RE: servo and throttle configuration
SF:
Using rods and two bellcranks two engines can be throttled with one servo. The linkage is set to have the throttles fully shut and fully open at the same time, then working with the angles good engine sync can be reached. It's a lot of bother to get it right, and after a period of time it all has to be done again as the engines wear, never equally.
For these (and other) reasons most builders use one throttle servo for each engine. With a single throttle channel on the radio the mechanical linkage has to be set precisely for sync, just as when using a single servo for both.
Next step up, and some consider it the best of all, is using a separate radio channel for each engine. This way, if your radio will do it, you can use the throttle curves in the transmitter to adjust the sync of the engines.
There are two electronic devices on the market to keep the engines in sync.
EMS/Jomar has their "Engine Sync Device" that measures the rpm of one engine, always the same engine, and matches the other to it. It is what is called a "Master/Slave" device. Last time I looked this cost $80, and you had to supply some minor parts to go with it. It requires separate throttle servos.
The ultimate electronic sync device is the "TwinSync," now available from RCS for $130. It also requires separate throttle servos, but it's complete, no additions to buy. For the added $50 you get a unit that is not master/slave, it reads both engine's rpm and changes the throttle of both to reach sync. It also has many other built-in facilities including a programmable glow driver, the ability to run the engines up individually for mixture checking, even the capability of tieing the throttles to the rudder for more crisp ground steering or extremely sharp yaw turns in flight. The device is fully programmable down to the point of varying the response time.
As a final note, synchronization of the engines on a twin is not really important, a few hundred rpm difference wont affect the flight of 99% of the twins we fly. But the sound when they are running in sync can make tghe enjoyment much greater. And when a cheap twin is going to represent $1200-1500 what's another $130?
Bill.