ow do you get a 1⁄3-scale model glider
into the sky?
You tow it with a 1⁄3-scale model towplane!
Aerotowing works well for full-scale
gliders, so it is no surprise that it works well
for models. It is the safest way to launch
large model gliders.
Oc-Tow-berfest 2000 was held September
29-October 1 near St. Louis MO. This was
the second year for the event. Pilots came
from New York, Michigan, Texas,
Wisconsin, North Carolina, Indiana, Illinois,
Iowa, and Missouri to participate.
Aerotow events are building in popularity. At
the end of this article I include a Web site with an
aerotowing contest calendar. At least 15 such
events were held in the United States last year.
Scheduling the event in the fall makes good
weather likely; this year it was perfect.
One pilot pointed out that for a good turnout
at an aerotow event, it is important to schedule
several days of flying. Many pilots won’t drive
halfway across the country for one day of flying
that could have bad weather.
Aerotowing joins the Radio Control (RC)
power fliers with glider fliers.
Contest Director Pete George enlisted the
help of the Midwest Air Wing (MAW) R/C
Club, which is primarily a power airplane group.
The event was held at the MAW flying
field—a nice facility northeast of St. Louis,
near Pontoon Beach IL. The friendly hosts
barbecued a vast quantity of ribs at the field
for Saturday dinner.
Pete also arranged for publicity on the
Rick Lake’s (Tecumseh, Michigan) 1⁄3-scale ASK-18 from a Pribek kit won Best Vintage.
The model weighs 18 pounds and spans 5.6 meters. Photo by Scott Miller.
n Mike Garton
Pete George’s Do-27 won Best Tug. Inset shows the Do-27’s top; machined-aluminum tow release is built into the top of the wing.
54 M ODEL AVIATION
Oc-Tow-berfest
March 2001 55
The German Connection
Full-scale and model glider-flying is very popular in Germany.
The number of model manufacturers and pilots is
disproportionately large for the country’s population.
High-quality large-scale glider kits are common in Germany. Not
only can you buy a model of your favorite full-scale airplane, you
can probably find it in several sizes.
Most large-scale gliders flown in the US have been imported from
Germany, and the typical level of prefabrication is very high; we
consider these kits Almost Ready-to-Fly (ARF).
White gel-coated fiberglass fuselages are the norm. The (relatively)
inexpensive kits have foam-core wings, presheeted with obechi veneer.
Fiberglass hollow molded wings are becoming common on the
high-end models. These wings are laid up in female molds. Unlike
fiberglass fuselages, the wing skins are a sandwich of fiberglass,
high-density foam or balsa, and fiberglass.
A carbon-spar structure is used in addition to the fiberglass
sandwich skins. This is the same construction method used with
modern full-scale gliders.
—Mike Garton
Mike Watson (Asheville NC) greases another landing with his six-meter ASH-26. This glider won pilots’ choice Best Modern.
Sailplane & Electric Modeler author Robin Lehman of Rochester, New
York preps his 1⁄3.75-scale Fox for another aerobatic flight.
A close look at Pete George’s Duo Discus shows sliding
vent windows and yaw-indicating strings on the canopy.
berfest
Photos by the author Graphic Design by Jill Ann Cavanaugh
56 M ODEL AVIATION
television news. I can’t imagine how to
make the event run any smoother than it did.
The average glider at this event had a fivemeter
wingspan and weighed more than 20
pounds. Most models had functioning
retractable main landing gear. Full-house
controls with elevator, rudder, flaps, ailerons,
gear, and nose release were the norm.
The contest is basically a fun-fly. Pilots
flew as often as they wanted, by grabbing
the frequency pin and putting their airplanes
in the line for a tow. This is a low-key and
low-stress format.
Pilots’ Choice awards were given. No
Scale documentation was used; basically, the
pilots voted for the most impressive models.
The three categories were Best Vintage, Best
Modern, and Best Towplane.
There was no cutoff year for Vintage. Glass
Slippers were considered modern; fabriccovered
open structures were Vintage. The
ASK-18s fit into Vintage because the full-scale
gliders use a fabric-covered fuselage.
Rick Lake of Tecumseh MI won Best
Vintage. Pribeck was the manufacturer of
his ASK-18 kit. The 1⁄3-scale model had a
span of 181⁄2 feet and weighed 20 pounds.
Mike Watson of Asheville NC won Best
Modern with his ASH-26, which was allmolded
by Schuller. The model had a sixmeter
span, which was 1⁄3 of the full scale.
Mike reported that it took 40-50 hours of
labor to install the radio and detail the cockpit.
He used two five-cell packs and a battery
backup system. The five-cell pack gives the
servos more speed and power, and it corrects
for the voltage drop over very long servo
leads. Aileron servo leads in an airplane this
size would be eight to 10 feet long.
Best Tug was awarded to Pete George’s
Dornier Do 27. This workhorse did most of
Pete George tows Robin Lehman’s model. In back, Rick Lake holds his 20-pound ASK-18.
Pit area and flightline. Wayne Wimbush’s (St. Charles MO) ASW-24 is in foreground.
the towing at Oc-Tow-berfest. The kit was
manufactured by Voigt ModelBau.
The model had a fiberglass fuselage,
rudder, and cowl. The two-piece wings were
obechi-sheeted with fiberglass wingtips.
Aluminum tubes were used as load-carrying
members in the plug-in wing and stabilizers.
The Do 27’s landing gear was scale, with
functioning shock absorbers. The pneumatic
shocks had a travel of roughly two inches;
they seemed to be set perfectly to dampen
bounces on landing. On a (rare) hard landing,
the airplane would just squat slightly, then roll
without bouncing.
The Do 27 had leading-edge slats. These
fiberglass parts functioned identically to the
full-scale version, and they were fixed.
Slats are slots in the leading edge that allow
some high-pressure air from the underside of the
wing to flow through to the top side of the wing.
The slats energize the flow, to allow higher lift
and slower speeds without stalling.
The Do 27 weighed approximately 32
pounds. It used a Brison 4.2 gasoline engine
with C&H ignition and J&H muffler. Pete
estimated that typical static thrust was roughly
32 pounds at 7,200 rpm. His favorite propeller
is a Bolly CF 24 x 10.
With this power plant, the model could
tow airplanes that weighed as much as 35
pounds (roughly).
Some rivets on the Do 27 fuselage were
molded in. The ones around the windows came
with the kit, and they actually do hold the
windows in place. The 50-ounce gas tank is
visible through the larger windows.
Pete gets 10-14 tows per tank. He and
Landon Grindstaff must have put at least 20
hours of flight time on the Do 27 during the
three days. The engine-mount bolts had to be
retightened once.
Pete is an expert builder and flier.
Robin Lehman (Rochester NY) and Gene
Barker (Champaign IL) were the other tow
pilots. Gene used a 1⁄4-scale Cub.
Pete also brought a 1⁄3-scale Sig
Spacewalker. It weighed 25 pounds with a
Brison 3.2 engine, which swung a 22-6 x 10
wooden Zinger propeller at 6,700 rpm. He
used a C&H ignition and a J&H muffler.
With this engine, the Spacewalker could tow
airplanes that weighed as much as 26 pounds.
Aerotowing Primer: Gliders are fitted with a
“nose release” for aerotowing. Belly hooks used
for winching will not work for aerotowing.
The nose release is located in the front-most
part of the glider’s nose. This servo-controlled
device can hold the loop of string on the end of
the towline and release it when the pilot gives
the command on his or her transmitter.
Inexpensive commercial nose releases are
available. Most just capture a loop of line
inserted into the hole.
Ball-and-socket releases are not
recommended; they are less reliable and they
require retrieving the ball from the towline.
Typical steady-state towline tensions are a
fraction of the weight of the glider. The
glider’s weight divided by its glide ratio
would be a first-order estimate. Dynamic
effects make much higher tension.
If the line goes slack then snaps tight, the
momentary tension will be high. A stretchy
Asheville NC contestants Mike Watson,
Landon Grindstaff, and Tobi Grether
made this trailer to haul their airplanes.
Interior of Mike Watson’s ASH-26. Back-up
battery system has working indicator light
on dashboard. Uses two five-cell packs.
Pete George was an excel lent host,
Contest Director, and primary tow pilot at
the second Oc-Tow-berfest.
towline is used to spread out the shock force
and reduce its magnitude.
For safety, the towplane has the ability to
drop the towed airplane by radio control.
Either pilot can disengage the tow if
something goes wrong. The towline stays
with the towplane during a normal flight.
When the airplanes get severely out of
position or an oscillation starts, the tow must
be aborted immediately. It is best to release
the line, land, and try again.
At Oc-Tow-berfest, 201 tows were executed.
There were no tow-related crashes, which is a
testament to the safety of the aerotowing method.
I saw a couple tows where the tow pilot
had to release the line quickly. On one tow,
both pilots dropped the line simultaneously
after a bobble.
The early releases were made because the
glider pilots had visibility-related orientation
problems at high altitudes. High aspect ratio
white wings are difficult to see on hazy,
overcast days.
Approximately half the pilots at Oc-Towberfest
added nonscale dark colors on the
bottom of their gliders’ wings for safety. The
colors may not be true Scale, but they make
the glider more practical to fly.
I did not see any early releases at low altitude.
The towplane release mechanism is located
on top of the center of the wing for high-wing
airplanes. The mechanism is located on top of
the fuselage for low-wing aircraft.
It is naive to think the towline would
work if it was attached to the rear of the
towplane. In response to control inputs,
airplanes pivot about their centers of gravity
(CG). Locating the towline-release fixture
near the CG is a good thing.
If the line was attached to the rear of the
towplane, its rudder and elevator would have
little effect; an out-of-position glider would
stall it. With the line attached near the CG,
the towplane can still maneuver independent
of the glider.
The towlines used at Oc-Tow-berfest
were 100-115 feet of braided polypropylene.
This material was chosen because it is
springy. Non-springy lines can be used if
they are in series, with a bungee to add a
The glider pilot should make sure he is
released before he announces it to the tow pilot.
Credit Where it is Due: In addition to asking
Pete George more than a hundred questions, two
other references were particularly useful to me.
Robin Lehman’s two-part article in
Sailplane & Electric Modeler magazine
volumes 2-2 and 2-3 (1997) is the keynote
American reference on RC aerotowing.
If you have an interest in aerotowing, it is
worth the money to order the magazine back
issues or a reprint of Robin’s articles. He goes
into great detail, with many specific examples
of engine/towplane combinations.
I explained the towing techniques here,
because Model Aviation has a much larger
circulation than Sailplane & Electric Modeler,
and it includes other types of modelers.
Another key reference is John Derstine’s
Web site. The pictures, timely information, and
especially the articles about the subject are very
good. It can be used as a jumping-off point for
all types of information about aerotowing.
John has links to all sources of largescale
gliders and towplanes. There is even
a model-airplane-specific German-to-
English dictionary. MA
Sources:
Robin Lehman’s Aerotowing articles in
volumes 2-2 and 2-3 (1997) of Sailplane &
Electric Modeler magazine:
Sailplane & Electric Modeler magazine
Box 4250, West Richland WA 99353-4250
(509) 627-0456
www.SailplaneModeler.com/
John Derstine’s Scale Soaring Web page (lots
of good aerotow information and events):
www.geocities.com/CapeCanaveral/Lab/5739/
Mike Garton
506 NE 6th St.
Ankeny IA 50021
[email protected]
shock absorber. The polypropylene is simple
and relatively inexpensive.
Each end of the towline has a tensionrated
fishing snap. A small loop of string is
used to connect the towline to each
airplane’s tow release.
It is good towing etiquette to put a loop
of string in the glider’s nose release before
going out onto the runway. This allows the
towline to be snapped on quickly.
Some aerotow pilots like to have a
streamer on the towline, ahead of the glider;
this gives a visual indication of when the
glider has released. Pete chose not to use a
streamer, because it increased the chance that
the towline would be caught in the weeds just
off the end of the runway during a landing.
Towlines are easy to lose when both
airplanes drop them at altitude; it is best to
keep the lines cheap and simple.
Towing Procedure: Rudder-to-aileron coupling
should be turned off in the glider. The two pilots
stand near each other, so they can talk.
The tug is taxied into position, hooked
up, then the slack is taken out of the towline.
Each pilot should keep one finger on the
release switch at all times.
After the glider pilot says “go,” the tow
pilot slowly ramps up to full throttle. Crosscontrol
may be needed to keep the wings
level for crosswind takeoffs. The glider
should lift off first.
Proper position for the airplane being
towed is above and behind the towplane.
Gliders usually require up-trim to stabilize
in this position; it is nice to have an elevator
preset on your radio for this.
The towplane will require down-trim
when towing a glider.
The glider pilot should keep the wings level
with the ailerons, and let the towplane drag the
glider through the turns. The glider may require
opposite aileron in the turns. Get the models
trimmed to maintain proper position while they
are still low enough to see well.
The towplane generally makes huge circles
in the sky, away from the sun. Zigzagging
upwind is a good pattern if it is very windy. All
turns should be very large in diameter.
March 2001 57
