9. Balsa variant

by Matias Rajkay matias@rajkay.de

In the August issue (8/99) of the German modeler's magazine "FMT" I first came across the Balsa Pibros in an article written by Jürgen Waßmund.

This article contained a simple drawing as well as all critical measurements. Using DesignCAD LT 2000, I made full-scale drawings, and generated DXF files, which I could use for my "Step Four" CNC mill.

My son decided to build an original "Depron" Pibros in parallel to have a comparison.  

The wing of my variant consists of 1.5 mm Balsa sheeting, with lots of holes (see pictures). Using the mill made all those round cut-outs much simpler! On each side, there are four ribs, made out of 3 mm balsa. The LE is built-up of strips of 1.5 mm balsa, the TE is just one single strip.

The individual pieces of sheeting where joined using CA adhesives, the ribs were glued to the (flat) bottom sheeting using "white" wood glue (slow curing and a bit heavy, but elastic). The top sheeting was added after the bottom had dried, and was pressed into position on a flat surface, thus ensuring that the Pibros would stay absolutely flat. The maximum thickness is only about 11 mm!  

For the RC equipment I used a Multiplex Pico 3/4 indoor Rx, with two Multiplex submicro servos. The advantage of all these components is that they can be integrated into the Pibros without the need of a keel-like body, as originally drawn in FMT's plan. I used four 110 mAh cells. Please see the attached pictures for more detail.

The Pibros was covered with transparent OraLight film, the two elevons were connected using 2 mm carbon fibre rods.

I added a small ply-wood "handle", which also contains a tow hook, and is reinforced on both sides with Dave Brown's carbon fibre flat ribbon.  

The model weighed in at about 175 grams, with the Cof G accoding to the FMT data (they give a range of 226 - 230 mm from the nose). The elevons were given the max. possible travel (about 45 degrees into both directions), the Tx (Multiplex Profi 4000) was set up with dual rates of 40% and an aggressive exponential of -60% on both ailerons and elevator.  

First hand-launch flights at our home field proved successful, and a few careful launches with a bungee also worked well, so we took both Pibri along to Brittany (France).  In Brittany we first tried flying the balsa Pibros at the Menez Hom, a fantastic slope, at about 4 - 5 Beaufort wind. Unfortunately the Pibros was not able to penetrate the wind at all, and the slightest touch of elevator resulted in a spectacular stall, with the model being blown back in an uncontrolled manner (scaring all pilots at the slope...).

After this first experiment we took both Pibri to the beach and experimented from some lower dunes, and again with bungee, and figured out that we had to move the C of G forward by about five mm. This minor modification entirely changed the characteristics of the Pibros - and we gave it another try at the Menez Hom. Now the bird was performing the way we hoped - fully aerobatic, and in the strongest winds.

Flight time was only limited by the capacity of the Rx battery, but I made several flights of more than 30 minutes.   We also got carried away using the bungee - 10 meters of 6 mm diameter rubber (!), and 30 meters of nylon string. I extend the rubber by about 9 - 10 meters. The Pibros will shoot away at almost supersonic speed, and a minimal tick of up elevator results in a rocket-like climb to about 50 meters of altitude. Doing this at  the slope into the wind I did hear some crackling noise, and during the climb one could see the tips of the Pibros bending up... so I decided to reinforce the Pibros on the bottom, by gluing two stripes of the Dave Brown carbon fibre tape to the balsa with epoxy. Both stips are parallel to the TE, covering the full width of the Pibros. Thea are located between the holes (this modification was not yet installed when the pictures were taken).

Comparing the balsa Pibros to the "foamy": The balsa Pibros is considearbly faster and "snappier", and it hurts more when it hits you! The foam Pibros is faster to build, and takes all sort of abuse.   

Both are tremendous fun, and my son and I are thinking about all sorts of Pibros-derivates - perhaps an electric foamy, or a glow-powered balsa Pibros will be built soon.  

Last updated June 17, 2001