Thanks for the reply
I found my answer here
http://www.hobbyking.com/hobbyking/store/ebasic.aspfor the benefit of others, cut & paste
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MOTOR POWER CHOICE(base on reccomended AUW, or Flying Weight of model choice):
Vintage types and many non-aerobatic indoor flyers - 50w~70w per 1lb
Trainers, gliders and high wing scale - 70w~100w per 1lb
Sport flyer with general aerobatic performance - 100w per 1lb
Warbirds - 120w~150w per 1lb
Multi engined models - 100w per 1lb (thrust from Multiple props gives in effect, more than 100w per 1lb performance)
EDF Jets - 150w~200w per 1lb
3D, F3A and high performance Models - 150w~200w per 1lb
LIPOLY VOLTAGE CHOICE
Based on the above, we now need to work out what voltage we are going to need to use, generally, to keep Lipo's in good order, try and keep max amps to around 50~60% of the capacity/C rating of the Lipoly Pack, for example, if you purchase a 2200mAh 20c pack, then it is rated for 44A constant discharge, so keep the max amps at around 20A~25A IF possible, it isn't always! Choose the capacity of pack based on reccomendation for the model by model manufacturer and in conjunction with the size/weight data published with all our advertised Lipoly packs, for low powered models, choose 20c packs, for general flying choose 20c~25c packs, for high performance models 30c + packs;
Up to 50w: 1s~2s
up to 100w: 2s~3s
100w Up to 500w: 3s (This is the practical upper limit for 3s Lipo's, so basically, models of 5lb AUW)
500w up to 800w: 4s (This is the 0.40~0.46 glow equivalent range favoured by many club flyers)
800w up to 1000w: 5s
900w up to 1500w: 6s (this is the 0.60~0.90 ic equivalent range)
8s~10s packs are for very large and generally specialised models.
MOTOR CHOICE - KV or RPM per volt
Which actually means, what prop size! If you are used to IC, the simple analogy is to treat low kv motors as 4 stroke engine equivalents and mid-high kv motors as 2 stroke engine equivalents, if you are not used to IC then we can give you some examples of the approach to take, this is an important choice as you can literally choose how your model flies, however, their are practical considerations, the most obvious is ground clearance. Please refer to motors such as the NTM range, which give you prop data as well as power, dimension and weight data.
Example 1: Trainer/Sport Model, 1lb AUW, we want 100w motor (3s 20c Lipoly) mid kv for general flying, probably around 1200kv~1400kv, so around 8" prop
Example2: 3D/F3A Model, 1lb AUW, we want 150w motor (3s 20c~30c Lipoly) low kv, 1000kv or under, spinning 10~11" prop, highly efficient at low throttle openings giving lot's of prop wash over control surfaces at all times, high thrust for low rpm and low amps draw at higher throttle openings.
Example 3: Warbird/scale Model, 1lb AUW 120w motor, kv choice, either of the above, it is personal choice
Example 4: High Speed Delta type model, 1lb AUW, 200w motor (3s 25c~30c Lipoly) 2200kv~3200kv motor, 5"~6" Prop, high speed/low torque, low thrust at low throttle openings, high speed from high rpm at full throttle.