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Builder Spotlight

May 2010 Issue

Cover Story: Building a Custom Cushion

The basics, plus some tips you may not have thought of that will make your flying more enjoyable.

Welcome back. This month we’ll take a look at a few basic upholstery terms, and make a custom foam seat cushion. Next time we’ll roust the classic Singer from storage and sew a stylish seat cover with it.

Let’s start this installment with an explanation of common upholstery terminology. Photo 1 shows an average aircraft seat without its cover. You can easily see the frame and foam. What you can’t see in this photo is the support beneath the foam. This seat has springs covered by canvas. Other seats, such as in some Mooneys, may have just a piece of sheet metal where this seat has springs. Many Experimental aircraft are similar to this in the sense that they have foam cushions sitting on sheet metal. On some aircraft seats the springs have been eliminated, and the canvas has just been stretched across the frame. When you design your seats it might be helpful to keep these three basic configurations in mind. Which one best suits your situation?

Photo 1: Without a cover. The letters designate features of the seat itself. Photo 2: Seat with seat cover. The letters identify parts of the seat cover.

Although there are many different seat designs, most seats have two basic components, a cushion and a backrest. Terminology is often based on context. When I say “cushion,” I usually mean the part of the seat that you put your seat on. You might ask, “Doesn’t the backrest have a cushion?” Well, the backrest has a foam pad, and you can call it a cushion if you want. Nevertheless, I’ll just call it a foam pad, and we’ll go with that to avoid confusion.

Photo 3: A bandsaw works great for cutting foam. If you have a steady hand, you can get by with an electric knife. This piece will be the core of the cushion.

Sum of the Parts

Each component may have individual features. Here we see that the backrest has a lumbar support, labeled A. The backrest and cushion have bolsters, labeled B. This seat has bolsters on each side of the seat, and they are symmetrical about the centerline of the seat. The cushion has a thigh support, labeled C.

Photo 2 is the same seat with its cover. Seat covers usually have four basic components: an insert, panels, boxing and back or bottom. The insert is the center part of the seat cover, labeled D. It often has quilt lines to make an appealing pattern. The panels are labeled E, and they attach to the insert. The boxing is labeled F, and it goes around the perimeter of the seat. Both the backrest and the cushion have boxing. Of course, there are practically endless variations on this basic configuration, but this gives us a good start on a common upholstery vocabulary.

The term “seat cover” has developed a negative connotation in the past. I’ve heard some people say, “I want new upholstery. I don’t want seat covers.” Apparently, they are aware of a common practice in which a new seat cover is installed over the old seat cover, without replacing any of the old padding. This is an inexpensive way to give a fresh look to an old seat, and was a common practice in used cars, which is how seat covers got a bad name.

Even though seat cover may be anathema to some, the term has a variety of meanings, so it’s wise to define it specifically any time its meaning is in question. To me it’s the part of the seat that covers the padding and frame. It does not necessarily mean a cheap substitute for proper repairs and new padding.

Photo 4: Wedges are cut for the thigh support. Photo 5: Splitting one foam block will make two bolsters. Photo 6: Simple lines provide guides for installing the thigh support and bolsters.

To Bolster, or Not?

I’ve noticed a trend in custom aircraft seat designs over the last 20 years or so. It seems that a lot of people want to design aircraft seats similar to the seats of sexy sports cars. Sports car seats have huge bolsters to keep passengers from sliding off the side of the seat when pulling lateral Gs. But most aircraft don’t pull the lateral Gs that Corvettes do. I think large bolsters look cool, but they are unnecessary and cause problems in many aircraft applications. They should be kept minimal most of the time, and they should not be parallel for the entire length of the seat.

I always like to flare out the bolsters at the shoulder on backrests and at the thigh support on cushions for two reasons. First, I prefer the way it looks compared to running the bolsters parallel. Second, and more importantly, it’s a lot more comfortable that way. Little details make a big difference, and two examples come to mind. I flew a Cherokee that had large side bolsters, about 3 inches high, and they ran parallel for the entire length of the backrest and cushion. At first the seat seemed comfortable enough, but about a half hour later I was uncomfortable and annoyed. The large bolsters kept my shoulders and knees pushed together. Having my knees pushed together required an awkward effort to keep my feet on the rudder pedals. Sometimes being able to reposition yourself even slightly prevents fatigue and increases comfort.

Photo 7: Stacking the wedges simplifies application of the glue. Photo 8: After spray glue is applied to both surfaces, the thigh support is installed. Photo 9: The inside of the bolster will follow these lines.

Another thing about bolsters that are parallel from the top of the backrest to the front of the cushion is that they look amateurish. Even though you’ll do this yourself, you want it to look like a top-notch professional did the job.

The other problem I’ve had with large bolsters, even though they were flared at the thigh support, was that I had no lateral movement in the seat. That was a problem because the seat was not exactly aligned with the yoke and rudder pedals. It didn’t take long for discomfort to set in, and my lower back was sore for the next week. If the seat would have allowed me to slide over just slightly, all of that pain could have been avoided. That’s why my seat designs have modest bolsters, and they flare out at the thigh support. The bolster provides some visual appeal without unnecessarily restricting your movement.

Photo 10: The bolster is the last piece of foam to be installed in the cushion. Be careful to stay on the line.

There’s one thing about sports car seats that I think is ideal for aircraft applications. Sports car seats are usually very firm. I like that. I hate seats with foam so soft that you bottom out, and it doesn’t provide any support. Every situation is different of course, but I usually prefer firm foam that’s contoured to provide a good seating position and lumbar support.

Do Dry Runs

I have learned the hard way that it is important for your long-term comfort and satisfaction that you take the time to do a complete mockup of the seat cushion and backrest. Small changes can make big differences in seating positions and clearances and, consequently, seat comfort.

I know a guy who spent a couple of days getting his seating position exactly how he wanted it, but he neglected to close the canopy during the process. When he finally did close the canopy, he discovered it hit his head. He had to lower the seat more than an inch to get the canopy closed, and an inch is a long way inside an aircraft—that was without a headset.

The moral of this story is to make sure you consider all of the variables, and design for them. The difference between a really comfortable seat and an uncomfortable one is in the details, and those details become evident after about an hour of seat time.

So go ahead and do a complete mockup. This will be time-consuming, and require that you get in and out of the aircraft several times. You’ll have to try several different seating configurations, and spend a lot of time just sitting there. However, if you make airplane noises and imagine soaring through the sky while you sit there, the process can actually be fun!

Photo 11: Trimming the bolster after it’s been installed is easier than trying to cut it to size beforehand.

Let’s Build It

Now that we have acquired some of the basic upholstery terms, and with seat design out of the way, let’s step through the process of building a seat cushion. We will begin with the foam. Foam comes is practically any size and thickness you might want. I started with a 24x72 sheet of 2-inch foam. For this project the 2-inch foam gives me the thickness I want for the cushion core, and allows me to fabricate the bolsters and thigh support without having to purchase other sizes. This saves money and produces less waste. I like to be efficient whenever possible.

As I mentioned, I like fairly firm foam. How firm is it? When you set this 2-inch piece of foam on a wooden bench and then sit on it, the foam feels soft, but you can just barely feel the wood beneath it. After the seat cover is installed, the cushion will feel a bit firmer, and you wouldn’t feel the wood at all.

I almost always use the same foam for every part of the seat padding. Some people like to use several different foams, but why? I suggest you let comfort be your guide. If you find yourself using really stiff foam so that a huge bolster will maintain its shape, you might want to rethink your design. However, when someone insists that the seat feel soft and be firm, I’ll use two different foams. That’s one situation when I’ll put a soft piece over a firm piece.

Photo 12: Use spray glue to attach canvas to the bottom for a finished look.

One of the most challenging parts of building foam pads is getting the shape you want. Of course it’s easy to get a rectangular block, but creating contours can give you fits. Here’s the trick to creating compound curves: Don’t try to shape one piece of foam into the final shape you want. Build it up using layers. Design the lower layers so that when you install a piece of foam on top of them, a smooth curve is created. Using this technique you can get nice, smooth shapes with only straight cuts.

As you can see from comparing Photos 1 and 2, the foam pads don’t have the exact same shape as the finished seat. The finished seat has nice, soft contours, and the foam pads have sharp edges and flat surfaces. The seat cover “softens” the hard edges of the foam to produce nice contours.

To begin this month’s project, I used a bandsaw to cut the required size block from the foam sheet, and then laid out the position of the bolsters and thigh support. There are not any absolute rules of design for this layout. I just try to keep it proportional and looking good.

Felt-tipped pens work best for marking foam. I prefer Sharpies over others because they have less of a tendency to smudge and transfer to other surfaces. It can be a real bummer to have a nice piece of beige leather ruined from marker transfer.

I want the thigh support to be modest, only 1 inch thick on one end, and tapered to zero at the other end. I don’t have a method to cut a single large piece of foam to those dimensions, so I’ll use the bandsaw to cut several small pieces and then use spray glue to glue them together. I want it to taper to zero because the seat-cover insert will be one piece, and this way it will provide a smooth transition.

Keep It Together

It can be difficult to apply spray glue to the edges of thin pieces of foam, especially pieces such as the wedges for the thigh support that taper to nothing. The trick here is to stack the wedges with their edges even so that they create a large surface area, apply glue to all of the edges at once, peel them apart, and then stick them together.

Photo 13: The marks on the edges will be used to make the seat cover, and that’s the next step.

I want the bolsters to be a little bit bigger than the thigh support, and I don’t want them to taper to zero. I want them to taper to about a half inch, because the seat-cover panels will join the seat-cover insert, and having a “step” in the foam structure at this point will help provide a smooth transition from the panel to the insert by helping to hide the seam allowance.

I used spray glue to attach the bolsters, and bent them to follow the layout line. This technique is much easier than trying to make compound cuts, and I’ll trim off the excess with the bandsaw.

I installed canvas on the bottom of the foam for a couple of reasons; however, it isn’t really necessary. Actually, when it comes to really necessary, a lot of the features that went into making this cushion were based on criteria such as my personal preference for ease of construction or aesthetic appeal. Foam, fabric and vinyl probably aren’t really needed at all. I’ve seen a milk crate substituted for a seat many times. But I digress…

The canvas on the bottom provides a finished look to the completed cushion, and it provides a better surface to attach the seat cover to, compared to the raw foam. Another advantage of attaching the cover to the canvas as opposed to attaching it directly to the foam is that the cover can be removed with less damage to the foam.

The canvas bottom also gives you the option to sew the seat cover to the cushion rather than glue it, if you want to practice your hand-sewing technique. This cushion is now ready for its seat cover, even though I may not be.

The hash marks on the perimeter edge and where the panels meet the insert will be used as alignment marks when the seat cover is assembled. I usually make patterns out of unbleached muslin or other such lightweight, inexpensive fabric. I’ll transfer the alignment marks from the foam to the pattern, and then transfer them from the pattern to the seat-cover parts.

I realize that you’ll probably never build a cushion just like this one, but the techniques used here have universal application. I think you could use them to build a seat for any purpose you might have.

Perhaps the best part of doing it yourself is obtaining a custom fit and style. You can mix and match features to create an aircraft interior that is uniquely your own. Whether it’s strictly utilitarian or a work of art, either way it’s fun and satisfying. If you have any questions, let me know. I hope to see you next month for construction of the seat cover.

Mike Manning has been doing upholstery work for more than 30 years. He’s a private pilot, SEL and glider, and he’s instrument rated with all three launch endorsements for his glider ticket.

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