The company I work for (eSolar) sent me to AUVSI in Denver, CO to have a look at the developers of unmanned ground vehicles. Why would they do something so awesome? Well, I designed and built the autonomous cleaning vehicles for the eSolar demo power plant in Lancaster, CA. Now we’re moving past demo and prototype and we want someone to build a better one. Our expertise at eSolar is heliostats and the software that makes our fields work. We don’t really want to be in the mirror cleaning business, but we need someone to be in that business so that they can keep our mirrors clean.

At AUVSI, I met with various manufacturers of robotic vehicles. They all had interesting stuff, but quite frankly, the component-level hardware that was at the show was just as interesting as the vehicles themselves. I finally saw a harmonic drive in operation, I got to chat with a manufacturer of miniature turbine engines and ask him detailed questions regarding their operation, I saw a nutating engine, and I saw more AHRS’s (Altitude Heading Reference System) than you could shake a stick at.

Anyhow, the most interesting part of the trip was not the show, it was the flight out there and back. Instead of flying Southwest, I decided to fly my RV-6... that you’re pretty well acquainted with by now. On Monday August 24th after work I flew to Davis (EDU) to see my girlfriend and mom. Then early on Tuesday I took off from EDU headed for Ely, Nevada (ELY). At Ely the engine ran a little rough during my taxi to the fuel pit after landing. When I went to kill the engine by leaning the mixture it didn’t die in the usual way but kept rumbling on for a bit. Finally it quit and I got out.

I took the top cowling off to see if I couldn’t have a look at things, but the mixture adjustment is on the carb which is under the engine and having the top cowling off didn’t help me much. I reached in through the gap between the cowling and the airplane to see if I couldn’t tug on the mixture lever and see if it was loose. It wasn’t. I chatted with a buddy and decided that maybe our engine was just adjusted a bit rich and because of that I may have trouble shutting it off at high altitude.

I bought gas, did an extremely thorough run-up and took off toward Denver. The flight went quite smoothly until on landing, the engine started mis-behaving again. Again, I couldn’t quite stop the engine using only the mixture knob so I grounded the mags to kill it. It died, less gracefully than usual but it did die. I hopped out of the airplane and scurried off to the conference.

I didn’t think much more about things until it was time to fly back. Again, I performed a very through run up and the engine ran great t all throttle settings except idle. I decided that idle was the least important of the power settings and took off. I flew from Denver (APA) to Four Corners (FMN) and then from FMN to Whiteman, CA (APA).

Shortly after leaving FMN I flew over the Grand Canyon by veering slightly off course to the north. The Grand Canyon is truly spectacular and I only wish that the pictures could do it justice. Unfortunately I have a bubble canopy and no picture through it is much good.

As I was flying over the Grand Canyon, I began trying to use the autopilot system that we had built for the RV. As you know it runs on an EEE-PC running some Matlab code we wrote. Unfortunately, every time I engaged the altitude hold, the autopilot would try to point the plane straight at the ground. Needless to say there we some bugs in the code. I started sifting through the bugs with the stick between my knees and in about 10 minutes I had the code working properly.

Between the Grand Canyon and Bullhead City, I was able to do some gain tuning, and with quite a bit of derivative gain, some proportional, and nearly zero integral I was able to stabilize the autopilot to some degree. It wasn’t perfect but it wasn’t so bad for a first step. All that was really missing was a low pass filter on the barometric pressure input. For some reason, spikes would appear on the barometric pressure input and these would cause the derivative error signal to spike. Needless to say this made the flight somewhat rough so I quickly coded up a running average filter and it smoothed things out quite a bit.

As I was passing the Grand Canyon and coding, I could already see cumulous clouds building on my route toward LA. I contacted ATC to see if they could inform me on where the weather was. Fortunately, the cumulous clouds were all on the San Gabriel’s just south of my route. I continued toward LA until the clouds were just off my left wing tip. Finally, as I was just south of Palmdale I noticed giant columns of precipitation and lightning. It was quite stunning. I kept trying to capture pictures of the lightning, but without a light activated shutter, or really long exposures, it’s nearly impossible to capture lightning.

When I had finally cleared the thunderheads and storms mentioned in my previous blog post I headed over the San Gabriels and down into the Burbank/Whiteman area. I was fearing some sort of engine trouble because earlier in the flight my engine had been acting up at low throttle settings. I was not sure why it was doing so, but I feared it might do it again -so I hedged with altitude. I must have been 10,000 feet above my intended landing spot when I was only 15 miles out and cruising at 200 mph.

I came in over the San Gabriels pretty high and in such a way that no matter what the engine did I was sure I could make the runway at Whiteman. Sure enough, as I reduced the throttle toward idle, the engine got very rough and began to cough.

By this point I was lined up on a two mile final with about 3000’ to lose so I wasn’t sweating it. In fact I was mostly concerned with getting down in time. I continued an aggressive slip and worked the throttle to keep the engine running. Then again, as I reduced the power to idle, the engine coughed and died.

This put me in a funny position because I knew the engine was running too rich, but I hated to pull back the mixture knob to the point where it could kill the engine from being too lean. That said, I was not going to get down if the engine kept running and I didn’t want to have to go around. I decided that I was clearly going to make the airport and that once I was on the ground who cared if I had in fact landed without an engine.

I had been cleared for a straight in to runway 12 at Whiteman, but I heard the tower chatting with another aircraft in the pattern and it seemed to me that he might try to sneak them in front of me. While normally this wouldn’t be a problem, I didn’t want anyone in front of me. I’ve seen the tower call a go-around too many times because someone doesn’t properly clear the runway and I wasn’t going to get forced into a go-around situation with a rough engine. I told the tower that I was having engine trouble and that I would appreciate it if he would clear me straight in.

The tower obliged, and from that moment on I just focused on getting the plane down on the ground. The engine died a couple more times but I was always able to nurse it back to life by moving the throttle to full. In this manner I coaxed it along every 10 or so seconds with a throttle pump. I cleared the threshold and landed right on centerline. As I was making sure not to screw up at the last possible moment, the engine died again and I had to pump the throttle quickly before the prop stopped.

Anyhow, I taxi’d back to my spot got out and went in to work. I told Cedric that we had serious engine trouble and that we really needed to have it looked at. Well, a local A&P heard about the trouble and our symptoms and decided to come have a look. He reached in through the air intake and shook the carburetor saying: “well there’s your problem.”

It turns out that our carburetor had shook itself off the engine. We tightened the screws and all was well again. In fact he just texted me to say that he had safely landed in Napa where he is spending the weekend.

It’s been very interesting for me to watch the progression of the autopilot for RV-6 project. First we were just going to make an electronic trim system, then we started adding features so that we could use the electronic trim for control of the airplane for an autopilot, and now we’re writing code for control loops for the RV. I’m very much under the impression that scope creep is unstoppable and we’ll probably always be working on the RV-6 autopilot or some idea that sprouted from it.

Since I last posted to the blog, we've made quite some progress:

• We have interfaced the Atmel to a USB-1408FS Measurement Computing DAQ
• We have interfaced the DAQ to a computer running Matlab (without the data acquisition toolbox)
• We have installed an electronic barometric pressure sensor in the RV-6 so that we can  measure altitude with the microcontroller.
• We have written Matlab code to run a control loop aimed at keeping the barometric pressure constant (ie altitude hold.)

With all that work completed it was time to see if it all fits together. This we did with some beer, the Atmel STK-500, a 10 turn potentiometer, the measurement computing DAQ, an eeePC, a servo and Matlab. Here is the layout:

We connected the servo to the Atmel. To the horn of the servo, we glued a piece of balsa wood to act as a wing. We connected an analog input to the DAQ to the wiper of the potentiometer (which is simulating the output of our analog barometric pressure sensor). The servo was in turn glued to the potentiometer knob such that the wing was perpendicular to the rotational axis of the potentiometer. Then we placed the wing in wind and varied the servo to try to keep it at the same spot, which we read in based on the potential of the wiper of the potentiometer. As the angle of attack of the wing was increased it raised the wing, rotating the potentiometer and causing the Matlab program to think that our altitude had increased. Matlab then commanded the servo to a different output by sending an analog signal through the measurement computing DAQ’s analog output to an analog input on the Atmel which we convert to a pulse width for servo control.

Long story short it’s awesome. Check the video below.

I've now added some feedback for the pilot on the trim position. We decided to incorporate the addition of this bar graph so that the trim could be set to "take-off" or other known positions. This will allow the pilot to have consistent force-feedback during take-off which is a critical portion of the flight envelope.

This is all on the dev-board still, but it is working as planned and things are moving along quickly.