addition al pe netration  points in th e fuselage skin. T his was accomplish ed by cha nging the final a ctuato r  linkage rod to  an arc shape analo gous to a circumferential segme nt of a wh eel with its center of rotation  locate d on the hinge axis of the flap (Figure 5 ). The portion of the  flap between  the hinge be aring a nd the   arc atta chme nt form s one ‘spoke’ of the  ‘wheel’ and the fu selage structu re between th e flap  hinge mount s  and the  anchor points of the EMA form s a second ‘spoke’, which allo ws free dom of m ovement of the   ‘wheel se gment’ (arc) in the  tangential direction. 

Linkage   The a ctuato r drive u nit transmits po wer to the circul ar segm ent li nk th rough a pinion  gear that drive s a  gear segme nt integrated int o the p eriphery of the circu lar segm ent. The d rive unit  consists of three m otors  that simulta neously apply redundant po wer to the pi nion ge ar through a  cycloidal tran smission. The drive   unit hold s the arc throug h a serie s of ‘V’ guide  rollers that are  integ ral to th e drive unit. T wo of the rollers  are m ounte d on the same side of the a rc as the pinio n and to  either side of the  pinion, a nd the third roll er  is mounte d directly opp osite the pinion.  The thir d roller is mo unted on an e ccentri c cam that can b e  adjusted to provide the proper comp ression of the arc b etween the oppo sing rollers and t hus minimi ze  free-play without inducing too much fri ction. The pinion gear is mounted on an adjustable Bell eville spring  stack to  main tain a constant radi al force bet ween the pinio n and sector gear teeth (Figu re 6). Th e only  radial connection fixed rela tive to the circula r segm ent con sists of the flap itself. The d rive unit  is mou nted  to the fusela ge throu gh a b earin g mou nted truni on at each end of the axis throu gh the pini on gear to fo rm  the other ‘spo ke of the wh eel’ and fully  constrain the a ctuator lo cation. 

      SECTOR GEAR PINIO N ROLLE RS ARC  Figure 5.  Flap Ac tuator Assembly  Before Ins tallation      Fig ure 6.  Arc Pinon Gear Drive Assembly  

Since th e drive unit is mo unted completely inside  the hull, a nd since the flap  (inclu ding th e hinge s and  actuato r attach bearing) is mounted  completely o utside the h ull, only the arc penetrates th e hull. A s the  actuato r arc moves in an d out it remain s fixed relativ e to the flap (sin ce the fla p acts a s a radiu s of the   circle  that the  arc move s through). The movement of the arc relati ve to the  skin  at the  hull penetratio n is  greatly si mplif ied si nce the circula r path of the actu ator remains fixed relative to t he hull. Sin ce the portio n  of the actu ator that pa sses through th e hull ha s a constant cro ss section the seal  has a static shap e and   remains  fixed to the hull. 

Defle ction an d Thermal Expan sion Relie f  Due to  differences i n the coeffi cients of therma l expansi on of the ce ramic flap and  aluminum h ull  structure, the two radial conne ction s to the actuator experien ce small rel ative chang es th rough the reentry p rofile. By mounting  the a ctuator drive u nit on an axis th at is free to  pivot slightly re lative to the  vehicle st ructure, the actuat ion system  can still functi on even as t he tw o radi al fixations vary slightly with  respe ct to ea ch oth er. Such variation s cause a sli ght displacement  of the effectiv e cente r of the a ctuator  circula r segm ent from the  flap hin ge lin e. As the flap rotates, the displa cement o f the cent erlines causes a  small rotation  of the drive unit abo ut its pivot axis in  order to m aintain the tang ency of the drive unit guid e  rollers and th e circular segment lin k. Thi s rotation i s permitted by allowin g the circula r segm ent to rotat e  381