The next step in our ongoing discussion of fabric covering using nitrate and butyrate dope, is to shrink the Ceconite fabric. This is a critical step in the covering process. Proper shrinkage is essential to a long lasting finish. The process of shrinking the fabric that will be covered with nitrate and butyrate dope takes on a special meaning due to the shrinkage of the dopes themselves.
Nitrate and butyrate (even the non-tautening variety) will continue to shrink throughout the life of the airplane. This will occur both on Grade A fabric and on polyester fabric. As the dopes shrink they tighten the fabric they have encapsulated. So today when you use Ceconite fabric along with nitrate and butyrate dopes to cover your airplane, you must be aware of this fact. Use only non-tautening dopes to cover your airplane even though they will shrink (to a lesser degree than regular dopes) with age. When using this process you will not shrink the fabric to its maximum prior to applying the dopes. You must allow for the resulting shrinkage caused by the dopes themselves.
There are several reasons why an aircraft has to be recovered before the full life of the covering system has been achieved. One of these reasons concerns improper tautening of the fabric when it is placed on an aircraft structure. If the fabric is too loose it will create a drumming affect in flight thus causing the chemical coats to crack and peel. If the fabric has been tightened too much structural damage may result. Either of these problems will contribute to having to recover your aircraft prematurely. With this in mind, it is very important that you use the proper procedures to tighten the fabric.
The fabric tautening process is much simpler today than in the days of Grade A cotton fabric. Cotton fabric has to be initially shrunk with water and then allowance made for the fabric to continue to tighten through the years from the shrinking of the dopes that are applied. Attaching the fabric properly prior to the tightening process is essential. If it is too loose when attached and glued you will never be able to achieve the desired tautness. If it is too tight when attached you will risk damaging a structure from the subsequent tightening through the years resulting from the actual shrinking of the nitrate and butyrate dope.
Today’s polyester fabric, Ceconite, is shrunk to its optimum tightness by applying heat with a regular household iron. Any additional tautening is not only unnecessary but may be harmful to the underlying structure. Too much extra tightening can actually distort or damage the component parts of the airplane.
When you initially shrink the polyester fabric, you must take into consideration the additional shrinkage that will occur as a result of the dope drying. Non-tautening dopes are available that reduce the amount of shrinkage that will occur. They are not true non-tautening—only less tautening. Additional plasticizers are added to the dope to reduce the tautening process. However, as the plasticizers evaporate with time a certain amount of tautening will occur. It is best to use non-tautening dopes to minimize this shrinkage.
First of all, lets look at what actually occurs when you tighten polyester fabric. The fibers have the capability of shrinking a total of about 12%. This will occur at a maximum temperature of 350 degrees F. At 240 degrees F the fabric will shrink about 5%. Below 240 degrees F (200-225 degrees) the fabric will smooth but shrink very little. At a temperature of 370 degrees F the fibers will actually begin to loosen. At 425 degrees F the fibers will melt. As you can see, it is extremely important for you to know the temperature of the device you are using to apply the heat. Because of this, you should use a regular household iron to shrink polyester fabric. The iron should have a rating of at least 1100 watts. NEVER USE A HEAT GUN to shrink fabric. You have absolutely no idea what temperature is being emitted and, furthermore, the temperature being applied will vary according to how far away from the fabric you hold the gun. Hide the heat gun while you are working with fabric.
The iron you use must be properly calibrated prior to using it to shrink your fabric. You will want to calibrate and mark at least 2 temperature settings. These are 225 degrees F and 240 degrees F. Your iron should be of a high enough quality to hold the desired temperatures within plus or minus 10 degrees.
CALIBRATING YOUR IRON
Obtain an accurate thermometer with a stem that can be placed under your iron. The best thermometer is a glass bulb type. As an alternative, use a candy or jelly thermometer available at hardware stores. If you elect to use one of these check the accuracy by placing it in boiling water and ensuring that approximately 212 degrees F registers.
Purchase some silicone heat sink compound from your distributor.
Build a one-half inch thick stack of dry paper towels. You will use this as a heat sink to place the iron on while calibrating it.
Next, place a small amount of heat sink compound on the bulb of the thermometer and lay it in the middle of the paper towels.
Place your iron on top of the thermometer that is on the paper towels.
Turn the iron on and advance the heat control to the wool setting and watch the temperature rise. Let it stabilize and then vary the control to reach a temperature of 225 degrees.
Using a piece of masking tape applied over the temperature dial, place a mark at the 225 degree setting. Vary the temperature until you have the iron calibrated and marked for 225 degrees F and 240 degrees F.
Turn the iron off and allow it to cool. Then thoroughly remove the silicone heat sink that will be on the bottom of the iron.
This process should be repeated each time that you change the length of the extension cord you are using, if you drop the iron accidentally, or when you start a new covering project. It is critical that the proper temperature is applied to the fabric.
You will want to purchase a small heat-sealing iron in addition to your regular iron. This smaller iron will be used in some of the non-load carrying areas that are inaccessible with a larger iron. Only use this type of iron in areas where exact fabric tension is not critical. Removal of wrinkles and smoothing of tapes can be accomplished using this type of iron. Calibrate the smaller iron just as you do the larger iron.
You must allow the fabric cement to completely dry prior to beginning the shrinking process. Failure to do so will often result in the fabric being pulled loose from the structure.
After waiting for the cement to dry and having calibrated our iron, it is time to begin the fun. Watching the fabric pull up and smooth out on an aircraft surface is very rewarding. You will enjoy this part of the covering process.
If you have used a fabric envelope with sewn seams, you will want to shrink the area immediately along the seam first. Failure to do so will cause the seam to be crooked. As you begin the shrinking process, the fibers of the fabric will shrink uniformly and evenly. If you are doing a large surface, such as a wing, begin in the bay near the wing root. Shrink that at 240 and then do the bay at the wing tip. You can then work your way toward the middle of the wing. This will help prevent any warping of the airframe. Let the iron glide over the surface—no pressure is necessary. Do not worry about leaving the iron in one place temporarily. It will not scorch the fabric nor will the fabric get any tighter. The amount of shrinkage is due totally to the temperature, not the pressure or time. Don’t worry about removing all of the wrinkles on the first pass. They will come out with at the higher temperature setting you will do on the next pass. Iron over the hard surfaces such as the leading edges. Realize these areas may act as a heat sink and require a little extra time to properly shrink. Be careful not to allow the tip of the iron to penetrate protrusions or rivets and cut the fabric. Also, and this is important, do not place the iron over cemented seams. A temperature of about 250 degrees will loosen fabric cement.
When you are shrinking fabric around a protrusion, such as a wing attach fitting, iron around them and after you have shrunk to 240 degrees you can cut the fabric above the fitting just enough to allow it to pop through. Make a small cut so the fabric does not pull too far away from the protrusion.
Shrinking the fabric is not tricky by any means. It is a fun step—but one that must be accomplished with care. We are now ready to seal the fabric with the first coat of nitrate dope.
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