You need to tell which RTF kit you are having as well as what are the specs of motor and ESC and the prop size if you want accurate answer.

I need to understand how one can select motor, esc and battery for rc plane, what all parameter we need to consider and how to derive their relation with eqch other?

As per my understand first we need to select motor based on plane weight, wingsize (i srll failed to understand how to corelate these parameter to come upnwith motor KV etc.) Then once you select motor we need to select esc and battery and props.

I have build old fogey and want to understand equations / formula / thumb rules of selecting electronics for given rc plane.

Thanks,
Niteen

No easy solution. I will be writing an article on this today

I found this

http://www.rcgroups.com/forums/showthread.php?t=1136470&highlight=noflyzone

I think this will help me to understand how it works

Anyone tried using Webocalc to decide on motor / prop / battery for given plane?
http://flbeagle.rchomepage.com/software/webocalc_1.7.6/html/webocalc_imperial.html

@niteenk
I had suggested this in this forum a year or so ago.
Webocalc is good for getting a rough idea of the type of performance likely from the airframe, motor, prop, battery used.
However if you use it for a while, you will notice that it:
1. Underestimates battery requirement
2. Overestimates duration likely
3. Overestimates field size reqmt
4. Cubic loading calculation appears incorrect.

Happy landings!

All these web-o-calcs and other softwares are approximate, and theoretical. There are a lot many more factors, which make purely deductive applications redundant, and demand some degree of empirical investigation.

Here is a simple apparatus I made for this:

@sanjayrai55 sir,
My test rig. Core is an Eent!
Motor run in reverse to avoid liftoff during thrust testing!
Regards

Iyer sir, what is an Eent? An electronic diagnosis system for Ear Nose Throat diseases? 

With this Rig I get accurate measurements of

1. Static Thrust
2. Watts at different throttle settings
3. Volts at ""  ""  ""
4. Amps at "  "    "
5. RPM (measured with a tachometer.

So now, to come back to motor ESC prop selection. Read on......

I would like to clarify here that I am not an electrical man. I don't like Electric guys cause they overcharge 

Also, I have only been into electrics for a year.

But some things are basic. (No units here, I am showing proportionality. If you want exact numbers, put in the units and constants)

Power = Amps X Volts (for DC)
Power = Torque X RPM
Thrust is dependent on torque

Motors do not have linear dynamic characteristics i.e. If you keep increasing the throttle the output will not keep increasing proportionately. The peak efficiency of a motor would normally be in the 50%-75% range.

It is something replied to by Pathar!
And
How come your pics are big and mine are tiny?

Now this Pathar is intriguing 

My pics are bigger because of the Dumbledore effect

Respectfully, there might be some problems with your test rig as I see. I might be wrong, please correct me if I am. The things that I am assuming are that the rig is used in the configuration as seen in the pictures and the motor is not parallel to the wire under tension. The problems that I see are:
1) When the wire connecting the rig with some rigid support through the weight scale becomes taut, the entire weight of the scale is supported by the thrust of the motor.
2) If the thrust line of motor is not co-linear with the scale and the wire, you will get inaccurate readings.

Suppose I have a high wing trainer of airframe weight 800 grams.

I reckon total weight of about 1.1 Kg with motor esc and battery

So I need (very approximately) 2.2*1.1*75 Watts OUTPUT from motor

Assuming an efficiency of 75%, I need 242 Watts.

Next, look at motors of 240 watts or thereabouts. Also, how will I be flying? As it is a trainer, I would be flying it easily generally.

I want ground take-off capability too, as I don't want to risk a hand launch (which cannot be aborted easily)

So I want thrust about 75% AUW at least. And I don't want too high an airspeed

Of course, the vector projection along the wire only would be measured. However, they actually are all parallel. Maybe the photo doesn't show it clearly. When the motor runs, the wire tautens, and the wire, scale and motor axis are parallel an horizontal

And about the first point?

@lastrites
When i tried measuring by attaching the scale to the tailwheel of my taildragger, she rotated on the main gear just enough to hit the floor with the prop...

To continue:

Let's look at this motor:

Avionic C2836 KV1120 brushless motor

with this manufacturer's data

Specification:
RPM / V - 1120 rpm
Weight: 76 gms

Combination of usage:
PROP – 9x4.7E
● Get 450gms Thrust
Lipo – 2cell 7.4V
Operating Voltage – 7.3V
Amp draw at load – 10.7 amp
ESC - 20 amp
Power –75W

PROP – 10x7E
● Get 600gms Thrust
Lipo – 2cell 7.4V
Operating Voltage – 7.3V
Amp draw at load – 11.2 amp
ESC - 20 amp
Power –78.5W

PROP – 11x5.5E
● Get 700gms Thrust
Lipo – 2cell 7.4V
Operating Voltage – 7.2V
Amp draw at load – 11.7 amp
ESC - 20 amp
Power –79W

PROP – 9x4.7E
● Get 1100gms Thrust
Lipo – 3cell 11.1V
Operating Voltage – 10.6V
Amp draw at load – 21.7 amp
ESC - 30 amp
Power –217W

PROP – 10x7E
● Get 1200gms Thrust
Lipo – 3cell 11.1V
Operating Voltage – 10.5V
Amp draw at load – 25 amp
ESC - 30 amp
Power –247W

PROP – 11x5.5E
● Get 1400gms Thrust
Lipo – 3cell 11.1V
Operating Voltage – 10.5V
Amp draw at load – 26.5 amp
ESC - 30 amp
Power –265W

PROP – 9x4.7E
● Get 1500gms Thrust
Lipo – 4cell 14.8V
Operating Voltage – 13.9V
Amp draw at load – 31.5 amp
ESC - 40 amp
Power –441W

Now, I only own 3S Lipos. In fact, I only own 1800 mAh LiPos! (Assumption)

So, I select the 3S (11.1 V) option.

All 3 prop sizes given here will fly my plane. Differently.

The 9X4.7 will give me least ampere draw, so longest flights, but slow (4.7" pitch) and less thrust (9")
The 10 X 7 will give me maximum power and maximum speed, having the highest pitch (7")
The 11 X 5.5 will give me maximum thrust, and an in-between speed.

Lastrites, the scale weighs < 50 grams. If that is your concern, it is a non issue.

Another frequent question is how long will the battery last?

Say the 10*7 is being used, which draws max. 25 A at full throttle. The Lipo is 1800 mah = 1.8 Ah = 1.8 * 60 = 108 Ampere-minutes

@ 25 A, we get 4.32 minutes.

Of course we never fly continuous full throttle. In typical trainer flying can expect 6-8 minutes.

The smart thing is do the first flight for say 3 minutes, and measure the battery voltage. That will tell you at what Duty Cycle you are flying. Then you can judge reasonably accurately.

Which brings us back to the test rig. Why do we need it at all?

Because, the manufacturer's test data is not always accurate, and/or under lab conditions. Also, at times for a particular application you might want a prop which is not listed. Or you might want to experiment with different props to optimize between thrust, rpm and thus power.

For example, I have recently got a Multiplex Tucano, and want to use a NTM 3548-900 KV motor on it. Which prop?

Weight of model: 1.85 Kg AUW. Type: Low wing aerobatic speed/sport. I need about 450 W. I have 3S batteries only.

Specs: (from the HK Product Page)
Model: NTM Prop Drive Series 3548 900kv
Kv: 900rpm/v
Poles: 3
Motor Wind: 7T
Max current: 55A
Max Power: 570W @ 12v (3S) / 815W @ 15v (4S)
Shaft: 5mm
Weight: 171g
ESC: 60A
Cell count: 3s~4s Lipoly
Bolt holes: 18.9mm & 25mm
Bolt thread: M3
Connection: 3.5mm Bullet-connector
Prop Tests:
Data available soon!


I tried a 11*7. Flop! 290 W!

13*6.5 - better. 390 Watts.

14*7 - Bingo! 500 Watts

On a 4S battery, I will of course get the desired wattage and thrust on a 11*7

And now, the ESC selection, and the elusive "C" Factor.

I will be drawing about 48 A max.

So, I will go with a 60 A ESC with a built in BEC. I am using 4 Servos 17 gram, one Servo 19 gram. 3A on the BEC is enough here.

@ 48 A, if I use a (say) 2200 mAh 20 C Lipo, I can only draw a maximum of 44 A theoretically (2.2 X 20). In practice, this LIPO if used continuously @ 40 Amps will pack up very, very soon.

Say I used a 3000 mAh 20 C Lipo. This will give a maximum rating of (3 X20) 60 Amps. So it will be safe.

If I had to use a 2200 mAh, it would be OK at a 30 C rating (66A)

In actuality I will be using a 3300 mAh 25-50C in the fond hope I will get some decent battery life