Chapter 9

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, 1998.].

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 as:
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.

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Copyright 2002, Robert S. Munson. All Rights Reserved