RADIO CONTROL AEROBATICS
Rick Allison, 26405 SE 60th St., Issaquah WA 98027
Correction
In the August 1998 column, Chris Bergen was misidentified as Mike Levenson. Apologies for the error.
Introduction
Pop culture is loaded with sayings that illustrate the cyclical nature of change: "There is nothing new under the sun"; "The more things change, the more they stay the same"; and "There is nothing as new as an old idea." The regular resurrection of bygone clothing styles foisted on us by the fashion industry is a concrete example.
Pattern is far from immune to this circle game, despite the fact that the "modern" era (dating from the advent of proportional radio equipment) of RC aerobatics spans only a few decades. Old Pattern ideas are constantly recycled and recombined with the occasional new insight in an attempt to make the new sum add up to more than its familiar parts. There is nothing wrong with this; it is how we progress and deal with changing rules and demands of the sport.
Constant-speed presentation
Slower, more constant-speed presentation is the latest "new" idea to come along. The concept is simple: the pattern looks better and more unified if the model is going up, down, and around at a constant speed. When perfectly achieved, the effect is elegant, ballet-like, and nearly hypnotic. The idea conveyed to the watching judge is serious precision and control, and large scores often follow.
In the hands of a master technician, this flying style can be very impressive. In the hands of the average Advanced, Masters, or even F3A-class competitor, the result is often soggy, wandering, and anything but precise. This lack of widespread success hasn't translated into disaffection with the style, which remains popular. The carrot on the end of the stick is large—mastering the technique can produce a big boost in wins—so people doggedly stick with the program.
Most problems stem from people attempting to copy what they see at the highest levels strictly by imitation, without much thought, technique, or close observation. To get a handle on the problem, I like to follow it to the root. In this case the root doesn't grow in RC Pattern at all.
Roots in Control Line Stunt
The constant-speed pattern concept was born, raised, and is still best practiced in the ancient and honorable sport of Control Line Stunt—the original precision aerobatic event. Watching a Stunt contest is hypnotic. Maneuvers are generally repeated in pairs or triplets, but mainly the effect comes from the absolutely regular, consistent velocity of the model as it scribes the pattern on the contained hemisphere of sky allotted by the lines.
That effect is deliberate. Behind it lies years of experimentation with engines and props and a large body of collective knowledge from Stunt participants past and present. Experimentation is ongoing: the era of the tuned pipe has dawned in Stunt and has spawned new calculations designed to achieve a constant-speed/variable-thrust effect.
Not everything from Stunt translates well to Pattern. Aside from the obvious differences of free versus restrained flight and control/no control in the roll axis, in Pattern we have throttles. Therein lies the main rub.
An axiom about controls
Any control that can be used can be misused.
The throttle is misused often by RC pilots—to the point of abuse. On any Sunday at the local field, many pilots treat the throttle as an on/off switch: on for taking off and flight; off for spins and landing. Most Mode 2 pilots can use the aileron/elevator stick in a reasonably coordinated manner; some can manage rudder and aileron; but few manage decent simultaneous rudder/throttle input.
Back when Pattern was flown AMA-style—long unscored turnarounds, center maneuvers buried in the middle, and full-throttle passes didn't matter—throttle skill wasn't necessary. Most of us could have flown the style with the throttle hooked to a retract servo with much the same result. Times changed. Modern turnaround flying put the model into a relatively small, highly defined area or "box." Development of continuous high-speed flight became impossible without a certain minimum of skill; the throttle became a must.
A few years ago the average Pattern engine barely produced enough horsepower to draw an acceptable vertical line. Technology was the limiting factor. Then a second generation of modern Pattern engines arrived, and suddenly available power was sufficient to exit the box vertically without slowing down. The technology made all things possible: we had choices such as how big to make maneuvers and, more importantly, how fast to fly. Which brings us, full circle, back to Stunt.
In Stunt, propulsion (sans throttle) is used to limit speed and provide thrust proportionate to demand—more under load, such as a vertical climb, and less unloaded, such as on a vertical down line. Given a no-throttle operating parameter, similar results could be achieved with similar methods. However, we have throttles, and maintaining full control of a physically unrestrained model means we keep them. I doubt many would vote to do away with them, no matter how poorly we use them.
Fortunately we have other methods available, along with basics borrowed from the circle guys.
Propeller selection
Number one on the list—borrowed from Stunt—is good propeller selection. For constant-speed maneuvering, you want a relatively low-pitch, high-diameter unit.
- For the average 1.20–1.40-sized Pattern engine/model combination, a prop on the order of 15–17 inches in diameter and a pitch of 8–10 inches is typical.
- The idea is to keep the propeller in its more efficient region and reduce the amount of throttle modulation required to maintain speed.
- We are after good thrust and acceleration from low-speed, low-rpm conditions. That means lower pitch—similar to using a lower gear on a hill or when departing a stoplight.
Adequate disc area (diameter) is important for two reasons:
- In a vertical up line, the propeller is the only surface working to support the aircraft’s weight and provide climb thrust. Disc swept area and loading matter as much as wing area and loading.
- It is more efficient to move a large cylinder of air at low rpm than a small cylinder at high rpm—especially when high-speed flight is not the intention.
When power is reduced to idle in a vertical down line, the prop may not turn fast enough to accelerate air and produce thrust. Instead, the model pushes the prop through the air faster than the pitch and rpm can support, creating aerodynamic braking. Larger-diameter, lower-pitch props produce more braking under these conditions.
A more complete discussion of propeller theory and its relation to Pattern aerobatics (including sound reduction) will have to wait, but the basics above are a start.
Throttle management
The second—and probably most important—item on the constant-speed list is good throttle-management technique. We can divide this into two parts:
- Physical
- Technical
#### 1. Technical This covers everything your radio and in-flight hardware can do to make the task easier. Many modern computer radios include exponential control for the throttle—a handy item for matching engine output to stick position.
A rotary-output servo arm attached to a rotary-input throttle-barrel arm by a pushrod does not easily produce linear travel. Whatever arm positioning you choose, sufficient mechanical advantage is rarely available. This built-in mechanical problem likely explains why many pilots treat the throttle as an on/off switch.
Dialing in expo on the low end or middle of the throttle curve can help. With careful use of a tachometer and test flights, engine response can be tailored to produce a more usable mid-range.
Some competitors file notches into their transmitter throttle-stick quadrants so repeatable stick positions can be clicked in during flight; the transmitter is then programmed for fixed throttle settings at those detents. While successful for some, I don't recommend this for two reasons:
- It will likely void the radio manufacturer's warranty.
- Weather conditions change; a setting perfect on a calm day may be wrong in windier conditions. Managing multiple weather-dependent settings requires skill beyond the average Pattern pilot.
A recent development is a twist-cable throttle-actuating device (the "Snake Drive") that uses rotary servo motion to produce rotary throttle barrel input without arms at either end. It's relatively expensive and more difficult to install, but it provides very linear throttle response; recent improvements have made it work even better. It's available direct from Unique.
#### 2. Physical There are no easy answers here other than sufficient practice, but some simple techniques help.
- Move the throttle stick smoothly and slowly from setting to setting. Jerking the throttle is as clumsy as jerking any other control.
- Smooth power applications require planning ahead and drilling the habit. They smooth the visual presentation of the pattern and the aural presentation. While engine sound is not a judging criterion, smooth-sounding power transitions contribute to the overall effect presented to the judge.
- Retard the throttle a little sooner and advance it a little later nearly everywhere in the pattern. This technique works best with a light model and a powerful engine.
- Work the midrange by flying more "off the stops." Pull power down, but not all the way off, on loops and Cubans—and don't advance the stick fully forward in level flight unless needed for a particular maneuver, such as a crisp point roll.
Good throttle management isn't easy to acquire and no single technique is magic. Practice and integrating many small details into a system that works for you is the only reliable path.
A regular, constant speed is the target, but precision remains the primary aesthetic and judging criterion in Pattern. If you fixate on slowing things down to the point where precision and crispness suffer, your scores will go down—not up.
Transcribed from original scans by AI. Minor OCR errors may remain.
