All about Ora­tex

Pilot - - TECH LOG: FOURNIER RESTORATIO­N PART THREE -

I re­searched all I could on Ora­tex. There are two grades of fab­ric, the dis­tinc­tion be­ing the num­ber of fil­a­ments in each fi­bre – Ora­tex 600 has around 28 fil­a­ments per fi­bre, mak­ing it suit­able for lighter aero­planes and ul­tra­lights of up to 600kg max­i­mum take­off mass (MTOM) with a Vne of less than 170mph and a wing-load­ing of un­der 11.6 pounds per square inch. It is also ac­cept­able for cov­er­ing all sur­face ply­wood, sheet aluminium or com­pos­ite ar­eas like Wagon’s fuse­lage, tailplane and fin. The stronger Ora­tex 6000 is OK for heav­ier aero­planes up to a MTOM of 6,000kg and had even been used on a huge Antonov AN-2B bi­plane.

Ora­tex UL600 was un­like any fab­ric I’d en­coun­tered be­fore: a woven, mod­i­fied polyester fab­ric, im­preg­nated with its durable ure­thane coat­ing sys­tem, but the weave is ex­tremely fine so we should prob­a­bly de­scribe it in terms of ‘de­niers’, like stock­ings. It is very light, thin (twelve mi­crons un­painted) and looks and han­dles more like a thin film of PVC with a weave em­bossed into it, as is of­ten used to cover model aero­planes. I later dis­cov­ered that it blows away eas­ily and is slip­pery – a bit like Christ­mas wrap­ping pa­per to han­dle, ex­cept much big­ger sheets. Nev­er­the­less it is ex­tremely strong in ten­sion, al­though per­haps less re­sis­tant to punc­tures and cuts with sharp-edged tools, when it fairly eas­ily rips along the warp or weft.

Both fab­rics were stated to be both flame and fuel re­sis­tant, and the cur­rent Series 3 ver­sion was claimed as flame re­sis­tant and fuel-proof. Ever scep­ti­cal, I cut off pieces and dropped them into jam jars filled with all the sol­vents I could find, in­clud­ing cel­lu­lose thin­ners, toluene and MEK. Weeks later there was no detri­men­tal ef­fect what­so­ever. Much later I dis­cov­ered that the only way to get (most) of the paint off this fab­ric is to sub­ject it to vi­o­lent high-fre­quency bending, vi­bra­tion and phys­i­cal bat­ter­ing for hours on end. Only then can you see just how del­i­cate is its weave.

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