Here is something useful from HK.

An under-powered model is a disaster waiting to happen, here is a rough guide to choosing the electric power train needed for various model types, bear in mind that over-powering is fine but the penalty is additional weight, and a good model is one that is balanced in terms of power, flying weight and build quality. This guide is as the title says, a ROUGH guide and offers a basis from which to choose a power train for your model, it is not intended to be a definitive guide but will help to get you into the air with performance that will make your introduction to electric flight enjoyable and reliable.

MOTOR POWER CHOICE(base on recommended 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 recommendation 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 specialized 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.


FINALLY, ESC CHOICE
You have decided on your motor, so look at the MAX AMPS figure given by the motor manufacturer in the data section and generally add 25% headroom, so, if a motor is rated to 15A, then choose at least an 18A ESC, better still a 20A and so on. Next make sure that the ESC voltage is compatible, in other words, if you are using a 4s Lipo, that the ESC is rated for 4s voltage. Next, check if it has functions you desire, if you are flying a glider for instance, you will want a brake facility so that the prop stops when soaring un-powered, allowing the prop to fold by not windmilling, we strongly advise purchasing a programme card to make programming the ESC easier. Also look at BEC rating, the BEC supplies radio receiver power for servo's without the need for a separate receiver battery, however, the can be limited in the number of servo's they are capable of powering, if the servo count is over 4, as it is on many models these days, then consider purchasing an ESC with a high AMP rated SBEC, or a separate UBEC, OPTO type ESC's (they have no BEC, keeping the ESC separate from RX supply) are recommended for large models that require a separate receiver power supply, they are also safer in high powered, large models as they will not arm until the RX is switched on.


Note: This document is a work-in-progress. Check back later as I expand this document.

How to convert ounce or grams of thrust to watts??


Ref: I have one CF2805 BL.

With this motor "The GWS EP 7035 prop produces about 15 ounces of thrust at 13 amps."

Rgds

Vishwas

bump

don't u have a convertor application in ur mobile phone?

just checked in my mobile..
1ounce=28.35gm

There is no simple thumb rule to convert thrust into watts. To find out how many watts of power is being consumed by a particular power system one would need a digital watt meter. It is connected between the battery and ESC  in series to find how much current is being drawn from the battery at different throttle positions. Also one can find how many watts the motor is consuming for different combinations of battery voltages, and propeller sizes. If one has a tachometer then you can also measure the RPM of any particular combo.

To find the thrust being generated by any given power system.i.e, Motor and prop combo, you need a setup consisting of a thrust stand, Wattmeter,Digital Tachometer and an accurate digital weighing scale which can measure in 1 gm increments. There are a lot of posts about these setups on www.rcgroups.com. Please go through those postings and you will get an accurate picture of how to measure the thrust, a power system can develop.

good luck,
Rao.

@xxkrishxx -- The original post mentioned the wattage of the motor. For my motor cf 2805 I need to know the wattage so that I can plan auw.

@Rao --- If I  understood your point, I need to know the current drawn. For my motor the thrust is mentioned at 13A. If we are using 2S, voltage is 7.4. As P=VI, the wattage is 7.4 x 13 =  96.2 W. Am I right?? So can I have 1lb AUW??

Vishwas

Hi Vishwas,

Here's my attempt at solving your query on how to convert thrust (gms) to Power (watts) so that plane's AUW can be planned.

Ofcourse I am using a online calculator for the purpose and I am giving a heads-up that it may not be accurate. Use a wattmeter for 100% accurate reading on the Power output.

I guess you have this motor
http://www.headsuprc.com/servlet/the-1409/EMAX-CF2805-Outrunner-Brushless/Detail

2800kv motor -
This is a high kv motor. Meaning: Motor has a high rpm/volt, requires a lower diameter prop so that current draw does not exceed motor rating.

GWS EP 7035 prop produces about 15 ounces of thrust at 13 amps. -
This is the only prop which can produce maximum thrust for this motor as per specification.

Battery choices as per website:
1) 7.4v 1000 20C
2) 7.4v 1300 20C
Motor's voltage rating is 7 to 13 volts. You may try a 11.1v battery.

ESC:
1) 20A.

Following data is what I entered on the website:
1) 7x3.5 GWS HD
2) Height:50m
3) Temperature: 28c
4) RPM: = 2800kv is rpm/volt under no load condition. Under load condition I am assuming rpm will drop by, let's say 33%. (NOTE: Folks, kindly update if this assumption is correct.)
So RPM for calculation is 1876kv/volt

1876 x (4.2 x 2) = 1876 x 8.4 = 15758.4 = ~15800 rpms

5) Volts = 8.4
6) Current = 13A.
7) Power = Blank it out

The main aspect here is the motor efficiency reads at 87% so that it does not burn up. Hence calculations must be correct with the given set of data. Thrust is calculated at 615g which is what you wanted.

So if you are developing a highly efficient airplane you may have 15-20% more weight than the calculated thrust. But for the time being I will be conservative at 15%.

Calculated power = 95w
AUW = Thrust *15% = 615 * 15% = 707gms. = 1.55 lb

gms to lb converter:
http://www.unitconversion.org/unit_converter/weight-ex.html

And vishwas, if you do build your own plane do post your pics, videos, results to cross validate the calcuations.

Thanks for reading.

Ashutosh.