note: there was actually a drop before this where I wanted balloons to act as air bags and soften the fall, but I never got to test that, and a 2m drop showed that the balloons used were a bit too fragile, altho this might be good because it disperses energy (somehow).
Our team constructed our design using cardboard (for the main package), a black garbage bag 534 cm in diameter (for a parachute), and string (to attach the parachute) When I dropped my design for the first time, I decided not to attach the parachute, because I don’t trust tape that much & I couldn’t think of any other way of attaching the parachute (for the moment), so I made a base triangle and just set the package on top. I don’t know why but I honestly thought that it would work, which it didn’t. The package fell off the design completely, off camera and off the force plate, so I have had no way of collecting any form of data. Our second drop went as well as the first, which means it went terribly. At this point, none of us had realized that the design flaw I explained below (except remarks like ‘that looks too smol’ and ‘mr frank said parachute must be 10x bigger than package’ or something). This time, our group decided to tape the parachute instead of using rubber band spam like I was attempting to secure the parachute. Ultimately, the method to secure the parachute really didn’t matter because the parachute was way too smol.
A lot of things went wrong with our initial design, most notably the parachute. When I started to make the parachute, I didn’t really want to guesstimate the area for our parachute, because then we might have a too high or too low rate of descent. For this reason, I googled how to get the area of a parachute, and I came up with this:
D = sqrt( (8 m g) / (p r Cd v2) )
D = diameter
m = mass
g = gravity (9.81)
p = pi
r = density of air (1.22kg/m3)
what went wrong is this: Cd, drag coefficient (0.75 for flat and 1.5 for domed). Basically, when a parachute opens, air flows against the extra surface area, which creates air resistance. Air likes to go the easy way, so when the parachute is a flat surface, air simply hits the object and goes in another direction (e.g, to the sides)
The top left diagram shows a basic idea of what happens when air hits a flat surface
When this is a curved shape, air has to travel more distance to get out of the way, which means more air inside the parachute at any given time, which means more drag. When I designed my parachute, I assumed that when I dropped it, the air resistance would (somehow) curve my flat parachute, which it sort of did. My diameter of 534 cm was calculated for a 3m/s descent, using a domed parachute, so my actual rate of descent was 6m/s (a 2 m/s decrease). I noticed this after both first drops with no more time (and garbage bags) to spare, so that’s that.
During construction, I didn’t want to tape the string or hot glue it directly onto the parachute, because of a higher possibility of the strings coming loose due to the low contact area. I solved this problem quite easily by taping straws onto the parachute instead, which stayed on quite well. This is where the problems come in. Because I just had the string hanging loosely on the inside of the straw, when I exert a force on it, the parachute actually collapses inward. I discovered this in ground testing, so no harm was done & I just cut the straws down to half their old size. However, this introduced another problem because my already long paracord got even longer because of the shorter straw, which increased the probability of my strings tangling, the time it takes for the parachute to deploy, and general reliability. Finally, I only used 3 string lengths in my parachute, because I thought that more than 3 would take too long, and 3 is enough to keep something stable. I think we lost a lot of air resistance & the assorted lift because of a fluttering parachute.
Some things in design 1 went quite well, for example, the straw idea I used worked extremely well, and there were no signs of fatigue after 2 drops.
For our next design, we will improve by INCREASING THE PARACHUTE AREA and using more strings to secure the parachute & reduce flutter.