The drone parachute

Let's talk about parachutes.

Multicopters can become deadly weapons if the motors stop spinning as they should. There are so many reasons for a failure to happen (bad motors, bad battery, radio problems,etc...)  that it is absolutely necessary for the pilot to anticipate it, especially if you intend to flight over people's head!  We made the choice to never fly directly above people, but if you're willing take that risk,  you might be interested into getting a parachute on your drone . That's why we designed last year a DIY parachute for our quadcopter  🙂

The principle is very simple : the parachute is carefully folded  in a tube on top of which lies a lid that a servomotor keeps closed. At the bottom of the tube, a big spring pushes strongly the parachute towards its top end. So if the servo lets the lid go, the spring will push the parachute in the air.

It may be hard to believe, but the parachute will fit in the tube !
It may be hard to believe, but the parachute will fit in the tube !
The spring is separated from the parachute by a CNC milled ring.
The spring is separated from the parachute by a CNC milled ring.

I recommend you use a strong spring, so the parachute is entirely pushed out when the servo releases the lid. The spring we used is 10cm long, has 8 turns and is 3.5cm wide. It can be fully compressed under pressure, which is what we wanted here, in order to get the smaller parachute we could.

A hook holds the string to the bottom plate
A hook holds the string to the bottom plate

One of the major problems with parachutes is that the air needs to get "under" the parachute envelope in order to make it act as it should. If it doesn't , the parachute will become useless. We followed the method described in the following video :

 

This is how the parachute looks before we insert the actual parachute. The spring will eventually be pressured until its minimum lenth
This is how the parachute looks before we insert the actual parachute. The spring will eventually be pressured until its minimum lenth
We used a glass fiber bottom plate as it needs to resist to a lot of pressure
We used a glass fiber bottom plate as it needs to resist to a lot of pressure
Bottom plate. It can attached to your frame by screws or zip-ties (we love them :) )
Bottom plate. It can attached to your frame by screws or zip-ties (we love them 🙂 )

Parachute surface calculation

 

Obviously, your parachute's surface needs to match the weight of your copter, and the desired landing speed you would like to reach. I think a commonly accepted falling speeds sits around 5m/s (18km/h).

We found in the past a pretty useful formula that gives the diameter of tissue to use depending on the weight you want to slow down and its expected falling speed:

 Diameter = \frac{70*\sqrt{m}}{V}

, with m in grams and V in km/h.

 

For our 1kg drone, it gives us a diameter of 122cm. As you can see, the parachute can quickly become huge compared to the size of your multicopter, and you should maybe start thinking about where to place it on your drone from the very beginning of its conception.

For our quadcopter, we used a slightly smaller parachute (taken from a distress rocket) than the 120cm recommended size given by the formula, which led to higher falling speeds than the expected 5m/s, but it stayed slow enough to keep the drone intact after many, many test flights ! 🙂

Parachute ready for insertion
Parachute ready for insertion
See, it fits ! :)
See, it fits ! 🙂
Ready to go!
Ready to go!
The servo arm is under a lot of pressure here.
The servo arm is under a lot of pressure here. We used the TGY-53317M servo for this parachute.

 

Here's the final video 🙂


Comment if you have any question ! 🙂