Eight drops with a single servo

This model-flying 'optional extra' makes it possible to carry out several drops during the same flight, using a single servo. It was built for the Stinson L5 Sentinel, a 1:3.75 scale replica whose large cockpit is well suited to housing it, but it can be used and re-proportioned for many other models. One thing is certain: it puts on an unusual and original show, as it can 'release' parachutists, streamers and water balloons as many as eight times in a row, and tow banners, tricolour flags and anything else your imagination can come up with.

The number of drops depends only on the size of the model, or rather on the volume of its cockpit, where what I call the container or drum is housed. The one I present here is made of a wheel 30 cm in diameter in which, by dividing the circumference into equal parts, I made eight compartments, each with an arc of just over eleven centimetres. I used 3 mm balsa covered on both sides with a thin cloth glued on with PVA glue. The partitions separating the compartments are made of the same material and have at their ends a 5 mm carbon rod (limewood works just as well) that joins the two side walls and sticks out 6 - 7 mm on one side. This sort of pin stops and positions the various compartments during the drop.

After placing the object to be dropped in one of the compartments, the drum is turned by 1/8 of a turn; the rotation pulls along a strip of soft rubber or plastic that closes the compartment, preventing the contents from falling out. The strip also has another function: it turns the drum during the drops, since its other end is wound onto a plastic tube inside which a rubber band works in torsion.

The base of the container is made of plywood and balsa, and has an opening the size of one compartment of the container (in my case 10 cm wide by 13 long). The rods connecting it to the drum are carbon and fit into an aluminium support on which the drum hub runs, held by a spring that allows the container to be moved away from the base, making loading easier. The axle carried by the hub must not pass through the inside of the drum but must be kept flush with the wall, so that the central part can be used for a through compartment.

Now let's look at how it works in practice. Once loading is complete, the drum is fully covered by the strip and kept under tension by the rubber band acting on the winder. At the first servo command (let's call it opening), the lever system opens the doors in the underside of the model and, at the same time, the rocker, driven by the engagement pin fitted in the slot of the central bellcrank, moves down and releases the protruding peg of the drum, which advances 1/8 of a turn and catches on the opposite side of the rocker (the hook-shaped one); at this point the contents of the compartment are freed from the strip and the drop takes place.

With the opposite radio command (closing), the doors close and the drum moves into position for the second drop. In fact, the rocker releases the peg from the hook-shaped side and catches the second peg on the opposite side; the drum advances by about another couple of cm. In this phase, the compartments used to hold banners release the small towing hook, and the banner is released from the model.

The banners I made are two metres long and twenty cm high. To fit them into the drum, I joined two diametrically opposite compartments (which is why the axle supporting the drum must not pass through) and then divided them parallel to the outer walls. It is also possible to split the banner's towing rod into two or three pieces, then joined with a spring so that they can be folded and fitted into the compartment. As you can see in the pictures, the drum has been set up to release three banners, but this number can be chosen at will: just insert into the compartment the small rod which, held by a small spring (floppy disks contain one that fits perfectly), locks the banner towing hook.

I sincerely hope that the photographs (by my friend Gianpiero Rosso) make up for the gaps in the description.