Summer 2025 – Bocian Upside Down

Gradually, the Bocian's flights are improving!

Here’s a video of a pleasant flying session at Balès Sud, in early September 2025. It's the first time I’ve truly enjoyed flying it inverted, and I’ll explain here why…

Summer 2025, the Bocian 1V at Port de Balès South.

What makes this project special is the goal of performing aerobatics with a retro-style glider — a fairly bulky 1952 two-seater design

The airfoil used, the SC17, is nearly symmetrical, which allows for decent inverted flight. But! In order to give the glider a nice flying attitude — “tail up” — I set the wing incidence at about 8° relative to the fuselage line. Once inverted, this results in a “tail down” flight attitude, and the fuselage drag becomes quite significant.

The fuselage drag in upright flight.

The fuselage drag in inverted flight.

PredimRC allows you to evaluate these effects. In level flight, the glide ratio is reported as 23, while inverted it drops to just 7! I’ve identified at least three combined effects contributing to this:

The built-in fuselage incidence, which generates drag,
The airfoil not being fully symmetrical (part 1): this increases the required wing incidence and thus further increases the fuselage incidence,
The airfoil not being fully symmetrical (part 2): it performs less effectively when inverted.

By flying with four-axis control and raising the flaps, the glide ratio can already be improved to 15 according to PredimRC. This counters the third effect.

Now, with the goal of flat spins in mind, my Bocian features adjustable wing incidence, controlled via a servo in each wing. (It actually seems that the pitch instability I experienced during early flights in 2023 came from those servos not holding their position properly inside the wings — an issue that’s since been resolved.)

The pitch servos are located in the wing, with a Teflon rectangle acting as a pivot on the arm.

The fuselage incorporates a steel slide, fitted to the Teflon rectangle.

In place.

The initial idea was to use them in opposition (wings forming a propeller shape) for flat spins. But I’ve also taken advantage of this system to reduce drag in inverted flight. When I push the right stick on my transmitter (mode 1 - using full four-axis control), the wings and tail surfaces simultaneously deflect upwards at their trailing edges, significantly reducing fuselage drag during inverted flight. PredimRC then reports a glide ratio of 20.

By adjusting the pitch angle, the fuselage drag is optimized during inverted flight.

As a result, the Bocian now glides much better when inverted, unlike in the earliest flights where it would slow down and descend. You can see it in the video: the recovery after inverted flight is clean and without drag. The short inverted flight halfway through the video is also level and maintains speed.

A good setup achieved — and more to come...