CSM Flybarless Conversion Guide (T-Rex 600)

The following is not a review but more of a technical overview detailing how to turn an ordinary T-Rex 600 into a flybarless T-Rex 600. The process involves the installation of several pieces of electrical equipment as well as mechanical modification of the rotor head to support flybarless operation.

You will need:

Plastic upper mixer arms
Original washout base and arms
1.5mm drill
2mm tap
2mm set screws * 2
CSM Cyclock
CSM 720 gyros * 2
T-Rex 600 un-shouldered balls * 2

First lets deal with the mechanical changes to support flybarless operation. In order to do this modification I will use all parts that come in the T-Rex 600 kit. We won't need to manufacture any parts just modify existing.

First job is to remove the flybar cage, flybar seesaw and the flybar paddles themselves. These can be stored in your spares box if you decide later to convert back again. Flybarless does fly a bit differently and it may not be to everyone's taste. The areas that I find most in need of further development being retreating blade stall and phasing changes at low headspeeds (usually at the end of an autorotation).

First we will modify the upper mixer arms which will need to be uninstalled from the flybar seesaw you just removed.

Essentialy what we have done is drilled a hole 24mm from the pivot and installed an extra ball. So it should be 24mm from pivot point to hole center as shown in the diagram. This allows us to add a little mixing into our blade grips so that the cyclic and collective are not over sensitive.

To mount the mixer arm to the head block we must use a bolt rather than a self tapping screw. It bolts directly into the screw holes that would normally secure the flybar seesaw.

Next we modify the washout base. This is a little more extensive than the mixer arms. We need to drill a hole in the washout base, tap it and install a set screw. Ideally install two set screws as we don't want the washout coming loose on the main shaft once secured in place.

Now we get to the washout arms and this is a little more complex again. The down link on the end of the washout arms that connects to the swashplate ball needs to be moved to the other end of the washout arm. Obviously this needs to be done for both washout arms. The reason for this is that you need a nice long arm to account for a much larger range of movement in the swashplate. Once the down link has been swapped to the long end of both arms the short end can be removed. The new washout can then be installed and secured in place as shown in the pictures. It basically goes as high as possible on the main shaft. Take note that the washout base is fitted upside down on the shaft, so the larger piece of brass bushing is towards the rotor head. Tighten and loctite both set screws against the mainshaft to hold it in place.

The last thing we do is flip the main grips over so they are upside down. Take off the main blades and reinstall as they will now be upside down and facing backwards. If you have a head button you will need to remove it to flip over the main blade grips.

Now you can connect all the links up and set it up so that mid-stick gives 0 degrees pitch and the washout arms and upper mixer arms are horizontal.

That's the mechanics all done. Pictures below.

Gyro & Cyclock Installation

Installing the gyros and cyclock is the next step. This is reasonably straight forward but if in doubt some guidance can be found on the CSM website the most useful being the following wiring diagram.

Follow this wiring diagram to the letter. The next job is to mount the cyclock and gyros on the machine. Below are pictures of my setup where I found the gryos to work best. The tail gyro and aileron servo being mounted together above the tail boom clamp and the elevator gyro on the side of the machine. I do have an RC-Motion custom frame so this setup may not be possible on a standard Align carbon frame. The key element is making sure that the gyros have a nice firm and vibration free mount point. Also they must be mounted directly in line with the axis of rotation you wish to manage with the gyro.

Transmitter Settings

All settings taken from a Spektrum DX7.

Travel Adjust

Elevator, Aileron and pitch travel adjusts set to 140% each way.

Rudder travel adjust 100%

AUX2 (aileron and elevator gyro gain) travel adjust set to 70%

Gear (tail gyro gain) travel adjust set to 112%

Exp & D/R

Aileron & Elevator D/R set to 70%


All set to 0

Swashplate mode

Set to 'Normal' as the cyclock takes care of CCPM mixing.

Quick parameters description

Dead Band

This changes the sensitivity of the sticks around center. A low value means that tiny stick inputs will cause servos to move. A high value means small stick movements will have little effect on servo movement. In other words it controls how far you can move the stick from center before the servos start to move.

Linear Stick Sensitivity

This value is very sensitive on flybarless and only small increments of 1 or 2 should be made between tests. This setting changes how much influence the gyros maintain when you provide stick inputs. So if you set this value too high the gyros won't provide much help and the model will feel very sensitive, too low and the model will feel very stable but disconnected as the gyros have too much influence.

Exp Stick Sensitivity

This is exactly like exponential on a transmitter. High values give you less sensitivity around center, low values make the stick linear with no exponential curve applied to the stick movements.

Pirouette start and stop rate

The start rate and stop rate control how quickly the gyros will accelerate or decelerate their rotation of axis when stick input is given or taken away. On flybarless the rotorhead is much slower than a tail to start moving, so a very high start rate is required as otherwise the stick inputs feel like they are lagging. Stop rate requires similar values to a tail system, make this too high and it can induce nod or bounce effects, set it too low and the model will be overly soft in how it stops, making things like four point rolls difficult to do.

Acceleration Gain

Not required for flybarless setup, set to zero.

Conventional Gain & Heading Lock Gain

These two parameters work together. So changing one normally means you have to change the other as well. The basics are that conventional gain acts like a damper, similar to how an ordinary rate mode gyro works. It tries to dampen unwanted heading changes. If you set this to zero you will get oscillation as the gyro hunts for it's heading lock. Some level of conventional gain is required to stop this. Heading lock gain controls how hard the gyro fights to maintain it's heading. This is best thought of like a spring, the higher the gain the harder the spring. Set this too high and high frequency oscillation can occur, too low and it can induce a slow wag. As mentioned previously you change the parameters in tandem as if you increase one you may have to decrease or increase the other to compensate.

Direct Coupling

This value determines how stick movements relate to a corresponding servo movement. This parameter allows your stick commands to override the conventional and heading lock gains and yaw rate control. High values give larger degrees of servo movement per stick position, low values give little movement and allow the gyros to control servo movement based on how fast the yaw rate has been set and how much the servo needs to move to achieve that yaw rate. So this parameter is basically an override, it determines how much the gyros will allow you to take direct control where a stick movement corresponds directly to a servo movement.

Heading Lock Range

This limits the range of headings over which the Heading Lock gain applies. It can, if desired, be used in conjunction with the Heading Lock Gain to limit the maximum power of the Heading Lock. Important: Set this parameter to zero for a 'standard' (non Heading Lock) mode. Otherwise in our flybarless application it should be set to maximum of 45.

HL Decay

HL decay time. This should be set to the minimum for flybarless, higher values are not useful. If anything this value needs updating to make it possible to set fractions of a second rather than 1 second as a minimum for flybarless operation.

This parameter can be felt most during and at the end of an auto when the headspeed decays, here you tend to get over sensitivity to stick inputs due to the heading lock not decaying fast enough with the slower headspeed. Therefore on CSM flybarless it is not sensible to drag out the hang time at the end of an auto as the model will become progressively more sensitive and difficult to control as the head speed decreases.

CSM 720 Tail Servo Settings

CSM Cyclock Settings

CSM Cyclic Settings

In the following screenshots I am using the settings in the yellow boxes (mode 0). The setting in the red boxes (mode 1) were some previous settings that were ok but discarded in favour of the mode 0 settings. These new settings are extremely close to how a T-Rex 600 with flybar flies. They give a much faster response than previous published settings with little or no lag in the controls.



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