KR2S :: Fuselage

Darren & Hardy's KR2S Project

Fuselage

	Building the jig blocks was a piece of cake and took no time at all. I bought some 42mm x 18mm pine lengths and began by marking off every 1" across the width. By clamping a piece of scrap timber to the drill press I was able to guarantee that every hole was the same distance from the edge as I slid the length along.
	Now that I had a stick with 2 rows of evenly spaced holes, all that was left to do was cut the blocks. Once again I clamped a scrap piece of wood on the mitre saw guide, 1" from the blade and with each successive cut just flicked the block out and fed through more length.
	The drilled holes are a little wider then the screws allowing them to push through easily. I didn't want to risk splitting the blocks with tight fitting screws. Once screwed into the bench the blocks don't move.
	Instead of snapping a chalk line to get my first straight line, I set up a laser level at one end of the workbench and marked every 18" or so along the laser light with a black marker. The photo doesn't show how bright the light was but I can tell you it had no problem illuminating the 14' of the top longeron.
	By screwing the blocks exactly to the lines I had marked, I knew the freshly machined longeron was going to be perfectly straight.........and it was.
	I then marked on the bench where all the verticals would meet the top longeron. I did this by hooking the tape measure at one end and running it out the full length of the longeron. Then I marked the distances between each vertical according to the plans. I was very careful to add the distances properly and also putting a "check" to the side so I knew which side of the line the vertical was going to be placed on. After the "mark & check" I held a set square up against the top longeron to draw the vertical lines.
	The plans give the lengths of only 6 verticals. Once these were cut and in place, the bottom longeron was then held in place at the "nose" with a block and then gently bent around the others, finishing at the tail. According to plans, the bottom longeron goes in a straight line to the tail. However, I have continued the smooth curve by pushing the bottom longeron in 3/4" half way to the tail. Again, using the laser level made this an easy task.
	The rest of the verticals are measured and cut to fit between the top and bottom longeron. Here's a couple of tips. Set the saw to cut the mitre at about 5 degrees and cut the end off a vertical. Do this first and see if the cut is correct. Adjust the angle of the mitre to shave more off until you are happy that the angle sits perfectly on the bottom longeron. Note the angle of the mitre and write it on the workbench so you know what angle to cut when building the second side. Once this is done, just set the mitre saw back to 0 degrees and easily cut the top off to length.
	To cut the diagonals, I layed out a piece of timber making sure it was aligned to the corners exactly as shown in the plans. Resting my "Pa's" sliding bevel on the diagonal, I sighted the bevel to the vertical and then marked the line to cut.
	One diagonal to saw! Once done, make sure the cut fits perfectly in the upper corner and then mark the angle on the other end using the sliding bevel. The angles on opposite ends of the diagonals are identical, therefore you can use the same sliding bevel angle to mark and cut.
	mmmmm..........Perfect!
	Looking closely at the plans, the diagonals to the front spar meet at the very top of the capping. This makes structural sense. The standard RAF48 spar height is 7.19" high but as I am using the AS5048 foil at the root, the height of the spar is going to be 8.19". Therefore I changed the position of these diagonals to meet 1" higher.
	All the timbers are now in place. All that is left to do is cut and fit the rest of the gussets. There is a very easy way to work out what angle they need to be.
	Remember how I marked on the workbench the angles I cut the bottoms of the verticals at? First you just subtract this number from 90 degrees. eg. 90 - 5 = 85 degrees. Halve this number eg. 85/2= 42.5 degrees. This is the number you need to set on the mitre saw. After cutting one angle, roll the timber over as in the photo and perform the same cut again. 2 cuts of 42.5 degrees add up to the 85 degree gusset needed.
	Getting the joints ready for the T-88 epoxy. I removed the jig block and slid the plastic underneath. It is then folded over the top and the jig block screwed back in. That way if any epoxy seeps out of the joint, it won't make contact with the workbench or the block.
	Here is what I got together to get started on gluing the fuselage. T-88 epoxy and hardener, plastic cups, icy pole sticks which I cut off the rounded ends to help get the epoxy out of the bottom of the cup. I also have a brush to apply the epoxy which turned out to be a useless applicator for the job as the epoxy has the consistency of a thick syrup on a cold day. Applying with an icy pole stick is the way to go. Chemical resistant gloves and electronic scales.
	The epoxy and hardener have to be mixed 50/50 by volume. This is very hard to judge given the viscosity of both. I chose to mix the two by weight. The epoxy is heavier than the hardener so they need to be weighed at a ratio of 13:11. That is, 13 grams of epoxy to 11 grams of hardener. The scales are calibrated to .01 of a gram. So add an empty cup, zero the scales, add 13 grams of epoxy, zero the scales, add 11 grams of hardener...........stir with an icy pole stick.
	I left the epoxy to cure for about 30 hours and then removed the nails which had no issue with sticking to the timber or glue. After removing some of the jig blocks I carefully lifted the fuselage from the bench which came away like a cake from a well greased cake tin. The protective plastic under the joints peeled off easily, leaving a smooth film of epoxy covering the underside of the joint.
	It was a real sense of accomplishment to finally free the fuselage from the bench. To feel its weight and strength was a real WOW! moment. In a few areas I used a light file to remove epoxy that had flowed onto the plastic leaving a bridge between the timbers, a bit like webbing between your fingers.
	It was now time for a cup of tea and to admire my handy work before repeating the process all over again. It has taken a total of 23 1/2 hours to get to this point and this does not include the construction of the bench. The fact that I will not have to plot the plans, build and place jig blocks and just a better general idea of what to expect, should decrease the build time for the second side.
	The second side is all glued up and now I'll let it cure for 24 hours. It has taken a lot less time to build....under 11 hours which includes 1 1/2 hours of machining the 5/8" x 5/8" lengths of timber.
	While waiting for the plywood to arrive I have decided to build a router scarfing jig using a 3' length of pine from an old bookcase. In order to drill the holes perfectly to hold the router, I just sprinkled sawdust on the timber, put some screws in the bottom of the router and pushed down really hard. Sawdust stuck to the screws and left imprints on the sawdust. Mark them with a texta and drill the holes.
	With the router secured to the timber, I have added screws along the bottom and bolts mid way up. The plan is to screw the jig to the end of the workbench and use the bolts to push it out to get the scarfing angle correct.
	To get the 10:1 angle required for joining plywood together with a scarfing joint, the jig needs to be angled back just shy of 6 degrees. After screwing in the bottom of the jig, I use the sliding bevel, set it to 96 degrees and screw the bolts through until the jig lines up with the bevel. Screw in the rest of the screws and I am done.
	Before gluing on the plywood, I sanded off the high spots where the glue had gathered on the underside of the fuselage whilst drying.
	Cutting the plywood for the first time. Aircraft grade ply is beautiful. It's smooth and there is not a mark on it unlike house building ply. My local ply supplier only has 4' x 4' boards so I need more joints than usual.
	My scarfing jig worked perfectly. Plenty of noise and plywood fibers.
	Both of these pieces of ply have had the scarfing done. When pushed together, the joint is perfect on both sides.
	After outlining the fuselage on all 3 pieces, epoxy is applied to the ply and the fuselage timbers as well. I then layed the fuselage on top of the plywood and applied pressure.
	I didn't want to use staples so used good weighty objects. The result was fantastic and I don't have the hassle of pulling staples and filling staple holes in time.
	The entire fuselage side has these beautiful glue fillets where the timber meets the plywood.
	When cutting the plywood initially I left a little overhang on all sides because I knew I'd get a perfect edge using the router with a flush trimming bit.
	This particular bit has a ball bearing on the very end the same width as the cutting flutes. The ball bearing rolls along the longeron and the cutting flutes trim off the overhanging ply.
	I have used the laser level to mark a straight line for the center of the fuselage. As the widest part of the fuselage will be 40", the center line must be at least 20" and then a few from the edge of the workbench. The tail of the fuselage must overhang the bench to allow room for fitting of the vertical stabilizer. I will have to temporarily widen my workbench for the bending of the boat.
	As seems to be common practice, I will be moving the widest part of the fuselage in line with the shoulders when seated. This position is behind the rear wing spar and slightly closer to the vertical aft of there. This improves pilot and passenger space. Moving the widest part to the aft edge of the wing instead of the middle also improves aerodynamic drag over the upper surface of the wing.
	I drilled the first two blocks exactly where I want the fuselage to be and the one down near the tail is about mid way to where the tail will eventually be bent to.
	Here you get a better idea of how far I need to bend the tail around.
	Slowly and gradually I bend the fuselage so as to get the tail end over the jig block. No "snap" noises here.
	After the bending I secured the bottom (top longeron really) to the workbench with a few blocks as it wanted to lift itself up and over the jigs. The top of the tail wanted to bend outwards, so I used a tent guide rope to pull it over to vertical, securing it to a screw in the workbench. I'll bend the rear of the fuselages around bit by bit over the next week to give the timber time to take up the curved form gradually. Hopefully this will avoid stressing the longeron too much, too quickly which can lead to snapping.
	Like I have said before, space in a single car garage is a premium so I added the workbench "wing" at this stage for the setting out and gluing of the boat. Its construction is covered in the Bench section. Once completed, I will remove the "wing". The second fuselage side has been set out identically to the first.
	It's been a couple of days and not much to show but a more bent fuselage. I did however machine more timber for the next stage. Gradually it has taken its final shape. I am widening the fuselage to 40" at the shoulders and after much thought and input from the KRNet guys, I have decided to keep the same "narrowed in" shape as the plans and widen the top and bottom of the boat the same number of inches, except the firewall which remains the same dimensions as standard. The other option was to make the fuselage perfectly vertical.
	While I finalize the bending of the rear of the fuselage, I have used trigonometry to calculate the cutting angles for the cross members. Remembering the fuselage is upside down on the workbench, the bottom of the fuselage is narrowed in which means the cross members need to be cut at angles to match the angle of inward tilt of the fuselage. The formula for this is TANØ = 62 ÷ 504, which equals 7.01^o. You can use the formula if you like or use an online triangle calculator like I did. My maths isn't what it used to be.
	So I have calculated the cross member angle cuts for the narrowing in at the bottom of the fuselage, but it also narrows in towards the firewall which means I now have to calculate that angle too. Use the online triangle calculator again, or if you want to show off to your friends the formula is SINØ = 47 ÷ 1200, which equals 2.24^o.
	Given the angles I have calculated, I can now set the compound mitre saw to cut both at the same time. Like the picture shows, I rotate the mitre to 7^o, and tilt the compound over to 2¼^o.
	The result is perfect. As the bottom and top longerons together are straight from the firewall the to forward spar, these cutting angles do not change even though the cross members get longer. I will use the same method for the straight section from just aft of the rear spar to the tail. In between, I'll have to take a few individual measurements. Using the calculator and a compound mitre saw removes the "hit and miss" approach to cutting these difficult angles. It's easier and quicker than I have made it sound here so give it a go.
	After cutting all the bottom cross members I marked each one with a line at exactly center and secured them in place with sash clamps. Just to see how straight the boat was, I lined up the laser level along the center line marked on the workbench and to my surprise it didn't need too much straightening. 3/8" out at the front and 1/4 out of line at the tail.
	I used my trusty tent guide ropes to "tweak" the fuselage into alignment and then went on to cut the necessary gussets.
	I decided to use the icy pole sticks and clamps to apply pressure to the gussets while the epoxy cured. This method was very successful.
	Once the epoxy had dried, I removed the clamps and the icy pole sticks flew all over the place like released springs. I did re-glue one cross member near the tail as the gusset had slipped over night and wasn't making good contact. After cutting the offending gusset off the cross member and sanding the joints back to timber, I epoxied it all back together. Now I am happy with the joint.
	As the sides of my fuselage are tilted in as per the plans, the bottom longerons meet the flat bottom of the fuselage at a slight angle as can be seen when using a straight edge. So that the plywood when glued can remain flat right to the edge, the slight angle needs to be removed from the longerons.
	The electric plane made the job very easy. I set it to a depth of .4mm and gradually planed the longerons, checking them for flatness with the straight edge as I went.
	Before fitting the plywood skin, the bottom engine mount rail and compression brace need to be cut and epoxied into place. After clamping a piece of freshly machined 3 1/2" x 5/8" hoop pine into position, I scribe with a pen on both ends where the cuts need to be made. Using the angles calculated earlier, I was able to make perfect cuts with the compound mitre saw.
	Epoxied and clamped into position, the plywood skin will be ready for fitting in 24 hours. But only after lightly planing the bottom edge of the engine mount rail to match the curvature of the longeron as you can see in the photo above.
	The plywood has been cut to size with a 1/4" overhang on the sides which the router will cut to a crisp edge when dry. I nailed the ply where it had to go and then crawled into the fuselage, layed on my back and marked with a pen around the cross members and along the longerons. This tells me where to apply the epoxy on the plywood.
	Once the epoxy has been applied, I secure with weights and 1 1/2 mm nails at strategic points and let cure for 24 hours.
	The big moment has arrived where I get to flip the fuselage over and view it the right way up. Before removing the jig blocks, I secured scrap timber between the top longerons to avoid any flexing movement. I have to say it's lighter than I thought and quite stiff.
	Back on the bench, I was able to closely inspect every joint and piece of timber from the inside. Both top longerons have cracks in them at the widest part of the fuselage. I am in two minds as to how to go about the repair. The damaged section of longeron will need to be replaced and a new section will have to be scarfed in with the joints over verticals. That is a given. I have already started bending a new piece of 5/8" timber but am giving thought to bending 2 x 5/16" lengths and laminating them together. I'll give it some thought over the next couple of days.
	I have decided to laminate the hoop pine to replace the cracked longeron sections but before cutting them out, I need to jig the top of the fuselage like it was when inverted on the workbench. This is so it will hold shape when the cracked sections are removed. I have used the 4' x 4' x 1/4" plywood intended for the firewall and also some bookcase shelves. The timber props inside the fuselage stop the sides from bending in.
	Using the plunge router with a straight cut bit and the adjustable guide, I shaved off a little timber at a time until I was sure I could sand the remaining area smooth back to the plywood. I'll route the scarfed sections tomorrow.
	The freshly scarfed and hand sanded longerons are now perfect and ready to accept the laminated sections.
	Having ripped the new longeron sections, all that was left to do was machine them to the required 5/16".
	Before gluing, I clamped in the new sections just to make sure they didn't crack again. Satisfied they were OK, I added epoxy and many clamps to be sure they held perfect form and contact with the plywood. I have only epoxied in one lamination on each side to avoid over stressing the plywood. The second lamination will be added tomorrow.
	Two laminations later and the clamps removed, all I have to do is trim the over hanging ends with the router.
	I set the router at a depth of 5/8" and set the adjustable guide along the outside of the fuselage. With the router bit sitting on the inside and at a width that only just contacts the existing longeron, I was able to slide it along removing the excess lamination.
	The result has been outstanding with no deviation in the curvature from firewall to tailpost.
	The top engine mount consists of 2 pieces of 3 1/2" x 5/8" timber which are epoxied back to back, however they differ in that the aft mount extends only to the edge of the longeron.
	The forward engine mount extends all the way to the edge of the fuselage. These mounts took me hours to get right as I had to take into account the angles of fuselage, the top longeron and doubler (not epoxied in yet) and the checked out portions of the mounts.
	When I cut out the cracked longeron sections, I let the fuselage decided for itself the best curvature in that area. The new curve spread out over a longer area while still maintaining the 40" cabin width. This has meant I have been able to use the planned 5/8" x 5/8" doubler instead of building one from 2 laminations as I thought I'd have to do at the time the longerons had cracked. Here they are epoxied in.
	I have left the longerons and doublers at the angle they have been epoxied together for maximum strength in the engine mount area. As I chose to not plane them horizontal, the plywood shelf horizontal support had to be notched so it would sit flat.
	The two firewall/fuselage ply braces are now in place as is the lower 1/4" ply shelf. The trick to getting it into position was to flex it downwards in the middle while sliding the edge up the verticals until it popped into place.
	The top cross members are now cut with all their tricky angles and clamped until the epoxy dries. According to the builders manual, the cross member for the seat back should be 6-8" aft of the rear spar at the top longeron position. I have set it at 6" aft to assist in keeping the C of G forward. I am 5' 11" and should have no problems with the more upright seating position.
	I forgot to read the electric planner safety instructions that state it's not wise to catch the plane falling off the workbench while it is winding down. Pretty lucky really as it could have been a lot worse.
	As I have planed the top longerons and doublers flat aft of the engine mount area, they are a little thinner on the inside. Where the top cross members meet the sides, they now slightly under hang which means I have to cut the gusset blocks and then remove a check so they also support the under hanging portion.
	The filler block needed too be leveled but the electric plane would just gouge it as the area is too small to support the length of the base. To get the job done I used a wood rasp to remove the bulk, then a metal file to remove the rasp marks and then sandpaper to get the nice finish.
	Areas that need dried epoxy removed to expose the underlaying timber so I can epoxy plywood or timber to that area, get scraped with my penknife then filed with the metal file and then sandpapered smooth. This has all taken quite a long time but I need to know my joints are going to be as strong as possible.
	The main spar plywood webbing has been cut and just placed into position. I am still deciding on my sound proofing options before epoxying them in and leaving a triangular void behind.
	I have managed to use f-clamps for the firewall gusset blocks and stopped them from sliding down by using supporting clamps.
	When the epoxy is wet and I apply pressure to the tip of the "v brace" with the black spring clamp, the timbers want to spread out. This in turn bends the cross member. So I have chocked the cross member which keeps it straight and thus stops the "v brace" spreading apart. To stop the new timbers from sliding down, I have used the blue clamps to hold them level with the already epoxied cross members.
	Believe it or not, I have spent a total of 19 hours sanding joints, cutting "v braces, cutting and epoxying the internal gusset blocks and placing urethane behind the main spar plywood webbing. The fuselage is on its side so I can add weight to the webbing while it sets. I weighed the fuselage today and it came in at 22 kg or 48 lbs including 2 temporary timber braces. But it is without tail post, spars and some plywood down near the tail.
	As I am using the NACA 63005 airfoil for my vertical stabilizer, the tail post needs to be cut to accommodate it. The whiteboard shows the stock KR2, KR2S and NACA tail post dimensions. All three have similarities in that they start tapering from 12" from the bottom to their tops and the lower ply ribs are all 5" above the top longeron. The plans state that the KR tail post is 40" long and the KR2S 43". I couldn't find the length for the NACA tail post but saw that the top ply rib was 34" above the top longeron which makes the tail post 45" long. The lower ply rib sits 5" above the top longeron where the tail post has to have tapered to 1.52" wide. The top ply rib width on the tail post is .85" (27/32") wide. From these figures, I have calculated that the bottom 12" have to be 1 5/8" wide. Download the NACA plans from Mark Langford's site and it should make sense.
	Given the calculated dimensions, I have accurately drawn the tailpost onto the timber leaving some extra length at both ends. A longer piece of timber is used as a straight edge which I am about to line up along the taper line. Once in position, I use wood screws countersunk through the straight edge and into the tailpost extra length at both ends.
	I then turn over the timbers making the tailpost the top piece. The table saw blade is adjusted to 2.4^o as per the plans and gradually reduced to 0^o by the time it has ripped to the end as per the plans for the top ply rib. I used the same procedure for the other side of the tailpost. A light sanding finishes it off.
	Here's a photo of the finished tailpost with the 1/4" plywood in place. Ever wonder what NACA 63005 meant? NACA stands for National Advisory Committee for Aeronautics which was founded in 1915 and was absorbed into NASA in 1958. Now for the 63005. The "6" when multiplied by .15, gives the section lift coefficient = 0.9. The "30" when divided by 2 gives the distance of maximum camber from the leading edge (as per cent of the chord) = 15%. The "05" gives the maximum thickness of the airfoil (as per cent of the chord) = 5%. So now you know.
	As the tailpost has to be angled aft 7^oI have cut that angle through a piece of pine and will use it as an aligning tool. By holding the tailpost against it I am able to see exactly where to make the angled cuts on the top and bottom longerons.
	Have a look at what I have done with the bench . I have leveled both the fuselage on the "loft" and the laser level on the workbench. Shooting a vertical line down the fuselage, I have made sure that it touches the cross members at the centers which were marked earlier for aligning the boat. As the laser exits the rear of the fuselage, it tells me exactly where to position the tailpost for vertical.
	To stop the tailpost from sliding while the epoxy dries, I have pre-drilled 4 small holes and gently driven thin nails through the rear of the tailpost and into the ends of the longerons. These will get removed afterwards.
	The KR2S is starting to look more like an aircraft now with the horizontal stabilizer resting in place and the front main spar in position.
	Whilst the main spar is not completed yet, I decided to cut the holes in the fuselage and slide it into position anyway.
	Before laying the plywood floor in the cockpit, I have cut and epoxied the three blocks that will support the rudder pedals. What I have also done is add square blocks to the open areas where the plywood will span bottom cross members. This will reduce sagging pressure when standing on the floor and possibly any drumming. Urethane foam offcuts have been added as sound proofing.
	I taped together two pieces of light cardboard and then layed it on the cockpit floor to create a template for the plywood. After marking around the the timbers it was just a matter of tracing the outline on the plywood. My first officer is always keen to help out......
	..............and imagine what it will feel like in the sky.
	My long clamps can be changed around to expand which came in handy when applying pressure to the main spar supports whilst the epoxy set.
	Here the rear brace that holds the horizontal stabilizer to the fuselage is being aligned with a straight piece of timber. It has to be aligned with the bottom longeron as it kind of sits out in the breeze until the outer plywood skin is fitted. The plywood in this photo is only there to keep the two verticals aligned with each other until they dry. At that time, 1/4" plywood webbing will be fixed to the forward side and up to make contact with the rear HS spar.
	Once dried the hanging verticals line up perfectly with each other and the bottom longerons.
	The elevator needs to have freedom of movement 20^odown and 30^oup. Upward movement poses no problem however to allow for the downward movement, an area has to be removed from the top longerons so as the elevator spar doesn't touch them. I initially cut away the curved area with a jigsaw and then finished it off by wrapping sandpaper around a tube. This time it was my prospecting pick. Then I went to work sanding.
	The time has come to epoxy the horizontal stabilizer to the fuselage permanently. The fuselage is taken out of the garage and placed on the concrete driveway. Timber wedges are used underneath to bring everything up to level. The side to side measurement is easy to take as anywhere you place the level across the fuselage it should read the same.
	Fore and aft is another story as the fuselage has a "banana" shape. The level reference point for fore and aft is taken between the front and rear spars on the top longeron. My digital level fits this area perfectly. All future wing and stabilizer angle of incidences will be adjusted with reference to this point.
	Before finalizing the job with epoxy, I do a complete dry run of the finished settings. This means getting equal amounts of HS hanging out either side of the fuselage, shooting the diagonals from the tips to the very center of the firewall, making sure both tips are vertically identical. Finally I set the - 0.75^oangle of incidence using the the templates which as stuck on the outside of the HS/elevator tips. When I see that it is possible to get all these right I know I can epoxy with confidence.
	There is enough room to lift the HS from the webbing and move it back so I can apply epoxy to all the webbing and spar surfaces.
	Epoxy applied, I move the HS back between the webbing and apply a few clamps but only lightly as I need to go through the whole procedure of measuring, leveling and incidence setting all over again. I spend a lot of time tweaking here and there to get everything perfect before adding the rest of the clamps and enough pressure to hold it in place until the epoxy cures.
	With the passage of time I have found that the cockpit floor is becoming dirty and a little stained so I have decided to sand is clean.
	To make it easier to clean in the future I apply a coat of clear satin flooring estapol.
	As I was happy with the results of the cockpit floor I decided to varnish the whole fusleage. I sanded with 80 grit sandpaper then 120 grit. I then vacuumed the fuselage out and applied the first coat of varnish. I let it dry over night and then did a light sand using 240 grit non clog paper the following morning. Like before I vacuumed but I then I wiped down all the surfaces with a damp cloth to remove every last trace of dust. A final coat was applied and allowed to fully dry. After a total of 17 hours of work, the result is outstanding.