April 2001 121
in The laST column I discussed getting ready to build and a
philosophy to get the most out of your available time. Hopefully some
of the points connected, and you’re on your way to a successful
building season and an even more successful flying season.
This time I’ll follow up on construction and take a few steps
back to look at some details that will make your model more
functional, lighter, and durable. Realize that these points will not
affect how your kit, design, or original performs.
So why even consider incorporating them in your package? The
short answer is that they do help the “big picture.”
let’s discuss wing ribs. Accurate ribs are a
primary concern, as is the selection of good
wood. Quarter-grain, lightweight wood is
preferred, but here is where a real problem
can occur.
Less-experienced builders will typically
try to save more weight by taking too much
out of the ribs, with cutouts. Shape of the
cutouts can also be a major problem.
Some years ago, I observed a hard landing at
a contest. I thought “no problem,” then I noticed
that the covering on the wing had shattered and
the interior of the wing needed major repair. The
ribs had fractured along the grain.
This was puzzling; the hard landing
didn’t seem severe enough to produce the
observed results.
However, the causes became obvious as we
all looked more closely. The wood used for the
ribs was clearly a well-defined “A” grain that
ran from leading edge to trailing edge. The ribs
had been cut out to lighten them, and the
fractures started at the points of the cutout.
There were actually two problems: the shape
and size of the cutouts and the wood grain.
The cutouts consisted of lines that
CONTROL LINE AEROBATICS
Frank McMillan, 12106 Gunter Grv., San Antonio TX 78231
Semiscale F-86D with scale ground power unit. Don Hutchinson
captured the flavor of this Korean war jet. (Hutchinson photo).
This 40-size F-86 has popular O.S. 40 FP for power. Don won Advanced class in major
California contest. (Dee Rice photo).
Models of popular semiscale movement: swept-wing F-86 “Dog,”
P-17 Paul Walker used in recent “Worlds.” (Hutchinson photo.)
122 M ODEL AVIATION
paralleled the outside shape of the ribs
(everybody does that), but those lines
intersected with vertical lines close to the
spar. This created a near-right angle and a
stress concentration—big time.
This effect was significantly highlighted
by the wood-grain selection. When the model
impacted, all these problems relieved the
stress by fracturing most of the ribs. I doubt
this would have happened had there been
better ribs on the model.
Remember that all the details count
together, and the weakest point will fail. What
I’m going to suggest is not new, but it is
worth revisiting.
Bob Hunt is a super practitioner of artwork
in Stunt wings. When cutting rib-lightening
holes, he uses large-diameter brass tubing,
sharpened, to obtain radiused cuts as trace
points on each rib. The reason is that the
tubing provides an extremely smooth cut that
you can’t obtain with an X-Acto™ blade.
The procedure is to make the circular
cutouts, then connect the radii for an elegantlooking
cutout. Attending to making these
nice cutouts are location and weight savings.
Strangely enough, the primary consideration
in this case is strength. Too many builders try to
save weight by taking out too much material.
If you are using good wood, you can’t take
enough additional wood out to matter over
what is necessary for reasonable strength. A
ballpark figure would be at least a 1⁄4-inch
margin around the cutouts.
By the way, take some extra time and care
to line up the cutouts.
as i’ve stated many times, building is just
like flying. The more frequently you do it,
with plan and purpose, the better you get.
Also as with flying, the longer you don’t
perform an activity, the less proficient you are
in that activity. There are numerous examples,
such as covering with silkspan or carbon veil
or setting up a control system.
However, you can and should learn from
each model you construct and fly. Evaluate
the design and methods you used, and see
where there is room for improvement in your
techniques and procedures.
Let’s review a few details, just to give you an
idea of what to look at. Be aware that I’m
including kits, plans, and personal designs in the
discussion, because everyone has a favorite way
of doing things. The way you develop a design
has as much to do with how you do things as
how the original designer has his set up.
The more you fly, the more you find out
what works. A great case in point was Al
Rabe when he was in the “all out.” Everything
had logic to the end that when he crashed (he
would leave no evidence on the field), he
would dissect the pieces to find out what was
too strong or weak.
For what was too strong, he would
analyze the area to see where he could
save weight. For what was too weak, he
would see where he could adjust the
structure to optimize weight and strength.
It’s a great lesson!
Many think of model design as the
outlines, areas, and moments, but there is
much more. Take the cowl-separation line, for
example. It is probably okay if you’re
building a proven design. But look at the
power package you are using.
Was the model designed for your setup?
Where are the items you need to get to, such
as the needle valve, glow plug, pipe setup, and
disassembly concept?
Have you ever gotten to the point in
construction where it dawned on you that
something was amiss, because your package
wouldn’t fit? This is what I’m getting to.
If you want to get the most from your
model, you have to be able to “service” or
maintain it. That means the parts that can
wear or require adjustment or change must
be accessible.
My approach to the front-end design of
the fuselage is to remove the power package
(engine/muffler, pipe, etc.) as a unit once the
cowl is removed. To do this, I establish the
cowl-separation line through the needle
valve centerline and the exhaust projection,
if side-exhaust/muffler configured.
If pipe-equipped, the engine lifts out and
the engine/pipe assembly slides out. This buys
you the ability to work on your engine outside
the model without changing any settings,
unless you want to.
Remember that simple is better when
you consider how to design your model. The
more complex you get, the more likely
something will fail. If you’re just starting to
fly more-complex models, study all the
plans you can get for ideas and approaches
to problem-solving.
details of Designs: The more you build, the
more you learn your personality for building.
By that I mean you will like certain methods
and approaches to construction.
That’s really what you’re after, because you
will invariably compromise your model if you
are not comfortable. That doesn’t mean you
shouldn’t try new techniques, but a good Stunt
program is typically an evolutionary process.
Look at Bill Werwage and how he has
evolved various designs of many models. He
made small changes to proven designs and
made them better.
Bill’s USA-1 is a World Championships
Classic-legal design that evolved through many
changes. Bill flew the later models in many
forms, up through the GEO-XL. (Editor’s note:
Look for the construction article in this issue.)
What he changed and why make an
interesting story. The basic layout was set up
for the popular ST .46 engine and the power
available. This dictated a thin airfoil to reduce
the frontal area/drag.
The next big step was an attempt to make
a smaller variant (Bill tried quite a few
slightly differing sizes to see if there was an
optimum) to fly better. He cut into the leading
edge of the wing in the outer approximately
20%, and wedged it open to thicken it.
Even with Bill’s ability to control weight,
this particular model had some “funnies” with
the tips on hard corners. By thickening
them—in total thickness, and moving the
highpoint of the airfoil forward a bit—Bill got
more lift at the tips. This stabilized them, and
the “funnies” were gone. That increase was
passed on to later examples.
Part of the moral of this story is, don’t be
afraid to cut into a model to make it work. A
pretty, poor-programming model does not do
you any good!
in the next column I’ll look into a new area
of engine development: the four-stroke.
This type of engine has long been
popular overseas, but really hasn’t been
the subject of competitive application for
Stunt in the US. That’s changing on the
East Coast; many are flying these engines
and seeing advantages. MA
