Radio Control: Scale Aerobatics

RADIO CONTROL SCALE AEROBATICS

Eric Hawkinson 319 Yellowstone Ave., Billings MT 59101

I received three letters concerning my use of the inaccurate (if commonly used) term "weathervane" in my April column to describe the yawing that occurs during crosswind maneuvers. In that column the discussion was about crosswind landings.

Letter from Bob Zajichek

My favorite was from Bob Zajichek of Issaquah, Washington. Excerpts follow:

"Although I can’t consider myself an outstanding RC pilot, I had 25 years military flying—mostly carrier aircraft. One of my pet peeves in the almost eight years I’ve struggled with RC flight has been the consistent misuse of the terms weathervaning or weathercocking by students and instructors!

"The wind does not cause the airplane to 'weathervane'; rather, the pilot (unconsciously?) points the nose into the wind in order to fly the desired flight path over the ground. He is using the 'crab' method of drift correction. The airplane is in balanced flight, with wings level and controls neutralized.

"What Mr. Hawkinson describes are the two methods of drift correction: the crab and the slip. The crab is used to maintain flight path over the ground in a balanced flight condition. The slip lines the longitudinal axis (and landing gear) up with the flight path. The upwind wing is lowered and downwind rudder applied to keep the airplane from turning. Because slipping is difficult, it’s frequently applied just prior to touchdown.

"Tricycle-geared aircraft can be landed in either a crab or slip condition because the center of gravity (CG) is forward of the main mounts, so that the airplane tends to track straight down the landing line after touchdown.

"Tail-draggers present more of a problem. With the CG aft of the main mounts, if the longitudinal axis is not lined up with the landing line, inertia will tend to swing the tail forward and cause a ground loop on landing. Therefore, it’s best to land tail-draggers upwind, with the downwind wing low. One-wheel landings are much preferred over landing in a crab, or worse yet, drifting downwind.

"Two final points:

1. Weathervaning only occurs while the airplane is on the ground. Once it’s in the air, the airplane moves right with the air mass; it’s in a crab (a balanced flight condition) to maintain the desired flight path. Let’s start using correct terminology.
2. Ground handling in a strong wind can be tricky—particularly in trainers, with lots of dihedral. All RC pilots should learn to taxi and takeoff just as pilots of full-scale aircraft do. Keep your aileron into the wind to reduce the possibility of the wind raising the upwind wing.

"As the airplane picks up speed and the ailerons become more effective during the takeoff roll, ease off the amount of aileron but fly off-wing-low and turn into the wind to establish your crab. Also, holding full down elevator while taxiing a tail-dragger downwind can increase pressure on the tail wheel."

Commentary on the letter

Of course, Mr. Zajichek is correct. It doesn’t change a single thing about how you go about learning to fly a "perfect" crosswind approach, but I’ll try to use more-accurate terminology.

There’s no doubt I’ll make similar "mistakes" in the future, since my alleged writing style is consciously intended to be as compact as possible while conveying the information. (Another example is the term commonly applied to describe a two-stroke engine running very rich as "four-cycling," an obvious misnomer, but instantly understood by most in the hobby.)

"Weathervaning only occurs while the airplane is on the ground. Let’s start using the correct terminology."

Two of the other letters contained the old saw about "the airplane doesn’t know the wind is blowing." This is true, but in my opinion it is a rather useless truism in our sport. The airplane doesn’t know it missed the runway, endangered a spectator, or bent its landing gear, either.

However ignorant the actual aircraft may be, the pilot—and all other things on the Earth’s surface—certainly recognize that the path of flight is decidedly different depending on wind conditions, relative to the fixed perspective of the pilot and others on the ground. RC and full-scale pilots recognize that different control inputs are required for differing wind conditions.

Scale Aerobatics vs Precision Aerobatics (Pattern)

This discussion leads nicely into another subject I’ve received several letters about: What is the difference between Scale Aerobatics and Precision Aerobatics (Pattern)?

The first and obvious difference is that Scale Aerobatics requires that you fly a scale airplane—normally a replica of a full-scale aerobatic aircraft. For non-contest purposes that’s really all there is to it. If you are wondering about competition, the answer is a bit more involved.

1. Aircraft, size and noise rules

• Pattern allows any aircraft and engine so long as the weight is no more than five kilograms (11 pounds) and the dimensions do not exceed two meters (approx. 78.75 inches) in span or length. Noise levels in Pattern are limited to 96 decibels at three meters over pavement.
• Scale Aerobatics (IMAC) contests require semi-scale aircraft, except that the rules allow non-scale models, at the Contest Director’s (CD’s) discretion, in the Basic class. Size is not limited except by the general AMA limits on RC models—so you can fly fairly large aircraft.

1. Use of electronic/mechanical aids

• There is no prohibition in the published IMAC rules against electric or mechanical aids, such as gyros. Note that some CDs may choose to outlaw such devices at their contests.

1. Similarities in flying

• Many maneuvers are identical between Pattern and IMAC, and control inputs for a four-point roll, for example, are the same regardless of the event. However, flying style and judging emphasis differ.

Major differences in how the two events are flown

1. Takeoff and landing scoring

• In IMAC events, takeoff and landing maneuvers are not scored.
• In AMA Pattern, takeoff and landing are scored maneuvers.

1. Emphasis: geometry vs smoothness

• IMAC emphasizes geometry and "prototypical" maneuvering. There is no mention of "smoothness" in the IMAC rules.
• In Pattern, "smoothness" is second only to precision in determining the quality and score of a maneuver. This is most evident in rolling maneuvers: the IMAC style tends to use more aggressive, quick aileron inputs.

1. Flight sequences and scoring format

• IMAC events often fly their sequences two per flight, with the higher-scoring sequence recorded and the lower score dropped. Landing for minor adjustments between sequences is allowed.
• Pattern scores generally use three flights per round; best two of three is common. Other formats include best three of four or best four of five or six flights.

1. Unknown sequences and Freestyle (IMAC)

• IMAC has the option of flying "Unknown" sequences and a "Freestyle" class. Unknowns are a group of maneuvers selected by a knowledgeable pilot and handed out on the day prior to flying. Pilots may not practice the sequence before competition. Unknowns are an added challenge because the order of maneuvers requires different technique.
• Unknown sequences, when used, are flown in addition to (not instead of) the regular published sequences for Sportsman, Advanced and Unlimited.
• The Three-Minute Freestyle class is a display judged on originality, versatility, harmony and rhythm, and execution. It is subjective, spectator-friendly, and a popular event at tournaments. Pilots may usually fly both their regular class and Freestyle at a contest.

1. Wind-correcting rules

• In Pattern the pilot is always required to correct the track of flight, no matter what aircraft attitude is required to maintain that track. This makes Pattern especially challenging.
• In IMAC rules, path-correcting is prohibited for vertical (90-degree) lines. The IMAC rule states: "90-degree lines are to be graded on the angle of the fuselage in relation to the ground. All other lines are judged on the track or flight path of the aircraft."
• Thus, in any wind an IMAC airplane may be moved downwind during a stall turn or humpty bump relative to the ground reference.
• A more detailed consequence: only the vertical (90-degree) lines are not required to be corrected. In a humpty bump, the entry, exit, and half-loop are supposed to be wind-corrected, while the up- and down-lines are not allowed to be wind-corrected.

1. Terminology

• The IMAC crowd tends to refer to Stall Turns (the normal Pattern description) as Hammerheads. They are the same maneuver, except that the yaw wind is dealt with according to the IMAC wind-correcting rules.

If you want to fly either event (and I hope you do!), order the Competition Regulations from AMA Headquarters and study the event often and thoroughly. This is good advice even if you don't intend to compete. Don't be afraid to fly both types of aerobatics—many pilots do. Both events require precision flying, and that is what helps us improve our skills.

Transcribed from original scans by AI. Minor OCR errors may remain.