Written by Fitz Walker A fun and durable profile bipe As seen in the Spring 2020 issue of Park PIlot. Review
Bonus Video
Specifications:
Model type: Indoor 3D Skill level: Intermediate/advanced Wingspan: 33.46 inches Length: 35 inches Flying weight (listed): 7.93 ounces Flying weight (as flown): 8.5 ounces Motor: ROXXY C27-15-1050 Kv Propeller: GWS 9 × 5 Battery: 3S 350 mAh LiPo Construction: Carbon-fiber reinforced EPP foam Radio gear: Hitec Aurora 9X transmitter; Hitec Optima 9 reciever; Hitec HS-45HB and HS-65MG-plus servos Price: $76.99
I’VE ALWAYS BEEN fascinated by flat-platestyle flying models and the way they seem to throw Bernoulli’s principles out the window in a triumph of brute physics over elegant aerodynamics. Like the bumblebee, science says they shouldn’t fly, but they do—and quite well.
In the tradition of some model airplanes dragging Mr. Bernoulli through the mud comes the Multiplex 3D-capable Challenger indoor flyer. Made of EPP foam and sporting a distinctive paint scheme, this biplane looks to offer high-performance aerobatics that are suitable for indoor flying in a small package.
Unsurprisingly, a model composed of flat parts comes in a flat package. All of the subassemblies, parts, and manual are neatly packed in a surprisingly thin box. The body and wings are constructed from durable, 6 mm EPP foam that is preprinted with the distinctive color pattern. Control surfaces are hinged using a beveled crease in the foam.
The manual is printed in multiple languages that share a common set of black and white photos. Assembly steps are on separate pages from the photos, so I recommend carefully reading all of the steps and associated photos before starting. None of the parts are labeled, but the manual text describes each part, and I generally had no issues finding the parts I needed with the help of the photos.
It wasn’t immediately obvious, but all of the plastic parts are 3D printed using Fusion Deposit Modeling—the same type of 3D printing commonly seen in hobby use.
Although the manual recommends adhesives that are common in Europe, you can (and I did) completely build the model using medium CA glue. Note that accelerator (CA kicker) can be used, but it should be done sparingly for the best strength.
Build
Assembly starts with the tail pieces. Those parts are reinforced by embedding various lengths of small carbon-fiber rods. The entire model is structurally strengthened by a veritable framework of carbon-fiber rods that must be cut and glued into place as the model is built up into a three-dimensional subject.
Factory precut slots and grooves are provided for all of the reinforcement rods to slide into. I recommend labeling all of the carbon-fiber pieces and using a caliper to note the various thicknesses of the rods.
This is the most involved and time-consuming step, but without the rods, the thin foam would lack structural integrity and be too flimsy for controlled flight. It is easy to do, but it will take a little time and patience.
Nearly all of the parts key together in unique ways to prevent assembly errors. However, I had a brief pause when I needed to reference various pictures in order to determine the proper orientation of the wings. In fact, I came uncomfortably close to gluing in the bottom wing upside down.
I used the recommended motor package, which includes a 15-amp ESC, a 1050 Kv brushless motor, and 9 × 5 GWS propeller. Note that the propeller hub’s hexagonal depression didn’t fit the stepped washer hub for the motor shaft, so I slightly drilled out the hub with a roughly 8 mm drill bit. I also enlarged the cutouts for the tail servos with a hobby knife so that the Hitec HS-45HB servos would fit. The manual appears to have a slight error in the brass coupler length for the ailerons (14 mm vs. 20 mm). It is correct on the parts list, but not on the assembly text.
The receiver, battery, and ESC are simply attached to the airframe with Velcro. With the radio hooked up, the control throws, especially the ailerons, look to have excessive deflection. However, lots of exponential is prescribed by the manual to tame things down.
Flying
Although the Challenger is generally meant for indoor flying, my first flights were out in the open on a day with a light, variable breeze. Despite the scary control throws, the model handled smoothly and was not at all twitchy. As a precaution, I programmed in an aileron low-rate setting, but quickly went to the normal rates when it was apparent that I had nothing to fear.
This is not a model that you fly fast, so you need the extra throws. All of the controls were crisp and smooth. I was immediately at ease experimenting with all types of maneuvers and throwing the sticks in the corners. Surprisingly, it was very controllable in the roughly 5 mph wind during my outing.
Control authority in forward flight is maintained to basically zero airspeed. This is great because this airplane excels at flying slowly. It also didn’t seem to care much at what attitude it was placed. I just pointed it in the direction I wanted it to go, wings level or not.
Knifeedge flight can be done at crawling speeds with rudder authority to spare. Knife-edge loops are also easy to do. The roll rate is brisk at full deflection.
I don’t claim to be an expert 3D flier, but the Challenger seemed to perform all of the advanced aerobatics in my mental library. I also found that the model didn’t seem to accelerate too much on the down verticals, probably because of the drag of the carbon-fiber rod reinforcements that were hanging out in the breeze. This low acceleration rate looks better because maneuvers will have a more consistent speed.
I found it to be a great model with which to experiment. Most maneuvers needed only a light touch of the controls. Although it is not an airplane for beginners, intermediate pilots should have no problem flying it on calm days.
Because I rarely went past half throttle, the flight time using a 3S, 350 mAh LiPo battery was quite good. I easily got 5- and 6-minute flights on a single charge. When I elected to use full power, I had unlimited vertical climb, but horizontal, full-throttle flight quickly hit a drag wall, so don’t expect a speed demon.
Although I wasn’t able to truly test indoor flying with the model, I did fly it in my treelined cul-de-sac with no issues maneuvering in such a confined space. Plus, there is something extra satisfying in being able to fly right outside of one’s front door.
Conclusion After spending some time flying the Multiplex Challenger, I found that its inherent stability, yet high maneuverability, makes it a fun airplane to experiment with new techniques. It excels in the low-speed flight envelope with gravity-defying, slow-speed handling. Both advanced and intermediate pilots will find a lot to like about the Challenger.
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