Forschen, Bauen, Fliegen with Idaflieg in Stendal

Ever since I started my studies in Computer Science, I started to fiddle with electronics in gliders. I enjoy this very much, except for one small part: explaining this to other glider pilots. Most glider pilots I meet roll their eyes when I passionately talk about electronics in gliders, and mumble something like “But why?”.

Getting a bit tired of trying answer that question, I started to look online for like-minded people. And so around 2010 I found out about Idaflieg, the umbrella organisation of the German Academic flight clubs (Akafliegs), existed. Following their motto “Forschen – Bauen – Fliegen” (Researching – Building – Flying), they not only teach you to fly. They also perform research and build their own prototypes, quite successfully. It’s no surprise that most well-known glider manufacturers employ former Akaflieg members. Their prototypes also inspire these manufacturers. Perhaps the most recent example of this is the Mü31, whos influence can be seen on the high wing position of the Jonkers JS3.

Idaflieg collaborates with the German Aerospace Laboratory (DLR), which provides their beautiful Discus 2c DLR with instrumentation and personnel during the yearly summer meeting, the Sommertreffen. During 3 weeks, Idaflieg and DLR collaborate at Stendal airport to measure the performance and flight characteristics of gliders, perform their own experiments and certify their prototypes.

Visiting the Sommertreffen was on my wish list for about 10 years now, but I somehow never got around to it. Around new year of 2023 it started to hit me: I have to visit this event! No more postponing, this is the year. And so I drove via my club’s summer camp in Wilsche to Stendal.

Aftermovie Idaflieg Sommertreffen 2022 – Idaflieg e.V. / Lars Samake

The first night, I start to talk with Carlos of Akaflieg Stuttgart. He asks me who I am and why I am visiting. I expect the “why” question, but it doesn’t come. Carlos starts to tell me about his club’s project: fs36 – the first certified glider with fly-by-wire technology. Carlos tells me it’s pretty difficult, because none of the CS-22 regulations are written with fly-by-wire technology in mind. Therefore it’s up to Carlos and his club to prove their system is safe enough to be certified.


The next day I receive my instruction in Zachering, a systematic method of evaluating the handling of gliders named after it’s original author: Hans Zacher. By flying prescribed maneuvers and using just a stopwatch, tape measure and protractor, one can evaluate many aspects of a glider’s handling. Carlos asks me to join him in the evaluation of a brand new DG-1001 Neo, an offer I can’t refuse since I regularly fly aerobatics on the DG-1000S with 18-meter tips. My club has the “old” 20 meter tips too, and I don’t like them.

We tow to 1800 meters and retract the electric landing gear. First we look at the stalling behavior of the glider. We stall the glider straight and level, with 10 degrees of side-slip and in a steady 30 degrees turn. I notice that the DG has become more docile with these new tips, especially compared to the old tips I don’t like. There are lots of warnings before the glider stalls, and with 10 degrees of side-slip there is a tendency to enter a stable turn with some shaking and mild rocking.

Carlos flies a 30 degrees turn.

Next up is the behavior related to adverse yaw. We roll a few times without using any rudder input and note for the time needed to reach 30 degrees of bank. We also estimate how much yaw is introduced as a side-effect of rolling. We also perform the inverse maneuver; from a stable 30 degrees turn we roll back to wings level using just the rudder. Finally we measure the time to perform a 45 degrees left to 45 degrees right turn. The DG seems more agile in 20 meters than I’m used to. I’m starting to be impressed with these new tips!

We enter a thermal together with the Akaflieg Darmstadt D-43, a side-by-side two-seater. The D-43 looks sleek when it flies over the iconic church of Stendal. Again I’m surprised by the low control forces of the DG.

The Akaflieg Darmstadt D-43. Photo by Simeon Schmauß

The next day we continue our program. Next up are maneuvers to determine the stability of the aircraft, which requires still air. We begin with the dynamic stability in the pitch axis. After we release the tow, the DG is trimmed to 115kph and slowly decelerated to 100kph. After the stick is released, the aircraft enters a phugoid. The DG dives down and lifts the nose up at 130kph. I start my stopwatch and Carlos writes down 130kph. The DG slows down and lowers the nose, and Carlos writes down the minimum speed. The DG picks up speed again, accelerates to about 130kph and starts to lift the nose again. I time 27 seconds and Carlos writes down the airspeed. We continue this 6 times, and conclude that since the speeds at the top and bottom of the oscillation do not diverge the DG is currently dynamically stable in pitch.

Finally we look at the forces needed to fly different airspeeds. While having the aircraft carefully trimmed at 115kph, we fly different speeds and note down how much force should be exerted to steadily maintain that speed. We also look at the required stick deflections.

After about 40 minutes we’re done with the program and land. Next up is an evaluation of the cockpit and checking our results with an experienced Zacher instructor. If our results are good enough, they will be entered into a central database. Using this database one can then compare our results of the DG with the results from other gliders.

Performance measurement

After evaluating the DG with Carlos, I help with preparing a Glasflügel 304 for performance measurements. This particular 304 has been measured already once in 1981, but using a different reference aircraft. Measuring it again can reveal differences between the two reference aircraft and their setups.

A complete day is spent cleaning, polishing, documenting (every little scratch and bubble in the gel coat is carefully described), instrumenting and finally weighing the glider. At the end of the day the 304 has been fitted with a very precise GPS received and a WiFi connection for communication with the reference plane. The plane is weighed with the pilot on board and parked in the front of the hangar.

The next day my alarm clock goes of at 05:20AM. This is when the decision is made: is the weather favorable for a turbulence-free performance measurement? The weather looks good, and at 05:45AM the Discus 2c DLR, the Glasflügel 304 and two tow planes are transported to the runway. Everything is set up on the runway of Stendal and the tow planes tow the gliders to 3000 meters. To prevent the pilots from getting bored, quality German music is played over the radio. The gliders are towed into formation and release. For each data point, they fly the same airspeed for a few minutes in formation. Meanwhile the measurement system records their vertical speed precisely at 10 times per second.

After about an hour the gliders fly over the airfield in formation and land. A DLR employee starts to extract the data from the Discus 2c DLR. He shows me a plot with the measurement from 1981 and today’s measurement. The data looks good: as expected there is a slight degradation in performance compared to 1981.

After a week I drive home extremely inspired. I’ve learned a lot, I’ve seen lots of awesome prototypes (more on those in separate posts) and I’ve even been able to help a little bit. I can’t wait for next year…

My first baby steps in avionics and flight testing

Moving map

I started both gliding and my studies in Computer Science in 2003. A few years later, in 2006, things started to itch. I looked at the state of glide computers at that time, and wondered: “Why are these things so expensive? I bet I can do better!”. So, as part of a project on embedded systems, I wrote a small moving map display for an iPaq 5550. The software was very simple: it would connect over Bluetooth to a GPS and display a trace on Geocover2000 tiles that I downloaded from NASA’s WorldWind server. The color of the trace showed a GPS vario value measured at that location.

This system was easy to use, and because I owned no glider at the time, I took it along for some flights in my club’s Ka8.

First variometer

All this was very nice, but I quickly realized that in order to do anything remotely useful, I would need to measure a properly compensated variometer value. And, having measured this compensated variometer value, it should be shown in a friendly manner. I owned a Pilatus B4, so I could finally integrate some electronics.

I decided to start with the variometer gauge first, and I was able to convince Mike Borgelt to share with me the protocol for steering the second-seat version of his variometers. I bought a Beaglebone, a BMP085 barometric pressure sensor, a USB-to-RS485 dongle for the Borgelt gauge and a casing and started experimenting.

The first measurement box, with a Beaglebone, GPS receiver and a BMP085 barometric pressure sensor.

The first test revealed something important: all these electronics are interfering with my radio! Every time I turned on my newly made electronics box, the radio would constantly start receiving noise. No amount of squelch could prevent this.

I told one of my colleagues with an electronics degree, and he suggested I put aluminium tape on the inside of my electronics box. Scratching the tape made the different layer contact each other and would shield the outside world against EMC from the Beaglebone and the switching 12-to-5 Volt converter I used. This made matters much better, but interference would still be present whenever I ran cables next to my glider’s power supply cabling. A piece of ferrite finally fixed things enough so I could start testing my new uncompensated variometer gauge. To the airfield!

The first test flight of my cockpit-pressure variometer. The bottom-center gauge is my own variometer.

This variometer used a simple Kalman filter to reduce the noise in the vario signal. Using some known information about the Pilatus B4 like its stall-speed, I could assign confidence numbers to the noisy readings I was getting from the BMP085. This allowed my Kalman filter to reduce the noise from my measurements, by looking at what’s physically likely to happen.

Measuring polar curves

During the same period I had followed a course on Modern Design of Experiments (MDOE) at work. The basic premise of this course is that – given a clear objective, a physical model of the phenomenon to be measured and a required accuracy – one can derive a mathematical model (response surface) of a system using just a few strategically chosen measurements. This contrasts the more traditional method of taking a lot of data and thinking about the mathematical model afterwards. In wind tunnels it saves a lot of measurement time, so it might reduce the number of datapoints I need to measure to get a plausible polar curve for the B4 too.

I had already obtained a polar curve of the Pilatus B4 from Idaflieg, so I had a pretty good idea what my results would look like. With this information, I could pick the few measurement points that I should measure. Since most software uses a formula of the form a*x2 + b*x + c, I opted to use that as well. That meant I would want to measure at the extremes, the points with most leverage, to determine the linear part of this function optimally and measure right in the middle to determine the quadratic part optimally.

I towed to 2 kilometers and flew timed descents on MacCready 0 speed, MacCready 5 speed and in the middle. In order to estimate the average wind direction and wind speed, I flew a few circles with constant speed and heading in between each pair of measurement points. The drift in these circles would give me an estimate of the horizontal wind.

The results looked pretty okay. As expected, my Pilatus B4 performed a little bit worse than the one measured at Idaflieg. That was not a surprise to me: I had some scratches on the leading edge of my wings, and the fairing at the wing-fuselage intersection was deformed from assembly mishaps. Taping was no longer possible. I also flew the glider mainly for aerobatics, so it didn’t receive the TLC one would give to a high-performance cross-country glider.

Although I realized that I still had a lot to learn and technical challenges to overcome, being able to derive a plausible polar curve using just a noisy barometric pressure sensor inside my cockpit was encouraging. In the meantime I had learned about statistics and implemented my first Kalman filter! I had evaluated the filter in terms of resonance and response time, and it had worked in practice! I was hooked, avionics became my poison.