CAUTION: Before actually mounting propellers make sure to test the motor operation to assure they run in the correct direction, else swap two leads on the specific motor to reverse its rotation direction until all match the picture. Wait with installing propellers until you see that the IMU and transmitter calibration look ok. Once propellers are in place they represent powerful aviation stuff that can accomplish considerable lifting force, so unless you have everything looking right prior to propeller install, then motors running havoc in an unintended way could hurt both you, the ArduCopter and others. So take care and double-check your setup prior to running with propellers.
Propellers come as pairs in two sealed plastic bags. One pair is intended for running clockwise (front/rear) while the other pair backwards (left/right). Make sure you install them as shown in the following picture (the + orientation shown. Same rotation direction principle applies for X orientation as well) :
By rotating them in the shown direction make sure they are oriented to drive air downwards. Once you come as far as arming motors and applying some throttle, then double check rotation directions before attempting flight.
How to mount propellers on each motor axle:
The washer shall go into that hub recess.
You place it on top, then press it into the opening....
And here is how it looks with the spacer in place.
Now install the axle using both aluminium parts.
Propeller with the mount ready to go onto the motor axle.
This type of 2mm shaft screwdriver proved handy for tightening the nut
* Put a small drop of locktite into the nut and start it onto the thread.
* Consider also to put some locktite onto the motor axle but make sure it does not leak down to the bearings.
* Position the prop adapter with prop onto the axle and carefully tighten the nut. Don;t use a very long-armed wrench. These aluminum thread's don;t like too much force. In addition excessive force can also negatively impact the strength of these EPP prop hubs. The adapter has to tighten onto the axle so that it sits tightly and the prop cannot turn on the axle. If the nut is also a spinner, that's all. If it is a normal nut and there is room, then consider adding a 2nd nut and tightening that onto the first. This provides additional security against the nut working itself loose in flight.