Article - Securing Fabric
When fabric is used to cover a wing certain precautions must be taken to ensure the fabric does not "balloon-up" in flight. Lift on a wing will cause the fabric to attempt to rise up on the top surface and separate from the ribs or from the plywood on a wooden airplane. This ballooning affect can be disastrous. What can occur in this situation is that the fabric separates from one rib followed by adjacent ribs as the pressure builds. This acts as a spoiler on one wing with the other wing creating full lift. An uncontrolled spiral usually happens.
You can think of the lift created by normal flight as a giant vacuum cleaner that is trying to peel your wing fabric off the top surface. Beginning with the first aircraft that were flown in the early 1900's fabric has been mechanically secured to the ribs. The Wright Brothers used a sewn pocket in the fabric itself in which they then inserted the ribs of the wing. (Several ultralight manufacturers are using this method successfully today). Bleriot aircraft actually used a piece of wood physically attached on top of the fabric to the rib below.
So, our next step in the covering process is to mechanically attach the fabric to the wing ribs. This will also be accomplished on control surfaces on most aircraft. It is necessary on any surface that creates lift. Seldom will you find a fuselage that needs any rib-lacing.
Using a rib-lacing cord is the most common method of securing the fabric to the ribs of a surface. This cord is made out of polyester material and is extremely strong. A special needle is used to tie a specific knot to accomplish this step. Other methods of attaching fabric to ribs include the use of screws, pop-rivets, and special fabric clips.
With the advent of the ultralight, a number of people have used alternative methods of fabric attachment including cementing the fabric to ribs. This has become somewhat common recently using the cements designed for fabric. Ribs are made a bit wider and fabric cement used to glue the fabric to the ribs. Fabric cements have been designed to hold two pieces of fabric together against the shear forces that may try to pull the fabric pieces apart. The fabric cement is not designed for peel forces that are applied to the fabric as a result of the lift created during flight. I strongly recommend using some form of mechanical attachment when securing the fabric to the ribs. Even if you are gluing the fabric back it up with a mechanical attachment. This is essential for safe operation of the airplane.
On production aircraft, the method used to secure the wing fabric to the wing ribs should be the same one used at the factory when the airplane was manufactured. If you want to use an alternative method you must obtain FAA approval. On experimental aircraft, you may use whatever means you desire. If your plans call for cementing the fabric to the ribs, you should also rib lace as an added precaution. You can even do both if you would like. If you are concerned about tying the proper type of knot I would recommend that you attend one of the EAA/ SportAir fabric workshops or visit one of the workshops held at various air shows. The knot is demonstrated and practiced at the workshops. The knot appears to be very challenging when, in fact, it is quite simple. If you have an experimental aircraft and you are having problems with the rib-lacing knot, simply tie square knots with each lace spaced properly. The proper spacing requirements may be found in the Ceconite Covering Manual or in FAA Advisory Circular 43-13. The distance between the mechanical attachments is dependent upon the never exceed speed of the aircraft. As an example, if the never exceed speed of your airplane is 150 MPH the distance between fabric attachment points on each rib will be 2-1/2 inches within the slipstream of the propeller and 3 -1/2 inches outside the slipstream.
Before I present the steps in fabric attachment, I want to discuss one other aspect of this process. Prior to 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 heat tautened. The tape is looped around the top capstrip of one rib to the bottom capstrip on the next rib until all are secured. (See Figure 1). This simply keeps all of the ribs straight and parallel with each other until the fabric is mechanically attached as we are discussing. As you can see, when rib lacing or some other form of attachment takes place the inter-rib bracing really serves no further function. Even though it serves no purpose it will not be removed.
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 Ceconite manual. We are presenting this chart for your convenience in Figure 2. Notice that you base the distance of the spacing on the never exceed airspeed of the airplane. The chart also requires a closer spacing in the prop wash area. This area 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. 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 recovering a production airplane and know the original spacing you can use that distance. Most light aircraft will end up with about 2-3 inch spacing within prop wash areas. Of course, you can use tighter spacing if you so desire. The restriction is the minimum spacing.
Spacing requirements for tail surfaces are not as restrictive. You can use twice the wing propwash spacing in this area. Ailerons should use the same spacing as on the wings.
Marking the Spacing
Now that you know the spacing, you will need to measure and mark using a pencil. Do not use anything other than a pencil to mark fabric. Ink may bleed through the final finish. You will begin measuring at the butt rib on the topside 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 were 3 inches the first measurement would be at 1-1/2 inches. Then the next mark would be 3 inches, etc. In this case, you would be sure the final mark is no greater than 3 inches from the 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. Now, rather than mark each rib independently, you can use a common chalk line and stretch it across the marks, snap, and you now have a mark at each rib. Blue chalk line will not bleed through the final finish.
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 wing spars. If the wing were symmetrical you could flip it over and mark the same spacing. Since most wings have an airfoil where the top surface has a greater curve than the bottom surface, 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. 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. Next, draw a line from each mark down to the bottom of the template keeping the line parallel with the spar mark. Now 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 bottom side of the wing.
You must place a piece of polyester tape over each rib prior to rib lacing or before using whatever means of attachment you decide. Without this tape to reinforce the fabric, the rib lace, 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 wing ribs. Use the width that exactly matches the width of your rib. It has an adhesive back and should be placed on each rib, both top and bottom. Align the tape carefully with the rib as you apply it. Be careful not to use anything but approved reinforcing tape. Do not use strapping tape or any other tape that is not approved. There have been instances where non-approved tapes have been cut by rib lace cord.
If you are going to rib lace the next step is to prepunch 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.
If you are going to use rib lacing as a means of attachment, be sure that you use only approved polyester rib lacing cord.
There are two ways to rib lace. You can put the wing on sawhorses for this process or you can stand 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 6-8 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 as found in the Ceconite manual. The hidden knot is easier to tie and looks better as a finished product. I am not going to try to attempt to explain these knots. Suffice to say, they really are not complicated but do require a little practice. My advice on the knot, get the Ceconite manual. Go to workshops for practical experience. Also, most major fly-ins will have a fabric covering workshop where the knot is presented. EAA Technical Counselors can also assist you in learning this knot.
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 recovering an airplane you will have holes where the rivets or screws were previously used. Make sure these holes are not oversized. If so, drill a new hole as close as possible to the old hole.
If you are using pop rivets, buy the ones that have a broad head for use on metal ribs. Standard hardware rivets will not work. Place a small .016 aluminum washer under each rivet. Pop rivets are certainly easy to install but may present a problem when you want to recover. Drilling them out can be a problem.
PK screws are another method of attachment. Again, start with the reinforcement tape using the required spacing. Use a .016 aluminum washer under the screw. Use self-tapping screws. You should not use PK screws on wooden ribs. 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 too, are difficult to remove without damage.
There you have the common methods of attaching fabric to wing ribs 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. Gluing is okay but must be followed by some form of mechanical attachment. If you have an ultralight or other small experimental aircraft, you can even tie square knots every few inches. Any knot is better than none. (Can't do this on a production airplane.) Just make sure each knot you tie is independent of all other knots. That way if one breaks loose the others will remain in place. The approved knots discussed earlier provide this protection.
Next month we will continue with our covering steps by examining installation of finishing tapes, drain grommets, and inspection rings.