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shadeland

Do Canopies Naturally Turn Upwind?

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There's a side discussion in the incidents form regarding whether or not a canopy will naturally turn into the wind. Specifically, an unconcious AAD deployment. Landing a ram-air canopy upwind or downwind in a moderate to high wind day (maybe even low-wind) can have a huge effect on the outcome for the jumper.

I'm starting this thread as a discussion to take it out of the incident.

Some suggest that canopies tend to turn into the wind. Others have said otherwise.

My personal thought: No, canopies don't naturally turn into the wind. A canopy will fly straight in the body of air its in. If the body of air is moving in one direction, the canopy moves with it, but the canopy thinks its straight.

Example: Canopy opens on heading, and flies a north course (360) and its indicated airspeed (relative to the mass of air) is 20 MPH. The wind is coming from the West at 20 MPH. The ground track would be NE at ~28 MPH (if my E6B computer math is right).

Without input the jumper would impact the ground at 28 MPH on a 45 angle (Hopefully helping make a natural PLF).

I can't think of a way for the air to communicate with the ground what direction the ground winds are going.

That's my thoughts on the matter, at least.

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In the scenario you are referring to,it is unlikely that the unconscious person will remain neutral in the harness, and not affect the canopy. That being said unless a canopy is kept directly into any head wind,as soon as it moves off to one side or the other of the line of the of the head wind,the head wind will continue to push the canopy to the side it veered, with increasing affect as the canopy shows more of its profile eventually ending up downwind, providing the head wind is stronger than what the canopies forward speed is able to produce.

Ever see anybody land and not be directly into the wind when they flare? They get pushed to the side they were favoring, and if not corrected, end up in a slight turning/flare that often is 90 degrees to the wind line.

The strength of the wind also plays a factor, and enough altitude to allow it to happen.

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You are flying a plane with a 60mph stall speed.
Your airspeed is 75mph.
You are flying into a 30 mph headwind.

You make a 180 turn, then level out pointing downwind.
(Altitude & airspeed remain constant).
Will you stall?
"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.

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ryoder

You are flying a plane with a 60mph stall speed.
Your airspeed is 75mph.
You are flying into a 30 mph headwind.

You make a 180 turn, then level out pointing downwind.
(Altitude & airspeed remain constant).
Will you stall?



No (assuming you're doing a standard rate turn or otherwise not inducing an accelerated stall).

Your wing only knows indicated airspeed. Generally, your wing has no idea what the air mass you're moving in is doing relative to the ground.

Your ground track should show 45 MPH into the wind, and 105 MPH after the 180. But your wing and indicated airspeed will always show 75 MPH unless pitch is changed or additional thrust, etc.

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>No, canopies don't naturally turn into the wind. A canopy will fly straight in the body of
>air its in. If the body of air is moving in one direction, the canopy moves with it, but the
>canopy thinks its straight.

In a steady, unchanging wind you are 100% right. The canopy doesn't know what direction the wind is coming from.

If the canopy is descending, and the winds are getting lighter as altitude is lost, the canopy will tend to turn into the _perceived_ wind change, which means it will end up tending to turn downwind. If the canopy is ascending (rare) the opposite is true and the canopy will tend to turn into the wind.

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Of course there are other factors to consider.

Is the canopy configured to be flying at full speed or with brakes set, with no input from the jumper/load?

Load distribution?

Canopy correctly trimmed?

Lots of variables if we add in differing weather conditions. It would be difficult to conduct a meaningful series of tests because you can't really have a "control" to measure variables against. The best you can do is standardise the particular test canopy and load balance.

What we are looking at is just the general tendency of canopy behaviour in this scenario.

Normally I would say a canopy will maintain a heading all things being equal, with nil wind conditions.
My computer beat me at chess, It was no match for me at kickboxing....

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Oh gosh it's hard to read posts implying headwind or tailwind or crosswind affect flight characteristics of an airfoil in flight. As others have stated, a wing in flight doesn't care what it is doing over the ground, it only cares about it's airspeed. A change in the wind's velocity results in a momentary change in the wing's performance until equilibrium returns.

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Okanagan_Jumper

Oh gosh it's hard to read posts implying headwind or tailwind or crosswind affect flight characteristics of an airfoil in flight. As others have stated, a wing in flight doesn't care what it is doing over the ground, it only cares about it's airspeed. A change in the wind's velocity results in a momentary change in the wing's performance until equilibrium returns.



The assumption that the canopy is flying in a homogeneously moving airmass is a convenient fiction. In reality the wind velocity is different at different altitudes, introducing a discernible directionality into the system and a descending canopy will respond to this.
...

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

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A canopy will turn a full 360 degrees, given enough time and altitude.

Built-in turns was a problem during the 1970s and 1980s, but as manufacturing tolerances improved, bailout-in turns got smaller and smaller.
OTOH Jumpers are inherently unbalanced ..... assymetrical. For example: my right shoulder is smaller (weaker) than my left shoulder. An unbalanced human will induce a slow turn.

The canopy is flying in a block of air and only cares about airspeed. The canopy has no clue that the block of air is sliding across the ground. Since it has no magnetic compass, the canopy does not know when it changes heading relative to the planet.
Most turns - during landing flares - are caused by uneven toggles.

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Quote

The assumption that the canopy is flying in a homogeneously moving airmass is a convenient fiction.



That's unhelpful I think, if that is a criticism. You do seem to (literally) put the emphasis on the fiction, rather than the convenience.

As you well know that just about every simulation of reality is a fiction in some way. One often has to start with a simplified version of reality and then add on other layers of complexity. So for parachute flight we can ignore relativistic effects (near the speed of light) and air compressibility effects (I dunno, maybe 1% error once as fast as 150mph), that make aerodynamic calculations messier.

In these flight discussions, first we have to get rid of the gross misunderstandings. So we show that in a homogenous air mass, there is no "downwind turn" effect or anything like that. The canopy can't sense the wind direction.

Maybe you have a better handle on it, but I don't know to how strong the directional stability of a parachute is, and how much of an effect wind shear will have on it, with wind speeds in typical situations lowering, especially in the last 1000 ft down to the ground. Will 2 kts shear in 1000 ft matter? Will 5 kts over 500 ft matter? And how much?

That's where I don't think we have a good handle on things.

If you had a typical parachute, well balanced with no built in turn, opened facing crosswind at 2000' with 20 mph wind, and wind going down to 10 mph at canopy level at the ground, "How many degrees would it turn downwind during its descent?"

Would the canopy end up facing straight downwind by the time of landing? Or would it only have been kicked maybe 15 degrees towards the downwind? In other words, is the downwind turning shear effect a major one in its real effects, or just a minor aerodynamic curiosity which rarely has much effect?

There are anecdotal stories but the data is sketchy.

Exactly what was the situation in any given story? Did a parachute open facing upwind? In which case any very slow turn that doesn't make it fly big circles, would make it "turn downwind". Did the parachute just seem to "head downwind", because everything drifts downwind unless able to penetrate upwind -- and heading off downwind at high speed with drift and canopy speed always looks more dramatic?

What about a canopy that opened up facing upwind? Then even the wind shear scenario shouldn't have any effect -- except if the wind backs or veers (changes direction) during the descent, which it may to some extent -- but typically not that much between canopy opening altitude and landing unless one has very specific dog legged wind conditions.

So I think it is still valuable to start with understanding the homogeneous air mass situation first. Then add on the layers of complexity --- shear during the descent and canopy directional stability.

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Well, in the jet I fly, the wing will move up or down or yaw slightly in response to air flow changes due to turbulence and shearing effects of changing wind velocity. How much a wing or parachute reacts to this airflow change is a question of stability, and for a ram air parachute this is complicated by inflation.

But I think you can give up any notion of defending the idea that wings, inflatable or not, turn downwind as a matter of course. If there is a built in turn well that's a different topic.

P.S, I've flown in 150 knot tailwinds up at altitude and the aircraft shows no desire to turn around.

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pchapman


...how strong the directional stability of a parachute is, and how much of an effect wind shear will have on it, with wind speeds in typical situations lowering, especially in the last 1000 ft down to the ground. Will 2 kts shear in 1000 ft matter? Will 5 kts over 500 ft matter? And how much?



As usual, peter cuts through the crap and identifies the important nuance in the question. Is typical shear (5-10mph) enough to turn a typical reserve a significant amount (more than 90 degrees?) Do I as an unconscious skydiver need to assume my reserve will be very likely landing downwind?

My feeling is that wind shear based turning is a small effect, and which way my reserve lands is more determined by which direction it is facing when it opens.
It's flare not flair, brakes not breaks, bridle not bridal, "could NOT care less" not "could care less".

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riggerrob

A canopy will turn a full 360 degrees, given enough time and altitude.

Built-in turns was a problem during the 1970s and 1980s, but as manufacturing tolerances improved, bailout-in turns got smaller and smaller.
OTOH Jumpers are inherently unbalanced ..... assymetrical. For example: my right shoulder is smaller (weaker) than my left shoulder. An unbalanced human will induce a slow turn.

The canopy is flying in a block of air and only cares about airspeed. The canopy has no clue that the block of air is sliding across the ground. Since it has no magnetic compass, the canopy does not know when it changes heading relative to the planet.
Most turns - during landing flares - are caused by uneven toggles.



A canopy most certainly DOES know when it moves from a block of air moving at one speed to a block of air at a lower altitude moving at a different speed.
...

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

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SethInMI



My feeling is that wind shear based turning is a small effect, and which way my reserve lands is more determined by which direction it is facing when it opens.



Physics is unaffected by feelings. Since no-one has made a measurement of the magnitude of the effect, no-one knows. What we DO know is that Newton's 1st Law says there will be some effect.
...

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

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kallend

Physics is unaffected by feelings. Since no-one has made a measurement of the magnitude of the effect, no-one knows.



Sometimes your turn of phrase is a little oddly antagonistic. "Physics is unaffected by feelings" sounds like an attack on what Seth wrote. He could have written, "I don't know whether..." and you could have written, "But physics knows!"

Yet we're kind of on the same page, agreeing that there should be an effect based on our understanding of the physics... but we're not sure how much.

Others may have different opinions on the size of the effect, based on their observations.

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kallend

***

My feeling is that wind shear based turning is a small effect, and which way my reserve lands is more determined by which direction it is facing when it opens.



Physics is unaffected by feelings. Since no-one has made a measurement of the magnitude of the effect, no-one knows. What we DO know is that Newton's 1st Law says there will be some effect.

Oh come on John. When I said "I feel", I am saying it is my estimated guess based on my understanding of the magnitudes of the forces involved and the time over which they can act. That is very different that how you used the word in the phrase "physics is not affected by feelings".

Obviously I haven't seen any hard data, but we have all been flying parachutes for a long time, and I think if 5 mph of wind shear distributed over 200 vertical feet could effect a 135 degree heading change, we would notice.

Seth
It's flare not flair, brakes not breaks, bridle not bridal, "could NOT care less" not "could care less".

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>My feeling is that wind shear based turning is a small effect, and which way my
>reserve lands is more determined by which direction it is facing when it opens.

And how you turn based on which way you slump in the harness.

In no wind the direction you land is effectively random (when you are unconscious) based on the direction you open and what direction your parachute turns due to your (unintentional) input. If there's any significant wind, then odds are you're going to land downwind because of the small influence the descending-in-a-wind effect has on the canopy.

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pchapman

***Physics is unaffected by feelings. Since no-one has made a measurement of the magnitude of the effect, no-one knows.



Sometimes your turn of phrase is a little oddly antagonistic. "Physics is unaffected by feelings" sounds like an attack on what Seth wrote. He could have written, "I don't know whether..." and you could have written, "But physics knows!"

Yet we're kind of on the same page, agreeing that there should be an effect based on our understanding of the physics... but we're not sure how much.

Others may have different opinions on the size of the effect, based on their observations.

This was all discussed at great length in the thread to which I provided a link in post #9 of this thread.
...

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

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kallend


This was all discussed at great length in the thread to which I provided a link in post #9 of this thread.



You got a laugh out of me, John. In that link you provided, you say (in bold) I wouldn't expect it to be much of an effect Now I could say that physics is unaffected by expectations, but how would that serve the discussion? The correct takeaway for me is to know that you agreed with me, (or that I agreed with you, since you created that post 10+ years ago).
It's flare not flair, brakes not breaks, bridle not bridal, "could NOT care less" not "could care less".

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I saw something on /r/flying which reminded me of this discussion (regarding what happens on the ground versus what happens in the air mass)

At Soto Cano AB in Honduras on Jan 13 of 2001 the tower controller was talking to a Chinook in the pattern when he felt the building shaking, so without thinking he keyed the mic and it went something like this: "Sugarbear 26, did you guys feel that?"

"feel what?"

"I think we are having an earthquake"

"Think about what you just said and get back to me"

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Having done hundreds of test drop when I worked for Para-Flite here's my take on it. Most dummy landings were more or less down wind.
Most test drops are done from a side door of an aircraft so the dummy and canopy are facing perpendicular the jump/drop run. Due to the forward throw of the aircraft the deployment happens mostly horizontally. As the forward throw is dissipated it becomes vertical. The initial inflation one side of the canopy is higher as the dummy becomes vertical the side that is higher sees a slightly higher load/ airspeed as the dummy swings under the canopy causing the canopy to turn in a down jump/drop run direction. In the few times the canopy opened facing directly up or down the jump/drop run the canopy continued fly in that direction no turn.
This just MY EDUCATED OBSERVATION from many years of drops with no scientific proof. Parachutes don’t care which way the wind blows but gravity has an influence.

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