A Fata Morgana (Mirage) Straight from the Printer

About two years ago, I decided to build a 3D-printed impeller jet from scratch. I’ve always liked the F-5 Tiger. So I sat down at the CAD program and began drawing the first lines. The basic shape—a solid 3D model—was modeled relatively quickly. At the same time, I printed the first test parts to figure out how many reinforcements would be needed in the fuselage and wings. It turned out that the entire structure would end up being rather heavy. This made me increasingly skeptical about whether the F-5 Tiger was really the ideal model for my project. I was very concerned that the model would behave erratically during takeoff and landing. Since the runway length in Riggisberg is limited, I prefer models with docile takeoff and landing characteristics. I therefore decided to put the Tiger project on hold and start with a Mirage III instead.

I used a 3-view drawing as a template for the shape, which I scaled to the actual model size in CAD. The basic process for the design can be described roughly as follows: First, a solid model is created with all the cutouts for the air duct, battery compartment, servo recesses, connectors, etc. Next, the solid model is “hollowed out.” This generates the actual wall thickness of the outer shell. Then it’s time for the “interior construction” with reinforcements, ribs, and the base, so that the individual parts will fit on the 3D printer. As simple as it sounds, this work was actually very labor-intensive. Especially when countless calculation errors start to pop up, you really need a lot of perseverance.

I printed the first prototype using PLA. The material is very inexpensive, easy to print

(minimal warping) and offers high rigidity. However, a significant drawback is its low softening temperature of approximately 55°. Caution is advised, especially in the summer. Dark colors on the model should be avoided. Printing all the parts took about 10 days on my homemade 3D printer. The machine has a build volume of 400 × 400 × 400 mm. Thanks to the CoreXY kinematics, which feature very little mass on the moving parts, high accelerations can be achieved, which has a positive effect on print time.

The individual parts can be quickly glued together using thick superglue. Thanks to the high precision of the fit, this is a real pleasure. Afterward, all components could be installed. For the powertrain, I’m using a Jet-Fan 90 V2 in combination with a Leopard LBP4082-B 2200-kv motor. The controller is well-rated at 150 A. Power is supplied by a 6S 5400-mAh 45C Swaytronic LiPo battery. The drive—

The kit is from JP-Hobby. To control the rudder and the wing, I chose the KST HS08-A. For the landing gear doors, I chose the Emax ES3301. I ordered all the components from Hebu and Swaytronic.

The maiden flight was scheduled for March 2021. It came to an abrupt end after just a few seconds. Once I had built up enough speed, I pulled the Mirage off the runway. The fatal nosedive occurred immediately... Troubleshooting revealed a completely incorrect center of gravity,

, which my gut feeling had also confirmed. However, I trusted the center-of-gravity calculation tool, which in hindsight proved to be a mistake. Since the flight battery couldn’t be repositioned, over 500 g of weight had to be added to the tail for the next flight. Heavily overloaded, I proceeded to the next takeoff. Even as I accelerated, it became clear that takeoff would be a close call. And so it was: the Mirage took off but crashed again shortly afterward. I was probably too hesitant with the

pulling up with limited thrust... Nevertheless, those present at the field were of the opinion that the Mirage should be able to fly.

Now it was time to redesign the model in CAD again. The impeller and battery positions had to be revised. Countless hours went into the project, so it wasn’t until a year later that the optimized version was ready for flight testing. This version flew quite well right from the start, but it was still significantly top-heavy. Initially without canards, later—

For me, the Mirage project has been a success in many ways. My CAD skills improved significantly, especially in free-form surface modeling. I also learned a lot about aerodynamics and airfoils. Last but not least, I honed my understanding of Mach

in the field of FDM 3D printing. I wouldn’t want to be without my 3D printers for anything in the world—especially not when parts for the new model are being printed while I sleep. The next project is already in the works!

*Images: Michael Bucher, Ueli von Niederhäusern*