Mechanical Considerations (Falcon) - norlab-ulaval/Norlab_wiki GitHub Wiki

Introduction

This section will detail the mechanical design and considerations taken to ensure safe flight.

A full CAD is available in the Norlab's Onshape team shared projects at this link.

Motor mounts

The motor mounts were custom designed to make a coaxial octocopter configuration from a quadcopter frame. The initial version was 3D printed and broke in mid-air so it is strongly advisable to avoid that. After this unfortunate event and discussion with machinists, we converged towards two aluminum laser cut parts to hold the motors and 3D printed middle parts to fit around the arms, which makes them strong and cheap to produce.

A 1 mm gap was voluntarily left in the middle of the sandwich to allow squeezing tight on the drone's arm and make sure it holds. A hole was drilled in the arm on the top to insert a safety screw. This one only ensures that the motors won't turn around the arm and can't pull out of it. All the screws used for this design are M4 and the nuts at the bottom are nylon-insert locknuts to sustain vibrations. The slot under the motors allows fitting any motor with a bolt pattern radius between 16 and 25 mm. The rear part protects the wires in case of a crash.

Battery Enclosure

The selected 20 Ah battery is pretty heavy is bulky, and we have to secure it properly and make sure its weight is centered on the frame. In this regard, a custom enclosure was designed to hold the battery on the bottom plate of the frame, as shown in the image below.

The battery is constrained in all directions but one and is secured with a velcro tie in the front. The mating XT90 connector on the frame is held tightly with two 3D printed parts to make it convenient to plug and unplug. Finally, the second version of this battery enclosure integrates the laser range finder that was added to better estimate relative altitude from the ground.

GNSS Antenna

The placement of the GNSS antenna is really important as it needs to be unoccluded by other parts and also far from the electromagnetic interferences of the motors. However, we don't want it to be too high as it would take the hit directly in case of a crash. Hence, the below support was designed to put it just above everything else and not in the way of the protective shell.

The part is fitted with heat-set inserts to avoid too many nuts that would have been tedious to tighten.

Protective Shell

A protective shell was printed to protect the electronic components from the elements (rain, snow) and potential crashes. It is precisely shaped to fit the frame, arms and components, as shown on the pictures below.

The shell is held by two M3 thumb screws at the bottom with custom fixations seen in light orange. At the top, the shell sits on three pillars (one was cut to with a cable) to protect the Jetson in the event of a shock. The drone was tested in light rain and snow, and experienced some crashes with this shell without any issue.

Electronics

The electronics are stacked in the middle of the drone, with the Doodle Radio at the bottom, enclosed in a 3D printed part, the Flight Controller in the middle and the Companion Computer on top.

Design Flaws

Over the course of the project, we noticed some flaws with the mechanical design of the base frame and the selected components.

  • Foldable props are great in paper as they are more convenient for storage, but in practice they are less stable in flight, as they lose thrust due to folding every time the motor accelerate or decelerate. Hence, we switched to rigid propellers, which are also conveniently cheaper.

  • Foldable arms are similarly convenient for storage, but again they pose a risk in flight. Indeed, if the hinge screw is loose, the arms can shake even if you tight the outer ring to the maximum. Also, we often noticed that the outer ring loosens when in the air, which could lead to a catastrophic event. Hence, users should be mindful of this flaw and a future build should avoid these folding mechanisms altogether.