Ken Rand's Styrofoam Airplane

By Jack Cox

One of the most talked about homebuilts at Oshkosh '72 was Ken Rand's tiny KR-1.

If you stood at the fringes of the crowds around the plane and listened to the chatter, it was usually a toss up as to which was more discussed -- the aircraft itself, or its method of construction.

The KR-1 is so small that some EAAers seemed to be a little leery of it. They would give it a sideways look and break out with a "you're putting me on" grin.

It took some convincing to get a few folks out of the notion that this was a large RC model or, at least, some over-indulgent father's taxiing toy for his kid.

Others sauntered up, read the name on the "Walk Me" sign and grunted something like, "So this is that Styrofoam airplane I've heard so much about."

No doubt about it, the KR-1 is small.

It has a span of 17 feet 2 inches, the length is 12 feet 6 inches, and it only weighs 310 pounds...almost half of which is the VW engine and prop!

The KR-1 is the creation of Ken Rand, EAA 30184, Huntington Beach, California and EAA Chapter 92. Back in 1968 Ken and Stuart Robinson, EAA 71345, started two homebuilts based about equally on the Taylor Monoplane and their control line model airplanes.

The fuselage is the familiar plywood and spruce box. Two ladder-type sides are built up of 5/8" X 5/8" spruce stock and are covered with 3/32" plywood from the firewall to just aft of the cockpit with 1/16" used from there to the rudder post...just like most wooden airplanes from the early de Havillands to the Volksplane, not to mention the balsa "stick" models us over-30 fogeys used to build in our misspent youth.

The "backbone" of the airplane is a 5 foot 5 inch center section -- actually the main and secondary spar carry throughs -- which ties the fuselage sides together, serves as the support for the seat bottom, the support for the retractable landing gear and its retract/locking mechanism, attach point for the side mounted stick and trim control, and, of course, the mounting points for the outer wing panels.

Up to this point the KR-1 is pretty conventional - but from here on out... wow!

Up front, the little 1200 cc VW engine has had just a light lick or two at modification for aircraft use. The crank has a 30 taper to match the tapered hub, the rear main bearing has an oil groove added, the oil breather line is relocated, thin wall exhaust stacks replace the auto equipment, a Revmaster injector carburetor is fitted, and the Wolfsburg ignition is replaced by a belt driven single magneto. Figure 2 shows the ultra-simple extrusion/Lord mount engine mount setup. Ken picked up his engine in a junkyard and has a ridiculously small amount of out-of-the-pocket cash invested forward of the firewall.

The landing gear can be best understood by studying Figure 2. A single handle pivots the whole assembly through a fore and aft are of about 900 and a couple of spring loaded latches with detents lock the gear in the "up" or "down" position. To retract, the wheels move straight back and up into wells in the center section leaving about 1 1/4" of the 8-inch go-cart wheels exposed -- like the early, conventional geared Bellancas. The only shock absorption comes from the tires, whatever flexing the horizontal gear assembly has from the pivot points outward, and the seat cushion.

The brakes? -- ever built a soap box racer? - or handled a two-horse wagon? That's right, the KR-1 brakes are simply tire scrubbers that are intended for differential ground steering and a little braking on the landing roll. Hydraulic go-cart brakes can be used, if desired.

The landing gear legs, including the wheels, are only about 17 inches long, which means the leading edge of the wing is about that high off the ground and the trailing edge is literally brushing the grass. The tail wheel is a dolly caster bolted to a length of auto leaf spring. This extremely low-to-the-ground stance is one of the striking aspects of the appearance of the KR-1. This tiny tiger should REALLY reap the benefits of ground effect on both take-off and landing.

Now comes the actual gee-whiz part. (For the younger generation, "gee-whiz" is an anachronistic exclamation picked up by your fathers in their formative years from reading decadent comic strips such as "Batman", "Tailspin Tommy", etc., when they were supposed to be doing their homework! It roughly translates as "heavy"... I think.)

The engine cowl, fuel tank, fuselage turtle deck, vertical and horizontal tail surfaces, and outer wing panels are completely or largely constructed of polystyrene foam!

The actual process goes something like this: slabs of polystyrene are glued in place, are trimmed and sanded to the desired profile, and have a layer of Dynel cloth epoxied on to form an amazingly tough and, when sanded, smooth exterior.

The whole thing takes a little getting used to. The vertical fin, for instance, is merely two upright wood spars with a profile rib at the top and bottom. The rest is PS foam -- including the leading edge! The rudder and elevator are even simpler: there is a leading edge wood spar plus a rib at each end and the rest is foam and Dynel -- again, including the trailing edges which are real knife edges.

The turtle deck will shake you up, also. It was built up by gluing on the PS foam slabs and sanding to shape -- with no bulkheads -- no bracing of any type. Before the Dynel and epoxy application you could have smashed the whole thing to bits with one half-hearted swipe of your hand; but after the Dynel covering had cured, Ken proved its strength to a slightly incredulous FAA inspector by standing on it!

The KR-1 is a convertible of sorts. When the foam blocks are glued on to form the turtle back, one has the option of an open cockpit, a full bubble canopy, or a "razorback" configuration . As the accompanying photographs show, a swing-over hatch is needed, even for the open cockpit version. A bubble canopy is bonded in the hatch, if that is the preference. One could, of course, build two hatches, one with the bubble and the other with the open cockpit and change them with the seasons .. a convertible, as we said.

In the area between the instrument panel (also of PS foam) and the firewall, an integral 7 1/2 gallon fuel tank is built in ... of PS foam/Dynel/epoxy, naturally.

The tight fitting cowling is formed around the VW engine by simply gluing the blocks of foam to the engine, shaping, etc....then the builder saws it off, splits it where necessary, bonds in fasteners and snaps it back in place.

Would you believe even the spinner is made with foam/Dynel? Ken sawed out a circular piece of wood, glued foam blocks to it, put the whole thing in a lathe and turned it to the shape he wanted -- then laid on the Dynel. Sanding, cutting out the prop blade holes and drilling a center retaining screw hole completed the job.

It's kinds like the old joke about the meat packer who used every part of the pig except the squeal!

The feature that should be understood is that the PS foam has little to do with the strength of the finished process. It is merely a filler and, most important, a built-in mold or form for the final shape of the layer of Dynel and epoxy. This outer shell is incredibly light and strong. This writer personally observed Ken Rand walking up and down the wing of the KR-1, and at the risk of a repair bill and/or a fat lip, I took the thin trailing edge of the elevator between two fingers and tried hard to deform it in any direction. No soap ... it is really tough!

The wing of the KR-1 is the mind boggler. It is composed of two 60-inch built-up wooden spars with a rib at the inboard and outboard ends. Two foam ribs are installed at the Y3 and 2/3 positions between the end ribs -- for support and shape only. A thick plank is glued on to form the leading edge and the remainder of the wing is planked with one inch thick slabs of PS foam, sanded to shape and covered with the Dynel and epoxy.

You noticed, didn't you, that there are no compression ribs, no drag and antidrag wires, no interior structure, whatever -- in other words, the Dynel skin is a structural member. But, one more time, don't think the wing is flimsy. As previously mentioned, Ken walks around on it...would you allow that on your spam can, termite nest, or moth's paradise?

The ailerons are simply sawed out of the wing and are reinserted in the same space, attached to piano hinges that have been bonded in the wells. A spruce strip is installed in the leading edge of the aileron for mounting the hinge.

All in all, the KR-1 is certainly one of the most highly innovative homebuilts to ever make an appearance at an EAA Fly-In. Many look at the simple, light, and inexpensive PS foam/Dynel/epoxy sandwich construction as the "break-through" that has been needed in the homebuilt world. With it there is no longer any need for our birds to be boxy; the compound curve is no longer a bug-a-boo; and glass-like finishes previously seen only on expensive European sailplanes is within the reach of every homebuilder.

The use of this material and technique has already spread to other areas. Wil Neubert's excellent "Stardusters International" newsletter recently had an article by Al Hooper which went into detail on the use of PS foam and Dynel to make fairings, fillets, and wingtips for the Starduster Too. As always, the more minds and hands we have at work, the more innovations -- like, Al has already learned that the heated glue guns are great for sticking the foam boards together and that the Stanley Surfoam plane does a good job in shaping the material -- Neubert uses an electric carving knife. And so it goes. We are sure to see widespread use of this material and construction technique in the years to come - perhaps more for styling and streamlining than for primary and secondary structures, however.

Many of you probably want some more specific information on the material Ken used on his KR-1, so:

FOAM -- Two pound per cubic foot rigid polystyrene -- the kind normally used for building insulation. It comes in 7" x 9" x9' boards that cost around $8.00 each. See your local home building supply house.

DYNEL -- "Dynel" is a synthetic fiber made by Union Carbide. It is used on the KR-1 in much the same way as fiberglass cloth is used to make cowlings, etc. on other planes. Dynel has the advantage of being four times as strong as glass, but only half as heavy. The cloth is very soft and can be stretched to fit almost any shape. 4 ounce per square yard Dynel is used on the KR-1.
EPOXY -- epoxy resin is used on the KR-1. Polyester resin can also be used with Dynel cloth in other applications, but will dissolve polystyrene foam, so is a no-no in this instance. Of interest is the fact that the Dynel cloth is fitted over the PS foam and the epoxy resin is squeegeed into the fabric -- you do not apply the resin first as with fiberglass.
GLUE -- White glue, Aerolite, etc. -- plus the hot glue guns can be used to stick the planks of foam together prior to shaping and applying the Dynel/epoxy. This is not critical -- just enough strength to hold the foam in place is all that matters. Be careful that whatever glue you use does not have a solvent that dissolves the foam.

Now, back to the airplane. How does it fly? ...don't know, because like the PL-4, ED-5 and a couple of others, the KR-1 did not have its restrictions flown off prior to coming to Oshkosh, so was a static display for the week. Ken said the little bird had about 17 hours on it prior to being trailered to Wisconsin. He reports his creation is easy to fly and has very good performance on such a tiny engine (36 hp). With a 50 x 38 prop the take-off run is only about 150 feet and the climb rate is 650 fpm. Cruise speed is 130 mph and the thick RAF 48 airfoil stalls at a low 42 mph.

Like a lot of the unusually small homebuilts, and, particularly, since this one is of unconventional construction, the KR-1 is not without its critics. The U-control model airplane bell cranks used in the trim tab system jolted a few people. Ken, a Douglas design engineer for many years, says the loads on the tabs are so light that the model parts are adequate. Others have looked askance at the aluminum hinges for the rudder and elevator. While strong enough, they undoubtedly will need frequent inspection for signs of excess wear. There are those who wonder about extensive use of PS foam but perhaps the best evidence of the strength of the KR-1 is the fact that in an early test flight, Ken had a power failure and had to land in an unimproved field. The little landing gear collapsed, but the airframe, foam and all, came through like it was made of cast iron. Finally, the size of the KR-1 will necessarily limit it to persons of average size...but that can be said of a lot of homebuilts.

In summary, we think Ken Rand's KR-1 was one of the really significant homebuilts at Oshkosh '72. The airplane itself is already being built in great numbers ... the $500 total cost of the prototype and the prospect of a short construction period are highly motivating! Over the long haul, however, we believe Ken's contribution to the homebuilt movement will be greater for his PS foam/Dynel/epoxy process than for his airplane. A great number of new designs are sure the spring up utilizing the "Rand Method" -- and that's bigger than any single machine, no matter how good it is.