The mechanical design of the yoke is shown on the right side picture and consists of the following main parts:
- a wooden case,
- an aluminium pipe,
- two bearing brackets for the pipe,
- an elevator mechanism with a gear rack and a gear and
- an aileron mechanism with two gears.
The parts 1 to 3 can be seen in the right image, while the parts 4 and 5 are in the bottom of the wooden case, as shown in the lower images. The parts 3 to 5 are 3D printed and have a sufficient stiffness to withstand the torque when turning the horn and the force when pushing or pulling the horn.
However the key element is the rubber band, which is rigidly mounted at the central unit. The rubber band is made out of a tension belt, bought for 2€ in a hardware store and cut to the required length.
When pushing or pulling the yoke, the band will be stretched, where the resulting force is nearly proportional to the push or pull way. The same is true when turning the yoke in either direction. Then the torque is also nearly proportional to the turn angle. This is exactly what we wanted to achieve.
To make sure that the horn is always in the middle position, when it is not operated, a bias tension is necessary. This bias tension can be regulated and adjusted by the two small clamps at both ends of the rubber band.
The elevator mechanism is realised by a gear rack, which is mounted on one side of the wooden case. When pushing or pulling the yoke, the small central gear will turn. On the axis of the gear there is mounted a potentiometer, which will be read continuously by the controller.
The nearly identical mechanism is realised for the aileron control. When it is turned, then also the gear and the attached potentiometer is turned.
Both mechanisms are mounted on a gliding plate, made out of white rigid FOREX foam. The plate will be moved forward or backwards within the wooden case, when the yoke is pushed or pulled. The moving parts, i.e. the aluminum pipe and the gliding plate were finally lubricated with Liqui Moly LM 47 to achieve for a smooth movement of the parts.
The whole mechanism works very satisfactorily until today. I will keep you informed, when having done more than 500 hours of flight time 😉 The pictures shown above for the two mechanisms were made before performing the final wiring.
After having assembled the yoke mechanism and placing the painted horn onto the yoke pipe, it turned out that the distance of the horn to the wooden yoke mechanism case is too small, because it is very closely placed to the main and lower panel of the cockpit, see right image with red arrow. Now it's hard to imagine what to do. The question was now, what can be done to place the horn more far away from the wooden case ?
- The first idea of course was to manufacture a longer aluminium pipe. This however means, to unsolder more than 20 cables from the interface board at the side wall of the wooden case. Having already tested the entire yoke with the soldered cabling, it would be very annoying to do the entire tests once more.
- The second idea was to manufacture an extension pipe, which has to be mounted on the existing aluminium pipe. However, also here we have the problem, that the cables have to go through this extension pipe.
The final solution was to manufacture an extension pipe with an axial slit along the entire pipe, where the cable harness (22 cables) can be fed in. The final CAD design of the extension pipe is shown below. Note: The rim at the left side of the extension pipe is only 0.2 mm thick and is finally cut-off by a scissors. The reason for this rim is, that the extension pipe is 3D printed in axial direction and we need a large area to be “glued” on the hot 3D printer bed during printing. The rigid mounting of the extension pipe to the horn is accomplished by an additional mounting clamp, which is placed inside the extension pipe. The shown M3 nut is glued on the mounting clamp. The mounting on the other side, close to the wooden case is realised by two 30 mm long M3 screws, which will go completely through the aluminium pipe.
I was not completely sure, whether this sophisticated and unusual mechanical design is finally rigid enough, because the applied forces on the yoke are relatively high because of the pre-tensioned rubber band. But it finally turned out, that the entire design is more than very rigid and relatively easy to assemble. The only negative point was the long printing time for the extension pipe, which took roughly 2.5 hours for one pipe. The assembled extension pipe including yoke and wooden case is shown in the picture above.