Of all the recent innovations in RC technology (and there have been many), one of the most substantial has been the development of artificial stabilization systems. They began several years ago as 1-axis gyros intended to tame the often unwieldy yaw behavior of RC helicopters. Now these devices are offered in 3-axis designs that can also assist the pilots of multi-rotors and all types of airplanes.
As artificial stabilization systems have become more refined, capable, and affordable, they have gained wide acceptance in the RC community. Many pilots initially viewed stabilization systems as a crutch for ham-fisted pilots. I think we've turned the corner, and the majority of RC modelers now recognize artificial stabilization as a useful tool with potential applications for pilots of all skill levels.
In previous articles, I provided overviews of the SAFE (Sensor Assisted Flight Envelope) system in the HobbyZone Delta Ray, Blade 350 QX2, and Blade 350 QX3. I have also reviewed the finer points of the Open Pilot CC3D unit. Over a series of articles, we will be looking at some of the other stabilization systems available today. We'll begin with a peek at the AS3X (Artificial Stabilization 3-Axis) system from Horizon Hobby.
What is AS3X?
As I began mapping out my plan to write about AS3X, I quickly realized that there are way too many different aspects of the system to cover in one article. So I decided to pare it down to its simplest form and provide a broad overview. Consider this an introduction. If there is interest, I will put together a follow-up article that explains some of the customization options that are available.
AS3X works to keep the airplane on its current commanded path, compensating for the wind's impact on the model.
The intent of AS3X is somewhat unique among stability systems. It is not meant to sense the horizon and level the wings to prevent a crash, as some other systems do. So there's no panic button to rescue your model from a piloting mistake. Rather, AS3X works to keep the airplane on its current commanded path, whatever it might be. If the integrated gyros sense a change in orientation due to an outside force (wind), the system provides corrective control inputs to the servos. This makes it seem as if the wind is having no impact on the model.
With few exceptions, modelers typically want their aircraft to be as light as possible. Lightly-loaded models take off and land slower, require less power to stay airborne, climb faster, stall less harshly…the list of benefits goes on and on. The prime drawback is that the lighter a given model is, the more easily its flight path is disturbed by wind.
Imagine tossing a balloon filled with air in front of a fan. The rushing air will certainly push the balloon off course. Now imagine tossing the same size balloon filled with water. AS3X attempts to make the path of a light aircraft as predictable and steady as the water-filled balloon, but without forfeiting the benefits of its bantam weight.
In addition to improving a model's wind tolerance, AS3X also augments its aerodynamic stability. It's the same idea behind many modern fighter aircraft. They must be designed with a fair degree of instability in order to achieve the desired maneuverability. These airplanes would be unflyable if not for the computerized control systems that compensate for their lack of aerodynamic stability. In the RC world, this approach can be applied with AS3X to make a model more agile, or perhaps make feasible a design with borderline aerodynamic stability.
AS3X was initially released in 2011 in conjunction with a trio of small models meant to showcase the system's capabilities. In the years since, numerous other AS3X models have been introduced. This includes several larger aircraft. The purpose is the same regardless of the specific aircraft it is installed in.
AS3X is not a stand-alone system. The necessary components are integrated into a variety of Spektrum brand 2.4GHz radio receivers. You'll need either a Spektrum transmitter or a radio with a Spektrum-compatible transmission module to use one of these receivers. If you buy the receiver separately, there is a smart phone app for configuring the AS3X parameters to your specific model and preferences.
As previously mentioned, there are a number of models in the Horizon Hobby lineup that come equipped with AS3X receivers. Although you still need a Spektrum-compatible transmitter, no system set-up is required for these models (although some offer configuration options if you desire).
In order to test the benefits of AS3X for myself, I chose the E-flite UMX Pitts S-1S. I paired the model with my Spektrum DX8 transmitter. I chose this model because it offers a trifecta of design traits that, without AS3X, would make it difficult to fly under common conditions. First of all, it is small. With a wingspan of just over 17", it is closer in size to a mahogany desktop model than the average RC plane. With such a small model, all of the normal imperfections, tweaks and control system lash are proportionately more significant--a factor that makes tiny models inherently twitchier and more challenging to fly.
Secondly, the Pitts is lightweight, tipping the scales at 3.2 ounces ready-to-fly. There are certainly lighter models in this size range, but not many, and not by much. In fact, the Pitts is lighter than just the wheels and tires of most standard-size airplanes. We've already discussed how being light puts a model at the mercy of the wind.
Lastly, the Pitts has long been considered a difficult design to fly well. This is true for the full-scale aircraft and the many RC models that have emerged through the years. The Pitts is legendary for its aerobatic prowess, but that comes at the cost of stability. You have to stay on your toes when flying a Pitts.
I normally wouldn't consider flying a model like this tiny Pitts if the wind is 5mph or more. In the weeks that I've been testing this model, however, I brought it along on every trip to the flying field regardless of the conditions. Of course this often had me flying the model well outside of my normal wind limits. Fortunately, my initial outing with the Pitts was on a nearly windless day. This allowed me to calibrate my feel for the model's performance without the influence of wind.
I walked away from those calm day flights impressed with the overall performance of the Pitts. Packed inside the micro-sized airframe is a brushless motor, tiny servos and a 2S-200mAh LiPo battery. The AS3X bits are housed on the hidden receiver, which also houses the brushless ESC. All of those components make up a powerful machine.
I could easily make the model perform loops, rolls and other basic aerobatics. That part was expected. What came as a pleasant surprise was that the Pitts didn't need the constant tiny corrections that a model this size typically demands to maintain straight and level flight.
With the AS3X, the Pitts didn't need the constant tiny corrections that a model this size typically demands to maintain straight and level flight.
The only thing that I didn't like about the Pitts is that the power system vibrates a little…just enough to make the airplane louder than it should be. I initially thought that the vibration was caused by the foam spinner, so I removed it. I've since balanced the prop as well, but the vibration persists.
Subsequent outings yielded more wind, which let me see how the Pitts would cope. Surprisingly, I didn't find the behavior of the model that much different. Yes, the airplane didn't maintain as true a path as it did on the calm day, but there was no question that AS3X was damping out most of the jumpiness that I expected under these conditions. I never felt like I was fighting the wind for control the airplane – a situation I have found myself in with numerous other small models.
I purposely took the Pitts out on a day that I wouldn't fly some of my larger, heavier models. I don't know what the wind speed was, but it was certainly more than 10mph at times. As a reference, I can tell you that prior to launching the Pitts, a gust of wind lifted it from the ground and sent it tumbling across the field. That incident broke one of the wing struts, but I flew anyway.
Despite those conditions, the Pitts was not only flyable, but enjoyable to fly. My only real consolation to the wind was flying a little higher than normal to give myself a little extra cushion to recover from maneuvers.
Even with AS3X taking the edge off of the wind's influence, there are still elements that must be considered on windy days. You must factor whether the model has enough speed to make significant progress in a headwind. You don't want to take off only to find that you're at full power and moving backwards! All of the stabilization systems in the world won't help you in that situation.
Designing an aircraft (of any size) is all about tradeoffs. AS3X puts a toe on the scale to shift the normal balance point of some key design tradeoffs and broaden the scope of what is practical for RC flight. With the Pitts, AS3X helped to make an ultra-micro model fun to fly in a wide range of wind conditions. It also served to transform a challenging aircraft design into airplane that is both predictable and aerobatic. These are precisely the types of benefits that enhance the enjoyment of RC flying and prime examples of why artificial stabilization systems continue to get more popular with modelers.