RADIO CONTROL SCALE AEROBATICS
Paul Kopp, 1013 S. Sedona Ln., Anaheim CA 92808
A frequent question concerns the benefits of purchasing a higher-end computer radio.
Medium- and higher-priced computer radios offer features that can greatly enhance flight performance. The purpose of this month's column is to examine some features of better computer radios and their application to scale aerobatics.
The majority of scale aerobatics airplanes are comparatively simple: one engine, two elevators, two ailerons, and one rudder. Smoke is about the only option.
Many designs (including Giant Scale) can be flown using a lower-end four-channel analog radio. A lower-end radio will suffice for a sport modeler who isn't interested in expanded capabilities or advanced setup, but there are a number of features on computer radios that will literally change the "feel" and characteristics of almost any scale aerobatic airplane.
Some features, such as exponential, have the same effect whether the airplane is a P-51 Mustang or an Extra 300. Other features, such as programmable mixing, may be used on a P-51, but for a different purpose than on an Extra.
Exponential
This is arguably one of the most useful features in achieving a good setup. Exponential is defined as the mathematical relationship between stick movement and servo travel.
If servo travel were strictly linear, a radio without exponential would exhibit a proportional relationship: how far the stick was deflected (50%, for example) would produce a proportional amount of servo travel. However, because servo arms move in an arc, the effective control throw at the surface does not increase linearly with stick movement. (Some modelers use larger control arms to increase travel and change the geometry.)
Without exponential the stick/servo relationship near center is effectively very sensitive: a slight movement of the stick produces a similar movement in the control surface. For an aerobatic airplane, the result is a model that is very pitch-sensitive, a handful to land, and not much fun to fly. The purpose of an exponential function is to "dull" the center of the stick by changing the stick/servo-travel relationship. The pilot "feels" an airplane that is much less sensitive near neutral, making it much easier to control. How much exponential is needed depends on the airplane and is a matter of personal preference.
Knife-edge mixing
Many scale aerobatics airplanes will hold sustained knife-edge. However, very few airplanes will hold knife-edge without pitch and/or roll corrections. When most airplanes are rolled into knife-edge (using rudder only), they will probably not fly straight. Many will "push" into the belly (midwing designs may pull toward the canopy), and exhibit a roll tendency, either out of knife-edge toward the rudder or away from the rudder. The propensity to pitch or roll from rudder is called "coupling."
An important concept is that although coupling may be most pronounced in knife-edge, it is inherent in the aircraft design. Some pilots unknowingly experience pitch coupling when they make a rudder correction during landing and the airplane's nose drops suddenly — it wasn't a radio hit.
The ability to hold knife-edge depends (among other things) on holding the required corrections. For simple knife-edge from one end of the runway to the other, holding the corrections requires fixed inputs. Try to do a long, slow roll or opposing knife-edge, and it's a different story: the airplane may wander and change roll rate.
When I first started flying competitively I was using a Carl Goldberg Models Sukhoi with an older analog radio with no mixing. Because I couldn't correct the pitching in knife-edge, I had to do slow rolls by pitching the nose up and flying through the maneuver in an arc, with little or no rudder. Those rolls were really ugly! It became a different airplane with rudder/elevator mixing — slow rolls were straight and axial.
Programmable mixes are used to minimize or eliminate coupling. Mixing allows the modeler to create a relationship between two channels: one designated the "Master" and the other the "Slave."
"P mixes" (predefined mixes) are significantly different from the programmable mixes found on better radios. Predefined mixes such as Aileron/Rudder are fine for a Cub, but opposite of what we need for precise scale aerobatics.
A programmable mix allows the modeler to create elevator or aileron corrections tied to rudder input. The rudder channel is the Master and the elevator or ailerons are the Slave. The "program" creates the relationship between the independent channels and defines the amount of deflection for each surface. If the airplane has pitch and roll coupling, it will require two independent mixes.
If you have two elevator servos and want to run them on separate channels, the radio must provide a function that allows both elevators to be controlled by the same trim. That feature is found only on medium- to higher-end radios. Without it you can set up a dual-elevator mix, but only one side will respond to the elevator trim.
Assuming the radio has dual elevator/trim capability, establish the Master/Slave relationship described earlier. For example, the rudder channel remains the Master and the primary elevator channel becomes the Slave. Movement of the primary elevator will drive the second elevator if the elevators are mixed. When the rudder stick is deflected, both elevators will respond.
Multipoint mixing
High-end computer radios provide additional mixing capability known as multipoint mixing.
Programmable mixing on medium-priced radios relies on a linear relationship between the Master and Slave channels. If an elevator has a mix value of 10%, as the rudder is deflected the elevator will gradually deflect until the 10% maximum is achieved.
Multipoint mixes allow modelers to change the mix relationship at different points, thereby creating a mixing "curve." Multipoint mixes are useful when the coupling is not linear. For example, we may establish "uncoupled" knife-edge flight using linear mixes so that the airplane can fly knife-edge in either direction using only rudder inputs.
But suppose we want to do knife-edge loops. The extra rudder deflection may require mixing corrections in excess of what a linear mix provides. A multipoint mix allows the modeler to provide the additional corrections at larger rudder deflections without disturbing the mix needed for level knife-edge.
Condition switches / Flight modes
Condition switches, or flight modes, are found on high-end radios. With the increasing interest in 3-D flying, this feature has recently attracted many users besides high-level competitors.
The benefit is that the airplane's setup can be changed instantly from precision — utilizing low, highly moderated throws — to 3-D — using extreme throws with different exponential curves, servo travel, and mixing. It is a very useful feature, especially among pilots who fly both precision and freestyle competitions.
Programming hints
There is one hint for programming, regardless of a pilot's experience: do it with the airplane on the ground.
- Verify the direction of travel. One pilot couldn't figure out why his Extra 300 pitched to the belly even after he set up what he thought was the correct mix. What he didn't check was that the elevator "correction" was going the wrong direction.
- Check exponential direction. It's very easy to create "negative exponential" (increasing the sensitivity near center) by setting the value the wrong way. To ensure you're going the right way, set an extremely high value and watch the control surface move. If the surface is unusually sensitive around neutral and dead at the end, you may have accidentally created negative exponential.
- Set up knife-edge mixes by trial-and-error. Because of differences in centers of gravity and other setup discrepancies, two airplanes may require different correction percentages. Starting at zero:
- Roll the airplane into knife-edge and gently feed in enough rudder to hold altitude while holding the required corrections.
- Have a helper write down the corrections by direction (for example, right-to-left: up elevator, right aileron).
- Correct one surface at a time, making sure the corrections are visually in the right direction.
- When the mixes are properly set up, the airplane should remain in straight, level knife-edge with rudder only.
A number of other features distinguish computer radios; however, it is the mixing capabilities that are most often used for scale aerobatics airplanes. Mixing can also be used to turn smoke pumps on and off, create engine kill switches (a nice safety feature), and cure minor pitching tendencies in vertical lines.
RCS 14
R/C Showcase (RCS) recently released a 14.02 cid gas engine, designed to replace 1.2 glow engines. I was lucky enough to get one to review, and I have been running it in a couple different airplanes. The engine is turning in the mid-8,000s with an APC 16 x 6 propeller, making it comparable to a good 1.2. Unlike other small gassers, the RCS is extremely light — another important consideration.
RCS has a full complement of accessories for the 14, including a predrilled Du-Bro rear-mount, an ignition switch, and a muffler.
Transcribed from original scans by AI. Minor OCR errors may remain.
