The First X4's FateAfter I got my first Hubsan X4, I spent the next week or so smashing it into all manners of objects at high speed. Trees, the ground, cars, even some unlucky people. No matter how soul-wrenching the sound of the impact was, it always turned out that the X4 took no serious damage. A bent propeller perhaps, but nothing that would ground it for more than a minute or two. The thing seemed essentially indestructible.
So when a friend of mine expressed interest in the X4 while he watched me fly it around one day, I thought nothing of asking him if he'd like to try flying it himself. After briefly explaining the theory of flying a multi-rotor aircraft, I handed him the controls and told him to take it slow at first.
Within 30 seconds, my first Hubsan X4 was laying in the dirt, shattered into pieces. In truth, I'm still not sure how he managed it. I'd seen the thing take much worse hits than that before. Perhaps all the beatings I put it through weakened it, and this was just the last straw.
Since the Hubsan X4 is so cheap, I ended up just buying another one for about $30 rather than trying to repair the first one. But I had a working flight controller and motors, and a few weeks to kill while the replacement X4 came in the mail, so I decided to see if I couldn't reuse the functioning components of the old quad to build a totally new one.
A Weighty QuestionQuadcopters in general have to be pretty lightweight, and a micro quad like the X4 especially so. I thought for awhile about what I could use as a frame material, but everything I could think of was either too heavy, too flexible, or too difficult to work with. Since I don't (yet) have a 3D printer, whatever material I chose needed to be something I could form into an appropriate shape without too much difficulty.
While in the Home Depot, I found some 5 mm thick plywood in the scrap pile which looked like it would work perfectly. It was very rigid, easy enough to cut and sand to shape, and seemed fairly light. But would it be light enough to build a quadcopter out of?
I knew how heavy the original frame was, and I knew that I was able to carry about 9 grams of payload around with that, which gave me a considerable buffer zone; the wooden frame could end up being 8-9 grams heavier than the original plastic, and I knew it would still at least fly.
The first step was to cut a square out of the 5 mm plywood and exactly measure its length and width (134 mm x 148 mm) to get its surface area. I then weighed the piece of plywood, and divided its weight (52 G) by its surface area. This gave me a density value (0.0026) that I could apply to a 2D model of the frame to get its estimated weight. I started drawing different frame shapes at 1:1 scale on paper, then measured them with calipers and calculated what that shape would end up weighing when cut out of wood.
In the end, I managed to use this method to estimate the weight of the final cut-out frames to within 1 G, which was more than accurate enough for what I was trying to do. All I wanted at this point was to figure out how large I could make the frame before I went overweight.
Building the FrameOnce I was fairly confident that my math was working, the rest was pretty simple. I just cut out the frame to a shape and size that I calculated to be within my weight limit, drilled some holes in the arms, and mounted the motors and PCB to it.
I had considered making some kind of little landing gear so I wouldn't smash up the PCB on landing, but in the end, the bottom of the motors serve that purpose. I also didn't have enough slack in the motor wires to route them down the quad's arms, so they sort of hang in the middle like a spiderweb. Definitely a rough and tumble bird.
The End ResultThe final wooden frame ended up making a quad that is slightly larger than the original Hubsan X4, but weighs in at just 0.31 G heavier than stock (39.23 G for stock, versus 39.54 for wood frame). I was honestly quite surprised, and in hindsight, there is no reason a larger frame couldn't be made out of the same material and still come in under the weight requirements. The motor leads would simply need to be extended to run out to the longer arms.
But the real question: How does it fly?
Well, it flies a bit like a piece of wood with some motors slapped onto it. It's slow and kind of clunky, but it certainly does fly.
If I had to guess, I would say the arm segments are much too thick in relation to the width of the propeller blade, which means a large amount of thrust is getting blocked and diverted upwards. Thinner arms would of course fix this, but at some point you'll loose structural integrity with this sort of material. As it is, drilling the holes in the arms for the motors caused some worrying cracks.
But you don't have to take my word for it, with the magic of modern technology, I present to you the wooden Hubsan X4's maiden flight in glorious 1080p: