Calculations & Drop Tests
Before the landing gears were determined to be designed this certain way and to use PLA as its material, several calculations were carried out.
Max Bending Force
By using max bending stress PLA material can withstand given by eu.makerbot.com, I worked backwards using formulas I learned from Mechanics III to get the max bending force my design can withstand. For the cross sectional area and polar moment of inertia, this information was acquired from Inventor based on my drawings and design, from the segment properties tab. From here, I had the necessary information to calculate the max bending force.
Impact Velocity
The second thing I began to calculate was impact velocity. Our drone flew at a max height of 12m, so that value was taken as the ideal to be used for calculation.
Impact Force
This equation was particularly interesting, especially the part where t is assumed to be 0.1s & 0.5s. Firstly, what is instantaneous rest time? Simply put, it is the time taken for an object to come to a complete stop after impacting with an object. For the first equation, when t is assumed to be 0.1s, the impact force falling at 15.34m/s (from 12m high) is a staggering 39kg!
However, when t is increased to 0.5s, the impact force is greatly reduced to 7.81kg. From the comparison of these two values, it's clear to draw the conclusion that by increasing the instantaneous rest time, impact force is reduced. Hence, by including the landing gears suspension systems, it increases the instantaneous stop time, which decreases the impact force felt by the drone, ultimately protecting the structural integrity of the drone.
Drone Drop Tests
As is evident in the video, the first drone drop test was a failure, due to poor design of the rib & web as previously discussed in my earlier blog post. However, after design improvements and structural changes, new drop tests were carried out, with me repeatedly dropping the drone from knee height, all the way up to chest height, and finally to head height, around 150-160cm. The final drop in the video shows the drone carrying the electronics & LiPo battery, weighing at around 1.5kg. The drone survived all these drop tests without cracks or breaks in the structure, which shows a successful design. Now, on to the calculations for this.
Impact Strength from Drop Tests
At 1.5m with 1.5kg of load, the total impact force felt was 81.37N, which is within the range given by the MakerBot website, providing max impact strength of 96N.
Survivable Height Of Landing Gears
But what is the true survivable height of the landing gears? I was unable to determine this during the drop tests, in fear of permanently breaking the drone during the experiment. Therefore, I carried out several calculations to determine this.
Data showed that with a minimum of 1.5kg load at max instantaneous rest time of 0.1s, the maximum survivable height would be 2m, which the experimental data showed that the landing gears still survived with 1.5kg load within 2m. However, with a 2.5kg load with same instantaneous time, the survivable height dropped to 75cm. This is still marginally acceptable, as this is within the landing range, in the event a hard landing or landing failure occurs.
That is all for today's blog post. Stay tuned for more!
Sincerely,
Ryan Ng
Invictus Mechanical Engineer
