Unmanned Air Systems (UAS) Parachute Recovery Technology

 

By Gene Engelgau, Fruity Chutes Inc.
Los Gatos, CA
http://fruitychutes.com  

Introduction 

The emerging UAS industry (aka drones) is experiencing an era of rapid growth as of 2014.  The Teal Group published a forecast this year that the industry will grow in excess of $91B over the next 10 years.  That may be low!  Not only is the number of products offered exploding, but the value, size and weight of these systems is also increasing rapidly.  In addition there are growing concerns about the safety and reliability of these systems and the desire to offer a means of safely recovering these systems in the event of a failure, or if there is a lack of suitable safe landing areas.  The most cost effective method to mitigate these concerns is the simple and humble parachute.   With this in mind I wanted to write a paper introducing the reader to many of the concepts to take into consideration.  Also to help understand the choices available with various recovery technologies, their advantages, and disadvantages.  We at Fruity Chutes have been making recovery systems for small to medium size UAS since 2009 and currently have shipped thousands of parachute systems around the world.

The key factors driving the need for parachute recovery systems are:

  • Value can be very high – Not only are the airframes valuable but the payloads can be even more valuable.  Total system values of $20K are commonplace.  Many commercial UAS can have total systems values as high as $500K when you consider exotic imaging sensors, professional cinematography equipment, or advanced sensor technology. 
  • Suitable Landing Area – In many situations the UAS may be used in areas where there is no way to land it.  Fixed Wing UAS in particular can have issues when used in remote locations and where there is no landing field.  In this case a parachute system becomes the primary recovery method.
  • Safety – As these systems grow in weight and size having them crash when something goes wrong is not a good idea.  In our current litigious society many UAS providers and users equip their system with a backup recovery system.  Some have found that providing a recovery system lowers the operational liability insurance premium by more than the cost of the recovery system, it is a win-win.
  • Government Regulatory – Many countries aviation rules are now mandating that UAS must include a backup means of recovery in the event of some sort of failure.  Countries included are France, Canada, Australia, United Kingdom and others are following suit soon.  Many of the EU countries are looking closely at this requirement and expected to enact rules mandating this.  In the United Stated it’s only a matter of time before the FAA takes this up as well and mandates a similar rule in order to mitigate public safety concerns. 

While many UAS users and businesses have thought about parachutes as a method of recovery, most have less understanding about the types of products being offered, the deployment methods, and what are the best products to use depending on the type of UAS.  

Factors to consider for a UAS Parachute Recovery System include:

  • Type of Parachute – This includes the canopy shape, type of canopy materials used, strength, and weight.  We will introduce the concept of the parachutes “Performance Rating” here. 
  • Proper sizing – Choose the optimal size bases on your UAS weight.  Too small can lead to damage upon landing.  Too large and the UAS can be dragged by the wind and have delayed deployment time. 
  • Deployment System – Deployment system is the means of which the chute is ejected from the UAS into the air stream.  This can vary widely depending on the UAS type (eg fixed wing, or multi-copters).  Most UAS recovery systems sold are done as an entire system bundle having everything needed.  
  • Safety Systems – An emergent need is developing for secondary systems that monitor the flight characteristics of the UAS and deploy the parachute if an abnormality is detected.

The Parachute 

Central to all recovery systems is the parachute.  The chutes job is to provide aerodynamic drag to slow down the payload, in this case the UAS.  And it needs to do so by being as light as possible and strong.   There are a number of different parachute types being used and each has its advantages and disadvantages.  Ultimately different chute designs can be boiled down to a single number called its “Performance Rating”.  This is a simple ratio between the parachutes load capability divided by the chutes static weight.  Less efficient chutes have a lower load rating.  For instance the flat sheet parachutes used on low cost recovery products have a performance rating of at most 8:1.  Meaning if you rate the parachute at 8 lbs @ 15 fps, but the parachute weighs 1 lb.  At the other end of the spectrum we have the Iris Ultra Compact IFC-120-S parachute has a load capability of 44 lbs @ 15fps, but it weighs just 1.375lbs (22 oz).  Its Performance rating is 32.

Types of Parachutes

The most important factor determines the efficiency, meaning maximized drag per given canopy weight, is the canopy shape and design.   Below is a list of common parachute design types:

Cruciform – This parachute is shaped like a cross.

Panel parachute – TAC, Sky Angle and The Rocketman
 

Annular or Pull down Apex - Fruity Chutes Iris Ultra
 

Flat Sheet parachutes – Canopy constructed from a single flat sheet of nylon 

Elliptical - Fruity Chutes 120” Classic Elliptical

 

 

 

Comparing Parachute Performance

One of the challenges in comparing designs is there is no standard in how a parachute’s diameter is measured.  And since the drag coefficient of the chute is based on the assumed frontal area this affects the apparent efficiency of the chute, or the drag coefficient (Cd).  Below is a list of parachute types and how the size is measured:

  • Cruciform is distance across parachute.
  • Flat parachutes are fabric diameter.
  • TAC and Sky Angle panel chutes are measured diagonal across the top.  In some cases published dimensions is not provided.
  • Spherachute is circumference of canopy.
  • Fruity Chutes are measured based on the opening diameter when inflated.
  • Opale provides it's parachute size based on the canopy area. 

The aerospace industry usually specs the Cd (coefficient of drag) in reference to the projected frontal area.  This is considered the gold standard in measurement and is independent of the canopy shape or construction methods.  For some parachute types (for instance a flat parachute) it’s difficult to predict the projected frontal area while under flight.  In this case the parachute measurement is simply based on the distance across the parachute canopy.

Below is a table comparing different types of parachutes with regards to how they are measured and various parameters and the Performance Rating:

Chute Type

Measure Type

Stability *

Cd

Cost

Performance Rating @ 15 fps (4.5 m/s) **

Use

Cruciform

Across parachute

Good at any speed

Low – 0.4

Medium

Low

High speed drogue or main parachute

Flat Sheet

Across parachute

Ok at low speed,

poor at high speed

Low – 0.7

Low

Approx. 8:1

Main or drogue

Panel Style

Across top panels, usually on diagonal

Good vertical stability, can rotate or spin

Med – 1.1

Medium

Approx. 10:1

Mostly as a Main

Elliptical (Classic Fruity Chutes)

Opening diameter

Medium high speed, Good low speed

Med – 1.6

Medium

13.4:1

Main or Drogue parachute

Annular (Iris Ultra)

Opening diameter

Good low speed

High – 2.2

High

32:1

High performance main parachute.    Ideal for UAS recovery

* The tendency of the parachute to stay directly above the load.  Some parachutes move from side to side especially as the descent speed increases.
** This is the descent rating of the parachute at 15fps (4.5m/s) divided by the parachute weight.  For instance the IFC-120-S parachute is rated at 45 lbs @ 15fps.  The parachute weighs 1.375lbs.  The Performance rating is 45 lbs / 1.375 lbs = 32 (or 20 kg / 0.625kg  = 32).

Parachute Construction and Materials

 Just as important as the canopy shape are the materials used to make the chute.    This will directly impact the weight, and hence the Performance Rating.

Canopy - Most of the parachute weight is in the canopy, between 50% to 80%.  All modern parachutes use rip-stop nylon.  There is a wide range of fabric weights, treatments, and quality.  Most consumer grade nylon you can buy in a store or online weighs 1.9 oz / square yard (0.045 kg / m2).  Most certified rip-stop parachute fabric is around 1.1 oz / sq yard (0.026 kg / m2).  Also material can be coated or not.  For parachutes this usually means a thin silicone coating to seal the fabric.  The downside is it adds perhaps 10% to the weight of the material.  Another treatment is to calendar the fabric, which mean to flatten the weave to lower the porosity at no increase in weight.  This is also referred to as F111 fabric and is used commonly in reserve parachutes.  It is preferable when parachutes need to stay packed for long periods of time (years at a time if properly done).  Coated fabrics can sometimes stick together or resist opening if it stays packed for long periods.  F111 fabric can be packed reliably for years at a time if kept dry.  Silicone fabric can be hard to handle and pack since it is slippery.

Shroud Lines and Harnesses – The shroud lines and parachute harness make up the rest of the weight of the chute.  Keeping these as light as possible will increase the performance rating.  But this needs to be done so while maintaining the strength.   The parachute can open very quickly, especially when moving at higher speeds.  The force applied to the load is proportional for the square of the speed at deployment.  Opening shock loads as high as 15G’s or more are possible and can put a huge strain on the chute canopy and the lines.  Maintaining high strength and a good design margin is desirable. 

Common materials for shroud lines are nylon, polyester, Spectra or Vectran.  One of the best materials to use for shroud lines is Spectra.  It has the strength of Kevlar so the lines can be made smaller and much lighter, and is very slipper and resists abrasion.  Spectra resists tangling as well.  By using Spectra lines the parachute overall weight can be reduced as much as 35% with no decrease in parachute strength.   

Parachute Packing Density

 Having the parachute as light as possible means it can be packed into a smaller volume.    A key fact is that packing density is not dependent on the parachute design, as long as we're taking about nylon or materials of similar density.  This has been verified by a number of studies done by different research groups.  One study found that a density of 30lb per cu foot (.28 oz / cu") can be reliably achieved by compressing the parachute at an equivalent force of 15 psi.   Compressing at 100 psi yields a density of about 45lbs, but at this pressure you start to damage the nylon.  For more see Study of Pressure Packing Techniques published in June 1962.

Parachute Packing Methodology -  The packing density varies depending on the method used.  The table below shows the packing density that can be achieved using various methods to pack the parachute.

Packing Method

Description

Packing Density (oz / cu”)

Packing Density (g / cc)

Traditional Fold and Wrap

Traditional fold, roll and wrap parachute. This is the most common method used by most rocketry folks.

0.13

0.23

Deployment Bag

The parachute is packed into a deployment bag. 

0.16 – 0.2

0.28 - 0.35

Canister Pack

Chute is packed into a ridged canister.

As high as 0.28

0.48

Using the packing density factors given simply divide the parachute weight by the factor to obtain the volume.

Harnesses, shock cords and other rigging

 You do not want to connect the parachute directly to the UAS, it will not be stable and can oscillate back and forth.  Instead the parachute needs to be connected to the UAV via a length of shock cord.  This makes the configuration more stable and the UAS to descent straighter.  The shock cord should be from 1.5 to 2 x the chute diameter.  From this we connect to a Y-harness to provide a multi-point connection to the UAS.  See the example below of a properly sized harness and shock cord.   

The shock cord can be made with Nylon or Kevlar.  Nylon will absorb more of the initial opening shock lessening the stress on the UAS.  Kevlar can also be used but one does need to more carefully consider that Kevlar has very little stretch so the opening shock will be much stronger.  There are ways to sew tear away section in Kevlar to lessen the opening shock load. 

Sizing the Parachute

This is actually pretty simple, but easy to get it wrong.  For example assume you want a parachute that lowers your UAS at 10 fps.  This sounds good but there are a number of problems. 

  1. First the size needed, and hence weight is inversely proportional to the descent speed.  For instance a 60” Iris Chute (IFC-60) is capable of 12lbs @ 15fps (5.44 kg at 4.5 m/s).  This is equivalent to falling just over 1 meter!  But if we want to bring down the UAS at a nice leisurely speed of 7.5fps (2.28 m/s) we need an IFC-120 parachute, which has 4 x the mass, and 4 x the packing volume!   Our performance efficiency of the parachute falls from around 32 to just 8!  So always use as small a parachute as you can to expect the UAS to survive with little to no damage. 
     
  2. Another issue is that parachutes like to kite in the wind.  So while you’re UAS may land softly, the wind will fill the chute and drag the UAS, which is very destructive.  There is a great video of that happening here: http://youtu.be/ETvOoAWVxVA 
  3. Updrafts in the air, called thermals, can cause a UAS to actually go up and hang in the air for a long time.  In addition the UAS can also drift a considerable distance.

Fruity Chutes rates its UAS recovery systems at 15 fps, which in most cases is a reasonably descent speed.  Of course it is up the user to ultimately determine the descent speed that is right for them.  NOTE - New laws in France for instance ask for a maximum descent speed of 4.13 m/s from the first of January 2015.

Deployment Systems

The deployment system refers to the methodology used to eject the parachute into the air stream.  There are two common methods:

  • Passive Deployment – The parachute is packed into a deployment bag and stowed under a hatch.  In very small systems the parachute may be folded and wrapped directly (not in a bag).  To deploy the hatch is popped open and the wind fetches the parachute for the compartment. 
  • Ballistic Parachute Deployment – Also called active deployment the parachute is packed into some sort of container or canister.  Then various methods can be used to forcefully eject the parachute into the air stream.

Passive Deployment Systems

This can be used when you have forward flight all the time such as for Fixed Wing UAS.  For these systems the chute is packed into a deployment bag (in most cases).  To help pull this from the compartment a small pilot parachute is provided that will be easily caught by the wind and pull out the deployment bag.  The harness extends until the bag is pulled off the chute.  Below shows the sequence of events during deployment:

 

 

 

 

 

Advantages of this system are simplicity, and lower cost.  Disadvantaged are that for larger chute the packing volume can be higher.   This method also cannot be used for multi-copters. 

Ballistic Parachute Deployment

In this case the chute is packed into a canister, in most cases cylindrical.  Then either a mechanical force is used, like a spring, or a gas is used to eject the parachute.  For gas we have either black powder, or CO2 gas.  An advantage in general to ballistic deployment is they can be used on UAS that can hover, like multi-copters.  They can also be used on fixed wing UAS.    Also since ejection of the main parachute is direct the deployment time is less and they can be used at lower altitude.  In any case for multi-copters the rotors do need to be cut after deployment so they don’t get tangled up in the parachute rigging. 

Spring Ballistic Deployment Systems – The Skycat.pro system is an example of a spring-loaded canister.  The chute is rolled and pushed into the canister that has a spring loaded piston retracted .  Think jack in the box, but with a more powerful spring.  Upon deployment a servo channel melts the wire retaining the spring and the parachute deploys.  Advantages are simplicity and relatively light weight.  Disadvantages with a spring system is it’s limited to mostly smaller chutes due to the amount of energy you can reasonably get into a spring.  Below is the Skycat.pro 5Kg system.  It used a IFC-60-S parachute.  

 

Gas Deployment – Black Powder – There are a few systems that use a pyrotechnic device to eject the chute. REBEL Space provides the system below called RDRS from 1 kg UAV’s up to 50 kg on the moment.  In the near future this system will also be available using a CO2 charge that is electrically activated. 

Gas Deployment – CO2 – CO2 deployment the best method for active deployment.  CO2 gas can exert a large amount of pressure and force and provide good loft even when ejecting very large parachutes.  These systems can be used on UAS up to 114 lb (52 Kg) or larger.  

Below is a PIDS-4-120-S on BBStratus Cinema Octocopter.

Drone Parachute System powered by CO2 Gas

Below is the Peregrine IDS from Fruity Chutes.

Safety (Fail Safe) Systems

Systems that are under development at REBEL Space together with some other UAS manufacturers are integrated deployment systems that are part of the UAS with an electronic reasoning system that will decide if activation of the emergency or normal recovery system is needed.  These systems continuously monitor the location and operation of the UAS, and via a decision tree they shut down the UAV and activate the recovery system if for instance the drone flies out of the allocated flying area of if a technical malfunction occurs that dangers the surroundings or the UAV operation.  These are completely separate from the flight controller in case the FC CPU malfunctions.  Look for more information about these soon.

Companies providing products referenced in the tutorial:

Fruity Chutes Inc, Los Gatos CA, USA. http://fruitychutes.com – Manufacturer of complete UAS recovery systems and parachutes to the professional market, aerospace companies and users.  Fruity Chutes is also an OEM of deployment systems to UAS manufacturers world wide.

REBEL Space BV, 1601 MK Enkhuizen, the Netherlands.  http://rebelspace.eu/ - OEM, Manufacturer and distributor of UAS recovery systems to the professional market, aerospace companies, UAS manufacturers and users worldwide.  Fruity Chutes EU distributor, all REBEL Space systems use Fruity Chutes recovery products.

Skycat.pro, Tammela Finland.  http://www.skycat.pro - Manufacturer and distributor of multi-rotor spring actuated recovery devices.

revisions:

V1 - October 15, 2014

V2 - October 28, 2014, Monor updates and added references to metric units.  Added section about Saftey Systems.

v3 - September 3, 2015, Minor spelling corrections

V4 - February 27, 2016, Freshen content

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