Ever since I developed a passion for scratch building radio controlled airplanes about a year ago, I have been very eager to design a system to deploy parachutes and other fun payloads (I’ll keep those as surprises for future posts).
I recently built an electric 41″ Cessna 185 out of foam, adapted from Mark’s plans over at www.budgetrc.com. He’s been a scratch builder since 1977 (well before I was even born), so he’s got a few more years of experience under his belt than I do .
Cutting to the chase, I’ll explain what it is exactly. It works with one servo, activating a retractable door. The door is part of a larger housing attached to the bottom of my Cessna. The system deploys anything you want to put inside the compartment – from parachute men and g-force testing devices, to itching powder and water balloons. It was a fun project, it worked great, but I still have a lot of kinks to work out. After toying around with the system, I realize I can do some actual experimentation with various parachute designs (I had little idea how much goes into successfully deploying a parachute until now) since I finally have a stable platform to work with.
Assuming you have some sort of a flying machine to attach the mechanism to, this should take no more $3 worth of materials to make. If not, head over to Mark’s website and build one! I’ll list the parts I used for the plane and parachute drop below:
- Foam (I used 4mm Dollar Tree foam; it cost me $1 for the sheet, I used about 1/8 of it; cellfoam88 and other materials should work nicely too)
- Servo (typical hobby servo, I used the HXT900)
- Bamboo Skewer
- Rare Earth Magnets (optional; used to secure door)
- Plastic Bag
- Foam (I used some Dollar Tree foam, but mostly pink insulation foam from Home Depot)
- Receiver (I used the OrangeRX 6 channel receiver because I own a DX6i transmitter)
- Motor (I’m all out electric, so I opted for a brushless 1300Kv one)
- Speed Controller (I went with a Turnigy 9 amp ESC, with a 2A BEC)
- Battery (chose to go with a 3 cell 750 mah LiPo)
- Propeller (for max thrust and low speed, I used the GWS8035)
- Servos (as usual, my favorite HXT900; 2 of them)
Building the parachute drop took me about 2 hours, and about 1.5 hours to design it. For those interested in the Cessna, it will take about 1.5 weeks of regular work depending on how nice you want the final product to end up (could take a few days too, all depends). Without the transmitter, the parts will end up costing about $50. You can get a cheap transmitter for about $20. Comment below know if you need more details or help.
Sketches & Images
I rendered a 3D model to guide me as I built the device. To see an animation of the 3D model, watch the video clip (above).
I won’t lengthen this post by writing too much about the process, it really wasn’t very intensive at all. To get used to good engineering practice and design, I started by hand sketching the mechanism (see above). A whiteboard works wonders when it comes to quickly conceptualizing ideas. A few constraints I set for myself for the design were: it had to be retractable, use only one servo, be easily detachable, and contain the payload securely in flight.
Once I settled on the design, I drew it up in 3D, animated it to ensure there were no identifiable flaws, and cut out the components. All cuts were quite flat, so an exact-o knife sufficed to cut the pieces of foam I required. All my design constraints were met. I decided to use a pair of 2mm rare earth magnets to hold the door in place. The magnetic force was strong enough to keep the door shut tight, but weak enough for the servo to effortlessly pry it open on demand from the ground.
The design and construction fortunately resulted in a satisfying mechanism. Nevertheless, there are several changes I will implement in future designs – nothing is perfect apparently .
What went wrong? Fortunately, not much – or at least not enough to classify this project as a failure. After running many flight tests, I have a ~60% success rate of the parachute opening upon deployment, and ~90% success rate for immediate deployment once the hatch is opened. Needless to say, for any sort of commercial application such statistics are severely sub-par. On the bright side, when everything did go right, the parachute beautifully descended from a comfortable 200 feet. One tip I picked up from NASA is to punch a hole in the center of my parachute (mine was the size of a quarter). It helps maintain stability and noticeably level vertical descent. Another crucial impact of the mechanism may be the negative aerodynamic effects on the plane itself. I’m not sure whether the impact is statistically significant (although I did notice a greater number of stalls), but I am working on a wind tunnel design to test such aspects of flight for future builds as well. Heavy payloads would certainly throw off the center of gravity.
What would improve it? I have a whole list of ideas, but of primary concern would be the parachute itself – not the dropping mechanism. With a 60% success rate, I’m now inspired to research parachute design and run some experiments of my own (expect a post on my results in the near future). I will experiment with aspects such as material used (I have an odd feeling that plastic bags generate a substantial amount of static ‘clinginess’ disallowing the parachute to consistently open), mass of the parachute man, length of the strings, size of the center hole, and altitude of deployment. However, the first thing I will experiment with is the folding technique of the parachute when packing it into the compartment. The key improvement I would make to the drop itself would be to make it as aerodynamic as possible. With my current level of knowledge and resources it would certainly be difficult to create an optimal design, but it is unarguably evident that rectangular shapes are always a big no-no when it comes to fluid dynamics.
What next? My next step with this project will be to experiment with the parachute design and assembly technique. I will do a lot more research, and try to hit a 100% success rate upon deployment. I will also run some static tests to determine if the parachute drop itself is posing significant detrimental effects on the aerodynamics of the plane. If so, I will redesign it; or perhaps even better, I will redesign the plane so that the mechanism can be embedded within the fuselage.
I was more or less on my own for this one, but my inspiration comes from watching countless rocket launches and the various stages gently float down. Most recently, SpaceX’s Dragon capsule, what a historic splashdown! Godspeed SpaceX!
- Mark at BudgetRC – Without his initial plans for my Cessna, I wouldn’t have a suitable platform for my project. In case his site is down and you want the plans, I hope he won’t mind if I post his plans here. For the one sheet, click here, and for the tiled plans, click here.
Thanks for taking the time to read this post. I’d love to read your comments and suggestions, or answer your questions. Have a wonderful day!