seeing this article I immediately thought about the famous "Triple
Self Portrait" painting by Norman Rockwell. I suppose to truly
be an analogy to triple object it would require building a model
of a model of a full-size airplane rather than being a model of
a free flight model. It's a unique concept.
Model Airplanes News items.
Models of Models
Dynamically similar sheet balsa miniatures of proposed designs
check out ideas, reveal the hidden flaws, are invaluable aid before
making, testing big ship.
by Paul Gilliam
Some typical miniatures - Krecek's Gourd, left; Zeek,
top, right; Civy Boy, bottom, right - that flew exactly
like their full size counterparts. The Torpedo .32 engine
affords a comparison of sizes.
The building of dynamically similar models in small scale to
test our full size gas powered models should not be received as
a new idea. We rather imagine that quite a few modelers over the
country have been building these 1/8 or 1/10 scale balsa models
to check or correct lateral area placement, fuselage moments, and
dynamic stability in general.
To the modelers who like to design or modify their own ships,
this dynamically similar, or small scale sheet balsa model method,
of design will be a short cut to reveal hidden faults in our design
ideas. We do not mean that this small scale model method is the
only sure way to design a model that has optimum performance. On
the contrary; many good free flight models are designed with only
basic design fact, a lark, and enthusiasm.
Dynamically similar small scale models can really help us with
our design ideas and our brainstorms - some of which are good, some
bad. When we draw the stool up to the drafting board to create a
new overpowered, freeflight rocket, we can really use this dynamic
similarity method of design. Small scale models (1/8-1/10 scale)
will help us rule out our bad design guesses and keep the good ones.
This is true if we can believe the stories these little models tell.
It will take only 30 minutes or little more of a modeler's time
to find out a few inherent stability facts from a miniature of our
miniatures. This writer is fully convinced that dynamically similar
models give a satisfactorily accurate account of stability that
we seek in our bigger, powered models.
Dummy motors made from old cement tubes. Center of gravity,
other feature., must be identical to the proposed airplane
that will be designed.
This really is a mini-Hogan! Thrown into bad turns, these
gliders show recovery characteristics of design. Wings made
of 1/32 in sheet balsa.
If you are ready to try this small scale method to check .the
abilities of its big brother, here are a few tips: 1. scale or size
is unimportant: 1/5 to 1/10 scale seems normal, or try to build
the small model with 7 to 9-in. wingspan; 2. do not make an effort
to build the model light. Let weight be proportionate to the big
ship that may follow the tests of this small one. You could well
give this small model several coats of dope; 3. be certain that
you use exactly the same nose and tail fuselage moments on this
small model as you will on the bigger, powered model. A ballast
of lead or cement tube will be needed to simulate engine weight.
This weight can be a blob of lead, or a neat, small, dummy engine
can be quickly fabricated from an exhausted cement tube, (illustrated).
Naturally, this small model must have same C G location as the big
model; 4. the addition of landing gear on these small models has
so far proved little, but if you desire to create a small landing
gear, you will naturally have a better parallel. Also, in some design
cases, a landing gear with faired struts or large wheels can affect
the lateral area of a model; 5. materials to use: 1/32 sheet balsa
seems best for wing, stabilizer and vertical fin. These parts may
be sanded to a basic airfoil shape, and in cases where you feel
unsure about wing and stab airfoil combination, small (similar to
the big model) airfoils can be made from 1/16 or 3/32 sheet balsa
stock; 6. as a resume: be certain you are reasonably accurate to
scale in total model weight, dummy engine ballast, wing area, stabilizer
area, fin area, wing dihedral, nose or forward fuselage moment,
tailor rear fuselage moment, center of gravity location, decalage.
Here is a list of visible flight characteristics that can be
noted by experimenting with these dynamically similar models: 1.
lateral area placement; 2. vertical fin area characteristics; 3.
yaw and roll stability; 4. spiral stability; 5. stall recovery;
6. glide pattern; 7. sensitivity to adjustment.
There are three definite forces that affect our full size models
that we cannot show by dynamic similarity. These are torque, gyro-precision,
destabilizing or upward resultant force of propeller spinning on
the nose of the model. Other than these three forces, dynamically
similar models tell an interesting story about our free flight models,
and most of the other forces that we must encounter to master a
free flight design.
With only 30 minutes building time, a dynamically similar or
small scale miniature will reveal a multitude of free flight problems.
These small models can be launched with the force of our arm (which
at peak velocity is probably near 45 mph) as we would heave an endurance
hand-launched glider. But unlike the manner in which we launch this
endurance glider, we can deliberately force these small dynamically
similar models into a bad turn. After doing this. we begin to see
the inherenr recovery of Our prospective design. The small models
will be (or should be) reasonably heavy in scale weight to our bigger
ships, so we can see that these small models can be launched into
good or bad turns at great velocity with the power of our arm.
Is a cabin job as good as a pylon? How
do these two gliders compare? Doesn't take long to find out; why
not try your own ideas?
In the illustrations for this article, we have included a method
to construct easily the dummy engine ballast that we will need to
create this miniature of miniatures.
To bear out and give a little more importance to this dynamic
similarity theme, we may mention that the Air Force thinks enough
of dynamic similarity to spend several million dollars on small
scale (1/8 to 1/10) models of some of our present actual jet planes
and experimental versions of jet fighters and bombers. And these
dynamically similar (yes, in weight and in all factors) models are
flown free flight at tremendous speeds and tracked by radar to gain
flight characteristics of these planes. Apparently these results
are most encouraging.
We members of the San Valeers Model Club (San Fernando Valley, Calif.)
have been playing with dynamically similar models since 1946. This
year, with the help of Jose Tellez, we constructed one dozen dynamically
similar models of some of the present day free flight designs. In
this group, we scaled and built such models as the Sandy Hogan,
Fubar, Civy Boy, Zekes, Skybeau, Tweek, Krecek's Gourd, Zipper and
others. We already knew what to look for in the flight characteristics
of these small models because we knew what flight patterns and idiosyncrasies
these different free flight models exhibited when flown full model
After wringing these models out with a fairly good arm, our enthusiasm
for dynamically similar models hit a new high. We found then, in
the small scale versions, that the Hogan actually did fly like a
Hogan, the Civy Boy like a Civy, the Zeke like a Zeke, and so on.
There may be some who will challenge the worth of these dynamically
similar models. But without argument, either way, we are bound to
see that these small scale models can be a lot of fun, even for
the unbeliever of visible fact. These small models are entertaining
for clubs who hold their meetings in, or who have access to, a gymnasium
or field house. In bad weather that would hamper the flying of full
size gas-powered models, model clubs can still have fun and competition
by taking these small models inside or to the barn.
For the modeler who seriously builds these dynamically similar
models to help sift good and bad ideas, it is best to fly in calm
air or indoors. By testing these small models under moderate air
conditions, we will be giving the "little models" a "little break"
that is similar in scale proportion to the roaring sand storms we
are apt to chuck our bigger free flights into.
Full size plan for a mythical free flight. Whittle
the fuselage, cut out the flying surfaces, assemble, and you're
set for work or fun.
Posted November 22, 2014