Covering With Dope  Part Two By Ron Alexander

To continue our series of articles on covering with nitrate and butyrate dope, lets look at what type fabric you should use on your airplane and how to attach it. As mentioned previously, Ceconite fabric should be used with nitrate and butyrate dopes. Ceconite has a very comprehensive manual that discusses the steps to complete when using nitrate and butyrate.

Types of Fabric

Ceconite fabric is a polyester fabric that is weaved in three different styles. All fabrics are 70 inches wide. Each style is tested and approved by the FAA for the Ceconite STC. You may remember that to be placed on a production airplane the fabric and coatings must comply with a Supplemental Type Certificate (STC). The dopes used on these fabrics must be manufactured under a Parts Manufacturing Approval (PMA).

Three weights of fabric are available for use. Ceconite light is a 1.7-ounce fabric designed for ultralights, gliders, and light aircraft with a wing loading of less than 9 pounds per square foot. This fabric is uncertified meaning it should not be used on production aircraft. Ceconite 102 is the medium weight fabric. It weighs 2.7 ounces per square yard and it is used on most light aircraft. It is a good replacement for Grade A cotton and is very well suited for all fabric covered aircraft that do not operate under extreme conditions. Ceconite 101 is the heavy weight fabric that is used for agricultural aircraft, warbirds, aerobatic aircraft, in short, any airplane that receives rough treatment. This fabric weighs 3.7 ounces per square yard.

Ceconite fabrics are stamped with the following inscription:

FAA PMA APPROVED
FOR USE SEE CECONITE
ELIGIBILITY LIST

CHART 2

ATTACHING THE FABRIC

After selection of the proper fabric for your airplane, you are now ready to attach the fabric to the structure of the aircraft. Ensure that you have properly prepared the surface, had it inspected, anti-chafe tape is in place where needed, and overall, the surface is ready to be covered.

You must use a fabric cement to attach the fabric. Only those cements approved for use with the Ceconite STC may be used. Ceconite Super Seam fabric cement is an approved cement.

The first step prior to cementing the fabric in place is to brush thinned nitrate dope over all areas where the fabric will be cemented. Nitrate dope should be thinned with 1 part nitrate thinner to 3 parts nitrate dope prior to applying. Brush this mixture on all areas where fabric will be cemented. This includes leading edges, trailing edges, butt ribs, tip bows, longerons, and tail feather tubing. Let the mixture dry in place before attaching the fabric. Brushing on the nitrate will result in a much stronger fabric bond when the Super Seam is later applied.

Two methods of covering may be used to attach fabric. The first is termed the blanket method where the fabric is cut to fit the structure and then cemented in place. The fabric actually lies over an airframe component. Usually two pieces of fabric will be cut (top and bottom). The first is cemented to the structure and the second piece is then overlapped and cemented to the fabric that is in place.

The second method of covering uses a pre-sewn fabric envelope. A pattern is used for your type airplane and a tight fitting envelope is sewn out of fabric. This envelope may then be slipped over the structure like a glove or sock and then cemented in place only at one end—the end that needs to be closed after the envelope is in place.

Blanket Method

You should always cut fabric that will be cemented using straight scissors. Be careful to make a straight cut with no raveled threads. Otherwise flaws and ravels will show through later in the process. By the way, Ceconite fabric has no top, bottom or thread orientation. The strength will be the same regardless of how you place the direction of the fibers. Cut the fabric to fit the component part. You will probably have to cut two pieces that will be overlapped. (More on overlapping later). You can use clothespins and fabric clamps to hold the fabric in place while you are cementing it.

Using a 1-inch brush, liberally apply Super Seam fabric cement over the first area where fabric is to be attached. Apply the cement to small areas at a time. The cement will dry rapidly in warm temperatures. Press the fabric into the wet cement being sure that it squeezes up through the weave of the fabric. Thoroughly work the cement into the fabric using your brush. Remember to only apply cement to as much an area as you can cover before it dries. The cement must be wet when you press the fabric into it. You should glue the fabric in place on one side of the airframe part.

During the cementing process you do not have to pull the fabric extremely tight. As long as it conforms generally to the shape it covers and there are no sags you will have it tight enough until you shrink it later in the process. How tight should the fabric be when you initially glue it in place? How tight you pull the fabric as it is glued into place will affect the final tautness as it is shrunk with heat. Final tightening with heat will actually shrink the fabric about 10%. So on a wing that is 50 inches wide the fabric will shrink about 5 inches. You do not want the fabric so loose that it will not properly tighten and conversely, you do not want it so tight that structural damage results from the heat shrinking process. You can actually bend or warp light structures during the heat shrinking if the fabric is too tight when cemented in place.

So what’s the bottom line? As an example, if you are covering a wing the fabric should be attached loose enough to allow you to pull the fabric above the top of the structure about 1 inch. This is a rough rule of thumb. As you gain experience you will be able to properly judge the amount of tautness.

After cementing the first piece of fabric in place, you will then attach the next piece overlapping the previously installed one. The following are rules concerning this overlap that must be applied to follow the Ceconite STC. Four rules apply.

1. All seams except those on the leading and trailing edges of wings must have at least a 1-inch overlap.
2. Wing leading edge seams must overlap at least 4 inches.
   (See figure 1)
3. Wing trailing edge seams must have at least a 3-inch overlap.
   (See figure 2)
4. Cemented seams on the fuselage must occur over a longeron, not a stringer.