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DIY Multirotor Recovery Parachute System Build [WIP]

#1
Hello there Pilots!

Introduction

First things first, I just registered myself to the forum and therefore am not sure if this is the right Forum to post this. Please correct me!

So, I just started building myself a 450 sized quadcopter. The quad will be used to do some homebrew aerial potography and therefore will be carrying a gimbal and a camera. Since I intend to use a Sony aplha 7 in the future on the thing, I wanted a recovery system to get te quad safely to surface when something goes wrong.

Now, you might think if I want to put such an expensive camera on there, I'd be able to come up with the money to
buy something like a Mars recovery Parachute. Since this system seems quite simplistic to me and I have a reasonabely good tecnical knowledge I tried to build one myself on a budget.
The project consists of three parts: The Parachute, The Mechanics and The Electrics.

Attention!
Metric measurements ahead!
(Also: spelling, as well as gramatical errors due to me being swiss. I'm trying tough!)

The Parachute

I opted to make the parachute too.
I found a nice PDF from a local model rocket club to calculate the parachute dimensions link to PDF: (It's German! http://www.argoshpr.ch/j3/articles/pdf/Recovery_Pt2.PDF
Since it's german, here is the basic formula to calculate the diameter of the chute using a so called "circular flat" design;
Parachute-Formula.png
I used a a weight of 1500grams and a slightly lower descend speed (safety first :cool:) and came up with around a 1.8m diameter.
I then made two testing chutes. For this purpose I used old clothes bags which consist of a very thin plastic, also some thin, 1.5mm braided nylon twine as the main strings. Looking at the next picture, you might see that I cut 8 pieces of twine, which are roughly the length of fthe diameter of the chute.

DSC_0256-min.JPG

I haven't tested the "real size" version yet. I'm planning on doing that in the next 1-2 weeks.

The Mechanics

I tried to work off the design that the medium sized Mars recovery systems use. I used a PVC (teflon would be better) waterpipe with a outer diameter of 50mm and an innter diameter of 47mm as the main housing. The length of the pipe is not yet determined. At the moment it is about 180mm long, but I hope to get that down a little more. I also bought a PCV Cap as the backplate for the pipe. For the swiveling top plate I cut and ground a circle with a diameter off of the outer diameter size of the tube out of a 2mm thick piece of aluminum.

Now to the "propulsion system"
The easiest way to me is using a simple spring with a plunger tothrow the parachute out of the main pipe.
So I got three different springs from a local spring supplier for cheap! Here are the dimensions of my favoured spring:
Wire thickness: 2.25mm / Outer diameter: 36mm / Length: 138mm
The plunger was easily made out off an old rexona deodorant spray which has a convenient outer diameter of 45mm
To give you a better picture of these parts, here's what they look like:

Main pipe:
DSC_0258-min.JPG
Top plate:
DSC_0259-min.JPG
Deodorant plunger:
DSC_0261-min.JPG
Propulsion system:
DSC_0262-min.JPG

I drilled holes trough the plunger as well as trough the PVC Cap to connect to the parachutes main string.
Optimally I'd connect the plunger to the spring as well as the PVC Cap and have the parachute only connected to the Plunger to additionally use the spring as sort of a dampener when the parachute opens up.

I hinged the top plate on the main pipe with a small splitring which is not the best idea. You'd be better of with a real hing, connected to the outer face of the main pipe.
Servo-wise I used a Esky 9gr Servo which was laying around, with it's servo horn as the main release arm (Obviously not the greatest idea, but perfectly adequate for this stage of the project ;)) I temporary mounted the servo on the pipe with a small, bent metal bracket and some ziiiiiiiip ties.

DSC_0260-min.JPG

This setup barely launches the folded parachute out of the pipe. Problem being the structural integrity of the folded parachute. This is easily fixed with cut open toilet paper roll as a shell for the chute. This shouldn't be a problem when using a stronger material like the tent foils.

Now, to the hardest part:

The Electronics

When should the thing go off?
I began experimenting with an arduino and an ADXL345 accelerometer, trying to figure out a way to recognize a fast loss in altitude / downward force. While I was messing around and getting stuck with the programm I had a way better idea.
I'll be using the RX's failsafe mode, which will be set to "motorts off". So the conditions to make the recovery system go off will be: FC armed and Failsafe on (motors off) and probably an auxillary switch on my transmitter to arms the system.
Since my Revo hasn't arrived yet, I really can't continue with this part of the Project.

I'll be continuosly updating this post to let you know where I am atm. Hopefully you can give me some tipps with your experience and knowledge, especially for these "problems":
- Mounting the hinge on the main pipe
- Detecting when the motors are off (maybe using the ESC signals or trough current sensing)

Thanks for reading and your ideas. Have a safe flight!

Weight of the whole thing coming soon!

Next Up

-Building the final parachute, using foils of an old tent.
-Detailed parts list
-Weight optimization
-Hinge and Servo mounting

Useful links

- Parachute calculation PDF German (again, in case you missed it) http://www.argoshpr.ch/j3/articles/pdf/Recovery_Pt2.PDF
- Tut on how to fold the parachute https://www.youtube.com/watch?v=pU9DQf_osu0
 
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