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Building a Tube and Fabric Airplane Part 2
By Ron Alexander (originally published in EAA Sport Aviation, August 1997)
Last month I began a discussion concerning building a tube and fabric airplane. The first three steps of the building process were discussed in that article. As a review, the steps are (1) decision and planning, (2) basic building phase, (3) preliminary assembly, rigging, and systems installation. In short, we decide which type of airplane we are going to build and we do as much initial planning as possible. The more time spent planning the more likely we will successfully complete our project. The basic building phase follows our planning. This consists of building all component parts of the airplane. This phase usually involves a considerable amount of time. After building parts we then assemble them in a preliminary manner adding all of the systems we can at that point. Preliminary rigging is done at this stage to discover any potential problems prior to final covering and assembly.
The final stages of building a tube and fabric airplane are (4) disassembly, covering, and painting, (5) reassembly, final rigging, and final systems installation, (6) inspection, certification, and final pre-flight. Each of these stages will be discussed in detail.
After you have assembled and rigged your airplane without any fabric on the surfaces, you will then carefully disassemble the aircraft taking care to note any problem areas that need to be corrected. I recommend that you take notes, pictures, and videos prior to disassembly. Care taken now will result in time saved during the final assembly stage. You must also consider where you are going to store all of the parts while you are covering the airplane. After you have disassembled the airplane you are now ready to cover the surfaces with fabric. In talking with kit manufacturers, it is at this point where several projects grind to a halt. Fabric covering is often viewed as a formidable task. The quality of the fabric covering determines the final look of your aircraft. You want to do a good job and chances are you have no previous covering experience. If you have no previous experience, I would recommend that you attend an EAA/SportAir 2-day workshop on fabric covering. At this workshop you will actually cover a control surface. At the end of the 2 days you will definitely have the confidence level and the skills necessary to successfully cover your airplane. If you are unable to attend a workshop, find someone who has previous experience with fabric covering and enlist their help. After selecting a covering system, be sure you follow their manual. Regardless of the type of covering system you use, you will find the Poly-Fiber Covering Manual very useful. It is a very comprehensive step-by-step manual and the basic steps are the same with all types of covering systems. The differences are found in the chemicals that are used. Expect to pay about $2,500 for all covering materials for a J-3 Cub size airplane.
At this point you will need to decide where to do the actual covering and spraying. Fumes will be a problem. If you are working in your garage or basement, you may want to cover the surfaces there and then take them to another location for spraying. You can, however, minimize the fumes and overspray by constructing an inexpensive paint booth. You can do this by building a frame out of wood or PVC pipe that is large enough to house a wing or a fuselage with room for you to walk around while you are spraying. You can hang the frame from your shop ceiling and then lower it for use with a set of pulleys. Cover the roof and sides with cheap plastic sheeting stapled to the frame. You can rig up lights and shield them with chicken wire to prevent breaking them. (We do not want sparks generated while we are spraying.) You can add a furnace filter at one end and an exhaust fan at the other end. Once again, be sure the fan has an enclosed motor with no chance of sparking. If you are in doubt, leave out the fan and quit spraying when the booth is full of overspray. It will settle within a few minutes, and you can go back to work.
Lets review the safety considerations detailed in the previous article involving painting and fabric covering at this point. Avoid skin contact and eye contact with all chemicals. Use safety glasses and rubber gloves when mixing paints. I recommend you also use one of the barrier hand creams. Breathing the fumes resulting from chemicals should be avoided. Use a charcoal-filtered respirator. DO NOT rely on dust masks. If you are spraying any type of polyurethane paint you MUST use a forced air breathing system. Any polyurethane paint contains polyisocyanides that can cause a severe allergic reaction in many people. Don’t take a chance.
Our first step in fabric covering is to adequately protect the surfaces from corrosion or rot. All steel surfaces should be primed with a two-part epoxy primer. Aluminum surfaces should be acid etched and treated with a conversion coating followed by a two-part epoxy primer. Although many aircraft have been covered without using epoxy primer on unpainted aluminum surfaces, it is always a good idea to use it weight permitting. Wood surfaces should be protected with a two-part epoxy varnish. To use anything other than a two-part epoxy is inviting problems. The chemicals used in fabric covering systems will lift and wrinkle zinc chromate and spar varnish allowing moisture to be introduced. Epoxy primers and varnishes are impervious to these chemicals. You should spray the primers and varnishes to insure quality workmanship.
This brings us to the subject of required tools, one of which is a spraying system. Two choices are available: (1) a turbine powered High Volume Low Pressure system and (2) a standard spray gun using compressed air. The new HVLP systems are very popular and if you do not have experience spraying, they are easy to use. They consist of a turbine that is connected to a 110-volt outlet with the spray gun attached. Expect to pay at least $600 for one of these units. Remember that no compressor is necessary with this system. You will need the proper nozzle for your paint and at least two lengths of hose. The turbines produce heat that is passed through the hose to the spray gun. If you are using a standard spray gun make sure it is of high quality. This is not the place to economize. Remember, this part shows. A good quality spray gun will cost $150-$200 and of course you will need an air compressor. The air compressor should provide at least 40 psi delivered at the gun. You also will need the necessary filters and moisture traps. Whichever system you decide to use keep it clean. I mean field strip and clean the gun after each use. If you neglect this step you will start seeing small flecks of dried paint in areas where you do not want to see them. Spraying is not difficult but it does require practice. You will be painting your airplane in small sections before it is assembled. This is much easier than trying to paint an assembled airplane.
Other than the spraying equipment, the tools required for fabric covering are not expensive. Several of them you probably already have around the house or shop. An ideal list of fabric covering tools follows:
• sturdy sawhorses about 3 feet high covered with carpet scraps
• fuselage turning jig (see figure 1)
• snag free table for cutting fabric
• electric clothing iron ( 1100 watts minimum)
• small hobby iron
• thermometers and heat sink for calibrating the irons
• an effective respirator
• outside breathing source if using polyurethanes
• ½ inch glue brushes
• paint brushes
• rib lacing needles
• sharp scissors
• good quality pinking shears
• single edge razor blades
• chalk snap line
• measuring tape
• paint stirring paddles and straining cones
• soup ladle for dipping paint
• spring clamps with 2 inch throats to clamp fabric
• wood spring clothes pins to hold fabric
• T-head pins
• tack cloths
• cotton rags
• paint cans for use in applying chemicals (see figure 2)
A word of caution concerning fabric covering tools. I do not recommend the use of a heat gun. It is impossible to calibrate the heat emitted by a heat gun. Also, the temperature changes as the gun’s distance from the fabric changes. Your fabric covering manual will explain the importance of proper temperature when shrinking polyester fabric. A calibrated iron must be used – nota heat gun.
The actual fabric covering process is not mysterious and difficult. Always start the covering process with a small control surface. This will prevent you from having to spend a lot of money to correct a problem during the learning stages. It is much easier to recover an elevator than a wing. Follow the manual step by step. Most people have fabric problems because they experiment with the process and do not follow the manual supplied by the manufacturer. They will mix and match different products and often use latex paint from their hardware store as a final color. This is all well and good if you enjoy the fabric covering process because within a short time you will probably have to recover the aircraft. Follow the manufacturer’s recommendations.
The basic steps of fabric covering are essentially the same regardless of the type of process. The first step is to prepare the surface for covering (prime the metal and varnish the wood). You also will tape over any sharp edges that could possibly tear the fabric. The second step is to attach the fabric to the surface using fabric cement or by sewing. Before you begin this step take one final look for cotter pins, control cable installations, tools that have been left, etc. This is the last opportunity you will have to check for problems before covering the airplane. This is an excellent time for another "in-process" inspection by another person. After the fabric has been attached it is then shrunk using a regular household iron. Polyester fabric is capable of shrinking about 10-12%. Proper shrinking of the fabric is very important to preclude future fabric problems. (Note: Polyester fabrics are used with all fabric covering systems. Dacron is the common term used but it is actually a tradename for polyester. Grade A cotton is very difficult, if not impossible, to purchase.)
After the fabric has been properly shrunk it then must be sealed with a chemical. This step is most important because the first coat of chemical must make a mechanical bond with the fabric. If this bond is inadequate there exists the risk that this coat and all subsequent coats will one day peel from the fabric. After encapsulating (sealing) the fabric, you then mechanically attach the fabric to all wing and control surfaces. This prevents ballooning of the fabric as lift is created during flight. Of course, this step is very critical and must be properly completed.
Inspection rings, drain grommets, and tapes are then applied to the fabric. This is usually referred to as "dressing" the surface. Basically this step improves the appearance of the surface in addition to providing necessary protection. Following this step we then want to protect the fabric from the ultra-violet rays of the sun. This is necessary to prevent the fabric from deteriorating within a short period of time. Protection from the sun is gained by application of chemical coatings containing aluminum pigment. Usually 2-3 coats of "silver" as it is usually termed, will be applied. After protecting the fabric the final step is to apply the color coats.
As you complete the fabric covering of component parts you will want to carefully store them. You do not want to damage them in any way prior to final assembly. If for some reason you find yourself unable to complete a surface, the best place to stop is after the application of aluminum pigment (silver coats). This is a good stopping point if you must put the project on hold for several months.
When painting your metal parts be sure to paint several inspection access plates. You will need these plates at a later date (usually the first annual inspection) and you will want the color to match. Allow the paint on the metal parts to cure for several days prior to installing them on the airplane. This will give them maximum protection from any scuffs you might inflict during assembly.
The fabric covering portion of your tube and fabric airplane should be very enjoyable. Do not be intimidated by the painting. You can usually do as good a job as a professional painter. Fabric covering chemicals are usually viscous and fairly easy to spray. By the time you reach the color coats on the fabric and the metal parts you will have a lot of practice. Practice is the key. Professional painters are usually not experienced with painting fabric. Often, their techniques for spraying the chemicals on fabric will be wrong. Remember to paint all of your cowlings, access panels, etc. during this phase. Again, carefully place them aside for proper curing time before placing them on the airplane.
The next step in our process is to reassemble the aircraft and then complete the final rigging and systems installation. Care must be taken to avoid damaging the completed components as they are reassembled. You will need space for this step. Often builders will rent a hangar space for the final assembly so that after certification the aircraft will be at an airport and ready to test fly. The final assembly should be relatively simple if you took pictures and notes when you dismantled the aircraft after the initial rigging. I recommend that you use only good quality aircraft hardware when assembling your airplane. Be careful using surplus hardware or hardware that was not manufactured by a reputable company. Your supply source should be able to provide you with this information. As you assemble a part completely safety any nuts, bolts, etc. at that time. Do not wait to safety later. You may forget.
Before we talk about the sequence of assembly and rigging lets define the term "rigging". Very simply rigging an airplane means to properly install and adjust airframe components (wings, ailerons, and tail surfaces). These components are each designed to do a specific aerodynamic job and to contribute to the aircraft’s overall performance, stability, and safety. Proper rigging insures that the designer’s flight characteristics of the airplane are achieved. All aircraft performance data is based upon the aircraft being properly rigged.
The first step in reassembly and rigging is to install the fixed tail surfaces followed by the moveable tail surfaces and trim tabs. It is advantageous to install the elevators prior to the rudder. This is done to avoid having to work around a rudder that extends into your work area when you are trying to mate the elevators and related tabs.
After the tail surfaces have been installed and properly rigged, the next step is to attach the wings to the fuselage. After wing installation the final assembly will consist of installing all moveable surfaces to the wing panels. For most rigging, the fuselage will be leveled in both directions and then blocked and secured. A detailed article on rigging will be presented at a later time. A properly rigged airplane will fly with the ball centered, "hands-off," with proper cruise performance being noted.
Next we will install all aircraft systems such as the engine and propeller, wheels and brakes, avionics, electrical systems, etc.. This phase of construction is time consuming. We are usually in a hurry at this point in time because we are eager to fly the airplane. Do not rush through the final assembly phase. Time must be spent to insure that you have adequately prepared the airplane for its final inspection and flight testing. It is extremely important that you enlist the aid of a knowledgeable person to inspect your work. Once again, ask someone who is an EAA Technical Counselor or a person who has built an airplane of the same design. I know that I have looked at an unsafetied cotter pin for months and another person has walked up behind me and immediately seen it. Another set of eyes is very important.
There are several things you can legally hire someone else to complete during this phase of construction and stay within the legal boundaries of the major portion rule. If you are in doubt, Advisory Circular 20-139 covers this in detail. Examples are avionics and upholstery. Hiring someone else to complete these items does not affect the major portion (49-51%) rule.
After the aircraft has been assembled, rigged, and all systems installed and tested, you should insure that you have properly labeled all controls (throttle, fuel control, carb heat, etc.) for movement. Your identification placard should be in place along with the passenger warning placard. You will also want to be sure the proper instruments are installed for VFR or IFR use and that they are marked with limitations. Don’t forget your ELT.
At this point we are ready to calculate the weight and balance data for our airplane. Very simply you will need the empty weight and empty weight center of gravity, the datum reference, weight and balance arms, and the most forward and most aft center of gravity limits. The maximum weight should be provided by the designer. Remember, you must keep a written copy of the weight and balance data in the airplane at all times. A properly prepared weight and balance document will present a professional image when the FAA inspector arrives to inspect your airplane.
The topics concerning the final inspection for certification of your airplane were presented in a previous Sport Aviation article (June 1997). A checklist for this process can be found in that article. The final preflight inspection prior to test flying must receive concentrated attention. Again, the more sets of eyes inspecting the more thorough the inspection.
In conclusion, building a tube and fabric airplane is a very rewarding and enjoyable experience. Before you begin the process be sure to spend a considerable amount of time in the planning stage. Do not forget to involve your family. This is very important. Successful completion of a tube and fabric airplane has many advantages such as knowledge of the airplane you will be flying, satisfaction of completing a project, being able to perform your own maintenance, enjoyment of sharing the building experience with others, etc.. Thousands of people have been successful building an airplane and you can be included within that group. Prepare yourself by gaining the necessary knowledge, involve your family, and be prepared for the time and money required. Attention to these details will provide you with the advantage you need from the very beginning. EAA/SportAir workshops are also available on fabric covering and welding. If you are undecided about the type of airplane to build I would recommend you attend the Introduction to Aircraft Building workshop. That workshop is also beneficial if you are uncertain about whether or not to even build an airplane.