In the past week we printed another prototype of our last design, using a Stratasys printer. Unfortunately, the minimum layer height of that printer (0.25mm) proved to be too large compared to the Ultimaker 2+ (0.1mm), which made our design impossible to fit properly at the joint and enable the flapping motion. Therefore, we decided to keep using the Ultimaker 2+ for our future prototypes, since it provided us with the necessary tolerances needed to create functional joint connections.
As seen in our last blogpost, we were able to print a simple prototype, which we assembled and evaluated, primarily assessing the movability of the joint and its ability to fly. The joint movability was assessed based off of the ornithopter’s ability to perform the intended flapping motion at a high frequency when powered by the rubber band. The videos from the last blogpost show that the printed prototype had very good joint movability and wing functionality. Because of this we considered the joint design to be sufficient for our purpose.
However, in the same video from the previous post, it can also be seen that the flapping motion does not greatly enhance the flying ability of the ornithopter, since it flies the same distance with and without the flapping motion. Furthermore, the flapping motion leads to instabilities in flight, which resulted in shorter flights. Additionally, the design of this ornithopter prototype was solely functional, which does not conform with our initial plan to build a batthopter.
First, we worked on the improvement of the functionality. The most essential change was moving from a free/fixed wing configuration in which one wing was fixed to the main frame and the other was free to rotate, to a dual independent wing configuration where both wings are separate from the body and move relative to the main frame. From this modification, we hope to improve the propulsion generated by the wings and also create a more stable flight due to the center of gravity remaining virtually constant while in flight. In order to create such a modification, we added another joint to the main frame and changed the linkage, transferring the rotational movement of the axis into a flapping motion of the wing.
Based off of the weight and rigidity of the previously printed components, the overall size of the object was scaled up to make a sturdier construction, which would allow for these modifications. In addition, to enhance the stability of the crank shaft, there was added support in two places, which will allow for more torque to be applied to the shaft, without the crank orientation changing. Also, thinned features on the wing tips were added to the design to allow for some flex while the wings are flapping. This feature was actually inspired from failed prints early in the design process which resulted in very long thin plastic strips being printed, which exhibited strong, yet flexible behavior that we felt would be a unique feature to incorporate into the design.
Two more measures, aiming to enhance the flight ability, were incorporated into the new design. An adjustable back flap was added to improve flight stability and allow for adjustment in the direction of flight. This back flap was fixed to the frame by a snap-fit, which will allow for fine adjustments to be made to the orientation. Another major change is that we decided to try out a more complex build design this time, which also should improve flight performance due to more weight at the center of gravity. Lastly, we include a mechanism to load the rubber band easier, since this turned out to be a cumbersome task with the old design. The completely redesigned ornithopter is shown in Figure 1.
Figure 1: First functional design draft of dual flap ornithopter
The design in Figure 1 is very functional and provided a good base to work from for the next generation of batthopter designs. After discussing the functionality of the design as well as the aesthetics with our team, were made further design adjustments to refine its appearance in order to better resemble a bat and optimize the design further for improved flight performance. This new design is shown in Figure 2.
Figure 2: Optimized batthopter with more visually appealing design
This optimized batthopter design has a reduced diameter in the linkage connecting the drive shaft to the wings, a slanted body, and thinner frame work to reduce material weight. The optimized dimensions for material thickness were based off of the structural stability of previous prints. In addition to these modifications, the optimized design adds a more artistic component to the batthopter and makes it better resemble a bat.
Here is a list concluding the main features of this prototype:
- Much larger size and sturdier construction
- Wings independent of the main body
- Wing features designed close to those of bat wings
- Adjustable tail section to control flight pitch
- Improved loading mechanism for rubber band
- Optimized part thickness based off of past print performance