rc-car

I have an elecrtic motor to be activated via radio control powered by 4x 1.2 volt 800mAh NiCd batteries in parralel with the motor.The R/C only has a Drive button - no speed control. So I came up with the idea that as long as the motors speed increases slowly, I can keep control of it (the car will use a 4:1 gearbox and a top speed of 22mph) The car's power rating is 1.7 amps. THe motor takes 0.2-0.8 amps depending on load. So I was hoping that a component or circuit layout will allow the current to pass from 0% to 100% in 10 seconds or so to enable the car to slowly increase in speed, so all the radio control needs to do is connect this circuit in the car and the component/circuit will allow the current to pass slowly.

Any Ideas?