Sometimes when working with SolidWorks I find myself spending a lot of time making sure that the model is spot on down to the last edge or vertex rather than considering the role of the part in the project at large. Today was one such time, I have been working on the rear motor mount. I will make my point about CAD shortly but first some background on the rear motor mounting.
One of the hangups that I have had getting a start on this part of the design was trying to decide exactly what level I would like to have the motor mounted at and hence at what level the propellor would be spinning. The reason for this indecision is tied to the the Mk. 1 prototype. When flying forward the Mk. 1 would pitch nose up. My suspicion is that the center of drag in a vertical sense is mostly influenced by the spinning propellers rather than the frame. Given that the centre of forward thrust is level with the frame and not the propellers a pitch moment is created causing the rotation. The logical progression from this assumption would be to get the centre of thrust in line with the level of the propellers and test again. For this Mk. 2 design that would mean getting the rear propeller level with the front boom. That however introduces its own set of problems mostly to do with finding space for all the components, particularly the battery. For now I have decided to use the same motor mounting as the forward motors but with a rear specific lower part incorporating a landing foot.
Show above is the design I am happy with for now, on the left with the surface lines shown and on the right without to give a clean look at the finished surfaces. Given that the project is still in a conceptual layout stage I have invested far more time in to it than was really necessary. The basic form for the part took only a few minutes work, keep the same profile outline as the motor mount, add a leg to the bottom. That would have served the need completely at this point. However I sunk significantly more time in to several iterations using different modeling approaches to arrive at what is seen here with fully resolved smooth surfaces.
If someone was paying me to develop this project with expectations of efficiency then this would be a fairly irresponsible approach. As the design progresses there is a reasonable chance that this part may need to change and that that change could require significant rework of the model. As a pleasure driven pursuit though I can afford the time to indulge in the modeling, I find a certain satisfaction from the process akin to that derived from assembling a model kit or just generally putting things together.
The next step is going to be having another look at the battery placement and power distribution. I am not happy with the positioning of the bullet connectors for the motors with respect to the arms and the battery connector and wiring is not going to work as shown here.