Built to Suit

Considering the kit as raw material let John Spratt build an S-20 Raven optimized for his high-country travels.

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Outside of its generally clean appearance, few exterior clues say John Spratt’s RANS S-20 offers high-country turbo performance or so many useful owner-supplied details. All painting was done outdoors by John in the dry Nevada climate.

We don’t think of it this way often, but in reality all kit aircraft are kits first and finished airplanes second. That is, long before anyone assembles them, aircraft kits must be profitable to build and sell or the kit manufacturer will go out of business. Put yet another way, not every kit airplane can be everything to everybody.

Hopefully, we people assembling kit aircraft are savvy enough about modifying kits. Most of us know we’re poorly qualified aerospace engineers, so we’re smart to stick to minor changes such as personalizing cockpit details or adding a handhold somewhere. But others of us have the smarts and experience to start with a good kit and transform it into an airplane more ideal to specific needs. John Spratt’s RANS S-20 Raven is one such example.

John Spratt combined a lifetime’s knowledge around aircraft to build an S-20 Raven best fitting his and wife Carolyn’s needs.

John’s S-20 story begins at age 12 with building free-flight, U-control, glider, and radio-controlled model airplanes, followed by 14 years as an Air Force pilot at the controls of the C-130E and KC-135A, and 25 years flying United 737s and 747s. Along the way, John flew all the usual Cessna and Piper flutter bugs and light twins while his experimental background includes building a BD-5A as far as the Bede company solvency allowed, along with time in an RV-9A and crafting a Searey amphibian fair enough to grace this magazine’s cover in May 2007. During his professional piloting days, John was technically engaged; his Air Force duties, for example, included instructing instrument school and aircraft systems for the KC-135A.

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Tubular spars and riveted aluminum construction are typical of RANS designs. John added 3M Scotch-Weld 2216 epoxy adhesive between every joint in the wing for additional strength and long-term stability.

In John’s words, his current experimental experience began “Sometime around 2001, [when] my wife and I were looking for something to do that both of us enjoyed. While living at Lake Tahoe, we rented a two-person kayak and after a couple hours of paddling decided whatever we got would have an engine in it. With me being a pilot, we bought a Searey kit and finished it in 2006. The build took four years as I was still working for the first two years of the build, and we lived at Lake Tahoe. My hangar was at the Minden-Tahoe airport, a 90-minute round trip. We flew the Searey as far as Key West, Florida, but mostly in Nevada, California, Idaho, Utah, Arizona, Oregon, Washington, and Wyoming.”

After flying the Searey for six years and “1300 hours of fun splashing on many lakes and rivers,” it was sold for something a little faster.

“All airplanes are a compromise of speed, utility, cargo capacity, comfort, etcetera,” explains John. “While the Searey was fine for a seaplane, dragging a boat hull and pontoons through the air made it slow at an 80-knot cruise speed. After selling the Searey, we wanted something a bit faster for cross-country flying, [so] we bought into a partnership in an RV-9A. We flew that for three years [before the partnership dissolved] and enjoyed the cruising speed almost twice that of the Searey.”

Looking like it grew there, the turbocharged Rotax 914 is a natural and effective upgrade from the S-20’s stock, naturally aspirated 912 engine. In this view, the oversize Earl’s oil cooler with 52.9 square inches of area—twice as large as stock—is the most prominent visible change. The stock oil cooler was repurposed as the cabin heater core.

By then, John and wife Carolyn understood their next airplane would be for going places, both on and off the beaten path. The backcountry RANS S-20, with its large baggage volume, “a cruise speed between the Searey and RV-9A, and tundra tires for flying into backcountry airstrips,” seemed to offer the best compromise of rough-field capability, en-route comfort, carrying capacity, and speed, so John ordered a kit and got to work.

Thanks to his building experience and completeness of the RANS kit, John had no issues with the basic build and was able to concentrate on tailoring the S-20 more exactingly to his needs. These were mainly increased comfort, better climb and altitude performance, plus improved maintainability.

While maximizing comfort is almost always a goal in little planes, John’s mission profile mainly includes long cross-countries with Carolyn. And while she’s ready for all the depravations of backcountry camping and flying, the pair knew they’d be a happier, more effective team with something more refined than the usual powered kite.

The need for more oomph in front of the firewall is driven by John and Carolyn’s Minden, Nevada, home base. Surrounded by mountains and high summertime temperatures, there is a true need for better than average performance at high density altitudes. That they would typically be loaded to gross with camping gear and flying two up only underscored that need.

Turboed engines are notorious for retaining heat in the engine compartment. Extensive heat shielding is thus no surprise in John’s engine bay. He’s run reflective shielding around part of the oil filter, coolant lines, wiring, crankcase, and muffler. Not visible is the insulation/shielding under the carburetors that is designed to lessen heat soaking after hot shutdowns. John used Fiberfrax ceramic felt covered with heavy aluminum foil for this. He also turns off the fuel pumps and fuel valve about a minute before shutdown to reduce the amount of fuel left in the fuel bowls.

Turbo Power

John’s answer for more power was replacing the naturally aspirated 912 Rotax with the turbocharged 914 version of the Austrian engine. John says this yields a two percent gain in speed with each 1000 feet of altitude. But swapping engines, even between two such similar powerplants, is a major change that drives several collateral changes.

Primarily for efficiency at altitude, John selected an Airmaster 332SF propeller with WhirlWind R70W blades. Secondarily, it fully feathers, turning the S-20 into a passable glider in the powerful Sierra Nevada mountain waves and desert thermals. Glide ratio increases from 8.5 to 10:1 when feathered.

Largest of these was an adjustable-pitch propeller as otherwise the turbo engine’s extra power at altitude would not be effectively turned into thrust. “After 7000 feet of climb, the rpm keeps going up with the fixed pitch,” is how John put it. While Rotax supports a hydraulic constant-speed prop governor, John picked a full-feathering electric Airmaster with WhirlWind carbon blades.

Rotax uses an efficient blow-through layout on the 914, so the air inlet to the turbo is very direct. The cone-shaped K&N air filter is the only thing before the turbo; curiously it is correctly positioned for the stock S-20 cowling’s air inlet and needed only this simple aluminum box to close with the cowling’s NACA scoop.

The larger engine must reject more heat, so a larger oil cooler and radiator were incorporated, and that in turn led to changes in the cowling. Luckily, the weight increase from the turbo system is minimal with the turbo Rotax install—around 20 pounds—so John did not have to change the engine mount length. But he definitely changed the mount.

As supplied, the RANS engine mount for the 914 was an unusual, asymmetric design that did not connect the four engine mounting points to each other. John found when everything was test fitted, his 914 and mount assembly easily yielded to hand pressure both vertically and sideways; it also had an aggressive 5.1-degree offset to the right.

Flight testing proved the obvious: The stock 1-inch RANS radiator was inadequate to cool the 914 in desert heat, so this custom unit was sourced from www.BeCool.com. It is the same width and height, but is 2.75-inches thick and allows unrestricted full power climbs at 35.5 inches and 5500 rpm. In winter this radiator requires partial blanking; the scoop-like ducting around the core mates with the lower cowling to form a ram inlet.

John wanted something more rigid with less thrust offset, so he added four tubes to connect the engine attach points to their vertical partners and also to increase triangulation to the firewall. The thrust line offset was reduced to more common 1 degree right and 1.2 degree down angles, which proved fine during flight testing. John also repainted the mount from black to white to better track any fatigue cracks (none to date).

Slip rings at the rear of the Rotax gearbox is how the Airmaster prop communicates with the instrument-panel-mounted controller. Airmaster says the brushes in this system should last at least 800 hours, and they certainly appear easy to change. To avoid overspeeds, Airmaster has the prop set for 5700 rpm and not the Rotax 5800 rpm redline.

As expected, the turbo power has made a big difference in the S-20’s altitude performance. Remembering John’s home field elevation is 4270 feet, he reports that at gross weight his S-20 takes off in 300 feet using 40 inches of manifold pressure and 5700 rpm. Rate of climb is in the 800 (cruise) to 1300 fpm (max effort, 5-minute limit) range while typical cruise speeds are about 107 knots true. And when the mood and weather cooperate, he can shut down, feather the prop, and soar.

Getting the RANS carbon fiber cowling inlets modified, the interior lined with Thermo-Tec heat barrier, painted and fitted to the boot cowl took a month, but it works well. The large NACA duct at bottom is the new radiator opening that is lower and farther aft than the original, which was closed. The smaller NACA duct is the unmodified engine air intake, and the scuttle at far left is the enlarged oil cooler scoop added during Phase 1 testing. At the same time, the two front inlets were reduced to approximately 20 square inches by blocking off their lower halves. The lip was added to the radiator NACA inlet and a second lip to the cowling exit. John figures a movable cowl flap would have been better on the exit; as it is, in winter he blanks the radiator with tape.

Fuel System

Multiple changes were made to the fuel system, both because of the turbocharged engine and as general improvements. Somewhat notably, these changes did not include increased fuel capacity. The S-20 tanks hold a useful 26 gallons in stock configuration, and the turbocharged engine and constant-speed prop are not the typical fuel-guzzling hot-rod modification found when installing larger displacement engines.

Both the Rotax 912 and 914 engines are carbureted, but the turbo’s greater-than-atmospheric manifold pressure requires a matching rise in fuel pressure along with boost referencing the carburetors. Thus, the stock 912 engine uses a single engine-driven fuel pump while the 914 requires two electric pumps working in series. These are equipped with check valves and a fuel pressure regulator to keep fuel pressure a constant 3.6 psi over the turbo’s outlet pressure. Furthermore, the 914 system is a return style where the engine is always generously oversupplied with fuel, and that fuel not consumed by the engine is returned to a fuel tank.

There’s also a header tank between the fuel selector valve and before the two fuel pumps. RANS supplies this tank in plastic with no quantity sender; John replaced it with an aluminum tank of his own design, which includes a quantity sender. The tank is mounted under the baggage floor where besides providing a ready reservoir for the carburetors, it accepts the return fuel. A vent line runs from the header tank to the left main; this vent line is clear so John can visually confirm what’s going on between the header and left main tank, and includes an inline fuel filter in case fuel drains into the header tank directly from the left main. Other filters are NAPA-sourced boulder catchers immediately downstream of the dual fuel pumps in case the pumps “shed parts.”

RANS’ stock fuel tank vent is a 3/8-inch tube projecting upwards next to the fuel caps. It has a forward-facing hole John thought looked like a rain, dirt, and bug catcher, so he fabricated a streamline fairing to cover the hole. Venting is accomplished by leaving the 45-degree angle at the bottom of the streamline open to the slip stream.

More failure modes were addressed by Racor marine filter/water separators at each main fuel tank outlet and a fuel valve with Left, Right, Both and Off positions—the latter to avoid cross-filling the main tanks when the airplane is parked out of level. RANS also supplies a fuel shutoff valve in the header tank-to-engine line to avoid flooding or to clear out a flooded engine during hot starts. Yet another addition was a vapor purge pump between the fuel selector valve and the header tank in case of vapor lock in that line or the header tank. This is a small Facet electric unit.

All said, John ended up with a complex, trinket-heavy fuel system, but one servicing a more demanding turboed engine and avoiding many of the pitfalls of simpler systems. One interesting capability follows from John’s placement of a fuel quantity sender in the header tank.

Originally the S-20 vented the main fuel tanks underneath the wing la Cessna, but the vent was eventually changed on all the kits to a short 3/8-inch line protruding above the wing next to the fuel cap. Additionally, there was no Off position on the fuel selector valve, so fuel was always free to flow from the main to the header tank, allowing cross-fill between the main tanks when the airplane was parked with one wing higher than the other. Also, the stock S-20 plastic fuel header tank is shown on the plans with no dedicated vent.

The fuel system issue John found was when the aircraft was flown, then landed, fueled, and then parked under a hot sun for a few hours. Then the mogas in the header tank became warm, producing substantial vapor in the header tank.

First thing to catch most eyes when walking up to John’s S-20 are the highly detailed aileron linkages—along with the labels on everything. The airplane has a generally clean and detailed visage that sets it apart on the ramp.

If flown in this condition, the low air pressure above the wing and the high vapor pressure in the header tank moved fuel backward out of the header tank and to the main wing tanks—or at least prevented fresh liquid fuel from gravity feeding to the header tank. Plus, of course, the main fuel pump(s) were sending fuel to the engine. During a worst-case scenario, things could suddenly get very quiet if the header tank ran dry.

John noticed this issue because he has a fuel quantity sender in the header tank (it displays on the Dynon screen). He could see the fuel level falling in the header tank. His solution is the little Facet electric pump between the fuel selector valve and header tank to positively drive fuel into the header tank. In fact, he’s seen the issue several times since the Facet pump was added, and turning on the pump has always cured the problem immediately. John suspects other S-20 pilots are experiencing this vapor lock, at least partially, but don’t know it because they don’t have a sender in the header tank. We’ll finish by saying John notes there are larger header tanks available—his is just 1.2 gallons—and he believes “bigger is better.”

John’s comment on analog instruments is, “You don’t want them in this day and age. What you get with the big SkyView or Garmin is amazing and increases safety. Basically, you have a 10-inch window to the outside world. There never should be a CFIT with these.” A close look here discloses two throttles, one on each side of the panel so either person can fly. The center console covers the electric flap actuator, houses the marine rated battery switch (Blue Seat Systems 6007 M-Series), and is topped with an adhesive-backed carbon fiber veneer.

A Bit of Glamping

Trying to build comfort in small airplanes is pretty much a windmill tilt compared to a modern car or house, but John knew judicious use of insulation and attention to detail would make his S-20 Raven considerably more enjoyable on cross-countries.

First of these, even if it’s more of a convenience or safety feature, is the Dynon autopilot. “The autopilot is big to me. My wife doesn’t fly, so if something happens to me, she can push the level button. It’s like having two pilots in there…[it] makes flying easy. I punch the button and look for traffic or at the scenery.” Perhaps the Searey gave John the push toward the autopilot as the “Searey was hands-on all the time.”

John’s panel is contemporary and all Dynon. The screen is a D1000T SkyView Touch flanked by a single com radio, autopilot, ADS-B transponder and intercom.

Requiring considerably more effort was changing from two control sticks to one centrally located pole. This was about eliminating the need to crawl over the sticks every time anyone got in or out. John designed and built the necessary linkage, along with new trim pieces where the stick meets the center console.

Working well with the change to a center stick is John’s preference for electric flaps. This is because the center console is where the S-20’s stock manual flap handle lived, but there was no room for it once the stick commandeered that real estate. Therefore, John converted to electric actuation ranging from -4 to +40 degrees.

John prefers the ability of electric flaps to stop anywhere in their operating range, not just the 10-degree increments typical with manual flaps. He also believes in stabilized configurations for takeoffs and approaches, so the rapid flap movement associated with manual flaps is not much inducement to him, even in backcountry operations.

Old timers would call this the “radio light,” but we’ll just say John built his own red and white dimmable LED panel lighting under the glareshield lip. The lamps are not visible from the pilot’s seat, just the glow they put out. Naturally, the white and red lights are separately controlled.

As for insulation, of which the stock S-20 has only a foam-rubber-backed fabric on the cabin side of the firewall, John completely padded the cockpit with a quilted heat and sound insulation material from Skandia, along with -inch fiberglass insulation in the lower half of the doors. “I’ve flown in 5 F ambient,” says John, who knows just how cold all-Plexiglas door panels get. The extra weight in the doors required replacing the stock 20-pound-assist door struts with 30-pound units, but that’s worth it to John.

Even the aluminum sheet making up the cockpit floorboard got a full panel of closed-cell foam insulation on its bottom. John found the material at Home Depot; it features aluminum foil on both sides for noise and temperature isolation.

Large, bald-spot-tanning skylights are a RANS hallmark; John installed Gila window film on his to tame the sun. He enthusiastically reports the UV rays are so muted, he finds no need for a curtain or other mechanical shade. This is John’s second experience with the Gila film as the Searey was similarly equipped.

While the seat upholstery is standard stuff, John found the kit-specified attachment to the seat frame via bulky tie wraps too space consuming. He changed that to simple safety wire hold-downs.

And while not a comfort item but still on the subject of seats, John added a second row of seat stops on the seat tracks as the single row is prone to twisting and breaking.

John trimmed the S-20’s big baggage compartment with seven pounds of Skandia SK-QB3 insulation. It’s quilted on the face, lined with plastic on the backside, and insulated in between. The insulation also eliminates the need to paint the inside of the fabric while damping noise and cold. Carolyn sewed the edges; the carpeting is indoor/outdoor material from Home Depot. Not seen here, the interior is lit by four dimmable LED lamps drawing just .1 amp. The forward pair mount atop the door trim and swing with the door to form puddle lamps outside the plane when the doors are opened, then help light the interior when the doors are shut. The gray paint on the tubing is Superflite primer and Aero-Thane AgCat gray.

Both under the seats as seen here, and beneath the baggage compartment, is where John put much of the electrical and fuel systems. This was done with -inch thick plywood decking and nut plates for easy access. Maintenance and storage is hugely eased with removable hardware. Even the floorboards got the same treatment.

John also found the thimbles terminating the rudder cables at the pedals rubbed on his shoes. Ultimately, he replaced the RANS-supplied cables for Aircraft Spruce cables using much thinner bushed ends. Those cables were also protected with a marine-quality sleeve John knew of from his Searey days because a friend’s RANS S-7 frayed its rudder cables in the same stock plastic guides his S-20 employs.

John’s Searey had toggle switches, so when outfitting the S-20, he was looking for something a little more finished, yet affordable. Oslo Switch in Connecticut had just the switches he wanted in a variety of standard and slim sizes. Their plastic bodies simply snap into a hole cut in the panel and have proven durable.

Safety & Details

By now, if you’re thinking John is detail oriented, you’re catching on. This is especially helpful with the electrics as John’s S-20 is electrically intensive. He opted for B&C Specialty Products overvoltage protection and ditched the standard three-way keyed ignition switch in favor of individual ignition switches and a push-to-crank button. His airplane does not have a traditional master switch and contactor (solenoid), John preferring a marine-rated battery switch. This high-amperage switch eliminates the need for a solenoid.

Furthermore, the two Odyssey PC 680 batteries sport 100-amp fuse protection, with the main battery also carrying a 30-amp fuse leading to the essential bus. John also made his own battery cables using Ancor 8 AWG marine-rated cable plus some scrap aluminum and a vise to crimp the heavy terminal ends.

Looking up at the busyness around the tailwheel spring and rudder horn shows the rod ends terminating the rudder cables. These came with the Aircraft Spruce cables John used to replace the thimble and pressed stockers. At the extreme right is the rolled aluminum streamline exit John made for the rudder cables at the fuselage. The stockers are more squared plastic pieces that didn’t quite open far enough for the upgraded cables’ travel.

Like everything else on the plane, the electrical system is identified to an inch of its life using EazyDraw software to write the custom labels. These were applied on the electrical bits using two-sided tape with a clear tape overlay. Two minor electrical notes are the wiring was temporarily laid out using pipe cleaners as quickie, easily manipulated tie wraps. Final fastening was done using conventional tie wraps, self-bonding silicone tape, and nylon cable clamps. In the cockpit, where chafing from the occupants getting in and out is an issue, a gray plastic split loom gives protection.

John’s center-mounted stick is topped with an Infinity grip he recognizes as a copy of those he held in the Air Force. With the center stick, John would appreciate an ambidextrous grip, but Infinity doesn’t make one, and as his wife doesn’t operate the controls, it’s not a major issue. Switches on the grip are for flaps, elevator trim, PTT, transponder ident and autopilot disconnect.

Fitting the doors and their latches proved labor intensive as the tautening fabric inside and outside of the doors warped them, and they had minor variations in their weldments to begin with. In short, typical fitting-together issues found in most kits.

RANS’ standard rudder and vertical tail shape is squared-off, but they also offer an optional “classic,” rounded shape that John opted for. Wanting his wing tips to better match his classic tail, he ditched the stock fiberglass tips and built his own in the conventional tip-bow manner. This saved 10 pounds and a bunch of unsightly sheet metal screws, but did require custom designing everything outboard of the aileron.

There’s no such thing as too much light when putting down on an unlit backcountry strip, so John selected projector beam LED lamps from off-road motorcycle house Baja Designs. He’s more than happy with them, saying he can read the numbers on an unlit runway from a half mile away. There’s a lamp on each wing.

In Flight

Considering its many amenities, John’s S-20 came in fairly light at 850 pounds, which certainly aids performance. Phase 1 testing was 40 hours and was finished in February 2017.

John finds his S-20 carrying 350 pounds, for 1200 pounds all-up weight, and comes off the ground in just 300 feet. This is at his home field elevation of 4270 feet msl using max power of 40 inches at 5700 rpm for 5 minutes.

Initial rate of climb is a bit better than 900 fpm at this weight, although putting out 10 degrees of flaps and pulling the nose up to 55 knots yields 1300 fpm. More realistically, the climb settles at 800 fpm with the flaps up, at 65 knots and 1430 pounds. Of course, the turbo means the rate of climb hangs in there much longer than naturally aspirated.

Instead of the usual vernier cable, Airmaster props use this controller for prop pitch functions. Normally, the unit is left in automatic, and the pilot simply flips the blue knob to whatever flight mode is appropriate. A manual fine/coarse toggle switch, just out of frame at lower right, allows manual override along with controlling the feathering function. John says it’s quick, intuitive, and “a joy to operate.”

John cites a typical cruise of 107 knots true at 12,500 feet at 75 percent power. That’s 31 inches and 5000 rpm. Go up to 17,500 feet and 113 knots true is available at 70 percent power.

Do John and Carolyn really fly that high? No, “We normally have no need to go higher than 12 or 13,500 feet. Just going over the mountains [only takes] 10,000 to 11,500 feet. We have gone to 15,000 to get over clouds or turbulence.” John also admits to cruising at 500 to 1000 feet agl “to get away from headwinds,” so his RANS has a wide range of operating altitudes. In the end, his cruising altitude depends on terrain, winds, and whether or not to bother with his portable Mountain High oxygen system.

Just as importantly, the S-20 Raven offers great storage volume and access for two happy campers along with gentle handling. All said, John seems happy with the S-20 kit and what he’s been able to accomplish with it.

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