Sikorsky S-38 Project

Walter Treadwell has tackled a variety of homebuilt projects over the years, but this may be his most challenging yet.


It’s probably safe to just come right out and say it: Walter Treadwell is not your typical homebuilder. In fact, he’s something of a legend around his local airport. And there is so much to his story. In the April 2008 issue of KITPLANES, the late Bob Fritz got a good leg up on Treadwell’s airplane-building history and the early construction of Treadwell’s latest project, a scratch-built, 55%-scale replica of a 1928 Sikorsky S-38 amphibian. At that time, Treadwell was well on his way to turning the plans he’d created in 2003 into reality in his small T hangar at Livermore, California’s airport (KLVK).

How many times do you get to see a brand-new 80-year-old airplane like this? This elegant vintage paint scheme was accomplished with the judicious use of rattle cans from the hardware store.

More than four years on, we thought it was time to check in on how Treadwell was progressing with this unusual project.

So I moseyed by his hangar and found him working away, brushing a base coat on a Poly-Fiber-covered lower wing. He graciously accommodated my request for an interview despite his busy building schedule, which runs about 5 hours per day, every day except Sunday.

Sailing Roots

Surprisingly, Treadwell wasn’t intimidated by the prospect of building a large twin-engine amphibian from scratch. “I started building little El Toro-type boats out of plywood when I was 12 or 14,” Treadwell said. “I used to sail them in the Oakland Estuary and Lake Merritt. And then in 1964, I bought a 45-foot sailboat that had been built to race in the 1940 Transpac race to Honolulu. I did a lot of work with wood on that boat and later on a fiberglass racing boat. So I guess you could say it all started with boat building.”

Indeed, this is where Treadwell’s S-38 construction started, as he built the hull up from the keel, much like building a boat. It also explains the build sequence, where the fuselage and center sections of the airplane are now complete with engines, radios and instruments. The only items left are to finish covering the wings, put them on and start making plans for flight test.

Ambitious? Certainly. But when you find out a bit more about Treadwell’s aviation history, you begin to understand how he ended up devoting more than 6000 hours of build time and nine years (so far) to this project.

Summer of 1944. B-25 Commander Walter Treadwell is third from left, shown with his crew. They all survived multiple combat missions in the South Pacific.

Again and Again

Treadwell is no newcomer to flying. Just after he soloed, the December 7th Pearl Harbor attack resulted in his soon flying combat missions in B-25s over the South Pacific. During WW-II, he spent time teaching instrument flying via LINK trainers, and he ultimately moved on to civilian life and transpacific sailboat racing after some 4000 hours of flight time logged.

Fast forward 40 years to the mid-1980s, when the flying bug that had been MIA for many years reemerged. It was time to visit Pacific States Aviation at Buchanan Field in Concord, California, to see what it would take to knock off 40 years of rust. Not much, as it turned out. After checking out in a Cessna 152 and then a 172, Treadwell just needed to become familiar with modern radio equipment and techniques to renew his commercial license.

Sikorsky S-38 Flying Boats being assembled in the New York factory, around 1929.

One thing led to another, and Treadwell also became current on the gauges, learning modern instrument flying procedures after years of teaching the finer points of A-N range flying four decades earlier. Then it was over to Sausalito’s Richardson Bay to get a seaplane rating. Then—enough fun—it was time to get back to work building airplanes.

Dipping his toe into the homebuilding waters, Treadwell first tackled a Lancair 235 kit about the time he resumed flying. This was followed by several more challenging projects, when merely assembling an airplane just wasn’t satisfying enough. Treadwell used his extensive engineering background to draw his own plans from whatever sources he could find, resulting in several aircraft that were pint-sized versions of the real thing. A half-scale Jennie came after the Lancair, and then Treadwell started getting really ambitious.

Even without the upper and lower wings, this S-38 replica looks almost capable of flying if you squint your eyes just right.

Struck by a Little Lightning

In the mid-1990s, someone suggested I look up this guy who was scratch-building a Lockheed P-38 Lightning. This is how I first met Treadwell, who had started with the plans from a balsa model, and scaled them up to a 55% replica of the famous fighter.

He figured a pair of Suzuki auto conversions would have the right shape to fit in the engine nacelles and would produce roughly 100 horsepower each. As in the original, he’d fashion radiators to be placed behind the scoops in the twin tailbooms. A few concessions were made to practicality, however; both props would now turn in the same direction to keep things simple.

The throw-over control wheel and column are much like the original, save for the nice wood accents not found on the original S-38.

Test-flown by none other than Dave Morss, the Lightning flew well, according to his report, and Treadwell’s extensive design work was validated. The Suzuki engines, however, just didn’t provide the power anticipated and were also plagued by cooling issues. Rather than spend any more time looking for powerplant solutions, Treadwell decided to donate the Lightning to an aviation museum in Santa Maria, California, where it remains on display.

With the Lightning gone, Treadwell needed something to fly. The “Ally Cat” soon appeared, a half-scale yellow Ag Cat replica that scratched the flying itch while he contemplated bigger things to come.

The throw-over control wheel and column are much like the original, save for the nice wood accents not found on the original S-38.

Why the S-38?

Treadwell recalled that he had seen a picture of the S-38 on the cover of Sport Aviation in 2003. “It just looked interesting,” he said. “I got a hold of a guy in Maine who builds models, and he had a lot of data and drawings. So that’s why, no other reason. It just looked like an interesting project.”

For those of us who were born post DC-3, the mental image from the word “airplane” and the appearance of the Sikorsky amphibian aren’t anything close. It helps to remember that Russian-born aviation pioneer Igor Sikorsky first conceived this design just a couple of decades after the Wright brothers first flew at Kitty Hawk, at a time when function was everything. Who cared what it looked like? Did it perform as planned?

The Western Air Express operated between Los Angeles and Catalina Island, and a round-trip ticket cost the princely sum of $20. This is the paint design chosen for Treadwell’s 55%-scale S-38.

The S-38 Amphibion above poses for a shot over the factory.

The short answer is, “Duh!” This twin-engine flying boat had a gross weight of 10,480 pounds and promised a useful load of 3800 pounds. Two Pratt & Whitney engines, each rated at 420 hp, would allow the S-38 to cruise at 110 mph while burning only 44 gph when throttled back to 1750 rpm. With 264 gallons aboard, the endurance was 6 hours with no reserve. Don’t forget oil consumption, listed as 16 gallons total during a 6-hour flight, or 2.67 gph.

The service ceiling was 18,000 feet, rate of climb was 750 fpm, and stall speed was 55 mph. The original Sikorsky sales brochure from 1930 stressed that the “performances” listed were from actual flight tests and not merely from the imagination of the sales department.

The rear passenger hatch looks like it came right off of a sailboat, especially with the varnished wood finish. A beautiful example of form following function.

With 840 total horsepower on tap, why so slow (by today’s standards)? Looking at the airframe, it’s not hard to figure out. The beauty of the Sikorsky is in the eye of the beholder, not in wind-tunnel results. The design has aerodynamic drag written all over it, from myriad struts and braces, exposed control wires and landing gear, fat upper and lower wings, and a twin cruciform tail supported by long rectangular booms. Massive amounts of horsepower can only overcome so much.

The first S-38 was designed for eight passengers and a crew of two. It was pretty luxurious for the time, with custom cabins available sporting couches and large chairs, a forerunner of today’s elegant civil jets. It saw regular passenger-carrying service in many parts of both the civilized and uncivilized world, during a time when paved runways were scarce, but oceans, lakes and rivers provided worldwide solutions to takeoff and landing.

The original S-38 cabin sat eight passengers; the scaled-down version will have room for two. The main bulkhead separates the flight deck from the cabin, just as in the original.

The S-38B and C models soon followed, raising the revenue-generating passenger count to 10 and 12, respectively. But the design was soon eclipsed by the Ford Trimotor and the Douglas Commercial (DC) 1. Still, the Sikorsky flew all sorts of missions in remote areas for years afterward, where the majority of available landing areas were either dirt or water.

Only one flying S-38 survives today, and it has only some of the original airframe components. A magazine cover photo of this one zebra-striped Sikorsky, featured now and then at AirVenture, was enough impetus for Walter Treadwell to embark on his own S-38 building adventure. Scaled down a bit.

As in the original design, the bullet-shaped engine nacelles cover the engine mount attach points and lubrication plumbing and give a bit of much-needed aerodynamic cleanup.

Borrowed from a couple of two-seat trainers, these propellers are for test only. They will be replaced by two props that are more period correct, and tuned to the engines and airframe.

Then and Now

Treadwell decided to take advantage of today’s technologies for the build. “The original airplane was made out of wood and Duraluminum [the forerunner of Alclad],” he said. “I had previously built the Lancair, which was all fiberglass, so when I started this one, I decided to use the same basic materials and techniques. I made 24×96-inch panels, vacuum-bagged them and laid them up.”

Although the original wingspars and ribs were built from steel and Duraluminum, Treadwell didn’t want to duplicate that method. “I had built the Jennie and the Lancair,” he said. “The Jennie had aluminum ribs and spar, and I was familiar with the fabric-covered wing. When I built the P-38, I used the same system for the ribs that the Lancair used, but covered in 1/16-inch plywood. So it’s an accumulation of the previous knowledge from the homebuilts I’ve designed and built.”

The large cruciform tail located in the thrust line well aft of the CG ensures positive water handling even at low taxi speeds.

With the fuselage sporting the livery of the former Western Air Express, it’s pretty difficult to determine exactly which materials were used where from a casual look. A contributing factor that immediately captures the eye is the beautifully varnished woodwork that adorns the rear passenger hatch, main bulkhead and trim on the gull-wing crew doors. Those were created just for this airplane out of 3/32-inch Lexan, finished with wood trim. The original S-38 had flat crew hatches that slid in stainless-steel tracks, but scaled down to 55%, the fit-through would be tight for most pilots, and building them from scratch would have been overly complex.

Treadwell has gone to great effort to keep other components as close to original as possible, within reason. The windows are all the right size and shape, including the windscreen. But they’re now all Lexan instead of glass, and the windshield frame is fiberglass, not stainless steel. The rear passenger hatch works much like the original, with a step-down ladder to the cabin, which seats two passengers in this scaled-down version.

The S-38 was initially conceived with little regard to the acceleration-stopping effects from drag at increasing airspeeds. Back then, 110 mph was really flying!

On the flight deck, there are the inevitable concessions to modern aviation. A com radio and transponder are in the panel, but round analog gauges are used wherever possible. One particularly nice touch is a custom-built throw-over control column, which has a round control wheel just like the original. However, Treadwell’s wheel is beautifully finished in wood and brushed metal, unlike the utilitarian original. Like many of the other beautiful touches on this airplane, it speaks volumes about Treadwell’s attention to detail.

Only the main wings are left to cover and finish. Note the blue foam and wood cap strips on the ribs, techniques used in previous projects.

Two Radials, No Waiting

It just wouldn’t do to have a modern, flat aircraft engine or even a pair of auto conversions powering this replica, so a pair of Rotec 3600, nine-cylinder radial engines are mounted in bullet-shaped nacelles, looking like miniature versions of the original 420-hp Pratts. These Rotecs are rated at 150 hp each, and are either blessed (or cursed) by all of those attributes that make radials so endearing to vintage aviators, including weeping oil through the bottom cylinders to the hangar floor. But this is a small price to pay for that period-correct appearance and that unmistakable radial sound.

Each of these Rotecs consumes about 7 gallons of 100LL avgas per hour, or 30 gph less than the huge Pratts that powered the original S-38. From his design calculations, Treadwell anticipates that his scaled-down Sikorsky will outperform the original in rate of climb and will be close in most other areas, with the obvious exceptions of gross weight and useful load. Treadwell’s S-38 should have a useful load of 1025 pounds, as opposed to the 3800 of the original.

Walter Treadwell engaged in the time-honored tradition of rib stitching. It appears that he’s done this before.

The propellers currently installed are temporary, borrowed from a couple of Cessna trainers for the sole purpose of running up the engines every now and then. Treadwell is having custom props built from design drawings, which should be perfectly matched to the output of the Rotecs and the anticipated performance of the S-38 airframe.

Flying is one thing. Stopping reliably is quite another. The brakes are another area where concessions to modern times are best made, and the maingear wheels are adorned with the same type of hydraulic disk brakes we’ve all come to depend upon.

“I run computer calculations to figure out various and sundry things about the airplane, but they don’t always come out the way you think,” Treadwell said. “Nevertheless, it’s a design unique unto itself. I’m hoping to get Dave Morss back again to test fly this one.”

Igor Sikorsky (left) stands with his “full-scale” S-38 flying boat, considered to be the state of the art in aerial transportation at that time. Even with 840 total horsepower, the S-38 could barely manage cruise speeds much over 100 mph. Can you say “parasite drag?”

Man or Machine?

I’m not sure which is more impressive, the airplane or its builder. Walter Treadwell has so many aviation accomplishments to his credit inside and outside of homebuilding that it’s difficult to grasp that they all belong to the same individual.

Treadwell survived his B-25 combat experiences without a scratch, save a slight deafness in his left ear from the constant pounding noise of the No. 1 engine. He also brought his entire aircraft crew safely through the conflict.

After not flying for 40 years, Treadwell jumped right back in at an age when many would consider the bingo parlor high adventure. He not only refreshed his basic aviation skills, but also renewed his instrument skills and earned a seaplane rating.

He now has five homebuilt projects to his credit, four of which he designed himself. So far, four of the five have not only flown, but flown well, and along the way, he has “built every type of construction, except the traditional RV sheet metal and rivet airplane.”

There’s been a lot of talk in recent years about the “greatest generation.” Happily, it’s still around, providing inspiration on a regular basis. All it takes to find it is a stroll around any airport, like the one in Livermore, where homebuilding occurs.

Walter Treadwell and his latest creation.

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Rick Lindstrom
Rick Lindstrom has been fascinated with motorized devices since the disassembly of his mom’s Kirby vacuum when he was 3, predictably followed by record player motors and lawnmower engines. After he learned to fly, it was only natural that he gravitated to the world of alternative aircraft engines. He currently pilots a Corvair-powered Zenith and is undecided about what will power his GlaStar.



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