Yes, and even more precisely it should be worded the other way around: The drag coefficient is proportional to airfoil thickness. Airfoil thickness means that the air has to flow around the airfoil. This displacement effect causes the flow around a thick airfoil to speed up more than around an equivalent but thinner airfoil. The thicker airfoil pushes the air aside and around itself more, causing the flow to accelerate and create more friction than the slower flow around a thinner airfoil. This effect is normally approximated with an additional term in the friction drag formula which is proportional to relative thickness.

Starting from the friction coefficient along a straight wall $c_f$, this additional friction drag has been captured in an empirical formula which gave the best fit to a wealth of airfoil drag data, cambered and uncambered. This is the formula for the zero-lift drag coefficient $c_{d0}$ of an airfoil: $$c_{d0} = c_f\cdot \left(2 + 4\cdot\delta + 120\cdot\left(\frac{1}{\sqrt{1-Ma^2}}\right)^3\cdot\delta^4 - 0.09\cdot Ma^2\right)$$ where $\delta$ is the relative thickness of your airfoil and $Ma$ the Mach number.

That slow flying aircraft use thick airfoils is not generally true. However, larger aircraft want to use thicker airfoils in their wing root in order to make the wing spar lighter. By using a wider distance between the lower and upper spar caps, smaller caps can be used for the same bending strength.

In order to maximise lift, leading and trailing edge flaps are used. Thicker airfoils make their integration easier, and they allow to carry more fuel due to the wing's higher internal volume. However, beyond 20% thickness at subsonic speed and 14% at transsonic speed thickness becomes a liability - the flow will separate too early to make thicker airfoils practical.

Yes, and even more precisely it should be worded a bit differently: The drag coefficient is grows linearly with airfoil thickness. Airfoil thickness means that the air has to flow around the airfoil. This displacement effect causes the flow around a thick airfoil to speed up more than around an equivalent but thinner airfoil. The thicker airfoil pushes the air aside and around itself more, causing the flow to accelerate and create more friction than the slower flow around a thinner airfoil. This effect is normally approximated with an additional term in the friction drag formula which is proportional to relative thickness.

Starting from the friction coefficient along a straight wall $c_f$, this additional friction drag has been captured in an empirical formula which gave the best fit to a wealth of airfoil drag data, cambered and uncambered. This is the formula for the zero-lift drag coefficient $c_{d0}$ of an airfoil: $$c_{d0} = c_f\cdot \left(2 + 4\cdot\delta + 120\cdot\left(\frac{1}{\sqrt{1-Ma^2}}\right)^3\cdot\delta^4 - 0.09\cdot Ma^2\right)$$ where $\delta$ is the relative thickness of your airfoil and $Ma$ the Mach number.

That slow flying aircraft use thick airfoils is not generally true. However, larger aircraft want to use thicker airfoils in their wing root in order to make the wing spar lighter. By using a wider distance between the lower and upper spar caps, smaller caps can be used for the same bending strength.

In order to maximise lift, leading and trailing edge flaps are used. Thicker airfoils make their integration easier, and they allow to carry more fuel due to the wing's higher internal volume. However, beyond 20% thickness at subsonic speed and 14% at transsonic speed thickness becomes a liability - the flow will separate too early to make thicker airfoils practical.

Yes, and even more precisely it should be worded the other way around: The drag coefficient is proportional to airfoil thickness. Airfoil thickness means that the air has to flow around the airfoil. This displacement effect causes the flow around a thick airfoil to speed up more than around an equivalent but thinner airfoil. The thicker airfoil pushes the air aside and around itself more, causing the flow to accelerate and create more friction than the slower flow around a thinner airfoil. This effect is normally approximated with an additional term in the friction drag formula which is proportional to relative thickness.

Starting from the friction coefficient along a straight wall $c_f$, this additional friction drag has been captured in an empirical formula which gave the best fit to a wealth of airfoil drag data, cambered and uncambered. This is the formula: $$c_{d0} = c_f\cdot \left(2 + 4\cdot\delta + 120\cdot\left(\frac{1}{\sqrt{1-Ma^2}}\right)^3\cdot\delta^4 - 0.09\cdot Ma^2\right)$$ where $\delta$ is the relative thickness of your airfoil.

That slow flying aircraft use thick airfoils is not generally true. However, larger aircraft want to use thicker airfoils in their wing root in order to make the wing spar lighter. By using a wider distance between the lower and upper spar caps, smaller caps can be used for the same bending strength.

Yes, and even more precisely it should be worded the other way around: The drag coefficient is proportional to airfoil thickness. Airfoil thickness means that the air has to flow around the airfoil. This displacement effect causes the flow around a thick airfoil to speed up more than around an equivalent but thinner airfoil. The thicker airfoil pushes the air aside and around itself more, causing the flow to accelerate and create more friction than the slower flow around a thinner airfoil. This effect is normally approximated with an additional term in the friction drag formula which is proportional to relative thickness.

Starting from the friction coefficient along a straight wall $c_f$, this additional friction drag has been captured in an empirical formula which gave the best fit to a wealth of airfoil drag data, cambered and uncambered. This is the formula for the zero-lift drag coefficient $c_{d0}$ of an airfoil: $$c_{d0} = c_f\cdot \left(2 + 4\cdot\delta + 120\cdot\left(\frac{1}{\sqrt{1-Ma^2}}\right)^3\cdot\delta^4 - 0.09\cdot Ma^2\right)$$ where $\delta$ is the relative thickness of your airfoil and $Ma$ the Mach number.

That slow flying aircraft use thick airfoils is not generally true. However, larger aircraft want to use thicker airfoils in their wing root in order to make the wing spar lighter. By using a wider distance between the lower and upper spar caps, smaller caps can be used for the same bending strength.

In order to maximise lift, leading and trailing edge flaps are used. Thicker airfoils make their integration easier, and they allow to carry more fuel due to the wing's higher internal volume. However, beyond 20% thickness at subsonic speed and 14% at transsonic speed thickness becomes a liability - the flow will separate too early to make thicker airfoils practical.