techBASIC Flies on a Stunt Kite!

Yes, that’s an iPhone on a stunt kite!

It started last weekend, when I got together with some friends to give techBASIC a try. Just in case you are unfamiliar with stunt kites, these are the kites that have two (or in some cases more) strings, so you can actually control the kite in flight. It’s pretty easy to do loops, figure eights, and other patterns. People who are really good with one can do some really amazing things. I’m not that good, but have a lot of fun anyway. The one I used is the Prism Quantum.

The goal was to find out what sort of G forces and rotation rates I was getting with this stunt kite. It’s powerful enough that I have to plant my feet just right in a moderate wind. I’ve had it drag me across a gravel path while my feet were firmly planted. Surely it could lift a little iPhone, right?

I wrote several programs in techBASIC, one to collect data using the Samples.sample method, and three to print the data, one each for acceleration, rotation rate, and magnetic field strength using the iPhone’s accelerometer, gyroscope and magnetometer. Each of these showed the values along each of the three axis.

By the way, did anyone else notice that the iPhone 4 has three sensors, and with techBASIC you can record all three? Gee, I guess we could call that a tricorder.

I wasn’t brave enough to just tape my iPhone to the kite and got for it, though! First I carefully weighed the iPhone on a food scale, then taped the equivalent weight in lead fishing weights to the main spar of the stunt kite. The kite needed a lot more wind than normal, but flew surprisingly well. Putting the weights roughly at the center of the kit’s natural turning point, I didn’t even see that much difference in the turn speed.

With that little experiment out of the way, I replaced the weights with the iPhone. I started by placing the iPhone inside a case and putting a screen protector on it, so stray tape glue would not be attached to the iPhone. Like any good home-brew project, the next step involved duck tape, carefully leaving the screen exposed so I could run the program just before launch. And off we went!

techBASIC records the acceleration, rotation and magnetic field strength about three different axis. You can combine these with a little algebra to get overall acceleration, and the GForce.bas sample that comes with techBASIC shows. In this case, though, I really wanted to see what was happening along each axis. Here’s the plot for acceleration. Incidentally, keep in mind that the color scheme on the image does not match the color scheme in the plots.

The green line shows acceleration as the kite zoomed up and down in the wind. You can see that the highest acceleration was up. That’s in the frame of reference of the kite, though, not the Earth, so it often accelerates to my left or right while the acceleration shown here ends up being upward. Red is left or right in the kite’s orientation, which is odd. I really didn’t expect the acceleration to hit the 2Gs the iPhone can register in that direction.

The really interesting thing is acceleration in Z, shown here in blue. Notice the rapid acceleration changes around 6-7 seconds into the flight. That’s when the kite buffeted violently as I caught a strong wind. I suspect the weight of the iPhone was a bit too high or low, causing a minor instability. This isn’t something I usually see with the kite, and I only saw it with the iPhone attached whenI got a good strong wind.

Here’s the rotation. The iPhone measures rotation in radians per second. Here you see it in revolutions per second. The program shows that the conversion is pretty simple: you just divide by 2π.

The interesting thing here is the rotation about the Z axis, shown in blue, which shows the turn speed I got from the kite. You can see that this maxed out at about 1.5 turns per second. The green line shows the rotation as I tugged on the strings. This tracks pretty well with the blue line, showing this kite is really pretty responsive. The red line shows rotation from buffeting in the wind.

We had a pretty good time collecting and analyzing the data. If you’d like to try this yourself, start with the source code (below), which you can load onto your iPhone or iPad using iTunes. I actually used both--the iPhone to record the data, and the iPad, with it’s larger screen, to analyze and plot the data. Like programs, data files can be moved back and forth from one device to another with iTunes. The download also includes the data file used to create the plots from this article.

The programs should work for lots of things besides a stunt kite, too. After all, they are general programs for collecting data and plotting it--there was nothing specific about kites in the programs. The same day, we taped an Phone to the leg of a ballerina!