31-2 Verification of the model using another photo

I opened the wing flaps to better match the projection of my model to this photo. I used the shorter edges of the flaps to precisely determine their deflection. The bottom flaps fits well into this photo (if you take into account that in the depicted airplane the outer flap is bent). To fit the upper flap I had to shift it by 0.8 inch, as shown in the previous photo (Figure 31‑1). This confirms that there is a difference! What is interesting, the wing on this photo is also slightly thinner than in my model — which confirms that I made a mistake in recreating the shape of its ribs.

As I wrote, I was convinced that I properly recreated the airfoil shape. I used the original coordinates of the NACA-2415 (and NACA-2409) airfoils (as you can see in Figure 31‑3a)! Thus I used another, side photo (PAM-3) to check this finding (Figure 31‑3b):

Figure 31-3 Further investigation of the wing root shape

The overall chord of the wing rib in Figure 31‑3 seems to be OK (luckily, on this photo you can see the fragment of the leading edge between the truck forks). The chord of its bottom flaps in my model also fits corresponding chord on the photo. However, the upper edge of the root rib in my model seems to be too high (by about 0.3 inch). I can see clearly that it occurs in the middle of the upper flange of this rib. On the scale plans this difference corresponds to just half of the contour line width! That’s why we have to use photos: the drawing conventions alone make the scale plans not as precise as we wish… The PAM-2 (Figure 31‑2) photo reveals that this difference (maybe somewhat smaller than at the root) extends over the whole wing span.

Well, so I had to fix it. While lowering the upper part of the center wing was relatively easy (Figure 31‑4a), I had also to modify all the adjacent objects — ribs, spars, and the fuselage (Figure 31‑4b):

Figure 31-4 Fixing the wing thickness issue

The more difficult was to make similar modification in the outer panels. The difference was smaller at the wing tip. To preserve the straight lines of the spanwise mesh edges I moved the whole selected area down by 0.3 inch, then compensated the difference at the wing tip by small rotation around the wing root chord. (I had to separately rotate each of these “longeron edges”). Of course, then I had to make a lot of minor compensations in the upper flap and the aileron contours.

However, I had to modify these trailing edge details anyway, following the other findings from the photos (Figure 31‑5):

Figure 31-5 Fixing the differences in the upper flap and the aileron bay

In this modification I had to revert the changes I made two months ago to the aileron bay edge (in the post about details of the outer wing panel). It was the wrong location of the flap hinge, while the aileron bay edge should be in the place depicted on the reference drawings! In fact, drawing these scale plans I assumed that the hinge of the upper flap was directly above the hinge of the bottom flap. (You cannot see the difference on the most common, horizontal photos). Now I know that it is shifted away from the auxiliary rear spar by about 0.8 inch. After this modification I had to shorten the chord of the upper flap and rotate its ribs and spars, adjusting them to the altered directions of the flap skin. It required a few additional hours…

Once I finished with the trailing edge, fixing of the outlets of the fixed slats was easier. I just had to modify the shape of the “cutting tool” auxiliary object, used in their Boolean modifier (Figure 31‑6a):

Figure 31-6 Fitting the shape of the fixed slat outlets

Then I adjusted the slat internal surfaces, fitting their upper edges to these modified openings (Figure 31‑6b).

If I encountered such surprises on the upper wing surface, what do I find on the bottom of the wing? I started by examining the outer wing panel (Figure 31‑7):

Figure 31-7 Verification of the wing bottom surfaces

It was surprisingly difficult to find an appropriate projection for this wing — I badly missed the fuselage here! (It would allow me to better determine the proper direction of the camera). The barrel distortion of this photo could also have some influences on this matching. Fortunately, it seems that my model fits better this area of the real wing. The first difference I found was in the fixed slats: minor adjustment of their direction and sizes. I fixed them in the same way as their outlets on the wing upper surface (I will not bother you by describing the details). Another difference is more subtle: it seems that the real wing tip has slightly different shape than in my model!

Of course, I had to check it on another photo, taken from another direction (Figure 31‑8):

Figure 31-8 Verification of the wing tip shape

This photo confirms my finding: it seems that I made another wrong assumption about the shape of the wing tip. (I assumed that the rear part was a single arc, while it is at least a smaller arc and an unidentified curve — maybe short piece of another arc of larger radius?). Of course I accordingly modified the wing tip (by adjusting location of a few of its vertices — in fact it was not as complicated as it sounds).

For the complete verification of the wing, I used the picture from the SBD manual. I checked the bottom surfaces of the center wing (Figure 31‑9):

Figure 31-9 Modifications after matching the photo of the center wing (bottom view)

To speed up narration in this post, the picture above is showing the updated mesh. I just enlisted the modifications that I made. As you can see I had to adjust the outer edge of the wheel bay (because it was not a simple circle). There were some minor differences in the split lines of the bottom covers (I had to adjust the bottom fuselage! Again!). Ultimately, I discovered that I placed the fixed ribs above the flaps in wrong locations (I really do not know why I not followed the stations diagram— now I corrected this mistake).

In this source *.blend file you can evaluate yourself the model from this post.

This is the last post about this “great verification”. Now I am coming back to modeling. In next two posts I will recreate the empennage of this aircraft.