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jtravis121

Recovery Arc vs. Wind Speed

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jakee


Everything about this is painful.



No, not exactly. There are some amusing parts.
Sad.. but amusing.

You have to wonder how some of these people have ever landed a canopy safely.

'Parking in a crosswind'
Every fight is a food fight if you're a cannibal

Goodness is something to be chosen. When a man cannot choose, he ceases to be a man. - Anthony Burgess

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Yeah... opinions are like assholes I guess, we've all got one. I maintain it's interesting to see the different takes people have on it. The way one person describes it might make no sense to me but I still know the affect they're talking about.
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JasonYergin

Yeah... opinions are like assholes I guess, we've all got one. I maintain it's interesting to see the different takes people have on it. The way one person describes it might make no sense to me but I still know the affect they're talking about.



No, it's not about how you describe it (well ok, the way you describe what you think is happening is also pretty bad), it's that the phenomenon you are trying to describe doesn't exist.

You don't get more wind in your face flying upwind and you don't get less wind in your face flying downwind. You will come out of a given dive with the same airspeed whether you're going upwind, crosswind or downwind, and airspeed is exactly what it says it is - speed through the air. This is the only thing that governs how much wind is in your face.
Do you want to have an ideagasm?

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hodges

In stable wind conditions,...


I'm reading your interpretation of stable as if the entire air mass is moving as one, in which case i agree 100%. I'm trying to explain what might happen if the air mass is gradually slowing down as you approach the ground. I drew some pictures to demonstrate

*ETA - I've got the bernoulli principle the wrong way round in my speculative explanation, meaning I would expect a canopy diving 'upwind' to recover faster, if at all

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jakee

***Yeah... opinions are like assholes I guess, we've all got one. I maintain it's interesting to see the different takes people have on it. The way one person describes it might make no sense to me but I still know the affect they're talking about.



No, it's not about how you describe it (well ok, the way you describe what you think is happening is also pretty bad), it's that the phenomenon you are trying to describe doesn't exist.

You don't get more wind in your face flying upwind and you don't get less wind in your face flying downwind. You will come out of a given dive with the same airspeed whether you're going upwind, crosswind or downwind, and airspeed is exactly what it says it is - speed through the air. This is the only thing that governs how much wind is in your face.

Oh this reminds me of a student who couldn't read the windflag, so he turned in each direction and spit to try and tell which way the wind was blowing. He landed downwind with an awful lot of spit on his face and helmet, but boy was he proud of himself for "figuring it out". :)

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Interesting.

It's fair to say that the wind speed drops off during your turn, to some extent, on most landings.

Let's park the stable wind conditions part then because that's theoretical and not very practical (especially not where I jump).

I'm interested in any conversation on how wind sheers between 1200ft to 0ft would affect a turn.

pchapman touched on it...

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kuai43

***
Everything about this is painful.



No, not exactly. There are some amusing parts.
Sad.. but amusing.

You have to wonder how some of these people have ever landed a canopy safely.

'Parking in a crosswind'

My God, you're right. This thread is like THIS
"I encourage all awesome dangerous behavior." - Jeffro Fincher

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>There's a little bit of amplification to the whole process going on with the extra wind in your face

There's no extra wind in your face.

>albeit with a flight that is now more compressed since it isn't traveling as far...

It is traveling just as far through the air.

> strong head wind it can up the lift ante by an extra few percent because there's an
>extra bit of push that wasn't there before.

There's no extra push that wasn't there before.

>now the wing is getting more lift with that weight

The wing is not getting more lift.

>because the wind is giving it a little push that would not and could not be there otherwise.

The wind is not giving it any extra push due to the direction you are going with respect to the ground wind.

>because the extra speed and lift aren't increasing in a linear way

There's no extra lift.

>your wing is actually flying less efficiently since it is cutting through a lot less air

It is flying exactly the same way it did during a normal landing.

There are only two differences landing downwind vs. upwind.

1) You. You will see the ground moving faster (and perhaps have some incorrect beliefs about the wind "decreasing lift" or something) and thus you will flare differently and that will DEFINITELY affect how the canopy flares

2) The ground. When your feet finally touch it, then you will notice the speed difference. The canopy doesn't care - but your feet definitely do.

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hodges

Interesting.
It's fair to say that the wind speed drops off during your turn, to some extent, on most landings.

Let's park the stable wind conditions part then because that's theoretical and not very practical (especially not where I jump).



Ok I'll give it a shot.
Since it gets mathy it's always a compromise between getting too wordy, or leaving out all sorts of calculation and explanation, which would then make it tough for anyone to follow the logic if they tried.


This is all for the situation where there is a wind shear -- wind speed dropping off sharply as one goes lower -- not the steady wind conditions discussed in much of the thread:


1) I've said before that it all depends on the vectors relating to relative wind and how that affects airspeed, angle of attack, and thus lift. And it is messy.
That's both correct, and a vague answer that doesn't help much.

But if nobody else has done the calculations, I'll at least do a couple.

I'll talk about airplanes first because it turns out the standard wisdom in aviation doesn't always work out for parachutes.

2) For airplanes, when one talks about encountering wind shear on approach, to be clear we're usually talking about the normal 'flying into the wind' and 'wind decreases as one gets lower'.

Standard wisdom is that the plane will tend to sink, because one will lose airspeed and thus lift. This seems to be borne out.

Calculations: Say the airplane is flying 80 mph on a 10 degree descending flight path. Yeah that's a bit steep but whatever. Angle of attack (relative to zero lift angle) is say 10 degrees, as a rough rough guess. When wind shear hits, lets say there's a sudden 5 mph drop in the wind. Doesn't matter what the wind is.

Wind shear in reality isn't usually an instant jump in the wind speed, but for a quick look at the issue that's sufficient, a sort of worst case for any given change in wind speed.

Vector addition shows the airspeed changes to about 75 mph, and angle of attack changes to about 10.7. Lift goes up with the relative change of angle of attack (10.7/10)(linear lift curve slope in the normal range of operation), and down with the square of the change of airspeed (75/80, squared). Net result is the lift being 94% of original.

Thus for the light airplane on approach, a sudden loss if some headwind does result in a loss of lift. The airplane will drop faster, pitch nose down, and eventually recover to steady state flight at 80 mph again.

3) Now to try the calculation for a parachute. There are an infinite number of possible cases to try, but how about this:

Medium sized canopy in a 30 degree dive, some sort of swoop, is at 40 mph (not a fast swoop canopy). Say 5 degrees angle of attack since it is moving fast.

It gets hit by the same sudden loss of 5 mph wind. Vector addition this time shows airspeed dropped to 35.7 mph. (Would be exactly 40-5=35 if flying horizontally, but it isn't doing that.) Ok, so the airspeed did of course suddenly drop when the headwind dropped.

But then the angle of attack: With the headwind dropped by 5 mph, the relative wind on the canopy is now from 34 degrees down not 30. A big 4 degrees increase in angle of attack. (Vector sum 40@-30 deg, plus 5@ 180 degrees = 35.7 @ -34 deg. Where 0 degrees is horizontally into the wind)

The lift from speed is at a 0.79 factor (35.7/40, squared), while the lift from angle of attack is up by a 1.8 factor (9/5). Multiply the two and one has a 1.42 factor.

Thus lift has increased 42%. Despite the airspeed decrease, the loss of 5 mph headwind significantly increases the angle of attack since the canopy is in a steeper dive, suddenly increasing the lift even more.

What does the canopy do? It will clearly lift away from its existing flight path. Better to say 'lift up' than 'pull up', as the latter may suggest pitching up the canopy.

So it wouldn't be like hammering the brakes, because brakes have a big pitching effect on a canopy. Maybe more like a rear riser recovery from a dive... That's adding extra lift and effective angle of attack through changing the airfoil shape, but not causing much pitching up.

What happens next isn't exactly clear to me on first thought. How does the flight path continue to change? The canopy's natural stability would tend to slowly pitch it back down to a more normal angle of attack. While at the same time, the extra G loading (and drag on the canopy with increased lift) might cause the jumper beneath the canopy to get swung forward.

Hard to tell without a full simulation to tell what exactly would happen, but in any case the canopy would get some extra, unexpected lift while in the dive.

4) If one were flying downwind instead,
the effect would be roughly reversed. My numbers gave a 43% lift loss. Extra airspeed suddenly, but because of the steep dive, a big loss in angle of attack.

Note that exact lift losses and gains do depend on the assumption of the angle of attack. Which aerodynamically will be somewhere between 0 and 15 degrees, roughly, when flying 'normally' and not collapsing or starting to mush and stall.

=====

So -- if I'm correct -- we have a result of a canopy in a decent little dive, losing headwind suddenly as it descends to land. It actually picks up lift and starts to lift up compared to the expected flight path, pulling out in less vertical distance.

=====

5) I hesitate to give answers like the above , as it might make some who made incorrect claims think they were right all along, when they weren't.

Say someone said, "When I'm diving into wind and hit wind shear behind the tree line at the the DZ, it seems like I recover faster from my dive. I can't figure out whether that's true or figure out an explanation." That's ok.

But if one says, "I think the if I dive into a headwind, I pull out faster, and it is because of ..... [a mess of logic about momentum and the chute catching more air etc that is completely wrong and doesn't explicitly involve wind shear]" ... then it is still wrong.

6) The faster recovery from a dive into wind shear could have other undesired effects:
Wow cool, you dove into the wind shear and lifted up from the normal flight path and thus recovered in less distance. .... But you're still at a lower airspeed, and a higher, draggier angle of attack, so the canopy pick up less speed in the rest of the dive. Almost like you started rear risering out of the dive too high and then let up again.

So you may end up with less speed for the swoop in the end, messing up your plans. Even if you did somehow start the dive lower to account for the faster expected dive recovery.

One isn't likely going to say, "Because there's a strong headwind today, and that increases the chances of significant wind shear, I'll start my swoop 50 ft lower, expecting that to happen." Is one? There's going to be more mental energy expended adjusting the swoop anyway to get the horizontal positioning right, to hit the gates right, if dealing with high and changing winds.

7) Wind effects from shear or gusts clearly change with the direction relative to the canopy. So the result can be vastly different depending on the angle of dive.

As we've seen, a headwind loss when in a moderate dive has one effect (increased lift). While if one were already in a horizontal flare, there would be no angle of attack change, just a sudden loss of airspeed and lift. And if one were in the middle of a steep dive, towards 90 degrees, a change in the wind (horizontally) would affect angle of attack without directly affecting airspeed at that moment.


If anyone has better insight or calculations... give it a go.

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DeeBeeGee

I drew some pictures to demonstrate


Your picture doesn't account for the movement of the wing across the ground while vertical. If I'm at 90º dive angle (hey, we can hope) then, my ground speed = wind speed. That being the case, your "90º" will be tilted with the direction of the wind.

pchapman

Ok I'll give it a shot.


Wind sheer is a change in director or speed, is it not? I'm not sure how a calculation like that can be made without some more specificity. If there is a sheer that "happens" (not sure what terminology to use) over say, 10ft, wouldn't the effect be different than if it were over 2ft? I don't know but, there seems to be a flaw in my understanding of your explanation or the logic therein. How long the wing takes to pass through the sheer would have an effect as well as the weight in the system, no?

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Quote


Wind sheer is a change in director or speed, is it not? I'm not sure how a calculation like that can be made without some more specificity. If there is a sheer that "happens" (not sure what terminology to use) over say, 10ft, wouldn't the effect be different than if it were over 2ft?



Sure, the shear will happen over some distance, which will take a certain amount of time to fly through. Assuming that there's an instant change is a simplified, worst case way of looking at it. In actuality there's typically a little more time for the parachute to adjust to gradual changes in wind or gust speed, taking the edge off whatever they would do if they really changed instantly.

(Still, if you've been hit by gusty, turbulent winds, it can seem pretty sudden at times, and not just a gradual bumping around.)

Even aerodynamics books, when looking at the effect of a vertical gust on an aircraft, tend to start with the idealized case of an instant change from zero to the maximum. It's easier to write out some equations. If one uses some smoothly curved, sine curve style shape, then one would need to do a simulation over time. The sharp edged gust is still a valuable simplification to help understand a gust's effects.

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pchapman

.... Ok I'll give it a shot....




This explanation is so cool!!
I checked/repeated your calculations while drawing the vectors on a piece of paper, and found the exact same results. I think your conclusions are correct!

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pchapman

Even aerodynamics books, when looking at the effect of a vertical gust on an aircraft, tend to start with the idealized case of an instant change from zero to the maximum. It's easier to write out some equations. If one uses some smoothly curved, sine curve style shape, then one would need to do a simulation over time. The sharp edged gust is still a valuable simplification to help understand a gust's effects.


Ok, that makes sense, thanks. Wouldn't a sine curve have a balancing affect on the equation, though? Since your sine wave has peaks and troughs... (I wish there was a "scratching my head" emoticon ;) )

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Haha, yeah I wasn't trying to be exact. Move the x-axis to the very bottom of the sine curve, then one has a nice rise from zero up to some maximum value (and if its a gust, dropping it back down to zero)

(In aerodynamics texts I've seen them use the "1 minus cosine" curve to be precise. Whatever.)

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I don't study aerodynamics but as a Chemical Engineer I study heat transfer which is similar.

This wind velocity gradient caused by friction/drag between the air and the ground is called a Prandtl or boundary layer. The wind speed varies logarithmically (which solves the cyclic issue of a sine wave) with height, stationary at the ground and reaching a maximum velocity (fully wind speed?) at the upper limit of the boundary layer, this layer can be anywhere between 50-300m thick (150-1000 ft).

Since wind shear is change in wind speed/height the wind shear gets bigger the closer you get to the ground where the rate at which the velocity drops off quickest.

So yeah it seems reasonable that as you're diving through increasing slower air your canopy will be decelerated by that slower air and generate lift. Above the boundary layer as long as there are no other weird atmospheric wind gradients then the canopy shouldn't care what the ground speed is.

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EvilGenius

I don't study aerodynamics but as a Chemical Engineer I study heat transfer which is similar.

This wind velocity gradient caused by friction/drag between the air and the ground is called a Prandtl or boundary layer. The wind speed varies logarithmically (which solves the cyclic issue of a sine wave) with height, stationary at the ground and reaching a maximum velocity (fully wind speed?) at the upper limit of the boundary layer, this layer can be anywhere between 50-300m thick (150-1000 ft).

Since wind shear is change in wind speed/height the wind shear gets bigger the closer you get to the ground where the rate at which the velocity drops off quickest.

So yeah it seems reasonable that as you're diving through increasing slower air your canopy will be decelerated by that slower air and generate lift. Above the boundary layer as long as there are no other weird atmospheric wind gradients then the canopy shouldn't care what the ground speed is.



Something tells me that the guy in the story from above who kept hitting himself with his own spit won't understand what you said.
"I encourage all awesome dangerous behavior." - Jeffro Fincher

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To expand on what you mentioned EvilGenius, I've seen different formulas and graphs for the typical variation of wind with altitude.

Of course the numbers are a rough average to something that varies a lot -- sort of like saying the ICAO temperature lapse rate is exactly 1.98 deg C per 1000' altitude. What you actually get within the altitudes you jump at, may be quite different day to day.

Getting back to the wind, one formula from the Iowa Wind Energy Centre gave these results:

The below is for terrain that consists of high grass or low crops (which affects the friction of the air mass, eg, prairies and lakes are windier). The starting point is 20 mph wind actually measured at a standard anemometer height of 33 ft:

15 ft-----17.4 mph
33 ft -----20
100 ft-----24.4
200 ft-----27.7
500 ft-----32.6
1000 ft-----37

I don't expect their formula to apply very well at 1000' and more. While the winds at 13,000' can be quite a bit higher than at 1000', it isn't like we have 50 mph wind up there every day.

Generally the shape of winds affected by friction with the earth will look like the below. This is the velocity gradient EvilGenius was talking about:
[inline wind-vs-altitude-sketch.jpg]

Now I wouldn't trust any graph to say what's happening on a particular day.

Many of us have seen examples, like a student coming in to land when the wind has picked up a little too much for students. The student is coming straight down, straight down ... then magically at 200 ft or 100 ft or 50 ft, he starts to get a little forward speed and comes in for an almost normal landing!


So all this does suggest
a) Yes there is some shear, not necessarily sudden, with altitude
b) Of course the biggest differences will happen when the wind speeds are higher
c) The shear amount over any short distance one will fly the canopy through is moderate in size.

So yes the wind might go down from 32 to 17 mph from 500 ft to when you are planing out during the flare. We're used to trying to account for dropping wind when planning our circuits or accuracy or swooping.

Over a short distance though, the changes (in this average situation) aren't high: From 200 ft down to 100 ft the wind dropped only 3.3 mph. If you were swooping at 60 mph and let's say in a 20 degree dive it would still take 3 seconds to cover that vertical distance. That's about a 1 mph change to the canopy each second, a smaller change for a canopy already moving at 60 mph. (If the dive were as steep as 30 degrees, it would be about 2 seconds and about 1.6 mph change per second.)

Which isn't a huge shear effect on the canopy, and gives the canopy a little bit of time to adjust to each bit of change. Will it affect the natural flight path of the canopy? Sure, a little.

(The lower one is, the more the wind changes with a given change in altitude. But down low, you won't be in as steep a dive either, so the canopy will take longer to be exposed to those changes. So who knows where one might experience the most shear (per unit time), depending on how one flies or dives one's canopy. )

This 1 mph in 60, per second, isn't at all like a bad scenario of 5 mph instant change, when going 40 mph, as in my earlier examples. Those were used to show how the canopy behaviour changes (and thus differs upwind vs downwind) if one does indeed have a sudden wind shear.

To sum up, we've looked at different levels of understanding of canopy flight:

1) At a first glance, start with assuming no wind speed change with altitude, and understand how that works. If you don't understand how canopies and air masses and air speeds work in that situation, your whole understanding of canopy flight will be messed up.

2) Next one can add on, "But what happens the wind does change suddenly with altitude?" and understand what that can do to a canopy, by looking at simplistic scenarios where the wind suddenly changes by a lot.

3) Next one can back off a little and see what is more likely to happen on a typical day: while the wind does drop off as one gets closer to the ground, the amount of wind shear your canopy experiences from second to second isn't huge. As far as how the canopy will behave, it is more like the original all-the-same-wind case.


(Just trying to teach these things is a great way to focus ones mind, organize thoughts, and make some calculations one hadn't gotten around to before!)

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Yup!
I agree, as I said in my post I think atmospheric boundary layers don't get enough "credit" for how they affect canopy performance, I check Mark Schultze wind data every morning, and I see I often fly through 14-18 kts differentials in a 6-8 seconds turn, that must affect the shape of my diving arc by quite a lot.
Also, I think, another big factor is that most people set their turn to a "90 deg" wrt to the wind-direction, whether it is a 90 or a 270, or a downwinder or a into-the-wind landing, all these configurations put the jumper into a 90 degs crosswind setup, which means that we are flying the canopy with some "rudder input", to keep it flying straight wrt our intended final, ie we are altering the way it flies and the initial conditions of our turn, from what it would be if the wind weren't there. That also must affect the final recovery arc/flight cycle.
I'm standing on the edge
With a vision in my head
My body screams release me
My dreams they must be fed... You're in flight.

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pchapman

... 1) At a first glance, start with assuming no wind speed change with altitude, and understand how that works. If you don't understand how canopies and air masses and air speeds work in that situation, your whole understanding of canopy flight will be messed up.


This is the salient point here, I think. There seems to be a lot more people than I thought who don't understand what it means to fly and land a parachute. My take away is, for most scenarios, the perception that we all have that our recovery downwind and upwind are different are just that - perceptions.

Do you think this is a fair statement?

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I guess that's fair, and takes us back to the start of the thread: In general (that is without any sudden wind shear), the canopy will recover the same either direction, but our perception of it could change due to the different ground speeds.

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pchapman

I guess that's fair, and takes us back to the start of the thread: In general (that is without any sudden wind shear), the canopy will recover the same either direction, but our perception of it could change due to the different ground speeds.



Finally.. a coherent and reasonable summation. I've really enjoyed all the complex mathematical analysis that's been brought out and it's been great food for thought, however I believe we've ended up with the simple answer that was needed. Occam's razor has made its cut.
I think we're done here.
Every fight is a food fight if you're a cannibal

Goodness is something to be chosen. When a man cannot choose, he ceases to be a man. - Anthony Burgess

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Yeah I think that people get all mixed up trying to compare mathematical conditions to "real" conditions where there is some variation and turbulence. Anytime someone says "this weird thing happened" someone else says "thats not possible based on these very specific conditions that you may or may not have been in." But yes, it is important to understand the fundamental principals first, then learn they can be "bent" based on changes in the variables.

For instance, I noticed a long time ago that flying straight and level (say 20 degrees off of upwind), my canopy will gradually turn to fly directly into the wind with no control input, it wouldn't/couldn't happen if the air column was stable and laminar.

I was confused, and some people I told thought I was retarded, until it happened following a strong gust, and the turn was pronounced. I had an "ah ha" moment. Basically, if the wind speed changes, you don't instantly change with it, so there is more drag/lift/whatever (I'm not trying to get really technical with an anhedral wing and a enormously low center of gravity) on the upwind half of the wing, letting the downwind half catch up a little.

Had another interesting chat and unresolved question. "If you fly perpendicular to the wind with smoke, will the trail be straight?" My guess is of course in a perfectly smooth column of air, yes, but those don't really exist. So "in real life" as the wind changes, the smoke will move as the wind does (very little mass), the jumper's movement will be delayed (time for their mass to equalize with the change in wind speed) so the smoke will be the sum of the changes in wind speed, and the jumper will be (roughly) the mean of the change in wind speed; thus the line of smoke will not be straight.

So to really simplify it, assume 10kt cross that accelerates and stays at 20kts, the smoke will accelerate essentially with the wind, while it will take the jumper at least a few seconds to absorb enough energy to move with the column again.

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Di0

Yup!
I agree, as I said in my post I think atmospheric boundary layers don't get enough "credit" for how they affect canopy performance, I check Mark Schultze wind data every morning, and I see I often fly through 14-18 kts differentials in a 6-8 seconds turn, that must affect the shape of my diving arc by quite a lot.
Also, I think, another big factor is that most people set their turn to a "90 deg" wrt to the wind-direction, whether it is a 90 or a 270, or a downwinder or a into-the-wind landing, all these configurations put the jumper into a 90 degs crosswind setup, which means that we are flying the canopy with some "rudder input", to keep it flying straight wrt our intended final, ie we are altering the way it flies and the initial conditions of our turn, from what it would be if the wind weren't there. That also must affect the final recovery arc/flight cycle.



This is a really good point, if you are crabbing, you are not flying with the air mass, and the initial variables at the beginning of the turn change, thus changing the recovery arc. And depending on the jumper, the turn itself is also altered because they are basing their input on cues from the ground.

So like it has already been said, in a perfectly smooth column with the exact same input, the arc is the same regardless of wind speed, but neither of these conditions are met in a real life application. The air column is not perfectly laminar, and input changes as well.

I see this same kind of thing with chemists, "but the calculations show that x should happen, what is wrong?!" well, the conditions of the calculation fail to reflect the real conditions your reaction is occurring under. You gotta embrace the unquantifiable variables, "embrace the chaos" as I like to say ;)

It is important to remember as well that aerodynamics are rather poorly understood, which is kinda what makes the field so interesting! To my knowledge there is no mathematical model that allows insects to fly, but they still do it, we just don't understand how (on a mathematical level anyway). And as far as I know, the real nuance of flapping flight is still elusive.

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I think DJL and Di0 got close to answering the OP's question, but didn't take the final step.

If the OP is working on 90 degree riser turns, he's probably trying to flu a square pattern over the ground. In high winds, he's crabbing on the base leg, so when he makes his final "90 turn" he's really only turning 60 degrees, or 45 degrees. Making a smaller turn like that will result in a high rollout every time.

The real answer is that he's rolling out high due to momentum, wind shear, or a faulty flux capacitor. He's rolling out high because he's not making the same size turn in high winds.

- Dan G

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