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September 2005 edition

Flight testing of a simple fix to lateral stability deficiencies

By Joachim Grenestedt

Engineers at Lehigh University recently designed and successfully flight tested a new control device that can tailor the lateral stability of aircraft.

Joachim Grenestedt, a mechanical engineering professor, designed canted tabs that are attached to the ailerons, the movable control surfaces on the wings that are used to roll the aircraft. Mounted on the trailing edge of aircraft ailerons, ‘canted tabs’ [an idea initially proposed by aerodynamicist Sven-Olof Ridder] are designed to modify the stick free lateral stability characteristics of aircraft. There are a number of reasons why altering lateral stability may be beneficial:

• Low lateral stability may result in negative spiral stability for an aeroplane. If left unattended the aeroplane will dig into increasingly steep turns.

• Lateral stability is required to control an aeroplane if the primary aileron controls become inoperable.

• Excess lateral stability leads to Dutch Roll, and requires high control forces during cross wind landings.

The standard way to alter lateral stability is by changing wing dihedral. But this can be an expensive and complicated task, as the modifications involve the most highly stressed and complex parts of the aircraft. Canted tabs were developed to have a similar effect as wing dihedral but at substantially reduced cost and complexity, reducing the need for extensive airframe modifications.

The function of the canted tabs may most readily be explained using a schematic of the airflow over the....

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