RADIO CONTROL AEROBATICS
Rick Allison 26405 SE 160th St., Issaquah WA 98027
If I were to pick ten pilots and judges at the average aerobatics contest and ask them what single maneuver family generates the most argument and controversy in the sport, I am certain that at least nine of them would answer, without hesitating, "snap rolls." The tenth would say, "spins," which are mostly the same maneuvers done in a slightly different way.
I'm even willing to bet that most of the ten answers would be announced forcefully, accompanied by jutting jawlines, full eye contact, and an unspoken invitation to disagree at my peril. There is an almost religious aura attached to most people's opinions on matters aerobatic, but when it comes to snaps and spins, that aura changes to a fervor and the so-called opinions to incontrovertible fact.
That nine of ten would name the same maneuver would seem to indicate at least a minor amount of agreement on the subject, and in fact, that is what it is; a minor amount.
The problem is that while nine might say "snap rolls," and all nine might have rock-hard opinions as to exactly what a good snap roll should be, those nine opinions will be different.
In fact, the chances of finding identical opinions on this subject at the average gathering of aerobatics cognoscenti are slightly less than your chances of hitting the lottery and surviving an airliner crash on the same day.
Why opinions differ
There are some reasons for this. Pattern is structured as an individual pursuit for pilot and judges, and it attracts people who like to think for themselves, rise or fall on their own merits, and compete. Generally speaking, these are not the sort of people who reach agreement easily or modify their cherished beliefs simply to avoid confrontation. In another age, they would have been referred to as "rugged individualists," or if you prefer, simply "pigheaded (insert noun of choice)."
The FAI and AMA rule books describe the execution of snaps, rolls and spins in only the most general and ambiguous terms, and the judging criteria provided are extraordinarily subjective in nature. Room has been left for honest opinions to conflict a great deal, and predictably, they do. We have done a less-than-admirable job of rule-writing at the AMA and FAI levels.
The emotion on this point is heated, because it isn't a trivial matter. Contests are won and lost on the application of these conflicting opinions. Few of us possess the nobility to lose well, and we cannot agree with a judge's interpretation of the rules—especially when that interpretation heads directly to a zero on a high "K" maneuver, and a ruined round.
By the same token, few judges possess the forbearance to tolerate a competitor's heated protest of a score they "know" is right, whether that disagreement is directed at them, or more properly, at contest management.
Fixing such a widespread problem isn't easy; if that were so, it would have been accomplished a long time ago. All the same, fixing it is a worthwhile goal. The aim in any friendly competition should be a maximum of fun with a minimum of fuss. Nowadays, people have a great deal of choice as to how they spend their leisure time. Angry aerobatics competitors and judges often choose not to participate in future events, and their absence diminishes the sport.
This doesn't mean we must fix every little flaw and set every Pattern bone of contention—which is fortunate, because it isn't possible. Disagreement will always be with us (see the paragraph above on the average Pattern competitor's personality profile). And it would be a vanilla-bean world if everyone was always on the same page.
However, this single snap/spin flash point is responsible for such a high percentage of the most-destructive bickering that it makes very little sense to continue ignoring it in the hopes that, by some magical means, an agreement will eventually emerge.
The easiest way to fix the whole mess would be to simply ban snaps and spins. That isn't an option, for a whole roomful of reasons, most of which boil down to this: it would be a cop-out. Snaps and spins are legitimate aerobatic maneuvers, and they belong in Pattern competition.
How snaps and spins work (a full-scale primer)
Before we get into possible fixes, we need to take a look at what makes these controversial maneuvers tick. And since these maneuvers weren't invented by RC modelers, we should go to the source: full-scale aerobatics.
The basic horizontal inside snap roll is really a spin done from level flight. Autorotation is induced by the difference in drag and lift between the in-snap or in-spin wing (inside the rotation) and the out-snap or out-spin wing (outside the rotation).
To start the horizontal snap or the upright spin, the aircraft is pitched up until the critical angle of attack (AOA) is exceeded, whereupon hard rudder is applied in the desired direction of rotation. The resulting yaw decelerates the inside wing and accelerates the outside wing. The slowing inside wing produces less lift (yes, some lift still exists past the critical AOA) and the accelerating outside wing produces more, thus producing autorotation (roll component) along the longitudinal (roll) axis.
As autorotation starts, the inside wing AOA increases (and thus produces more drag), while the outside wing AOA decreases and produces less drag because of the shift in the relative wind (the airflow that the wing "sees") because of the rotation. The effect is felt along the vertical (yaw) axis and is additive; rotation increases. This is even more obvious with the snap, because the entry speed was higher; the stall entry was accelerated, and the aircraft is continuing along its original horizontal flight path because of mass inertia.
The main difference between the entries to the two maneuvers is that for the snap, the entry speed is higher and the elevator movement is abrupt enough so that the critical AOA is exceeded almost instantly, causing an accelerated stall, with little deviation of aircraft track; for the spin, the aircraft is slowed and the pitch-up past critical AOA is gradual. When the stall is accomplished, the direction of flight changes to a near-vertical down line as autorotation commences.
The above is fairly basic full-scale aerobatic information (somewhat simplified for understandable consumption), but it has rarely been seen in the modeling literature that I've reviewed during the last several decades.
We have what an anthropologist would refer to as an "oral tradition" on this subject. What most RC modelers and even most Pattern pilots "know" about spins and snaps is that when you do "this" with the control sticks, the model does "that," and it looks a certain way to them. Their basic knowledge comes from their instructors, their peers, and from their own experimentation and outside reading they may or may not have done. Since everyone's experience is different in this respect, it is small wonder that opinions differ so much!
It is bad enough that this hit-and-miss approach to aerobatic education produces such a wide variance in basic knowledge. Unfortunately, the pilot-to-pilot grapevine also spawns various myths that are passed on as fact until they harden like yesterday's cheeseburger in a coronary artery.
Common myths and the truth
- Myth 1: The application of aileron has no place in a proper snap (or spin).
Truth: An application of aileron (usually small, but varying a good deal with aircraft type) in the direction of rotation is the rule in the full-scale competitive arena, done for the same reason that model aerobats do it: to accelerate the rotation by further increasing the difference in AOA between the inside and outside wings. Too much aileron will produce only drag, and actually slow the rotation.
- Myth 2: Once the elevator is applied to commence the stall for a snap, the control position is never modified until recovery, lest the aircraft unstall and barrel around.
Truth: Once rotation commences, forward stick is usually applied instantly to the point where the critical AOA is just barely exceeded, to keep from "burying" the snap. The elevator is quickly backed out a little to clean up the maneuver and accelerate the rotation. This is called "unloading" the snap, and it is an especially critical technique for snaps on a vertical line or for any snap in a maneuver where airspeed or energy must remain high.
"Burying the snap" is acrobatic slang for exceeding the critical AOA during the maneuver to the point where the platform drag becomes excessive and too much energy is lost. When this happens, the snap rotation is slowed and the exit airspeed is low, which causes the aircraft to "fall" out of the recovery. It is by far the most common snap roll fault in any aerobatic arena, full-scale or model.
It is so common that we need to spend little more time with the idea.
Most RC pilots have never heard the term; no specific downgrade exists for it, and yet when I sit in the judge's chair, I will find in excess of half of the snaps I see are "buried" to the point where the portion of the maneuver following the snap roll proper is well-nigh ruined — which definitely causes some downgrading to take place.
Some of this is because the pilots don't realize that they have a choice; throwing the sticks into the corner and nailing them to the stops is the only way they know to perform the maneuver. Some of it is because of snaps done "on the button," with way too much elevator programmed into the mix.
And unfortunately, some of it is because unloading the snap properly can cause the maneuver to look too clean, axial, and directional to satisfy many judges (and some fellow competitors) who, because of their own ignorance and prejudices passed on from others, are actually looking for a buried snap! Unless the model loses a great deal of speed and energy while flailing around like an airborne train wreck and then exits 15° off heading, these arbiters of aerobatic bad taste aren't convinced that the so-called "break" has occurred, and the model has actually autorotated.
The last problem isn't confined to the realm of RC aerobatics.
I found the following quote from the late former World Aerobatic Champion Leo Loudenslager, writing in Mike Goulian's and Geza Szuroy's excellent book Advanced Aerobatics (McGraw-Hill, 1995):
"At a World Championship, I was flying the laser, and the maneuver was an outside snap roll from inverted to inverted on a climbing 45° line. I hit the maneuver perfectly. Back on the ground, a fellow competitor stated, with some frustration, 'That maneuver was either a 10 or a zero.' This incident illustrates what you can run into doing the perfect snap roll in competition."
Anybody who doesn't find Leo's words very familiar simply hasn't been flying competition aerobatics very long.
Spins
Spins are another case in point. Most of the arguments center around the entry, and whether the model was "snapped" into the spin, or perhaps "flown" through the entry (no stall break). Some (perhaps most) of these disagreements are a direct result of the ambiguously written rules — but some are again the result of prejudice. Many of us have an iron-clad mental picture of what an aircraft should look like when it stalls and then spins, and if what we see doesn't fit our mental mold, it is rejected or downgraded.
Since many factors enter into the way in which different designs spin, the FAI and AMA books contain admonishments to the judge to "disregard the attitude of the model so long as it is stalled," but every year I judge with and fly in front of people who have either never read that rule, or who have decided that their own ideas take precedence.
Rules and rule-writing problems
Addressing these problems will take a multifaceted approach. We need to revise the rules, then improve pilot and judge education. Neither will be easy, but I have a few suggestions for the rules side of the equation:
The snap and spin rules, as they appear in the current AMA Competition Regulations, were lifted virtually intact from the FAI book. I know this because I was the one responsible for doing it about a decade back. You may take my word that this was done to align both books to the same "traditional" standard — not because the FAI maneuver definitions were perfectly written.
In 1990, the FAI Sporting Code that I cribbed from stated that a snap roll was "a rapid autorotative roll of the model in a stalled attitude." A bit further on, the statement appeared that ". . . the attitude and flight path must show a definite break before rotation is started." If the stall does not occur and the model barrel-rolls around, the maneuver is zeroed. I believe that the current rules are identical on these points.
On spins, the same ultra-spare approach applies in the FAI rules: "In order to spin, the model must be stalled. The entry is flown in a near horizontal path with the nose up attitude increasing as the speed decreases. The nose then drops as the model stalls. Simultaneously, the wing drops in the direction of the spin. If the model does not stall or if the model is snapped into the spin, the maneuver is zeroed."
The AMA book follows both definitions closely, but leaves out the FAI spin requirement for a "definite break before rotation is started," stating only that the attitude and flight path must show a "definite break." This was done because many people, myself included, felt (and still feel) that the FAI requirement for a break "before" rotation was totally unjustified in model aircraft at 150–175 meters, without the benefit of stop-action or slow-motion photography. The Sporting Code also lists as a judging criterion for snap rolls that they must maintain "... a constant flight path through the manoeuvre." No one could explain just exactly what that statement meant. I suspect that it means that, during the snap rotation, the center of gravity of the model must follow roughly along the track projected by the line of entry to the manoeuvre, but it doesn't state that, does it? We left it out of the AMA book.
As a result, regardless of whether the AMA or FAI book is used, the pilot gets little help from the definition, and the judge gets even less. People are operating on opinion and subjective impression ("Well, it looked like a snap roll...") because they have been given all sorts of room to do exactly that.
Even the AMA judging video, which is absolutely excellent in most other areas, isn't any more help than the rule book with snaps and spins — because the definitions used are the same, taken word for word from the FAI book, which were in turn lifted from the FAI full-scale aerobatic rules, possibly even as a "grandfather" clause! The weight of this particular tradition is immense; in fact, it may be the original "grandfather" clause!
Suggestions for clearer, judgeable rules
What is needed is a definitive rewrite of these ancient and vague rules — one that answers the big questions and addresses the gray areas. Most importantly, the rules and definitions should be written with the idea of judgeability in mind. We must give the judge a hook to hang the score on, and let the pilot know exactly what is required. And since the FAI appears to be locked into adherence to this nonfunctioning tradition, we need to do it in the AMA Competition Regulations, where it isn't necessary to convince the rest of the world to effect change.
Examples of possible, more precise criteria:
- Define a snap-roll entry to a spin by a measurable geometric relationship, for example: the up-going wing must exceed the vertical plane (past 90° of rotation on the roll axis) before the nose drops below the horizontal plane as defined by the initial flight track. That gives a hard-and-fast criterion.
- Replace vague phrases like "rapid autorotative roll" with specific numbers, e.g., a minimum rotation rate such as 360°/second.
- Define a barrel roll by the center-of-gravity track: if the track describes more than a half-wingspan visible circle during the maneuver, classify it as a barrel roll.
- Replace a loose "break" requirement with a requirement for simultaneous divergence demonstrated in pitch, roll, and yaw axes.
- Rework spin requirements for entry and exit paths to allow specific angular deviations (for example, five or 10 degrees) rather than vague terms like "near horizontal" or "near vertical."
It is very likely that you can think of other problems and similar solutions along these lines, and I don't pretend that the possible solutions I've suggested are the best or only ways to solve these problems. They are merely intended to show that it is possible to write accurate and specific requirements.
There is something we can do, and these ancient problems we've inherited don't have to continue to exist, at least in an AMA Pattern. We should solve these and move on to new ones.
That's called progress, and around here, it's traditional as well.
Transcribed from original scans by AI. Minor OCR errors may remain.
