As I am very much influenced by the post by Iyer sir.... I wanted to frame this post......

@Iyer sir.... Your techtalk has helped me a lot.....
In this post I am giving the output which I learned from my aeronautical engineering and my aeromodelling.....
The field of aeromodelling has grown up in the country. The techniques and methods used has worked well in most of the cases.....  what about the reality?

So I would like to express my views.....
Experts Please correct me if I am wrong.....

The field of aeronautics possess four main categories:
Aerodynamics
Propulsion
Structures
Stability and control (Performance)
So the field of aeromodelling tooooooo........

Lets begin with few questions.....
               What will happen if we interchange the position of flaps and ailerons?
Why don't a high wing rolls like a low wing or mid wing airplane....?
Is only the thrust and watts calculations are sufficient.....
And more......

  Lets find out....

So let me begin with the field PROPULSION......

Since electric planes are cost effective..... let me begin with it and then move on to internal combustion....

Before entering to propulsion.... Lets roll back with some basics.....
As I am a great fan of Sir Issac Newton.....
Lets begin  with his
LAW  CONSERVATION OF MOMENTUM....
Momentum......
Whats it.....
An example.....
A man of mass 100kg  running at a velocity of 10m/s.
Then his momentum is 100kg.m/s
So momentum is nothing but a mass moving at a velocity.....
Sir Issac Newton Says......
Momentum is conserved......
Which means Intial momentum= Final Momentum

Let me go with an another example.....
Assume a trolly on the railway track at rest... which posses a mass of 100kg (Assuming there is no friction)
A Man of mass 100kg runs at a velocity of 10m/s and jumps into the trolly........

The question is what would be the velocity of the trolly now.....?
The velocity of the trolly with man would be...... 5m/s
How
The momentum gained is (man) 100kg*10m/s = 1000kg.m/s
Since the momentum is conserved this would be the final momentum tooo.....
But final mass is man+trolly= 100+100 = 200kg
Final momentum..... 1000kg.m/s= 200kg*xm/s
Therefore x = 5m/s

So Now whats the relation between all these Newton, Momentum, Man, Trolly, Mass and velocity with aeromodelling......

They do...
Lets a take a plane of mass 1kg......  (Assuming Stalling velocity to be 5m/s) (which means required momentum is 5kg.m/s)
Lets find out possible ways of flying.....
Considering the Turnigy - C3530-1100
   

   Propeller    Gear  RatioVolts   Amps   Watts   RPM   Speed (mph)Thrust (g)Thrust (oz)RPM as % of Kv*V
GWS HD 8x4   1   6.9   3.9   26   6780   25.7   236   8.32   91%
GWS HD 8x4   1   7.9   4.7   37   7620   28.9   300   10.58   90%
GWS HD 8x4   1   8.9   5.65   50   8430   31.9   375   13.23   88%
GWS HD 8x4   1   9.9   6.9   68   9360   35.5   460   16.23   88%
GWS HD 8x4   1   10.8   7.95   85   10110   38.3   544   19.19   87%
GWS HD 9x5   1   6.9   6.55   45   6240   29.6   383   13.51   84%
GWS HD 9x5   1   7.9   8.05   63   6930   32.8   484   17.07   82%
GWS HD 9x5   1   8.9   9.65   85   7680   36.4   597   21.06   80%
GWS HD 9x5   1   9.8   11.3   110   8340   39.5   702   24.76   79%
GWS HD 9x5   1   10.8   13.05   140   8970   42.5   822   29.00   77%
GWS HD 10x6   1   6.9   8.35   57   5880   33.4   466   16.44   79%
GWS HD 10x6   1   7.8   10.1   78   6510   37.0   575   20.28   78%
GWS HD 10x6   1   8.8   12   105   7140   40.6   691   24.37   75%
GWS HD 10x6   1   9.8   13.9   136   7710   43.8   804   28.36   73%
GWS HD 10x6   1   10.7   16.05   171   8250   46.9   920   32.45   72%
GWS HD 3T 10x6   1   6.8   10.35   70   5460   31.0   531   18.73   75%
GWS HD 3T 10x6   1   7.8   12.6   98   6040   34.3   663   23.39   72%
GWS HD 3T 10x6   1   8.8   14.6   128   6580   37.4   770   27.16   70%
GWS HD 3T 10x6   1   9.8   17.05   167   7100   40.3   914   32.24   67%
GWS HD 3T 10x6   1   10.7   19.1   204   7540   42.8   1030   36.33   66%
APC E 11x5.5   1   6.8   10.2   69   5520   28.8   549   19.37   76%
APC E 11x5.5   1   7.8   12.6   98   6090   31.7   693   24.44   73%
APC E 11x5.5   1   8.7   14.7   127   6570   34.2   818   28.85   70%
APC E 11x5.5   1   9.7   17.15   166   7020   36.6   960   33.86   67%
APC E 11x5.5   1   10.7   19.5   208   7530   39.2   1090   38.45   65%
GWS HD 11x7   1   6.8   11.8   80   5250   34.8   610   21.52   72%
GWS HD 11x7   1   7.8   14.2   110   5790   38.4   739   26.07   69%
GWS HD 11x7   1   8.7   16.7   145   6270   41.6   855   30.16   67%
GWS HD 11x7   1   9.7   19.2   186   6660   44.1   1015   35.80   64%

GWS HD 8x4   1   9.9   6.9   68   9360   35.5   460   16.23   88%    (4th data from top)

so thrust is .460gms and velocity is around 15m/s  the momentum gained is 6.9 is more than sufficient since the airplane will move at a velocity of 6.9m/s which is  greater than its stalling velocity

APC E 11x5.5 1 6.8 10.2 69 5520 28.8 549 19.37 76% (9th data from bottom)

thrust .549kg and velocity is 12m/s  and the momentum is 6.558 will make the airplane fly at a speed of 6.588m/s

but please everybody should understand all these are based on and assumption of stalling velocity.... which is great factor

@iyer sir.... if u please share your experiments it ll be even more great

To make it even more real i need to steal a post from the coromaster sanjay sir....

@sanjay sir,,,, as u were not online i was not able to seek permission from you sir....

Quote from: sanjayrai55 on September 12, 2015, 12:39:09 PM
 

I first put a 9*4.5 prop, but realised it wasn't enough (The AUW is 1050 grams - I'm using a 3000 mah LiPo)

So on the second sortie I changed to a 9*6 and it was fine. With the 9*4.5 I ws flying at full throttle throughout!

intially he used 9*4.5 and flying at full throttle say 1000rpm and changed to 9*6 and flying at little less throttle say 7500rpm

the thrust was reduced but how did the plane fly?

Here i make  when the comparison

Quote from: Devanesan Andrews on December 22, 2015, 01:51:07 PM

Since today almost all the rc models are based on electrics..... let me begin with it and then move on to internal combustion....

Disagree

sorry sanjay sir.... i changed it

Dev, thrust is not the only consideration. You can't get lift without velocity. The 9*6 gives higher velocity than the 9*4.5 :- the 9 contributes to the thrust, the 6 to the velocity

Quote from: Devanesan Andrews on December 22, 2015, 03:46:19 PM
sorry sanjay sir.... i changed it

You didn't need to

@sanjay sir,,,,,  sir if you please share the motor specs  used in Ken Willard's Headmaster, in Corro & Foam it ll be grateful sir....

@sanjay sir

   Generally we in engineering measure thrust(force) in Newtons....  but why we follow it in grams in aeromodelling....

The "gram" is a short form of  gram weight. Gram is a unit of mass. The force exerted by 1 gram (due to gravity) is 1 gram-weight. Similar units are lbf, Kgf.

1 gram weight = 1 X 9.81 gram. cm/sec ²
or.... 9.81 dynes


Motors: Thermocol: NTM 2836-1200, 9*6;  Corro: TGY 3536-1250 10*6

thank u sanjay sir....

sanjay sir acceleration due to gravity is constant.....  what about acceleration due to propellor??   how could we take it as 9.81cm/sec2

I replied only to this:  "Generally we in engineering measure thrust(force) in Newtons....  but why we follow it in grams in aeromodelling...."

Nothing about prop, just about unit of measurement

sanjay sir since the day i entered aeromodelling i was frustrated of using this grams.....  so only wanted to find an another way.... found the momentum

sanjay sir.... i knew that you are well versed and experinced in all those above concepts i have spoken..... so if it has any mistakes pls guide sir...

Dev, I am no expert.

I don't even know why momentum equations?

I would have used this:

Power = force X distance /Time = Torque X RPM

Motor can produce 'X" power
Assume only 0.8 X is available to you

Check various prop tables to see the velocities and thrust generated at different rpm's.

Select motor/prop combination based on this.

If you don't have a motor, but wish to buy one, use the old rule: Trainers 100 W/lb, Sportsters 125-140 W/lb, 3D 150++ W/lb

Select power accordingly

Then select KV depending on type of model. Always keep in mind slower props have less energy losses: so try for large props rotating slowly unless it is a jet or ducted fan - there you want high speed small diameter.

Remember: the upper figure in the prop size relates to thrust (which is Torque/radius) and the lower is speed

So a 9*6 vs a 10*6 - the speed is same, but the 10 gives higher thrust
a 9*6 vs a 9*7 - the thrust is the same but the 9*7 will give higher speed

You can also refer to freewares like motocalc, which are guides only, certainly not infallible!

Why don't you select a real life example of a plane that you wish to select a motor/prop/Lipo for? Let's go through it together, hopefully with participation from other forum members