In the last article we discussed applying finishing tapes in our ongoing series on fabric covering using Ceconite fabric. In some areas, such as around a wing tip, you may choose to apply bias tapes. These tapes are specially cut on a bias so that they will lie down when pulled around a curved area. The choice as to whether or not to use bias tapes is up to the individual. I would recommend their use on a wing tip area and on any rudder that is built with a sharp curve.
Bias tapes do have one disadvantage. Since they are cut from a roll of fabric on an angle, the tapes must be sewn together. As a result of this, you will encounter a sewn every 70 inches or so. The tape will also reduce in width about 1/3 when stretched around a curved area. So, when selecting the proper width of tape remember to order a size wider to allow for the shrinkage. You will probably want at least a 4-inch wide tape on a wing tip bow. That will allow you to begin on the leading edge of the wing and extend the tape around the bow finishing with the proper width on the bow itself. You can actually overlap the bias tape on top of the leading edge tape that is in place.
Letís take a detailed look at how to apply a bias tape. The first step is to cut the length of tape needed. Donít forget the seams. Start your cut right after a seam. Cut the end that will be overlapped in the shape of a teardrop. This will provide a more pleasing appearance. After cutting the tape, use a pencil (no pens) and carefully mark a centerline along the entire length of the tape. You should then mark the area where you will be applying the tape. Mark about a 3-inch width on the wing tip bow so you can see where to apply the tape.
Apply nitrate dope to a small area on the leading edge where you will overlap the tapes. Apply only a few inches (3 or 4 inches) of tape using nitrate dope and then hold it in place with a spring clamp or with your fingers. Allow it to dry for a few minutes until it will stay in place. You will want this to dry enough so that you can pull the remaining tape while the portion that was cemented in place will remain that way. You can also use Rand-O-Proof to cement the tape in place. Both of these need to be thinned using a 1 to 1 ratio.
The idea is to cement a small part of the bias tape in place, let it dry then come back after about 1 hour and complete the taping. You now have the tape held in place over about a 3-inch area allowing you to apply tension to the tape.
Next apply a coat of thinned nitrate dope, or Rand-O-Proof on the area where the tape will be placed. Then pull the tape around the wing tip applying it to the entire bow. Use the pencil line to keep the tape centered over the bow. Keep pulling on the tape until all of the wrinkles are gone and the tape is lying in place. You now have a tape that has spanned a curved area without leaving any puckers or wrinkles.
Linear cut tapes may also be pulled around curved areas if the curve is fairly shallow. Again, attach the tape over a small area and allow it to dry. Then stretch the tape tightly around the curved area over a coat of nitrate dope. You will probably end up with a few wrinkles and tape edges that are not cemented in place. You can then take an iron, calibrate it to 225 degrees and use it to smooth out the wrinkles.
If you consider the time involved and the final appearance, it is preferable to use bias-cut tapes on most curved areas.
This step makes the job look professional. When you are taping (linear tapes or bias) you will probably end up with a few wrinkles and raised edges on the tapes. These can be smoothed down and removed with the application of heat.
First of all, calibrate your iron to 225 degrees. A small hobby iron is best for this task. You can use the iron to actually smooth out any imperfections that exist particularly along the edges of tape. If you have pinked edges that are not cemented in place you can use the tip of your iron to literally melt them down. This is much easier than sanding out imperfections.
You must be very careful not to heat the iron above 225 degrees. Anything hotter than 225 degrees will shrink and deform your tapes. You will then have tapes that have a snakelike appearance. All wrinkles, bubbles, and raised areas can be fixed using the iron as discussed. Go over every tape edge on the surface. You can use your fingers to tell when they are smooth. When they feel smooth to the touch they will appear smooth with the final finish. This one step will save many hours of sanding at a later time.
There may be areas of the fabric that need to reinforced such as around strut attachments on a wing or over a fuel cap area. The area may be oddly shaped and too large to accommodate a tape. When this occurs you will want to cut a piece of fabric to the exact size and shape needed and then apply it to the area using thinned nitrate dope. Be sure that on the fabric you will be using to cut out the patch you iron out all of the wrinkles. If this is not done the wrinkles will be prominent in the final finish. Trim these patches using pinking shears. Use of lightweight fabric is also recommended for these gussets or patches.
The component parts of a fabric-covered airplane must have a way for moisture to escape. Condensation can introduce moisture into a wing or fuselage as well as rain or water that may collect from washing your airplane. These areas must be able to breathe and allow the moisture to escape. This is accomplished using a small plastic item called a drain grommet. These small grommets are cemented in place on the underneath side of surfaces. Drain grommets should be applied anywhere you think moisture may collect. Of course, the drain grommets will be placed on the bottom side of a surface so the water can escape. Most wings, as an example, will have a drain grommet located next to the outboard side of each rib at the trailing edge on the bottom side. Some people will place a grommet on each side of rib. You will also want to place drain grommets on the underside of a fuselage to allow water to drain. This is especially important near the aft end of the fuselage where moisture tends to collect.
Drain grommets are cemented in place using Super Seam fabric cement. Each drain grommet should be covered with a piece of fabric that is cut slightly larger than the grommet itself. If this is not done they will vibrate loose and eventually separate from the fabric itself. The best way to cut a piece of fabric is to use the inside of a roll of fabric as a template. Lay the roll of fabric onto a smoothed piece of fabric and draw a circle on the fabric using the inside of the cardboard on the roll as a guide. This will provide the ideal size piece of fabric to cover a drain grommet. After you have completed the final finish you can use a pencil soldering iron and melt the fabric out of the drain hole. This will then allow water to drain out.
Three types of drain grommets are used. They are plastic grommets, aluminum grommets, and seaplane grommets. I prefer to use aluminum grommets because they are thin and cover easily with a piece of fabric. Seaplane grommets have a small vented hood that helps them siphon water out. They are normally reserved for use on seaplanes.
Inspection Access Rings
After you have covered your airplane you will occasionally have to gain access to certain areas inside the wings or fuselage. This may be accomplished using inspection plates that are held in place by plastic access rings. These hole reinforcements (access rings) are cemented in place on the fabric over every drag wire junction, wing fitting, cable guide, control bellcrank, or any other area that should be inspected regularly. Access may also have to be gained into one of these areas during assembly of the airplane. These inspection holes are usually installed on the bottom side of a surface. If you are recovering an airplane you may use the old fabric as a guide to locate placement of the inspection rings.
The plastic rings are cemented in place using Super Seam fabric cement. Use a small brush to apply a coat of Super Seam onto the ring itself and then lay it on the fabric (flat side down) in the area desired. It will dry within a few minutes. Be sure to clean up any cement that works out from under the ring.
After cementing the ring in place, you will then want to cover it using a piece of fabric. Failure to do this will provide an opportunity for the ring to separate from the base fabric at a later time resulting in loss of the ring and attached inspection plate. This separation is often the result of vibrations and air loads when flying the airplane. A simple piece of fabric cut to fit over the ring will prevent this from occurring.
The first step in cutting a piece of to fit over the ring is to smooth the fabric that will be used. You donít want any wrinkles in the fabric. Lightweight fabric works best for this purpose. You can use an iron set at 250 degrees and iron out all of the wrinkles on a piece of scrap fabric. Then take one of your 1-gallon cans and carefully set it on top of the piece of fabric (be sure the bottom is clean). Take a pencil and draw a circle on the fabric using the outline of the can as a guide. This size is just about perfect to cover the inspection ring. You will want the fabric to extend beyond the ring itself. Cut the fabric out using a pair of pinking shears. This will ensure you have pinked edges for proper cementing.
Using thinned nitrate dope, brush on a coat both inside and outside the inspection ring large enough to wet out the fabric overlay. Lay the piece of cut fabric over the ring and allow the dope to soak through the fabric. Use a brush to work the dope through the fabric. After it dries, brush on another coat of nitrate dope. After this coat has thoroughly dried you can use your iron to smooth out any imperfections.
When you have completed the airplane you may want to cut the fabric out of a few ¨of these inspection rings. You will cut the fabric out in the areas where you want an inspection plate. Only cut out the rings where access is needed for assembly of the airplane. Leave the others uncut. They can be opened at a later date if the need arises. Once the fabric inside the inspection ring is removed it will be covered with a metal inspection plate. These plates are made to fit over the plastic inspection ring and are easily removed to provide access inside the area.
Another tip, when you are painting your airplane be sure to paint a number of metal inspection plates the final finish color. You will then have them available to place on the inspection rings when needed at a later time.