Dave Sechrist’s F1G model design features
an extended motor run as well as some
interesting construction features.
The wing uses a homemade, carbon-fiber D-box covered with thin chrome
Mylar. The three-function timer is made
from a wind-up plastic mechanism.
An all-wood, full geodetic stabilizer provides a stiff
structure that’s easier to build than it looks.
by Louis Joyner
Dave Sechrist has been experimenting with long motor runs on his own-design
F1G Coupe. “I was trying to go for a
minute- 45,” he said. “You just cruise
around and the prop folds and you are
still a little ways up in the air.”
Because F1G is flown to a 2-minute
max for the rounds, this approach
has some merit in calm conditions. To
achieve the long propeller run, Dave
uses a longer motor with a smaller-than-usual cross-section.
A typical 10-gram F1G motor is 12
strands of 1/8-inch strip, giving a motor
run of approximately 40 seconds. On his
model, Dave uses 18 strands of 1/16-wide
rubber (the equivalent of nine strands of
1/8 rubber), allowing a big increase in the
number of turns.
The initial motor torque is lower,
which can make trimming the burst
easier, but this also slows the initial
climb through turbulence. Total climb
height is also reduced, but the glide
time needed to make the 120-second
max is less. “It’s not very good in windy
conditions,” said Dave. “At the Nats, it
went down with the prop still turning.”
Increasing the propeller diameter,
pitch, and blade area can also increase
the motor run. Dave’s model uses a 16-
inch diameter x 20-inch pitch propeller
(406.4 x 508mm). Some F1G propeller
hubs allow blade pitch to easily be
adjusted at the field.
In addition to the long motor run,
Dave’s F1G offers several unusual
features including that he built, rather
than bought, the model. The three-function timer was made from a wind-up plastic clock mechanism and scroll
purchased from Texas Timers.
He modified it for slower running and
added the scroll, disks, and wire-release
arms to operate the variable incidence
tailplane, auto rudder, and dethermalizer.
“I use the auto functions with 12 strands
of 1/8, but they are not needed for 22
strands of 1/16,” Dave added.
The wing uses a carbon-fiber D-box
that Dave vacuum-bagged over a form.
The finished D-box shell was then
attached to balsa front ribs and a carbon-balsa spar. Carbon-capped aft ribs tie the
D-box to a narrow TE. The stabilizer,
however, is all wood and uses the egg-crate construction popularized by Italian
modelers in the 1960s.
Throughout the years, I’ve used
egg-crate construction for wings and
stabilizers and can vouch for its high
torsional strength. It is much easier to
build than it looks.
As outlined in the 1959-61 Model
Aeronautic Year Book edited by Frank
Zaic, the secret is to use rectangular
strips for the ribs and then sand to airfoil
shape. For his stabilizer, Dave used .248-
inch strips of 1/32-inch balsa, 1/16-inch
square spars with webbing between, and
1/16-inch square basswood for the TE. A
scrap of D-box material from the wing
protects the center section from hold-down bands.
When the various components are
133 Model Aviation MAY 2014