8/16/2019 Fabric Doping

1/1

abric dopingUse the wrong method ofdoping synthetic fabric andyou could come unstuck

HE N ATIONAL TRANSPORTATIONSafety Board (NT SB) recently releasedits report on the April 1995 accident

that took the lives of aviation legend SteveWittman and his wife.

The NTSB report found that while in cruiseflight, the fabric covering the plywo od skin of

the right wing the Wittman aircraft debondedimmediately ahead of the aileron and bal-looned up. The pulsing of the ballooned fab-ric is then believed to have initiated aileronflutter, which in turn caused the w ings to flut-ter and separate from the aeroplane sim ulta-neously.

The surface of the aircraft s plywood cov-ered wings w ere coated w ith a layer of StitsHS150X Poly-Fiber that had completelydelaminated from the right wing. The fabricwas recovered and was intact, although theoverlying doped finish had largely been shed.

Investigation determined that the fabrichad been bonded to the plywood skin by anestimated two coats of clear nitrate dope,brushed into the weave of the fabric from thetop or outer surface. The nitrate dope was fol-lowed by five or six coats of clear butyratedope, a coat or two of Poly-Spray silver andfinally, several coats of yellow Po ly-Tone. Theplywood skin had not been treated in anymann er prior to the application of the fabricand liquid finishing materials.

MicroscopicA m icroscopic examination of the surface ofthe plywood revealed a thin film of nitrate

dope, which the NTSB believes was forcedthrough the weave of the fabric when the firsttwo coats of nitrate dope were brushed on.The imp rint of the weav e pattern of the fab-ric could be seen in the film of dope, indicat-ing that the fabric had delam inated from thenitrate dope rather than the plywood itself.

Synthetic fibres such aspolyester usually requiredifferent materials andapplication procedures tonatural fibres.

Compliance with Stitsnow Poly-Fiber) STC :

1008wE requires that twocoats of Poly-Brush beapplied to the surface tobe covered in this casethe plywood skin) beforehe Poly-Fiber fabric is

applied. The aim is toprovide a base for com-plete fabric bond to thewood.

After the fabric is adhered to the plywoodusing the fabric cement (Poly-Tak) the cover-

ing is heat taughtened. The first coat of P oly-Brush is brushed on to ensure good penetra-tion through the fabric. This attaches to theunderlying layers to give a good bond. Thenext coat of Poly-Brush is usually sprayed onto promote a smooth surface and fill theweave. It is then finished with further coats ofthe aluminium pigmented Poly-Spray fol-lowed by the final colour coat.

No te that Stits/Poly-Fiber finishing m ate-rials do not include nitrate or butyrate dope .

In the older procedures involving naturalfibres, a bond is created when the material

being used as an adhesive is absorbed orwicked into the fibres.

When modern synthetic fabric is used, anadequate bond is created when the adhesiveflows around the non-absorbing fibres to encap-sulate them. Without two initial coats of PolyBrush there is insufficient adhesive present toencapsulate the fabric fibres and create a goodbond.

Poor bondingPoor bonding procedures can be discoveredby testing the bond on a small surface of theaircraft. I know of a wooden motor glider,about 20 years old, which was in need of recov-ering. The Stits method was selected. Follow-ing removal of the old covering, the plywoodwas sanded d own and washed with acetone.

To ga in experience in the Stits method, anelevator was cov ered first as an experiment.The plywood was coated with two coats ofPoly-Brush thinned down to provide pene-tration. The fabric was glued down using

Poly-Tak and tensioned using a hot iron.A brush coat of Poly-Brush was applied fol-

lowed by a spray coating. The Poly-Spray wasthen applied.

The result looked good. The next day theexperiment was tested for adhesion - there wasnone, or at the best, very little. Most of the Poly-Brush stuck to the Poly-Fiber, and not the ply-

wood. The m aterial lifted away an d the basecoat of the Po ly-Brush lifted off the plywood.

A telephon e call to a Stits agent revealedthat even a small trace of Nitrate Dopeapplied 20 years ago can stop Poly-Brush fromsticking to the plywood. The answer is to sealover the embedded dope with Stits Epoxy Var-nish (or use a system based on N itrate dopeinstead of the Poly-Brush system).

These two examples of failed bonding ofsynthetic fibres illustrate how important it isto pay careful attention to manufacturersdirections, or do some tests that meet orexceed the manufacturers requirementsbefore you fly.

Tex Chalson is district airworthiness manager basat Paralleld Airport

FLIGHT SAFETY AUSTRALIA AUTUMN 1997 33