Parachutes are Cool! - DIY Multirotor Parachute

by AverageGuyRC | April 28, 2015 | (28) Posted in How To

Last fall I had an epic crash with my semi-scratch built 750mm quad after a propeller came apart in flight.  This was only my 3rd flight with the quad.  I watched helplessly as the quad tumbled out of the sky. I can still hear the thud as it impacted the ground.  After the crash, I collected the pieces and my pride.  I went home to contemplate the gravity of the situation and I decided to look into a parachute system.  This quad was my first endeavor into the Arducopter platform.  I was using an APM 2.6 on the quad that crashed.  I did some research on the Ardupilot web site and found that the platform was beta testing a parachute command with the Pixhawk flight controller.  This peaked my interest.  My intention was to eventually upgrade to a Pixhawk controller after mastering the cheaper APM 2.6.  Based on the information provide on the ardupilot page, I scoured the internet and found that commercially available parachute system were expensive.  The cheapest unit was over $150.   If I were flying an expensive AP platform I could justify spending $150 or more to protect my investment.  It is hard to justify the expense when the parachute system was half the cost of my quad.    I researched building my own system.  None of the systems I could find were very elegant in design.  They ranged from a Tupperware box with a rubber band to having a parachute balled up and rubber banded to the bottom of the quad.  After my research, I decided, in Flite Test fashion, to build my own parachute system that would rival and cost less than the commercially available systems.  Here is what I came up with.

36in Parachute System

42in Parachute System

BUILD

Parachute

One of the biggest challenges in designing the system was the parachute.  I immediately found that commercially available parachutes are also expensive.  In fact, the companies linked on the Arducopter site were charging over $100 for parachutes.  Are you kidding me?  That was not going to cut it.  I did some more research and I found a couple of YouTube videos describing how to build your own.  Hell, the Flite Test guys used a garbage bag for a parachute in one of their videos.  I definitely need to build something reliable that wouldn’t come apart if I was going to spend all this time and effort in building my own system.   After watching several videos I decided to give the internet one last search.   I came across a company called Sperachutes that produces parachutes for model rocketry. http://spherachutes.com/

To my surprise their parachutes were relatively inexpensive when compared to the other sites I had found.  They provided a chart on what size parachute you’d need for a given mass and elevation (MSL) you would be flying at.  I decided to try the 91 cm  (36in) parachute (Size is equal to ½  circumference).  There chart showed it worked for up to 1.27kg (2.8lbs) at my elevation (1524m/5000ft).  The best part was the price.  With shipping (within the US) it cost about $36.  At this cost, I decided to purchase the parachute rather than make it.  I knew that I could probably make a “usable” parachute but decided to save time and purchase a professionally made one.  I speculated that it would cost around $10 or less plus time and effort to make my own parachute.  

An important step with the parachute is how to fold it.  If it is not folded correctly or wrapped tightly enough it could hang up in the tube and not deploy correctly.  Folding takes practice.  Do it several times until you get it right and tight.  This is how I folded my parachute:

Fold both sides the same.  Each side should have the cords together.  

Fold in half so all cords are now together. 

Fold in half again making sure all cords are together.  You may need to make another small fold before you fold lengthwise depending on how big the tube allows for.   

You can either fold accordian style or roll it up from the top of the parachute down towards the cords.  Make sure the cord is wrapped tightly around the parachute.  

Launch System

I decided to go with a spring and tube launch system.  The end result was a system with a mass of 142g (5.0oz) and performance similar to commercially available parachute systems. 

Tube

After I received the parachute, I measured it when folded.  I determined that I needed a tube that was approximately 5cm (2in) in diameter.  I found that the treats I feed my cat came in a plastic tube that was about the sized I needed.  

To determine the length of the tube, I measured the folded parachute and added that to the compressed spring and spring top measurements.  The spring needed to be longer than the tube in order to push the parachute out of the tube efficiently.  The tube for the 91cm (36in) parachute is approximately 8.9cm (3.5in) long and the spring is approximately 15.2cm (6in) long.  

3D Printed Parts

I went with 3D printed parts for the top of the launch tube system.  I have a XYZ Davinci 1.0 printer.  Linked below are the files on the Thingiverse web site.  I used Tinkercad to design the parts.  This site is free and really easy to use.  With the 3D design file, it would be easy to disassemble my parts and enlarge to diameter to fit a different size tube if need be.  

Depending on the 3D printer used, some of the parts may need some sanding or trimming in order to get smooth operation and fit.  I used pushrod wire to make the hing and safety wire.  I heated the wire up and melted it through the plastic in order to install the wires.  

3D Parts: http://www.thingiverse.com/thing:796612

Spring

Since my spring needs were very specific, I made my own spring. The following link will describe how to make a homemade spring. 

http://www.instructables.com/id/Make-your-own-springs-in-seconds/

I was not able to find spring steal and ended up just using some stiff 14ga wire that I found at Home Depot.  A spool cost about $10 for 30m (100ft). As you can see in the videos, this wire works well enough to launch the parachute an acceptable distance.    The other thing I learned was to use a cylinder that is smaller than the diameter than the launch tube you will be using.  There is some rebound after the spring releases when making it.  I ended up using 3.1cm (1 1/4in) PVC pipe for the spring builder.  

I topped the spring with a 3D printed plunger to push the parachute from the tube.  I used some orange p-cord to tie the plunger to the bottom of the tube to prevent it from shooting off when the parachute is deployed.  There needs to be enough slack to allow the spring to go past the top of the tube.  

Attachment Points

I used orange p-cord for the attachment points to the quad in my experiment.  It is important that the vehicle hangs level over the CG.  I used a small screw lock oval to attach the parachute to the cord.  This screw lock oval is way overkill for this but I like the ease of being able to detach the parachute from the copter when it comes time to do any work on the copter.  I intend to replace this cord with something smaller like light cable or Kevlar cord in the future to clean the look up. 

The completed launch tube is attached upright on the tri-copter using on screw and locking nut.  The tube itself doesn’t take any load other than just securing the system to the frame. 

Servo

I used a small 3.7g Turnigy servo (TGY-1370A) for the build.  This servo can be found for about $3.  The biggest concern I have in using the cheap servo is the potential for the servo being damaged by the force of the spring pushing on the lid.  After a couple of months of testing, the servo has held up.  I did have to modify a large servo arm to work with this servo.  The ones that come with the servo are not rigid enough.  I cut the supplied servo arm down and glued this piece into a larger servo arm. 

Safety Wire

I used a safety wire to prevent the parachute from launching by accident while not in the air and to remove the pressure off the servo while in storage.   The wire goes through the two nipples on either side of the lid.   

Controller setup

For my tests, I used a Taranis radio.  I programmed two switches to control the parachute.  Both switches need to be activated for the parachute to launch.  Since radios are not universal, you will need to do some research on how to set this up on your particular radio. 

Testing

Test Vehicle

I decided to use my FT Bat Bone Tri-Copter for the test vehicle.  I only chose this vehicle because it was at its end of life.  I had unintentionally done some drop tests with it, sans a parachute, several months back.  The frame was cracked in several places and the bottom portion of the tilt mechanism was missing.  The servo had to be hot glued in place in order to function properly.  The tilt mechanism was the original design from Flite Test and does not hold up well to abuse.  This proved to be the weakest link in the drop tests.  I destroyed two servos during the drop tests (Well, actually three.  But the third one broke when the tri-copter fell from about 6m  (20ft) when a plug came undone from one of the motors due to violent vibration from the failing tilt mechanism.  I didn’t have enough time to deploy the parachute from that altitude).  I currently have a Dragonfly with a Tough Tilt on it.  I believe that the Tough Tilt would do a lot better than the original FT tilt mechanism.  In addition to the sad shape of the frame, I also had issues with the motors.  The Turnigy Park 300 motors were burned out.  They were unable to lift the tri-copter high enough to do the tests.  When they would heat up, they could no longer product enough lift to keep the copter in the sky.  I ended up replacing them with  DT750s and 11x4.7 props that I had on hand.  Unfortunately, before the swap I crashed twice cracking the frame again.  The test vehicle became the biggest headache for the whole drop tests.  By the end,  hot glue and duct tape were the only thing holding it together.  Between the spongy frame and wild vibration from the tilt mechanism, the tri-copter was a challenge to fly and film at the same time. 

I used plenty of padding to protect my flight controller (RTF Flip 1.5 MultiWii) and receiver. The end result was a test vehicle that weighed 1.16kg (2.55lbs). 

Test Setup

I conducted several test on the parachute system.  My first test was to determine how well the parachute would launch while in flight.  I statically tested the launch tube while on the ground and it seemed to work well, but I needed to see it work in flight.  I set up a down comforter in my living room and flew the tri-copter above it about 2m (6.5ft) and activated the parachute.  As you can see in the video I did a relatively poor job of dropping the tri-copter on the comforter.  As a side note, it did not sustain any damage from those drops.  Based on the slow motion video the parachute system performed well on these test drops.  

I also did static drops at both 2m (6.5ft) and 4m (13.1ft) to measure the drop time from a given height.   I used this information to estimate the drop distance in a real world drop before the parachute fully deployed.  Based on the video, it took approximately 0.5 sec to drop 2m and 0.8 to drop 4m.  Using the 4m drops, this would be equivalent to approximately 5m (16.4m) in the first second of drop time.  Since I believed the parachute should open within 2 seconds, I figured this calculation would provide a large enough margin for error for two reasons. First, these drops did not take into account the drag created by the spinning propellers.  Second, they did not take into account the drag of the parachute as it is deploying.  My end results are an estimation based on these numbers and are used only as a buffer to determine the lowest altitude for the parachute to fully deploy. 

I conducted four drop tests.  All of the tests were done at an altitude of greater than approximately 40m (131 ft) based on a variometer used to calculate altitude. Due to issues with the on board cameras and the difficulty of filming and flying, the first video above was pieced together from several tests to demonstrate the results.  

Results

Based on an onboard camera, on one of the drop tests, the system was able to fully deploy the canopy of the parachute in less than 1.5 seconds.  This was the time from the servo activation to the parachute coming into frame and being completely open.     Other videos revealed that the parachute canopy began to open within one sec after activation. Using 5m of drop, as calculated above, for the first second of drop time and considering the drag created by the parachute as it opens during the next 0.5 seconds, it is reasonable to assume that the drop distance will be no more than 7.5m (24ft).  This distance does not take into consideration the reaction time (time to recognize a problem and then activating the parachute).  If I added a second for reaction time, that would result in an additional drop of over 5m (as the vehicle continutes to drop the velocity will continue to increase past the calculated 5m/sec).  The vehicle now has close to two seconds to accelerate before the parachute starts creating drag.   If I stick with the 5m/sec, 12m is “probably” a reasonable buffer to deploy the parachute with the understanding that an increase in altitude would provide more time for the parachute to decelerate the test vehicle to a stable decent rate before it hits the ground.  My calculations are of course a “best guess” as they are loosely scientific in nature.  I would welcome anyone to take my data and calculate a theoretical drop distance and compare that to what I calculated.   

Three of the fours tests resulted in the tri-copter coming down in the correct orientation landing top up.  A fourth test resulted in tangled attachment cords which cause the tri-copter to land tail first (servo destroyed).  Ultimately, the vehicle sustained minor damage in each drop.  The parachute used (91cm/ 36in) according to the manufacturer was at is limit for mass at the elevation tested.  A larger parachute would help to lessen the impact force upon landing.  The tradeoff is mass and size of the launch system.  With the understanding this is a last ditch effort to prevent major damage to the multirotor,  I would stick with this system for the test mass used.  The tri-copter was easily repaired after all of the drop tests (even with the poor condition prior to the testing).

Cost

The cost of the system will be varied based on the availability of parts and access to a 3D printer. My costs were as follows:

Parachute: $36

Servo: $3           

3D Pinter material: $3

Spring: $0.50 (wire spool cost $10 for 30.5m (100ft) and the spring uses about 5 feet of wire)

Tube: Free

Total: $42.5

Keep in mind, if you don’t have items on hand as I did, like the PVC pipe, quick link and p-cord, the initial cost could go up by~ $20.

Conclusions:

Even with a total cost of ~$62, this system is far less than any commercially available system and it performed well.  Based on this design other size parachutes can be used with slight modifications to the system.  I currently have a system set up with a 106cm  (42in) parachute.   

With all the time and effort I put into this, how necessary is a parachute on a hobby level multirotor? If using quality parts is it really necessary?  If I had used higher quality propellers, would I have even gone through all this?  I can come up with two use cases for a DIY parachute. First, if you are flying hobby level AP platform with an expensive Gopro style camera/gimble and you regularly fly over 12m it may be worth it to protect your investment(see note below).   The second reason occurred to me last week when I was flying line of sight and I lost orientation.  The wind picked up my FT Dragonfly and started to carry it away.  I did not have a parachute attached at the time.  The wind was pushing it towards houses.  I was seconds away from cutting the throttle so it wouldn’t come down over houses and people.  Thankfully I regained orientation and didn’t have to ditch the Dragonfly.  Had I had a parachute attached, I could have activated it instead of ditching the aircraft.  Similarly, it could be used as a last ditch effort with loss of video signal during FPV flights or during a flyaway, assuming the receiver was still connected to the transmitter. 

 So, do you really need a parachute on your multirotor? Maybe not, but parachutes are cool!

*Note:  My testing did not take into consideration the impact force on landing.  With a top up orientation, a camera and gimble would be the first part of the multirotor to hit the ground after the landing gear/skids etc.  If you are trying to protect an investment like a camera and gimble, a larger parachute would be recommended based on the force your landing gear/skids can absorb to prevent any significant impact on your camera setup.     

COMMENTS

1959cutter on April 28, 2015
air command rockets website has a great deployment for water rockets made from empty pop bottles,that ejects the parachute in a similar manner without a steel spring.
i love the video,looks promising.
i've thought about a system for fixed wing also.
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cucubits on April 29, 2015
In my opinion this is not a very good solution. Please bear with me. As it is, it seems heavy and very bulky compared to the size of the copter. Also it's inefficient, as I noticed it still falls with too high speed. However these are not the main problems with it. This would 'save' the copter only in a very limited types of emergencies: 1. you assume you will have control over the servo to deploy when something goes bad, so only cases where it maybe able to help is if as you said a prop falls off. Any electronic and you maybe unable to control the servo; 2. you assume you will be high enough to flip the switch after you notice it already falling at probably terminal velocity. Also, sorry but all your tests assumed the copter is still upright when deploying. In a real life case, after a prop/motor fails, it will most likely tumble and is the parachute deploys while doing that, it will most probably get tangled in the arms/other props, copter itself.

TLDR: Only maybe able to save the craft in a very limited type of situations. Not effective.
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AverageGuyRC on April 29, 2015
It definitely is big and bulky relative to the tricopter. I used it as the test vehicle because I didn't mind it getting destroyed. The intent is not for use on smaller multirotors. I wouldn't use it on a tricopter if I ever where to replace the one I used. I have to disagree with reference to the FT Dragonfly or the 750 quad as pictured. . The mass and bulk doesn't noticibly change the flight characteristics. You are correct that it's use is limited. It is a last ditch effort that "may" limit damage. It comes down to being able to fix your vehicle or having to replace your vehicle. It does require control over the servo. If the problem is power related than it won't work. Most higher end receivers and flight controllers allow for setting fail safes. It could be argued that a properly set up fail safe could activate the servo even if you no longer have control of the servo. Additionally some of the high end flight controllers, like the pix hawk, can deploy the chute automatically. Altitude is another constraint for sure. Even the commercial units require at lease 10m to deploy. How often do you spend above 10m? That would dictate the need. Reaction time is also key. I think most people could react within 1 sec. But that would require training and muscle memory. I would suggest someone practice by running scenarios in there head with there eyes closed to build that muscle memory.(with the copter on the ground of course). You are also correct my tests were done upright. If I had more $ and time I'd love to test by cutting out one engine. Will it get tangled? Maybe. In fact during one of my tests it did get tangled and the parachute still worked. The cords on the parachute are long enough to accept some tangling. As the parachute deploys, it will have the same rotational velocity of the copter. Until the canopy starts to open it will be rotating with the copter. Parachutes are not for everyone. Are they really needed? Maybe not, but parachutes are cool.
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Kurt0326 on April 29, 2015
Now I what to build one for a plane. God job!
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AverageGuyRC on April 29, 2015
I have some ideas for a plane versions. I got some of my inspiration from full scale parachute systems like what are used on Cirrus airplanes.
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apnewton on April 28, 2015
Brilliant and mad. Great article. 5 stars from me.
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HilldaFlyer on May 4, 2015
That is cool.
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NullVoxPopuli on April 28, 2015
that's pretty cool! if there were something small and maybe <= 10 grams, I'd totally attach one to my warpquad.
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yeahsure on April 28, 2015
10 grams for a parachute and launch system? I don't think that's even possible.
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earthsciteach on April 28, 2015
$46 recovery system - Priceless!
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Mrslaney on April 28, 2015
Distance of a fall under gravity (assuming no drag forces) is x = (0.5) g * t ^2. Where, x is fall distance in meters, g is acceleration due to gravity in m/s/s (9.81 on earth), and t is time in seconds. Assuming there is little to no drag produced until the parachute fully deploys allows a calculation of total fall distance to deployment of about 11 meters if you exclude reaction time. However if you include reaction time the fall distance begins to deviate from your estimate. Average human reaction time is about a quarter of a second. Adding this to the time before deployment (now 1.75 seconds) yields a total fall distance of 15 meters. If you assume that it takes an entire second to realize something is wrong and hit the switch on your transmitter then the fall distance increases to 30 meters.
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AverageGuyRC on April 29, 2015
Thank you for the math. 12m sounds fairly reasonable especially when considering the drag.
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Mrslaney on April 29, 2015
I think you are over estimating the drag from the partially deployed parachute. If I was looking to protect any gear I cared about I would estimate fall distances GREATER than 15 meters. Drag is directly proportional to frontal area. The folded up chute has a very small area compared to the deployed chute. The 1/4 second reaction time is slightly optimistic. It also takes time for the copter to slow down to terminal velocity when the chute does deploy.
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AverageGuyRC on April 30, 2015
I should clarify my terminology. When I say there is drag from the parachute, I am talking about when the canopy of the parachute begins to open. Base on the testing, the parachute is out and unfolded but the canopy hasn't begun to open at 0.8 sec. I make an assumption that the canopy begins to open at 1s. So essentially, there is no drag from the parachute during the first sec. Based on the testing, the vehicle will drop approximately 5-6m during that first sec. I also know that the parachute canopy is completely open at 1.5s. The only reason I know that is during one drop, the parachute with completely open canopy happens to come into frame at 1.5s. What I do not know is exactly when the canopy begins to open and when the canopy is completely open. I also do not know the velocity after the parachute completely opens and the drop rate stabilizes. This would require further testing on my part.
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HarleyRev on April 29, 2015
great article ! If I had a multirotor that cost $1000 and more, probably one that has expensive video equipment, I would install a chute like this on it for sure. Even if it could only save the coptor 25% of the time, it would still be a smart investment I think.
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George_Tucker on February 20, 2016
First off GREAT article!! You remind me of my brother - a great thinker with excellent articulation.
So I am here learning about RCDrones which started from the wife insisting on aerial photography.
because I am investing in wifey's hobby I did a LOT of reading on what a beginner needs and does with their new aircraft. We will be Using the SYMA X8G. I look at it like she is the pilot and I am the RC A&P mechanic. So Like you, I agree the folks that sell the parachute deployment systems or PDS ( I had to go there. I love acronyms) charge a mint. I want to thank you AverageGuyRC for the spring idea and the link to where you found it. I do ALOT of research and because I don't have access to a 3D printer . I had to think inside the Tube....I am using a flip up lens cover from a 50mm scope for the top. I will update when I get all my parts that I ordered and start to put it together. Also that auto deployment system (ADS) out there --What do you think.?? ...a glorified alarm circuit-Break using the buzzer connection to activate the servo?? A Question ???? If not using the auto deployment system.(ADS) and you need to launch the parachute I can't seem to find what or where to connect the servo to to have a remote push button deployment. Are you using like a blue tooth transmitter ? or a button on the present controller? One last thing - I didn't have any cat treats so I bought a three pack of baby bottles and chopped the top off - filed it down to fit into the bottom half, flipped it over, connected the spring to the threads so as not to lose the ejection plug. I will also be using the servo lever to pull a short wire with bend bend and over to a small metal lever about an inch in length 1/8 " in width with a middle swivel point screw to pop the lid.
Again thank you for the GREAT Ideas you got my mind working again . I just hope I can make this work ...... considering this is my first venture into RC and flight I seem to be one who flies by the seat of ones pants..... Thanks Again AverageGuyRC Hope to be back here soon....
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Parachutes are Cool! - DIY Multirotor Parachute