Hi guys,
         I have a set of 4 6" propellers. They are made of cheap plastic and all of them are pusher props. They work fine when they are used normally. They work as a pusher when they are spun anti-clockwise and as a puller otherwise. I am building a quad-copter, so I would need both pusher and puller props. I drilled the other side of the prop and flipped it over the motor  . Some sites say that this turns a pusher prop into a puller prop. Unfortunately, nothing has changed. It is working the same way even if I flip it over. Shouldn't it do the opposite now? So whats wrong with it? Please help...

flip it over AND change the rotation of the motor...

but this is not going to solve your problem because youre using it on a quad where two motors need to spin in the opposite direction to counter the torque...if all your props are same orientation, and you flip two, all your motors will run in the same direction to provide lift, which will send your quad spinning...

You need two sets with LH and RH rotation..

I have done that...one pair of motors spinning clockwise and the other pair spinning anti-clockwise...but using it the normal way...or flipped...the propellers are working the same way...downward thrust for anticlockwise and upward thrust for clockwise. Shouldn't it be the opposite when I flip it?(i'm talking about flipping the propellers by the way...motor directions have already been reversed for one pairs)

did you try reversing the direction of the motor?

Flipping  the direction of the propellers alone will not change the direction in which they blow air. You need to change the direction of the motors as well. For a quadcopter, since you want the two motors spinning clockwise and two motors spinning anti-clockwise, you will need two different sets of props.

Thats what I'm saying...I have flipped the propeller and the direction of the motor...yet its working the same way....the propeller is shiny on one side and dull on the other side...so flipping it puts the dull side on top...does that have to do anything with this?

I think , especially if you are for quad. don't use the same props.just flipping or so.

As said in above posts Get LH and RH prop sets.

problem is that I am not getting a six inch prop with LH and RH rotation...and I cant use more than 6inch cuz the frame wont allow me to...

It is imperative as an aeromodeller to understand what determines the direction of airflow in any fan;

1. Hold the fan in front of you vertically, looking at it from front:
2. On the top blade, if the left edge is closer to your face than the right edge (pitch), AND the fan is turned counterclockwise, air will be blown away from your face...
3. Any *single* inversion (direction of spin or pitch) will negate the flow of air. Two negatives make a positive so if both change, air will still be blown away from your face...

***Flipping does not change the pitch of the prop (look at the blade from tip edge while flipping)...It simply ensures that the leading edge, which has a curve at the tip of the edge strikes the air in a much aerodynamic fashion, stays the leading edge when the motor direction is inverted...significantly boosting the performance of the propeller***

The principle is simple. If you learn it once, it will be with you for the rest of your life, elsewise you will always be in doubt...Look at your ceiling fan and try to understand why does it throw air downwards, why does it rotate ccw? what will happen if it rotated the other way.

You have to work with two constraints now...making sure that two motors turn anti to each other and at the same time all motors 'cut into' air at the top and push it to the bottom.

Ok...i understand what your saying...that means whether the curved edge strikes the air first or the straight edge...it only affects aerodynamics...right? So that means...if even if I flip the prop, it should work as before but will suffer from increased air resistance? So there is no other way but to get counter props?

Props are available at HobbyKing:
6x4.5:
http://hobbyking.com/hobbyking/store/__27908__Counter_Rotating_Three_Blade_6x4_5_Propeller_Black_1Pair_Bag_.html

6x3
http://hobbyking.com/hobbyking/store/__11329__6x3_Propellers_Standard_and_Counter_Rotating_6pc_.html

How much does it cost to import such propellers to India, other than the shipping charge and the cost of the propellers itself? (I'm asking about import duty, if applicable)

You got it right, other quad experts maybe able to help more

Meanwhle, here's a small rhyme I just came up with for everyone, there will be more I believe:

Left Edge closer, going low,
away from you air will blow,
invert pitch or motor spun
for direction, change only one
prop better or worse, do the flip,
leading edge must bear curve on the tip

Haha...you came up with that just now? Awesome man... And thanks a lot for explaining the propeller principle...

If you use Air Mail to order the propellers, I don't think you should be charged any duty.

Girish sir, Nice rhyme 

Thanks Maahin,

Yups anirban, as soon as Monday passes, I start feeling creative

never knew there is aeropoetry too.....

Flyshaad.....