A Stemme S10 VT soars over Torrey Pines, California, one of the most famous soaring sites in the U.S. When the engine is stopped, the Stemme's propeller
blades fold into the nose of the aircraft, leaving an aerodynamically clean fuselage.
MOTORGLIDERS ARE AIRCRAFT WITH good gliding and even soaring
performance, equipped with (usually small) engines. This allows
them not only to take off without a towplane or winch, like a traditional sailplane, but also to avoid “landing out” and having to be
retrieved after an intentional landing away from home base.
Motorgliders span the entire range of both powered and
unpowered performance, but we can divide them into two broad
classes: touring motorgliders (TMGs) and self-launching
sailplanes (SLSs). TMGs are generally quite airplane-like (typically
with the engine and propeller in front), but they are still much
more efficient aerodynamically than most lightplanes. The most
common metric here is what’s called “glide ratio,” since it
expresses the distance the aircraft will glide forward, power off,
for each unit of altitude lost. To put things in perspective, many
traditional lightplanes such as the Cessna 172 have glide ratios
around 7.5-to- 1—if you’re at 1,000 feet above ground level, you can
glide about a mile and a half. Some of the newer light-sport
aircraft (LSA), with their slick designs, are better, at around 10-to-
1, as are many retractables as long as the gear isn’t extended. Jet
airliners, with their long, narrow wings, do better yet, with glide
ratios in the mid-teens.
Typical TMGs are two-place, usually side-by-side, although there
are a few with tandem seating. They have wingspans of 50 feet or
more and glide ratios in the 20-to- 1 to 30-to- 1 range. Pure wingspan
isn’t all that’s important, though. Their wings also are relatively narrow (high aspect ratio), which reduces their drag at low speeds, and
total wing area is generous, which means the motorglider has a low
sink rate. So low, in fact, that on a day with good conditions, they not
only glide efficiently, but also soar–that is, gain altitude without
power in thermals or other updrafts. While older designs (the Grob
109, Valentin Taifun, or Ximango) are too heavy to qualify as LSA, several more recent designs (such as Pipistrel’s Virus, Sinus, and the
Czech-built Phoenix) take advantage of light carbon-fiber materials to
fall under the 1,320-pound gross weight limit of LSA.
Such gliding and soaring performance
comes at relatively low airspeeds—typically
around 50 to 60 knots for straight-ahead
gliding, and even lower when circling in a
thermal. What makes the TMG so versatile,
however, is that it remains quite efficient
even when operating power-on at higher
speeds. My treasured 1985 Valentin Taifun
17E was considered state of the art for
motorgliders in its day. With its 56-foot
wingspan, it glides a bit more than 28-to- 1
at 60 knots, and with camber-changing
flaps it thermals beautifully at 4 5 knots,
with a sink rate of less than 200 feet per
minute (fpm)—but it can also cruise all day
at 105 to 110 knots, burning all of 4 gallons
of gasoline per hour. For a comfortable two-place ship with plenty of room for baggage,
that’s pretty economical travel. In fact,
Taifuns have crossed the North Atlantic
and have been flown from Germany to
Nepal for use in weather research in the
Himalayas; gaggles of European motorglider pilots often undertake impressive
trips all over their continent and beyond,
just for fun.
Recent TMGs often use the common
80-hp water-cooled Rotax 912 series of
engines. Those from a couple of decades
ago generally use German powerplants
based on the most widely distributed four-cylinder opposed air-cooled engine in the
world: the Volkswagen. Propellers are often
featherable, either electrically or
