If you’ve ever fooled around with a gravity knife, you can imagine the eureka moment Dr. Reiner Stemme likely experienced while trying to develop a motorglider providing both uncompromising soaring and outstanding powered performance. Imagine a prop’s blade rather than a knife’s flicking out from the handle, and you’ll understand how a propeller that looks like it’s too big to fit pops out of the nose of the Stemme S10. It’s the high-performance motorglider that Dr. Stemme quit his day job as a laser physicist to create. The patented breakthrough allows the S10 to switch between powered and unpowered flight in seconds with no aerodynamic penalty or change in the aircraft’s center of gravity.
The S10 has been FAA certified and flying in the U.S. for more than 20 years, but in an era looking for lean and green solutions, it seems like the Stemme’s time has come. Its 115 hp Rotax 914 engine gives it a popular and efficient powerplant, the 50:1 glide ratio makes the Stemme capable of both high-performance glider competition and cross-country soaring, and today’s gliding computers and onboard weather products make it easier to find and exploit lift. That’s probably why Berlin-based Stemme AG is putting renewed focus on the U.S. market. Mark Stevenson, newly appointed president of Stemme USA in Lyons, Colo., and Andreas Hebner, Stemme AG’s Germany-based demonstration pilot, invited P&P to meet up at Maryland’s Frederick Municipal Airport (FDK) for a “soaring safari,” an Anglicism the Germans coined decades before the Beach Boys ever planted the idea of setting off in search of waves. Hebner had flown N137VT, an S10-VT—the turbocharged S10 variant—from Columbia, S.C., the previous day, and its wings were still folded back from its night in a hangar.
The Dream Of Wandersegelflug
Dr. Stemme was inspired by the wandersegelflug, or cross-country wandering flight concept, championed by glider pioneer Wolf Hirth in the 1920s and ’30s, a movement that spawned the development of motorgliders. All these hybrid aircraft incorporated design compromises, limiting their soaring or motoring capabilities, which Dr. Stemme sought to overcome. Wandersegelflug embraced total self-sufficiency for the glider and its pilot, as the S10 exemplifies.
“The whole concept of this aircraft is based on the fact that the pilot should be able to do everything by himself. He doesn’t need a second person to unfold the wings or fold them, or get him in the air with an aerotow,” Hebner said as he began to demonstrate the principle in action.
Equally comfortable gliding or touring, the S10-VT’s nosecone retracts over the stowable prop for aerodynamically clean soaring, while the midship-mounted Rotax 914 engine and IFR-certified panel make this a capable cross-country cruiser.
The left and right outboard sections of the 75-plus foot wing can be folded back, making the S10-VT about as wide across (some 37 feet) as a Cessna or Piper four-place single, easily fitting into a standard hangar. A hinged rod connects the left and right outboard sections with the wing’s center section, and a large carbon-fiber bolt in each outer wing slides into a corresponding socket in the center section. One person can swing each folded wing out, connect the aileron control cable and the electrical connector for the wingtip lights, and push together and mate the sections using a cylindrical hand lever tool to lock the joints. The company says the entire procedure can be done in less than 10 minutes; Hebner unfolded and connected each wing in about 60 seconds. Tiedown rings can be snapped in and out of a pair of hard point plugs under the wings, which alternatively can accommodate mounts for cameras or other equipment.
Stemme demo pilot Andreas Hebner prepares the S10-VT for flight.
The Prop Position
The nose of the aircraft is in its extended position, revealing the tucked propeller blades, for preflight inspection. Springs keep prop blades retracted when the engine is off, but can be manually extended for visual inspection. Centrifugal force extends the blades when power is on. The engine itself is amidships, aft of the cabin, connected to the propeller via a drive shaft routed unseen through the cabin. The Stemme also features side-by-side seating, another break with previous high-performance motorgliders, which makes instruction easier and improves the flight experience.
To enter the Stemme, position your back against the fuselage, palms behind you on the rim of the cabin, and hoist yourself up backward so you’re seated on the canopy rail. Turn and swing your legs into the cockpit and put your butt in the forward-and-aft adjustable seat. Seatback cushions are removable to make room for parachutes.
Flight instruments occupy the left side of the panel, engine instruments are on the right, and a Garmin GPSMAP 696 is in the center. The panel can be outfitted for glider operations, cross-country touring or both, and most buyers opt for the combination. Any serious glider pilot is going to spend time above 18,000, and the S10-VT’s service ceiling under motorized operations is 30,000 feet, so an IFR panel is de rigueur. The flights are allowed by the FAA when cleared into a “Soaring Box” that allows a block altitude operation, VFR only. The LX 9000 soaring computer and vario navigation system is another popular option, providing penetration speeds, speeds between lift and a selection of information critical for competing or cross-country soaring.
Stowable prop, authoritative tailplane, and finely engineered cockpit instrumentation distinguish the motorglider.
The cabin combines a modern, clean layout with precisely engineered parts and components, the cockpit practically bristling with levers and handles, including blue-handled air brake/spoiler control levers for left and right seats, dual control sticks equipped with handbrakes of the type typically seen on a bicycle (the pilot’s featuring a keypad input for the LX 9000), and an industrial-strength flap handle. The flap indicator on the console is marked from +10 to -10 degrees in five-degree increments (the negative settings for high-speed gliding) like a finely calibrated laboratory instrument.
Look Ma, No Towplane!
The turbocharged Rotax 914 takes a few moments longer than Rotaxes usually take to fire up, as the throttle is kept back to keep excessive power off the unfolding blades. The plane tracks well while taxiing, with the biggest challenge coming from potential obstructions—narrow ramp spaces or snow banks on the side of taxiways, for example. The S10 can taxi with one or both wings folded if clearance is an issue.
Anyone accustomed to getting airborne in a glider the conventional way can be excused for wondering where the towplane is as you line up on the active for departure. But, this is what wandersegelflug is all about! Five degrees of flaps are used for takeoff, as well as landing. Though the 914’s full 115 hp are available for takeoff, with plenty of runway, Hebner chose to pamper the engine and limit the output to 100 hp. It takes several seconds for the S10 to build up speed, and it seems to levitate rather than take off. With positive climb established, gear is retracted and flaps neutralized. Heat rising from downtown Frederick aided our climbout, producing a 500 to 600 fpm rise at 65 to 70 knots indicated. We headed for a valley just to the west of town where scattered small cumulus clouds marked rising thermals.
To help get where you’re going more efficiently when under power, the S10’s propeller has an adjustable pitch mechanism. When activated, an electrical element in the propeller hub heats and expands, pushing against the blade and turning it to a cruise from a climb configuration, an operation that takes about three minutes.
At 4,200 feet, a little less than 4,000 feet AGL, Hebner demonstrated engine shutdown and prop stowage. Simply pull back the power, turn off the ignition and verify the rpm reads zero, even as the prop continues windmilling. Two pull cord handles in mid panel labeled “1” and “2” control prop stowage. The first, when pulled smoothly, arrests the windmilling, allowing the spring retraction mechanism to pull the blades in against the hub. The “2” cord, pulled slowly, positions the hub to allow nose closure. A handle located below the prop wranglers pulls the nose cone over the nestled blades and closes the engine air intake vents on either side of the fuselage, restoring smooth, aerodynamic lines to the glider.
Sailing The Skies
Hebner set the glide computer to provide audible alerts of our vertical speed; the beeps growing higher in pitch and closer together as lift increased. Today, we were simply looking for thermals, but on a different day, we might have picked up a mountain wave generated by the Appalachians—air currents that can extend for hundreds of miles downwind of a mountain range and be ridden well into the flight levels, enabling a glider pilot to stay aloft for hours at a time. Hebner said his longest S10 glider flight was about six hours, flown over the Swiss Alps, and longest powered flight was 6.5 hours, covering more than 800 nm. That puts a premium on cabin comfort and functionality. The semi-recumbent seats are comfortable, with calf support keeping the knees above the thighs, and the instrumentation is easy to see even in direct sunlight. Visibility is superb, the bubble canopy wrapping over the semi-supine occupants as if displaying them in a deli meat case. The radiant heat can make ground ops warm in the summer—the canopy’s side vents are effective—but keeps the cabin comfortable at altitudes where outside temperatures are well below zero. Meanwhile, you can turn off the battery and power the avionics with optional solar panels embedded atop the fuselage, and take off your headsets and use the speakers and boom mic to monitor and communicate with ground controllers.
Slow flight characteristics are especially important in a glider. Tight, low-speed turns are essential for taking best advantage of thermals, and additionally without power to smooth landings, a glider must behave docilely through the stall. Tight, slow-speed turns, stalls in the landing configuration (at about 42 knots indicated) and recovery from a lazy spin provided plenty of confidence in the S10’s behavior at the bottom of the envelope. For tight climbing in a thermal, put in full flaps and keep the airspeed at about 45 knots. With strong lift, airspeed can go as low as 35 indicated, Hebner said. We exited the thermal near the cloud base, set the flaps to minus 10 degrees for high-speed flight (111 knots indicated), and raced toward the nearest cloud to pick up its underpinning lift and repeat the process as quickly as possible, as thermal riders are wont to do. “It’s kind of a dolphin flight,” Hebner said of the semi-porpoising flight profile.
Returning To Power
We could have glided back to the airport—now lost in the haze about a dozen miles east—from just about any point in our safari, but we decided to restore the power and motor back leisurely. Pushing the nose cone open, hitting the ignition and getting the engine air started took just a few seconds. The S10 can land as a powered aircraft or as a pure glider, and Hebner showcased its power-on performance in the pattern by having us execute a touch-and-go on our first landing. As either a glider or powered aircraft, the pattern speed is about 65 knots, and the airbrakes are needed to get the aircraft on the ground. Even with its landing gear out, the S10 has about a 30:1 glide ratio, with the airbrake bringing that down to the 7:1 range associated with a powered aircraft. It’s recommended to set up a stabilized approach without jockeying the airbrake on short final and execute a three-point landing. When you feel the wheels touch, go to full deployment on the airbrake, ensuring a gust of wind won’t kick you back into the air.
Even after barely testing the S10’s gliding and touring capabilities, its clear Dr. Stemme achieved a giant advance with the S10 in realizing the promise of practical and uncompromised engine-assisted soaring. As Hirth himself might have said after feeling the S10 thump against the runway after returning to earth, “Das wandersegelflug!”