I ran a low head speed E7 all last season and enjoyed it very much. I had more people fly the helicopter than ever, because everyone wanted to try it. If you add the jumper and run 6s the helicopter will fly but in my experience it will be a little more than 1000 (not much though since you’re running a 12 tooth pinion).
If you are running set RPM of 2000 then that is what the castle gov wants to try to maintain, so it will try to give you the maximum head speed it can on 6s. In the beginning it may be in the very low 1200's but mostly somewhere in the 1100's. This is because idle up two on the castle gov settings was told to run 2000 so it will try to do the best it can even though it is given 6s. When going to low head speed I found I added a little tail gain to my flybarless controller but that was more for tweaking, the tail will still hold. I personally like 1250 head speed, I find I can fly for 8 minutes of 3d but it still has enough pop to do anything I want. 1000 head speed makes everyone take notice because they are sure it will fall out of the sky at any moment, lol. I was running Rail 716 mains and Rail 116 tails. I am currently in the process of building a new low head speed E5 super stretch for this flying season running 696 Rails, 7s 5000mah , Scorpion 4225 550kv and shooting for 1100/1250/1400 head speed.
I personally would suggest setting up the castle governor settings for three modes, for example
1. 1000
2. 1850
3. 2000
I would only use the 1000 setting when running 6s on 12s I think it is rough on the speed controller trying to maintain 1000 head speed in flight.
When I ran my E7 on 12s I was like you and ran 1950 as my normal head speed, give low head speed a try!
Let us know how it goes and how you like it.
http://heli.dacsa.net/calcv3/Default.aspx
