May 2001 137
aS i Think about many columns of the
past, I realize that I’ve fiddled with the
numbers game many times.
However, it’s about time I focused on a
few points that will “eat your lunch” if
you’re not careful.
I’m referring to how measurements
and dimension control during the
construction process can combine to
affect your model’s flight performance.
Although this discussion may seem a bit
esoteric to some readers, you may run
into many of these situations.
Let’s talk about controlling shapes. You
want to make sure you have an accurate
representation of the part you want to make.
The stabilizer that is symmetrical about the
centerline needs to be symmetrical.
It seems so obvious that it should be a
given, but think again.
When drawing up new stabilizers
(stabs), more than once I have copied the
original drawing for a pattern and found
CONTROL LINE AEROBATICS
Frank McMillan, 12106 Gunter Grv., San Antonio TX 78231
The Mustang by this young competitor portends much for the
future. Who is the mystery flier in this 1987 photo?
No mystery here; this is Paul Walker in his pre-Impact “Bad
News” days. The Nats winner had a much-modified O.S. 46 FSR.
Joe Parisi of Australia had a nice color scheme and layout on his 1999 Novar. The
model is a Randy Smith design, and is powered by a PA .61 engine with pipe.
138 M ODEL AVIATION
an error side-to-side; the leading edge on
one side was a pencil-width less. I
caught the error by folding the pattern on
the centerline.
Now you could ask, “How much could
that minor error have affected flight
characteristics?”
The answer is, “Probably not much.”
However, if undiscovered, the error could
be multiplied as the pattern was transferred.
The nature of these errors is that they
multiply if you’re not careful.
That’s really the problem I’m
highlighting: the need to be
exceptionally accurate in every stage of
constructing a model.
Consider the stage at which you have
accurately constructed the stab, and now
you have to shape the leading and trailing
edges. As you shape, you remove material.
It takes discipline and a good eye to
produce the desired radii and straight lines.
Once you’ve done that, do you go back
to see if your planform was maintained
(accurately)? It’s a good idea to check how
well you’ve done!
While we’re at the construction
sampling stage, have you checked
thickness? I’ve seen stabs that were true,
but one side was thicker than the other.
That can affect how the airplane flies,
because thickness equals lift.
There could be several causes for
that. The thickness of the wood sheets
might be inconsistent and not what is
represented. When a design calls for 1⁄16-
inch thickness, do you have that exact
thickness? Does it matter?
Most times it doesn’t, as long as you
know. However, if you are concerned with
a target thickness, such as the target stab
thickness of 9⁄16 inch, you should check.
Be aware that multiple layers of
materials can add to the errors, and that is
the numbers game. If you check, you might
even take advantage of sheeting with uneven
thickness to adjust the total thickness of a
part from one side to the other.
Another cause of inaccuracy could be
you. When you sand the framework of any
part, you have to be careful about thickness.
Visualize a symmetrical airfoil on a stab.
There is a centerline and a shape to
maintain. It’s obvious that the airfoil must
be maintained, and it should be symmetrical
about the centerline.
The experienced builder would say that
he or she can construct the model with a
plotted section copied from plans, and
there’s that point again: yes, you can, but
again, you have to watch the sanding.
If you do too much to smooth the mating
of the ribs, you change the thickness on one
of the four surfaces. That changes the lift
symmetry of the stab.
Another area to look at is the edges of
your control surface. The radii should be
the same surface-to-surface; i.e., right to
left stab, right to left elevator. Everything
should be identical.
Here’s where you run into trouble. As
you shape the planform, you can blunt the
edges but induce a difference in the
thickness. To correct this, you have to thin
the edge to get the radius to the dimension
you want. It’s all a circle of
interrelationships!
In the big picture, the details I’ve
discussed are relatively minor by
themselves. But if you don’t understand
how and why you can experience them,
you will probably make these mistakes. I
know, because I’ve made all these errors
and then some.
Many of the “not-so-good” models
have problems rooted in the myriad of
details encountered in constructing a goodflying
Stunter.
You should be aware of all the things
that can go wrong. Armed with the
knowledge, you can develop your
building skills and procedures to avoid
potential problems.
new items: If you’re going to do good
work, you need good tools. You can tell
quality by the feel and balance, and by the
smoothness of operation.
For some time, I’d heard about the
quality of the Iwata airbrush. I recently
had the opportunity to test several of the
extensive line of tailored instruments.
From the first moment you pick one of
these tools up, you know you’ve got
quality in your hands.
The Eclipse HP-BCS bottle-feed
airbrush is dual action, with the control
button regulating the material flow and
the air. This gives exceptional control
for the finest detail.
On the other end of the demand scale,
the airbrush is also designed for high paint
flow with thin to thick paints, right in the
range for our use.
The other gun I tried, which fits into the
next-larger category, is the RG-2. The most
impressive aspect of this gun is the detail of
the design. The control needle is supported
by multiple bearings, to ensure smooth
operation and long life.
This is the right-size gun to touch up
the main coats on our airplanes. The
advantage is that you can control the paint
flow to produce extremely thin, uniform
coats. This results in easier finishing and
lighter weight.
Both units are first class, and together
are all we need in capability and quality.
Give these a look if you’re in the market or
want to upgrade. Contact the Iwata-Medea
company, 795 SE Taylor St., Portland OR
97214; Tel.: (503) 253-7308.
The exceptional PA .65 is now in
more fliers’ hands. The limits of its
flexible power are being explored, with
larger propellers and more blades. This
engine is a real “horse” that likes more
and more load.
We were using the 13 x 41/4 Bolly blade
until recently, and it was great! Now Randy
Smith, the PA designer and supplier, has a
new extended 131/2 x 41/4. Diameter is
vertical line tension, so you can derive
some huge benefits if your engine can carry
the diameter.
As a rule of thumb, each 1⁄8-inch
addition in diameter in this range is
approximately a 2% change.
Remember that increasing diameter does
slow the turn; you must experiment. You
can also fly slower with increased diameter,
because you have the thrust.
Contact Randy at Aero Products, 1880
Scenic Hwy., Snellville GA 30278; Tel.:
(770) 979-20345.
The mystery flier is a young Todd Lee.
Congratulations to Todd on his marriage and
on his performance at the Nationals. MA
