A machine of that size will have to be 12s for sure. I recently built a large Robbe Alouette and converted it to electric (see here http://www.runryder.com/t534439p1/) and I got asked by someone the other week how I chose a motor/battery set up for it. The response was a bit lengthy.....sorry, but in case it is of use to you, here it is;
ELECTRIC MOTOR CHOICE
KV as we use it refers to the rpm constant of a motor - it is the number of revolutions per minute that the motor will turn when 1V (one volt) is applied with no load attached to the motor. In summary, we call it revs per minute per volt, but do not think you will obtain those revs when you attach a motor to your heli drive train, as obviously the revs will be reduced because of the load unless you use an ESC in governor mode and you have enough reserve power for it to compensate with more throttle (more on this later).
So what does the KV tell us? Well it is related to the power out from a motor, or more usefully the torque level of a motor. It is determined by the number of winds on the armature (or turns as we sometimes call it) and the strength of the magnets – so there are many possible variables. KV allows us to get a idea on the torque we can expect from a particular motor.
In summary, a low KV motor has more winds of thinner wire - it will carry more volts, but at less amps, produce higher torque and drive a higher gear ratio.
That may sound confusing, but compare it with a high KV motor which has less winds of thicker wire which will carry more amps at less volts and spin a lower gear ratio at high revs.
Let me give you an example:
A 6 turn is called a hot motor - it has only 6 winds of thick wire and is made to carry big amps at low volts.
For example, a 3025/6 motor has a KV rating of 1255, meaning that the motor will spin at 1,255 rpm per volt with no load (not recommended for brushless motors - never run them without a load - it is a theoretical value only). This motor is often used in medium sized fixed wing and will spin a 12 x 6 prop at 7,900 rpm on 7 cells and draw 48A - giving 350W. You might want to push the amps with this motor and fit an even bigger prop to give you more Watts for competition use because it will take more amps - it is able to take 65A for up to 60 seconds or 100A for up to 10 seconds, the manufacture specs sheet show this.
Compare this with a 3025/8 motor - that is the same size motor physically (armature 30mm diameter and magnets 25mm long), but it is wound with 8 turns of thinner wire. It has a KV rating of 985 - much lower than the hotter 3025/6. It will spin a 12 x 6 prop at 6,900 rpm on 7 cells and draw only 27A producing 249W. This motor has more torque and you might want to increase the voltage to take advantage of that characteristic, but watch the amps because this motor has an amps limit of 52A for 60 seconds as per the spec sheet.
These motors are quite close in specs, but you can see the difference. The point is that the higher KV motor would be used to give high power for short bursts in competition flying, but the lower KV motor has more torque and is better suited to a sports application or continuous power. So - you can see that the KV figure is useful in helping you choose a motor for a particular application/head speed target. It is particularly useful when considering two motors of exactly the same size but different windings.
When considering motors of different sizes, it is just another figure in the electric power jig-saw that allows you get a handle on a motor and its uses. You might come across a larger motor such as a 4035/10 and you will find the KV rating at 405. Now I know that seems like a low KV, but check the useful current range for that motor when considering it. Compare it with a 4035/14 and you will find that the 14 wind motor only has a KV of 299 - so you can see that bigger motors have a lower KV rating than smaller motors and operate at higher volt ranges for a different purpose. A 4035 motor will use 6S to 8S (higher voltage Lipo batteries) and comfortably fly a .60 size model weighing up to 8.0kg.
In essence, the KV figure allows you to compare similar motors and understand what gearing would be required for your application. You may have a limited gearing range available so therefore have to pick the motor to suit the gearing rather than the other way round.
If you take a high-KV motor and gear it down, you effectively make it in to a lower KV motor! For example, take a high 5000 KV motor, but gear it at 10:1. It is a 5000 KV motor when considered without the gearbox, but it is a 500 KV in effect when a 10:1 gear ratio is added to it. The motor is still doing 5000 rpm when one volt is applied to it, but the output shaft of the gearbox is doing 500 rpm. You can still see that this is a low voltage/ high current motor because if you apply 12 volts to the motor (circa 3s pack)it will do 60000 rpm without a load (not that we ever do that - we should not run brushless motors without a load) and that is a high revs level. Inner-runner motors like this will run at quite high revs, but Plettenberg for example say that there is a limit to the revs capability of their inner-runners and it is about 70,000 rpm!
Outer-runner motors on the other hand are low rev, high torque motors and are not intended to run at very high revs like some inner-runners are. Outer-runners are generally limited to more like 15,000 revs and this is reflected in the relative KV ratings.
So - there are horses for courses, and the KV rating of a motor is one factor that gives us an indication of the way it is meant to be used. You would not use a low KV motor in a Zagi because you need high revs with a little prop at 1:1 ratio. You would not use a really high KV motor in a large heli because you want to swing a large disc and obtain high torque at lower rpm when compared to something like a Trex 450 where you may run up to a 3500KV motor, a Trex 500 would be circa 1600KV and a Trex 600 would be 1200KV. Compare these to the size of a large scale heli and you will see why the KV is more likely to be around 400 - 600 max.
Take my recent Robbe Aouette conversion for example. My gear ratio is 11.12:1, my motor is 500KV and I am running 10s cells which are circa 40V, so my equation is giving 1438 at 80% esc......500KV x 40V divided by 11.12 gear ratio x 80% esc setting = 1438. Loading is not much of an issue as the esc I use is in gov mode, so the 80% off load dictates the "desired" head speed of 1438 and the esc is free to increase throttle up to 100% where required to maintain that.
He who dies with the most toys is the winner!