There are many flybarless controllers on the market now, and most have evolved to the point where they all fly well for most pilots. In addition to flight performance, other differentiating qualities that separate the various flybarless controllers are the ease of setup and the features that are available. The SK-720, from Skookum Robotics, is a second generation flybarless controller and is one of the most feature packed controllers that Ive seen, while still being relatively easy to program. Lets take a look at what it has to offer.
Physically, the SK-720 is a tiny black box that measures 38mm long x 30mm wide x 14mm thick and weighs 17 grams, which is about the size of a small 7- or 8-channel receiver. There are connectors on all four sides of the unit. On one of the short sides, there are the standard pins for the servo and battery connections, while the opposite side has a USB connection and a port for the external field terminal. On one long side, youll find two ports for up to two Spektrum satellite receivers or a Futaba SBUS receiver, while the opposite long side has a port for a future GPS module. On the units top surface is a two-color LED that indicates the SK-720s status.
The description of all the ports probably gave you a clue as to some of the SK-720s features. For Spektrum DSM2 and DSMX users, the SK-720 can be hooked up directly to two Spektrum satellite receivers, eliminating the need for a base receiver. The advantage of this is to eliminate much of the wiring (and potential failure points) that would traditionally go between the main receiver and the SK-720. All the servo, speed control and battery connections go directly to the SK-720, which makes wiring a lot neater, and is especially useful in tight installations. When operated in this manner, up to nine channels are supported.
For Futaba S.Bus users, the functionality is nearly identical. An optional cable ($11) is needed, which connects a Futaba receiver directly to the SK-720 with a single harness. Again, all the servo, speed control, and battery connections go directly to the SK-720. In addition, the SK-720 can also interface with Futaba PPM, Graupner, JR and other traditional receivers.
LOG DATA FROM YOUR FLIGHTS
One of the neatest things about the SK-720 is its data logging and playback capabilities. In addition to recording the attitude of the helicopter using the 3-axis gyros, the SK-720 can determine the flight path of the heli using the built in 3-axis accelerometer. It also records the stick positions from the transmitter, the supply voltage and even the level of vibration in the helicopter. If an external RPM sensor is hooked up, the SK-720 will record RPM as well. All the data is stored in the 2GB of onboard memory, which is provided by a microSD chip.
After a flight, this data log can be downloaded to a computer and the flight replayed, with a virtual transmitter emulating your past stick movements and virtual helicopter flying around the screen emulating your actual flight. Superimposed on the screen are the other parameters that were captured by the SK-720, such as battery voltage and vibration level. These values constantly change, and give a good indication of the helicopters health. If you happen to have a marginal flight battery, the battery voltage will dip during hard maneuvers. Vibration level is shown on a scale of 0 to 10, with 0 being the best and 10 being the worst. If you see that vibration is approaching 10, it might be time to inspect your helicopter for something that is out of balance, or mount the SK-720 in a different manner to isolate it from the vibrations.
The optional GPS module supplements the 3-axis accelerometer, providing even more accurate flight path information during playback. The GPS module will be available as a future upgrade.
GETTING INTO TROUBLE? FLIP A SWITCH!
The SK-720 also supports self leveling. With the flip of a switch, the helicopter will automatically get back upright when configured in this manner. This is a neat feature for aspiring 3D pilots who happen to get totally out of whack when attempting a new stunt. However, the helicopter will still drift and it is up to the pilot to manage the collective to prevent the heli from rising or falling. Reading between the lines, it seems as though the optional GPS module will allow the SK-720 to hold position in a fixed spot in the sky. Time will tell on this one!
THE POWER BUS
The SK-720s power bus is rated up to 8.5 volts and 10 amps, which is suitable for all but the largest of helicopters. For more power-intensive installations, the SK-720 can be purchased with an external power bus that is rated for up to 20 amps. It includes heavy-gauge battery wiring, which minimizes any voltage drop during times of high current draw. It also minimizes the number of connections to the SK-720 unit, which helps isolate the main unit from vibration. There are two separate power buses internally on the SK-720, which allow the swash servos to operate at a different voltage than the throttle or tail rotor servos.
SETTING UP THE SK-720
Setting up the SK-720 is relatively easy. The first thing to do is connect it to a Microsoft Windows based computer using the supplied USB cable. Instantly, Windows will recognize the SK-720, and will treat it as an external mass storage device, similar to attaching an external USB flash drive. The manual and the computer software are preloaded onto the SK-720s internal memory. Double clicking on the SK-720 software installs it on the computer. Once completed, youll be ready to start configuring the unit.
When the SK-720 software is run for the first time, it asks you if you want to start configuring the SK-720 using the installation wizards. If you answer yes, the software will take you through three separate wizards, which step-by-step, tell you how to configure the SK-720 for your particular helicopter and installation. The first wizard instructs you on setting up your transmitter in preparation for the SK-720 and configures the unit for your particular servos in the helicopter. It will then instruct you to install the unit in your helicopter as described in the manual. After this is done, it takes you through two more wizards; one to configure the main rotor servos and main rotor linkages and the other to set up the tail rotor. If youll follow all of these wizards to the letter, youll have no problem setting up the SK-720.
Like a regular tail gyro, the SK-720s tail-rotor gain can be adjusted from your transmitter via an auxiliary channel. In addition, the gain for the main rotor gyros can also be adjusted via a secondary channel. I find this a nice advantage over other flybarless controllers, as many only have remote gain adjustments for the tail rotor only. Having remote gain control for tail and cyclic gyros makes fine tuning the SK-720 so much easier.
Once at the flying field, the SK-720s settings can be tweaked with its software by re-attaching it via USB to a laptop computer. If you do not wish to lug a computer to the field, Skookum Robotics has an external field programmer that allows the pilot to adjust any of the settings that are available on a PC, without a PC. It has a large LCD screen and allows you to navigate through all the settings via the four directional buttons and a center enter button.
I chose to install the SK-720 on my Gaui X5 helicopter, which has unofficially become my flybarless controller test bed. The SK-720 can be mounted in almost any orientation, as long as the label is facing up or down. To keep all the wiring hidden between the frames, I mounted it atop the base plate of the frames rather than on top the front tail boom mount. My mounting location is relatively tight, so I chose to attach the SK-720 to the frames using Velcro rather than the included foam tape. This was done because I need to partially remove the SK-720 from the helicopter to gain access to the USB ports. It would have been nice if the unit was capable of being mounted on its side, as attaching directly to the side frames would have made the installation a little easier and provide better access to all the ports. Not a big deal, but perhaps an idea for a future enhancement.
Ive been a longtime Spektrum user, so I configured the SK-720 to work with two Spektrum satellite receivers. I was able to remove the 6-channel receiver that I originally had in the X5, saving weight and eliminating a bunch of extra wires. Everything tucked neatly between the side frames for a very clean installation.
There are three, preset setups that you can choose from during the setup process: basic, sport and 3D. For my setup, I chose 3D. The wizards also instruct you to set the starting points for the gain to 50 percent for the tail and cyclic.
FLYING WITH THE SK-720
My first test hops were very encouraging” with the SK-720 holding a stable hover right from the initial setup. On my Gaui X5, I was able to increase the gain to 60 percent, which improved the tail-holding power, but was still low enough to not induce any unwanted tail oscillations. During fast-forward flight and tic-tocs, I was getting some cyclic bobbing, but was able to eliminate it completely by dropping the cyclic gain to 40 percent. With 60 percent gain on the tail and 40 percent gain on the cyclic, the X5 flew great, holding a straight line through the fastest of forward flight, and tracking perfectly through pretty much any maneuver that I threw at it. Tail rotor control is superb, with the tail locked like a vise during ultra-fast backwards flight and holding a steady rotation rate during moderate speed flight.
Even though I chose the 3D configuration during setup, the cyclic rotations rates were still a little on the slow side for my tastes. This gave me the opportunity to try out the field programmer. With the helicopter still on (and throttle-hold engaged to keep the motor from engaging the blade), I plugged in the field programmer. The field programmer gets its power from the SK-720, so it comes to life the instant its plugged in. Once powered up, it showed a list of all the settings that can be changed. I scrolled down to the second cyclic menu, and changed the rotation rate from 240 degrees/sec to 360 degrees/sec. After unplugging the field programmer, I took it up for another spin. At this point, the cyclic felt way too fast, so back to the programmer, and I dialed in 300 degrees/sec. This setting turned out to be perfect for me. This was all done in a matter of minutes, and I didnt have to lug around a computer or wait for it to boot up.
I did the same exercise with the tail rotor-rotation rate, upping it slightly to match my tastes. The field programmer was great, being really intuitive and easy to use. The hardest part of using it was mating and disconnecting the small square 4-pin connector. Its a little tricky to figure out which way it goes and it takes a bit of force to try to pull it out when youre done. Im still waiting for the flybarless controller that I can configure via an app on my iPhone using Bluetooth or Wi- Fi connectivity. Are any flybarless manufacturers listening?
REVIEWING THE DATA
When I got home, I hooked the SK-720 back up to my computer. Every time the SK-720 is powered via its servo bus, it starts a new log file. I flew three flights during the previous session, so I had three files onboard. Using the Skookum SK-720 software, I was able to replay each flight and watch my stick movements along with what the helicopter was doing. Vibration level was around one during hovering and never got above four during stunts, meaning that the amount of vibration reaching the SK-720 was at an acceptable amount. At no time during the flight did my receiver voltage drop below 5.5 volts, indicating that my BEC and wiring were up to the task.
THE FINAL WORD
Out of all the flybarless systems currently available, the SK-720 is one of the best. In addition to all the cool features such as advance receiver connectivity and data recording, the flight performance is exceptional. If you are looking to remove the flybar from your helicopter, the SK-720 from Skookum Robotics is definitely worth a look!
Gaui Helicopters (Empire Hobby)
www.empirerc.com, (480) 982-0909
Skookum Robotics, Ltd.