As usual I wanted this modification fully reversible, to get back to the original configuration with absolutely no scar - structural as well as aerodynamic - if preferred. This made me give up a modification described in the Bücker Newsletters years ago, which implied to drill the main pitch bellcrank.
I kept the right tab as the standard controlled trim, and used the left one as a servo-tab, driven by an additional bellcrank linked to the rear torque tube by a plain 30 mm shaft collar. The bellcrank is made of 2 mm aluminum sheet and is bolted to the shaft collar.
As the tab will now be moving as much as the pitch control, I replaced the original 1.0 mm plain steel wire by 1.2 mm, 7 wires nylon-coated steel cable which is more flexible and more slippery. It is sold by game fishing shops for shark fishing, with the appropriate sleeves and crimp pliers and is rated 250 lb.
I made cable sheath stops using cut and partially drilled standoffs, welded on bolted tube clamps to allow an accurate positioning on the structure tubes. I bought all the hardware from McMaster-Carr, and one turnbuckle from ACS.
It is now operational: just waiting for appropriate weather to test it...
Updates:
Apr 5 2015
I flew the servo tab this afternoon. The trimmable speeds are more or less unchanged: I noted 100 to 110 KPH according to the applied power, and of course it also depends on weight and CG. The approach speed was trimmed with a good margin to the trim lever stop. However, this could easily be adjusted by lightly modifying the neutral position of the tab bellcrank on the torque tube.
Evolutions and aerobatics are much more pleasant, with a better pitch/roll harmonization. Inverted flight and particularly inverted turns are much easier, less tiring. Rolling circles should also be easier, but I still have to work on them as the coordination feeling is different from what it was.
I even did the best kiss-landing ever with two fingers, after 5 months without flying the jungmann - beginner's chance? ;-) I did not even notice that I had delicately settled, 3 points, on the ground, for a few seconds...
Moreover I feel relieved not to feel the whole pitch control stressed by heavy forces...
On the other hand, the stick-free stability is of course a bit degraded, which makes cruise flight a bit more demanding. In a turbulent sky paved of lovely cumulus, I noticed a tendency to light stick oscillations around neutral, hands off. This may be due to a little bit of cable slack around neutral: I will tighten the cable and try again. Another possibility could be to replace the 1,2 mm shark-fishing cable by 1,5 mm teflon coated mountain-bike brake cable for less elasticity, but the cable end buckles may be more difficult to do.
Well, at this time the result is not yet absolutely perfect, but not bad at all: I already feel it as a real improvement. I will go on improving.
Apr 24 2015
I proceeded to several test flights to finalize the servo-tab settings.
I reduced the cable-to-cable distance from 100 mm to 85 mm and now the result is very good.
The Jungmann's nature is absolutely unchanged, except a bit lighter pitch forces which make the control more harmonized, particularly in aerobatic flight.
The stick-free stability is unchanged and the unpleasant hands-off porpoising tendency, which happened in turbulence with 100 mm cable-to-cable, has now fully disappeared.
The minimum trimmable speed is about 110 KPH, and can be easily adjusted by the position of the bellcrank on the torque tube.
The flare is easier due to more linear pitch force and response.
GIlles