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How are foam cores shaped for tapered and swept back wings?

I understand that many constant chord, constant thickness rectangular wings use the same sized airfoil shape from root to tip. Let's assume the wing uses a classic NACA airfoil shape such as 650-18.

How would a foam core wing with a classic NACA airfoil shape, swept back 10 degrees be shaped? Is the wing, using the same airfoil shape, NACA650-18 just rotated back 10 degrees from the root? In this case, in cross section longitudinally (along the plane's y axis, in the direction of airflow), the actual airfoil shape would no longer be the same shape as the rectangular non swept wing, even though the geometric perpendicular cross section of both wings is exactly the same. Is this correct thinking?

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  • $\begingroup$ It isn't clear to me whether you are asking about manufacturing techniques, or wondering about the altered shape definition resulting from the longer chord when the wing is swept. You are correct in your thinking that the relative or apparent chord of a straight wing becomes elongated when swept. Is that all you are wanting to confirm? $\endgroup$
    – Michael Hall
    Apr 12, 2021 at 1:32
  • $\begingroup$ I was asking about both actually, the manufacturing technique, and how the shape is altered. $\endgroup$
    – Fred
    Apr 12, 2021 at 14:01

1 Answer 1

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I'll assume you're already researched the methods for hot wire cutting foam blocks in the Rutan moldless method and are familiar the hot wire tool and airfoil templates you make and the template cutting process.

You can do it two ways; make hotwire templates with the longitudinal axis airfoil contour (the stretched airfoil shape resulting from the sweep) to stick on the end of the foam blocks that you have already truncated with the 10 degree sweep, or make the templates to the airfoil profile perpendicular to the mean chord line, stick them on to the blocks with square ends, and truncate them to the 10 degrees sweep angle after you've cut them out.

enter image description here

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  • $\begingroup$ So how do you account for sweep by elongating the airfoil template as in your case 1 above? I assume you just use the same airfoil shape, but with a longer chord? In either case, the "longitudinal airfoil shape" will not be the original one. Is this correct? $\endgroup$
    – Fred
    Apr 12, 2021 at 0:00
  • $\begingroup$ Take a straight wing and cut through it at a 10 degree angle. The outline along the 10 degree plane is elongated because of the truncated cut. That is the airfoil profile to the airplane, because that is the profile that is aligned with the airflow. That's all sweep does; it makes the air think the airfoil is longer for the same thickness. I'm not sure if the Rutan procedure and the templates that came with the plans were for method 1, template parallel to the airflow profile, or method 2, template perpendicular to the spar profile. It should be easy to find a builder's web page showing it. $\endgroup$
    – John K
    Apr 12, 2021 at 0:59
  • $\begingroup$ So if you're designing an airplane from scratch, do you use the elongated chord in your calculations? Also, seems to me, you can't use the standard lift curves because the shape is different - different lift, drag and moment curves. Is this correct? $\endgroup$
    – Fred
    Apr 12, 2021 at 13:58
  • $\begingroup$ I assume you mean the chord is longer, the span would actually be shorter. $\endgroup$
    – Fred
    Apr 12, 2021 at 14:03
  • $\begingroup$ @Fred, I would suggest that you incorporate your question about calculations into the original question. Because that was why I was uncertain what you were looking for... $\endgroup$
    – Michael Hall
    Apr 13, 2021 at 18:40

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