Airfoils are usually divided in "thin" and "thick" according to their stall behaviour: trailing edge stall, leading edge stall, and thin airfoil stall. One of the main defining parameters in how the wing stalls is the thickness of the wing profile.

From Torenbeek, both the pictures and the citations:

1. Trailing edge stall

This type of stall is characteristic of most airfoil sections with thickness/chord ratios* of approximately 15% and above. The flow at large angles of attack is characterized by a progressive thickening of the turbulent boundary layer on the upper surface. As the angle of attack is increased to about 10 degrees (B) flow separation starts at the trailing edge and moves gradually forward.

2. Leading edge stall

Airfoils with thickness/chord ratios of about 9 to 12 percent experience an abrupt separation of the flow near the leading edge. On these sections separation of the laminar boundary layer occur weIl before the attainment of maximum lift and prior to transition to a turbulent boundary layer. Transition occurs in the shear layer thus formed, and the expansion of the turbulent motion spreads at such an angle that re-attachment of the flow quickly occurs, en- closing a ·short bubble" and subsequently forming a turbulent boundary layer (B).

The lift and pitching moment curves exhibit abrupt changes when the angle of attack for max1mum 1ift 1s exceeded. There is l1ttle or no round1ng of the 11ft curve and a sudden negative sh1ft of the pitching moment result1ng from the rearward sh1ft of the centre of pressure 1s observed.

3. Thin Airfoil stall

On very thin sections of thickness/chord ratios of Iess than about 6 percent and on round noses a small separation bubble occurs at very small angles of attack (S). At a certain critica1 angle of attack the short bubble breaks down, but the flow subsequently re-attaches downstream, forming a "long bubble" wh1ch causes a slight reduction in the 11ft-curve slope (B). W1th 1ncreasing angle of attack the point of flow reattachment progressively moves backward until it coincides with the trailing edge and maximum lift is reached at this condition (C)

So according to the categorisation of airfoils in three types of stall, "thin airfoil" means a chord thickness of 6% or less.

Airfoils are usually divided in "thin" and "thick" according to their stall behaviour: trailing edge stall, leading edge stall, and thin airfoil stall. One of the main defining parameters in how the wing stalls is the thickness of the wing profile.

From Torenbeek, both the pictures and the citations:

1. Trailing edge stall

This type of stall is characteristic of most airfoil sections with thickness/chord ratios of approximately 15% and above. The flow at large angles of attack is characterized by a progressive thickening of the turbulent boundary layer on the upper surface. As the angle of attack is increased to about 10 degrees (B), flow separation starts at the trailing edge and moves gradually forward.

2. Leading edge stall

Airfoils with thickness/chord ratios of about 9 to 12 percent experience an abrupt separation of the flow near the leading edge. On these sections separation of the laminar boundary layer occur well before the attainment of maximum lift and prior to transition to a turbulent boundary layer. Transition occurs in the shear layer thus formed, and the expansion of the turbulent motion spreads at such an angle that re-attachment of the flow quickly occurs, enclosing a "short bubble" and subsequently forming a turbulent boundary layer (B).

The lift and pitching moment curves exhibit abrupt changes when the angle of attack for maximum lift is exceeded. There is little or no rounding of the lift curve and a sudden negative shift of the pitching moment resulting from the rearward shift of the centre of pressure is observed.

3. Thin Airfoil stall

On very thin sections of thickness/chord ratios of less than about 6 percent and on round noses a small separation bubble occurs at very small angles of attack (S). At a certain critical angle of attack the short bubble breaks down, but the flow subsequently re-attaches downstream, forming a "long bubble" which causes a slight reduction in the lift-curve slope (B). With increasing angle of attack the point of flow reattachment progressively moves backward until it coincides with the trailing edge and maximum lift is reached at this condition (C)

So according to the categorisation of airfoils in three types of stall, "thin airfoil" means a chord thickness of 6% or less.