Nuts a BoltsAircraft Building

IN PREVIOUS ISSUES, "AIRCRAFT :

Building" has outlined theproper techniques used torepair sheet metal and com-posite surfaces. What aboutfabric-covered airplanes? •..•They, too, are damaged, andusually repairing this dam- :age isn't difficult. Often thedamage is minor, a tear in : • •the fabric. Major repairs include dam-age to the underlying structure, andthey are another story.

Repairing fabric involves essentiallythe same steps, with the only differ-ences depending on the fabric-cover-ing system used. We'll address thePoly-Fiber process this month andcover Ceconite fabric next month.

As always, the first step is makingsure you can legally make the repair.If you own a production airplaneand have the required knowledgeand skill, you can make the repairunder the supervision of an airframeand powerplant mechanic (A&P),who ensures that the repair is donecorrectly and endorses the airplane'smaintenance records.

If you own an amateur-built air-plane, you may legally performany repair—fabric damage in-cluded. If you haven't done anyfabric work before, you'll want tofind someone, such as an A&P me-chanic, fami l ia r with fabric tech-niques and have him or her inspectthe area and recommend a repair.

Damage AssessmentIf your airplane is covered with

the Poly-Fiber system, repairs aresimple. The Poly-Fiber manual has acomplete discussion on fabric repair,and the techniques you use to repairdamage must be in accordance withit. This is mandatory on a produc-tion airplane and highly recom-mended on a homebuilt, and there

Tear RepairFixing common damage to fabric

RON ALEXANDER

should not be any differences in re-pairing the two types of airplanes.They fly through the same airspacecarrying the same people.

Repairing fabricinvolves essentiallythe same steps as

repairing sheet metal,with the only

differences dependingon the fabric-covering

system used.

A small tear or rip is the mostcommon form of fabric damage. It'susually on the bottom of the fuse-lage or a fabric-covered gear leg, andit's usually caused by rocks or debriskicked up by the prop blast to thewheels. As with all repairs, assessingthe damage is your first step.

As you inspect the damage to thefabric, be aware that the underlyingstructure may have also been dam-aged. This damage is often hiddenunder what may appear to be mini-mal damage. Inspect the underly-

ing structure closely to' ' " • determine the extent of

] ' all the damage. If a com-' "'1.' ponent part is damaged,

you may need to removeit to repair it. If you're re-pairing a production air-

. . . _ . - , . . p lane, you must be. ; . . , . . working under an A&P'sx supervision. •

If your inspection determinesthat the damage is to the fabric only,you next must ascertain the cover-ing system and the type of colorcoat used. If you built the airplane,you already have this information.If you bought the airplane, this in-formation should be in the airframelogbook. It the airplane is coveredwith the Poly-Fiber system, the colorcoating (or top coat) should be Poly-Tone or Aerothane. Each topcoat re-quires a different repair method.

When repairing Poly-Fiber, youdon't need to f i n d a needle andthread; Poly-Tak fabric cement takesthe place of sewing. In most cases,to repair a rip or tear, you cut out apatch from a new piece of fabric andcement it in place over the damagedarea. When cutting your patch, usethis simple rule of thumb. If the tearor damaged area is less than 8 inchesin any one direction, the patch'soverlap on top of the old fabric mustbe at least 1 inch in all directions. Ifthe damaged area is more than 8inches, the patch's overlap must beat least 2 inches.

Repairing Poly-Tone SurfacesWhen Poly-Tone is the cover-

ing's f i n a l topcoat, f i r s t decidewhether you'll make the repair onthe airplane or whether you'll needto remove the part. Naturally, you'llrepair any underlying damage tothe structure before repairing thefabric. If you must replace wood or

SEPTEMBER 2000

Cutting out damaged fabric.

Brushing on the first layer of Poly-Brush.

Materials Neededfor Fabric Repairs

Here are the tools and suppliesyou'll need for typical fabric repairs:• Small piece of fabric• Masking tape and paper• Small glue brush• 2-inch brush• Pinking shears• Iron• Thermometer to calibrate iron• Pencil• Small touch-up spray gun• 280-grit sandpaper• Rags• Gloves• Goggles• Poly-Tak cement• MEK• Poly-Brush• Poly-Spray• Top Coat—Aerothane or Poly-Tone

tubing, make sure youcoat the new materialwith epoxy primer orepoxy varnish.

Next, trim the fab-ric around the dam-aged area to removeany loose or f rayedfabric. Then measureand cut a patch withthe required overlapfrom a piece of clean,u n - s h r u n k e n fab r i cthat matches the style

of fabr ic used on the air-plane. If the airplane is cov-ered with medium-weightfabric, cut your patch frommedium-weight fabric.

When cutting the patch,use p inking shears to keepthe edges from fraying. Youmay want to cut the patchso that it is a square, rectan-gle, or circle. This gives thecompleted patch a morepleasing appearance. Some-times you will be able to cutthe patch to match theshape of the damaged area

without it being unsightly.Lay this patch over the damaged

area and outline the patch on theexisting fabric with a pencil. Do notuse a pen because you cannot re-move the ink from the fabric. Withtape and paper, mask the area out-side the patch. Leave a little extraspace for working room. Use a goodquali ty masking tape and butcherpaper. Don't use newspaper becausethe print may bleed through.

Wearing rubber gloves and eyeprotection, thoroughly clean the oldfabric in the marked area with MEK.Clean it all the way down to bare fab-ric, removing all of the old coatingscompletely. Be as neat as possible;you do not want the MEK runninginto the undamaged area because itcan create sloppy runs and makes areal mess on the good fabric.

With the fabr ic thoroughlycleaned, use Poly-Tak cement and a

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OO SEPTEMBER 2000

Aircraft Building

small glue brush and attach the patch to the existing fab-ric. Make sure you've got the required overlap. Apply afairly liberal amount of Poly-Tak to one side at a time.Poly-Tak wil l dry rapidly on a warm day. Cement thepatch in place by working the Poly-Tak up through thenew fabric. Depending on the ambient temperature, letthe Poly-Tak cement dry at least 30 minutes to one hour.

When the Poly-Tak is dry, heat smooth the cementedareas forming the overlap with a small iron calibrated at225°F. Calibrating the iron is important. Heat in excessof 225°F weakens the Poly-Tak bond. Next, calibrate theiron for 350°F and iron the patch itself to heat shrink thenew fabric. Be sure you stay away from the cementededges with this temperature. Otherwise, you will loosenthe cement. For added protection, shield the cementedareas with a piece of cardboard.

After shr inking the patch, apply three coats of Poly-Brush with a 2-inch brush and let the area dry. If your dam-aged area is larger than 8 inches, frame the patch with2-inch finishing tape applied with Poly-Brush.

Applying Poly-Spray.

To protect the area from the sun's ultraviolet rays, applyat least three coats of Poly-Spray using (borrowing or rent-ing, if you don't have one) a small spray gun. Let the Poly-Spray dry overnight, then match the airplane's originalcolor using Poly-Tone topcoat. Remove the masking tapeand spray two or three coats of Poly-Tone over the area. Al-low this to dry for a couple of days. Finally, using a whitepolishing compound, polish the blended areas for a bettercolor match of the old area to the new.

Repairing Aerothane SurfacesThe main difference in repairing fabric coated with

Aerothane is that MEK doesn't remove the Aerothane, apolyurethane paint that resists most chemicals. Youhave two alternatives: repair the area from the inside ofthe structure or sand off all of the Aerothane coats.

If you can get to the back side of the damaged fabric,

here are the steps for repairing thedamage from the inside:

On the outside of the fabric maskaround the damaged area, leavingat least 1 inch around the area. Ap-ply the masking tape directly to theAerothane surface.

Scribe around the inside edge ofthe masked area. You are trying tocut through the coatings—but notinto the fabric itself. You must workcarefully to do this correctly.

Apply MEK (using proper skinand eye protection) to the back sideof the fabric in the damaged area.This will soften the Poly-Brush andPoly-Spray tha t is under theAerothane, allowing you to pry upan edge of the damaged coatingson the outside and peel it awayfrom the masked off area.

Carefully cut a patch that will fitexactly within the masked area.

Using Poly-Tak cement, applythe patch to the area.

Follow the sequence outlined forPoly-Tone repairs until you get tothe final color coat and then matchit using Aerothane topcoat.

If you cannot get to the backside of the fabr ic , mask the areaneeding repairs. Using dry 280-gritsandpaper, remove the Aerothanecoats. Keep sand ing u n t i l youreach the silver Poly-Spray layer.Once you reach silver, remove theremain ing coatings wi th M E K .Once you reach bare fabric, followthe steps given above.

When you respray the colorcoats of Aerothane, blending thenew coat with the old is more diffi-cult. You may want to respray theentire panel area. For example, ifthe repair is on a control surface,mask out the area from rib to riband spray new paint over that en-t ire surface. You will then have abetter match.

Repairing fabric dings isn't a ma-jor deal. Like most repairs, obtain-ing the necessary tools and suppliesbefore you begin, along with a littleplanning, makes the job easy. •

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