The KR wing skins are vacuum bag molded to insure a good bond between the inner and outer skins and core. The core is made with Polyurethane foam which is used in many aircraft, automotive and marine applications. The facings (inner and outer skins) are one ply each of 6 oz. 7533 bi-directional glasscloth. The warp fibers (the ones running spanwise on the outside facing, parallels the leading edge; while the inside facing is applied with the warp fibers paralleling the trailing edge. This "cross plying" gives good torsional as well as "rip stop" strength. The skins are beefed up" in the areas where the two spars are attached with one extra ply of glass. Because of the inherent strength and stiffness built into the skins, it is not necessary to add more length to the spars. The "plyfoam" web supplied in the kit is quite acceptable. You may notice in the pictures that the spars installed in N4DD are full span. We found this be a needless expense and a weight penalty.

STEP 1: SPAR MODIFICATION (for outer spars already built to KR plans)

Because of the extra span of the skins as well as the increased taper, it is necessary to slightly modify the main spar. The rear spar can be used in its stock form. Approximately 1/8" must be shaved off the top and bottom of the outer end of the main spar. This cut is tapered to "0" at the inboard end. (See Fig 1)

Fig. 1

Each vertical web should be drilled through with a 3/16" drill. This allows transfer of air pressure as the aircraft climbs and descends. A second ply of plywood is then applied to the forward face of the spar. If your spars are not yet built, we recommend building them according to Fig. 2.

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Side

Top

(Cap only without plywood)

Fig. 2

This is easier since all the vertical members are the same length. It is also stronger because of the increased contact area between the spar and the wing skin. The forward web need only be skinned out 4 ft from the inboard end. Two coats of varnish or polyurethane is applied to the forward, rear and inside surfaces. DO NOT paint the top and bottom of the spar. This should be clean so a good bond can be made between the spar and the skin.

Make wing attach fittings as per KR instructions. To set dihedral, cut two blocks of wood (3.5") long. With plane in upright position, place the blocks on top of center wing spar. Sight along the top of the blocks and raise the outer spar tip until it lines up with these. See Fig. 3.

Fig. 3

Drill spar per KR plans. Next place 32" rib on spar outer tip. Leading edge of rib should be 9 inches from center of main spar. Pound a small nail through the rib into the center web so the rib can rotate. Now raise the tail of the plane until the center section ribs are level on the chord line. Next, level the "level line" on the outer tip rib. Glue and nail in place. This gives a 3 degree washout between the tip rib and the center section. Next install the rear spar per KR plans. The tip of the center of the rear spar should be attached 22 ¼" from the leading edge.

Cut and glue in 48" rib at the root so that it lines up with the center section. When gluing ribs, take a wire brush and "eat out" the foam between the fiberglass facings about 3/16" inch. This allows for a cotton flox and epoxy mixture to be applied. A "finger filet" is made by wiping the flox-epoxy mixture on the outside of the joint. See Fig. 4

2.

Fig. 4

Use coarse sand paper to sand the spar any place a rib is glued to it.

Once ribs are installed (48" & 32" only), plane spars so they line up with the airfoil contour, (as per KR plans). Since the skins have a ¼" thick core, except where the spars attach, 2, 3 and 4 ribs will have to be cut ¼" down all around the outer circumference. Make a trial fit of the lower skin at this time. You will immediately see where the skin fits, also that the 32" rib must now be trimmed to fit inside the foam core. Do not be surprised to find that the inboard end of the skin needs to be trimmed to fit the center section. Also, cut out access holes for the wing attach bolts. Once a good fit is obtained then move to Step II.

Since the skins are made with vinylester resin, only vinylester resin should be used. Some epoxies will work if sanded completely into the glass cloth. The vinylester is our first choice. You may purchase vinylester resin from Aircraft Spruce. Rand Robinson glue or West system glue will work if skin is sanded well. If you plan to build in wing tanks, do not use Rand Robinson Epoxy. Some fuels have been known to react with this epoxy. You will also need some "cotton flox", available from Wicks or Aircraft Spruce.

Make a "trial fit" before any gluing begins. You need to make some simple "Campbell Clamps" to hold the skin in place. Boat builders will know how these are built. For those who don’t, cut from scrap lumber ten pieces of 5/8" x 5/8" wood, long enough to extend 3 or 4 inches behind the trailing edge. Each clamp will involve one piece on top of the spars and one below the bottom skin. When the ends are pulled together it holds the skin in place. Five sets of these are needed while gluing each wing. See pictures.

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One more very important thing is to "PAD" the skin with some 3 or 4 inch wide strips of ¼" or 3/8" plywood. The Campbell Clamps will produce quite a lot of pressure at the point where they run across the spar. The strip of plywood will help spread out this load more evenly along the span of the spar.

Use fiber strapping tape on the ends of the clamps, pad, and skin. Once happy with the fit, remove. You will find that it helps to have helpers on this job.

Mix vinlyester resin with some flox. This should make the mixture just barely stay in the cup when it’s turned upside down. A slow, gradual "crawling" out of the upside down cup is preferred. Another way to check the mix ratio is to stand your stirring stick straight up in the center of the cup. It should stay. Now turn the cup on its side and the stick should slowly fall. Place vinylester slurry on the skin where it contacts the spar. Working upside down, place slurry in the groove on the bottom of the ribs. Place lower skin in position. The radius of your finger in the corner will create a smooth "finger filet" of flox. The use of strapping tape and a couple of "C" clamps will help hold the skin against the trailing edge ribs and the leading edge. Once happy with the job, let cure. The next step is to fit the rest of the ribs. All will have to be cut to fit the spars. They have been left in one piece to allow for slight differences in spar locations. Again, once they are cut for the spars, they will have to be cut down ¼" on top and bottom to allow for the ¼" thick skin. At the trailing edge you will note that the ribs get rather

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far from the aft edge of the skin. Fit the ribs and mark their position. Cut out the foam on the top and bottom with a wire brush. Glue in place using "flox slurry". Eyeball from the wing tip to be sure leading edge lap is straight. Use tape, supports, or weights to hold while glue cures. Do the same with the trailing edge. The spar extension butts against the 32" rib in line with the center of the main spar. It runs out to meet the 30" rib. Glue it in at this time, also let the ribs cure.

Now is a good time to install wiring for tip lights and radio nav antennas. The pitot tube can be mounted now by attaching a 1" x 1" angle aluminum bracket to the spar. Remember to install disconnects at the inboard end of the wing. Now is also a good time to install your wing tie-down. Use a "boat-eye" made for the front of a boat for the tie-down. Get these from many discount stores or marine dealer. They usually have a 3/8" bolt. Drill 3/8" hole from the bottom through the center of the spar cap about 15" in from the outboard tip of the spar. The bolt in the "boat-eye" may be too long and may have to be shortened to save weight.

You have probably already decided to install wing tanks since most of the KRs now built do have them. However, I have found in many hours of KR flying and many cross country trips that about three hours is all I can stand to sit in a KR. Since most of the VW engines burn between 4 to 5 gallons per hour, this indicates that 16 gallons would easily go the 3 hours and have some reserve. Therefore, it is my opinion that if you have around 15 to 16 gallons in the main tank, that is enough and would not warrant the trouble of installing wing tanks. My own N4DD has only 13

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gallons in the nose. I found that I would often run about 30 minutes short of where I wanted to go. When I installed the wing skins on N4DD, I installed the tank from the front spar to the rear spar and between the first two ribs. This gave me 15 gallons per side; this is not needed. Since I usually only put 6 to 8 gallons in these tanks, the constant sloshing from front to rear is continually changing the C.G. and requires constant attention. Please don’t make the same mistake I did.

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The top of the fuel tank (the top wing skin) should have an extra layer of fiberglass over the tank area. The fuel cap should be installed before the top skin is installed. Wicks Aircraft sells a nice fuel cap for around $35. The outlet fuel fitting and drain is made by bonding a ¼" aluminum tab on the outside of the tank after the wing is completed. For now, the only thing left to do to the tank is to paint the inside with resin to seal any small leaks or pinholes. Once the wing has been completed and removed from the center section, cut through the outer layer of glass on the inboard rib with a razor for installation of the outlet tank fitting. A 1" x 2" section for the aluminum tab is all that is necessary. This should be located at the aft, lower corner of the tank. With only the outer skin cut, you can peel it away then scrape off the foam from the inside skin. With the inside skin exposed, flox the tab in place and apply 2 layers of glass over it. Drill and tap two 1/8" NPT holes for a drain fitting and fuel line near the bottom of the aluminum plate.

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Before any resin is mixed be sure to make a trial fit of the top skin on the wing. You should be able to see through the wing in the spar areas to make sure the spars contact the skin properly. Be sure the ribs contact the inside of the skin. The inboard end will probably need to be trimmed more towards the trailing edge to fit the center section. Cut out the skin to gain access to the wing attach bolts. The small foam strip on the lower skin wing tip may have to be sanded somewhat near the aft end to allow the top skin to fit. This foam strip is merely to act as a "shelf" to lay resin-flox while gluing the top skin. Use your wire brush to eat out about 1/8" to 3/16" of foam on all the ribs. Double support under the wing tip to keep it from twisting when you bond the top skin on. Once you are happy about the fit, gather your Campbell clamps and two pieces

of angle to clamp the trailing edge. Also you will need 6 to 7 clamps to clamp the trailing edge. Get 3 or 4 friends and educate them on what you want each to do.

Mix enough resin and flox to cover both spars, top of all ribs, tip and trailing edge. DO NOT glue the leading edge at this time. You will also need to cover the area around the fuel tank with plenty of flox. Try to stack up the flox high around the tank so you will be sure to seal the top of the tank skin. Get some help at this point. The leading edge should fit the overlap. Again this will be glued later. Once in place, apply the Campbell Clamps. Be sure to "pad" the spars with the two strips of plywood. Use masking or strapping tape to go around the wing tip. The angles are placed on the trailing edge, one on top and one on bottom. These are held in place with C clamps. See picture on page 4.

These angles will hold the trailing edge perfectly straight while the glue cures. Now, if you have some resin left which is not beginning to gel, you can work it in between the skins at the leading edge. The top skin will flex enough to pull it up slightly so the resin-flox can be applied. Use masking tape to hold the leading edge while it cures. We recommend that the C clamps all be left in place at least over night. Once all clamps are removed, sand with a sanding block, two inches on either side of the leading edge joint. Lay two strips of glass cloth, four inches wide, over the joint. I recommend you stagger the strips, one up, one down slightly, to help blend the edges of the strips into the skin profile. If there are some holes or impressions in the joint area, you can fill these with Bondo, BEFORE applying the glass. Remember you don’t want any air under the glass cloth. Once cured, sand all edges with a sanding block to knock off any burrs and straighten out any bumps.

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STEP V: AILERON Basically the aileron is built as per the KR plans. Cut the aileron out of the wing. It is 6 feet long with a 7 inch chord at the root and 5 inch chord at the tip. See Fig. 5 (next page) to get angles for cutting spars. We use #8-32 bolts to hold the hinge on to the spar. A washer and nut is placed on the back side. Place the bolts every 6 inches. You can slightly stagger each bolt to allow the bolt heads to clear each other. Put two bolts 2 inches apart near the inboard end on each side of the hinge. This will help keep the hinge from flexing when the push rod pushes and pulls on the aileron. The inside facing of the wing skin is cut back carefully ¼ in. and the foam scraped away from the outside facing. This makes a "groove" for the aileron spars to fit into. Make trial fits with the spars until they fit properly.

Apply flox to the back side of the hinge bolts to lock all the nuts and then paint the wood with resin.

Apply resin-flox to the wing and install the forward spar. You may use a staple gun to hold the spar in place. We put only about 6 or 7 staples on top and bottom and arranged a clamping system to hold the spars.

Once the forward spar is cured, glue in the rear spar to the aileron. Care should be taken to keep the glue out of the hinge. We have found it is easier to leave both spars together while gluing rather than try to replace the hinge pin after the glue has cured. Masking tape over the hinge will help keep it free of resin.

STEP VI: FINISHING Once all gluing and fiberglassing is complete and the fittings installed in the tank (if you built them) then it is time to paint the wing. You may find you didn’t quite get the right angle on your spar caps and now this shows up as a bad place across the spar area. You should sand these low areas with 80-100 grit sandpaper to roughen the surface. Use a Bondo type filler with micro balloons already added. Trade names for these fillers are Micro-lite, Dynalight and Super-lite to name a few. These weigh about 7.5-8 lbs per gallon whereas the old fashioned Bondo weighs 12 lbs per gallon. Fill in low areas. Sand the entire wing with 180 grit

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paper. Sand the leading edge with 80 grit to get it smooth. Next, spray on a coat of lacquerprimer. This will show up any irregularities in the surface. If you find any pin holes you can squeegee in some primer while it’s still wet or let it dry and use a spot putty such as Nitro-Stan. Wet sand the surface with 400 grit wet or dry. The wing is now ready to paint. We use Dupont Imron or Centari paint. We also recommend a white or very pale color paint due to its resistance to UV damage.

Good luck with your KR project. You have many hours of great flying ahead of you!

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Dear KR Builder:

This is an Urgent message to the purchasers of wing skins. Some of the early KR plans called for a 5 ½" high dimension on the outboard end of the outer main spar. I have not seen this in the two versions of the plans I have here. They both called for 5 1/4".

If you have built your spar to the 5 ½" dimension, you will NOT be able to simply cut down the spar to 4 15/16". This would remove too much of the structure of the spar. If this is the case, cut the uprights from the middle of the spar and build the spar back to the 4 15/16" dimension. If you have already cut them down, then you MUST order new spar material and build new outer main spars!!!!

Also, we have found the epoxies currently sold for homebuilders do not bond to the Vinylester wing skins unless the skins are sanded to the fiberglass. We strongly recommend the use of Vinylester resin for bonding the skins. It can be purchased from Aircraft Spruce and Specialty Co. Phone (714) 870-7551 Fax (714) 871-7289.

I regret the additional trouble this may cause, but I feel it is VERY IMPORTANT!

Sincerely,

Dan Diehl