RC Aerobatics

By Dave Lockhart. Photos Dave Lockhart and Paul Osinski. Full review in the Summer 2015 Park Pilot.

“Aerobatics” is a term that encompasses a lot of ground— or a lot of airspace. Pattern, precision, 3-D, artistic, freestyle, aeromusical, F3A, F3P, and IMAC (International Miniature Aerobatic Club; mini-iac.com) are among the terms often used to better define the type of aerobatic flying and classes of competition. Contrary to myriad advertisements that claim a given airplane is “ideal” for all types of aerobatic flying, each type of flying can benefit substantially from a tailored design and specific airplane setup. Pattern is a general term used to describe aerobatic flying in which pilots complete a series of predetermined aerobatic patterns or maneuvers (sequences or schedules). Pattern generally refers to airplanes flown in the FAI (Fédération Aéronautique Internationale; fai.org/aeromodelling) classes of F3A and F3P (Outdoor and Indoor Pattern; nscra.us). Pattern is a style of aerobatics with emphasis on lines, shapes, geometry, and precision. Pattern maneuvers are generally more traditional in nature. Maneuvers are typically varieties of loops, rolls, snaps, spins, stall turns (Hammerheads), tail slides (IMAC and F3P), and torque rolls (F3P). The types of airplanes flown in each of the specific competition classes are influenced and limited by restrictions on size, weight, and noise. Additionally, for the FAI F3M (large RC Aerobatic Power Model Aircraft) and IMAC classes, there is a requirement for the airplanes to be scale replicas of full-scale aerobatic airplanes. Pattern airplanes (F3A and F3P), without the encumbrance of a scale requirement, are designed to be as surgically precise and as aerodynamically pure as possible. F3A airplanes are typically 2 meters in length and nearly 5 kilograms, pushing the limits for the class. IMAC is an organization that promotes competitions mimicking those of their full-scale counterparts in IAC (International Aerobatics Club; iac.org) competitions. With both IMAC and F3M competitions, precision is again the goal, but the aircraft are generally not as precise as a purpose-designed Pattern model. IMAC and F3M airplanes can be much larger in size—20 kilograms for F3M and 55 pounds for IMAC. At the highest levels of IMAC and F3M competition, the airplanes typically push the weight limit. With all other things equal, bigger flies better. Freestyle and 3-D are terms generally associated with more aggressive flying that encompasses a broader flight envelope than Pattern flying. 3-D flying specifically refers to flying/maneuvers in which the airplane is in a partially or fully stalled condition. In a stalled condition, the wing is generating insufficient lift to support the weight of the airplane and the thrust of the propeller is keeping the airplane in the air. Examples of 3-D maneuvers are Harriers, rolling Harriers, torque rolls, waterfalls, elevators, and a variety of tumbling/gyrating maneuvers such as crankshafts, pop tops, and Lomceváks.

Dave’s 60-inch Extreme Flight Extra is in inverted Harrier mode, as evidenced by the substantial elevator deflection. Extreme Flight designs typically have relatively long tail moments with large control surfaces to provide a good balance of control authority for 3-D maneuvers, while maintaining precise control. Photo by Paul Osinski.

Dave’s newest project is the Allure, which is the latest F3A design from Bryan Hebert at CK Aero (ckaero.net). The Allure pushes the F3A class limits, maxing out the wingspan and length at a full 2 meters (78.75 inches), and is slightly more than 18 inches tall at the rudder hingeline.

Artistic aerobatics and aeromusical are terms generally associated with the flying of a set routine that is choreographed to music. This type of flying most often includes elements of both Pattern and 3-D flying. Design characteristics of a Pattern airplane include long tail moments (wingspan is usually less than fuselage length), tall fuselages (for better knife-edge performance), tapered-wing planforms (improved stability in turbulent air and better roll response), low drag (better penetration in windy conditions), and moderate-size control surfaces. IMAC/F3M airplanes are primarily characterized by having a scalelike appearance. Compared to Pattern airplanes, they are higher drag (larger cowls to cover the proportionately larger engines of the full-scale airplanes), have shorter fuselages, and less-tapered wing planforms (structural considerations of the full-scale airplanes). The best set-up Pattern airplanes have enough control throw to complete snaps, spins, and stall turns. Control surfaces are generally small (roughly 20% of surface area of the wing or stabilizer). Keeping the control throws to the minimum needed (rarely more than 25° of deflection, except for the rudder which is 35° to 40°) results in the best control resolution for the most precise control inputs. Pattern airplanes typically use propellers with diameter-to-pitch ratios of approximately 1.5:1 (or 21-inch diameter and 14-inch pitch), which are good for both moderately high cruise speeds and good thrust in climbs, assuming they are relatively low-drag airframes. Scale models with higher drag require “flatter” propellers for optimal Pattern performance—a ratio closer to 2:1 (20 x 10). The CG (center of gravity) is generally slightly forward (the airplane requires down-elevator to maintain inverted flight), which improves stability in turbulent air, but reduces the ultimate effectiveness of the rudder and elevator. 3-D flying is simply not possible without having good control authority and good thrust at very low airspeeds. Control surfaces are best when they are large—typically 30% to 40% for the ailerons, and even larger for the elevators (as much as 70%). Control throws are also large, typically 45° on the ailerons and 60° (or more) on the elevator. To increase thrust at low speeds, propellers with diameter-to-pitch ratios of closer to 2:1 or even 2.5:1 (15 x 6) are preferred. Higher diameter-to-pitch ratios are less efficient for high-speed flight, and limit top-end speed, but provide better thrust at low speed and more airflow over the tail surfaces, increasing the effectiveness of the tail surfaces at low speeds. Given sufficiently large control surfaces and control throws, a neutral (no down-elevator needed to maintain inverted flight) CG is not needed for 3-D maneuvers, but a farther-aft CG will facilitate more aggressive tumbling and gyrating maneuvers. Both types of airplanes/setups benefit greatly from precise linkages, tight hinging, and high-quality servos. Sloppy linkages reduce the precision of the control inputs, make trimming much more difficult, and increase the odds of control surface flutter occurring. When a computer radio is being used, the endpoints (or travel volume) should be at the factory default (or slightly higher), using the mechanical linkages to achieve the desired maximum control-surface deflection.

These are examples of mechanical linkage setups biased for 3-D and Pattern styles of flying. The 3-D linkage setup has the linkage attached farther from the output shaft of the servo, and closer to the control surface on the control horn. The pattern linkage setup is the opposite: closer to the servo output shaft and farther from the control surface.

When it comes to the flight envelope for each type of airplane/setup, there is a good deal of overlap between precision Pattern flying and 3-D flying. How much overlap depends on the setup of the control throws, CG, and the power system. Pattern airplanes can certainly hover and torque roll, but “punch out” from a hover is not as strong as a 3-D setup, and the control authority is limited while hovering. For pilots who do not aspire to fly 3-D, a 3-D airplane with 3-D control throws is harder to fly than a 3-D airplane set up to mimic a Pattern airplane. A 3-D-style airplane that is set up with reduced control throws will nicely fly precision aerobatics, although not as precisely as a Pattern design. Certainly the most versatile option is a 3-D airplane set up with 3-D throws, and either a flight condition or dual-rate switch(es) to electronically reduce control throws to mimic the throws of a Pattern setup. Electronically reduced control throws will make the airplane less sensitive at high speed, and less prone to being overcontrolled.

A favorite park flyer for all occasions, the Fancy Foam 42-inch Extra 330, is set up with two flight modes controlled by a single switch on a JR XG14 transmitter. High rates are for no-limits 3-D flying, and low rates are for more-precise control at high speeds and for Pattern flying.

The “best” aircraft or “best” setup for an aerobatic model depends on the type of flying. No single airplane or setup will be the best for both 3-D and Pattern flying. If the desired priority is 3-D flying, look for designs with large control surfaces and hingeline bevels that allow lots of control throw. If the desired priority is for Pattern flying, look for designs that have long tail moments and moderate-size control surfaces. Many airplanes are capable of a variety of flying styles. Experimenting to find the setup that is “best” for you is part of the fun!