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Designing the Arms 

A quad copter has 4 extended arms (logically). they all are connected to the main frame that houses the computing board and battery. Design a way to assemble the arm to the main body that would be sturdy and fixed in a axle while remaining easily detachable.

Our approach is to design an arm that is 180mm long point to point, with one end for the motor to be mounted on and the other end for the arm to mount onto the main body. The arm is attached via cylindrical mount joint that stays in position by a fix lock extrude. It will be sandwiched by two top and bottom bodies as shown on the concept sketch below.

Concept Sketch

The Three Variation
The above 3 design variation are to be compared together to see if the designs would be sturdy enough to withhold a 10N thrust force while  maintaining to be a light weight .

The Yellow design has a Tapered outward form that is biased to the motor mount. it has  triangle truss like pattern, with a leg stand on the bottom.

The Blue design has a honeycomb pattern for structural rigidity. its has a simple straight frame and needs a leg mount assembly.
The Red design has a very minimalistic approach mass wise, with a triangle truss pattern to withstand side impact. 

Simulation Analysis  

To make sure all the above designs are able to withstand real world conditions. All design will be manufactured using ABS plastic.

The bottom table shows the physical properti value of ABS. ABS has a tensile strength of 30 N/mm^2.   We can theoretically assume  that the yield strength of ABS is 24 N/mm^2(base on 80% tensile strength).  For our parts to pass, the parts cannot exceed a stress force of  24N/mm^2.

Using Solidworks Simulation workbench, we can select a fix geometrical point for the structure to remain a constant. In this case we’ve selected the inside and outside wall of the cylinder mount as our gemological fixtures as shown below.
We can apply force on the area where the motor is mounted. Select the face and direction of force, in this case a maximum load of 10 N to emulated maximum possible thrust force on each arm.
The Result
Yellow Mass = 44.5g 
Stress result = 20.5 N/mm^2

  Blue Mass = 43.5g 
Stress result = 14 N/mm^2
Red Mass = 26g
Stress result = 21.5 N/mm^2

All the result shows that the designs have pass the yield strength stress allowance of 24 N/mm^2

Base on the simulation results and looking at the total mass of the designs,  We’ve choose to go forward with the Red arm design, it has less then twice the mass then the rest thus we will be saving on weight and material cost.

We finish of the arm design with a stand assembly part so the quadcopter can take of and land on different terrain surfaces.

End of Part 2