Trailing Edge Geometry

The trailing edge may be sharp, flat or rounded (or have some other more complex slotted, serrated or perforated design away from sharp).

The (incorrect) Kutta-Zhukovsky theory of lift has led to a common (incorrect) idea that the generation of lift requires a sharp trailing edge as expressed on Wikipedia:

• The Kutta condition gives some insight into why airfoils always have sharp trailing edges, even though this is undesirable from structural and manufacturing viewpoints. An aircraft with a wing with a smoothly rounded trailing edge would generate little or no lift.

Accordingly, the standard NACA airfoils all have sharp trailing edge.

In practice, trailing edges are smoothly rounded but do not destroy lift. DFS simulations with a rounded trailing edge show good agreement with experimental observations and are explained by the New Theory.

Flatback Airfoils

Airfoils with a flat cut-off at the trailing edge are called flatback airfoils.

Flatback Airfoil Wind Tunnel Experiments, Sandia Lab, 2008, reports experiments with a fatback airfoils with 0.5%, 8.75% and 17.5% trailing edge thickness-to-chord ratio, with about the same drag and lift for 0.5% and 8.75% and doubled drag for 17.5%. See also Trailing Edge Modifications for Flatback Airfoils

Wind Turbine Blade

Blunt edge is reported to give better lift than sharp, because a blunt edge gives positive lift on the upper surface near the trailing edge, while a sharp edge has negative lift.  Noise may be reduced by trailing edges with perforation or brushes, far from sharp:

• Blunt-Wavy Combined Trailing Edge for Wind Turbine Blade Inboard Performance Improvement.
• Survey of Techniques for Reduction of Wind Turbine Blade Trailing Edge Noise

Rounded Trailing Edge

• NACA Report (1938): Trailing edge radius 0.5% of chord same lift/drag as sharp, for 1.6% small increase of drag.
• NACA Memorandum (1956): Trailing edge radius 1-2% of chord.
• The Effect of Rounded Trailing Edge on Drag
• Draft of upcoming article