To find an optimum flap angle, the resulting aerodynamic forces on the flap are calculated for each angle. A self-activated flap will stay in its equilibrium position where the sum of all forces on the flap is zero. By introducing such a flap, the flow can be stabilized and the maximum lift increases.

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clean airfoil

 flap angle = 25o
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flap angle = 40o flap angle = 52o


At the optimum angle the flap is just slightly touching the detached shear layer. If the flap angle is too low, the flap remains inside the reverse flow region and shows less effect. In the case of too large angles, the flap works like a spoiler and generates additional drag. Compared to the clean airfoil the maximum lift coefficient can be enhanced by more than 10%. The predicted lift coefficients agree fairly well with the experimental data. Further investigations were carried out for different flap angles to find an optimum flap configuration for a given angle of attack.

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Calculated lift coefficient for different flap angles Calculated flap-force coefficient over angle of attack
for different flap angles (CF = 0 means flap in equilibrium position)

The Strouhal number of detaching eddies in the case of a flap in equilibrium position can be compared to the Strouhal number of the clean airfoil at different angle of attack. The self-adjusting flap, however, does not give the optimum flap angle for maximum lift. In the equilibrium position the flap angle is slighly larger than the optimum angle, resulting in a gain of about 10% lift instead of 14% in the case of the envelope polar.

large picture Strouhal number of detaching vortices
for different flap angles and angles of attack

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Last modified: Thu Jul 27 17:58:14 CEST 2000