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I'm doing some research on wings and airfoils used in the 30s and 40s, and I'm trying to understand the rules about airfoil proportional thickness changes across the wing (root to tip) on tapered wings.

Many planes of that time used airfoils with thickness ranging from 15% to 9% (root to tip). (Source: https://m-selig.ae.illinois.edu/ads/aircraft.html)

So the question is: what are the rules for dimensioning airfoil thickness from root to tip?

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There are no clear rules; choosing wing thickness was no rigid science.

However, some rules did exist:

  • Root thickness goes up with aspect ratio. While "regular" wings (aspect ratio between 5 and 8) had a root thickness between 14% and 18%, the high aspect ratio Davis wing (AR 11) needed a 21% root airfoil.

  • Tip thickness was chosen for maximum section lift in order to add stall margin and aileron authority. This means thickness here is between 9% and 12%.

  • Camber also reduces from root to tip proportional to the lift per area ratio. While the inner wing should produce the highest lift per area, the outer wing reduces this for stall margin and better aileron authority with upwards deflected ailerons.
  • In the early jet age, high speed required thinner wings because this delays the onset of Mach effects. The symmetrical tip airfoil of the Me-262 had 9% thickness and supersonic aircraft reduced wing thickness to 6% to 4%, the thickness of the Bell X-1E wing.
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Airfoil thickness at the root was chosen to obtain a good cantilever ratio: the bending moment is highest there, and you need to give it structural height. Also, you want to create most lift at the wing root, that is why chord, thickness and incidence angle are all highest there. All three tapering down towards the tip for structural reasons.

There is a bit more info in this answer.

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The term for the draftsman's job of airfoil depiction in the, 30's and 40's, was called "Lofting". Maybe the term is still in use, today.

Airfoils camber would be defined as a percentage of the chordline dimension, either above or below the chordline. In the day, a table of points along the chordline would have corresponding (+%) or (-%) values to be plotted and then connected via a draftsman's French curve template.

When making the drawings for the tapered wing's ribs, the camber diminishes proportionally with the length of the chord.

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  • $\begingroup$ Thanks for you reply. Where can I found references about this? $\endgroup$
    – Alex Byrth
    Commented Oct 2, 2018 at 15:33
  • $\begingroup$ Off hand, I sure don't know. I would have to just do www searches, myself. Sorry I can't be of more help. $\endgroup$
    – Walker
    Commented Oct 2, 2018 at 15:57
  • $\begingroup$ I would like to know why not use the same airfoil from root to tip, keeping same proportional thickness and camber. $\endgroup$
    – Alex Byrth
    Commented Oct 8, 2018 at 0:04
  • $\begingroup$ As an anecdote, the term lofting comes from shipbulding, when the curves between hull frames were fitted on a 1:1 drawing... in the loft above the shipyard, which is the only place those drawings would fit. The term is still in use, although it is falling out of the common parlance. $\endgroup$
    – AEhere supports Monica
    Commented Aug 6, 2019 at 7:31

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