Tinkering with a cheap RC car

A long time ago I bought a cheap RC car to experiment with. I figured I could use its two-motor tank-drive in another application- I wanted to see if i could re-purpose the circuit for something completely different (I considered an RC door lock for a while, but never got around to it). I also wanted to mess around with the Infrared “transmitter” and find some application for that too- I didn’t find one, but I did learn a lot. Here’s what I discovered.

The circuit board was relatively simple- the motor inputs were on the left and right sides, and the infrared sensor is by the charging port. The battery is a 100mah LiFe that charges directly through the board from the transmitter. I was planning on using a phone charger wall-wart to charge the battery by swapping connectors, but I took it apart and salvaged the motors instead; I didn’t think the car was going to last very long anyway.

Circuit board with motors and battery still attached. I might have used the IR sensor for something, but at the time I didn’t know if that was possible or not, nor did I know how to use it for anything.

The motors are similar to the kind you might find in a cell phone for vibration. I had some gears from a wristwatch that press-fit snugly onto the shaft, so I played around with making a set of gearbox plates, but I got bored of that pretty quickly. You can check out my post on that, too.

I’m always amused by the batteries that manufacturers throw into these toys. Before I came across this RC car, I’d never seen a LiFe battery before, so they were new to me. A quick search online told me that they were basically bad LiPOs- they don’t hold their charge as long, can’t deliver current as efficiently, and don’t last very long. They are cheap in comparison to LiPos, however, which is a plus.

I have since thrown the board away, but I still have the motors- I’m considering building a gearbox driven by one of them to turn a 5” propeller that’s collecting dust.

24” Flying Wing (stepped airfoil)

24” wing

I started this project simply to have a small crash-resistant plane that I could fly in my front yard. For the first time, I looked into flying wings for the design. All I knew was that they could be more efficient and were a little faster. After a bit of research, I learned that they had quite a few advantages over traditional plane designs. Stepped KF airfoils also offer quite a few advantages over traditional airfoils. I’ll discuss what I learned and give a quick build log on my 24” flying wing.

Notice how the steps don’t reach all the way to the front of the wing

I wanted to keep the wing as light as possible. I had some 2.5g servos lying around, so I soldered standard PWM cables to the wires that came stock with mini JST connectors with signal, ground, power from left to right. Soldering proved to be quite a challenge, as micro servos have extremely small wire. I don’t know how much current these 2.5g servos draw, but it can’t be more than .25 amps. To get the insulation off I just clamped down on the ends of the wire with my fingernails and tugged it loose. This works quite well.

Mounting the receiver just took a little hot glue:

I just hot-glued the receiver to the 2nd step. It’s placement shouldn’t affect the wing aerodynamically, as it is rather small.

Also glued in the ailerons. I never got a chance to take pictures of the motor and battery after they were mounted, but those steps are self-explanatory. I made it a tractor, because I didn’t want to have to deal with CG issues if I moved the motor to the back for a pusher configuration.

The aileron servos should be squared up, perpendicular to the control surface. Mine aren’t, because I was in a hurry. If you do ’em right, you’ll have perfect throw on both ailerons. I had to adjust them a little with my radio.

I never thought that sweep angle on a wing could be as intricate of a subject as it is- There are many factors that contribute to the performance of the wing to consider. The most important consideration is drag- the sharper the angle, the less drag, which results in a faster wing. I went with a mild 140 degrees for a little more drag to slow it down enough to fly in my front yard, which is rather small.

With a protractor I drew  a 140 degree angle on a full sheet of foam board. I wanted my overall span to be 24” but I didn’t know what wing chord to use. For aesthetics’ sake, I chose 10”. In the future, I’ll just wing it (pun intended)- after two very different chords I found no significant change in performance.

Notice how the steps don’t reach all the way to the front of the wing

The first KF step is 1/2 of the entire wing. I just laid it down on the remainder of the sheet of foam board and traced- then I split it in half and glued it onto the wing, offset 1/4” inch. I don’t know why exactly I did this, but I felt that a gradual sweep was needed- maybe I felt it necessary to emulate the profile of a traditional airfoil- what I did know was that there was nothing aerodynamically responsible about having a wall of foam before the KF step.

3 steps straddled about 1/2” per step

After putting it all together, I did a few test taxis around my driveway. It lasted 5 flights before I accidentally landed it into a tree.

RIP, 24” flying wing

24” wing

Use the pictures for reference if you want to build one!