I learned a lesson yesterday(2apr08)that I did not expect.When you use Heli-Govenor-High on a twin,you must check your ESCs to make sure they are"matched".
     I have flown a twin for two years and since I did not use Gov. mode matched
ESCs never were a problem. However,my new twin will use Gov. mode thanks to the
posts of Kim,Dean,Mike and others.
     My twin mock-up was bench run yesterday and a real problem was evident. My set-up is one Zigras timer,two CC-35 ESCs, one 14.8v 4270mah Battery,two E-flite
480-910kv motors,and two APCe 9x6 props. I set my timer for 9660 rpm and ran the
set-up. The outbd motor ran a steady 9660rpm. Then I checked the
inbd motor and it was running 9810rpm. Total amps were 37amps. My timer is connected to both ESCs with a Y harness. Both ESCs had been set the same using
Castle-Link software. I removed them and rechecked that they were the same. Then I switched the ESCs with all else the same and ran the set-up again. As I
expected the outbd motor now ran 9810rpm and the inbd motor ran 9660.
    I asked Mike Palko for some ideas by E mail. Today I took four CC-35 ESCs and labled them A,B,C&D Then  ran the set up and noted the RPM of each ESC.
The results were A=9810, B=9660 C=9810 D= 9720. Based on this I selected A&C and ran my set-up again with the timer reset to 9660 rpm. This worked !!! Both motors
ran at 9660rpm. I really don't understand why this happened but,I plan to match up my ESCs from now on.
   Walt Brownell
 
   
     

   Dennis
 Yes, that is another way to solve the problem,but it adds another layer of
complexity. My approach is to follow the KISS principal. There is enough work
to set up for changes in wind and lap times, without making it necessary to
change two timers. It is a cost effective way considering ESC cost to timer cost.

  Walt
 

Alan

The ESCs have been purchased over a period of 3 years. Some were bought together,
others were bought singly. I think that there may have been updates by CC that caused differences in these units. They sure have changed the Castle Link software
enough times in the last 18 months. Anyway, the units are reliable and work fine.
No one said they should be matched. Not many modelers will be impacted by this
problem. I was just giving a "heads-up" to anyone planning a twin.

Walt     

Dennis,

   We certainly think alike. I had the same questions that you posed. I ran tests to
find out. I tried different throttle settings several times. No problem. I ran contra-
rotating props and both tractor props. No problem. My understanding is that prop differences should not impact RPM. That is why you can set RPM with a small prop
and then put on a larger prop with no change in RPM. I can't say that I know any one
that has tried one ESC and two motors, but I would like to hear from any one who has
tried this.

  Walt

Followed Alan's lead and looked up the subject:

* Yes, some people have run two motors off one ESC successfully.  At least one company sells a dual CD motor system designed to plug into a single 10A controller.

* Some others have not been successful and/or have burned up motors and ESC's. 

* A rep from Castle specifically chimed in to say DON'T DO IT.

I presume it is not possible to make all the Catles becve the same by indvidual changes in their programming?

I'm an AE not a EE but it sounds like we need some kind of magic feedback loop that takes the rpm of motor 1 and adjusts the throttle signal to motor 2 so the rpms match?

Then how do we adjust a slight mismatch in RPM for the inboard vs the outboard motor?

Dennis,

     I hope that when we discuss ESCs we don't get caught up in brushed motors.
I have a tri-motor CL scale model with speed control and only one ESC. That works very well with only one battery and 3 brushed motors.

Walt

Hi All,
Dennis, there is some outfit that sells a "twin synchronizer", but I don't remember who, just now. I'll go look.
More importantly, who says synchronized or matched is important? Yes, the differences between two otherwise identical ESCs, with the same revision software, is almost certainly the difference between the processor crystals ... so a tiny pitch difference between the two sides will fix it. 

Last I chatted with Paul Walker, he said that the B-17 needed the outer engines to be set just a touch leaner and a few RPM faster to fly cleanly. 'Kinda like tire stagger on a circle track car! Let's see, a foot and a half between the motors on 65 feet of line is 2.3%, and on a 6" pitch amounts to 3/16" of pitch difference, just to balance the load side to side.

later,
Dean