I made up my mind, but I made it up both ways.
- Casey Stengel
The network of model aviation organizations was covered in Chapter 3.
Organizations have a major influence on the model aircraft designs
that evolve. To borrow a Biblical analogy; each organization begot
events; events begot rules; rules begot designs tailored to those
rules; those designs begot standardization; standardization begot
stagnation in creativity and individuality. It's interesting to note
that FAC rules provide bonus points for models of prototypes that are
more difficult to fly.
From the kit manufacturer's standpoint, standardization is helpful.
The most popular designs allow higher volume production of
particular designs resulting in reduced production costs per unit.
Many events have been in place for years, long enough that modelers
have found the best combination of design elements to fit the rules.
The result is that models flown in non-scale events begin to look
alike. In scale, full- sized aircraft whose features best fit the
model event rules, also dominate.
9.1 Non-Scale Design Selection
Non-Scale models may be built from kits or plans designed by others,
or the design may originate with the builder.
If the builder chooses the design work of others, he/she must depend
upon contest results found in model magazines or SIG newsletters;
unless the model design has achieved a reputation for success over a
period of time.
Unfortunately, most contest results list the event winners through
the first five or six places and the score achieved (duration, speed,
etc.) with no mention of the name of the model design. Sometimes
pictures are included which helps identify the model design. Top
winners may rate a special article.
If the builder originates a design he/she may be guided by the
proportions and other characteristics of other successful models.
Alternatively, guidance may be found in reference books. Don Ross
devotes chapter 17 of his book to design. Several figures illustrate
"rules of thumb" for rubber model proportions. [Ross, Don. Rubber
Powered Model Airplanes. Hummelstown, PA: Aviation Publishers,
With experience one can apply aerodynamic criteria to
evaluate a design.
9.2 Scale Design Selection
In general, much of what is said above applies to scale models, but
there are other considerations. One is required to reproduce the
full-scale aircraft in miniature form. Not all full-scale aircraft
make good models. Full-scale aircraft must be maneuverable to
respond well to control inputs. Free flight models must be stable to
recover well from upsets caused by wind gusts. To achieve stability,
wing dihedral may be increased, horizontal tails (stabilizers) may
need to be enlarged, and sometimes vertical tails (fins/rudders) may
need to be reduced in size.
Another example involves weight distribution. A wing's chord is the
average width from the leading edge to the trailing edge. A model
should balance (nose to tail) a third of the wing's chord aft of the leading edge. This balance point is usually called the center of
gravity (CG). On a full-sized aircraft a heavy engine is located
ahead of the CG. On a rubber model the rubber "motor" is distributed
both forward and behind the CG. The net result is that if a
prototype has a "long nose moment" (distance from CG to back of the
propeller) it takes less extra "ballast weight" ahead of the CG on
the model to balance the structure aft of the CG. The message is
build tails light.
The Flying Aces Club features a lot of mass launch events in their
contests. The concept is simple - all contestants launch their
aircraft at the same time and the last one down wins the round or
event. Each event is limited to scale models of a given era or type.
Dick Bennett researched the winners of the World War One event and of
the World War Two event over an extended period of time. He assigned
three points for a first place win, two for second place and one
point for a third place showing.
For World War One the RAF SE-5 got 58 points, followed by the Fokker
D.7 at 43, and the DeHavilland DH.6 got 27 points. All the models
that earned any points over the years were plotted on a graph based
on certain characteristics of the model. The wing chord /span was
plotted in the X direction (horizontal) and the nose moment/total
length in the Y direction (vertical). It was found the winners
tended to have the same characteristics - long nose, large wing area
and slab-sided fuselage (minimum structural weight).
The same process was repeated for the World War Two event. The
P51/A-36 got 31 points, followed by the Kawasaki Ki-61 Tony at 22 and
the Fairey Barracuda got 14. Once again the nose moment, wing area
and "slabby" fuselage were a factor.
It should be noted that a maximum wing span is specified for these
events so a large wing area indicates a wider wing (chord). The
World War One study was published in Flying Aces newsletter issue
173-99 Jan./Feb. 1997. The World War Two study was published in
issue 184-110 Nov./Dec. 1998.
William McCombs covers design selection in great detail in his book.
In table 5-2 he predicts performance for 93 commonly modeled free
flight scale aircraft. [McCombs, William F. Making Scale Model
Airplanes Fly. Self published, available from the author at 2106 Siesta, Dallas, Texas 75224] (Do yourself a favor and buy this book).
I will provide further reference information about model aerodynamics
in Chapter 12.
9.3 What's a Reynolds Number?
We have seen above that not all full-scale aircraft make good models
due to weight distribution or stability requirements. Another major
consideration is the so-called "scale factor."
The formula for a Reynolds Number for modeling purposes can be taken
Re = 68459 x VL
Where V = velocity in ft./sec. Or meters/sec.
Where L = length in feet or meters (wing chord or other component length)
The Reynolds Number for a 747 airliner is over 10,000,000; for a
large R/C sailplane model from 100,000 to 400,000; for an indoor
peanut scale model 10,000.
Now that I've got it, what do I do with it?
The boundary layer of airflow on the upper surface of a typical
profile tends to separate easily from the profile at very small
Reynolds Numbers. This increases drag and reduced efficiency (leads
to early stall).
A wider wing (chord) increases the Reynolds Number. That's why
small-scale models of prototypes with wide chords fly better.
A highly tapered wing has a narrower chord toward the tip resulting
in a lower Reynolds Number there.
This brief discussion about Reynolds Numbers hardly scratches the
surface of the subject and there are more factors involved. For more
information see the references cited in Chapter 13. Especially the
book, Model Aircraft Aerodynamics by Martin Simons.