The concept of the mechanical design of the engine power control with its six levers is shown in the right picture. It consists of six levers, two large ones for the throttle and four smaller ones for the propeller pitch and the mixture control. At the bottom of the figure there are six potentiometers, which will turn according to the lever movements.
The levers are 3D printed as PLA parts and are shown below. The small pads at the ends of the levers (see right below picture) were placed to avoid, that the lever will release from the hot printer bed during printing. These pads were cut off after printing. The levers were sanded and then spayed with black dull paint. To mount the levers on the required plastic gears, additional round fixation parts were 3D printed and fixed by small 2 mm screws. There was no way to glue the levers directly on the gears, because these are made out of polyamide, which is nearly impossible to glue.
Also the three different knobs were 3D printed out of PLA material, sanded and finally sprayed with the correct paint. These are then mounted onto the levers by M4 flat head screws.
The challenge was the construction of the entire assembly, which shall offer the following features:
- The torque, to move the levers, shall be adjustable by the pilot.
- When moving one lever, the other levers shall not move simultaneously.
- The short lever angle of less than 100 degree shall use the maximum possible range of a potentiometer, which has a typical angle of 270 degree, hence a gear train is necessary.
- The gears of the levers shall not be visible from the outside.
- The complete assembly shall fit into the upper compartment of the pedestal.
Adjustable Lever Torque
The adjustable torque of the levers has two benefits:
- The levers are standing exactly perpendicular to the horizontal friction shaft.
- It’s by far more realistic with a user selectable torque, than implemented in commercial toy products, where the levers are messing around.
To implement the adjustable torque, all levers are mounted on a 6 mm aluminium friction shaft, which has at both ends an M6 thread. Between the levers, dedicated friction cylinders (green indicated in the above picture) are placed, which are made out of 25 mm diameter aluminium round stock with an inner drill hole of 6.5 mm diameter. With an applied axial force, the friction can be adjusted to the required torque level. The implementation of the axial force is realised by dedicated mechanical 3D printed PLA parts at the left and right side of the friction shaft, as shown in the right figure. Turning the yellow friction knob will increase the friction and hence the torque to move the levers. This design idea for this friction concept has been taken from www.simvim.com/b58.
On the left side of the friction shaft we have two parts. The most left one is the fixation of the friction shaft with the housing of the engine control assembly. The right part includes a negative M6 nut outlet to fixate a M6 nut. The drive shaft with its M6 thread will be placed through this part and the friction shaft is rigidly fixated by a counter nut. On the right side we have the friction knob and a pressure plate. Also here the friction knob has a negative M6 nut outlet, where the corresponding M6 nut is fixated.
Because of the realised friction by the applied axial force, not only the lever, which is moved will turn on the friction shaft, but also the neighbouring ones. To avoid this, a friction cylinder fixation bracket was designed and mounted on each friction cylinder against the housing. The result is perfect and only this lever arm turns, which is actually moved.
Lever Gear Ratio
Each of the six lever arms shall be turned by approximately 90 degrees from the minimum to the maximum position. When using a potentiometer to measure the actual lever arm position, we will have a possible turn angle range of approximately 270 degree. To use the entire range of the potentiometer, a gear ratio of slightly below 3 would be a good choice. To realise this, a gear train of 50 teeth and 20 teeth with module 1 was foreseen, which gives a gear ratio of 2.5. Moving the lever arm by 90 degree will then rotate the potentiometer by 225 degree.
By accident wrong gears with 30 instead of 20 teeth were delivered. It finally turned out, that these larger ones are much better to rigidly mount on the potentiometer axis. It was therefore decided to use these larger ones, which gives a gear ratio of 1.67. Moving the lever arm by 90 degree will then rotate the potentiometer by only 150 degree, which is still acceptable. With an ADC resolution of 10 bits we get 569 measurable digital counts (1024/270x150), hence a lever arm step resolution of 1.7 per mil is achieved.
Assembly of the Engine Controls
The housing is made out of 5 mm white rigid FOREX foam plates, which are glued together. On the left and right side of the housing round cut-outs are placed. On the left side for the fixation of the friction shaft and on the right side for the friction knob.
The challenge was to mount the three potentiometer units (two ones for each control, see right picture) into the housing in such a way, that no backslash in the gear train exists. To guarantee that the potentiometers within the clamps are correctly mounted in the axial direction, a dedicated mounting pattern plate was designed and 3D printed.
To close the gaps between the levers, seven round lever cover segments were designed and 3D printed, three ones within a lever pair, two ones between the three lever pairs and two ones for the left and right side. The one to be placed within a lever pair has a width of roughly 29 mm. Although the covers have only a thickness of 0.8 mm, they are very stiff due to their round shape. Finally they were sprayed with gray paint. Printing these covers was not a big issue, the only problem was, as usual, the long printing time of more than three hours for all of them, one piece after the other. Putting everything together, the Engine Control is ready for use and was assembled into the upper compartment of the pedestal, as shown in the below right picture.