Why & How I Built an RC Airplane

by Octapoo

Saturday, September 19, 2020 at 02:48:00 UTC

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A note from the future: I posted all of this at once in my Discord server, because the start was just "I found this cool YouTube video" which isn't something I usually blog about, and so it wasn't until two months later that I realized I had something I should be blogging about and I needed to fill everyone in on the back-story. So this starts out like I'm just talking about some YouTube videos I watched, but it gets more interesting as it goes on, particularly in the posts that will be following this one.

So click "read more" and enjoy this blog finally starting to become interesting.

Sometime last year YouTube recommended this video to me...

...which led to this video...

...and somewhere in the middle of them I found myself thinking "I wish I had one of those cool radio transmitters." I just thought it was so cool how generic it seemed to be, in that it had controls that seemed to have no predetermined purpose, and so he was able to set up a switch to change his boat from autonomous mode to manual control.

Anyway, YouTube recommended the video to me again a couple of months ago, and I got to thinking about how I have a lot of the parts he was using in the video already, due to having bought RC motors and speed controllers because I thought I was going to build a CPAP machine from them and they were the cheapest powerful motors on eBay. I also had 10 servos I bought just for whatever since they were only $1 each and they looked useful. So I looked into what one of those cool radio transmitters would cost and, while the one he uses in those videos is very expensive, there are also cheap ones, and so I could have one for only $65. So I decided I had to have one, even if I didn't know what to do with it.

This is the one I got:

It comes with this little receiver:

It is rather generic in how it operates. Six of the outputs are what are known as "PWM" and simply output a pulse 50 times a second whose length is proportional to one of the controls on the transmitter.

This is the standard for how RC stuff works, and so any servo motor or speed controller can plug straight into any of these six outputs. They're all identical, so there aren't any rules like "this one can only be connected to motors" or "this one can only be connected to a steering servo."

There are also two digital signals which are proprietary and specific to this transmitter and receiver. I wrote some AVR code to interpret the signals from them, but haven't done anything with them yet. One just gives all of the servo position commands digitally in a 32-byte serial packet. The other is for attaching sensors, which it seems like the company didn't actually develop very many of, and didn't document at all. It took a lot of digging on the internet to figure out how to talk to it, since it's a command and response type of thing, but I now have code for that too, so I can send a few bytes of information back to the transmitter and have it displayed on the LCD. I was hoping to be able to extract the data from the controller digitally, so that I could perhaps send back GPS data and have a laptop map it, but despite the PCB having an insane number of test points, the vast majority of them aren't doing anything. The internet tells me that the PCB has a test point for every pin on the microcontroller, whether it's used for anything or not. I did find where the microcontroller communicates with the RF module over some kind of 5 MHz digital signal but that's a bit too fast for me to look at.

Anyway, since I have a radio transmitter and receiver, and most of the parts, I thought "why don't I build an airplane?" Indeed, I watched most of rctestflight's videos, and he makes it look like you just glue some shit together and it flies. So I ordered what I didn't have (propellers and batteries) and set about trying to glue some shit together and make it fly.

My first attempt was inspired by the plane in this video:

Since it was designed to be durable and for a kid to learn how to fly, I thought it would be a good choice. On a later viewing of the video after building it, I noticed he also says that he designed it to be really maneuverable so it would force him to learn to fly well, or something like that. So maybe it wasn't a great choice for a first plane.

However, I don't know because also I didn't have an appropriately sized motor, nor did I order an appropriate size of propeller, nor did I have the ideal battery. So I don't think it was ever going to fly. I eventually added wheels, after growing tired of trying to make it fly and having it nosedive straight into the ground and break yet another propeller, but the wheels just made the thing super heavy and thus ensured it was never leaving the ground. This is what it looked like shortly before I tore it apart and threw it away.

I decided I needed a different approach. I found another YouTube video about an easy-to-build plane which seemed to have a really well-thought-out design and detailed instructions. It's made from foam board you can find in some dollar stores, which interestingly is better for this application than the more expensive foam boards sold in other stores.

It was fairly easy to build, as I took a bunch of shortcuts, since I wasn't sure how well it would work. In particular, I didn't cover it with tape at all. I didn't take any pictures of it. I only recorded a bunch of terrible videos of it because each test flight lasted about three seconds and, after it was over, I couldn't remember anything about what actually happened during the flight, so I wanted to have video to review and analyze.

Here's one of the better videos:

Eventually it started nose-diving into the ground, and after two of those, the motor broke free of the fuselage which broke where there was a trap door to insert the battery. I then noticed that the battery had shifted forward, making it nose-heavy and thus doomed to crash on every flight, but long after taking it home I also noticed that the elevator control was broken, making it always pitch forward. So I guess I should have been checking it over after every crash.

Anyway, after that I planned to just make a new one that was better reinforced for the inevitable crashes.

In the meantime I downloaded an RC flight simulator called "crrcsim" which can be used with my transmitter because it came with a cable that connects to it so that it shows up as a joystick on a PC.

It took a few days, but I learned to fly somewhat OK and land the plane: Warning: The audio is a little loud.

I then spent like a month slowly working on what isn't all that different from the last plane, just covered in tape this time, and with two tube pieces that slide together under the main wing so that I can put the battery in between and thus avoid any trap door that creates a weak spot in the frame. This took a long time to build because I didn't have a lot of energy and so on a lot of days I just didn't make any progress on it.

...but now, it's finished, so I have this thing:

I wanted to take it to a nearby RC plane flying field today, but it was kind of windy, so I decided to wait until tomorrow, and instead I gave it a couple of test flights in my mother's all-too-small yard. Here's the first one:

That went so well that I decided to tape the camera (a small SJ4000) to the plane itself: Warning: The audio is much louder.

With that, I'm happy with how it's working so far. Unlike with the last plane where I didn't have a good sense of what to do when the plane wasn't doing what I wanted, after using the simulator, I knew exactly what to do, and thus I was able to actually control it and see it respond to my controls for a few seconds before it hit each tree.

So hopefully it'll work well at the RC field, though I'm not totally confident because I went there to look at it about a month ago and it's kind of small. I even modeled the place in the flight simulator and, at least with the planes that the simulator has, it's kind of hard to keep them in such a small place, and the field has signs up about how you're not supposed to fly over any of the adjacent land, which seems borderline impossible in the simulator. ...but whatever, I'm just going to try to go when no one else is there to bitch about what I'm doing.

Anyway, if you want more video to watch, here's the latest "rctestflight" video, which I thought was awesome:

That sort of autonomous stuff is something I'd like to try out too, but you'll note that he lands the plane manually, so it seems pretty obvious that I should know how to control a plane manually before trying to do anything automatic.

Oh, and if anyone is wondering, the latest plane I made, the brown one with 3 pictures, is about 650 grams and has a wing span of 58 inches.


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