Center of gravity finder
Both builders of an airframe and flyers who are just putting one together must choose a center of gravity, and it is important that they be very similar. A nose-heavy plane will fly inefficiently faster than it should need to, while a tail-heavy plane will tend to mysteriously stall. Usually, models come with instructions on where the CG should be, but in many repackaged and low-end models this is absent. Determining to within a few millimeters where the CG should be, and adjusting where it is with a particular battery/motor/AP configuration, is essential. Many planes are amenable to being balanced right where the wings join the fuselage, to determine if the forward-back CG is in the right place.