UNDERSTANDING TURBULENCE for sUAS

 

Image result for drone crash, buildingAs sUAV/drones become more and more popular, it seems that more and more of them are striking the sides of buildings, trees, or poles without the pilot understanding why.
“It was flying fine and all of a sudden it zipped up and into the side of the building.” “Everything was great until the drone had a mind of its own and flew straight to the ground.”
“The drone was flying over the trees and all of a sudden it spun around and dropped into the trees.”

Reading forum conversations around the internet suggests this is a common, yet unfortunate and avoidable experience.

First, let’s establish that flying in GPS mode may be ineffective when very close to a building. Signal may be lost, and this could explain a few of the building strikes.

However, far and away more likely in most instances the UAV was caught in a “rotor.” These are also known as up/down drafts, lee waves, or cross-winds, depending on which aviation discipline one adheres to. Needless to say, these phenomenon do exist, and play havoc with any sort of aerial activity whether it’s wingsuiting, parasailing, skydiving, model aircraft flight, swooping, small aircraft, and particularly light-weight multirotors.

Image result for wind turbulence map
These “waves” are indicators for manned aviation and construction crews, yet the principle is
only a matter of scale.

Even when a manufacturer provides a statement of stability in “X” winds, this should not fool a pilot into thinking that the sUAS is turbulence-resistant. Given enough turbulence or infrequency of a wave, the UAV will become unstable.

It’s always better to be down here wishing we were up there, instead of being up there wishing we were down here.

The first rule is to set wind limits. Small quad-craft should stay on the ground at windspeeds of greater than 12mph/5.5 meters per second. Hexcopters should consider grounding themselves at 22mph/10meters per second. Of course, this figure may vary depending on your organizations policy and procedures manual, insurance requirements, or payload on the sUAS.

This video provides some demonstration of the cycle of the wave and how a gyro and accelerometer might cope with the cycles. Notice how all the aircraft are “cycling” in an attempt to maintain altitude and position, even as the waves of the wind rotate?

Truly, knowing about them is half the battle. Staying away from them is the rest of it. Failing the former, being able to manage the craft in turbulence is the next-best step.

A building blocks the wind on one side (windward side) and on the opposite side (leeward side) the wind will pay all sorts of havoc with any flying object. Winds will extend in distance up to four times the height of the obstacle, and two times the actual height.

Understanding Turbulence 2

40×4=160 feet. Therefore, for 160’ beyond the obstacle at ground level, your multirotor is at risk for catching either a down draft or an updraft.

Huh?

OK, say there is a building that is 40 feet in height, and you have a medium wind blowing. Gusting or steady, it makes no difference.

40×4=160 feet. Therefore, for 160’ beyond the obstacle at ground level, your multirotor is at risk for catching either a down draft or an updraft. Either way, the airframe/hull is not in clean air. In extremely high velocities (high winds) the ratio of obstacle/distance may be as great as 15X (of course, a UAS would likely not fly in these winds)!

In terms of height, depending on wind velocity, the UAV may have to climb as high as 80’ to find clean air above an obstacle. yet at 80′ AGL, the winds are likely entirely different as well, depending on the weather and other obstacles in the area.

The air goes over the obstacle and is “pulled” to the ground (downdraft), where it then “bounces” upward (updraft) and tries to resume its level flow.

These phenomena are entirely independent of  sinks,thermal rises, dust devils, and the like.

This also occurs in natural/unbuilt up areas. Trees, canyons, ridges, rock-lines; any large object will incur rotors. Avoid them. It’s virtually impossible to determine exactly where the down draft vs. the updraft may be occurring, and the location of these dirty winds will change with swind velocity.

Understanding-Turbulence-3

FLYING IN URBAN ENVIRONMENTS

When wind flows between buildings, the mass of the air/gas is compressed. This results in an increase in velocity. Think of squeezing hard on a tube of toothpaste, compressing the contents through the tiny hole in the end of the tube. This increases the speed/velocity at which the toothpaste squeezes out. The same thing occurs with moving air between buildings or other solid objects.

Depending on the wind speed, the increase may require as much as 4-10 times the distance before the winds return to “normal” velocity seen before the gap or corner.

Image result for Wind
Image courtesy of Rheologic

Ground winds and winds “aloft” (true winds aloft are beyond the reach of most UAS operations) are rarely equal. Winds at 50′ are rarely the same as winds at ground level in an urban or suburban environment.  Even small berms in the ground can cause jarring turbulence (as shown above) that settle in the low areas. These urban “microclimates” can be very problematic for light weight UAS in required-precision environments.

Turbulence

Here is a more complex example of winds blowing at 22mph in an urban environment.

Complex Winds.JPG

complex winds 2

Compression of the flow due to building dynamics push the wind into more than 40mph in some areas. While the overall winds, and reported winds in the area suggest that the windspeed is perfectly acceptable for most commercial aircraft, turbulence and accelerated velocities within tight areas are far beyond the risk limits of most small UAS’.

Flying from warm sands to flying over water on a hot summer day may also create challenges to smooth and level flight.

DUST DEVILSImage result for dust devil

Dust devils are summertime phenomena that can be very dangerous to humans anywhere a UAS may be flying. If they happen in a city, there is usually ample evidence of their existence, as debris flies high in the “funnel.” These nasty actors can show up anywhere there is hot asphalt, sand, dirt, and if that mass of rapidly moving air connects with a cool surface, they can turn violent very quickly, slinging a sUAS far from its intended flight path.
Image result for dust devil Image result for dust devil

Dust devils in the Nevada desert can be frightening, especially when two or three combine into one vortex.

If by chance a dust devil is seen climbing in the distance, prepare to bring the aircraft home and land. If the dust devil is anywhere near the vehicle, climb in altitude while moving in any direction away from the dust devil. They are usually very short-lived.

Image result for dust devilImage courtesy Washington Post

How do we avoid getting caught in turbulent air? The long answer is “experience.” Flying in these challenging spaces teaches us to find the lee, based on the behavior of the UAS, which will always be slightly latent to the wind.
The short answer is to study environments. Look at the wind indicators that might normally be missed.  Learn to read the environment; it’s not hard once one begins to look for the details around buildings, trees, brush, monuments, chimneys, and other ground obstacles.

Two standard practices that may save pilots from troubles;

  • Always use a windmeter/anemometer, and check the winds frequently in midday flights.
  • Have a corporate or personal policy of a hard-deck/stop speed.  This eliminates wishy-washy/should I/shouldn’t I decisions in the field.  Our cap for teaching students with a Hexcopter/Yuneec Typhoon H is 16mph. If a gust crosses 16, we immediately stop, and wait it out to determine the wind trendline.

Another practice (although not standard) is to put a 5′ stream of crepe’ paper on a stick at eye level or so. This WDI, or Wind Direction Indicator, will immediately demonstrate changes in windspeed or direction, both clues that the weather may be rapidly shifting.

Determine distances from obstacles as accurately as possible prior to flight in order to best understand where the rotors will occur.  Doing so goes a long way to maintaining control and safety when the drone is in flight. With a bit of experience, one rarely needs to worry about obstacle turbulence.

Happy flights!
~dse

 

 

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Skydive Elsinore Wingsuit School Video Guides

 


Wingsuit training has been a dynamic journey in skydiving for the past decade, with a great deal of spread in how the discpline is taught to new wingsuiters.  At Skydive Elsinore, we’ve developed a wingsuit training mechanism very similar to the AFF program.  Using basics of coaching, coupled with input from John Hamilton, Jay Stokes, “Lob” Lobjoit, Jarno Cordia, Robi Pecnik, and several hundred students in the initial process, we’ve developed functional, consistent methods for wingsuit training.
These videos are what we show to wingsuit students at various levels in the coaching process. There are other videos not shared, we’ll make them available at a later date. 

This article is not intended as wingsuit training. It is intended to inform existing and would-be wingsuit coaches about our methodology. A wingsuit coach is highly recommended, and it is equally recommended that a quality coach be sought out, safety record questioned, and methods explained before hand. A 10 minute first-flight course isn’t training. A proper, complete FFC is going to last a minimum of 45 minutes, with 90 minutes being more common. PLEASE SEEK QUALIFIED COACHING** for a First Flight Course and at least a few post-FFC jumps. This is important for your personal safety, for aircraft safety, and the safety of others. Wingsuiting is different from ‘normal’ freefall due in part to the horizontal component, and the speeds at which we can travel, in addition to deployment differences and the potential for higher speeds if instability occurs. 

Each part of  the jump is broken into smaller chunks of manageable  information, which are then combined to complete the jump. Whether it’s the first flight course or a post-FFC coaching jump, every module is broken down into at least two parts. There are at least two modules to every jump.
Currently we offer 10 levels beginning with the FFC and finishing with an introduction to backflying. Most of the levels are supported with kinesthetic and isometric exercises, not unlike the Skydive University methods taught in the USPA Cat G-H coaching jumps for new skydivers.

FFC’s are broken down into five elements/modules;

  • Exit
  • Navigation (with practice touches)
  • Deployment
  • Clearing the Suit
  • Emergency Procedures

The elements/modules are heavily drilled/practiced on the ground, and supported with pre-jump training video.

Often times, we have would-be wingsuiters with exactly the minimum number of jumps required by the USPA BSR, and they may or may not be current. Our coaches use discretion in training, however, low jump number students are often required to do all FFC ground training and a wingsuit-less wingsuit jump (performing all tasks that they’ll be performing when they don the wingsuit) prior to doing a jump with the wingsuit and continuing training.

This is a pre-FFC training jump. The student fulfills all wingsuit tasks without wearing the wingsuit. The student has already been through the ground portions of the FFC.

EXITS:

Wingsuit students often express fear of the horizontal stabilizer (rightfully so) during the pre-course interview. Exits are drilled until the student can confidently exit the mockup with eyes closed. We spend more time on exits than in any other module of the course, as the setup, launch, flyaway, and horizontal stab avoidance are part of every wingsuit jump in the future, and the only part of the jump that is life-threatening for both the student and pilot (and others that may still be in the aircraft after a wingsuiter exits).
We teach a positive-contact exit method that assures closure of the wing; there can be no mistake. This exit method serves every turbine aircraft with a side door, and we train wingsuiters (on request) to manage 206 and 182 aircraft exits. 

NAVIGATION  (WITH PRACTICE TOUCHES)

(Skydive Elsinore provides wingsuits students with a 90 degree turn from jumprun upon coach request. Not only does this practice offer the student a more straight-on flight path, it also ensures that First-Flight wingsuit students are well off the path of jumprun, preventing proximity with tandems and/or other skydivers in most situations). Practice touches are broken down into four components. Note that waveoffs are part of the practice-touch process; waveoffs should be taught in all First Flight Courses.

DEPLOYMENT:

First Flight Students frequently express trepidation about deployment; getting the parachute out cleanly while wearing large surface areas is daunting for even the most experienced skydiver. We drill deployment procedures to the point that students are able to do them with their eyes closed on the ground.  This builds confidence and muscle memory. Kinesthetic reinforcement is very important in this drill. A waveoff before deployment is required of the student. Some coaches do not teach a wave off, citing that it “might be too much information for new wingsuiters.” This is simply ridiculous. We teach waveoff in the FJC, so if  a first-jump AFF skydiver can wave off, so can a first flight wingsuit student with at least 200 jumps. Ingrain the habit from the start. We also may never see this wingsuiter again after the First Flight Course.

CLEARING THE SUIT:

Clearing the suit does not have a supportive video;  it is fairly straightforward, and takes only a few minutes to teach although this part of the training is also broken down into three parts, then assembled as a whole.

EMERGENCY PROCEDURES

INSTABILITY RECOVERY:
We spend a significant portion of allotted time on instability recovery. Many FFC students have been watching YouTube, reading Dropzone.com, or have heard horror stories about the mythical flatspin (that doesn’t occur with properly taught FFC courses).  They are fearful, and often express fear in the FFC interview process.

When coaching, we do not refer to “flatspins” but rather “instability.”  Old school methods teach to ball up; this presents its own problems as students progress into larger and larger suits. The method we train is effective whether in large or small wingsuit, rather than having one procedure for large suits and one procedure for small suits.  We train on creepers with kinesthetics and isometrics, and the student is well prepared to deal with any instability or rotation that may occur.

LINETWISTS:

Linetwists are a part of the Emergency Procedures module. We offer multiple methods for the beginning wingsuiter. There are other methods available; we’ve found these two methods to be quite effective for the new wingsuiter. 

Linetwists occur in a small percentage of FFC’s. Of course there are other methods for clearing linetwists; we’ve found these methods to be very effective for the newer wingsuiter, without adding to their set of training tasks.

FFC/JUMP ONE

Once the FFC has been successfully completed, we move into rapidly advancing skills and confidence of the FFC student while their confidence is high.
The second jump in the series trains a front float exit with start/coast/stop and forward motion control. We teach this immediately so that students understand various methods of slowing down or “stopping” in the event they may be flying too close or too fast towards another wingsuiter in a group setting. We feel this is the next most-important skillset.

JUMP TWO:

Front Float Exit/Start/Coast/Stop skills.
Student will exit front float (coach in rear) and once relative to coach, the student will perform three tasks prior to deployment. The front float exit is arguably the most safe exit for wingsuiting, and it is taught very early. We also use this exit as an FFC with students that are very tall.

JUMP THREE:


Running/Pivot Exit (for Otters, Caravans, Skyvans, other large door aircraft)
Up/Down fall rate skills (performed with the hips, not head, arms, or legs)

JUMP FOUR:

GAINER EXIT (for Otters, Caravans, Skyvans, other large door aircraft)
This is a “rabbit jump” where the student is no longer base; the Coach acts as a base and provides a stable reference for the student to fly to.
(No Supporting Video)

JUMP FIVE:

Students are prepped for barrel rolls. A Front Float exit is common, but students are given a choice of Running/Pivot or Front Float exits. Gainer Exits are generally not appropriate for barrel roll jumps. The purpose of this jump is as much about instability as it is about performing the barrel rolls.  Students that are able to deal with mild instability are generally prepared for beginning backflying.

JUMP SIX

Baton Passes. Student choice of exit.

JUMP SEVEN

Performance Category jump. Student has two options from which to choose.

JUMP EIGHT

Performance Category jump. Student has two options from which to choose.

JUMP NINE

Performance Category jump. Student has two options from which to choose.

JUMP TEN

Running pivot exit. Student will transition from belly to back, backfly for five seconds, and transition to belly for five seconds. This is an introduction to backflying.

Phoenix-fly and Skydive Elsinore have funded and facilitated the development of the training method.  I’m very grateful to them both for making it possible to develop a program for wingsuiters that is sensible and efficient for cross-training AFFI’s, USPA Coaches, and Wingsuit Coaches.  Wingsuiting is still seen as a discipline similar to freefall, and the dedication to creating better, more consistent training on the part of both groups is inspiring and appreciated.

Coaching helps wingsuit students arrive safely at backflying confidence with the entry-level to flying on their back.
Positive-contact exits work very well, offer great stability, and provide a method that assures there can be no tail strike.

Kinesthetics and isometrics play a big role in coaching at Skydive Elsinore Wingsuit School.

**Jump numbers are not what makes for a “Qualified Coach.”  Manufacturer ratings are a good place to start; There are great coaches without manufacturer ratings and there are terrible coaches with them.