For the RC model gives it a multitude of clever little circuits that are usually easy to build.
Speed controllers, chargers, charge indicator, start box, through to complete telemetry systems.
Here is a small selection

Here you will find Schematic Circuit Diagram for DIY and further information on Hobby Model
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Small beeper for emergency landing flight models	www.electronics-lab.com/projects/models/003/
Microcontroller controlled throttle with Mosfets	ww1.microchip.com/downloads/en/AppNotes/00847a.pdf
Various speed control, battery charger, power tester	www.klaus-leidinger.de/mp/index.html
Telemetry system with altimeter MPXS4100 quite complex	www.electronic-engineering.ch/microchip/projects/alti/alti.html