alfredbmor:: ... In the Model Airplane Magazine the author of the review shows the physics effects of the swash plate movements and claims that should be better if we apply a 135o (3 servos) rule instead a 120o (3 servos) in the swash plate configuration.
The reason to go from 120 to 140 (or whatever) is mainly due to the collective pitch change induced during elevator movement.
Ideally ... you want pure movements ... ie elevator only with elevator stick movement.
The 120° config induces some collective pitch movement (if not compensated for) during elevator input. People like me who fly around willey-nilley, floppy-sloppy will probably not notice.
If you are after crisp precision flight ... you'll notice.
Take a look at the image below. The black line is a line through the main shaft perpendicular to the C/L of the heli. The other lines (green, red, blue, orange) are lines parallel to the main shaft line.
Each vertical (arrowed) lines show the distance from the attach point lines to the main shaft line.
As you can see, in the 120° config, the ail/pitch line (red) to main shaft line is much shorter than the elevator line (green) to the main shaft line.
What does that mean? Say your elevator input moved the elevator line UP 1 mm and the ail/pitch line moved DOWN 1 mm ... what would happen at the IDEAL line? Well ... if the shaft line is the theoretical pivot point ... then moving the ail/pitch line 1mm would cause the IDEAL line to move FARTHER than 1mm. If one side of the swash (elevator attach point) moves 1mm and the other side moves more than 1mm then the pivot point will no longer be at the center. This forces the swash to slide up the shaft slightly ... which induces collective pitch changes with elevator input ... ooops. This will happen if all 3 of your servos move equally with elevator input.
We can reduce the affect with TX mixing. Newer TX's have this feature. It does this by causing the ail & pitch servos to move the swash less than the elevator servo does .... so that the IDEAL line (orange) move distance is nearer to the elevator line (green) move distance. When the green line and orange line move equally in opposite directions ... the pivot is through the main shaft (black line) and NO collective pitch is induced.
We can also reduce the affect by mechanical means ... ala 140° config (see BLUE line in image)
The 140° config moves the ail/pitch attach point line further away from the main shaft line making the ail/pit distance closer to the elevator distance. It's done by attaching small levers with attach point balls at the end. This effectively causes the ail/pitch line (Red for 120°) to be pushed back to the blue line (140° config).
So ... As the difference in the two distances get smaller ... the less of the pitch change affect you will have.
Of course, if you could make the two distances equal ... that would be IDEAL
