Can you explain little about "Gear Ratio" and is it mandatory to be 1 while selecting motor and prop for given model? what will happen if I have Gear Ratio slightly great than 1 i.e. 1.05-1.1 or lower i.e. 0.95 - 0.99 etc

The only time I have come across the term Gear Ratio is in relation to Brushed motors, wherein a gear box is used to reduce RPM without sacrificing torque and power. Unfortunately I have not encountered this term in Brushless Motors. Could you explain your question in more detail, with specific reference to the context?

@niteenk
Where did you see such gear ratios? In Webocalc?

In Webocalc a gear ratio of 2 means that for the selected prop to produce the predicted result with the given motor kv, motor efficiency and cell count, the prop shaft needs to be geared down by a factor of 2 as compared to the motor shaft. Ie, prop to turn at 50% of motor rpm.

Ratios of close to 1 simply means the match between motor, cells and prop is slightly off.

Don't bother too much about webocalc. The built in assumptions and scope for variation in input data are such that you can only get a broad idea. Don't think the results will be like 99% or 101%. Use it only to get a rough estimate.

I've not come across ratios like you mention anywhere else.

Thanks Sanjay sir andIyer sir for information. I have built rc plane of weight 700 gm with wing span of 38" and trying to figure out which motor (baded on required thrust etc) choose for it. Getting confused day by day with lots of info available on web.

Quote from: lastRites on August 26, 2014, 12:51:16 PM
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.

With all due respect....
I feel the 1st point stands true
The thrust line in the apparatus is parallel but not in line with the weighing scale used for measurement of the static thrust..

Thus there is a moment arm being created causing two vectors at the point where the weighing scale is attached... One in the line of thrust and one in a vertical direction trying to lift the tail of the apparatus.  So the static thrust reading would be actually less then the realistic value.
Please correct me if am wrong.

@niteenk
Not only too much info on web, also oldies like me trying to give too much so called gyan.
To avoid confusing you further, i will stop with this last post. Pm me if you need more.

A models ability to fly and its flight performance depend (to a large extent) on two factors, and their combination
1. Wing loading
2. Power loading
3. Combination of wing loading and power loading

Restricting ourselves to a typical high wing trainer of 36"-48" span:
   wing loading.          Power loading.               Performance
1. 6-7oz/sqft                  50watts per pound.         Light wind slow flyer for new pilot
2. 7-9oz/sqft                  75watts per pound.         Reasonably slow. Can handle 5-10kmph wind
3. 10-11oz/sqft.              100 watts per pound.      Can do loops and rolls, handle 10-15kh wind
4. 10-12oz/sqft               150 watts per pound.      Aerobatic. Too much for beginner.
5. Over 12oz/sqft.            Whatever the power.      Will fly like a bomb or a brick. Too much for a beginner!

(Note for experts: I am only talking here about small planes less than 4' span)

I guess your 38" span is about 6" chord or say 228sqin or 1.6sqft.
And weight of 700gms or 24oz is including motor, prop, esc, rx, servos, battery
So you will be in 15oz/sqft range.

With low power you have a brick, with high power you have a bomb!

Would you like to post a pic, give dimensions of wing area, and clarify whether 700gms is bare airframe without electronics, power, battery or all up?

If you are interested, i can post links to videos of how some of my trainer models fly. The links are already there somewhere on this forum, but i can find them for you.

Best wishes. Bye.

Doc Nayeem:

In no picture is the wire of the Scale taut. Therefore I fail to understand why the supposition.

The wire and scale are parallel (to the eye) under operation.

I fail to understand the nit-picking. This is an idea - please do improve on it! I will be delighted, and the loudest applause will come from my side.

The point is there is an inexpensive and reliable method of judging the motor characteristics under different prop conditions, without shooting in the dark

Iyer sir... this is lovely!

What about motor rpm and prop selection?

@sanjayrai55

Mai chup rahoongi
Kyonki
Mere test rig ki koi tareef naheen hui
No vector, no twisting moment...

Perhaps many newbies fall between two stools.
Neither badhais nor aerodynamicists.

So in my posts i try to stop just before i get carried away completely.
(Even though i often cross the line suggested by you)

About rpm and prop selection, better than giving my gyan, i could give links to posts of certain other members / master coroplasters, as i've been doing in the past...

@sanjayrai55
Sir,
Niteenk is worried about, of all things,
An Old Fogey that he built.
(I expect an Old Fogey could fly like my Indore Bipe, ie, backwards if necessary...)
Have referred him to devashish, who built one last week and may test fly coming sunday

BTW, did you see my post on Nagar to FAI?

Iyer sir

Your test rig was simple and effective 

VK Nagar is a friend, and a wonderful human being. Always wish the best for him

Rai saheb
You are too kind as usual
Always feared waking a tiger...
But if you are awake, or tomorrow, pl see my last posts on Basic aerodynamics thread

@rai sir
Am still a bit confused
Hope this picture dipcts my query

Doc, the test rig is on smooth wheels. When the motor runs, the rig moves forward. The rig is anchored via the Scale by a thin steel wire

When the rig moves forward, the wire is under tension and tautens. In the tautened state, it (the wire) is horizontal, and parallel to the motor axis.

Thus the tension in the wire (which is measured) has no component in the Y Axis.

In the pictures, the motor is not running, so the wire is slack. Since I need my hands for taking pictures, and am alone, I can not run the motor while taking these pics.

I trust it is clear now, but please ask if not, or phone me. My number is on my profile

@rai sir... 
Its a good discussion going on...
The apparatus moves forward...  Until the wire is taut and then it stops...  So at this point we are measuring the static thrust...
Though the point where the thrust applied and the point where we are calculating the thrust are parallel but not in line that is there is a vertical distance of 4 to 5 inches this is creating a moment arm and trying to rotate the vertical arm rather then pull it and hence the resulting vectors will not be unidirectional....

AAha! I finally get your point - moment arm.

Will not create anything in Y direction significant as the Tension (also a vector) is in X direction.

The effect of the couple (formed by the Static Thrust and Tension) will indeed be to rotate the assembly so it tends to fall on it's nose (prop side)

But if the weight is enough to keep the wheels on the ground, there will be no Y axis forces. This is the case

Keep in mind the basic funda of a Chain being only as strong as it's weakest link.

The error in the reading of the Scale is less than the compounded errors in the assembly not being square, friction in the wheels etc etc

Subscribed.

It`s all greek to me

@rai sir...
The moment arm causing the rotation of the vertical arm which you have mentioned which you say is counteracted by the weight is the same force (created by the motors thrust) that won't b quantified on the measuring scale
I still feel we would get a more accurate measurement if the scale was attached right behind the motor ...
Kindly pardon me if am wrong

Make one and let us know Doc. It will be most useful. Keep the readings and info in the public domain

I personally am satisfied. And I am sure all agree on the concept for a multi-parameter tester. So, any improvements are a part of normal development

Hats off sir...

Just a small update. I'm getting sold on lower KV motors  .

I used the NTM 3548-900KV on my MPX Tucan. I selected a prop of 14*7 based on testing, getting 530 W maximum. Using a 3600 mAh 40C battery, I am getting great mileage: a 6 minute flight still leaves me with 70% battery!!

Similarly, on my FunCub, I was earlier using a TGY 3536-1250KV, with a 10*7 prop. Shifted to a 3536-910 KV with a 13*6.5 prop, getting 20% longer flight times (ie 20% less power consumption)

I guess I was underestimating the importance of thrust. Low RPMs + Bigger props => better thrust, but not proportionately higher Amps.

Noticed something similar on a 2836-1100 KV when I moved from a 9*6 to an 11*5.5