Modeling the Outer Wing Panel (2)

In the previous post I have formed the general shape of the Dauntless wing. Now I will work on its trailing edge, separating the aileron and flaps. They were attached to the internal wing reinforcements. These reinforcements were distributed in parallel to the trailing edge (Figure 13‑1):

Figure 13-1 The line that will separate the aileron and flaps
Figure 13-1 The line that will separate the aileron and flaps

In the first step I will split the wing mesh along this line. However, before I do this, let me mention a certain geometrical effect which can be surprising for many modelers. (Frankly speaking: it was also surprising for me — I knew that such an effect exists, but I thought that its results can be neglected for this wing area).

When you place on the wing a plane shaped like the line from Figure 13‑1 (see Figure 13‑2, left), you will discover that the resulting intersection edge on the wing surface forms a curved line (Figure 13‑2, right):

Figure 13-2 Cross section details
Figure 13-2 Cross section details

The curve on the wing tip is not a surprise, but why the intersection of the flat plane and the wing trapeze (i.e. the line between point 1 and 2) is also curved? The answer is: because this wing is like a section of an elliptic cone. The only straight line on the cone surface connects its base and apex. Any other direction (like our cutting plane) produces a curve. When the curvature of the wing airfoil on this area is low, the deviation from the straight line can be neglected. However, in this wing it produces a 0.23” deviation at the aileron root rib. You had to adapt contours of the spars and stringers used there.

Obtaining such a gently curved shape on a relatively long element is difficult from the technological point of view (i.e. costly). It can be applied if the high performance is on the stake (as in the Spitfire case). However, even the Spitfire designers had to make a compromise with the workshop and made the bottom of their wing flat. (In this way they provided a technological base).

What could do a pragmatic Northrop (then Douglas) designer in such a case? I have no direct photographic proof, but it seems that they approximated this shape with two straight segments. They are split at the aileron root section (Figure 13‑3):

Figure 13-3 Approximation of the curved cross-section
Figure 13-3 Approximation of the curved cross-section

In the next post I will show you that in this wing each of these two segments was made in a different way. The flaps were attached to a reinforced vertical wall (a kind of a partial spar), while in the front of the aileron there was a lighter structure matching the shape of the aileron leading edge.

After these deliberations we can cut off the trailing edge from the wing (Figure 13‑4):

Figure 13-4 Wing control surfaces separated into a new object
Figure 13-4 Wing control surfaces separated into a new object

(I did it in two steps. In the first step I created a new edge along the intended split line, using the Knife tool. In the next step I separated the rear part of this mesh into a new object).

We will deal with the red elements in the next post. In this post let’s recreate wing details along the flaps and aileron bay (Figure 13‑5):

Figure 13-5 Further updates of the rear wing edges
Figure 13-5 Further updates of the rear wing edges

The ultimate edges of aileron bay are located a little bit further than the “reinforcement line”. I extruded them from the original mesh.

When a part of the original control mesh is removed, the shape of the resulting object can have small deviations from the original shape of the complete wing. Thus before I separated the trailing edge I copied the complete wing into an auxiliary, “reference” object. Now I am using it to ensure that all these newly extruded vertices lie on the appropriate height (Figure 13‑6):

Figure 13-6 Adjusting Z coordinates of the newly created vertices
Figure 13-6 Adjusting Z coordinates of the newly created vertices

On the picture above you can see solid red areas around the modified vertex. This is the result of the approximation of the curve section (the flap hinges have to be straight lines).

To determine exact shape of the aileron bay edges I placed an auxiliary “stick” along the aileron axis, as well as some circles around it. The radii of these circles match the shape of the aileron leading edge (+ the width of the eventual gap — see Figure 13‑7, bottom left). Then I set the view perpendicularly to this aileron axis object, and used auxiliary circles to determine the shape of the aileron bay edges (Figure 13‑7):

Figure 13-7 Adjusting the shape of the aileron bay edges
Figure 13-7 Adjusting the shape of the aileron bay edges

Finally I closed the aileron bay with a curved wall that matches the shape of aileron leading edge (Figure 13‑8):

Figure 13-8 Closing the aileron bay
Figure 13-8 Closing the aileron bay

In this source *.blend file you can check all details of the mesh presented in this post. The next post will report further progress on the wing trailing edge details (I will form and fit the aileron).

This blog provides just an overall picture of the process. If you want to learn more about Blender and digital aircraft modeling, see this guide: “Virtual Airplane” (vol. II).

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7 thoughts on “Modeling the Outer Wing Panel (2)

  1. Good stuff, Witold…

    With respect to curvature in spanwise cuts, one thing that caught me was that any spanwise deviation in section incidence (twist) will produce a constant-chord cut with curvature unless the wing is lofted using ruled surfaces, in which case the section shape between the root and tip becomes distorted from the theoretical ideal. This can become very pronounced in highly twisted wings, and results in a conundrum: Do you want to match the theoretical spanwise progression of the airfoil section, or have straight hinge lines and seals? Usually what happens is the loftsman, the aerodynamicist, and the manufacturing engineer agree on contour deviations from the “theoretical” wing shape in the last ~30% of the wing chord to simplify things and ease manufacture and rigging of the control surfaces. This portion of the wing is “relatively” unimportant from an aero-performance standpoint. These simplifications, taken to an extreme, straighten the chordwise contour of the wing to produce a constant-angle “wedge” at the trailing edge so the upper and lower aileron/ flap contours are planar. A good example of this is the tapered-wing Piper PA-28 series of airplanes. The outer wing is twisted, and the aileron break doesn’t lie on a constant chord line, but the aileron is a planar, flat-wrapped, triangular box hinged at the upper wing surface using piano hinges.

    There are a lot of subtleties in the design of even the simplest of airplanes.

    Thanks again for documenting this build se thoroughly…

    -Brian-

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  2. Brian, thank you for this comment – indeed, quite often the things are not as simple as they seem at the first glance…

    Your note about the wing washout pushed me to check the influence of the twisted tip on the shape described in my post. (Dauntless wing was designed when the washout was not widely used – in that time only the Spitfire prototype had its wingtips twisted by -2deg. That’s why the BT-1 and then the SBD team had problems with the stall characteristics. They resolved it in a less “elegant” way – by introducing the fixed slats).

    To test the influence of the wing washout on the non-chordwise cross section with a plane, I prepared following testbed:

    The result after twist by 1 deg:

    (as you can see, the bottom intersection edge is nearly straight, while the upper edge remains curved);

    The result after twist by 2 deg:

    The bottom edge is straight. The slope of the upper edge is increased.

    What’s interesting, its max. deviation from the straight line is the same as in the previous case (it seems to be constant).

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  3. Interesting…
    By shear coincidence I happened to be out at Chino airport on Sunday and had a look at the Yanks Air Museum’s SBD-4. Unlike the Planes of Fame SBD-5 on the other side of the field, with Yanks you can get into a position to actually sight down the upper and lower edges of the wing-aileron break and check the contour. The lower edge is absolutely flat. The upper edge has a very slight deviation from straight, being slightly “tented” at the pushrod attachment. The upper edge actually looks like two line segments with a common vertex at the pushrod. I’d say the deviation from perfectly straight at this vertex is between about 0.10 and 0.20 inches.

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    1. Witold,

      You’ve got it exactly right, although the variation between “tented” and straight is so subtle it could just as easily be a result of manufacturing/restoration tolerance buildup as a design feature. I didn’t notice if the aileron followed the wing contour; it would be harder to detect because there’s no hard edge to sight along. I wanted to take a photo but the depth of fields was so wide I could only get a couple of inches of the aileron break in focus at any one time.

      Have an enjoyable and relaxing vacation! We’ll all be here when you get back…

      -Brian-

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