All the information I could find online was aimed at fixed wing, where it is quite easy to "fine tune" things using different prop diameters and pitches. Not so easy for helicopters!
Perhaps if I go through the procedure I used for the Lark, it will help.
As a starting point, I look at the specifications of the engine for which the heli was designed. For the Lark, which was designed to fly on a 20/25 ci motor, I used the power figure for an HB25, which supposedly produced just shy of 0.6 HP at around 15,000RPM (higher than I expected!) - equivalent to around 440 watts.
http://sceptreflight.com/Model%20Engine ... 025-H.html
Initially I was hoping to fly it on 3S, say 12V in round numbers.
So, we are looking for something that is rated at around 440 watts, with a kV rating of 15000/12 = 1250kV.
The motor I went for was an Overlander 3536/06. This is rated at 1300kV and 442 watts.
The model did fly on this, but was a bit marginal, so I upped the cells to 4S, which now provides sprightly performance! Most of the time, it is flying on around half throttle in the circuit, or just over half in the hover.
I should add that I couldn't get a primary pulley of 8T to fit the electric motor, so I had to go for the nearest I could get, 10T. This would have affected my calculations slightly.
Its also worth noting that these old British helis (Morley, MicroMold, etc) tend to use Imperial belts and pulleys (XL) rather than metric. The difference in the pitch of the teeth is not really visible (5.06mm Vs 5mm from memory), but will result in high wear rates if you get it wrong!
I used the same calculations for my Schluters, intended to fly on Blackhead Webra 61s (about 1.2 HP, or 900 watts). I came up with 500kV on 6S packs. Again, it flew on this, but was slightly underpowered. I ended up using 580kV.
As a result, my advice would be to use a slightly higher kV rating than you calculate!
As an aside, Phil_G, of this parish, came up with a simple formula for conversions as being 100 watts per cc of engine capacity! This agrees very closely with my figures, and makes for a very simple calculation! Moreover it works for helicopters just as well as it does for fixed wing! Cheers, Phil!
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Pete