Now You're Talking: Soap Box
"Model does not fly at a scalelike speed" are the words quoted from the AMA rules as an itemized error for flight realism in RC scale modeling. Judges are left with the responsibility of deciding what this means, since these words have been acknowledged to have no specific definition.
The author of these words recently indicated "scalelike speed" was never intended for interpretation as a requirement to fly at scale speed, if some had read it that way. He further stated, "never was such a rule and there should not be!" This is probably a relief for scale competitors to learn, if our activities at contests were policed by radar. The words may have been chosen simply to discourage modelers from flying arbitrarily fast without regard for other realism considerations which are described separately.
Well-defined rules normally bring order and harmony to competitive events. In some cases, judges have interpreted a specific manner not intended by penalizing a contestant for not flying at scale speed, or what is called Linear Scale Speed (LSS) herein. Most individuals realize the impractical nature of LSS and some of its conflicts.
LSS has been described as when a scale model travels a straight path for the identical view angle and time at scale distance from the viewer as would be seen for full size. Accurately judging distance, view angle, and time is not easy. Fortunately, other easily observed realism features also exist that relate to an optimum speed.
Knowing that a definition is lacking may stimulate further thought to achieve improved judging uniformity as encouraged by the AMA rules. Such efforts should also recognize that LSS contradicts many other expected maneuver realism features described in the AMA judging guidelines. These include prototypical maneuver attitude, option selections, stability, and g-loading movements for optimum scale-size maneuver realism. Comparative visual observations and aerodynamic flight physics confirm such problems.
I suspect that technically oriented scale modelers would like more details on these relationships, which go beyond the scope of this article and the space allotted. It may be requested by mail from the author.
It does not require much imagination to quickly realize that LSS will provide advantage to large models over small ones, if that had been the primary objective in scale speed. For scale modelers, notable competition should play no favorites based on size alone. Otherwise it becomes a race as to who can build the largest model, with cost ramifications and counterproductive effects on overall participation. Hopefully, scale modeling has a better principle in mind than that.
Since prototypical attitude is perhaps the most visibly apparent feature of maneuver realism, it plays a corresponding role in recognizing optimum speed consideration.
For a basic realistic turn, this translates to an identical prototypical bank angle in a scale-size turn that we can all associate with aesthetic realism on a readily observed basis. This prototypical appearance is very sensitive to an optimum overall speed relationship that LSS does not provide for optimum realism during flight.
At the US Scale Masters Championships in 1993, all those attending an initial evening pilots' meeting believed they were already performing with optimum speeds to provide both scale-size maneuvers and prototypical bank-angle turns compared to full size. I agree with these notable pilots.
Since we all would also likely agree with these two profoundly basic and important features for flight realism, what is this so-called Maneuver Realism Scale Speed? With historical definitions it is Dynamic Similitude (Scale) Speed or DSSS. If more palatable to you, just continue to think of it as Maneuver Realism Scale Speed in this article.
Dynamic Similitude Scale Speed (DSSS) Table
Model Linear Scale — DSSS Sq. Root Multiplier — Full-Size Speed and Dynamic Similitude Scale Speed in mph (Note that the first row speeds are those of full size.)
- 1/1 — 1.0000 — 75, 100, 125, 150, 200, 250, 300, 350, 400, 450
- 1/2 — 0.7071 — 53, 71, 88, 106, 141, 177, 212, 247, 283, 318
- 1/3 — 0.5774 — 43, 58, 72, 87, 115, 144, 173, 202, 231, 260
- 1/4 — 0.5000 — 38, 50, 63, 75, 100, 125, 150, 175, 200, 225
- 1/5 — 0.4472 — 34, 45, 56, 67, 89, 112, 134, 156, 179, 201
- 1/6 — 0.4082 — 31, 41, 51, 61, 82, 102, 122, 143, 163, 184
- 1/7 — 0.3780 — 28, 38, 47, 57, 76, 94, 113, 132, 151, 170
- 1/8 — 0.3536 — 27, 35, 44, 53, 71, 88, 106, 124, 141, 159
- 1/9 — 0.3333 — 25, 33, 42, 50, 67, 83, 100, 117, 133, 150
- 1/10 — 0.3162 — 24, 32, 40, 47, 63, 79, 95, 110, 127, 142
- 1/12 — 0.2887 — 22, 29, 36, 43, 58, 72, 87, 101, 116, 130
- 1/16 — 0.2500 — 19, 25, 31, 38, 50, 63, 75, 88, 100, 113
Typical airshow and maneuver speed class:
- Low: WW-I
- Moderate: General Aviation
- High: WW-II
- Very High: Jets
* K Factor
Dynamic Similitude and Its Rationale
Dynamic similitude speed and motion study has been used extensively by aircraft designers to better predict flight response characteristics with models much like our own. It has also been used for the study of animals (including dinosaurs, as in the successful filming of Jurassic Park).
The Dynamic Similitude Scale Speed (DSSS) for a model is that which best duplicates full-size performance in similar attitude-motion effect through prototypical scale-size maneuvers when accelerating qualities are not altered. This is the added further reference for "similitude" effect, since our gravitational acceleration does not have any scaling factor K as would linear dimensions of our scale models (K = 1/4, 1/5, 1/6, etc.).
Some may ignore the technical understanding of DSSS, but we cannot ignore its realistic motion effect on all objects in our constant-gravity environment. When acceleration remains unaltered, it can be demonstrated that DSSS provides a square-root relation to the scale K factor (as summarized in the table). As in competition, most scale modelers are already likely performing quite well to duplicate DSSS for maneuver flight realism, simply because gravity and physics mandate much of it.
An oversight can occur when comparing scale model flight to full-size combat aircraft capable of high pursuit speeds. For example, WW II (or jet) fighters can be used to erroneously defend LSS and dismiss DSSS for these select models by referring only to full-size high-speed flight and relating that to typical maneuvers in AMA scale rules. However, these full-size aircraft did not fly such maneuvers at or near top-rated speeds.
In comparison, the slower vintage models can clearly become handicapped if LSS were adopted with this analogy.
WW II Fighter Maneuvering Speeds (Practical Observations)
Documenting WW II fighter aircraft maneuvering speeds may be of immediate benefit to many readers, since most published literature gives only top-rated speed at high altitude. The following description is from Steve Hinton, a respected pilot for the Planes of Fame Air Museum in Chino, California. It describes what we would typically view from the ground under sea-level conditions for typical airshow environments. It agrees with other literature on this same subject for WW II fighter maneuvers.
- A stock P-47 can only do about 300 mph at sea level; a P-51, about 350 mph. The differences relate to how "slippery" each aircraft is aerodynamically through thicker air. Other fighters may vary, depending on overall performance design.
- Horizontal rolls are typically done from 150 to 250 mph.
- Round loops are entered approximately at 300 mph, but light-weight-loading fighters such as the F4F can enter them at 240 mph.
- An egg-shaped loop with WW II fighters can generally be accomplished much lower—in some cases down to 200 mph.
A round loop is optimally performed when there is zero g at the top, where the aircraft briefly floats (centrifugal force offsets gravity) rather than exhibit negative g or a possible "hog" shape. For unusual maneuvers that require very high entry speeds above the level-approach capability of the aircraft at or near sea level (airshows), these are typically dived into to achieve entry requirements (such as for entering a vertical roll). He refers to "Reno-style speeds" as a specialized area.
This information is outlined in the DSSS table for typical maneuvering speeds. I have included a few other examples for general comparison.
Observations on Model Performance and Judging
The table likely reflects what most scale modelers are already achieving for satisfactory maneuver results, particularly if monitored accurately with radar. There may be two areas of improvement.
- The upper end of typical maneuvering speeds and performance for WW II scale fighters is sometimes lacking—particularly for prototypical vertical maneuvers such as loops. Perhaps another perceptive writer recently stated it best: "...you notice that a fighter that flies like a slug isn't scoring so well."
- Many jets are flying only on the upper end of their prototypical maneuvering speed range, rather than policing themselves in throttle control during maneuvers not requiring high speed. These might include low-performance horizontal maneuvers, including figure eights. Excessive bank angle in scale-size maneuver turns reveals corresponding excessive speed quite easily.
When scale fighter models are equipped with adequate speed performance range to include vertical maneuvers, it certainly solves the problem often described as a "monotonic speed" that detracts from overall flight realism performance. In the DSSS chart, performing fighters have a comparatively broad range of speeds for their maneuvers. However, a modeler must use that speed range wisely, since the choice of a performance-related aircraft is not an inherited license to speed through all maneuvers unnecessarily.
Scalelike speed for flight realism in the AMA rules could use better definition to avoid the flare-ups mentioned by others in AMA. There will be continued sensitivity to possible editorial errors that have no definition. This is particularly frustrating when a judge can misinterpret "scalelike speed" as scale speed or Linear Scale Speed (LSS), and then emphasize it as a flight-realism error.
With these types of erroneous interpretations, the AMA rules for maneuver and overall realism features become a document in conflict with itself. This is further aggravated when some modelers discourage insight beyond LSS, rather than promote investigation of overall related facts affecting flight realism.
Since speed is so difficult to humanly judge in a direct manner, it would be far more practical to judge speed based on its effects on the other better-defined or recognized maneuver and flight-realism features. This often is already the case, since graphic prototypical attitude features speak for themselves without becoming technical in other respects.
Any description given to speed should reference these existing AMA maneuver-realism features or prototypical maneuver options to avoid conflicts. The Scale Masters has already recognized these features for the last two years with "maneuver realism scale speed" to reflect what was occurring instinctively by experienced judges and contestants. It also provided means of "accurate and consistent judging" as recommended by the AMA Scale RC Flight Judging Guide.
When comparing other possible interpretations, it was also the only realistic way to give uniform recognition for overall flight-realism qualities on a fair and equitable basis to all size and vintage aircraft competitors.
Further Information
Further technical background information referred to in this article is available by sending a self-addressed stamped envelope to:
Kent Walters 6407 N. 83rd St. Scottsdale, AZ 85250
Transcribed from original scans by AI. Minor OCR errors may remain.
