How to achieve the correct rotor head mixing and control was from day one always a challenge, my original idea was to use a conventional servo layout i.e one servo for elevator, one for aileron and one for pitch and worry about how I controlled these servos at a later date.  Maybe I could do it using free mixers on the transmitter or by using Vee tail mixers pinched from the airplane guys it didn't matter, let's get the mechanics built first.

For those of you reading that may not be familiar to how a tandem rotor helicopter like the Chinook operates then very simply, with aileron or ROLL the front and rear swashplate tilt sideways in the same direction, with rudder or YAW the front and rear swashplates tilt sideways in opposite directions.  With a single yaw input the helicopter will rotate or pirouette about a center point between the front and rear rotors, by using a combination of roll and yaw it is possible to rotate about the front or rear rotors. Elevator or PITCH is a bit more complicated, a tandem rotor helicopter uses what is known as Differential Collective Pitch or DCP and no manual elevator were the swashplate tilts forward or backwards.  DCP adds or decreases the pitch on the front and rear rotor blades in opposing amounts, to enter forward flight the front rotor pitch is decreased and the rear increased making the helicopter and rotor disks tilt forward resulting in forward flight, to fly backwards the opposite inputs are required.    

After designing and making 2 standard layout servo control systems like the photograph opposite I discovered that there was in fact an electronic tandem rotor mixer available, the TH-2 from  Tech Model Products   in USA.  Unfortunately this mixer uses ccpm so that meant re-designing and making both servo control systems again.  The TH-2 is a proven mixer used by TMP on their small tandem rotor helicopters, it is designed to power small miniature servos but this is not a problem when using large standard or digital servos as they also supply Extender Boards to carry the higher current directly to the servo and not through the mixer.  It is freely programmable through a PC interface and most parameters can be changed.

The Mk2 mechanics flew using the TH-2 mixer and was very good, the only disadvantages were that when any changes needed to be made to the mixing parameters it meant hooking it up to a PC or laptop and it didn't (at that time anyway) support Phase Angle Shift.  I felt it important that when using such large multi-blade rotor heads and expensive blades that the control push rods were as straight as possible and I didn't like the angle of operation without phase angle shift.  

Whilst building the Mk2 version I became aware of Joel Rosenzweig's development at Helitronix in USA, he was working on a tandem rotor mixer based upon the highly successful single rotor version for multi-blade heads.  After some discussion with Joel I agreed to do the Beta testing of his new mixer and after a couple of months bench and flight testing we arrived with a very versatile and flexible unit, more details on this mixer. The Helitronix Multi-Mixer leaves no stone unturned, every conceivable parameter can be configured for each individual rotor head, whether it be single, tandem or coaxial rotor design and it supports phase angle shift.  It is programmed using a hand held device that simply plugs into the mixer and includes a display and push buttons for quick and easy setup out in the field.  

So this Helitronix mixer was the one for me, I tried all sorts of different combinations of values just to see the effect on flying characteristics, different gyros gave better stabilisation than others.  The mixer takes the four basic commands from the receiver, aileron, elevator, rudder and pitch and mixes them in the desired rates and ratios to each rotor head, stabilisation gyros can be connected between the receiver and mixer to help combat outside influences such as wind and provide a much more stable flight.