Poly

Sometimes it's nice to carve out some time to build something that would make the 12-year-old version of you give you a high-five -- and, in this case, a five-year-old nephew.

Few things are cooler than RC airplanes.

A view of the servos and control surfaces.

My nephew loves those giant (4' wingspan), $10, polystyrene gliders so the girl and I decided to spend some time experimenting with aircraft building.

It really does fly over 100' and is surprisingly durable.

 

The Build

We started by cutting out aileron and elevator flaps and using duct tape to create hinges. Fixing the various control surfaces at different angles allowed us to experiment with their effects. Next, we decided to add some servos to create an RC glider. The fact that the wings popped out during crashes seemed like a feature so we gave each wing its own servo -- the common, but less robust, alternative is using one servo mounted to the fuselage that is linked to both wings. We mounted one additional servo to the tail of the fuselage for the elevator. Even though the glider used a tiny 3.7V, 220mAh battery that was scavenged from a quadcopter, the whole aircraft was simply too heavy to sustain flight as a glider. The four motors from the now-battery-less quadcopter were still perfectly good so we built some brushed DC motor control boards and glued the motors to the fuselage. To power the whole thing we found an inexpensive 2-cell LiPo battery and a cheap Battery Elimination Circuit (BEC) to provide 5V to the servos, motors, and receiver. The result looked sweet but was woefully underpowered.

Rev 1: Lots of duct tape and disappointment

For the second rev, we replaced the duct tape with plastic hinges to save weight.

Cut slits in the wings, dab some StyroGlue on the hinges, and let it set.

To protect the servos on the wings the girl purchased small polystyrene globes from the craft store, cut them in half, and hollowed out spaces for the servos.

The globes are easy to hot-glue to the wing and look very natural.

After upgrading the hinges it seemed like a good idea to add landing gear. The local hobby shop had some foam wheels that we picked up for less than $10. The back wheel had its own plastic mounting plate that was easy to glue to the fuselage.

The rear elevator's servo is easy to embed and we found the perfect landing gear.

The front wheels fit nicely on coat hangers so we glued the coat hangers into wooden dowels and then glued the dowels into the fuselage.

The white is a plastic coating on the coat hangers -- we stripped that part off any spots that needed glue.

 

Mounting The Motor And Keeping It Cool

Now we were committed so we purchased a cheap "park flyer" kit from Amazon with a 1000KV brushless motor, 10"x4 prop, and ESC. We screwed the new motor into wooden dowels that had been glued into the front of the fuselage.

Dowels mounted directly to the motor provide very little crash protection and no thermal insulation.

We carved out some space at the bottom of the fuselage for channeling the motor's wires. The result was hilarious to look at but after a few crashes it was difficult to get the motor aligned correctly with the rest of the fuselage.

Looks a little fishy.

To make the motor easier to mount we ended up machining an aluminum plate that lays flat against the fuselage and includes two tabs for mounting the decorative cowling. This plate improved crash resistance and also helps to disperse the heat from the motor.

A couple of hours on a mini-mill can solve so many problems.

The motor and Electronic Speed Controller (ESC) can get quite warm so we used milkshake straws to ensure that the motor wires don't completely impede airflow in the ESC's cavity.

Milkshake straws, bubble tea straws, who can tell the difference?

The ESC lives in the skinny cavity, the receiver lives in the wider cavity beyond the tape.

The airflow from the front of the plane needs a place to exit so we mounted some additional straws on both sides of the fuselage at the back of the ESC cavity.

The exit straws have the fortunate side effect of looking really cool.

 

Final Tuning

To test various combinations of batteries, motors, and propellers we threw together a simple rig to measure thrust vs. amperage. It's not perfect, and certainly not calibrated, but it allowed us to make relative measurements for different setups.

TV tray + right-angle shelf brace + hinge + clamp + kitchen scale + cheap multimeter with 20A fuse = thrustometer

The great thing about chalk boards is that you can fill them with random notes and there's no hurry to erase them.

We waited until the plane was mostly finished and we had chosen a battery/motor/propeller combination before we carved out space for the battery so we could use the battery's location to balance the plane. Unfortunately, this required putting the battery pretty far back in the skinny part of the fuselage; this is the most likely part of the fuselage to break. We addressed this problem by lining the battery cavity with popsicle sticks for reinforcement.

The popsicle sticks were glued together to make sheets before they were hot-glued into the fuselage.

 

The Receiver

We used the SparkFun nRF52832 Breakout board as the receiver because it offered a nice form factor. Most servos and ESCs run at 5V but the nRF52832 tops out at 3.6V so we used a cheap level converter to interface between the nRF52832 and its outputs.

The level converter along with a capacitor to add some bulk capacitance to the 5V rail for brown-out resistance.

We also tacked a simple programming harness on to the breakout board so it could be programmed via an nRF52-DK. The two boards were joined together using 0.1" break away headers.

Sparkfun sandwich

We used right-angle 0.1" headers on one side to interface to the ESC. On the other side, we used labelled micro-JST connectors for the various servos (roll, pitch, yaw).

Power and throttle signal in the front, roll, pitch, and yaw in the back.

The ESC connected to the receiver along with the battery connector.

This harness connects the receiver on the left with the servos on the right. The large connector on the left is a 5V power and ground pass-through from the BEC.

 

The Transmitter

The transmitter is an nRF52840 development kit that is mounted on some foam board along with a couple of analog thumbsticks and a AA battery pack. The nRF52840 has +8dBm output power and the nRF52832 on the receiver has -96dBm RX sensitivity so range has not been a problem. Bluetooth Low Energy isn't particularly well-suited for this application so we created a proprietary, FHSS, 2.4GHz protocol that allows the transmitter to send packets to the receiver without waiting for a response.

Easy to carry, easy to hold, easy to take apart because I'm only borrowing the nRF52840-DK.

 

Conclusion

I can't really comment on the plane's flight characteristics compared to other aircraft because this is the only plane that I've ever flown. However, flying is not easy so it's nice that polystyrene can be repaired quickly. We also learned some things:

• StyroGlue works very well for small parts like hinges
• Hot glue works the best for large parts like dowels and fuselage repairs
• The big sanding drum in the Dremel kit chews through polystyrene nicely
• Amazon has fire suppression blankets and small fire extinguishers that can be carried discreetly