100 M ODEL AVIATION
i doubT that I would have difficulty finding
25 Scale Aerobatics modelers who agree that
lighter airplanes fly better. However, hidden
in that survey is the fact that there is no
consensus on what “lighter” actually means.
A few years ago, there were a number of
builders whose common goal was to build as
light as possible—lighter was better, period.
More recently, although building light is
still an important aspect of Scale Aerobatics
modeling, a new philosophy of optimum
weight has emerged—especially with the
larger Giant Scale airplanes.
A 25% airplane can’t be built too light.
Because of scaling effects, that doesn’t seem
to be true for some Giant Scale designs.
However, that doesn’t mean weight is not an
issue; weight is always an issue, and excess
should be avoided. As builders, we try to
recognize the difference between weight we
need (structure weight) and weight that is extra.
Lightening takes a bit of intuition,
engineering skills, and a healthy dose of
common sense. A $15 postal scale (available at
most office-supply stores) is a great help—
especially when making component selections.
The common-sense aspect is identifying
weight opportunities, while not
compromising the integrity of the airframe.
I watched a YS 1.20 literally fall off the
nose of a 1⁄4-scale Laser, because the builder
removed too much structure in his quest to
build “super light.” There are limits.
Recognize also that a couple ounces off
the tail may be a benefit, whereas the same
weight removed closer to the center of
gravity may not be.
Lightening can also be an exercise in
diminishing returns—especially on bigger
airplanes. One can pencil a few kit
modifications, plan the component group,
and the airplane will finish light. After that,
removing the next few ounces may require
an inordinate amount of effort, sometimes
with questionable benefit.
Some builders have experimented with
extremely lightened airframes. They have
lightened foam wings, put lightening holes
in plywood formers, replaced ABS (plastic)
with fiberglass, cored out foam decks,
composite landing gear, etc.
What emerged were airplanes that some
pilots felt were “too light” (and in extreme cases,
too weak), proving that not being heavy enough
was possible. At this year’s Tournament of
Champions, a few airplanes had lightening holes
in the rudder—and big strips of lead under the
tail.
There are three schools of thought on building
light. The first is not building light—just
Paul Kopp, 1013 S. Sedona Ln., Anaheim CA 92808
RADIO CONTROL SCALE AEROBATICS
The winner of the 2000 Tournament of Champions, Christophe Paysant Le roux of
France, starts his ZN Line Extra 330L during the finals on Sunday.
Well-known FAI and TOC competitor Chip Hyde (right) with his 2000 Tournament of
Champions backup airplane. On the left is the airplane’s builder, Scott Anderson.
Look again! This is 2000 Tournament of Champions competitor Frazer Briggs’
removable pilot, who has his arms in the cowl of Frazer’s Extra 260.
March 2001 101
paraded as such. It consists of using a larger
engine (in some cases, a much larger) in an
airplane designed for a particular size engine,
reinforcing the airframe, and paying little or no
attention to what components are used or how
the airplane is finished.
When the airplane finishes well exceeding
the manufacturer’s target weight, the builder
concludes that this was how the airplane
should have been designed in the first place.
A common example is 72-inch 1.20-size
airplanes that are mated with 1.80 twostrokes,
and tip the scale at more than 13
pounds. The airplane may fly okay by the
pilot’s standard, but it certainly isn’t light.
The next school is to build the airplane
more or less stock, using the recommended
size of engine. Although the stock airframe
may be reasonably light, aircraft
performance is compromised by excess
weight resulting from a poor choice of
components. Tires, tailwheel, spinner,
servos, engine mount, batteries, and
especially engine, impact finished weight,
and ultimately flight performance.
It is common to read kit reviews where
the model’s finished weight is one to four
pounds more than the published weight—
and that’s a lot on a 70- to 80-inch airplane.
This generally occurs, not because the
finished weight was understated, but
because the builder made poor choices of
components and finishing techniques.
To illustrate, I’ll start with the nose and
move backward, looking for places to save
weight on an otherwise stock airplane.
Many modelers favor the rigidity of spunaluminum
spinners. Tru-Turn and other
companies offer lightened backplates for some
of their spinners, and the extra cost is minimal.
The weight savings of a single item
such as a spinner may not seem to be
much, but it adds up when combined with
other components. Depending on the
airplane, significant weight “savings” are
possible by a careful selection of the
component group.
Morever, since the spinner is the farthestforward
part of the airplane, a one- to twoounce
weight loss in the nose may equal three
or four ounces behind the firewall.
I recently exchanged a lower-priced 3.5-
inch-diameter spinner for a high-quality
aluminum spinner with a lightened
backplate—a 11⁄2-ounce weight savings. I also
swapped a 20-inch carbon propeller for a
wood Menz, and saved another two ounces.
On a 14-pound airplane, almost 1⁄4 pound
was removed from the nose by changing
components. The airplane was lighter, and
vertical performance was improved. And
since the airplane was nose-heavy, I
preferred to reduce nose weight rather than
add weight to the tail.
Next is the engine mount, which is rarely
an issue on larger airplanes with engines
that bolt directly to the firewall. However, a
builder’s choice of mounts may impact
finished weight and performance of smaller
airplanes.
Compared to metal mounts, nylon
fiberglass-filled mounts offer plenty of
strength at roughly half the weight.
Cowls and wheel pants also offer
opportunities for weight savings. Compared
to fiberglass, ABS cowls may add more than
a 1⁄4 pound of weight and substantially more
on larger airplanes.
Modelers can purchase aftermarket
cowls or wheel pants from several
aftermarket fiberglass companies.
Midwest Models took a unique approach;
it chose not to include ABS parts in the 80-
inch-wingspan Extra 300XS kit, but to
include coupons and contact information for
several aftermarket fiberglass suppliers.
Do tires make a difference? Du-Bro and
Sullivan offer lightweight tires, and both
lines are much lighter than the companies’
“regular” lines.
Spending extra money on something
such as tires may seem like overkill, if not
absurd; however, a few dollars for a couple
ounces saved is a good exchange if the
objective is to finish light.
Consider that one reason a pilot may select
a large engine is that he wasn’t impressed by
the performance with the engine deemed
appropriate by the kit designer. But that
ignores how much performance is lost by
“building on” excess weight.
Then consider the additional loss of
performance resulting from a poor choice of
propeller. Yet that is a common scenario.
The “fix” is to reduce the finished
weight—not to strap on a larger engine.
One thing that distinguishes a competition
airplane from a sport model is that the
competitive builder pays close attention, to
make sure excess weight is kept to a minimum.
A significant component that affects
weight is the battery. As I mentioned in a
previous column, using Sub-C Ni-Cd packs
is discouraged. They are literally dead
weight, even in 40% airplanes.
Smaller, high-capacity Ni-Cds are a
better choice. Duralite batteries are also
gaining popularity, and they provide higher
capacity than Ni-Cds, with a significant
decrease in weight.
However, there are limits. I hear of modelers
flying 30% and larger airplanes, and relying on a
single pack. The slightest failure can destroy
thousands of dollars and many hours of work.
The insurance provided by a redundant
or parallel pack is well worth the weight.
The weight of two packs of Duralites may
be less than a single Ni-Cd pack.
Last is the tailwheel. Manufacturers have
different approaches to tailwheel design, and
most use a dual-spring setup to achieve
something of a scale finish. Weights vary
considerably with different tailwheels; it’s
best to get an idea of whether you will need
tail weight when you make a selection.
The third school of building light
involves making changes to the kit. So far,
we have relied on “bolt-on” techniques.
Where building light becomes controversial
is how, and sometimes why, a builder will
reduce weight using a building technique.
If you are interested in lightening a kit, talk
to (preferably) the kit designer or an experienced
builder who is familiar with the airplane and
proven lightening techniques. Many 70- to 80-
inch kits offer many opportunities for lightening,
but you have to know where to start—and more
importantly, where to stop.
For the best performance, optimize
airplane size, engines, and your component
group. Don’t focus only on “building light.”
New Product: Dave Patrick—one of the
innovators of light 1.20-scale aerobatic
models—is going to introduce a 76-inchwingspan
Extra 330L. The target weight is
11 pounds, so it will be a good candidate for
a 1.40 engine.
Early pictures look good! MA
Seven-time TOC competitor Chris Lakin with 40% Carden Aircraft
Edge 540, powered by a Desert Aircraft DA-150.
JR SCAT Series competitor Mike Stoner kneels with his Pirate
Models Extra 300 Special, powered by a BME 100 engine.
