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Carving Foam Seats
By Tony Bingelis (originally published in EAA Sport Aviation, August 1990)
WHAT are you going to do about the seats? Have them made? Make them yourself? Maybe find a couple of salvage seats from an old Cessna, or maybe an American Yankee, and have them refurbished?
It really depends on the cockpit design of the airplane you are building, doesn't it? In some homebuilts it is possible to install a standard ready-made seat and simply bolt it to the floor and the aircraft structure. On the other hand, the structure may be so designed that adding a separate seat would raise the pilot so high his head would not clear the canopy.
Seats For Canards And Other Composites
Many of the low profile, fast composites with limited headroom have built-in semi-reclining seats for this reason. These are sometimes termed as "astronaut couches". All they need in the way of seat upholstery is a one-piece foam cushion, covered in the fabric of your choice. Very little or no carving of the foam padding is required.
Traditional Steel Tube And Wood Designs
Other types of aircraft featuring steel tube construction generally have a welded frame seat to which a plywood slab seat and seat back is secured.
This practice of using a plywood seat is also typical of many conventional wood designs. The result is disgraceful.
It is no wonder that this type of seat is considered to be aviation's worst. It seems that no matter how much foam padding overlays that plywood slab, you will still bottom out against that hard unyielding surface.
Of course, there is a limit to how thick a foam cushion you can sit on without your head protruding even further into the full blast of the slipstream, or worse, banging against the canopy.
A far better alternative would be the installation of a surplus seat from some expired store-bought airplane. Used seats can usually be acquired at a reasonable price, and are generally adaptable to most conventional homebuilt designs.
Since the seat fabric will probably be in deplorable condition, you will undoubtedly want to strip the seat of its old fabric and have it recovered.
Before doing that, I would suggest adding additional foam to re-contour the seat to a more comfortable shape.
Seats In Low Wing Designs
Low wing designs have the large main wing spar passing through the cockpit. In effect, you sit on
the spar, or more accurately, between it and the rear spar. While this is quite reassuring structurally, it can complicate the fabrication and installation of suitable ready-made seats.
I guess my RV-6 is fairly representative of the low wing types. It is also unique in that it presented an excellent opportunity for designing and carving my own seats.
The RV designs feature standard metal seat backs which are hinged at the bottom on piano hinges. This allows the seat back to be tilted forward for easy access to the huge baggage compartment.
Because the seat back is hinged, I figured a two piece seat design would make the easiest and most practical installation.
Since headroom was not a problem in my RV-6 (I'm 5' 11 "), I was able to build up a nice deep foam seat base.
By contouring that deep foam seat base to fit my body, the results hold great promise for sitting comfort rarely experienced in other airplanes.
Seat comfort depends considerably on the elimination of the localized pressures that a slab seat imposes on the pelvic bone. This concentrated weight bearing against your pelvic bone soon develops into a full fledged pain after about an hour of flight.
Carve your foam seat curves as shown and you will just about eliminate those concentrated pressures that numb your butt.
I'm sure you realize that a fancy upholstery job, including leatherette trim, inserts, beading and the like, requires skills and equipment most of us don't have.
Doing that kind of work also requires the skillful manipulation of a heavy duty sewing machine. For this reason, you, like many other builders, will probably opt to have your upholstery work completed by an auto/boat trim shop.
You will be more assured of a good fit if you size and fabricate the foam seat cushions yourself. On the other hand, if you expect the upholsterer to design and fabricate the seats before he covers them, you can expect your bill to be much higher.
What Foam To Use
Most upholsterers and auto trim shops use polyurethane (polyfoam) foam. Since it is readily available and rather inexpensive, you know it has some drawbacks.
The biggest drawback is that the stuff supports combustion. That is, it will burn when you put a lighted match to it. However, in spite of that shortcoming, most homebuilders are using it in their seats. To minimize this deficiency you could have your seats covered with one of the fire resistant aircraft fabrics like Nomex or Vonar.
Temperfoam is another alternative for those who feel strongly about fire resistant materials. In addition to being a remarkable seat cushion material, it is fire resistant.
The aircraft grade of Temperfoam is best known for its impressive impact resistance in the event of a crash.
Yes, it does have a drawback. It is very expensive. A piece of 3" foam, the size of a seat cushion, will cost over $50.
Slightly less expensive is a sister product to Temperfoam. It is called Sunmate. Besides costing about 1/3 less than Temperfoam, it is about 15% lighter in weight. Like Temperfoam, Sunmate will not support combustion.
I found that the local foam shops and upholstery supply stores don't go in for specifications and for fancy terminology.
They gave me a choice. I could buy a block of soft polyfoam, a medium foam, or a hard foam. The foam slabs were about 2' x 5' in size. The 2" thick and the 3" thick foams appear to be the most commonly used thicknesses.
Sit on a slab of 3" hard foam and you will not sink down into it very much. The soft foam, on the other hand, compresses almost completely and offers very little resistance to body weight.
A combination of hard foam bottom layer and a medium density middle layer topped by a soft foam layer makes a good comfortable combination. However, your body weight and individual preference may dictate a different arrangement.
For my seats, I opted to use the hard density foam for all the layers. I wanted to raise my seat as high as possible, consistent with maintaining adequate headroom. This would provide me with a better over-the-nose visibility in the taildragger, and minimize the need for zigzag taxiing. After all, I don't want spectators to think I can't steer straight.
If you, too, are building a RV-6, you can use the layout information shown in Figure 2. Some of you may not require such a deep seat base. In that case, the bottom layer of foam may be eliminated or the foam layer thicknesses changed to suit your needs.
Take note of the two most important features of a contoured seat:
1. The seat bottom should be more or less concaved as shown, and
2. The seat back cushion should be bulged in the general area of the lower third of the seat back to provide support for your lower back. This ensures greater comfort during all flights.
Dimension the foam for each of the seat bottoms and the seat back layers using a carpenter's square, and a marking pen for drawing the cutting lines.
The next step is to cut each of the foam laminates to the correct size and shape.
Cutting And Carving The Foam
If you have a bandsaw, use it to cut each foam slab to the correct dimensions. Incidentally, you will find the wedge shaped side strips (identified as "A" in the Figure 2 drawing) very easy to cut on a bandsaw that has a tilt table.
If you do not have a bandsaw, you can still do a pretty good job using an electric carving knife, a handsaw, or a hand held hacksaw blade. It will, however, take much longer.
The foam is very easy to cut . . . especially the hard denser foam layers.
You will notice that the foam layers of the seat bottom must be cut to the angles shown to match those of the aircraft structure and the cockpit floor slope aft of the spar. This works out quite well because the assembled shape of the foam layers will serve to immobilize the seat so effectively that a separate frame is unnecessary.
Notice the matching angles cut between the seat bottom and the seat back. When installed in the aircraft, these matched bevels automatically entraps the seat back so that no other restraint is needed to keep it from sliding around.
Note: After the seats have been upholstered, you might want to install a non-mechanical fastener to restrain the top end of the seat back.
Next, you can draw the curved lines to which the top layers of foam must be carved. Draw these contours on both sides of the foam block.
Carving The Seat Contours
Use whatever cutting tool will work for you.
I have used an electric carving knife, a hand held hacksaw blade, a small Japanese plywood saw (it cuts on the pull stroke rather than on the push), a single edged razor blade, and even an electric drill fitted with a sanding disc.
The sanding disc is virtually useless on soft foam as it tends to grab and pull the foam in the direction of rotation.
A sharp long blade kitchen knife sometimes seems to work as well as the electric carving knife.
Don't worry too much about precise shaping. The imperfections will not show under the upholstery material.
Since the average seat is about 15" wide, it is difficult to cut evenly across the full width of the foam. But it can be done.
To make the carving operation more manageable, try taping the foam block to your work table with duct tape. That should immobilize the foam and make it easier for you to use both hands for the carving exercise (just think, the experience you gain here will enable you to carve your next turkey like a pro).
After completing the carving operation you should, if you haven't already done so, cut out the 4 wedge shaped strips identified as "A" in Figure 2. These strips are to be glued along both sides of the seat bottom and seat back. They give a better shape to the seat and at the same time contribute to the comfort and good looks of the seat.
Next, you assemble the foam seat blocks into a single unit using an adhesive that won't dissolve the foam. Do the same with the seat back assembly.
A hot glue gun works quite well after you work out the correct technique for its use. Since the hot glue sets so quickly (5 minutes or less), it is very difficult to spread the glue over a large area and join the two pieces together before the glue hardens. The best way to use the hot glue gun for the foam assembly is as follows:
1. Stack the two foam layers to be glued, one on top of the other, correctly aligned. Then, slip the hot glue gun's nozzle between the two layers and squeeze out the glue while moving the gun along the joint. Keep the nozzle well inside the edges. All you need is a single glue line all around the perimeter of the foam layers. Hold the edges closed for a few minutes and the job is done.
But be careful you don't get any glue on the outside edges as it cannot be sanded away. Instead, you will have to snip out the offending lumps with scissors.
You might prefer using 3M's spray trim adhesives. Simply spray it on both pieces of foam and assemble it. Be careful to get the edges of the foam blocks aligned correctly as you go because you won't be able to slide the pieces around . . . the adhesive grabs that quickly.
Except for beveling the ends of the triangular wedges, that completes the seat assembly.
Before you charge off to the upholstery shop with your seats, I suggest that you first install the foam seats in your airplane and get a little cockpit sitting time. Try sitting in the seat for at least 30 minutes. The time won't be wasted because you can easily spend that much time becoming familiar with your cockpit and instrument layout.
If you become aware of some uncomfortable pressure point on the seat bottom or back, reshape that area to better distribute your body weight.
I think even if you stay with the approximate curves shown, you will be pleasantly surprised with the comfort of your new seats.