plane ▁and ▁ 1 5 ▁for ▁an ▁air lin er . ▁ ▁The ▁hub ▁▁ ▁In ▁simple ▁designs , ▁the ▁bl ades ▁are ▁directly ▁bol ted ▁to ▁the ▁hub ▁and ▁are ▁unable ▁to ▁pitch , ▁which ▁leads ▁to ▁aer od ynamic ▁st all ▁above ▁certain ▁wind spe eds . ▁In ▁other ▁more ▁s oph istic ated ▁designs , ▁they ▁are ▁bol ted ▁to ▁the ▁pitch ▁bearing , ▁which ▁adjust s ▁their ▁angle ▁of ▁attack ▁with ▁the ▁help ▁of ▁a ▁pitch ▁system ▁according ▁to ▁the ▁wind ▁speed ▁to ▁control ▁their ▁rot ational ▁speed . ▁The ▁pitch ▁bearing ▁is ▁itself ▁bol ted ▁to ▁the ▁hub . ▁The ▁hub ▁is ▁fixed ▁to ▁the ▁rot or ▁sh aft ▁which ▁drives ▁the ▁generator ▁directly ▁or ▁through ▁a ▁g ear box . ▁ ▁Bla de ▁count ▁▁ ▁The ▁number ▁of ▁bl ades ▁is ▁selected ▁for ▁aer od ynamic ▁efficiency , ▁component ▁costs , ▁and ▁system ▁reli ability . ▁No ise ▁em issions ▁are ▁affected ▁by ▁the ▁location ▁of ▁the ▁bl ades ▁up wind ▁or ▁down wind ▁of ▁the ▁tower ▁and ▁the ▁speed ▁of ▁the ▁rot or . ▁Given ▁that ▁the ▁noise ▁em issions ▁from ▁the ▁bl ades ' ▁trailing ▁edges ▁and ▁tips ▁vary ▁by ▁the ▁ 5 th ▁power ▁of ▁bla de ▁speed , ▁a ▁small ▁increase ▁in ▁tip ▁speed ▁can ▁make ▁a ▁large ▁difference . ▁ ▁Wind ▁tur b ines ▁developed ▁over ▁the ▁last ▁ 5 0 ▁years ▁have ▁almost ▁univers ally ▁used ▁either ▁two ▁or ▁three ▁bl ades . ▁However , ▁there ▁are ▁pat ents ▁that ▁present