If you ordered the optional powdercoating of your steel airframe parts, you will need to do some preparation before you begin the actual construction. Because of the limitations on the coverage of any applied coating, there will be a few areas that are either thinly covered or bare. This is most likely to happen at junctures that form a sharp angle, or on parts that have, or create, a deep cavity. Prime candidates on the fuselage are where the landing gear and float brackets join the fuselage tubing, and the horizontal stabilizer attach brackets. Don’t concentrate on the fuselage to the exclusion of the other parts: items such as the inside of the control column, horizontal stabilizer, engine mount weldment, etc., should all be thoroughly inspected and any areas noted for painting.
Although the steel parts are thoroughly cleaned before powdercoating, wipe any areas that you intend to touch up with a clean rag and de-natured alcohol. Use a matching color of quality enamel paint and a very small paint brush, and apply paint to any bare or thin spots. You need to carefully inspect all welded parts anyway, and this is an excellent time to dab on some paint as required. 4130 steel is wonderful stuff to make airplanes from, but absolutely must be covered or it will corrode very quickly. Don’t let this happen to any portion of your airplane!
To complete your touch up painting, locate any areas that might provide a trap for moisture, and seal them with a structural adhesive/fox mixture. Again, the prime candidate for this is the juncture between the gear and float attach brackets and the fuselage tubing, but don’t limit your search to just this one spot. Examine all parts carefully and perform the steps required. After the adhesive has cured it can be painted over to blend in seamlessly with the rest of the structure.
METAL PREPARATION FOR PAINTING
Steel parts should be sandblasted with fine grit sand (#30 grit) to ensure good paint adhesion. There are some metal preparation products such as C-2200 Metal-Sol metal cleaner from Stits that can be used. However, we have found that to adequately clean and prep the welded junctions, sandblasting works best. Unless all oil is removed and the 4130 tubing is "roughed up", no type of paint will adhere to it very well.
After the steel parts are cleaned and sandblasted, paint immediately to avoid rust. You will probably need 2 epoxy chromate primer kits to provide adequate coverage for all the steel parts unless you are prepared to spray all parts at one set-up with a small touch-up gun such as a Binks Model 115 or similar product.
When painting epoxy chromate primer, apply a wet tack coat and then two medium coats at 30 minute intervals before the sandblasted or prepared surface becomes contaminated or starts to rust, preferably within 24 hours. The epoxy chromate dries within 30 minutes. For best adhesion, color paint should be applied within 48 hours, otherwise the chromate primer will cure and must be lightly scuffed with Scotchbrite to rough up the surface for good paint adhesion. Refer to the Stits Poly-Fiber "Covering and Painting Manual" supplied with this manual for complete instructions.
PAINTING SEQUENCE
One of the ways to save yourself a lot of duplicated effort is to plan your building sequence according to the requirements of the parts you may have to paint. If you ordered your parts already powdercoated, then you have the greatest flexibility in sequencing the construction steps. If you have bare metal that you are going to prime and paint yourself, you also have plenty of latitude, but you can save effort by preparing parts and painting them in large batches. You can either fit parts together, and then disassemble and clean them for painting, or paint first and then perform whatever touch up might be required after the parts have been handled, drilled, reamed, etc. Regardless of which route you choose, mask off any areas (such as threaded rod ends) that would be troublesome to clean paint from.
The sequence that everyone encounters is that the final assembly of the engine and firewall should follow the fabric covering and final painting of the fuselage. The windshield and instrument panel can also wait until after final painting.
All metal parts must be covered with a corrosion resistant coating. Zinc-chromate primer has been used on aircraft for many years and is unequalled in protecting steel and aluminum. To save weight, you can apply top or color coat to only those parts that will be visible when the aircraft is completed, or you can pay the weight penalty and opt for greater corrosion protection. Aluminum can be treated with products such as Alumi-prep and Alodine; these alodining coverings are lighter than primer and paint.
We have found that Stits Poly-Tone provides the most flexible and tough finish on fabric. Use Stits Aerothane on aluminum and fiberglass parts for greater adhesion. Stits Epoxy Chromate Primer is an excellent product for protecting metal parts.
HAZARDOUS MATERIAL CAUTION
Several of the products that you will use in the course of construction are defined as hazardous, and appropriate precautions should be taken to protect your health. Observe the warning labels provided by the product’s manufacturers. In particular, zinc-chromate and other chromate containing primers are known carcinogens, and the mists from spraying these products must not be inhaled. Take care not to let fumes from any spraying operation build up in your work space, and use gloves when handling the structural adhesive.
Powerful solvents such a MEK (methyl ethyl ketone) are very hazardous to work with, having fumes that are not only dangerous to breathe and extremely flammable, and the liquid solvent can move quickly through your skin and get into the bloodstream. If you choose to use solvents of this type, wear gloves and work in a well-ventilated area far from open flames or sparks.
DIMENSIONS
Fractional inch dimensions are used throughout this manual and the associated drawings, as it has been our collective experience that homebuilders prefer to work with that system. While you should not be content to be in a hurry and do sloppy work on your aircraft, you should also not waste endless hours chasing every last 1/128" of perfectionism. Unless otherwise specified, a tolerance of 1/16" is quite acceptable. In certain more critical areas, such as drilling the spar inserts or rigging the wings, do your best to hold to a tolerance of 1/32".
In some instances, decimal inch dimensions are used; you might need a reamer that is .2500", or be using safety wire that is .03 2" diameter. These are dimensions that are usually fixed by the material or the tool itself, and we are not implying that you are expected to measure or work to the thousandth or ten-thousandth inch accuracy.
Many areas of your Kitfox are bonded for high strength, simplicity of construction, and light weight. Successful bonding requires careful surface preparation, and attention to keeping the prepared surfaces clean. Contaminations can be as mundane as the oil from your skin, or the lubricant mist from pneumatic drill exhaust. The most pervasive contaminants are those containing silicone, such as commonly available lubricant sprays, or automotive waxes and polishes; keep such products in one specific area of your shop, and use good housekeeping practices to prevent silicone migration into other workspaces.
SURFACE PREPARATION FOR BONDING
There are essentially three steps to bonding preparation: cleaning the surface with a solvent wipe:
roughening the surface to provide the adhesive with a good ‘tooth’ into the materials; and cleaning the roughened surface a second time just before bonding. The best solvents are denatured alcohol or MEK; we recommend alcohol because it is far safer to use than MEK and is readily available from hardware stores. The best rags for the solvent wipes are old T-shirts that have been freshly laundered or old diapers from the local diaper service.
Aluminum parts should be roughened using red Scotchbrite pads; on wooden parts use 100 grit sandpaper; on steel parts use 120 grit sandpaper. Avoid using sandpaper on aluminum, as most sandpaper grits are formed from aluminum oxide particles, which get imbedded in the surface and would promote corrosion. Metal parts need special attention to the solvent cleaning, as lubricant residues from the manufacturing processes can remain on the surfaces. 4130 steels in particular have a corrosion resistant coating on them that must be removed before bonding or painting. 4130 has a lot of great structural properties that make it ideal for use on aircraft; however, it is very prone to corrosion when unprotected, being good for about three foggy nights before developing rust.
Remember to do the solvent wipe as the last step just before bonding, so that any contaminants that might have gotten on to the surfaces are removed just prior to application of the adhesive.
STRUCTURAL ADHESIVES
The structural adhesive provided with your kit is 3M 2216 B/A Gray Epoxy. A 3M data sheet is provided, but we’ll call out the pertinent points here.
The mix ratio by weight is 5 parts of the white base to 7 parts of the gray accelerator. By volume, the ratio is 2 parts white base to 3 parts gray accelerator. Be sure to use the proper ratio, as the structural properties of the material decrease rapidly with small excursions away from the ideal mix. Be sure to mix the materials carefully, scraping the sides and bottom of the mixing vessel to include all the product in the mixing action. Continue mixing for at least 15 seconds after the material has a uniform color.
At 75’F the epoxy will reach a handling strength in 8 to 12 hours, and a full cure at this temperature is reached in 7 days. Warmer temperatures will speed up the cure and handling times, but be sure not to disturb the bonded parts until sufficient strength has been reached. Temperatures below 600 F will greatly extend the cure time, so either heat your shop or heat the parts above 60’F when bonding.
Where steel and aluminum parts are bonded together, the adhesive should be mixed with a small quantity of microballoons. The ‘micro’ serves to keep the adhesive from squeezing completely out from between the parts, thus insuring that the dissimilar metals never come into physical contact.
Where a structural fillet is required, the adhesive should be mixed with cotton fox, both because it adds strength to the fillet, and also because it adds sufficient body to the adhesive to keep it from running away from the surfaces.
MATERIAL PROPERTIES OF ALUMINUM
Many important parts of your kit are made from aluminum, such as the wing spars, critical brackets, fuel line tubing, etc. Aluminum parts are susceptible to the propagation of cracks if they are nicked or scratched, so handle your spars with great care to prevent damage. Be sure that you deburr any holes that you drill. Try not to leave any sharp corners or scratches on any aluminum part. Don’t use a pencil for marking aluminum, as it promotes corrosion, and don’t use a scribe for layout work. If you do nick or scratch a part, and the damage is not too deep, carefully file the surrounding material until the scratch has been removed, and then burnish the aluminum by rubbing it with a much harder metal object, such as the polished shaft of a steel punch. Burnishing levels the very fine groove left over from the file, and leaves a smooth surface that doesn’t offer a place for a fatigue crack to start.
Riveting and rivet spacing is talked about in a later section, but it should be mentioned that edge distance’ is an important consideration in aluminum parts. In general, the center of a hole should be no closer to the edge of the part than 2 times the hole diameter. Use specific dimensions instead of the general rule, when such information is available.
DRILLING
Often overlooked as a very basic operation, many holes are not well drilled. The secret is as simple as using a sharp bit and the proper feed rate and speed. As a rule of thumb, bits that are 3/16" and smaller should be turned at a high RPM with a light feed pressure, while bits larger that 3/16" should turn more slowly and use a higher feed pressure. The larger the bit, the slower it should turn, and the higher should be the feed. You can tell if you are operating the bit properly by observing the chips that are created; the most ideal drilling operation produces a chip that is one long twisted strand of the drilled metal. If you see lots of small pieces when drilling, then you need to either sharpen the bit, or experiment with different feeds and speeds. When hand drilling, be sure to hold the drill steady and square to the work, and apply your feed pressure straight down through the bit. In other words, don’t apply a side load to the bit, as it will create a very neat oval hole. If they made oval rivets and fasteners, this would be less of a problem. It is usually not necessary to drill a pilot hole for bits that are smaller than 3/16" diameter, although the purists insist on a pilot hole for anything larger than #40. Use a center punch to mark the hole location, and start the bit carefully so that it doesn’t drift away from the punch mark.
A relatively new tool called a step drill is very useful for making clean, very round, and automatically deburred holes, especially in thin sheet metals. They are not inexpensive, however.
For holes between 1/2" and about 2-1/2", a hole saw works very well. They do tend to make holes that are slightly oversized, so if you are doing work that requires accuracy, use an under sized hole saw and file the hole to it’s final diameter. If possible, a drill press should be used.
DEBURRING
It is good practice to deburr every hole in every metal part, if possible. Use a 100’ single flute countersink, a deburring tool, or even a large drill bit. Most holes in aluminum can be deburred by a single turn of the countersink pressed into the hole and rotated with your fingers. You are trying to change the sharp edge of a drilled hole to a chamfered edge, as sharp edges create stress concentration points and are the most likely places for cracks to start. When you chamfer the holes, the small cuts and ragged edges are removed. Some inside holes are nearly impossible to deburr without special equipment, such as P/N 60023.000 Deburring Tool, which is available through our customer service department.
REAMING
You will often have to ream the bushings and bolt holes in the steel parts for a proper fit. AN bolts may vary in size from one another by a few thousands of an inch. You should ream each hole to fit the particular bolt intended for that hole. Proper reaming of bolt holes and bushings is important for the installation of the control system and other parts of the Kitfox. Reamers will stay sharp much longer if operated at very slow speeds; therefore, hand reaming is preferable to using a drill motor. If possible, you should insert the reamer shank first into the bushing, then attach your "T" handle or drill motor to the reamer, and turn it slowly as you pull the reamer through the bushing. It is easier to keep the reamer straight in the hole if you pull it through rather than push it in. This will insure that the hole is not accidentally oversized and will provide a safe fit. Never turn a reamer backward (counter-clockwise), or it will be instantly dull.
TAPPING
Although rare, there are a couple of instances where you will be called upon to tap a drilled hole. Taps are commonly available as taper or starting taps, and bottoming taps, which will cut threads to within one thread of the bottom of a blind hole. As with a reamer, taps are designed to be used by hand and turned slowly.
Start any tapping operation with the right size hole, as listed on the shank of the tap. Lubricate the tap with thread cutting oil, and begin turning the tap into the hole, being very careful to keep the tap square to the work. The right feed pressure is important to cleanly starting the threads; holding the tap steady and not letting the end wobble around will prevent breaking the delicate first threads. Advance the tap one turn, and then back it up a half-turn to break the chips, and then advance again. When you need to fully thread a blind hole, start the threads with a taper tap, and then switch to the bottoming tap after the threads are started. Be careful not to advance the tap when it has reached the bottom of the hole, especially in aluminum.
AIRCRAFT HARDWARE
Aircraft hardware supplied in your kit is manufactured by certain standards, normally AN (Army and Navy or Airforce and Navy), MS (Military Standards), or NAS (National Aerospace Standards). These standards specify minimum strengths of the materials and provide uniformity in the hardware. Aircraft designers use these standards in establishing the structural integrity of the aircraft. If you substitute ‘commercial grade’ hardware from the hardware store, you are reducing the strength and safety margins that were designed into your aircraft. This is obviously very stupid, so don’t do it. If you are unsure of the hardware you are using, discard it. "If in doubt, throw it out".
SELF-LOCKING NUTS
AN self-locking nuts secure themselves using a principle called prevailing torque, which means that some feature about the design of the nut grips the threads. The use of fiber or nylon thread inserts, or a distortion of the thread pattern itself accomplishes this. Fasteners of this type are not intended for repeated on and off use; after several on/off cycles the self-locking feature will weaken, the prevailing torque will be diminished, and the nut should be replaced. Use plain style nuts during initial fitting of parts and save the self-locking nuts for final installation. Also note that the nylon-insert type lock nuts are not for use in high temperature areas. During your initial pre-flight inspection, pay close attention to the type of nuts installed. Make sure that you have the appropriate hardware; also check to see that there is at least one thread of the bolt extending beyond the nut, and that all castle nuts have a cotter pin in place. Also, bolts should be installed with the heads either up or forward, unless specifically stated otherwise.
FILING
Filing is pretty straightforward, but two things should be noted. First, never pull a file backward on the work. Files can cut on the push stroke only, and will be quickly dulled if used the wrong way.
Second, the thin metal end of the file, called the tang, presents a hazard to your hands. If you are pushing hard on the file and it suddenly bites into the work and stops, you can puncture yourself. For this reason, always use a file handle on the tang. Our icon for the filing operation includes a silhouette of the suggested file shape in the upper right of the circle.
LUBRICATION
When you see this icon, the type of lubricant that you should use will be called out to the right of the symbol. This will most likely be grease (for bushings, bearing bolts, and pins), or oil (flaperon bearings, cable wire end clamps, etc.) While the lubrication requirements of the aircraft systems are not very demanding, high quality lubricants are not significantly more expensive, and should be used exclusively.
SANDING
Self-explanatory icon; the suggested grit size is shown to the right. Be careful when sanding on fiberglass that you don’t inhale the dust.