# How to determine the thickness of an airfoil?

In calculation or designing, how to determine the airfoil thickness? What is the guideline or consideration in designing airplane's wing? We all know that the bulge (the ticker in the airfoil) is required to apply Bernoulli principle, to create low pressure above the wing. But how to bind the round nose shape requirement to the design requirement. In this case, to make the airplane to be efficient (especially related to the power consumption).

This airfoil is required for an airplane as I explained here. Maximum weight will be 400kgs, and the maximum speed will be 200km/h.

• We all know that the bulge (the ticker in the airfoil) is required to apply Bernoulli principle, to create low pressure above the wing. You may want to read up on that assumption. See related questions on airfoil thickness, equal transit time fallacy and how wings generate lift – Sanchises Dec 9 '18 at 8:48
• Sorry, I may have misunderstood what you meant with "bulge". I edited the question and replaced "bulge" with thickness, is that what you mean or do you mean Nose Shape? – Koyovis Aug 6 '19 at 6:01

Factors for determining the wing thickness during the aeroplane design procedure:

• Determine the required structure height, which follows from the aspect ratio determined by the aerodynamicists: the cantilever ratio, which is the structural wing semi-span divided by maximum root thickness. Statistical analysis has found that transport aircraft usually have a ratio between 18 and 22, lower means a lighter wing but more drag.
• Stall behaviour: for thin wings the leading edge stall dominates, for thick wings trailing edge stall which is more gradual. This imposes a minimum thickness.
• For aircraft cruising at M > 0.5: the wing thickness must be such that dive Mach number remain subcritical. Imposes a maximum thickness.
• For good max $$C_L$$ a value from the sweet spot between 12 and 15 should be selected, see below = fig 7-9 from Torenbeek.

Torenbeek then concludes:

It is concluded that the root section thickness of transport aircraft should be chosen such that a good cantilever ratio is obtained. For a given taper ratio this implies that the thickness ratio at the root will increase proportionally to the aspect ratio. A thickness ratio of between 15 and 20 percent is in the interest of good performance when using relatively simple trailing-edge high-lift devices and provides adequate room for retracting the undercarriaqe. Thickness ratios above 20 percent may show diminishing returns due to the increasing profile drag and the relatively low maximum lift and this, in turn, limits the aspect ratio to a maximum of approximately 13 for cantilever wings.

Tip sections (without flaps) should be between 10 and 15 percent in order to attain a high maximum lift. This reduction relative to the root is also in favor of low structural weight.

• The conclusion of Item #2 and #3 aren't swapped? I guess the #2 imposes thicker airfoil and #3 imposes thinner airfoils. Maybe I'm wrong, of course. – Alex Byrth Sep 4 '19 at 18:44
• @AlexByrth you’re exactly right. Imposing a thicker airfoil means there is a minimum value, below which there would be problems. – Koyovis Sep 4 '19 at 21:09
• thanks for your explanation. I'm a non-native English speaker, so I've translated "impose" as "force to a", "obligate to a". But seems it means "limits the value of". – Alex Byrth Sep 7 '19 at 12:54