# What are the conditions for an airfoil to be a “thin airfoil?”

I know about the theory differences between regular airfoils and thin airfoils, but is there any condition for saying a given airfoil can be analyzed as if it is thin? The extreme case of infinitely thin makes sense, but I am curious if there is some cutoff dimension that determines thin or not thin.

There is no hard boundary. Normally it is around 8% relative thickness, details depending on camber and nose shape.

Typical for a thin airfoil is a stall originating from the nose, with a sudden separation of upper side flow, while thicker airfoils start to stall with a separation starting from the trailing edge and moving gradually forward. This gives thin airfoils a nasty stall behavior while thick airfoils stall in more benign ways. The stall behavior depends not only on thickness but also on camber and the geometry details of the airfoil nose, but can be used to separate thin from thick airfoils.

Thin airfoils make sense in two applications:

• when the local angle of attack is well controlled, such as in flaps and turbo machinery, and
• for trans- and supersonic flight where thickness causes wave drag.

In all other applications thicker airfoils with a blunter nose should be preferred because they allow to store more fuel and to make the load-carrying structure more efficient. The upper limit of practical airfoils is at 20% to 22%, the root thickness of the Davis wing as used in the B-24 and B-29.

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.

• Yes the mechanics are pretty much identical, it's just the stall behaviour that differs, the leading edge stall being very nasty with the sudden drop in $C_L$, the thin airfoil $C_L$ drop being much more benign. Torenbeek does state in the book that there are many more variables, indeed such as the ones you mention. Thanks for the edit by the way. – Koyovis Feb 19 '18 at 14:14