From where we get the term left or right turning?
In an Airplane the Pilot is normally behind the propeller, so he looks at the backside of the propeller. The propeller is turning from his view "left" - anticlockwise.
SO this Propeller is an left turning propeller.
On you MikroKopter (standing on the ground) you see normally the propellers from above.
left turning Propeller turns from this sight of view clockwise and an
right turning Propeller anticlockwise.
On our MikroKopter we use left and right turning Propeller's.
Here you can see the directions of rotation:
And here you can see the rotation direction of an Quadro, Hexa and OktoKopter.
It a Motor is turning in the wrong direction - change only 2 of the 3 cables on the BL-Ctrl.
When you mount a propeller you should take care of a firm seat. The propeller and also the nut should not come loose during a flight. Also take care that you did not screw the nut to tightly. Here you can damage the propeller and also the propeller mount.
Here you can see how to assemble a propeller on a Motor MK3638:
The propeller on a multicopter should generate thrust - and here more then the weight of the copter. These propellers are the only way to change the position and altitude of the copter.
So we need propellers with an good efficiency. And they should be light that the motor must provide very little acceleration power.
The right propeller for the right purpose
If you drive a car you need different weels if you drive oin summer or winter. Also on your Copter you can use different Propeller's.
You can buy cheap and also expensive propellers. For a training's flight or for a light copter you can use the cheap EPP propeller's.
But under load this propeller's can break. So if you will fly with payload or with a heavy copter you need stronger propellers. Here you can better use e.g. APC or CFK propellers. This propeller's are much stronger and you can also fly with payload safe.