Wind Turning Canopy?

[mickle1 0]

As a civilian AFF instructor I recently had the opportunity to sit in on a military 1st jump briefing. As the Instructor went through the familiar points on canopy control, he announced that if the canopy is left to it's own devices, the canopy will turn and run downwind.
The canopy they use is a square 360 much like a small Tandem canopy.
Frankly I was amazed at this piece of information and tried to explain that as far as I knew there was no way a canopies flight could be affected by wind direction, only its apparent ground speed would be affected.
If anyone can enlighten me otherwise I'd be very interested.
I have a feeling it's a hang over from round canopy briefings. With almost no forward speed they will descend down wind.

[denete 3]

Weathervaning.

SCR #14809

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

[JerryBaumchen 1,468]

Hi mickle1,

Every drop test I have ever done with a dummy ( no human or other type of input ) under a square ( and that is NOT a lot of them ) the canopy turned downwind.

JerryBaumchen

[kallend 2,150]

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As a civilian AFF instructor I recently had the opportunity to sit in on a military 1st jump briefing. As the Instructor went through the familiar points on canopy control, he announced that if the canopy is left to it's own devices, the canopy will turn and run downwind.
The canopy they use is a square 360 much like a small Tandem canopy.
Frankly I was amazed at this piece of information and tried to explain that as far as I knew there was no way a canopies flight could be affected by wind direction, only its apparent ground speed would be affected.
If anyone can enlighten me otherwise I'd be very interested.
I have a feeling it's a hang over from round canopy briefings. With almost no forward speed they will descend down wind.

In a constant wind a canopy has no idea which way the wind is blowing.

If there is a wind shear, the canopy may well turn as it descends through the shear, and the turn direction would be downwind if the wind is decreasing as altitude decreases, which is usual. I wouldn't expect it to be much of an effect.

Example: say the canopy is flying on a north heading in a crosswind from the west at 20kt. Due to the wind the canopy is actually drifting sideways to the east at 20kt and the relative wind is from straight ahead.

Now let the canopy descend through a shear to where the wind is only 5kt (same direction). Newton's 1st law says the canopy and skydiver are still going east at 20kt, so the canopy now feels a (20-5) = 15kt wind from the right. The canopy will turn right into this relative wind until the system comes to equilibrium again. Seen from the ground, this is a turn downwind.

...

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

[pilotdave 0]

Interesting idea.

The key is that the canopy will turn INTO the wind (like you said) when disturbed, not away from it. Otherwise our canopies would naturally fly backwards or completely out of control.

Dave

[dharma1976 0]

I have noticed that before the canopy has its own relative wind sometimes it searches for one :-P

probably my imagination and not physics like kallend, but I have noticed a correlation (and that being in the scientific version of the word like yeah it coincides but not cause and effect perhaps)

Cheers

Dave

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As a civilian AFF instructor I recently had the opportunity to sit in on a military 1st jump briefing. As the Instructor went through the familiar points on canopy control, he announced that if the canopy is left to it's own devices, the canopy will turn and run downwind.
The canopy they use is a square 360 much like a small Tandem canopy.
Frankly I was amazed at this piece of information and tried to explain that as far as I knew there was no way a canopies flight could be affected by wind direction, only its apparent ground speed would be affected.
If anyone can enlighten me otherwise I'd be very interested.
I have a feeling it's a hang over from round canopy briefings. With almost no forward speed they will descend down wind.

http://www.skyjunky.com

CSpenceFLY - I can't believe the number of people willing to bet their life on someone else doing the right thing.

[PhreeZone 20]

But did it stop turning once it reached "downwind" or did it keep turning?

Yesterday is history
And tomorrow is a mystery

Parachutemanuals.com

[billvon 3,119]

>Every drop test I have ever done with a dummy ( no human or other
>type of input ) under a square ( and that is NOT a lot of them ) the canopy
>turned downwind.

We did a lot of drop testing on rigs, and the reserves seemed to land every which way. (Although I definitely remember the downwind landings the most vividly - they were a lot more exciting!)

[hackish 8]

Someone told me this once and I was a little curious. So I opened a little high and played limp dummy for a few seconds. The canopy did turn downwind and seemed to stay going that way until I released the brakes and turned it back for fear of not making it back. I don't know why it did that but it did...

-Michael

[UDSkyJunkie 0]

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As the Instructor went through the familiar points on canopy control, he announced that if the canopy is left to it's own devices, the canopy will turn and run downwind.

I remember talking to an experienced rigger a LONG time ago who said that when they developed large squares for military cargo they were able to design them so they would tend to naturally turn INTO the wind so in the event of a control failure it would at least head the right direction. I don't have any confirmed source to substantiate this.

He didn't say how jumpers canopies are affected... I've never noticed a tendancy to turn in any direction, but then again I've never just let the toggles up for several minutes in high winds to find out. If there is an effect, it would be logical that larger, lightly loaded canopies would be affected to a greater extent.

"Some people follow their dreams, others hunt them down and beat them mercilessly into submission."

[indyz 1]

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I remember talking to an experienced rigger a LONG time ago who said that when they developed large squares for military cargo they were able to design them so they would tend to naturally turn INTO the wind so in the event of a control failure it would at least head the right direction. I don't have any confirmed source to substantiate this.

Ok, let's do a thought experiment here. You get in the plane blindfolded. The pilot takes you up to altitude, picks a random heading for jumprun, and you get out. You deploy and let the canopy fly on whatever heading it opened on. With the blindfold still on, can you tell which way is into the wind? You can't, because the canopy is moving in the airmass. It's the same as if I put you blindfolded in a canoe, in the middle of a river. Would you be able to tell if the canoe was pointing upstream, downstream, or across the river?

Until you introduce an external reference it is simply impossible to determine which direction the wind is coming from. You do it by opening your eyes and looking around. If the canopy wanted to do it, it would need a GPS or inertial navigation system or something.

[UDSkyJunkie 0]

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Until you introduce an external reference it is simply impossible to determine which direction the wind is coming from. You do it by opening your eyes and looking around. If the canopy wanted to do it, it would need a GPS or inertial navigation system or something.

As an engineer and an analytical-type guy, I have gone through exactly that thought process and came up with the same answer.

That said, the real world is much more complex... the wind is not steady in direction or speed, and the jumper and canopy have mass, so you're never truly static relative to the airstream. Does that make it possible to determine direction? I don't know.

For your canoe example, I say it depends. If it's a slow-moving river, probably not... if it's faster, possibly yes... certainly the canoe will behave differently if it's sideways vs. forward or backward, and various forces will probably cause the canoe to always move slightly slower than the water, so maybe you could figure it out.

"Some people follow their dreams, others hunt them down and beat them mercilessly into submission."

[kallend 2,150]

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Until you introduce an external reference it is simply impossible to determine which direction the wind is coming from. You do it by opening your eyes and looking around. If the canopy wanted to do it, it would need a GPS or inertial navigation system or something.

As an engineer and an analytical-type guy, I have gone through exactly that thought process and came up with the same answer.

That said, the real world is much more complex... the wind is not steady in direction or speed, and the jumper and canopy have mass, so you're never truly static relative to the airstream. Does that make it possible to determine direction? I don't know.

.

Of course it does. As soon as you introduce a wind shear you introduce a detectable quantity.


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

...

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

[kallend 2,150]

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I remember talking to an experienced rigger a LONG time ago who said that when they developed large squares for military cargo they were able to design them so they would tend to naturally turn INTO the wind so in the event of a control failure it would at least head the right direction. I don't have any confirmed source to substantiate this.

Ok, let's do a thought experiment here. You get in the plane blindfolded. The pilot takes you up to altitude, picks a random heading for jumprun, and you get out. You deploy and let the canopy fly on whatever heading it opened on. With the blindfold still on, can you tell which way is into the wind? You can't, because the canopy is moving in the airmass. It's the same as if I put you blindfolded in a canoe, in the middle of a river. Would you be able to tell if the canoe was pointing upstream, downstream, or across the river?

.

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.

...

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

[indyz 1]

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I remember talking to an experienced rigger a LONG time ago who said that when they developed large squares for military cargo they were able to design them so they would tend to naturally turn INTO the wind so in the event of a control failure it would at least head the right direction. I don't have any confirmed source to substantiate this.

Ok, let's do a thought experiment here. You get in the plane blindfolded. The pilot takes you up to altitude, picks a random heading for jumprun, and you get out. You deploy and let the canopy fly on whatever heading it opened on. With the blindfold still on, can you tell which way is into the wind? You can't, because the canopy is moving in the airmass. It's the same as if I put you blindfolded in a canoe, in the middle of a river. Would you be able to tell if the canoe was pointing upstream, downstream, or across the river?

.

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.

Sorry. I should have added "assume you are near the middle of a really wide river."

[ZigZagMarquis 9]

What I'd like to know is how well received (or not) your interruption of a military 1st jump briefing went.


... or did you talk to the instructor afterwards?

[kallend 2,150]

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I remember talking to an experienced rigger a LONG time ago who said that when they developed large squares for military cargo they were able to design them so they would tend to naturally turn INTO the wind so in the event of a control failure it would at least head the right direction. I don't have any confirmed source to substantiate this.

Ok, let's do a thought experiment here. You get in the plane blindfolded. The pilot takes you up to altitude, picks a random heading for jumprun, and you get out. You deploy and let the canopy fly on whatever heading it opened on. With the blindfold still on, can you tell which way is into the wind? You can't, because the canopy is moving in the airmass. It's the same as if I put you blindfolded in a canoe, in the middle of a river. Would you be able to tell if the canoe was pointing upstream, downstream, or across the river?

.

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.

Sorry. I should have added "assume you are near the middle of a really wide river."

That's like "assume the parachute is in a uniform homogenous air mass all moving at exactly the same speed in the same direction. Simple, but it never happens in reality.

...

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

[Hooknswoop 19]

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

Derek

[denete 3]

Are you sure that wind will not turn a canopy?

Unless you are 3-dimensionally symmetrical (you would be a sphere, by the way), you are going to find that you have a profile which has less wind resistance than the others.

What are the stabilizers on a modern square canopy used for anyhow?

- David

SCR #14809

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

[denete 3]

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...Otherwise our canopies would naturally fly backwards or completely out of control.

Wouldn't this require all of the suspension lines to be equal length, the stabilizers to be removed, and the arch of the canopy to be flattened? Or gravity to stop working?

- David

SCR #14809

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

[billvon 3,119]

> Are you sure that wind will not turn a canopy?

Wind will not turn a canopy.

>Unless you are 3-dimensionally symmetrical (you would be a sphere,
>by the way), you are going to find that you have a profile which has less
>wind resistance than the others.

Right. And that profile is the profile the canopy stabilizes at in normal glide. (Think about it - if your canopy got directly in front of you, or off to the side - how long do you think that state of affairs would last?)

If the canopy is out of trim or you're leaning you will turn - but you'll turn no matter what the wind is doing. (Unless, as Kallend/Hook pointed out, you see wind shear.)

>What are the stabilizers on a modern square canopy used for anyhow?

Making the canopy a bit more stable in yaw and improving glide.

[denete 3]

Thanks for the explanation!

- David

SCR #14809

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

[dontiego 0]

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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?

"We call on the common man to rise up in revolt against this evil of typographical ignorance."
http://bancomicsans.com

[billvon 3,119]

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

It will also turn because it will suddenly hit slower water, and it will keep its original speed. Since canoes prefer to move in some directions more than others, that may cause a turn.

>The canopy goes down in altitude and reaches the slower wind *the
>whole at once*.

Well, no, wind shear is never that "accurate." But let's say that it reaches the wind shear all at once for the sake of argument, and let's further assume it's a big wind shear (makes it easier to visualize.)

So you're flying along with a 30kt crosswind, coming from your right. Your canopy is doing 20kts. While you are in the crosswind, you don't feel any difference - the wind is coming from 20kts in front of you like it always does. Canopy doesn't want to turn or anything. Your groundspeed is about 36 knots, on a diagonal to the left.

Then - wham! - you hit the shear, and suddenly the wind drops to 10kts coming from your right. You can't change your speed instantaneously, so for a split second you're still doing a groundspeed of 36 knots to the left. What you _feel_ is a sudden strong wind from the LEFT, because your body/canopy is still heading left at a pretty good clip.

The canopy weathervanes into this apparent wind and quickly settles back down to normal flight. After all is said and done, your canopy has turned downwind.

That's an example of an extreme shear; most wind shear is spread out over a few thousand feet, and the effect is much, much milder - to the point that unless the winds are unusual you generally won't see much tendency to go any specific direction. 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. But it's just trying to turn; it doesn't care where the wind is coming from normally.

[dgw 8]

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?