RC India

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....

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

Disagree :D

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

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

Sent as PM originally, now posted here at Deva's request.

"initially 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?  ???"

Hi Deva,
FACT: it flew
ASSUMPTION: thrust was reduced
HENCE: assumption must be incorrect.

Why incorrect? Because based on two other assumptions, which were not validated, namely, rpm on 9x4.5 and rpm on 9x6.
In general, when assumptions lead to a conclusion that differs from the observation, then the assumptions need rechecking.

Good effort. Wait till you get comments from Gusty sir, Sanjayrai sir etc!

The RPM will indeed drop, because the load on the motor is higher. This was discussed before : how to predict an RPM drop on a motor? Fact is it's tough; best is to ensure the motor has adequate power and torque so the effect is minimized!

Same doubt.
Think of a quad in hover. No velocity no momentum.

And for fixed wing, you need to know the power (not momentum) required to:
1. Maintain cruise speed (not just stall speed)
2. Accelerate reasonably quickly
3. Climb

Power = force X distance /Time = Torque X RPM

Should not this be RPS? Cause in general Power is expressed in Watts, and that is Joules/ Second.

And work done is circular motion = angular displacement(2 X pi X n) X Torque

Correct me if I am wrong.

I am trying to explain proportionality, not giving a formula

@devanesan Andrews,

Consider a glider.
With a given trim (i.e., stab/elevator adjusted to achieve a certain angle of attack of the wing)
it conserves momentum without any power source (except gravity  ;D)
If you retrimmed for a lower angle of attack it will stabilize at a higher value of momentum.
Why does the momentum not go on increasing?
Because other factors are at play!

This, along with the points mentioned in my earlier post, make me doubt if momentum is a good yardstick for estimating the power requirements of an aircraft.

That is self evident.
Inboard ailerons will give inadequate roll rate.
Outboard flaps will increase the possibility (and effect) of tip stall during landing.

Deva,
Sometimes I ask members 'what's the objective?'
So I'll ask you what's the objective of asking this question.
Regards.

Not really for me to answer, but Iyer sir, many beginners would want to quantify some of the principles behind successful RC flight. There is some stuff on this forum, but it does tend to get extremely heavy from the experts, and dubious from the wannabees.

Why don't you put together a series on this in Iyer's Tech Talk which will be simple to understand: stick to easy terms, four letter words etc.  :giggle: :giggle:

outboard flaps will break the wings due to high bending moment in the centre.....

objective of the question is to tell this to all....

10000 members are there in rci 9000 may know this.... i am telling this to beginners like me

@ sanjay sir and  Iyer sir....

    in the beginning itself i told i am expressing my views

and this post is not just made for aeromodels.... more than that upto an UAV

iyer sir lets me get clarify about AOA first

considering an rc plane

              the air is stir and the plane is moving
 
 if the elevator is pushed up and the nose pitches up.....  so now the angle of attack has increased?

Deva,
Come on. You don't need my answer to that.

@rcpilotacro,
Gusty sir, where are you?
How to clarify without offending feelings of OP?
You have credentials which I lack...

iyer sir want to get clarified so only i am asking sir

Here is a proof that every thing could changed.... and the yardsticks are not yardsticks

https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html

https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html

https://www.grc.nasa.gov/www/k-12/airplane/wrong3.html

Yes.

Well.... guess the aerodynamics being propagated by DA are his assumptions..... a lot more happens actually because its not just one force acting on the aeroplane.  There are a multiple forces acting and many of them nullify the effect of the first force or compound it in many ways. 

All I can add to this confusion is...... leave the big daddy of aerodynamics apart.... Just follow the KISS rule.

OR Else DA please read "Mechanics of Flight" by AC Kermode.  This is the book all aviators read and study before taking their machines into the Air.  Otherwise for everyone else out here Guess the Humble book named "From the Ground Up" is more than sufficient.

If all theories are wrong ; what causes the lift? :headscratch:

Ok got the answer here : https://www.grc.nasa.gov/www/k-12/airplane/factors.html

@shobhit sir....  thank you so much sir...

but i have read

mechanics of flight and flight without formulae by kermode
introduction to flight  and aerodynamics by anderson
aicraft performance and design by anderson
stablitiy and control by perkins and a lot more

still the problem is why i dont know the thrust is measured in grams in aeromodelling instaed of newtons

Devanesan

Do read up 'Circulation theory of lift' 'Dimensional Analysis of Lift' of of course read clancy (not the famous Tom)

This is the simplest document you will find on 'Dimensional analysis of lift'

 Dimensional Analysis of lift (http://aerostudents.com/files/aerodynamicsA/dimensionalAnalysis.pdf)


PS
thrust x Velocity = Power (watts or Joules) do see Thrust power equation and how Jet engine recovers it with speed because of intake designs. We could even discuss intake designs, it, i guess will be too much for RC commuinty

@rcpilotacro sir,

 i agree with all you  said sir....

and i agree that its too much for an rc community....

this is what i want to change.... in india still its just a hobby... want to take that above  it

Dear DA..... Doing your studies on the subject you must have come across a lot of other books too and read them..... well.... the answer to your question.... on the topic u brought out in your earlier post.  Well.... you say most of us do not dwell much into Aerodynamics to understand the hobby better and to take it to a higher level or a new dimension.  Well... if you see here many of us fly helicopters or multicopters.  How many of us really know how a helicopter flies.  How do the blades function and how is the lift generated and a helicopter turned..... and what is stability...

Why do you not take some similar topics in Rotary wing AD and try and explain some of the baffling issues of helicoptrs..... it will be interesting.

Basically we all wish to keep it as a hobby and know what is required.  Knowing a little more will definitely help help each one of us.
  

in that case shobit sir... i ll definitely do that

Shobbit sir, a video series by smarter every day is already there regarding helicopter physics.

@rcrcnitesh,
Pl see PM!

Hi Deva, I was initially surprised to read planes don't need an aerofoil to fly! So tried that, and it flies!

Of course yes sir... But you all need angle of attack sir

👍👍👍

Hi Deva, one more question, if we invert an aerofoil ( so that flat side on top) and then give it an angle of attack , what would be its flight characteristics?

It'll be a boat, no doubt  ;D

Edit

Afterthought. Provided the AOA is negative. Else it'll be a submarine!

Deva and SI74. Just joking, please don't take offence, ok?

 :giggle: that's a perfect reply sir...  ;D

Here is the answer my dear...

Oh really? Deva? Is it this? "Lift can be achieved in either orientation" ??

Yes sir for  sure

Can anyone tell the alternate software for motocalc or if any cracked version available let me know