A rotational control apparatus (10) includes multiple facings (274-276) which sandwich multiple friction plates (252, 253) rotatable with. To create turbulent air flow, the friction plates (252, 253) include an undulating outer circumferential edge (258) and a plurality of passages (260) in the interface...http://www.google.com/patents/US6755289?utm_source=gb-gplus-sharePatent US6755289 - Rotational control apparatus
