No Effect of Rounded Trailing Edge on Drag and Lift

· fluid mechanics, flying










New Theory of Flight presented on The Secret of Flight  is based on an analysis of computational solutions of the Navier-Stokes equations with slip boundary condition showing that the rotational slip separation pattern at the trailing edge (shown in the picture above) has an important role for the generation of both lift and drag. To capture the crucial slip separation pattern it is necessary to resolve the flow at the trailing edge, which requires that the trailing edge is rounded with a certain positive radius.

Classical Theory of Flight according the circulation theory of Kutta-Zhukovsky on the other hand requires the trailing edge to be sharp, and the practical necessity of constructing wings with somewhat rounded trailing edges is considered as an  imperfection from the ideal case of knife sharp edges.

Experiments were made early on (NACA Report (1938) and NACA Memorandum (1956)) to determine the dependence of lift and drag on the radius of the trailing edge, with the principal finding that a rounded edge off radius less than 1% of the chord length showed essentially the same lift and drag as a maximally sharp edge, while a certain increase of drag was noted above 2%.

Does the New Theory predict anything concerning drag as the trailing edge radius is decreased starting with say 2% of chord length? Yes, analysis shows that rotational slip separation reduces a zero drag potential flow pressure singularity scaling with 1/radius in a zone of area scaling with the radius, and thus results in essentially constant drag under decreasing radius. Similarly, lift also stays constant.

It follows that even if the wing has a nearly sharp trailing edge, one may in computational simulations replace it with a rounded edge of say 1% chord length allowing resolution of the crucial separation pattern without excessive number of finite elements.

There is neither any practical need to construct wings with very sharp trailing edges.

In The New Theory thus practice with necessarily somewhat rounded trailing edges comes together with a theory also requiring rounded trailing edges.

Blunt Trailing Edge

Rounded Trailing Edge

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