Flight Stabilization Basics
THREE-AXIS STABILITY FOR FIXED-WING RC
THREE-AXIS STABILIZERS or gyros specifically designed for fixed-wing airplanes are a relatively recent innovation. These amazing little black boxes help the aircraft to fly stable around its three axes: roll (aileron), pitch (elevator), and yaw (rudder). Prior to their development, modelers often used single-axis gyros designed for RC helicopters in airplanes, usually on the rudder. Multiple-axis stabilization required multiple gyros and was a chore to install and set up. Today’s three-axis stabilizer units have a small footprint that can be installed in aircraft of virtually any size. There are also ready-to-fly planes that come with these units preinstalled as well as receiver/stabilizer combinations in one unit. Today’s onboard stabilizers are versatile and can be a great aid to pilots of all skill levels. They make learning to fly much easier and less stressful, and also allow experienced pilots to fly more comfortably in wind conditions that would otherwise ground them. They also help tame those squirrely tail-dragger planes that are a handful to tame. Despite their widespread use, there still seems to be some confusion and misconceptions regarding the use of three-axis stabilizers. Some pilots feel as if they are relinquishing control, while others think of them as autopilots. I have also talked to pilots who have expressed their dislike, claiming the plane doesn’t fly correctly. I suspect that, in most of these cases, the gyro was improperly set up or the pilot had unrealistic expectations. A basic understanding of how these stabilizers function and their features will help ensure success at the field. It will also help you choose the gyro best suited to your intended purpose and your budget, of course. Gyros vary widely in price and features, and it is not possible to list every feature available here. The principles on which they operate are universal, however, as are some key features discussed here.
STANDARD AND HEADING HOLD MODES
All but the most basic gyros have two principal modes: Standard/Rate and Heading Hold (also known as “3D AVCS”). Typically, the pilot can toggle, in flight, between modes using a transmitter switch. Gyros with only one mode operate in the Standard mode. A gyro operating in Standard (or Rate) mode will sense a deviation or upset from the airplane’s flight path and provide an opposing control deflection—the higher the rate of rotation, the larger the correction (hence, the name “rate”). The gyro makes no attempt to return the plane to its original heading but merely dampens any deviation. The easiest way to visualize this is to use an RC helicopter as an example. Consider a heli in a stationary hover that is hit by a wind gust from the side. The heli will try to weather-vane into the wind, which the gyro senses as a rotation around the yaw axis. The gyro will send a command to the tail rotor to stop the rotation, but the heli will have rotated to some degree and will be on a new heading. In essence, the gyro gives the airplane a bigger feel with improved tracking and fewer and less severe oscillations due to the wind. Sport and aerobatic pilots often find the smoother flying characteristics and improved tracking allow them to concentrate on perfecting their maneuvers. The gyro will not make their loops any rounder or compensate for a crosswind as the plane still goes where the pilot points it. Stabilization systems have also been
a boon to scale modelers. Tail-draggers, especially World War I and WW II fighters, have notoriously poor ground handling, especially on the takeoff and landing rolls. A gyro can tame that tail as well as improve the peculiar flight characteristics of some scale models. Turbine pilots have relied on gyros to achieve the ultrasmooth and arrowlike precision flying you may have seen. Another benefit is the reduced likelihood of an unintentional tipstall. While it offers no miracle cure, a gyro can turn a mediocre-flying model into a pretty-good-flying model and a good one into a great one. Heading Hold mode has various names depending on the manufacturer and is a more aggressive form of stabilization. The gyro will not only correct the momentary rotational movements as does a rate gyro but also sense how far the plane has deviated and return it to its original heading. The transmitter sticks no longer control the servos directly but, rather, tell
the gyro the required rate and direction of rotation. In this mode, the gyro will attempt to keep the plane locked in the attitude commanded by the pilot. Using the hovering heli example, the gyro will sense the rotation about the yaw axis, stop the rotation, and return the tail to its original position. In an airplane, if the pilot enters a hover and turns on Heading Hold, the gyro will keep the plane locked in the hover with no input by the pilot. Aspiring 3D pilots may find this mode useful in learning difficult new 3D maneuvers. It is especially helpful in harrier maneuvering as well as hovering and performing the torque roll. Normal, or non-3D, flight should always be done in the Standard mode to avoid unwanted and unusual fight characteristics. In Standard mode, the servos will return to center when the sticks are released. That’s not so in Heading Hold mode; the servos may not return to center as the gyro tries to hold the last commanded attitude.
SPECIAL FEATURES
Some gyros feature a Beginner mode, which limits the bank and pitch angles. This helps eliminate overcontrolling and subsequent loss of control common to newcomers. One of the most challenging aspects in the early stages of flight training is learning to give the proper control response with the airplane flying toward you. The Beginner mode gives student pilots time to sort this out without the constant need for bailouts. The best way to learn to fly is to receive instruction from an instructor at the local club, but this may not be feasible for everyone. A docile trainer equipped with this type of gyro gives the pilot a fighting chance. Also, flight training with an instructor can be less stressful with a gyro-equipped
trainer as well as giving the post-solo pilot increased confidence. There are a few occasions in the past where I could have used a Panic Button feature. Some stabilization systems include a panic or return-to-level function. When activated by a button or switch on the transmitter, the plane will return to straight and level flight no matter what attitude it was in when the panic button was activated. This is obviously great for beginners, but even experienced pilots may find it useful on occasion when a case of the “dumb thumbs” strikes.
BOTTOM LINE
Flying a gyro-equipped model is a difficult experience to put into words. Just as with dual rates and exponential, gyros are a pilot aid that increases the enjoyment of flying RC airplanes. Perhaps one day, we will wonder how we ever flew our models without them.