Aircraft Building: Envelope or Blanket? - Freea.moirier.free.fr/Construction/Construction...

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Aircraft Building

WHEN IT COMES TOcovering your air-plane with fabric,one question scarespeople new to theprocess—Do I haveto learn to sew?

Not really. Thereare two ways to coveryour airplane with-out adding a sewing machine to yourtoolbox and sewing to your list of newskills. But you still have to answer aquestion—envelope or blanket? In-stead of sewing you can buy a presewnenvelope that fits over the part, like awing, like a tight-fitting sock, or youcan buy a roll of fabric, lay it over thepart like a blanket, and glue it in placewith special cement.

In aviation's early days builderssewed envelopes because they didn'thave a choice. The special glues thatwithstand the forces acting on thefabric in flight and make the blanketmethod possible didn't exist then.So they sewed.

Glue wasn't the only thing thatkept early builders from using theblanket method. When you gluefabric to a wing using the blanket

Envelope or Blanket?Fabric covering options for your airplane

RON ALEXANDER

method, the top and bottom blan-kets must overlap at their gluejoints on the leading and trailingedges. The average wing is 60inches wide. In the early days cot-ton and linen was the fabric ofchoice, and it was only 48 to 50inches wide. Because the fabricwasn't wide enough for span-wiseseams (wing root to t ip), earlybuilders had to sew envelopes withchord-wise (fore and aft) seams.

Then came 70-inch polyester fab-ric, which builders could lay on thetop and bottom of wings and glue inplace with the necessary overlaps atthe leading and trailing edges. Butearly builders and mechanics werecreatures of habit (of course, we arenot today), and they continued touse fabric envelopes with chord wise

seams. These en-velopes still existtoday, but builderscan get envelopeswith span-wiseseams upon requestfrom manufacturers.

Many people findchord-wise seams ona wing unsightly,

and this is why span-wise seamscame into existence. The old CAM18 manual (predecessor of AdvisoryCircular 43-13, Acceptable Methods,Techniques and Practices) requiredfabric seams to be oriented with theairflow whenever possible. AC 43.13abandoned this practice.

Finally, shrinking the fabric wasanother obstacle to the blanketmethod in the early days. Nitrateand butyrate dopes continue toshrink fabric over time, and it didn'thave an adverse affect on chord-wiseseams because the dope slowly tight-ened the entire surface, leaving theseams straight.

But the question today remainsenvelope or blanket? Even with to-day's fabric covering techniques,each method has its pros and cons.

Laying the fabric over the structure is the first step in theblanket method of covering.

After laying the "blanket" over the structure, mark the fab-ric where you need to makes cuts. Always use a pencil.

Sport Aviation 99

Aircraft Building

Finally, cut the blanket to fit.

Blanket MethodToday's synthetic fabric (polyester)

makes the blanket method of fabriccovering more practical. You simplyroll out one piece of fabric for the topof a wing or surface and then roll outanother piece of fabric for the bottom

Seams must face the inside of presewn envelopes. Makesure they lay flat against the airplane's structure to get apleasing appearance.

side, making sure the two pieces havethe prescribed amount of overlap.(Using the Poly-Fiber system, youneed a 2-inch overlap on the leadingedge and a 1-inch overlap on the trail-ing edge.) Then you glue themtogether with the recommended fab-

ric cement. You can cover smallercontrol surfaces by wrapping onepiece of fabric around the componentlike the shell of a clam and cementthe one side where the fabric meetswith the specified overlap.

Today's fabric cement (such as

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Whether youuse the blanketor envelopemethod of cover-ing, you attachthe fabric to theairplane withspecial cement.

Poly-Tak or Super Seam) is strongenough to replace sewn seams. Boththe Poly-Fiber and Ceconite supple-mental type certificates (STC) allowcemented seams for all wing load-ings and airspeeds. Thanks topolyester fabric's 70-inch width, cov-ering wings using the blanketmethod is practical (depending onthe width of your wing, naturally).

You can cover an entire airplane bypiecing fabric together with cemented,overlapped seams, but the cementedseams must have a hard structure un-derneath them. "Hard structures" areleading and trailing edges, longerons,structural tubes, etc. You cannot createa cemented seam in "thin air" such asbetween two wing ribs.

As you can see, when covering theairplane with this method you shouldnever have to get out your needle andthread, and most fabric-covered air-planes today require little, if any, actualsewing. Do not confuse sewing fabricwith rib stitching or lacing, where youattach the fabric to a structure. Rib lac-ing is a different step and its purpose isto keep the fabric from "ballooning"on the top side of a surface that createslift during flight. It does not join differ-ent pieces of fabric together. :

Fabric EnvelopesYou can purchase a sewn envelope

that fits each component of your air-plane like a glove. Aircraft Spruce &Specialty has patterns for most aircraftand it will sew an envelope out of thetype of fabric you desire. Then youslide the envelope over the structureand glue it in place with fabric cement.Because a properly sewn envelope fits

the structure like a glove, in most casesyou have to cement the fabric in placeon one end. For example, after youpull on a wing envelope you cement itat the butt end (root) of the wing.

Envelopes save a lot of work and,therefore, time. On a wing, their dis-advantage is a matter of preference.Some people don't like to see span-wise sewn seams. (Aircraft Spruce willsew the envelope with span-wiseseams on the leading and trailingedges, but builders must request this.)

When you receive the envelope theseams will be on the outside, so youmust turn it inside out to make surethe seams are inside the envelope andagainst the structure. The excess seamfringe can snake or bunch up alongthe structure and have an unsightlyappearance. You can avoid this bymaking sure the fringe lays flat alongthe structure before shrinking the fab-ric. This requires some time and effort.

The patterns used to sew envelopesare aircraft specific, and if you (or a

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Aircraft Building

previous owner) have modified thestructure in any way, the affected en-velope will not fit . A 50-year-oldairplane with its sixth wingtip bow ora new cabin birdcage will not haveretained its original shape. If this oc-curs, expect to spend a lot of timecutting and fitting the fabric.

Heat shrinking an envelope pres-ents another problem. You mustshrink the envelope in such a waythat its seams remain straight. If youstart shrinking one side of a seamwith an iron you can easily pull theseam in that direction. If you try tofix the problem by shrinking theother side you'll probably end upwith a seam that looks like a spasticsnake. To prevent this, begin theshrinking process directly over theseam itself. Then work outward onboth sides of the seam to balance theshrinking process to keep it straight.

You can sew your own envelopes ifyou have the desire, a commercialsewing machine, and the approvedpolyester machine sewing thread.Don't try to use a home sewing ma-chine. Polyester aircraft fabric is toughstuff and it will wreak havoc on it. Ifyou want to save yourself a lot oftime and grief, order an enveloperather than attempt to sew it yourself.

Your DecisionWith all of this information, which

method should you use to cover yourairplane? It's up to you. Each type hasinherent advantages and disadvan-tages, and there's nothing wrongwith using both methods.

Generally, it's easier to cover afuselage with an envelope becausethe blanket method's "cutting-and-pasting" takes a lot of time and canbe confusing.

The blanket method, however, eas-ily covers wings and controls surfaces.Envelopes save you time when cover-ing these components, but they aremore expensive than buying just thefabric, cutting it to size, and cement-ing it in place.

The choice is yours.