June 2007 137
DYNAMIC SOARING (DS) captivates many Slope
Soaring pilots. This month’s guest columnist is Spencer
Deputy ([email protected]) of Salt Lake City,
Utah, whose skill and experience flying gliders on the
dark side far exceeds mine.
The Addiction: The wind is up, yet the air is smooth,
and I am itching to get my sailplane into the air. After a
radio and control check I walk to the lee side of the
slope. I still get nervous on that first launch of the day,
and today the winds are ripping!
Gripping the model in my left hand, I hold it over my
head to get a feel for the air. The airfoil immediately
grabs the rushing air and pulls at my grip on the fuselage.
I try not to consider the possibility of a
difficult landing as I release the airplane and
watch it climb up and out.
In seconds the model has gained enough
elevation, and I send it diving into the valley
behind me. As it dips below my elevation, I
roll the wing perpendicular to the slope and
pull back on the elevator until the aircraft is
heading straight over top.
Thump! The sailplane rips through the
layer into the rushing air above. I hear and
feel what I have been craving all week.
I gently pull the elevator back and send
the model around for another circle. Again it
punches through the layer, and this time the
high-pitched whistle assures me that my
sailplane approves of the speed.
With each overhead pass the whistle’s
pitch grows higher and my mind struggles to
stay ahead of what the model is doing. In just
a few laps it is going almost faster than my
brain can track.
[[email protected]]
Radio Control Slope Soaring Dave Garwood
Essential DS elements: White model is top of
airplane entering backside turn. Black model shows
bottom of airplane entering topside turn. Red line
represents backside airflows. Green line traces
airplane’s path. Solid blue line marks shear layer.
Purple dotted line marks front, topside airflow. Chris
Tyler diagram.
Above: Spencer Deputy prepares
to launch Valenta Models’
Destiny at high ridge at Point of
the Mountain flying site (Draper
UT). He launches to the north,
into the wind, and into slope lift.
Daxton Self photo.
Left: Spencer crossed over to the
south, or downwind, side of the
hill and flies a DS pattern at high
speed. Spots in the sky are not
dust spots on the camera sensor;
they are parasailors in flight. Self
photo.
Spencer Deputy writes an introduction to dynamic soaring
Marty Hill (Malad ID) flies his Leading Edge Gliders Crossover at Francis Peak near
Farmington, Utah. Photograph by Tauno Knuuttila.
06sig5.QXD 4/23/07 9:18 AM Page 137
138 MODEL AVIATION
Ted Fraughton and Marty Hill fly Tuff Planes Electron DS60 and
Bowman’s Hobbies JW60 at Parker Mountain (Acton CA).
Knuuttila photo.
Ron Medel (Salt Lake City UT) flies his Leading Edge Gliders
Crossover on the backside at Francis Peak near Salt Lake City.
Knuuttila photo.
This time when it pops through the layer I roll the wing level and
yank back the elevator stick, sending the airplane into a vertical climb.
Up and up it goes, as if gravity has no effect.
What Is DS? It’s a technique of flying in which your sailplane
repeatedly crosses a horizontal shear layer between two air masses.
The top air mass will have a high velocity while the lower mass will
be fairly calm. The goal in DS is to gather speed in high-velocity air
and then retain that speed by diving through the shear layer into the
calmer air below.
While your model is in the calm air below, turn around for another
push from the high-velocity air above. When conditions are right the
sailplane will also gather energy in the backside turn.
With 15 mph winds, a good airplane and pilot can easily exceed
100 mph. Winds exceeding 25 mph can provide model speeds in
excess of 150 mph. In optimal conditions, many sailplanes and pilots
have exceeded 200 mph, with the current world record at 301 mph.
The diagram shows what is happening to the air at a DS location.
The air on top is rushing over from the windward, or “front,” side of
the slope. As this air flows overhead it creates a shear layer, separating
itself from the calmer air below.
As the airplane reaches higher speeds it can penetrate turbulent air
more confidently. At slower speeds it can be jostled or turned upside
down in a fraction of a second. For this reason it is important to dive
into the backside with plenty of kinetic energy, especially on a first
entry of the day.
In the diagram the red line represents the rotation of air in the
backside. At times an airplane may actually accelerate out of the
backside turn. This happens because the air circles down and climbs
back up the hill, similar to an eddy behind a large rock in a river.
This can vary by location because it is affected by the size, shape,
and conditions of the slope. A skilled pilot can change the shape and
size of the model’s pattern to maximize this potential energy.
During your first DS experience you will probably notice that it
takes much more elevator on the backside than on top. When your
sailplane punches through the layer into the top side, the airspeed is
high and the air is smooth, causing your control surfaces to be
extremely sensitive. In the backside turn the airspeed can be lower and
more turbulent, which means more elevator may be needed to keep
your model in a steady circular pattern.
DS is not limited to circles. Aerobatic patterns using the front and
back side of a slope open new doors for sailplanes and pilots. DS
combat and DS racing with foam aircraft are extremely popular.
Sailplanes for DS: Design and construction for DS is vital. Airfoil,
drag, spar strength and stiffness, and weight are important factors.
Most balsa wings cannot withstand the forces involved, nor do they
have enough weight. Anything you might use for Combat can get you
started with DS. Foam models are recommended for learning and
testing new sites.
A great intermediate to advanced foam airplane is the Bowman’s
Hobbies JW60. It is a 60-inch plank designed for DS. Not only is it
very stable in turbulence, but it’s durable in a crash.
Eventually you will find that composite designs offer superior
performance. Although their price might scare you at first, their
performance is worth the money.
Scouting a DS Location: When searching for a new flying spot, pay
attention to the birds. Ravens, sparrows, and an occasional hawk can
sometimes be seen DSing to quickly navigate mountain ridgelines.
Birds can be a big help in testing the air. If there is slope lift and/or
thermal activity, they will typically be playing in it.
Frontside lift is not the only element required for a good DS
location. The shape of the hill and the surrounding topography are
important, and smooth topography is preferable. Trees and/or brush
on the front and back of the slope can cause a turbulent layer, making
it difficult to keep an airplane under control.
Safety for yourself and others is imperative; an impact from an
airplane could cause serious bodily injury or property damage. Make
sure people and property are clear of the area. Flying near roads and
trails may be unwise. It is essential to use good judgment in deciding
when and where to fly.
The shape of your slope is another important factor. If a ridge is
too flat or round, the air will curve over the top and into the back side
without creating a shear layer. This problem is easy to spot because
there will be no sudden calm as you begin to walk down the backside
of the slope.
Another test of the layer is to throw a handful of grass from the top
of the slope. A good shear layer will carry the grass out over the
backside, staying above the layer before falling straight to the ground.
When a layer does not exist the grass will be sucked down the back of
the hill.
A final factor in picking your spot is the shape of the ridgeline and
the hills on either side. A straight ridgeline will work, but it is
preferable to have a bowl-shaped ridgeline either in front of or behind
you. This forces the air over the slope and prevents air from sneaking
around the sides, especially when wind direction is not perpendicular.
06sig5.QXD 4/23/07 9:21 AM Page 138
Some locations may have wind curving
around a low point on the ridgeline. That
causes undesirable backside airflows, which
increase turbulence and shrink the areas of
smooth air in your playground.
The Landing: One problem may be finding
a suitable landing zone. Some locations
offer only small and rocky landing areas.
Depending on obstacles, you may choose
a front- or backside landing. Opinions differ
on which is easier, but I prefer landing on
the front side. A backside landing requires
that you maneuver the model under the layer
and use circles, flaps, or spoilers to decrease
its speed. At the right moment the airplane
must be brought around, pointed at the right
position, and set down.
This landing requires commitment
because once the model has slowed, there is
no inertia to carry it around to try again. I
am less fond of backside landings because
of the sporadic turbulence, which can be
even more violent for a sailplane at lower
speeds.
In a frontside landing, start with the
model low on the slope and then climb at an
angle against the hill toward your position.
Speed is bled off by climbing the slope and
further controlled with flaps or spoilers. The
goal is to stall the model into the chosen
landing zone when ground speed nears zero.
Spoilerons (upward-deployed
ailerons) work great for frontside
landings. There is no need to force these
landings because the airplane can go
around as many times as needed.
Talk the Talk:
• Dark Side: Also referring to the downwind
side of the slope, this term refers more to the
high-speed, unpredictable element, which
can cause adrenaline addiction and
spectacular crashes.
• Groove or Grooving: The pattern with the
most energy. It also refers to a state of mind.
• Lift: Area on the slope’s front side,
offering rising air for your model.
• Flutter: Rapid oscillation of a wing or
control surface. The fluttering component
forms a resonant vibration, which can
amplify across a surface, which can strip
servos or shatter wings, ailerons, and
elevator. Minimizing the chance of flutter is
an important consideration in designing a
DS sailplane.
DS is a unique sport, and those who commit
to the relationship seem to fall hopelessly in
love. It takes a pilot who is not only skilled
enough to perform the demanding flying,
but is also able to access suitable locations,
which may require hiking. DS requires
patience and weather-watching skills, and
the great DS days require that you be at
Mother Nature’s beck and call.
When all elements fall together, there is
nothing more exhilarating than a big day
with a fast sailplane. But don’t take my word
for it; find your local site and see for
yourself what all the excitement is about. MA
Sources:
DS article
www.geocities.com/soaringbythebay/dsoar.
htm
Tuff Planes DS article
www.tuffplanes.com/Flying/Flying_Technics/
Dynamic_Soaring.html
General article about DS
www.en.wikipedia.org/wiki/Dynamic_soaring
(Lookup term: dynamic soaring)
Bill Patterson art, DS videos
www.billpattersonart.com/dszone.swf
Crossover (aka Jackhammer) kit:
Leading Edge Gliders
www.leadingedgegliders.com
JW60 kit:
Bowman’s Hobbies
www.bowmanshobbies.com
Valenta Models Destiny:
Soaring USA
www.soaringusa.com
Lift Ticket video with DS speed-record
footage:
Reese Productions
www.reeseproductions.com
RC GROUPS—online discussion and
message base
www.rcgroups.com