Tail mechanism

The tail mechanism is the component that provides the thrust to the HoverFish. Therefore, when designing the mechanism, durability and compatibility of the parts are very important to consider. The tail mechanism is made out of 2 parts: the platform and the hinge. The platform is the fixed structure that is taped to the mylar balloon, while the hinge will be the moving structure that flaps the tail. The hinge must be very durable so that it can withstand the work done against the flapping air, and it also has to rotate freely and smoothly about the platform. The platform itself has to have a wide surface area so that the thrust provided by the flapping tail can be transferred efficiently to move the HoverFish instead of just poking into the balloon.

Tail hinge iteration:

An early iteration of the hinge involves the usage of RC hinge and a cut styrofoam piece. It was not working as what we wanted because the entire hinge is just too heavy and it is not easily secured to the platform. Moreover, since the entire weight of the tail and the hinge is rested on the servo, it overloads the servo.

Our 2^nd  iteration involves 3d printed hinge structure. This prototype serves as the foundation for the future iteration of the hinge support. This iteration of the hinge does not allow proper mounting of the carbon fibre rod. So, in our 3^rd iteration, we extend the side so that the attachment surface for the carbon fibre rod is longer.

Moving on to the 4^th iteration, we decided to make the structure thicker. From our testing of hovering the HoverFish, the hinge structure flexes too much. The thicker structure ensures that it is a rigid structure and it can move the air with less hassle. Secondly, we put a stop near the connection to the hinge support so that it can rotate freely about its axis of rotation.

On all the iteration, there is a surface for the carbon fibre rod to stick to using a super glue.

Tail platform iteration:

During the 2 months of tail development, we have come up with 6 different iterations as seen from the picture above.

The 1^st  iteration shows the early concept of our platform. There is a bed for the servo to rest on, slots for RC hinge to connect to the hinge, and a circular platform of 2mm thick to ensure it does not snap.

On the 2^nd iteration, we play around with minimising the thickness. When we lower it to 1mm, we observe that the platform becomes flexible that it bends instead of snaps. This is the moment when we realise that a 1 mm thick platform is usable for our project and this dimension is replicated for the future iterations. The secondary improvement we made is that the way the RC hinge is mounted to the platform is further simplified.

3^rd iteration is our first major turning point. We realise that RC hinge might not be durable enough for our purpose. We intended the RC hinge to continuously flap, which is not its intended function and shortening its lifespan. The 3^rd iteration adopts the idea of barrel hinge, where the hinge rotates about the holes on the platform. This overcomes the idea of durability and also the compatibility; the hinge can now connect to the platform securely.

Secondly, the servo is secured to the platform in a different way. Instead of resting on the platform, we are making it upright so that it is closer to the axis of rotation and we can optimise the design of the platform

On the hindsight, however, there are two things that we do not realise at first:

1. The gap for the servo is too small
2. There is not enough clearance on the hinge support for the hinge to rotate

These shortcomings are tackled in the 4^th iteration. We measure the gap correctly now and we rounded off the hinge support so that the hinge can almost rotate 180°. However, the filament used on the 4^th iteration has a higher density. Even though the model used is very similar, the weight difference is very apparent. We tackle the problem of filament density on the 6^th iteration, but we made a 2^nd major turning point on the 5^th iteration.

On the 5^th iteration, instead of a circular platform, we opted for a more trapezium platform. When we were testing the 4^th iteration platform, we realise that the weight of the entire tail mechanism is significant enough to cause a torque. This torque pushes the mechanism inward, putting the alignment of the tail mechanism out of the straight line. Our thinking is that by having a wider base than the top, we can minimise the poking of the tail mechanism into the mylar balloon and ensure that the entire mechanism is dead-on straight.

Moreover, as discussed in the hinge development, the thicker hinge means that the hole for it to rotate about has to be wider. This has a minor impact in the design process as we have to ensure that there is enough thickness around the hole for the 3d printed component to not collapse

There is not much improvement when we are moving into the 5^th to the 6^th iteration. We just opted to go for a lower density filament, hence reducing the weight of the entire platform.