Instructables

how to control speed of an dc motor?

i have very small dc motor of 5v 650mah and i am using it in my rc helicopter and it rotates very fast so how to controll its speed plz help me i would be thankful for that

If you only require a set speed you could use a gearing system small gear to large... otherwise you could supply less voltage to your motor (50% less voltage = 50% less speed) or more efficiently you can use Pulse Width Modulation to switch the motor on an off at a particular rate... this is pretty much how a dimmer switch works... I personally would look in a cheap children's toy for a couple of gears :)
As uniqueutopia says, you can use gearing, which will maintain (actually increase) your available torque, or PWM, which will reduce your torque in proportion to the speed reduction if the source voltage remains the same (5V). I would not run the motor at a constant voltage much lower than it actually calls for, since that can cause damage.

I *would be inclined to use a combination of gearing and PWM.