Unapologetic rock crawler

Despite a 9,000-foot density altitude on a hot, hazy, smoky day in Driggs, Idaho, (elevation: 6,231 feet) the fully loaded two-seater accelerates smartly, levitates off a grass runway after a 250-foot ground roll, and climbs 1,000 feet per minute at 70 knots.

The airplane’s semi-liquid-cooled, turbocharged Rotax 915 engine’s cylinder heads stay well within the normal temperature range; an MT, three-blade, constant-speed propeller growls at fine pitch/high rpm; and 29-inch Alaskan Bushwheels and nitrogen shock absorbers make the rutted runway surface feel almost smooth.

“We’re from this area, so we build airplanes that perform well in these demanding conditions,” said John McBean, president of Kitfox Aircraft, the 18-employee firm based in Homedale, Idaho, and a former U.S. Air Force A–10 crew chief. “This sort of density altitude would be extreme in other places—but to us, it’s just another summer day in the mountains.”

Kitfox has delivered more than 7,000 kits since its 1984 founding, and more than 3,500 of its steel-frame, fabric-covered, folding-wing aircraft have been registered and flown in the United States. Kitfox helped popularize backcountry flying among pilots of experimental/amateur-built aircraft, and its 141-horsepower STi (which stands for “STOL Inspired”) is the company’s most powerful, highest performing model.

The STi is optimized for off-airport use—not just unimproved runways. And at a moment when getting away from it all has captured the imagination of people around the world, the STi allows pilots to actually do it with confidence and style.

“We’ve been focused on backcountry flying from the time our company started 35 years ago,” McBean said. “That’s always been central to why we fly, and everything else we do.”

Rock crawler

The Kitfox Series 7 STi is a no-compromises STOL airplane.

Other Kitfox models such as the Super Sport and Speedster are faster, nimbler, and more fuel efficient. But the STi is an unapologetic rock crawler meant for the bush.

The STi stands tall on tundra tires that give it a cocky, nose-up attitude on the ground. Beefy shock absorbers on all three wheels cushion rocks and ruts with a wide wheelbase for uneven surfaces. Transparent doors and a full greenhouse maximize visibility. A razorback line of vortex generators on top of the wings (and the bottom of the horizontal stabilizer) keep airflow attached at high angles of attack, and prominent stall fences help achieve the same goal.

Other aspects that stand out when first meeting the STi are a sleek cowling and wide-chord, three-blade propeller, making the STi reminiscent of a turboprop (which it’s not). The four-cylinder Rotax 915 iS engine simply can make do with smaller air intakes than a fully air-cooled engine, and that makes the tight cowl optimal.

And those flaperons.

Kitfox has used flaperons for roll control and enhanced lift since its inception—and the combined flaps/ailerons on the STi stretch across nearly its full wingspan. Those fabric-covered, counterweighted flaperons droop a few inches below the trailing edges of the wings, and the flaperon angle increases as the pilot deploys them to add both lift and drag. The flaperons continue to move differentially (one up, one down) for roll control whenever the pilot moves the control stick laterally.

Flaperon advantages include light weight and more span for greater effectiveness. The main disadvantage of flaperons is an increase in adverse yaw compared to traditional ailerons.

The STi horizontal stabilizer is braced by a metal strut. A rubber gap seal on the elevator is meant to lighten pitch forces and increase elevator effectiveness. Pull handles on the aft fuselage make the airplane easy to move on the ground.

The avionics in the STi—as with any experimental/amateur-built aircraft—are left to the builder’s discretion. The STi factory demonstrator that I evaluated (N81KA) has minimalist VFR instrumentation in keeping with the airplane’s backcountry emphasis on light weight and maximum performance.

The panel also contained a few telling clues about the airplane’s character: a simple slip/skid ball was prominently placed directly in front of the left-seat pilot; and a colorful angle-of-attack indicator was positioned directly above it. Those weren’t coincidences. Clearly, the builder’s instrument placement shows the most important aspects of flying this airplane are coordination and getting close to—but not exceeding—the wings’ critical angle of attack.

Cruelly unfair

Climbing into the airplane involves ducking below the top-hinged doors, sitting in the semi-reclined seat, then swinging your legs over the floor-mounted control stick. A four-point seatbelt harness cinches me into the left seat while McBean, a veteran flight instructor, does the same on my right.

Engine start is normal for a Rotax engine, which is to stay it’s push-button simple. There’s no mixture control and, on this airplane, no propeller control, either. The engine knows to select low prop pitch/high rpm (for takeoff, climb, and landing), and high pitch/low rpm for maximum efficiency cruise. It also performs its own self-diagnosis after starting to make sure its dual electronic ignitions and the hydraulic prop governor are operating normally.

Taxiing requires surprisingly heavy rudder pressure and some differential braking because of the large surface area the soft tundra tires put in contact with the ground. Visibility over the nose is limited by the airplane’s high deck angle, so the pilot relies on S-turns and unrestricted visibility out the sides through the airplane’s acrylic doors and windows. (The pilot also can taxi, or fly, with the doors fully open, or completely removed.)

Aligned with the grass Runway 22 with an eight-knot quartering headwind and flaperons up, I release the toe brakes, add full engine power, and try to keep my expectations low. In such thin air, the prop can’t provide as much thrust—or the wings as much lift—as they would at a saner altitude. Evaluating any airplane on such a day seems cruelly unfair.

But as the prop shifts to low gear and the turbocharged engine spits out a whopping 37 inches of manifold pressure, the airplane charges ahead energetically. A nudge of forward stick gets the tailwheel off the ground at about 20 knots, and holding a tail-low attitude has the airplane off the ground and climbing in about six seconds and 250 feet of ground run.

Climbing at 70 KIAS with two occupants, full fuel, and about 10 pounds of baggage nets a sustained 1,000-fpm rate of climb despite the absurd density altitude. Light to moderate right rudder is required to keep the slip/skid ball centered during the climb.

The Rotax 915iS turns a mind-bending 5,800 rpm at full power, and a reduction drive slows the propeller to a maximum 2,300 rpm. A fully automated turbocharger allows the engine to maintain full-rated power up to 15,000 feet.

Once in the practice area, a series of maneuvers including steep turns and lazy eights shows the airplane to be somewhat lighter in pitch than roll, and lightest of all in yaw. In other words, the rudder was very light throughout the airplane’s speed range, the elevator was kind of light, and the flaperons were normal.

The STi maximum roll rate is a spritely 60 degrees per second with flaperons up, and it slows dramatically and control forces stiffen when flaperons are fully deployed. Electric pitch trim via a rocker switch on the center console is highly effective, and it helps counteract a strong nose-down pitching moment when flaperons are deployed.

Slow flight is an area where the STi shines. There’s an aural low-speed alert at 60 mph indicated (52 knots), light airframe buffeting at 45 mph (39 knots), and a crisp stall break at 42 mph (36.5 knots) with flaperons up. Power-off stall speed drops to 38 mph (33 knots) with flaperons full down.

For landing, McBean advises a target airspeed of 60 mph on final approach with full flaperons and the throttle slightly above idle. That recommended speed sounds too fast for an airplane that stalls at 38 mph. The aviation rule of thumb that uses 1.3 times the dirty stall speed as a target on final would put the ideal approach speed at just 50 mph. But the faster speed, McBean advises, allows pilots to avoid pulling the throttle all the way to idle—and that prevents the propeller from moving to low pitch. Low prop pitch is a highly effective air brake, so it’s wise not to use it until you’re in ground effect and ready to touch down.

I make several three-point and wheel landings on the grass runway. Not surprisingly, three-point landings are consistently shorter, and the airplane rolls to a stop with little or no braking in about 300 feet. A steeper, slower approach and aggressive braking (especially at lower elevations) could easily cut that number in half.

Ground handling is obedient and uncomplicated. And frankly, takeoffs and landings are so short that there’s little time for things to go awry.

The journey

Kitfox got involved in backcountry flying decades before it was considered cool, and now the company is well positioned to take advantage of the growth that’s taking place around adventure flying in rugged and remote areas.

The STi can compete and win at STOL events against airplanes that cost many times as much. In the hands of a pilot attuned to its strengths, the STi is a spectacular performer that can operate off any runway—and most meadows, pastures, hillsides, lake beds, and riverbanks.

Its turbocharged engine makes the STi a stellar performer at high elevations, and its preference for car gas assures pilots can traverse remote regions where avgas may be difficult or even impossible to find. The STi is an adventure machine, and that fits well with pilots seeking to explore beyond the bounds of paved runways, swank FBOs, and expensive hamburgers.

Kitfox expected to sell about 120 kits in 2021, and the Series 7 STi is its most popular model. The company also builds six complete light sport aircraft per year at its factory.

“Our kits are 100 percent U.S. manufactured,” McBean said. “We source our parts in Idaho first, the United States second, and overseas third. About the only things we buy internationally are Rotax engines from Austria and MT props from Germany.”

McBean says his biggest frustration today is a too-long, two-year-plus backlog of kit orders waiting to be filled. He said the company is doing everything in its power to reduce the wait, and enable customers to fulfill their flying ambitions.

“Our customers want airplanes to fly in the backcountry—and our purpose is to enable them to do that well and safely,” he said. “That’s what we’ve done all along, and that’s what we’ll continue to do. We’re all about the journey.”

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