Seat Construction

I was trying to figure out what would be the most comfortable seat position for a pilot in a reclined position.  My Goal was to reduce the height of the fuselage to reduce the projected frontal area. The other problem was the carry through of the main wing spar. I was aided in this problem because I decided early on to place the spar at 33%, this would place the spar aft of the center of mass of the pilot. In the design of an ultralight, because the Pilot weighs as much as the airplane (At least in my case!!) its a good idea to place the pilot in the center of the CG range of the plane so as to avoid the need for ballast.. One day I was reading my latest addition of Soaring magazine and I saw an ad for Jantar Sailplanes. In the ad they were boasting that their seat was the most comfortable in the industry. They had a inboard side profile of the cockpit and seat. Well if it is good enough for Jantar, then it was good enough for me!!  I scanned the ad and did an underlay trace on my Cad System (Rhino 3D) and I then had the seat curve. I orientated it to clear the spar and place the pilot on CG and then designed the structure around it.



Seat back up structure being installed.



The seat back up structure is designed to the bottom contour of the seat. One of the design criteria I adopted was to ensure that the seat did not deflect under max G loading such that it interfered with the aileron control torque tube that runs under the seat itself. In this picture temporary jigging is fixed to align the 3 seat supports. Most of the secondary seat support structure is made from Spruce Scrap and or Birch plywood. The contours and shapes are laid out from dimensioned drawings on the plans. For ease of build in areas like this, I plan on having laser cut card board templates made.



Seat center support intercostals

 There are two parallel seat support intercostals that not only serve to keep the pilots behind from deflecting the seat and jamming the controls, but these also provide the mounting provisions for the aileron torque tube. The Intercostals are built up of 1/32nd plywood and 1/2" spruce square stock. All glue joints used T-88.









seat supports completely installed

 The center intercostals were mounted to the fwd face of the main wing box. External stiffeners tie the intercostals to the wing box, like wise on the landing gear frame. In the final plans I will clean up this area a bit. I had to add plywood fillers that were not planned for originally. I will modify the gussets of the landing gear frame to extend them such that they act as fillers. I will also redesign the truss members to eliminate some redundancy . The intercostal stiffeners could become a part of the wing box internal truss, likewise for the landing gear frame. It will probably save 5 to 6 ounces.

One of the reasons I love to design ultralight airplanes, is the discipline it gives you in the area of light weight and tight design. I remember when I hired on at Beech Craft in Wichita, I was making a bit of a name for myself locally with my Wren Design, and I started receiving calls with job offers. When I was interviewing with the various engineering groups, the guy running the design group for the Starbarge (Star Ship) looked down his nose at me and said, "we don't hire Ultralight designers on the Starship" That was a fact!!!  It was 5000 lbs over weight and out of the class. Beech petitioned the FAA for a special category so the lead sled didn't have to meet Airline design criteria. I guess they had all the "Ultraweight Designers" they needed. I ended up doing manufacturing R&D, but that's another really good story!!!


another view of the seat supports.

 a short intercostal that spans the gap between the center intercostals will be added later to support the torque tube. There is a cutout for a Delrin Shoulder bushing.

Seat Flat pattern being trial fitted

I did not want to actually mold the seat. So I hit on the idea of laying out a seat flat pattern from 4.5 oz PVC foam . I pieced together the cores bonding them with a mixture of Micro balloon and Epoxy. The actual pattern was taped together Poster board cardboard. I have seen this forming technique used to make ECS ducting on commercial private jets. I filed this technique away for future use. The future is NOW!

Only one side of the foam is actually laid up, and that is the surface that will be in tension. Now the seat curve reverses and because of this, you do not lay a layer of glass the full length on the bottom surface. The glass stops where the panel curvature reverse. The seat is really nothing but a wide thin beam. The fiberglass laminates on the faces alternately resist tension and compression forces. If only one surface (Tension side ) is laid up and the seat is deflected, the foam itself experiences compression. And since the foam is relatively weak and is of low modulus (Stiffness) it readily forms to the curve of the underlying seat structure. This is why its important that all of the seat structure be in align and level.


Seat being pushed into position.

 In this picture the seat is pushed to the support structure. After a trial fit, I removed the seat and taped up all of the seat support structure and the side truss. The seat was then put back into position and hot glue was used to temporally hot the seat into position.

seat being held while the hot glue cools.

The front of the seat lower surface was left bare and not laminated. I used 1 single ply of 120 style glass for the bottom surface. Since this surface was in tension, there was no need to have a thicker laminate because thin section stability is not an issue . I have experimented quite a bit with foam and glass construction, its against intuitive thought, but the lightest laminates I have made were with 4.5 lb/cu/ft PVC foam. PVC foam is a closed cell foam, this is important because wet resin will not continue to wick into the core, and in the process draw in air. This of course will add extra weight. Laminates over foam usually start with a slurry coat of micro balloon and resin, usually Epoxy, that is trowelled onto the surface. The idea here is to fill the exposed cells with a lighter weight material than the raw resin. When the foam is cut at the factory, the cut cells are exposed, the lighter the foam, the larger the cells. I have found that 3 lb density foam has too large a cell structure, this is why the 4.5 lb density is lighter.



Another view of the fwd edge of the seat


 I elected to form the fwd reverse curve with a heat gun. I will not do it this way in the future, I would tape the lower surface with aluminum duct tape and then hot glue the foam to the supports.

once the seat core is formed into position, a single ply of 8 oz 285 weave style glass was laid up. You could also use a  8 oz plain weave because of the absence of compound curvature. This would be a little cheaper.



Seat after final trim

This is the seat after it cures and the edges are trimmed. The edge treatment is to lightly sand into the exposed foam and a mixture of Micro balloon and resin mixed to a consistency of bread dough is pushed into the edge. After it cures the edges as sanded smooth. The final seat weighs 15 oz. The seat needs to be removable to gain access to the wing attach pins, so it is held in place with twing strips of Velcro tape