Model aircraft tend to be realistic reproductions of the real thing. That means, among others, that starting the petrol engine must be done by hand or with an external electric starter. Manual starting may be realized in two ways: with the circuit in Fig. 1 or with that in Fig. 2.
The circuit in Fig. 1 places a 3-V relay between the starter and the motor. When the starter switch is closed, nothing happens. But if then the propeller is turned by hand. the motor acts as a generator and, once it is turned fast enough, the generated voltage is high enough to energize the relay, whereupon the motor starts. An advantage of this is that it is a purely mechanical operation, which is readily incorporated. A disadvantage is that once the relay is energized, it is operated by a 6-V supply: a slight waste of energy.
The circuit in Fig. 2 is rather more economical since it uses a 6-V relay. Again’ the motor is used as a generator operated, by turning the propeller. The generated voltage is applied to the base of T1 via .8:1 When the base is supplied with sufficient current, the transistor is switched on. The relay is energized and the motor starts. The starting point can be preset with P1.