Future Hovercar Model
During second year, I embarked on a fun personal project aside from my course, to design and build a model "car" that visibly hovers (without the use of a skirt). I took inspiration from the 1960s era "Cushioncraft CC2". This was a also a skirtless, futuristic-looking design.
I appreciate the minimalism of this vehicle and its sleek, modern-looking appearance and aesthetic.
I had a performance design in mind a design for this model, that would allow it to travel at great speeds. Because of this, I chose to only give it one fan. This would lead to a forward centre of gravity which would cause the model to tilt forward, completely rendering it unable to stand stationary. Another reason for this was that I lacked another motor, another battery, and an ESC for both of the two, preventing a design like the CC2. I chose to go with a more sleek, aerodynamic appearance, as well as a modular design with a removable top section. This would allow for easy maintenance (theoretically).
Overall, this whole project would really be a test of the ground-effect lift that such a model could produce with this type of design.
I wanted to make the design as slim as possible, so that it could fit in a bag and look incredibly futuristic. This meant radically rethinking the way such a model could produce thrust. I envisioned a type of variable diameter nozzle, similar to the way that fighter jets vary their convergent-divergent exhausts when lighting up their afterburners. The problem with this was incorporating a "valve" essentially in such as small space right next to the spinning blades of the impeller internally, as well as the rudders, which also had a relatively large mechanism of their own. This proved cumbersome and so the vale idea was ditched altogether. My idea was then to have the model simply "lie down" on the ground when it wanted to remain in one place and then simply fire up the lift motor when it wanted to move, tilting it forwards in the air and subsequently moving it as well. I liked this simple design as it reflected the simplicity of the whole model alltogether.
I planned to add lights to the craft eventually to improve the futuristic aesthetic of the whole design.
Building the Hovercar
Not only was this the first time making a model of this variety (without a skirt), but also one that fully incorporated a working centrifugal impeller.
Unlike the renders included earlier, the first iteration of this model used an inefficient design of impeller which I later realised was designed for a much higher RPM range (much higher than the average brushless hobbyist RC aircraft motor could run at). In hindsight, this design actually appears to be for transonic speeds, which is understandable now I look at the sharp leading edges of the blades - hence why it is used in high power, high RPM environments like a car's turbocharger or compact, axial-flow jet engines, not low-subsonic brushless outrunner motor mechanisms.
Despite this though, it did actually produce enough power to lift the craft temporarily.
Explosion
Unfortunately, upon testing the impeller compressor fan at max throttle, it exploded. Whilst most of it largely remained intact, parts of the impeller went everywhere.
Why this Happened and the Solution
It became very obvious to me after this, that as I was unable to perfectly balance such a fan with my limited equipment, I would need to make a compromise.
Luckily for me though, it seemed that I was already using a fan designed for much higher RPMs, which only really produced a significant airflow at higher RPMs. This meant that there were significantly higher stresses being experienced by the fan when running at optimal speeds for flight. Switching to a slower speed, enclosed impeller design (commonly seen used in "blowers"), meant less stress experienced, less probability of material failure, and a much more efficient design for producing the most airflow at the speeds this motor was designed for.
Just before the fan exploded, I got a good view of the hover performance of the vehicle. Although it somewhat lifted up off the ground, there was virtually no height on this at all and I deemed that unacceptable as I was aiming to achieve a very noticeable hovering performance by this model. This called for a complete redesign.
Future Iterations of this Model
Upon the model breaking and additionally not achieving the optimal height, it occurred to me that the model was extremely heavy for its size. Being an aircraft, this was detrimental to its performance, and upon evaluating the causes of this, I found that it was mainly the use of so much 3D printed PLA in the design.
I have planned a complete redesign of this craft in the given the time to manufacture it all again. Making this whole thing out of either foam or balsa just in the same way that one would make a model aircraft or drone is ideal, as these materials both have an extremely low density and high strength.
Not only this, but given that I observed the model's extreme tendency to pitch forward due to the centre of gravity. I knew from previous experience that this model would inevitably hit speeds which cause it to become extremely unstable. I was worried about the front catching too much air underneath and it flipping over backwards in the lateral axis at these speeds, but this was unlikely due to the pitched down position.
To hopefully combat this problem, I added some aerodynamic devices to the craft's design, such as a front and back spoiler. I imagine that it might need some side aerodynamic devices, however, when I get around to rebuilding this I will test the performance without first, as it may not need them.
The front spoiler should stop the hovercar model from flipping over.
