Wind Turning Canopy?

[relyon 0]

It's assumed that the canopy and/or suspended mass (eg. the jumper) is [close to] ideal as well. If the lines aren't exactly matched, the drag isn't balanced laterally, etc. the canopy could turn for reasons other than the wind. In those cases it won't stop turning either.

Other than wind shear (for the reasons given), wind does not turn a canopy.

Bob

[MooChooser 0]

The key to this I think is that in a gust, the canopy is affected more than the suspended mass beneath it.

When a gust of wind hits you, the canopy is blown over to the side whilst you are not affected nearly as much. This imbalance is what causes the downwind turn.

In a block of air that's moving at the same velocity throughout, a well trimmed canopy will maintain heading regardless of wind direction.

[kallend 1,620]

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That is not an appropriate physical model.

Say the canoe was also moving towards the bank (like gravity pulls a parachutist towards the ground). The river flows less rapidly close to the bank, so the end of the canoe closest to the bank will slow down relative to the other end, and the canoe will turn. The shear in the stream will induce a rotation.

I'm not really with you.
With shear of the air mass, I guess that you mean different wind speed at different altitudes?

The canoe turns because one end of it reaches slower flow while the other end does not. Ok.

The canopy goes down in altitude and reaches the slower wind *the whole at once*. It's not like one side of the canopy catches the slower wind earlier than the other, inducing a turn, is it?

The POINT is that once you introduce shears or turbulence into the flow, there IS a way in which an object can be rotated to a preferred direction. See my first post on the topic for a specific example.

www.dropzone.com/cgi-bin/forum/gforum.cgi?post=2999905#2999905

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The only sure way to survive a canopy collision is not to have one.

[kallend 1,620]

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Are there any factors to suggest that such induced motions result in a parachute heading into the oncoming direction of the wind, rather than any other direction?

Have you tried flying you parachute sideways or backwards?

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The only sure way to survive a canopy collision is not to have one.

[dgw 8]

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Are there any factors to suggest that such induced motions result in a parachute heading into the oncoming direction of the wind, rather than any other direction?

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Have you tried flying you parachute sideways or backwards?


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Can't say I have. Sorry for the ambiguity. I rephrase:

Are there any factors to suggest that such induced motions result in a parachute 'running downwind' as per OP's query?

[kallend 1,620]

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Are there any factors to suggest that such induced motions result in a parachute heading into the oncoming direction of the wind, rather than any other direction?

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Have you tried flying you parachute sideways or backwards?


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Can't say I have. Sorry for the ambiguity. I rephrase:

Are there any factors to suggest that such induced motions result in a parachute 'running downwind' as per OP's query?

If the upper level wind is stronger than the lower level wind.

www.dropzone.com/cgi-bin/forum/gforum.cgi?post=2999905#2999905

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The only sure way to survive a canopy collision is not to have one.

[pchapman 261]

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People often think their canopy wants to turn downwind because they spend so much time trying to fly _upwind_ - and when you have any turn at all in a canopy at that point it's trying to turn downwind.

Yeah, if one takes a normal upwind jump run, and has the canopy open up facing upwind, then it is little surprise that (if the canopy turns at all), it "tries to turn downwind". No more surprising than the fact that a mouse at the north pole knows exactly which way to run south!

But other than that, the shear explanation (Kallend & Billvon) sounds good.

[mdrejhon 8]

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he announced that if the canopy is left to it's own devices, the canopy will turn and run downwind.

I witnessed some minor weathervaning when flying without touching my straps (leaving brakes stowed - flying only using risers, but I temporarily let go of my rear risers during a long 15,000 feet cross-country and relaxed, while minding my spotting).

So, yes Canopies seem to want to turn downwind eventually. But two things need to happen:
(1) Winds must be inconsistent speed across your canopy to cause the weathervaning effect, i.e. turbulent air, varying wind speed zones, etc. Your canopy will randomly turn 1 degree this way, that way, like a coinflip, but it is generally biased downwind. (like a coin that's slightly heavier on one side)
(2) You fly with for long enough with absolutely no touching of the toggles or risers. (only pratical to do this reasonably safely during a cross country, i.e. a 15,000 feet hop-n-pop) Most turbulence is so minor, you dont' feel it, so you'll only gradually turn downwind over a long period of time (i.e. a few minutes) - it's cumilative.

However, if wind is perfectly steady across your entire canopy and body (no turbulence), you will keep going perfectly straight. No argument there.

But as I have learned, this definitely isn't always the case - you've probably flown through turbulence before, including turbulence that causes you to change direction slightly.

Also, I'm not sure (need more than a few hundred more jumps experience), but I think I noticed flying INTO strong turbulence (upwind) knocks me around more than if I was flying WITH the strong turbulence (downwind). This probably is the weathervaning effect being more intense the further away from the downwind vector... Of course, it probably depends on how much windsheer there is in the turbulence, but generally turbulence often has a windshear component: Some of that air is rising diagonally, swirling, vortexing, etc, and all that includes a horizontal-variance component (windshear)

I'm not an authoritative source, but I tend to agree that canopies do tend to fly downwind - but it needs a little "jiggling" (turbulence, wind shear) to do so. Without that, the canopy will go straight until you hit the ground (if everything else, including lines are perfect.)

Swoopers won't notice this (hurrying to ground), but longtimer CRW guys will definitely notice this.

[mark 102]

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So, yes Canopies seem to want to turn downwind eventually. But two things need to happen:
(1) Winds must be inconsistent speed across your canopy to cause the weather-vaning effect, i.e. turbulent air, varying wind speed zones, etc. Your canopy will randomly turn 1 degree this way, that way, like a coin flip, but it is generally biased downwind. (like a coin that's slightly heavier on one side)
(2) You fly with for long enough with absolutely no touching of the toggles or risers. (only practical to do this reasonably safely during a cross country, i.e. a 15,000 feet hop-n-pop) Most turbulence is so minor, you dont' feel it, so you'll only gradually turn downwind over a long period of time (i.e. a few minutes) - it's cumulative.

mdrejhon:

Do you think this phenomenon is confined to ram-air skydiving canopies, or might it extend to other air vehicles as well? For example, would round canopies tend also to turn downwind? How about para-gliding canopies? Would it matter if the vehicle was powered (like a Para-Plane) or unpowered (like a fiberglass sailplane?)

Also, would the canopy turn downwind faster in stronger wind? Or is the rate of turn solely a result of the turbulence?

Finally, if the wind speed increases as your canopy descends, would that make any difference to the rate of turn?

Thanks,
Mark

[pilotdave 0]

Don't know what caused your canopy to turn, but disturbances HAVE to turn a (ram-air) canopy into the wind. Think of a plane, with a tail at the back end. If a gust hit the plane from one side, the plane will automatically turn toward the gust. Or put another way... if the pilot kicks the rudder to the left and then back to neutral, the plane will suddenly "see" a gust from the right, because it yaws to the left. It will automatically correct back to the right.

Now think about what would happen if a plane had the tendency to turn AWAY from a gust. If a gust came from the left, the plane would yaw to the right. That would increase the gust factor from the left, causing the plane to turn more to the right. That would continue until the plane is flying backwards (or spinning out of control). Same thing if the pilot kicked the rudder and let go. It would yaw left, causing a crosswind component from the right. It would turn more to the left, causing more wind from the right, and so on, until flying backwards or out of control.

A canopy has to behave the same way. Otherwise it would be uncontrollable. Might fly straight in absolutely perfect conditions. But the tiniest gust from one side would whip it around. It would be laterally unstable.

Dave

[mdrejhon 8]

I don't have enough authority to know for sure. It just does happen to my canopy.

But remember this: Winds aren't perfectly uniform from 15,000 feet down to 0 feet.
There's no time, anywhere in the world, that this ever happens. There's always say, 0.1 mph difference somewhere, and more likely a much bigger difference, like 23mph at 6000 feet and 17mph at 3000 feet.

As a result, there's always guaranteed wind shear as the wind layers friction against each other, even if it microscopic: i.e. varies only 0.001mph per second as the parachute floats down.

But I could be wrong too.

Anyway, according to mathematical physics (what I know), I believe perfect parachutes (perfect balance, perfect lines, perfect fabric, perfect symmetric, etc) will fly without turning in a perfect wind as long the perfect wind is also perfect consistent at the full range of altitude (top to bottom). But that will never, ever happen...

There's always disturbances, even at 0.001 mph level, everytime somebody has ever jumped in history...

However, a very well setup canopy in very amazing gentle wind conditions, at not too high an altitude, can probably reach the ground before turning downwind, if you time it well. And body movement might even be the more dominant effect in affecting turns. (harness turns caused by even very minor shifts in your center-of-gravity)

But as one of the few who love high altitude hop-n-pops (15K), I definitely notice a minor turning-to-downwind bias in cross countries, that overpowers my minor body movements during relax straight hands-off-risers/toggle flight...

FWIW, I fly Sabre 170, which is mostly immune to harness turns (unless I try very hard).

[mdrejhon 8]

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Don't know what caused your canopy to turn, but disturbances HAVE to turn a (ram-air) canopy into the wind. Think of a plane, with a tail at the back end. If a gust hit the plane from one side, the plane will automatically turn toward the gust.

That's normal. Weathervanes can be designed to point INTO wind or AWAY from wind. It just depends on how the weathervane is designed, no?

The physics aren't the same for a regular rigid airplane with a tail, and an inflatable flexible wing with no separate tail. Next.

I'm even sure that it might be possible some canopies respond differently (i.e. it might be rigidness, and cross-braced might behave differently). Maybe yours flies into the wind. I have no way of knowing.

Nobody has written a scientific paper on this matter, but I can definitely guarantee that some wings turn into the wind, and some wings turn away from the wind.

The pre-existing scientific proofs for turning into the wind only applies to classical airplanes (rigid bodies), and does not touch upon the physics of flexible fabric wings.

On a rigid wing, anything affecting one tip of the wing to affect the other pretty much virtually instantly (less so for bouncy thin wings such as those on solar aircraft, etc), but this doesn't happen in a fabric wing -- one edge can respond faster than the other edge - with a noticeable human-visible lag. So we're in nearly unexplored physics territory.

Might make for an interesting University paper.

[pilotdave 0]

Sure, put the arrow on the other end of the weathervane and it will point away from the wind.

But did you read the rest of my post? A canopy that weathervanes away from the wind is laterally unstable. It would be impossible to control because any disturbance would cause it to veer off course until it's flying backwards relative to the airmass.

There's a big picture and a little picture. The big picture is which direction will a canopy turn relative to "the wind." Will a canopy turn downwind?

The little picture is how will a canopy react to RELATIVE wind. That's where gusts and turbulence (which is just a bunch of gusts) come in, as well as control input.

Kallend had an interesting theory that explains why a canopy might seem to turn downwind (the big picture) based on the fact that a canopy will turn INTO the relative wind (little picture). That's the only explanation I've seen that makes any sense. But I still don't believe it explains why so many people seem to have observed canopies turning downwind on their own.

Canopies HAVE to behave like weather vanes. That's why they fly forward. If they acted like backwards weathervanes, they would fly backwards.

EDIT: The fact that a canopy is fabric doesn't matter. This is a problem of stability. The behavior you describe (a canopy that naturally turns AWAY from a gust) would be unstable. I don't care if it's made of nylon or steel. If it turned away from a gust, it would immediately "spin out."

Dave

[mdrejhon 8]

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Sure, put the arrow on the other end of the weathervane and it will point away from the wind.

Then you might prove it's not the weathervane effect that causes some canopies to fly downwind. (I did warn you I may be wrong. )

But fact remains: Some wings are observed to fly downwind (like my and many other people's canopies), some wings are observed to fly upwind (like a classical rigid plane).

I'm more inclined to think it's more related to rigidity. And there must be underlying factors that 'tempers' unstability, that we haven't discovered (i.e. maybe extra tension exerted on certain lines that counteracts instability - as a non-elliptical canopy turns, like my own, it quickly wants to go back to straight.)

Also, some elliptical canopies actually exhibit the unstable behaviour you describe: Some really extreme canopies need input in order to keep going straight, or they'll spin out of control. (Perhaps not by this effect, but this is something worth mentioning - who knows - coincidence, coincidence - maybe someone daring would like to find out (at high altitudes) if extreme ellipticals flies more stable without input downwind or upwind). Square canopies like mine have the stability of wanting to go straight when I let go of input. So it is powerful self-balancing behaviour that quickly massively recovers from minor turning-into-wind effects.

I can confirm you that the turn-into-wind effect is less powerful than the go-back-to-straight effect. It takes less than a few seconds for my canopy to go back to straight from a very steep turn when I let go of turning input, while it takes more than a minute for me to turn downwind on no input. The fact that the stabilizing effect of a square parachute prevents runaway destabilization, is the best theory I can throw on the table at the moment, but feel free to bring up an alternative one...

Again, it would make for an interesting University Thesis, to come up with an authoritative answer for why this happens, or why this 'seems' to happen.

[chrismgtis 0]

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Wind will not turn a a canopy. Wind shear/turbulence can turn a canopy.

Derek

I had that happen a few weeks ago. I was at about 2000 feet and started getting hit with turbulence which would sling my canopy a few degrees in one direction. When I got about 50-100 feet above the ground it happened again. I compensated with slight toggle input and landed it straight. When it happened I thought "Oh shit, I'm about to break a leg". I guess I can chalk it up to experience and listening to what I've been told by instructors and experienced jumpers. I landed it fine with no problems. Though, I did ground myself and quit jumping for the rest of the day too.

Rodriguez Brother #1614, Muff Brother #4033
Jumped: Twin Otter, Cessna 182, CASA, Helicopter, Caravan

[kallend 1,620]

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Don't know what caused your canopy to turn, but disturbances HAVE to turn a (ram-air) canopy into the wind. Think of a plane, with a tail at the back end. If a gust hit the plane from one side, the plane will automatically turn toward the gust.

That's normal. Weathervanes can be designed to point INTO wind or AWAY from wind. It just depends on how the weathervane is designed, no?

The physics aren't the same for a regular rigid airplane with a tail, and an inflatable flexible wing with no separate tail. Next.

.

Care to go into more detail so that a mere physics professor like myself can understand your theory?

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The only sure way to survive a canopy collision is not to have one.

[billvon 2,380]

I think the "intuitive science" is making an appearance. Some things, like how to run faster, are pretty intuitive because our brains are designed to understand things like that. Some things, like aerodynamics, are less intuitive - so our intuitions can sometimes lead us down the wrong path.

Take the headwind thing for example. If you are running into a headwind, it feels like it's windier than when you are running with the wind. So it's intuitive that a wing will generate more lift when it's flying into the wind than when it's flying against the wind. That's completely wrong, of course, but it "feels" right. Such assumptions have gotten people - including some otherwise good pilots - in trouble.

(Not replying to you in particular.)

[denete 2]

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Don't know what caused your canopy to turn, but disturbances HAVE to turn a (ram-air) canopy into the wind. Think of a plane, with a tail at the back end. If a gust hit the plane from one side, the plane will automatically turn toward the gust.

That's normal. Weathervanes can be designed to point INTO wind or AWAY from wind. It just depends on how the weathervane is designed, no?

The physics aren't the same for a regular rigid airplane with a tail, and an inflatable flexible wing with no separate tail. Next.

I'm not a physicist. I'm not an engineer. I've just been paying attention to things that fly for a few decades. Take what I write with a grain of salt, I could be pretty far off the mark. Some of my ideas may be hard to follow without seeing them and most are "that looks about right" theories. Good luck.

Okay, so a traditional configuration aircraft has a forward wing, trailing fuselage, and aft empenage. The pitch dampening comes from that span of the fuselage and the horizontal tail surfaces. With a typical square canopy, this is mimicked by the suspension lines and the pilot. The pilot and gravity work together as pitch dampeners. The traditional aircraft is usually modeled with the wing, fuselage, and tail in a horizontal plane (or pretty close to it), and the airflow happening in that same plane (no high AoA test examples, just the plain-Jane model).

Gravity is our "motor", and it pulls in a constant direction...down. Think of the angle at which the relative wind (assuming a dead-calm day) is hitting us. To put it in the traditional airflow model, we would have to turn the suspension lines close to parallel to the ground with the pilot being the furthest forward point, and the canopy the furthest aft point. Now we are looking like a traditional canard. Because of the freaking AoA of the canopy, it is functioning more like an under-cambered wing with dive-brakes. Having taken gravity out of the equation, where does roll-control come from (still thinking in this sideways model, this would be the same as yaw in our normal canopy flight)? Well, it should come from warping the canopy or from twisting of the pilot below the canopy. The pilot as a rolling surface control would look a lot like the Gossamer Albatross' canard (a floating canard that can be rolled).

So what does all of this matter? Well, when you put us back in the normal configuration (pilot closest to the earth, canopy closest to the sky), we've got a lot of mass under the canopy. Because of the tension on the suspension lines, the pilot is going to be affected by wind and will pendulum out longer due to mass. Once the pilot moves out of center under the canopy, you have swiveled the canopy in some manner and changed the profile. It is now more likely to be affected by the wind while the pilot pendulums below it.

Yeah, right...sure. That was with constant wind. You have to remember that air doesn't just travel parallel to the earth. You are constantly flying through a "river of air" (thanks Dave Thornburg) which boils and flows. So, you are hit by lateral and vertical changes more often than you might think. For everyone that says wind doesn't matter and that canopies are always moving through air the same way no matter what the wind speed or direction is, I say pffft. This is true after the canopy and pilot come into "agreement" with the wind, but that is not an instantaneous happening. It takes a little time (thanks Newton). Throw a raft into a river and you'll see it happen. The raft will actually accelerate and come up to speed with the water (close enough), but it doesn't happen instantly. During that transition time, we're on an invisible roller-coaster in the air.

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On a rigid wing, anything affecting one tip of the wing to affect the other pretty much virtually instantly (less so for bouncy thin wings such as those on solar aircraft, etc), but this doesn't happen in a fabric wing -- one edge can respond faster than the other edge - with a noticeable human-visible lag. So we're in nearly unexplored physics territory.

Unexplored? I'll bet some guys at NASA and others around the world would disagree.

http://www.nasa.gov/centers/dryden/news/FactSheets/FS-045-DFRC_prt.htm

http://www.engr.uconn.edu/~adstc/publications.html

And there's always: Advanced Recovery Systems Wind Tunnel Test Report by R.H. Geiger and W.K. Wailes. A Pioneer Aerospace Project for NASA. Tests of large ram-air canopies were performed in the NASA-AMES 80 x 120 wind tunnel. Results for various angles of attack and wing loadings are presented. August 1990, NASA Contractor Report 177563.

Please take all of this lightly, and remember what I said about "that looks about right" theories.

- David

SCR #14809

"our attitude is the thing most capable of keeping us safe"
(look, grab, look, grab, peel, punch, punch, arch)

[riggerrob 558]

Turbulence affects you more when you face upwind, because you are exposed to that turbulence for more minutes.
IOW, you stay in the thermal longer.

[riggerrob 558]

Yes, I heard the same lecture from the Canadian Army in 1981. I did not believe that theory then and I still don't believe it now.
The bottom line is, most canopies are less than perfect. less than stable and have slow, built-in turns.
Initially, they will turn cross-wind, then down-wind, then cross-wind, etc.
If you leave you hands off the controls long enough, eventually they will turn a full circle and repeat the process until impact.
Turns can also be caused by unevenly-adjusted harnesses, one testicle larger than the other, etc.

[speedy 0]

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The bottom line is, most canopies are less than perfect. less than stable and have slow, built-in turns.

Also, most notice that these turns always turn them into the direction that they DO NOT want to go. These involuntary turns are often noticed when people land their canopy. They are convinced the the wind blew them of course But we know, they really only has one ball

Dave

Fallschirmsport Marl

[dragon2 0]

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The bottom line is, most canopies are less than perfect. less than stable and have slow, built-in turns.

Also, most notice that these turns always turn them into the direction that they DO NOT want to go. These involuntary turns are often noticed when people land their canopy. They are convinced the the wind blew them of course But we know, they really only has one ball

Student in pic: It was the WIND I tell you! It's been blowing me sideways on landing the whole day

ciel bleu,
Saskia

[speedy 0]

Oh yeah, I can see the wind MUST have been a problem

If you have interest in work at our DZ ( video) please PM me.

Dave

Fallschirmsport Marl

[brettski74 0]

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The bottom line is, most canopies are less than perfect. less than stable and have slow, built-in turns.

Also, most notice that these turns always turn them into the direction that they DO NOT want to go. These involuntary turns are often noticed when people land their canopy. They are convinced the the wind blew them of course Tongue But we know, they really only has one ball [Laugh] [Wink]

Student in pic: It was the WIND I tell you! It's been blowing me sideways on landing the whole day [Mad]

Yes - it must have been a really strong wind to blow his arms around like that.

[dthames 0]

MooChooser

The key to this I think is that in a gust, the canopy is affected more than the suspended mass beneath it.

When a gust of wind hits you, the canopy is blown over to the side whilst you are not affected nearly as much. This imbalance is what causes the downwind turn.

In a block of air that's moving at the same velocity throughout, a well trimmed canopy will maintain heading regardless of wind direction.

I think the suspended mass is something that should be considered a lot in this question. Any time the ground speed changes caused by any change in the airspeed vector, the suspended mass will lag behind the canopy (2nd law). This lagging of the suspended mass can cause the canopy to dive or zoom, to some extent. The smallest change in the wind direction or strength can start a gentle turn that.

I could never get a radio controlled airplane trimmed the stay directly into the wind, if I took my hands off the controls. As soon as it started to turn, the turn rate picked up until it was flying downwind.

Instructor quote, “What's weird is that you're older than my dad!”