I saw syma X5 quadcopters use extra gear wheel to rotate propeller instead to directly link propeller to motor. I think this technique use for get more flight time. but i don't know exactly.please explain.
this is the motor holder - http://www.symatoystore.com/image/cache/data/X5C/X5C-05-Motor-holde...
This is a method still in common use for cheap toy quadcopters.
It permits the use of really cheap, high speed brushed motors.
It's disadvantages are really cheap DC brushed motors have a short use life and the gearing system itself adds resistance and wear.
The better technique is to use brushless motors that are actually wound properly for the battery and desired maximum speed.
That is considerably more efficient and lasts longer and efficiency is higher at all motor speeds.
Brushless motors and their ESCs cost more than cheezy little geared DC brushed motors. :)
Why generally more expensive quadcopters have brushless motors on them.
That said the little Syma is a great, durable and low cost way to learn to actually fly a quadcopter
I still have the V929 ladybug quad from year 2012 converted to FPV drone that uses the gearing spur to spin the prop, still running well after many year till today without any problem with no sign of wear and tear. They can fly about 10~12 minutes on a small battery compared to direct driven motors and can carry extra load. Yes they indeed helped some runtime improvements. Didn't see any advantages and i love it very much when compared to brushless motor of public demo since the prop spin is low rpm and less flesh shredding. Having a geared mean less damage directly to the motor if crashes. You can check my build here for geared type brushed quad: http://www.supermotoxl.com/fpvuav-models-builds-and-flight-reviews/...