After the prototype second flight, we had to stop the plane since the motor had lost 4 magnets (well, they just detached from the bell and moved a bit actually). Although the motor itself was repaired the next afternoon, we decided to try a different engine, and ordered a Turnigy 3548 sk3 motor. Honestly I can’t blame completely the original motor. It was running to the max power rating, and even if the first flights were all but “full throttle”, not all the cooling ideas initially designed were 100% ready in the plane yet. Still, we decided to use this forced stop to give the Juliet2 some upgrades.
Optimizing the cooling
The A1-D was designed from the start with efficient cooling in mind. The air entering the two inlets in the engine cowl could move through the motor mount and enter the fuselage, cooling the ESC, with some vents letting part of it out just after the landing gear and the rest continuing it’s travel to the electronics tray and finally being evacuated past the canopy on the bottom of the fuselage. After we traced the motor problems to the heat management, we decided to see what was possible to do to improve the concept further.
We added a third air inlet in the cowling, a flap before the first of the outlets to help the extraction of the hot air, and cleaned the path of the air through the lower fuselage. The area of the exits was almost doubled to improve the flow.
The new motor is not only more powerful, it’s also heavier and uses bigger (read heavier again) propellers. The prototype could use some weight in the nose for an extra safety margin in the first flights, but we wanted to be able to move the center of gravity back with the battery only later on, when exploring the flight envelope. To counter this extra weight in the nose, the ESC was moved from below the engine mount in the cowling to above the landing gear in the fuselage, which was its designed alternative location. I also moved the receiver on top of the electronics tray to free up the area below it.
Not keen on gaining weight on a plane already a bit above the target, I tried to save weight where possible. Without re-opening the structure and making modifications to the project, I could work only on the hardware used and some accessories. Optimizing the choices of materials saved around 40 gramms. It might not be much, but the brainstorming about the possible weight saving translated also in good idea for upgrades to include in the next version of the A1-D.
Testing the Juliet2 Upgrades
With the new upgrades installed, even if missing some cosmetic details, we went back to the field. As of today, the prototype has accumulated around 1hr of flight in this configuration. The flight characteristics were not modified, the plane remains very stable, the low speed test went very good, with no dangerous behaviours close to the stall. We introduced also triple rates to test the full potential of the control surfaces, but a serious 3D flight test will come only after we move the CG back, at the end of the test phase.
With the new motor the verticals are even more unlimited, it’s impressive to see the acceleration the plane develops while going straight up disregarding the gravity. The power available will increase by switching to 4S batteries, but given the results so far and the lack of serious negative effects with running such large propellers, I believe keeping a common battery between the Juliet2 and the Tundra makes more sense.
The Juliet2 keeps flying and giving us important data. All these informations are being considered to design a “production ready” improved version. The need for testing composites has ended with this prototype, we have learned enough to know what to to with the C18 project, so this time we are using mainly the traditional balsa and plywood construction. This makes the plane perfect for laser cutting, and to make a small number of short kits for those interested. In parallel, there will be the restoration and possible electric conversion of an old Ripmax Extra Slim fun fly, and the restart of the works on the twin Beech. So stay tuned for more exiting news!