Build Your Skills: Fabric

Last month we began our discussion of attaching fabric to an airplanes wing. As a quick review, we must ensure that the fabric on a wing will not balloon as a result of the lift created by the wing during flight, which would cause the fabric to try to separate from the upper surface of the ribs. To ensure that this doesn’t happen, you must use some form of mechanical attachment when securing fabric to ribs. There are several options including rib-lacing, screws, pop rivets and fabric clips.

Several aircraft plans call for the builder to glue the fabric in place on each rib. As mentioned previously, it is OK to do that, but you must also secure the fabric through a mechanical means. Fabric cement is not adequate protection against the possibility of the fabric lifting during flight, as it has little tensile strength; it is designed to have shear strength. Properly attaching the fabric costs little in money and time, and it will help to ensure safety in the long run.

Dont take the chance. Remember that with a production airplane you must secure the wing fabric to the ribs using the same method employed by the factory when the plane was originally built. If the manufacturer rib-laced, you must do the same. If screws or other specific types of fasteners were used, you must also use the same method. Experimental-aircraft builders may use any method desired-just be sure to mechanically attach the fabric. Also, you will probably want to secure

As we discussed, prior to attaching the fabric to the ribs you will have shrunk the fabric and then coated it with one coat of Poly-Brush. You are then ready to begin the next step.

Before installing the fabric on the wing you will want to be sure the ribs are all parallel to each other. This step must be done prior to placing any fabric on the surface. A twill tape called inter-rib bracing tape is used to keep the ribs straight up and down when the fabric is tautened with heat. The tape is looped aroundthe fabric on any surface that will create lift, such as all control surfaces.

Determine Proper Spacing

The first step is to determine how far apart you should space the attach points. This means going to the chart presented in Advisory Circular 43-13 or in the fabric-covering manual. Notice that you base the distance of the spacing on the never-exceed airspeed of the airplane. The Poly-Fiber chart also requires a closer spacing in the prop-wash area, which is defined as all of the wing or control surface included within the diameter of the propeller plus one rib. In the interest of cosmetics, most builders will take the more restrictive prop-wash distance and apply it throughout the entire surface. Otherwise, you end up with staggered attach points. There’s nothing wrong with that except that you will not have neat looking rows of rib-lacing, screws, etc. when you look down the wing. It is also easier to lay out the spacing if you use the same distance. If you are re-covering a production airplane and know the original spacing, you can use that distance. Most light aircraft will end up with 2- to 3-inch spacing within prop-wash areas. Of course, you can use tighter spacing if you so desire.

Spacing requirements for tail surfaces are not as restrictive. You can use twice the wing prop wash spacing in this area. Ailerons should use the same spacing as on the wings.

Marking the Spacing

The next step is to measure and mark the spacing using a pencil. Do not use anything other than a pencil or blue chalk to mark fabric. Ink may bleed through the final finish. You will begin measuring at the butt rib on top of the wing. Begin at the aft edge of the leading-edge fairing and measure aft toward the trailing edge of the wing. The first point is always placed at one half the distance of the regular spacing. So if our spacing is 3 inches, the first measurement would be at 1.5 inches. Then the next mark would be at 3 inches, and so on. In this case, you would be sure the final mark is no greater than 3 inches from the wings trailing edge.

After marking the butt rib, pick a rib near the center of the wing and near the outboard end and place the same marks. Rather than mark each rib independently, you can use a common chalk line and stretch it across the marks, snap, and you then have a mark at each rib. Blue chalk line will not bleed through the final finish. Do not use red chalk, though, as it may bleed.

Next, we want to measure and mark the bottom of the wing. Unless the wing is perfectly symmetrical, you will have different marks on the bottom. You want any rib-lacing to be as parallel as possible to the wingspars. If the wing were symmetrical, you could flip it over and mark the same spacing. But most wings have an airfoil where the top surface has a greater curve than the bottom surface, so you must use a different method to measure and mark.

You can keep your lacing parallel to the spar by making a cardboard template. Hold a piece of cardboard next to the butt rib and trace its shape. Mark on the cardboard the location of the forward spar, and cut out this template. Now place the template against the butt rib and transfer to the template the marks you have made on the top of the butt rib. Then draw a line from each mark down to the bottom of the template, keeping the line parallel with the spar mark. After the lines are drawn, you can transfer these marks to the other side of the template. This will give you a template for both wings.

Place the template on the butt rib and mark the position of all attach points to the bottom of the wing. Turn the wing over and, using the bottom marks on the template, transfer the spacing to a middle and end rib. You can now use a chalk line to snap marks across all ribs on the underside of the wing.

Reinforcing Tape

You must place a piece of polyester reinforcement tape over each rib prior to rib-lacing or before using whatever means of attachment you decide on. Without this tape to reinforce the fabric, the lacing, screw, etc. will cut right through the fabric and defeat the purpose of this entire step. Reinforcement tape comes in various widths to accommodate the size of your wingribs. Use the width that exactly matches the width of your rib. The tape should be placed on each rib, both top and bottom. Align the tape carefully with the rib as you apply it, and use only approved reinforcing tape. There have been instances where non-approved tapes have been cut by rib-lace cord.

Pre-punch Holes

If you are going to rib-lace, the next step is to pre-punch all holes. This will make it much easier for you to accomplish the rib-lacing process. Use a straight rib-lacing needle and punch a hole in the fabric on the rib-lace mark right next to the reinforcement tape. Do this on the top and bottom of the surface. You will use these holes for needle placement during the rib-lacing process.

Rib-Lacing

If you are going to use rib-lacing as a means of attachment, be sure that you use only approved polyester rib-lacing cord. It is available as a round or flat cord. Many people prefer to use the flat cord, because it will lay flat on the surface of the wings and cover nicely with finish tapes.

There are two ways to rib-lace. You can put the wing on sawhorses for this process, or you can support it vertically in a wing stand. With the latter method you will need a helper to pass the needle back and forth as you lace. This method is often easier and faster. You can start rib-lacing at the leading edge of the wing or the trailing edge. You can perform the rib-lacing process on the top or the bottom of the wing. It does not matter, because the knots will be concealed on the inside of the wing.

Use a curved-tip rib-lacing needle to tie the approved knot. This will allow you to pass the cord under the fabric from one hole to the other. Start with a piece of cord about 6 to 8 feet in length. Only two knots are approved for a production airplane: The modified seine knot is described in Advisory Circular 43-13, and the hidden modified seine knot is found in the Poly-Fiber manual. The hidden knot is much easier to tie and looks better as a finished product. Although I wont attempt to explain these knots, they are not complicated, but they do require a little practice. Go to workshops for some practical experience. Most major fly-ins will have a fabric-covering workshop where the knot is presented. EAA technical counselors may also assist you.

Other Methods of Attachment

Pop rivets, screws and fabric clips will often be approved and used on metal ribs. Again, you will start by measuring, marking and applying reinforcement tape. If you are re-covering an airplane, then you will have holes where the rivets or screws were previously used. Make sure they are not oversized. If they are, drill a new hole as close as possible to the old one.

If you are using pop rivets, buy the ones that have a broad head for use on metal ribs. Standard hardware-store rivets will not work. Place a small 0.016 aluminum washer under each rivet. Pop rivets are certainly easy to install but may be difficult when you want to re-cover, as drilling them out can be a problem.

PK screws are another method of attachment. Again, start with the reinforcement tape and the required spacing. Use a 0.016 aluminum washer under the screw, and use self-tapping screws. You should not use PK screws on wood ribs, as they can introduce moisture into the wood over time.

Fabric clips are often used for this covering step. They are pieces of wire formed into self-locking barbs that are snapped into holes or slots on metal ribs. Cessna and Taylorcraft use them. They are also difficult to remove without damage.

There you have the common methods of attaching fabric to wingribs and control surfaces. The important thing to remember is to mechanically attach the fabric. Do not risk the consequences of not doing this step properly. If you have an ultralight or other small Experimental aircraft, you can even simplify this process by tying a square knot every few inches. (Any knot is better than none, though you cant do this on a production airplane.) Just make sure that each knot you tie is independent of all the other knots. That way if one breaks loose, the others will remain in place.

Next month we will continue with our covering steps by examining installation of finishing tapes.