Article - Installing Tapes
After securing the fabric to the wings and control surfaces of your airplane, the next step is to apply finishing tapes. Finishing tapes will be used on all major surfaces of your airplane including the fuselage. They are used to protect certain areas from abrasion in addition to covering the rib-lacing, screws, rivets, etc. that hold fabric in place. Tapes are made from the same polyester fabric that you will use to cover your airplane. The fabric covering manufacturer sends rolls of fabric to a company that in turn cuts the fabric into tapes of varying widths. These widths are from 1 inch to 6 inches. The most common tape used on airplanes is 2 inches wide. The tape is actually cut leaving pinked edges. Pinked edges are used to increase the amount of surface that can actually adhere to the aircraft fabric when the tape is cemented in place. The actual edge area that will be cemented in place increases by over 41% when pinked edge tape is used versus straight edge tape. As you can see, the pinking actually allows the tape to be held in place more firmly than would be possible without the pinking. Ceconite still sells tapes that are cut straight (without a pinked edge) in addition to pinked edge tapes. Some people prefer the look of a straight edge tape but often encounter problems with the edges lifting from the surface with time. Properly applied pinked edge tapes will stay in place without lifting.
Finishing tapes are cut from two different weights of fabric -- lightweight and medium weight. Lightweight fabric weighs about 1.7 ounces per square yard and medium weight weighs about 2.6 ounces per square yard. Either of these tapes may be used with any weight of fabric. In other words, if you are using medium weight fabric you may use either lightweight tapes or medium weight tapes. The same principle applies to all weights of fabric. A number of people with covering experience find that the lightweight tapes are easier to work with and provide a more pleasing appearance on a surface. The choice is yours.
Tapes are normally cut lengthwise with the roll of fabric. These are referred to as "linear tapes." After being cut they are then rolled up into lengths of 25 yards to 50 yards. So, when you buy a roll of tape from a supply house it will be either 25 yards long or 50 yards long.
As we will discuss, it is often necessary to tape a curved area. How do you get the tapes to lie down neatly and adhere to the surface? There are a few tricks we will outline later but you also should be aware that special cut tapes are available just for curved surfaces. These are termed "bias tapes". These special tapes are cut at 45-degree angles to the fabric weave. They are also cut in various widths just as regular tapes. Since fabric is milled in widths of approximately 70 inches, it is impossible to cut long lengths of tape on a bias. Bias tapes will have a sewn seam every 70 inches or so. This has obvious cosmetic disadvantages when trying to use a bias tape over a long area. If you are taping the leading edge of a wing, as an example, you can use linear cut tape for most of the leading edge and then place bias tape around the wing tip area. More on this later.
To summarize, finishing tapes are cut from lightweight or medium weight fabric. They are cut straight along the edge of the fabric (linear tapes) or at a 45-degree angle with the edge of the fabric (bias tapes). Tapes are normally cut with pinked edges and are available in a variety of widths from 1 inch to 6 inches. 2-inch tapes are the standard width for most ribs and longerons. You will use more 2-inch tape than any other width. 3-inch wide tapes are used for leading or trailing edges of wings and control surfaces. 4 and 6-inch widths are used for larger leading edges of wings.
Where to Use Finishing Tapes
As mentioned, finishing tapes will be applied over all ribs that have been rib-laced or where screws, etc. are used to secure the fabric. Any fabric seam that has been sewn together or glued using fabric cement requires a finishing tape to reinforce the area. Remember earlier discussions where we talked about overlapping and cementing fabric. The resulting seam where the fabric was overlapped should have at least a 2-inch tape placed over it. Other areas need to be reinforced for added strength. Using finishing tapes accomplishes this objective. For example, the very aft portion of a metal or wooden leading edge on a wing will require a finishing tape. If this area is left without a tape a spanwise crack will often develop.
Trailing edges of wings and controls should be reinforced with finishing tapes. Leading edges also require taping. Longerons and stringers on a fuselage are usually taped. The leading edge of gear legs is another area where tape should be applied. In short, any area that is subject to abrasion or any area you think needs to be reinforced should be taped.
Selecting and Applying Finishing Tapes
Selection of the right size and type of finishing tape is important. You will want to cover the area completely and neatly. 2-inch tape is normally used over wing ribs. Leading and trailing edges of wings and control surfaces will often require a wider tape such as 3-inch or 4-inch. Some leading edges will need to have 6-inch wide tapes applied for optimum protection. It depends entirely upon the size of the wing. 2 or 3-inch tape is satisfactory for most trailing edges. Sewn seams should have at least 2-inch tapes applied over them.
Where should you use bias cut tapes? This again is a personal choice. Some experienced coverers do not use bias cut tapes at all. They work with linear tapes to cover curved areas. If the curve is fairly pronounced, such as a rudder for instance, I would recommend using bias tapes. Bias tapes are often used on wing tips as another example. Remember that the seam found on a bias tape should be removed for cosmetic reasons. A bias cut tape will also become somewhat narrower when pulled tight around a curve. For this reason, you will normally want to use 3-inch bias tapes. A 3-inch tape pulled tight around a curve will narrow to about 2 inches or about 1/3. Bias cut tapes will lie flat around a curved area versus linear cut tapes that will leave wrinkles when pulled around a curve. You will want to use bias cut tapes only on curved areas.
The normal taping process begins by placing tapes over the ribs of wings, controls, etc. After these are in place, you then apply the tapes over the leading edge, trailing edge, etc. Most people start by applying tapes chordwise followed by the ones that will be applied spanwise on a surface. On a fuselage, you will want to install the tapes on longerons followed by tapes over the leading and trailing edges where you begin the taping process. See Figure 1.
Now, lets take a look at how to apply the Ceconite tapes over our surfaces. We must use one of two products to apply the tapes. These are thinned nitrate dope or Rand-O-Proof.
I recommend that you draw pencil lines where you want to apply a tape. Using a straight edge to accomplish this task only takes a few minutes. Mark the area just a bit wider than the tape to ensure proper edge adhesion. You should then use one of the products mentioned above and thoroughly wet the area where the tape is to be applied. You can either spray the chemical or brush it on using a high-quality bristle brush. Lay the tape onto the area and smooth out all wrinkles and bubbles.
When using nitrate dope, do not thin it too much. A one to one ratio is adequate. It should be thick enough to provide a good glue bond and keep the tape securely in place. However, if it is too thick you will have a rough appearance under the tape itself.
You will need to work fairly rapidly in higher temperatures, as the dope will dry fast. Immediately lay the tape onto the area just brushed. The tape will begin soaking up the dope right away. Use your brush to then go back over the tape to lay it smooth and work out any air bubbles. Little brushing is needed. Be sure you complete any brushing before the nitrate dope dries. If not, you will leave brush marks that will be difficult to remove. It is also very important that you have the first one-half inch of each side of the tape thoroughly wetted out with dope. You are ensuring that the edges of the tape will not lift.
Once the tapes are in place and the nitrate dope has dried to the touch you should then spray or brush a thinned coat of nitrate dope over them to completely seal the tape. Be sure the dope underneath has dried prior to doing this step.
It is almost impossible not to encounter wrinkles and edges of tape that will stick up somewhere. There is a way to get these wrinkles out and to get the edges to stick down properly. Take your small hobby iron and calibrate it for 225 degrees F. Do not let the temperature get above 250 degrees F. After you have calibrated the iron, use the tip to smooth the wrinkles and edges of the tape. This heat will soften the dope just enough so the pressure of the iron will press it smoothly onto the fabric. Do not use a heat gun or an uncalibrated iron for this purpose. You must be sure that the temperature of the iron does not exceed 250 degrees F. If it does, the tapes will begin to shrink causing an unsightly curve to develop in the tape itself.
In the next issue we will continue our discussion by presenting the method to apply bias tapes, inspection rings, and drain grommets.