Stemme S12: Touring and Soaring

With a Rotax 914 engine, an autopilot/EFIS and baggage space, Stemmes S12 offers the best of two worlds.

Stemme’s Twin Voyager S12 next-generation motorglider might be the ultimate niche aircraft. It’s not intended for IFR flying and carrying more than two people is out of the question. Despite an impressive 53:1 glide ratio, it’s not regarded as a competition glider, although it has set world records. But for as many things as the German-made S12 is not, it does so many things we’ll enough that, like previous S-series models, it promises to thrive in a class of its own.

Aircraft Flight Trial

As Stemme prepares for its first S12 U.S. delivery this December, we spent a couple days pawing and flying D-KSIZ—the certification flight test aircraft—at Stemme’s U.S. headquarters in Columbia, South Carolina, to see just what the growing list of buyers get for its $369,000 starting price.

Certification Matters

The EASA-certified S12 is pending FAA certification under FAR Part 22 (motorglider) and an airworthiness certificate is expected in the coming months. While it waits, Stemme isn’t holding back deliveries. Aircraft delivered before FAA certification will be operated in the experimental category. To legally fly the S12, you need a glider rating and a logbook endorsement from a CFIG holding a motorglider endorsement. We suggest getting solid training before flying the S12 alone, especially if you don’t have experience operating at high altitudes (it has a 32,500-foot service ceiling). It’s also a taildragger, although it doesn’t exactly handle like one on the ground.

The side-by-side-seating, carbon fiber S12 is a derivative of the shorter-winged S10VT, which will remain in Stemme’s lineup. Previous Stemme motorgliders, including the discontinued S6 and current-production S10VT, have a reputation for being complex aircraft and they are. The landing gear is retractable and in the S10VT and new S12, so is the propeller, which the pilot stows in the fully closable nose dome via push/pull mechanical levers.

Stemme S12

The carbureted and turbonormalized liquid-cooled engine in the S12 is based on the certified four-cylinder Rotax 914 F2-01. Stemme slightly modified the 914 (repositioning the air induction and turbocharger mounting scheme, to name a couple of tweaks), so it sits in a steel tube frame in the midsection of the aircraft, just aft of the occupants. This places the engine near the aircraft’s CG, which is favorable when it comes to pitch control and stability. The tailplane is of a “T” design and the wing (all 82 feet of it) is mounted to the fuselage in the upper third. With 215 feet of wing area, the S12 is as much a sailplane as it is an airplane. There’s an inner wing with flaps and Schempp-Hirth airbrakes, foldable outer wings with continuous ailerons, two wing extensions with ailerons and two winglets.

The engine is connected to the variable-speed composite propeller (designed by Stemme) with a composite driveshaft. The propeller’s manual pitch change from takeoff to cruise position is accomplished by electrically heated expansion elements, and from cruise back to the takeoff pitch position by unheated elements via spring force.

Two fuel tanks are located at each end of the inner wing, which supply the feeder tank in the fuselage via electric fuel pumps. Fuel is supplied to the engine by a main pump, or with a backup auxiliary pump. Total available fuel capacity is 32.3 gallons.

Stemme S12

Avionics, Loading

The S12 has practical avionics, but that’s not to say it has a stark panel. While buyers can choose—to a point—what equipment they want, the motorglider will be type certified with Dynon’s EFIS-D10A and AP74 autopilot, plus an electric trim system. It’s a huge challenge getting an autopilot to work we’ll with a wing of this size, but we found it to work reasonably we’ll in calm air. Worth noting is that engine torque and a sideslip rolling moment tend to slightly lower the S12’s right wing during fast cruise at high power settings, although we didn’t notice any issues while the Dynon AP74 was engaged.

Since it’s only designed for cruise flight and not for approaches, the autopilot won’t even engage at speeds under 60 knots. It tracks a course from the NMEA output of a Garmin aera 660 or GPS796 GPS. Missing is a pricey GTN navigator because frankly, it just isn’t needed in this VFR machine, plus it wouldn’t fit.

The S12 we flew had a Slovenian-made LXNav LX9070 soaring computer/variometer and mapping system, and there’s an option for a FLARM traffic awareness system. New to soaring? You’ll learn early on (hopefully not by experience) that midair collision risk in glider ops, particularly during soaring events, is stupidly high—as in be ready to bail out of the thing. But, glider-to-glider FLARM systems are taming that dragon. Stemme delivers the S12 with ADS-B In and Out, a Becker transceiver, Becker transponder, a fuel flow computer and round-gauge backup instruments.

Stemme S12

Stemme S12

Total pressure, static pressure and TEC (total energy compensated pressure) are measured with a detachable pitot tube on the propeller dome.

Standard is an engine bay fire warning system, which generates an audible alert with the landing gear warning horn, and optically with a red warning light on the panel.

The S12 has an empty weight of 1521 pounds and a maximum takeoff weight of 1984 pounds. Stemme listened to owners of previous models and included a luggage compartment that’s accessed through a door on the tail boom. It holds a maximum of 44 pounds and can accommodate soft duffle bags. When traveling with the S12, leave your golf bag and snowboard at home—this machine is not a hauler. There is a lower storage compartment located behind each seat that accommodates 22 pounds of stuff.

Stemme added a water ballast tank in the tail to help with CG control when the aircraft is loaded with heavy occupants. Filling (up to 15 liters) is done by pouring water in the right shell of the vertical tail. A scale directly beside sealable holes shows the corresponding water amount.

Stemme S12

Flying It

Stemme S12

As you might expect, taxiing the motorglider with its 82-foot wingspan presents some challenges. Barely clearing the hangars and fuel trucks on the ramp, the wings overhung the edges of the big main taxiway at South Carolina’s Columbia Metro Airport. Hydraulic disk brakes are operated with a lever on each control stick—no toe brakes.

Compared to other conventional gear aircraft, the S12 is easy to taxi because it has a large tail boom and a steerable tailwheel. The S12 is significantly easier to handle on the ground thanks to a wider main landing gear stance, compared to the S10. A wider stance is also required for the S12’s longer wing.

With the propeller in the takeoff position and cowl flaps open, we used the 115 percent maximum takeoff power setting (limited to five minutes), which is roughly 5500 RPM. The throttle has two stops: push straight forward for 100 percent power and then push the lever slightly to the left and forward for 115 percent power.

Stemme S12

Hold a tail-low configuration and the S12 levitates off the runway. With Stemme U.S.A.’s Wes Chumley coaching from the right seat, the ground roll was approximately 600 feet and we saw a climb rate of around 600 FPM. Set a 62-knot climb speed and retract the main landing gear, which takes roughly 40 seconds to fully retract. In a maximum continuous climb configuration, we saw fuel burn in the 7.5 GPH range, which yielded a 300 FPM climb rate at 90 knots.

Controlling whether the left or right fuel tank feeds the engine is straightforward with a panel-mounted selector valve. The auxiliary fuel pump is switched off when passing 1000 feet in the initial climb. It’s easy to speed up to a cruise configuration thanks to manual flaps that can be deflected upward, or in a negative configuration. Cycling the propeller into cruise pitch mode (or from cruise to takeoff) takes approximately two minutes.

The S12 has pleasant handling characteristics. The flight controls are pushrod driven and the rudder system uses cables. The rudder pedals are adjustable, but the seating position is not. Stemme added an electric pitch trim system to the S12, which makes it easy to trim for level flight. Once in cruise, the motorglider has long legs and impressive speed. Stemme publishes the S12’s maximum cruise speed at 140 knots true at 10,000 feet. In a no-wind scenario, you could fly for 950 nautical miles. The Rotax can burn as little as 5 GPH and it’s approved to run on ethenol-free premium unleaded gasoline and 100LL.

The appeal of the S12 is the ability to shut its engine down and soar wherever and whenever you wish. To do that, the propeller’s pitch is cycled back into takeoff mode and you’ll want to reduce the power to keep the CHT and oil temperature below 212 degrees Fahrenheit. With the cowl flaps fully open, the Rotax cools quickly, which helps to reduce the risk of overheating during engine restart. Before shutdown, the airspeed is reduced to approximately 54 knots.

Once slowed and cooled, simply turn the ignition off and you are now in sailplane mode. The S12 is equipped with a propeller brake to stop its windmilling, which reduces wear on the propeller clutch. Pull the brake handle, turn off the fuel and close the propeller dome.

The optimum speed to maneuver the sailplane is between 65 and 70 knots. The amount of adverse yaw you’ll experience depends on the amount of aileron input. With the huge wing, be ready for lots of rudder input. Even so, crank the sailplane into a steep bank with the right amount of rudder input and the S12 is easy to maintain in coordinated flight. Again, it’s nice having the weight of the engine close to the CG. Less aileron input still results in a decent roll rate without the need for lots of rudder input. Plus, it has impressive roll stability. Set a roll rate, dial in a touch of up trim and the sailplane will fly that configuration hands off all day long.

Slips aren’t approved with the gear down because of the gear doors and the S12 is not approved for spins, although they have been demonstrated. Aerobatics aren’t approved. With a stall speed of 42 knots, the S12 offers plenty of warning before it stops flying. There’s buffeting, but no dramatic wing drop. We were able to get it to break to one side during an aggressive pull, but relaxing the stick immediately got the wings flying again.

Transitioning back to the powered configuration requires slowing below 76 knots. Turn the fuel on, open and lock the propeller dome, open the cowl flaps and move the propeller into the takeoff pitch position. Since the Rotax is carbureted, you’ll need to turn on the choke when the engine is cold. Set the power to idle and turn the ignition to the start mode for a few seconds. An automatic electronic device adds the ignition with a three-second time delay, which allows the propeller blades to fully deploy before the engine starts. Once the engine starts, turn the ignition to “both,” advance the throttle to 2000 RPM and come off the choke as the engine warms. Landing can be done in powered or gliding configuration.

For landing, the changeover of the propeller blade pitch can take up to five minutes, so you need to be mindful of it in case of a go-around. The landing gear is extended at 59 knots and total extension time is roughly 30 seconds. The airbrakes are a huge tool for speed control throughout the approach and landing phase. With them fully extended and the propeller dome closed, the glide ratio is 7:1 at 59 knots. You don’t want to flare too low in this aircraft because of its tall landing gear, and touchdowns are made in a three-point attitude.

Stemme S12 easy wing folding

Stemme S12 easy wing folding

Wing Folding: How Easy?

The reality of owning a motorglider with an 82-foot wingspan is that you’ll have to deal with folding and unfolding the wings, should you store it in a typical personal hangar. Fortunately, Stemme has made the task much easier on the S12 than it was with its S6 model, which required several people and at least an hour to accomplish. We had our hands in the task when we flew the S12, and while it may be a deal breaker for some (those who are physically and strength challenged), the mechanical process is quite straightforward.

The folding/locking mechanism on the S12 is identical to the one used on the S10VT and once you’ve attached and detached the wing sections a few times, the task can be accomplished in roughly 15 minutes.
What you’re really doing is attaching and detaching the outer wing extensions—not the entire wing. When folded back, the wing rests on the surface of the tail boom making it possible to pull the motorglider in and out of a standard 40-foot hangar.

To fold each wing, unsecure the attachment pin with a lever mounted on the underside of the wing, pull outward and pivot the wing aft toward the tail boom. To reattach, pick the wing up, walk it back, slide it into position and lock it. It really is as easy as Stemme says it is.

Who Buys the Stemme?

“In general, folks aren’t going to buy the S12 for serious Open Class competition soaring because there are cheaper alternatives. Plus, it would be like bringing a Ferrari to a Formula One race,” said Stemme U.S.A.’s Wes Chumley. Still, Chumley has flown the S12 in contests (and won) and Stemme motorgliders have set world distance records. A Stemme was recently used to reach and soar above the peak of Mount Everest.

But for more pedestrian pilots who can afford the eye-widening $369,000 starting price, the S12 is about convenience. The ability to self launch and motor—at decent speed—to the best soaring conditions you can find without having to trailer it back to the hangar has sizable appeal, in our view.

The S12 is delivered to and assembled by distributor Stemme U.S.A., a division of the respected Bell Aviation in West Columbia, South Carolina. There is also an established and growing North American service and maintenance network to support the product after delivery.


Larry Anglisano
Editor in Chief Larry Anglisano has been a staple at Aviation Consumer since 1995. An active land, sea and glider pilot, Larry has over 30 years’ experience as an avionics repairman and flight test pilot. He’s the editorial director overseeing sister publications Aviation Safety magazine, IFR magazine and is a regular contributor to KITPLANES magazine with his Avionics Bootcamp column.