söndag 5 augusti 2012

Rounded or Sharp Trailing Edge?

The classical text book circulation theory of flight by Kutta-Zhukovsky requires the trailing edge of a wing to be sharp in order to generate lift (see previous posts on text book theory of flight).

Real wings have rounded or blunt trailing edges but still generate lift, and lift is thus not an effect of the sharpness of the trailing edge, a fact known since the beginning of powered flight by wing engineers and manufacturers, but contrary to current text book theory.

The recent Sandia report Trailing Edge Modifications for Flatback Airfoils compares the performance of different trailing edge designs of wind turbine blades (see picture above). The main effect of a rounded or blunt trailing edge, with edge diameter up to 10% of chord length, is increase of drag without lift decrease and with improved stall:
  • Blunt trailing edge airfoils are of interest in the engineering of large wind turbine blades because they allow for a strong structure with a high aerodynamic lift to structural weight ratio. 
  • However, these airfoils also have a high drag because of the low pressures in the wake acting on the blunt trailing edge. The goal of the present research effort is to find the most effective way of reducing the base drag while retaining the favorable characteristics of the airfoil that make it of interest for application in the inboard region of large wind turbine blades. 
The New Theory of Flight presented on The Secret of Flight shows that lift is generated by 3d rotational separation at a rounded trailing edge in accordance with observation. The classical text book theory with lift generated by a sharp trailing edge is contradicted by observation. When now a correct theory is available it is time to rewrite text books.

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