KR-2S :: Seat

The KR2S Project

Seat

The seat is going to be made of carbon fiber. In getting a satisfactory shape I need to take a few things into consideration. I need to determine the height it is going to be above the fuselage floor, allowing room for cables to pass cleanly underneath. The shape needs to fit on the front and rear spar and the angle of the seat back needs to be far enough aft to provide sufficient leg room to the rudder pedals and to terminate at the parcel shelf cross member.

	First things first. The bottom of the seat is calculated using some timber between the lower longerons. I sat on it and angled some makeshift seat backs. Moving the "seat bottom" timber fore and aft, I found the legroom length I was happy with. Whilst the photos do not show it, I used a piece of cardboard and made a very rough outline of the seat by holding it up against the fuselage side.
	Getting in and out of the KR was made easy by securing clamps to the spars on both sides and having them touch the ground, giving the KR stability much like a Harrier Jump Jet.
	From the cardboard template I cut out the first of three profiles and made sure that it lined up.
	The box section has now been built and work can now begin lining the shape before then applying a layer of clear contact as a release agent for the carbon fiber layups.
	I keep changing my mind on how to build the seat. Here I have cut four pieces of foam and used expanding foam to glue them together but not to the box section.
	I have divided the seat into two sides and then marked shapes that are similar to my car seats. I then shaved the centers down about 1cm at the bottom.
	Expanding foam has been used to glue the thigh bolsters in place. These will be sanded away to form very shallow mounds maybe 1cm high. Just enough to provide some lateral support.
	I have used the electric sander to carefully remove the overflowed expanding foam from the joints at the back of the seat.
	This is the piece of timber removed from one of the seat profiles made earlier. A length of sand paper stapled to the bottom and I now have a perfectly shaped sanding block to make the corner angles rounded.
	Whilst I have not finished the sanding yet, you get the idea of how I want the two seats somewhat contoured. Looking down on the lower seat backs you can also see how they have been slightly shaped too.
	Trial fitting the seat seems OK with about 1" clearance from the fuselage floor. I made my "Flip-O-Matic" in 30 seconds flat. It ain't fancy but it works a treat and what's more an engineering degree isn't needed to build it.
	I have applied gaffer tape and plastic to the center spars to stop the first layer of carbon fiber from sticking to them.
	As with any other layup, I have used the plastic sheet method and then draped the carbon fiber over the inverted foam seat.
	Apply the peel ply and fit the seat into place. I have left enough carbon fiber to drape over the front and rear spars which has been stapled to hold it in place until dried.
	Everything went perfectly well until I went to remove the peel ply. I'd been staying back at work until late working on the WAFs. I didn't get to removing the peel ply for three days and discovered that by this time it had become brittle as the epoxy had completely hardened within it. Peeling it off was akin to peeling a price sticker off a CD. What has been peeled off in this pic took in excess of one hour! My sense of humor wore thin to say the least so I will start all over again but will use this skin as a mold for the next. As a postscript to this disaster, 6 months later I found out that a well meaning friend bought a few meters of fabric that looked like peel ply from a haberdashery store when I had asked them to go to my fiberglass supply shop.
	I started on the new seat by marking out the plastic sheet for the beginning of a new layup.
	I cut a piece of carbon fiber to the size marked on the plastic, layed it on top and then squeegeed through the resin/hardener mix until it was wetted out. Peel ply was then layed on top and pressed by hand to remove any air bubbles and to make sure that it had absorbed any excess resin. Another piece of plastic was then layed on top of the peel ply to create a "sandwich". I then flipped over this sandwich and layed it on the mold to make sure everything was going to be fine.
	Looking at the photo above, there is no peel ply between the plastic and carbon fiber showing, so I carefully peeled off the plastic and layed down a piece of peel ply, then replacing the plastic. So what I have here is a "sandwich" of; plastic - peel ply - carbon fiber - peel ply - plastic. This layup is so easy to handle. While making room on the bench I just picked it up by the corners and draped it over the side of the aircraft. Between the plastic, nothing moves.
	I layed the sheet over the underside of the mold and used staples to hold it in place. The mold was then set in place over the spars and allowed to cure overnight.
	The next morning I peeled off the first layer of plastic and peel ply to reveal this beautiful looking piece of carbon fiber.
	I then just popped the seat off the mold and then removed the plastic and peel ply from the other side.
	The beginning of the new seat has gone quite well. Still a lot of work to do but I am happy so far.
	The vertical reinforcing structures that would normally be between the seat bottom and fuselage floor, I have decided to build into the seat itself. The first step is to get some 1/2 " square foam strips which I made a simple timber form to create.
	Pop some foam in, shave it level and pop it out.
	Here's a view of how the reinforcing will sit from front to back. There will also be some from side to side. As each of these miniature square beams will be covered with carbon fiber and connected to each other, the seat should end up being structurally very sound.
	Trying to wrap the wet carbon fiber around a small beam that has just been painted with microslurry is no easy task as it wants to unroll. So I have given up on that idea for the moment...
	...and will try to do it again when the microslurry has dried. So in the mean time I have set the beams to the shape I want on the seat and held them in place with some light weight whilst they dry.
	Here is one of the beams wrapped in CF which is held in place with pins. Soon after this shot I tried to wrap it all in peel ply. What a disaster! The whole thing wanted to unwrap itself. A valuable lesson was learned. Do not try to reinvent the wheel. There will be time to test and build a "better mouse trap" after my KR is flying. Every day wasted on these things is a day I am further way from flying.
	So it's back to KR seat Mk 3. This is similar to the first seat but for the yet to be shaped 2" thick piece of foam which will sit on top of the forward spar.
	When shaping the bolsters on first seat I managed to sand through to the expanding foam used as adhesive. Being harder than the green foam, it was a pain to sand. So I decided to carve out areas where I wanted the bolsters to go.
	I then cut 2" foam to size and slid it into the cutouts.
	The foam was then glued from behind with the expanding foam. I did learn that the expanding foam is very easy to sand about 10-15 minutes after mixing. At 20 minutes it is too hard again.
	This left no hard foam joints to sand through. After shaping the bolsters I used a very stiff micro to fill some minor gaps.
	The pre-molded seat skin has had peel ply layed on the back with a drizzle of resin to make it sit in place when turned up the right way. I have done this because there are tiny holes between the carbon fiber weave that will allow the resin to seep through a little. The CF has then had resin squeegeed over the top in readiness for the contoured foam which has had the reverse side painted in the microslurry.
	I placed the seat in its proper position in the KR and weighted the foam with anything I could find that had some weight in it including a bag of dog biscuits.
	The following day I lifted the seat out and covered it with a layer of runny micro.
	I then wet out a piece of CF on a plastic sheet and draped it over the seat, peeled off the plastic and squeegeed the CF into close contact with the foam.
	Peel ply was then layed on top to draw out the excess resin and then the seat placed back in position to cure overnight.
	The following morning I removed the peel ply to reveal the final form of the seat with small bolsters and slight sculpting in the lower back area. At this stage the seat is held in position by hooking the carbon fiber over the front and rear spars. Strengthening ribs will be added shortly to the underside.
	1" thick foam has been used to create some strengthening ribs. Each is a 2 piece affair which is glued together with expanding foam and then sanded to a depth of 1½ - 2".
	Each rib has been coated with a slurry before carbon fiber and peel ply is added to the sides only.
	These are then layed in position over the seat bottom which is protected by plastic sheeting. I do not want them to adhere to the seat yet.
	After about 2 hours the carbon fiber and resin has hardened enough that it has stuck to the foam but still soft enough to easily trim with a razor blade. The ribs are removed and all the overhanging carbon fiber trimmed off before putting them back into position to fully harden.
	When completely hardened I removed about 1/16" of foam from the underside of the ribs and a little more at the edge where the foam meets the carbon fiber. This was the filled with flox (resin and chopped cotton) which provides incredible strength when adhering the ribs to the seat bottom, which has a line of flox added in readiness.
	1" bi-directional carbon fiber tape was then wet out and used at the rib to seat junction to provide additional strength to the joint. Peel ply soaks up any excess resin.
	The CF tape creates aesthetically pleasing edges too.
	This side of the rib gets the same flox treatment as the other before covering with a layer of 3" uni-directional carbon fiber tape. The uni-directional tape has all the CF filaments running in one direction which in this case is length ways. This therefore means that all its strength is length ways too. The reason I have used this tape is that this side of the rib only needs uni tape and I've had a roll of it sitting around for ages anyway.
	As with all layups, I have used peel ply.
	The seat is almost finished but it still needs to be trimmed where it meets the fuselage and some carbon fiber tape added to the edges. I have sat on it in situ and bounced up and down and there is zero movement.
	I'll be having head rests built into my seat backs which will be split folding. I have made the the seat shoulders 2½" above the level of the parcel shelf. The head rests are shaped using whatever I could find laying around. In this case the plate did the job.
	Cut out and place in position to make sure it will fit, I'm pleased with the look.
	The carbon fiber was added to one side at a time and then sanded to the seat contour. The plastic sheet method was used but I lowered the foam onto the wetted out layer like putting a jigsaw piece in a puzzle.
	All of the edges get a layer of CF which have to be held in place with clamps until the resin cures.
	Where carbon fiber, or fiber glass for that matter, meets at a right angle , the corner has to be strengthened by removing a little foam in that corner and then filling the void with a stiff flox mix. The oxide color in this picture is the micro slurry.
	I fitted the seat back in the fuselage a found that it had a 1/4" twist which made it sit out at one end of the shelf. The solution was as easy as supporting it out in the sun and adding a weight to the corner. 10-15 minutes in the Aussie sun softened the resin enough to reshape the seat back. Perfect!
	I have decided for a number of reasons to not have a split hinged seat as originally planned. I don't think I would use the split folding seat in flight and the added seat back area above the parcel shelf would hit the fuselage walls if folded forward. I still wanted the ability to remove the entire seat back to gain access to the area behind. As there will be no hinge I have shaped these "monster teeth" out of foam and carbon fiber. They will be epoxied to the CF that overhangs the rear spar.
	Now that they are epoxied in place, the "monster teeth" also provide additional rigidity to the CF overhang.
	The seat is easily removed when I need to access the back.
	And it rests securely against the supports and parcel shelf.
	The first hole is cut for the control column which allows 1/2" gap all around at full deflection. The hole was cut with a jigsaw and the corners were radiused with a rat tail file.
	Both holes are now in the seat and have their edges covered with flox and then carbon fiber. Even though the the seat is hard, the curvature under the thighs and through the small of the back make it extremely comfortable. On the ground that is!
	This is some 1" x 1.5" x .065" rectangular tube left over from the column. I have cut it down the middle with the hand grinder and into four pieces which will become the seat belt anchors.
	These anchors are going to opened out to form "U" channels, so the best way to do it was heat them with oxy/acetylene and then just open with a pair of pliers.
	A piece of railway line clamped in the vise is a great makeshift anvil. More heating and a few good whacks with the hammer and the sides are now flat. Yes, that is a Henrob 2000 torch and my practice welds are coming along nicely. No photos yet.
	AN4 holes have been drilled so that there is exactly a 1" gap between them and the bottom of the channel. The sharp edges are cleaned up with a file and then they are given a good going over with a wire brush before two coats of gloss white rust inhibitor.
	The 1" gaps in the anchors is so that the AN4 bolts pass through the plywood webbing and not the spar cap. I had reservations about drilling 1/4" holes through 1" spar caps. As the anchors pass under the aft spar and are not just bolted to the webbing, the spar would have to break for them to pull out. I picked up a pair of seat belts off eBay for $70. They are out of a FedEx B767 and I reckon are a great buy.
	The retract is bolted to the very back of the parcel shelf and a swagged stainless steel cable will be run from it to the tailpost to transfer excessive forward forces should that ever happen.
	The outer lap belts fit nicely around the side of the seat bottom. Slots have been cut through the seat bottom for the inner belts. The slots have been lined with flox and carbon fiber and the edges have been sanded smooth so as to not cut into the seat belts.
	Because the seat belts have to go around the head rests as in the photo above, there is insufficient length in them to have the buckle to sit low in the lap. Slots have been cut through the seat back through which the belts will pass in a more direct line.
	The slots are lined with carbon fibre as I did on the seat bottom. I didn't want the shoulder belts to fray from sliding through the slots all the time so I picked up four seat belt bezels from an auto wreckers for $2. (Identical to my AU Falcon which is where I got the idea). I cut the tabs off, sanded them flat and scuffed the inside to give a better bonding surface for the flox.
	The result is exactly as I had hoped for. It is functional and keeps the area uncluttered.
	The swagged cable has been attached to some 6061-T6 plate. AN4-11A bolts pass through the plate, retract base, the 1/4" plywood shelf and a further 1/4" plywood bottom plate epoxied to the underside of the shelf and cross brace.
	Following discussions with fellow KR flier and builder Phil Matheson, I have changed the location of the cable security point away from the tailpost. The reason being that in an accident the tail may be torn off the fuselage. The secondary security point is now located at the cross bracing junction and longeron of the bay behind. The 1/4" plywood base is epoxied to the longeron, bracing and gusset blocks. The swagged cable attaches to the 6061-T6 extension plate that is then bolted to the plywood base.