Cutting Out the Landing Gear Bay

In this post I will cut out the opening of the landing gear bay in the wing. In the SBD Dauntless its shape consists a rectangle and a circle (Figure 19‑1):

Figure 19-1 Landing gear bay in the SBD Dauntless
Figure 19-1 Landing gear bay in the SBD Dauntless

However, when you look closer, you will notice that the contour of the main wheel bay is not perfectly circular. There is a small deformation of its shape on the leading edge (see Figure 19‑1). I think that it looks in this way because of the technological reasons. Another feature of this opening is the fragment “cut out” in the bottom part of the fuselage, below the wing. (We will make it when we will form the fuselage).

I started by applying all the information that was confirmed by the general arrangement drawing (presented in this post) and various technical descriptions: the main wheel used 30”x7” tire. Its center was placed 18.5” from the firewall (measured along the global Y axis) (Figure 19‑2):

Figure 19-2 Fitting the main wheel
Figure 19-2 Fitting the main wheel

The X coordinate of the wheel center can be determined by the location of the root rib (10”) + small gap + tire radius (30”/2) ≈ 26”.

Then I tried to put around the main wheel a test contour of the opening in the wing (Figure 19-3):

Figure 19-3 Determining the best method to create main wheel opening
Figure 19-3 Determining the best method to create main wheel opening

Initially I thought that I will recreate this opening by embedding a subdivided octagonal hole in the wing mesh, as I did in my P-40 model (see Vol. II of the “Virtual Airplane” guide).

A subdivision curve based on an octagon produces nearly perfect circle. It does not matter if vertices of this octagon lie on different depths — as long as they form an octagon in the vertical view, the curve based on such a control polygon looks like a circle in the vertical view. (The mathematicians call this property “projective invariance”, it also applies to the NURBS curves). When you know it, it is much easier to model various mechanical shapes.

However, when I created an appropriate octagon around the wheel, I discovered that one of its vertices lies outside the wing mesh (see Figure 19‑3a). You cannot compose such a contour into the wing. Therefore I decided to create this opening using another Boolean modifier, as I did in the case of the fixed slats (see this post). I prepared the basic contour of the “cutting tool” — a smooth circle based on a 16-vertex polygon (as in Figure 19‑3b).

The fragment of the main wheel opening that “touches” the wing leading edge seems to be flatten a little (see Figure 19‑1). To obtain such an effect I rotated the “cutting tool” object (the ring) by a few degrees so its Y axis was perpendicular to the leading edge. Then I shifted a little the single edge of this ring along the Y axis, fitting it into the wing (Figure 19‑4):

Figure 19-4 Adjusting shape of the “cutting tool” object
Figure 19-4 Adjusting shape of the “cutting tool” object

By small movement of these two vertices I was able to precisely recreate the shape of this opening visible on the photos (Figure 19‑5):

Figure 19-5 Final adjustments of the shape of the future opening edge
Figure 19-5 Final adjustments of the shape of the future opening edge

If I do not want to get the inner part of the “cutting ring” inside the resulting opening, I have to assign to this wing mesh a sheet metal thickness (using the Solidify modifier – as in Figure 19‑6):

Figure 19-6 Preparing the wing skin for the Boolean modifiers
Figure 19-6 Preparing the wing skin for the Boolean modifiers

Because the forward and rear part of the wing are separated, I can use this Solidify modifier only in the front part. In this way I do not increase the polygon count of this model with unnecessary faces.

As you can see in the picture above, I also created a second “cutting object” — a box. I will use it to recreate the rectangular opening around the landing gear leg. Both of these tool objects are located on a single layer (9) which will be hidden during rendering. Their parent is the rear part of the center wing section (to avoid dependency conflict with the front part of the wing).

Finally I assigned both of these “cutting” objects to the Boolean (Difference) modifiers of the wing skin (The same method as used for the fixed slats in this post). You can see the result in Figure 19‑7:

Figure 19-7 The opening for the landing gear, created with two Boolean modifiers
Figure 19-7 The opening for the landing gear, created with two Boolean modifiers

It would be quite difficult to recreate such an opening by altering the control mesh of the wing skin. It also would make its shape more complex, and difficult to unwrap in the UV space (for the textures).

The openings created by Boolean modifiers have another advantage: it is very easy to modify their contours. I had to do this just after I created these holes. I discovered that I made minor error in the reference drawing: the landing gear leg opening should lie a little bit back. (Its centerline should pass through the landing gear wheel center (Figure 19‑8):

Figure 19-8 Adjusting the opening location in the wing
Figure 19-8 Adjusting the opening location in the wing

All what I had to do was to shift back the auxiliary box object, which creates this opening. So easy!

On the other hand, I observed small shadows caused by triangular faces created by the Boolean modifiers along edges of this opening. It was impossible to remove them in the typical way — using the Auto Smooth option or the Edge Split modifier. The only solution was to increase (from 2 to 4) the level of the Subdivision Surface modifier assigned to the wing surface object. It increased 16 times the number of resulting smooth faces created from this mesh. Fortunately, I split the wing into two parts, so I could set keep such a dense mesh only around the area where it is needed.

In this source *.blend file you can check all details of the wing presented in this post.

In the next post I will create main spars and ribs, visible inside this opening.

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