You can also make a drop mechanism using a pin-release for "external stores."

2 eye bolts on the belly of the aircraft, 2 eybolts slightly closer together on the "bomb" (I typically put a bit of flour in a plastic easter egg and add fins) A piece of wire acts as the retaining pin. pull the pin with a string and the bomb falls.

You can add a pen-spring for automatic locking of the pin to hold the bomb. (just pull by hand to load... a servo easilly overcomes a pen spring.)

This simple release can be adapted for dropping parachutes and mounted on the top of an aircraft (near the CG) for a glider tow release. (takes practice and cooperation of the tow pilot and the glider pilot or you wreck 2 models very fast)