All about Oratex
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.