Article - Selecting Fabric
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, uncertified 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 2.6 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 3.4 ounces per square yard.
Ceconite fabrics are stamped with the following inscription:
See Ceconite STC Manual For Aircraft Eligibility
This stamp appears about every foot on the fabric itself. Fabric without this stamp is not legal to be used under the Ceconite STC.
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 New Super Seam to attach the fabric.
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. 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 dope will result in a much stronger fabric bond when the first brushed on coat of nitrate dope is later applied to seal the fabric.
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.
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 New 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.
All seams except those on the leading and trailing edges of wings must have at least a 1-inch overlap.
Wing leading edge seams must overlap at least 4 inches. (See figure 1)
Wing trailing edge seams must have at least a 3-inch overlap. (See figure 2)
Cemented seams on the fuselage must occur over a longeron, not a stringer.
Cemented seams generally cannot be positioned over non-structural areas such as an open bay between ribs on a wing. The cemented seams should be over a structural area. If this is not possible, then the seam should be a sewn seam instead of cemented.
Now lets get back to cementing our overlapped piece of fabric. The first step is to liberally coat the two pieces of fabric that will be joined with cement only on the area where they are to be joined. Allow the cement to become a bit tacky to the touch. Next press the new piece of fabric into the one previously installed and work them into each other with a brush and your fingers. (Be sure to protect your hands with latex gloves or a barrier cream).
Do not add additional New Super Seam on top of the previously cemented seams.
Fabric can also be sewn instead of gluing. You can purchase a sewn envelope for your particular airplane that fits each component part like a glove. Aircraft Spruce & Specialty has patterns for most aircraft and they will sew an envelope out of the type of fabric you desire. When you receive the envelope you simply slide it over the structure and then glue it in place using fabric cement. For example, if you purchase an envelope for a wing you would pull the fabric over the wing and then cement it at the butt end of the wing. Using an envelope saves a lot of work and consequently time. The disadvantage on a wing surface is the appearance of a sewn seam. Some people do not like to have the sewn seam running chordwise on a wing. Aircraft Spruce will, however, sew the envelope spanwise so the seam is located on the leading and trailing edge. This must be requested. Also, when you receive the envelope the seams will be on the outside. You must turn the envelope inside out to make sure the seams will then be on the inside of the envelope against the structure.
Why can't you buy the material and then sew up an envelope yourself? You can do this providing you use the approved polyester machine sewing thread. However, I do not recommend doing this unless you have a commercial sewing machine. The fabric will wreak havoc on a regular sewing machine needle. Save yourself a lot of time and grief and order an envelope rather than attempt to sew it yourself.
The attachment of fabric is relatively simple. However, it must be done properly prior to beginning to shrink the fabric. The glue must also be allowed to dry prior to beginning the shrinking process. If you begin too soon you will pull the seams apart. Once the glue has properly dried the heat tautening will not harm the seams.
Our next step will be to actually shrink the fabric itself. This is a very critical step that will be presented in the next issue. In that issue we will also continue explaining subsequent steps of the covering process.