ON THE USE .OF POLYURETHANE FOAM By Stan Hall Chairman, SSA Research Committee
P is gaining increasing popularity among sailplane deOLYURETHANE FOAM, of the foam-in-place variety,
signers and builders. In practice one mixes two inter-acting liquids together and pours them into the cavity to be filled. In a few seconds the mixture turns to foam, expanding some 30 to 35 times its original volume in the process. In 10 minutes the material is rigid and can be formed with wood working tools. Polyurethane foam is being used for such sailplane applications as stabilizing the ply on D-tube spars (to prevent thinner plys from eventually sinking between the ribs), for adding torsional stiffness to long and thin plywood or metal-covered control surfaces such as ailerons and flaps, for contouring flat gore-sided fuselages, and for making fillets. Some Cherokee II builders have found yet another application; they fill the whole wing forward of the rear
spar with foam and fiberglas over it. Since the Cherokee takes no torsional loads through its leading edge this technique is a natural. I am using it on my new bird, Ibex. The resulting surface is superb and long lasting, although not quite as resistant to the pressure of the thumb, for example, as a plywood surface. One can permanently indent it without too much trouble. Interesting and productive as the use of polyurethane foam is, it brings with it some problems, problems which can be side-stepped if you know they are there. Here are some I discovered, along with the help of Jack Lininger, who put me on to polyurethane and helped me understand its vagaries. One of the more important problems is that the foam will continue to expand long after you think it has stopped and this can, under the right (or wrong) conditions, be disastrous. I must re-skin the empennage on Ibex on this account. The photo in Fig. 1 will show you why. I made the mistake of building the surfaces first and pouring the polyurethane in afterwards. Bad show. The instructions on the cans warn about the pressure this material can build up in foaming. I chose to slight this warning because, after all, foam isn't very strong, is it? My attitude seemed to find support in what turned out to be poor advice on the part of a self-styled expert (not Jack). He told me that foam, in encountering an obstacle, merely increases its own density as it expands, and that my surface wouldn't swell. As a potential safeguard against this advice, however (I was concerned at least somewhat), I pumped the liquid a little at a time through a steel tube which I inserted into the surface from one end, allowing a half hour or so between charges. I was delighted with the results — for about a month. Then the surfaces started to bulge, showing large, unsightly bumps, with every rib showing like a spavined horse in reverse. As a result I had to re-skin the surfaces. I later heard about a chap who, a few years ago, decided to stiffen up a metal sailplane wing by filling it with polyurethane. As the story goes, he up-ended the wing, tip down, and poured the liquid directly into it through a hole in the root rib. Not only did the wing swell up, it even popped a number of rivets! *
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*
When I built the ailerons and flaps for Ibex I tried a different approach, one which worked. (See Fig. 2). I built a lower-surface jig for the surface on a large plywood sheet and tacked the skin to it. I glued the spar
and ribs to the skin and then, with the whole surface open, I poured in the foam. Later I glued on the top skin. After six months the surfaces are perfect. I did the foam-
ing outside where the hot sun could assist the process of expansion. I let it set for a day and then sanded the foam down to the ribs with a long straight 2 x 4 to which sandpaper had been rubber-cemented. After this I forced the foam to swell even further by applying heat with lamps.
Hgure '2—Top to bottom, shows three steps in filling a
I maintained a good El Mirage 130 deg. F. for 15 minutes per foot. And swell it did. In fact, I got some nice big, golf ball sized bumps which I went gleefully after with my sanding board. Imagine my dismay when I sanded right into big, hollow bubbles, each complete with crater. Obviously I had gotten the foam too hot. I should have backed the heat lamps away a little and kept them on the foam longer, allowing a more gradual swelling. My newest tech-
typical small control surface with polyurethane foam.
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POLYURETHANE FOAM . . .
(Continued from preceding page)
nique is to cook the foam until, when I place my ear about an inch from it, I can no longer hear the little popping noises it makes as it expands — and then I give it 10 minutes more. On the matter of the craters, I compounded the crime in the process of effecting repair; I should have refilled the cavity with more foam, but I decided to use a paste made of polyester resin and microballoons instead. When the filler hardened I started sanding it down flush. This turned out to be a very poor technique. First, as the sand-
ing started taking the filler down, small chunks would break loose and gouge bigger chunks out of the foam. Also, since the filler was harder than the foam around it, the foam would wear away, leaving the filler to stand above it no matter how much I sanded. All very discouraging. After a short nap to calm the beast within me I carved out the filler and did what I should have done in the first place — fill the cavity with foam. Incidently, one will always have the problem of the foam wearing away more quickly than the wooden rib or whatever is being used as a contour guide. After the surface is fiberglassed the ribs will stick up enough to be noticeable. One way to minimize this is to cut the foam with the lightest grade sandpaper practicable. If the ribs still show, nothing much can be done except to fill in after glassing with resin and microballoons. Invariably small surface voids will develop here and there in the foam, few ever larger than >/4 to % in. in diameter. If the surface is to be eventually glassed these voids should be filled with polyester resin and microballoons (if subsequent sanding is not required). Just mix the filler to a very heavy paste, fill the void and scrape off
the filler, flush with the foam. Smaller voids can be ignored since, if the surface is later glassed, the resin will leak through the cloth and fill them. Somewhat larger voids
can be ignored if plywood is used in lieu of fiberglas for
the final covering. Polyester resin and microballoons really make a very excellent mixture for filling and making shapes. Microballoons are made by Union Carbide and consist of microscopically small, incredibly light, hollow phenolic spheres. A gallon can of them weighs about a pound. One pours the resin and the catalyst into a container, mixes them well and stirs in the microballoons to the desired consistency. The trouble is that unless the mixture is extraordinarily thick it will run downhill and flow away from where it is needed — and it is almost impossible to find anything on a sailplane that lays exactly horizontally. Mixing a very thick paste is okay, and the more microballoons you use the lighter per unit volume, which is good too. (One can even heat the resin first to thin it out so that it will take more microballoons). However, the thicker (and hotter) you make it the quicker it sets up and in some cases you will have to be very fleet of foot.
Microballoons mixed into the resin until the mixture "tears" as you stir it will set up in less than 5 minutes, so you have to plan ahead.
Some marine supply houses sell a "thixotropic agent", which is a fine, white (and also very light) powder which one can mix into the resin to keep it from running downhill and still have a fairly low viscosity. One of its problems, though, is that it doesn't sand nearly as well as the microballoons, being in this regard more like straight resin.
But back to the foam. If you are foaming to a contour, as on a wing, you can conserve foam by constructing cardboard dams loosely over the ribs. This will force the foam to go where you want it. Since the foam will stick 42
DECEMBER 1966
Figure 1—Showings effects on plywood surface of expanding form.
to almost anything, you can save yourself the difficult
task of peeling the cardboard out of it by taping newspaper to the side of the cardboard coming in contact
with the foam. Simply sand the newspaper off afterwards as you contour the foam. Since in using newspaper I find myself wasting valuable time reading it after relieving it from the cardboard I have been trying another technique. It is not very good. I have been applying a paste wax directly to the cardbeard.
This relieves fine but the wax sticks to the foam, creating
a crust that is incredibly hard and difficult to sand. I may try wax paper. Failing in this, it's back to the girlies in the entertainment section of the Chronicle.
The mixing of polyurethane foam is very critical in terms of agitation, time, and mixture ratio. You have about 15 to 20 seconds before it starts coming up out of the container like thunder out of China. On the other hand, if you don't stir enough, big, hard and crusty bubbles will form instead of beautiful, consistent, small-cell foam. Sanding this mess is completely unproductive.
The mixture ratio has to be just right. Too much of one part will cause the foam to shrink after initial swell-
ing. Too much of the other will cause it to stay soft and on the rubbery side. In mixing the two liquids you will get
some on your hands. Alcohol and water is supposed to take it off. I just let it wear off. As for your clothes, you can plan on simply throwing them away. The instructions on the cans warn of the toxic hazards. You'd best believe them (I do, now) and do your foaming outside. A lot of ventilation or a vapor-filtering mask will help if you can't go outside. If you've never had a bad attack of asthma you can experience the symptoms by taking a casual whiff or two of the di-isocyanape and Freon 12 used in the foam. One of my friends says it feels
as though the foam were forming in your lungs instead of where you poured it. This is serious business. It is important to replace the lid on the can that contains the Freon 12 "blowing agent" immediately after a portion of the contents is used. This is the can that goes
"whoosh" every time you loosen the lid. Once the lid is cracked the material starts losing the blowing agent and,
as a consequence, its foaming power. After first use the deterioration can be postponed by storing that particular can in the refrigerator (not the deep freeze) when not scheduled for use in the next several days. Obviously, however, it has to warm up again before it can be used. I find it appropriate to set the cans out in the sun while I'm working. A small amount of heating of the cans seems to improve the quality of the foam. However, if it is heated too much the Freon 12 is driven off, thereby defeating the purpose. I know one chap who placed his cans in the oven
to heat them up. I don't know if the oven and the kitchen
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Feature Speaker II The 19661 ly-hi S OUR featured Fly-In speaker A this year, we could scarcely have done better than to present the dis-
tinguished Paul Garber, head curator and historian of the National Air Museum of the Smithsonian Institution. A veritable memory bank of aviation history, Mr. Garber shared a portion of this knowledge with the people who attended the Fly-In with his illustrated story on "The History of
Flight." Everyone, and some with their little children sound asleep on their laps, listened and watched intently into the late hours the films and slides depicting the growth of aviation from its inception to the present day,
narrated by Mr. Garber.
His was the kind of presentation that has great educational value in addition to being downright fascinating. His aviation background dates back to World War I when he served in the Army Air Service, and subsequently in the United States Air Mail Aviation Service. He came to the Smith-
sonian in 1920 Museum
as a "preparator" of
specimens,
principally
in
aeronautics, and advanced to his pres-
ent position. His service there has
been War as a reau
continuous except during World II when he served in the Navy Commander assigned to the Buof Aeronautics, Special Devices
Division.
Mr. Garber is a memer of the Early Birds of Aviation (having soloed in 1915 in a homebuilt glider), the Air Mail Pioneers, National Aeronautics Association, National Space Club and other organizations. At the National Air Museum, a wonderful
and valuable collection of aircraft, spacecraft, engines and accessories of flight has been acquired. He has been closely involved with the acquisition
of many of these famed objects, and has enjoyed learning about their origin, development and use.
Everyone is to be commended for
their courtesy and interest in Mr. Garber's presentation, and we are
Paul Garber of the National Air Museum, in his appearance as the feature speaker of the 1966 Fly-In.
sure that he returned to his duties in Washington with a lasting favor-
able impression of our Association and its people. ®
POLYURETHANE FOAM . . . (Continued from page 42)
are now full of foam, but the idea would make a fine movie. A quart kit of polyurethane foam in liquid form (two quarts total, one quart Part A, one quart Part B) will retail at around $8.50 and will yield 2Vfe cu. ft. of foam of 2 Ibs. per cu. ft. density. A gallon kit (two gallons total
liquid) will retail at about $29.00. The material I use is distributed nationally (to marine supply houses, mostly)
by Taylor and Art Plastics, Inc., Oakland, Calif. The smallest kit they package is a pint (one pint of each). A quart kit will, on the basis of 2.5 cu. ft. yield, produce 30 board feet of foam for a cost of about 28 cents per board foot. Compare this with 45 cents per board foot one often pays for polyurethane planks or blocks which, although superbly consistent in cell structure, are harder to use in most sailplane application than the foamin-place material. * * * Block styrofoam is less expensive but is attacked
by polyester resin. One needs to protect the foam against this attack by applying two or three thin coats of Weldwood
Synthetic Resin glue (available at any hardware store) or by coating with epoxy resin. Although I haven't experienced any trouble with the Weldwood technique I find that polyester resin doesn't bond well with it. And epoxy resin is expensive. Everything considered, if you plan to foam, I recommend polyurethane. Just stay alert as you
use it. You might as a last resort also read the instructions on the can. ®
jflemoriam CHARLES H. "FRED" ELLISON
It is with deep regret that we announce the untimely passing of Charles H. "Fred" Ellison, EAA 17738, of Hobbs, N.M., as a result of an aircraft accident. Fred received an award at the 1966 Rockford Fly-In for "promoting the EAA in the Hobbs, N.M.
area", and this was rightfully deserved. He organized Chapter 215 in the Hobbs area, and served as its first president, and was its current secretarytreasurer. At 27, he had much to offer the EAA movement and aviation itself with his spirit and drive. His Spezio project was reportedly exceptional in workmanship, reflecting his pride and skill and promise of greater accomplishments in his future years. We are all saddened by this loss, and we and aviation will miss him very much.
A NSWERS The following are the answers for the mystery aircraft
that are shown on page 13. (1) Fleet 1, a popular light biplane in 1929, powered by a 110 hp Warner; and (2) the Lincoln Standard biplane,
also powered with a 110 hp Warner.
SPORT AVIATION
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