# What are the rules of airfoil thickness from root to tip on tapered wings?

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?

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.

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.