HK-500CMT Flybarless

     I've been really enjoying throwing my HobbyKing HK-500CMT heli around the sky, doing 3D, inverted and fast-forward flight. Definitely a step up from a 450-size heli. The CMT kit has a metal head and tail, and a carbon-fiber frame. I have the tail-belt-drive version. I've been impressed with how it flies and how well it's been holding up. It can be built for an all-up cost of around $400.

     After flying the heli for about a year I upgraded it to flybarless, using the RJX Head and Tarot ZYX-S System. I've been very happy with the result, as it has made the heli both more stable and more agile. Inverted hovering needs much less stick input, while faster rolls and flips can be performed.

See here for a gallery of pics.
See here for an older gallery.
See below for a parts list and video.

     There are some things that need to be tweaked during the build of the heli. With no modification, the tail belt was very tight. I got mine looser by cutting a bit off the end of the tail tube with a hack saw (about 5mm). The tail belt for the Align T-Rex 500 is a bit longer than the HK-500 belt, but it was still too tight without shortening the tail tube.

     The mesh of the tail-drive gear was too loose; there was more gap there than I'd be comfortable with. Where the tail assembly mounts to the frame, I used a dremel to widen-out the screw holes a bit, and also the openings where the plastic "ears" notch in. After that, I could adjust the mesh.

     The stock metal tail-control rod was too long. I made the mistake of cutting it shorter and gluing the links on with CA, which doesn't bond well with metal. It came apart in flight, resulting in a crash. I changed to a 2mm carbon-fiber control rod, 380mm long, with links glued onto the ends, and this has worked well. Widening the holes of the pushrod guides that mount along the tail can help the control rod run more smoothly.

     The tail assembly fit was loose on the tail boom. Pushing the end of a zip tie into where the tail assembly mounts and tightening the bolts worked to snug it up. I found the tail fin too short, allowing the tail rotor to scrape the ground. I replaced it with the tail fin from a T-Rex 550. (An extra hole had to be drilled in the fin.) Another approach would be to glue a bit of carbon-fiber rod extending below the tail fin.

     Before I changed it to the flybarless setup, my attempts to use a cheap gyro and tail servo did not work out. With both the GA-250 and HK-401B gyros, there was tail wagging that would not calm down unless the gyro gain was lowered so far that the tail-hold got too loose. I switched to using a Logitech LTG-2100T gyro and an INO-LAB HG-D261HB servo, and this combination flew well. The INO-LAB servo has a smaller footprint than what can fit in the stock bracket, but mounting it with the adapter blocks from the kit sets it firmly in place. I used KBDD 70mm tail blades. (With my flybarless setup I'm still using the INO-LAB tail servo and it was been working well.)

     For cyclic servos I'm using MG930s, with the longest control arms that come with the servo. Using the outermost hole on the arms, the distance is about 16.5mm.

     During the initial build I reinstalled most of the screws in the kit, applying blue thread lock (especially in the head and tail).

     The stock canopy was somewhat squashed in the kit's packaging, and never fit quite right. I got a new fiberglass canopy BYRB-5002, which fits well. A "slot" piece on the frame is supposed to hold the bottom of the canopy, but I never got it to work well and used velcro instead.

     My setup runs on 5S 3000mAh 30C batteries. I'm using a Typhoon 500H 1800kv motor with a 13-tooth pinion, Turnigy 85A ESC (Tz85A), Turnigy HV UBEC (TR-UBEC7.5) set to 6 volts, and Turnigy 425mm CF blades (CF425B). This setup provides a head speed of around 2600 RPM. I always use a voltage monitor on helis; for this one the HKing-5S.

     Both the UBEC and ESC come with on/off switches. While I don't find the UBEC switch useful, the one on the ESC works well as a motor enable/disable switch. I can keep the motor disabled until just before takeoff, and disable it again right after landing.

     The Tarot ZYX-S system can be setup with transmitter-stick inputs, or using a PC software utility -- I very much prefer the software. The USB program adapter connects between the PC and the unit. A USB extension cable (like this one) can make it easier to work with, especially after the unit is mounted in the heli.

   The utility program from Tarot would only run on Windows XP. However, rcgroups member John_NZ has created an enhanced version of the program (FBL Gyro v1.22) that runs on Windows 7 and adds features; available at http://fbl.net.nz. The manual for the program is here. The manual for the Tarot ZYX is here.

     To setup the ZYX-S unit, it needs to be powered via one of its servo connectors and it needs to be receiving signals from a receiver. A standard receiver can be connected using the male-to-male servo connectors, or a satellite receiver can be used instead. Here is a connection diagram for the standard receiver:

To connect a satellite receiver, the ZYX-S DSM2/DSMJ Satellite Receiver Cable is used, with one end plugged into the satellite and the other plugged into the rudder ("RUD") connector on the ZYX-S. (The cable provides the right connectors and 3.3V power to the satellite.) Here is a connection diagram:

     On my heli I'm using an OrangeRx R100 Satellite and have had no problems. My transmitter is a Futaba 9C with a Spektrum DM8 module. To bind the satellite to the transmitter I (temporarily) connected it to an OrangeRx R610 receiver, but it's also supposed to be possible to do a bind by connecting the satellite to the ZYX-S and plugging a male-to-male cable into the top two connectors, as shown in this picture. The speed controller plugs into the "AIL" connector, as shown in the this picture. In my setup I also needed to include the wire from the separate UBEC, so I used a 'Y' cable.

     On the PC the 'COM' channel corresponding to the USB program adapter needs to be selected in the combo box. The first thing I did was update the firmware in the ZYZ-S unit. See here for version 3.50 of the firmware (it needs to be unzipped). To do the update the ZYX-S unit needs to be powered up (via one of its servo connectors) while holding its 'Set' button. In the FBL Gyro software, click on 'Open' and navigate to and select the "ZYX-S-350.bin" file from the downloaded '.zip' firmware file. Click on 'Upgrade' to perform the upgrade.

     In the FBL Gyro software, click on the 'Connect' button to initialize the connection to the ZYX-S unit. If it's successful the three indicator circles will change from red to green. The current settings will be read in from the ZYX-S unit and shown on the display. The settings can also saved to and read from a configuration file via the 'File' menu.

     Here are the channel locations for the collective servos:

The tail servo is connected to CH4.

In the FBL Gyro software, in the Setup under Installation, I found the Receiver Type setting of 'DSMJ' was what got it to work with the OrangeRx R100 satellite. Here are my other Setup selections:

Servo Type:  Tail servo is 1520us, swash plate servos are 1520us digital
Swash Type:  120 degree CCPM
Servo Rev:  CH1(Normal), CH2(Reverse), CH3(Normal)
Swash Travel:  Aileron Travel = 80, Elevator Travel = 80
Collective Pitch:  70
Tail Setup:  Servo Trim = 0, Tail Limit A = 100, Tail Limit B = 125
Gyro Dir:  Pitch = Reverse, Roll = Normal, Yaw = Normal
Pyro Opt:  Optimization Direction = Reverse

Here are the settings I'm using on the main 'Configuration' screen:

Main Rotor | Roll Agility = 50, Roll Gain = 32, Pitch Agility = 50, Pitch Gain = 30
Tail Rotor | Yaw Rate L = 50, Yaw Rate R = 50, Gain = 100

Here is a copy of my saved FBL Gyro configuration file: HK500FBL.ini. My settings result in a very agile heli that may feel too "touchy" for some. Selecting the "F3C" or "3D Soft" presets will give a softer feel.

Below are the settings I have in my Futaba 9C transmitter:

AIL/ELE/THR reversed
Aileron/Elevator expo:  35%
Rudder expo:  50%
Throttle Curve Normal:  0,55,75,92.5,100
Throttle Curve IdleUp:  100,90,85,90,100
Pitch Curve Normal:  35,43,52.5,70,90.5
Pitch Curve IdleUp:  0,30,50,70,100
Gyro Rate:  40%

     When installing the RJX Flybarless Head I used the upper-most hole on the center hub, as shown in this pic. The linkage rods going from the swashplate to the blade grips are 54mm center-to-center. Here is the manual for the RJX Head. For the initial alignment I put the blade-grip bolts into their holes and eyeballed them as being vertical when the swashplate was level.

     For the tail rotor, I set its pitch to be about +8 degrees (in the direction that compensates main rotor torque) when the tail servo is centered. This is recommended in the ZYX-S manual and elsewhere -- it helps to improve the responsiveness of the tail control.

     Here's a tuning and troubleshooting thread: http://www.helifreak.com/showthread.php?t=485789

Parts List

Here's a video of my flying the heli (from before I upgraded it to flybarless):

Click here to contact me

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