Unmanned Air Systems (UAS) Parachute Recovery Technology

 

By Gene Engelgau, Fruity Chutes Inc.
Monte Sereno, 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. This estimate may be low! Not only are 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 an informative page introducing the reader to many of the concepts to take into consideration. Below you will also find an explanation of the choices available within various recovery technologies, their advantages, and their disadvantages. We at Fruity Chutes have been making recovery systems for small to medium size UAS since 2009 and have shipped thousands of parachute systems around the world.

Key factors driving the need for parachute recovery systems include:

• High Value Risk – Not only are the airframes costly, but the payloads can be even more valuable. Total system values of $20K are commonplace. Many commercial UAS can have total system values as high as $500K when you consider exotic imaging sensors, professional cinematography equipment, or advanced sensor technology. 
• Suitable Landing Area – In many flight 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, flying without a backup plan for 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's a win-win.
• Government Regulation – Many countries' aviation rules are now mandating that UAS must include a backup means of recovery in the event of a failure. This includes the countires of France, Canada, Australia, United Kingdom and others who are soon to follow suit. Many of the EU countries are looking closely at this requirement and expect to enact rules mandating backup recovery systems. 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 parachute's “Performance Rating” here.
• Proper Sizing – Choose the optimal size based 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 – The deployment system is the means by which the parachute is ejected from the UAS into the air stream. This can vary widely depending on the UAS type (e.g. fixed wing or multi-copters). Most UAS recovery systems sold are done as an entire system bundle including 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 parachute's job is to provide aerodynamic drag to slow down the payload - the UAS in this case. It must do so by being as light and as strong as possible. There are a number of different parachute types being used, and each has its advantages and disadvantages. Ultimately, different parachute designs can be boiled down to a single number called its “performance rating."  This is a simple ratio between the parachute's load capability divided by the parachute's static weight. Less efficient parachutes 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. This translates to rating the parachute at 8 lb @ 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, which has a load capability of 44 lb @ 15fps, but it weighs just 1.375 lb (22 oz). Its performance rating is 32.

Types of Parachutes

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

 

Comparing Parachute Performance

One of the challenges in comparing designs is that there is no standard in how a parachute's diameter is measured. Since the drag coefficient of the parachute is based on the assumed frontal area, this affects the apparent efficiency of the parachute [or the drag coefficient (Cd)]. Below is a list of parachute types and how the size is measured.

• Cruciform is distance across the parachute.
• Flat parachutes are fabric diameter.
• TAC and Sky Angle panel parachutes are measured diagonally across the top. In some cases, published dimensions are not provided.
• Spherachute is the circumference of the canopy.
• Fruity Chutes are measured based on the opening diameter when inflated.
• Opale provides its 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 (ex: a flat parachute) it is 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, various parameters and the Performance Rating:

* 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 lb @ 15fps. The parachute weighs 1.375 lb. The Performance rating is 45 lb / 1.375 lb = 32 (or 20 kg / 0.625 kg  = 32).

Parachute Construction and Materials

Just as important as the canopy shape are the materials used to make the parachute. 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). In addition, the material may or may not be coated. For parachutes, this usually means a thin silicone coating to seal the fabric. The downside is that it adds about 10% to the weight of the material. Another treatment is to calendar the fabric, which means 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. This 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 they stay 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 because it's slippery.

Shroud Lines and Harnesses – The shroud lines and parachute harness make up the rest of the weight of the parachute. Keeping these as light as possible will increase the performance rating. However, this needs to be done while maintaining the strength. The parachute can open very quickly, especially when moving at higher speeds. The force applied to the load is proportional to 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 parachute 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. Additionally, it is smooth and slick and resists abrasion and tangling. By using Spectra lines, the parachute's 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 factor is that packing density is not dependent on the parachute design as long as we're talking 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 45lb, 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.

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 allows the UAS to descend straight. The shock cord should be from 1.5 to 2 x the parachute diameter. From here, 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, thereby lessening the stress on the UAS. Kevlar can also be used, but you need to carefully consider that Kevlar has very little stretch so the opening shock will be much stronger. There are ways to sew a tear-away section in Kevlar to lessen the opening shock load.

Sizing the Parachute

This is actually pretty simple yet easy to get 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. The size needed, and hence weight, is inversely proportional to the descent speed. For instance, a 60” Iris parachute (IFC-60) is capable of 12 lb @ 15fps (5.44 kg at 4.5 m/s). This is equivalent to falling just over 1 meter! 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 4x the mass and 4x the packing volume! Our performance efficiency of the parachute falls from around 32 to just 8! So always use as small a parachute as possible to expect the UAS to survive with little to no damage.

2. Another issue is that parachutes like to kite in the wind. As such, while your UAS may land softly, the wind will fill the parachute and drag the UAS - which is highly destructive. There is a great (and entertaining) video of this happening here.

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 reasonable descent speed. Of course, it is up the user to ultimately determine the descent speed that is right for them. NOTE - National and regional laws can require a maximum descent speed. In France, for example, new laws were enacted on the first of January 2015 requiring a maximum descent speed of 4.13 m/s.

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 from the compartment.

• Ballistic Parachute Deployment - Also called active deployment, the parachute is packed into some sort of container or canister. From here, 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 a Fixed Wing UAS. For these systems, the parachute is packed into a deployment bag (in most cases). To help pull this from the compartment, a small pilot parachute is provided that the wind will easily catch and pull out the deployment bag. The harness extends until the bag is pulled off of the parachute. The images below show the sequence of events during deployment:

Advantages of a passive deployment system are simplicity and lower cost. Disadvantages are that, for larger parachutes, the packing volume can be higher. This method also cannot be used for multi-copters.

Ballistic Parachute Deployment

In this case, the parachute is packed into a canister (usually cylindrical). Then either a mechanical force is used, like a spring, or a gas is used to eject the parachute. For gas, either black powder or CO2 is used. A general advantage of ballistic deployment is they can be used on UAS that can hover, such as multi-copters. They can also be used on fixed wing UAS. In addition, because ejection of the main parachute is direct, the deployment time is less and they can be used at a 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 parachute is rolled and pushed into a canister that has a retracted spring loaded piston, similar to a 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 lightweight. Disadvantages with a spring system include a limitation to mostly smaller parachutes due to the amount of energy you can reasonably get into a spring. Below is the Skycat.pro 5 kg system using a IFC-60-S parachute.

Gas Deployment - Black Powder - There are a few systems that use a pyrotechnic device to eject the parachute. REBEL Space (which has since ceased operation) provided the system below called RDRS from 1 kg UAVs up to 50 kg.

Gas Deployment - CO2 - CO2 deployment is 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 a BBStratus Cinema Octocopter.

Below is the Peregrine IDS from Fruity Chutes.

Safety (Fail Safe) Systems

Between REBEL Space and some other UAS manufacturers, there are integrated deployment systems under development to be part of the UAS with an electronic reasoning system that decides whether activation of the emergency system or normal recovery system is needed. These systems continuously monitor the location and operation of the UAS. Via a decision tree, they shut down the UAV and activate the recovery system if, for instance, the drone flies out of the predetermined flying area or 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, Monte Sereno, CA, USA. https://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 (no longer in operation), OEM, Manufacturer and distributor of UAS recovery systems to the professional market, aerospace companies, UAS manufacturers and users worldwide. Former Fruity Chutes EU distributor, all REBEL Space systems used Fruity Chutes recovery products.

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

Revisions:

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

V1 - October 15, 2014