Old Debate: Canopies with no input, *will* fly downwind?

[mdrejhon 8]

[Edit: I created this thread, before billyvon and others split the topic from the incidents forum. Billyvon, please feel free to re-merge this into the main thread.]

I am attempting to keep this distraction away from Incidents, so out of courtesy, I am posting this thread here to deflect attention on this detail, away from there.

A canopy doesn't care what it is relative to the ground. In a perfect laminar-flow wind that always blows at exactly the same speed. So canopies with no input, will never tend to fly downwind in this perfect 'physics' case.

But if someone take my arms off toggles and just relax from a high-altitude hop-and-pop, some canopies are reported to slowly inch their way to downwind, after a minute or two. (be attentive if you test this, and it probably works best with larger canopies)

The micro-fluctuations of wind (going through turbulence, going through wind layers, minor vortexes, etc) causes subtle effects that cause canopy to 'inch' its way (.... errr .... I mean 'millimeter' or 'micrometer' it way) to a downwind heading.

Most of the time, wind surges up (gusts) rather than surging down (sudden deadness). Airplanes and canopies flying into a sudden gust, often gets deflected towards downwind. This is often seen in radio control airplanes, when you try to control light and slow flying airplanes in windy weather;

Yes, if there were sudden dead moments in a gusty weather, it would (physics-wise) be more likely that a canopy starts to point upwind instead of downwind.

Now.... Even stable air often have micro-gusts (not perceptible to humans) that will often 'inch' (er, 'millimeter') the heading toward downwind.

I am especially interesting in hearing from other control airplane pilot (of which I am one); who's familiar with the phenomenon that a radio/control airplane (without input) often gets deflected to fly downwind, because of the gust-deflection effect. It is much more common with the popular slow-moving ARF/RTF foamies (almost-ready-to-fly / ready-to-fly out of the box, foam airplanes), when you try to fly them outdoors in moderately windy weather. When letting them coast (no input) they get deflected towards the downwind direction during the sudden gusts.

And we all know, sudden brief gusts (tendancy to deflect *certain* aeronautical vehicles to downwind) are more common sudden brief deceleration in wind (tendancy to deflect *certain* aeronautical vehicles to upwind). And this scales downwards to the micro level too, to gusts that humans cannot perceive. Wind in real world nature is not laminar, and the different speeds of wind layers as a canopy descends, produce opportunity for micro-gusts and vortices between the wind layers. The more gusty, the faster the tendancy to turn downwind. In very stable weather with little wind and virtually zero turbulence, the forces may accumulate enough to completely turn a canopy downwind before landing.

Again, it is not unamious to all aeronautical vehicles, and thus, may not be unamious to all parachutes (i.e. it may happen more with more-easily-deformable non-cross-braced parchutes, who knows?)

Comments? Especially from Ph.D's -- and from R/C pilots -- and from ultralight pilots -- and from CRW pilots? Although not a Ph.D here, I did get 96% in Physics at one time...

[Butters 0]

How many time have you flown up wind, cross wind, or down wind without any input? Numerous. How many times has your canopy turned to fly down wind? Never. It's not that hard of a question to answer ...

"That looks dangerous." Leopold Stotch

[Calvin19 0]

From the way I can see it, the only way an aircraft (fixed wing) can be 'deflected' from it's stable uncontrolled flight, is from changes (sudden or gradual) in it's relative airflow.

IE-a stable high dihedral aircraft that descends through a shear layer entering left to right moving air relative to the air it is leaving will be banked to the right, and then regain its stable flight. in this it will have been 'deflected' some angle "downwind".

since shear layer effect is extremely common in near ground situations, you could say that a stable aircraft would be 'deflected' downwind.

a para chute, though negative "dihedral" (anhedral) it is still a stable aircraft, and I think would do the same thing.

(1000hour commercial/INS/multi/acro/tailwheel/GLI, experienced modeler)

[mdrejhon 8]

Before you speak too soon:

I think have -- when I just relaxed on a 13500 hop-n-pop, and went through high-altitude turbulence layers without un-stowing my toggles. (I unstowed at about 6000 feet, high enough to take care of any mals that may arise from unstow)

Have you let your canopy fly through wind layers/turbulence, while toggles are stowed? I noticed an approx 5-degree turn away from my upwind heading. That was a turn opposite of my canopy's super-tiny natural built-in turn (which is probably caused by millimeter differences in the line) I was only hovering my hands above my rear risers at the time so it was not affected by my hands, and the size of the canopy is a Sabre 170, which is more affected by turbulence than say, a Pilot 120 or a VX99 which a lot people here seem to have. Few people have the patience to keep toggles stowed for a while, due to the safety implications, and related reasons. (Cessna jump, only person in the air, great spot, so I had luxury to just relax my arms for a bit while looking around, and a pre-plan to unstow at 6K.) Non-unstowed toggles simulates an unconscious skydiver under a premature deployment or under an AAD fire; stowed toggles with absolutely no input.

Students fly big canopies -- precisely the kind that are more easily deflected downwind by the micro-gusts I describe.

[Edit: I created this thread, before billyvon and others split the topic from the incidents forum. Billyvon, please feel free to re-merge this into the main thread.]

[mdrejhon 8]

Quote

IE-a stable high dihedral aircraft that descends through a shear layer entering left to right moving air relative to the air it is leaving will be banked to the right, and then regain its stable flight. in this it will have been 'deflected' some angle "downwind".

That's what I see with my radio control airplanes -- the foamies tend to bank during the gust-up and un-bank during the gust-down. But now it's flying at a direction that's a few more degrees downwind. If I'm crabbing to the left, it'll deflect in the opposite direction than if I am crabbing to the right. It's clearly towards the downwind direction, obviously. It's very obvious when trying to fly crosswind, and I even noticed that once on my canopy too from a high-altitude "no-toggle-unstow" cross-country parachute flight. During the rare high-altitude turbulence I encountered, I felt a ever-very-slight bank downwind of my crosswind flight, opposite to my canopy's built-in natural turn (which is almost imperceptible, but opposite direction).

I am thus led to believe, that it makes sense that *some* canopies in *some* cases, do have a tendancy to turn downwind in *some* meterological conditions... No evidence proves otherwise, lots of evidence proves so (especially including external evidence, such as flying radio-control)

Quote

(1000hour commercial/INS/multi/acro/tailwheel/GLI, experienced modeler)

Pretty impressive. Would you agree that lighter aircraft are more easily deflected downwind by the gust/shear effect?

Something I'd like to learn more about: What about wind layer shear (going through vertical differences in wind) versus gusts (going through horizontal differences in wind)? Maybe I need to clarify my terminology, because behaviour is different in these two cases. Does the turn towards downwind only happen during shear, or also gusts? Or to different extents?

Computational fluid-dynamics on this could be very interesting! It would need to support shear layers "that moves up from below" (as you glide at a slope), and also support gusts, to test this out.

[Butters 0]

Layers and turbulence will do all sorts of things to canopies ... I thought we were discussing constant wind only.

"That looks dangerous." Leopold Stotch

[billvon 2,434]

>Most of the time, wind surges up (gusts) rather than surging down
>(sudden deadness).

If that was true, over the billions of years of the earth's existence, windspeeds would be hundreds of thousands of miles an hour (if the trend was even slightly "up" on average.)

Over time, the average does not go up or down, because wind speeds go up as often as they go down. We perceive that it gusts up more than it 'gusts' down because we notice wind more readily than its absence.

>Airplanes and canopies flying into a sudden gust, often gets deflected
>towards downwind.

That's true - when they are flying upwind.

Do the following experiment. Pick a moderate to high wind day. Get out four or five miles from the DZ (a distance where you will barely make it back.) Fly downwind the whole way. You'll discover that your canopy wants to turn just as much as it does when you are flying into the wind - except now, since you are flying downwind, it will always want to turn _into_ the wind.

Since we rarely do this, we don't notice it as much.

[mdrejhon 8]

Quote

Layers and turbulence will do all sorts of things to canopies ... I thought we were discussing constant wind only.

No disagreement there.

In a perfect, constant wind, a canopy will always fly straight ahead in its existing direction, regardless of wind direction, and never get deflected downwind. Total agreement to everyone (including billyvon) here. Moving along in perfect laminar-flow wind would be exactly the same as being exactly still; from a relative frame-of-reference perspective. But such 'perfection' never exists...

... BUT ... (and that's a whopper BUT)

There's no perfectly constant wind. Even wind that looks constant to human eyes, can have imperceptible turbulence that's so soft you don't feel it, that will deflect your canopy by one millimeter, causing your heading to turn less than one degree downwind. In a 15-minute flight from full altitude, there might (or might not be) enough of this micro-level happening, to fully turn a canopy downwind.

But as we all know, a random canopy flying on a hot day, flying straight over water, forest, asphalt, buildings, etc, will have VERY noticeable turbulence in the wind caused by the different updrafts causing the shear/gusts -- often as high as 3000 feet. On a hot day, there's likely enough turbulence in there, from unconscious flight 3000 feet down to 0 feet, to finish turning a student-size canopy fully to the full downwind direction. (Big canopies, such as student canopies, are where turbulence is REALLY felt during wind shear, gusts, updrafts, etc.)

I'm now interested in the more academic stuff -- how does vertical differences (shear, updrafts, etc) versus horizontal differences in wind (horizontal gusts), affect uncommanded 'turning' behaviour... Or is it a plane of shear (i.e. wind layers, boundary layers of wind zones, etc) or a plane of pressure zones (i.e. updrafts, gusts) that has more of an effect on uncommanded turning behaviour?

Obviously, if we want to get fully technical, we have to factor in assymetry here (like gusts being stronger at the left edge of your canopy than the right edge, or the updraft being stronger at one end than the other end, etc) which can counter-act the turning tendancy, but as a pratical matter, let's average the assymetry out, which probably brings us back to the simpler cases of shear zones and pressure zones...

[billvon 2,434]

> On a hot day, there's likely enough turbulence in there, from unconscious
>flight 3000 feet down to 0 feet, to finish turning a student-size canopy (where
>turbulence is REALLY felt) fully to the full downwind direction.

Agreed. But if he opened at 5000 feet, there might well be enough of the same turbulence to turn him downwind and then back into the wind. It depends on whether the canopy has a built in turn, how much turbulence there is, if he's leaning in the harness etc.

[pilotdave 0]

I think it was kallend that shared the theory that the decreasing wind speed as we descend could cause a canopy to turn downwind. A gust (increasing wind speed) will cause a canopy turn toward the wind. Otherwise our canopies would be unstable and would have no tendency to fly forwards. But as we descend, the wind speed tends to decrease. Our canopy could see this as a series of negative gusts (decreasing wind speed). If the canopy experiences more decreasing wind than increasing wind, it could theoretically turn away from the wind direction over time.

On the other hand, i doubt that this effect would be enough to notice and the reality is that canopies tend to turn a little for whatever reasons, and we only notice it when those turns are inconvenient... such as away from the wind direction when we're trying to penetrate.

Dave

[JackC 0]

A rapid increase in wind strength can affect a canopy's heading by moving the relatively light inertial mass of the canopy more than the heavier inertial mass of the skydiver. If the gust comes slightly from the side, this will make the canopy roll which induces a turn. The pendulum effect of the skydiver swinging back underneath and past center will only partially correct for this because the amplitude of the swing will be less so the net result is an altered heading. Once the canopy has taken a heading slightly off the wind line, any gust along the wind line will induce more roll and hence more turns.

A rapid decrease in wind speed however, will tend to make the canopy dive to regain it's airspeed instead of roll so the heading will be affected much less.

Debate that one amongst yourselves, I'm off to the pub to do some interactive fluid dynamics experiments.

[ryanl2006 0]

Another think that hasn't been discussed here is what about a tendency of the canopy to open in specific direction relative to the wind?

I would argue that due to a difference in upper winds to winds at deployment altitude, the relative wind will not be from directly straight ahead for seconds after opening. In this case would the wind tend to blow the canopy to face upwind or downwind for an unconcious jumper shortly after opening? Discuss...

[For what its worth I'm a senior in mechanical engineering who has taken several upper level fluid dynamics classes]

[pchapman 262]

Canopies turning downwind?

Wish that had worked for those static line or IAD students who didn't listen to their radio at all and sailed off randomly into the distance instead of heading back to the DZ!


My best understanding so far is what PilotDave said about what Kallend had described in earlier threads on this topic -- That normally the wind doesn't matter, but wind shear can have an effect. Theoretically due to yaw stability of the canopy, hitting a sudden new wind would turn the canopy slightly into wind. Conversely, going down through wind shear to lower wind (as is typical) would turn the canopy out of the wind direction.

Whether the shear is enough to make a practical difference in most cases, that seems less likely. It is one thing to turn a little downwind (eg 20 degrees), vs. actually turning until lined up fully downwind. (eg 180 degrees)

Consider a canopy in a strong crosswind and then descending past the edge of a forest acting as a windbreak. I doubt the canopy is going to swing 90 degrees away from the wind to line up downwind.

Or sometimes it is easier to imagine the reverse -- going from shelter behind large buildings, to a sudden crosswind when out in the open. The same yaw stability theory would have the canopy turn "into" the wind. Maybe yes, but not 90 degrees worth.

So I don't see that say 15 kts of wind shear between opening and landing are going to be enough to turn a canopy all that much.

JackC brings in the roll issue. There it isn't quite clear what the net effect would be, but I'm thinking there's still some overall yaw stability that dominates.

But I'm open to better evidence or theories.

[billvon 2,434]

>If the canopy experiences more decreasing wind than increasing wind, it
>could theoretically turn away from the wind direction over time.

If it is heading crosswind, and the wind shear is pretty severe (i.e. more than a knot every few seconds) and the wind is changing linearly, that that could happen. However:

1) Winds that severe generally keep people on the ground. We're talking days where winds are 60 knots at 3000 feet, or where there is a really sudden shear (i.e. at a tree line.)

2) It will only affect canopies flying crosswind, not canopies headed into the wind or away from the wind.

3) In practice it does not happen. Dummies and uncontrolled unmanned payloads generally either fly in a straight line or circle slowly in one direction.

[billvon 2,434]

>I would argue that due to a difference in upper winds to winds at
>deployment altitude, the relative wind will not be from directly straight
>ahead for seconds after opening.

Again, that effect is negligible. Tracking/backsliding has far more effect on the direction a canopy opens.

[Calvin19 0]

Quote

Agreed. But if he opened at 5000 feet, there might well be enough of the same turbulence to turn him downwind and then back into the wind. It depends on whether the canopy has a built in turn, how much turbulence there is, if he's leaning in the harness etc.

If we can assume that the canopy/stable aircraft and unconscious/helpless pilot has no tendency to turn slowly either way, AND we have a perfect model shear layer effect to the ground, the most stable and most probable average heading would be downwind.

HOWEVER, if there is sudden gusts, (there is no perfect shear layers) blah blah blah, I would say that even though the aircraft is most likely to end up impacting downwind that upwind, that there is still a huge chance factor involved.

ALSO, there is no aircraft that has a perfect stable heading hold, not even 500,000EU sailplanes. I would say that even though the tendance will be to a downwind heading through a wind shear, that only 51/100 would impact downwind.

[billvon 2,434]

>If we can assume that the canopy/stable aircraft and
>unconscious/helpless pilot has no tendency to turn slowly either way, AND
>we have a perfect model shear layer effect to the ground, the most stable
>and most probable average heading would be downwind.

I'd think that once you get to forces that small, you have to start accounting for things like Coriolis forces as well. But both those effects are _really_ small outside of some very special cases.

[Calvin19 0]

Quote

>If we can assume that the canopy/stable aircraft and
>unconscious/helpless pilot has no tendency to turn slowly either way, AND
>we have a perfect model shear layer effect to the ground, the most stable
>and most probable average heading would be downwind.

I'd think that once you get to forces that small, you have to start accounting for things like Coriolis forces as well. But both those effects are _really_ small outside of some very special cases.

Kinda like curving a bullet.

[Calvin19 0]

so yeah, I guess the point was, a gust (or descent/transition through a shear layer into an airmass of different vector [gust]) the aircraft would have a tendency to turn 'with'( NOT necessarily downwind relative to the ground)' the airmass it is entering. but this is a physics problem, and has no useful application into real life.


PS- a thing to remember here, is the original problem from a different thread was an airmass relative to the ground. 'downwind' only applies to an airmass if it is in reference to the ground.

[Jonsmann 0]

I believe there is a natural explanation for the false impression that canopies will turn downwind.
I have seens students on many occasion brainlocked or unconscious under canopy. They all hang slightly uneven in the harness and the canopy will thus turn slowly.
Observed from the ground:
As long as they generally fly upwind it looks as though they are just hanging there. However, when the slow turn brings them downwind the canopy will start to go really fast across the ground. This gives the visual impression from the ground that the canopy magically turned and headed downwind. From the ground we see the change in speed as an event rather than a natural consequence of the slow continous turn.
The rate of turn is the same so if you keep looking you will see that the canopy with an unconscious skydiver will continue the slow turn and go upwind again.
Impression in the air:
The brainlocked student/skydiver will generally "wake up" by going fast downwind, take control of the canopy and conclude that the canopy by itself turned downwind.

Jacques

[airdvr 200]

I don't know much about physics. My statement was solely from watching a jumper whose chest strap broke when he dumped in a track at around 6K. Hanging upside down in the harness his canopy eventually headed downwind and stayed downwind until he found the top of a 90' tree several miles from the DZ.

Please don't dent the planet.

Destinations by Roxanne

[pchapman 262]

Quote

Hanging upside down in the harness his canopy eventually headed downwind

That's a bizarre incident to witness but I think you would agree that his thoughts may have been something other than,

"I must remain perfectly still and avoid uneven weighting of the harness, as this would add confounding factors in this attempt at data collection to study wind effects on parachute dynamics!"

[davelepka 4]

Quote

Hanging upside down in the harness his canopy eventually headed downwind and stayed downwind until he found the top of a 90' tree several miles from the DZ.

I'm sure we can all agree that the above scenario does not represent much of a scientific test of a canopies natural tendency to do much of anything, It is, however, a great story.

This whole issue is simply one of perception. Let's keep in mind that 'into the wind' represents only one heading on any given day, and every other heading will include some degree of downwind component.

For example, if the wind is coming from 270, then that heading is into the wind. Even a slight deviation off of that, say 265 or 275 will result in a crab downwind to some degree.

The key argument against canopies 'seeking' a downwind heading is that a canopy in flight is only 'aware' of the relative wind created by it's forward movement. This is why we have to differentiate between airspeed and groundspeed when discussing things like flying landing patterns which involve navigating to fixed point on the ground and involves traveling both with, and into the wind. In a perfect world, canopies do not know anything about wind direction.

However, this is not a perfect world, and as we know the wind is not a perfect medium. There are gusts, doglegs and thermals that distrub the airmass that a canopy is flying through. While the canopy may be 'blindly' flying along in it's own relative wind attempting to maintain its trimmed arispeed, the variations in the airmass will cause the arispeed of the canopy to vary slightly, and not always in a uniform way across the entire canopy. As the canopy attempts to return to it's trimmed airspeed, and maintain uniform pressure acorss the wing, variations in airpseed and heading will occur.

It's just like driving a car. Set the cruise on long stretch of smooth highway, and you can take your hands off the wheel for surprisingly long periods of time and still go straight down the road. Try the same on a bumpy road, and the reaction of the car to the bumps will cause the heading to change. The bumps in the road are just like the variations in the airmass.

Now to bring it all together, canopies may very well 'seek' a downwind heading when no input is applied, but not for the reasons many think. The canopy is not getting blown downwind, it is simply reacting to the variation in the airmass, and this can result in alterations of the heading.

As we previously discussed, into the wind represents only one heading on the compass, while every other heading will have a downwind component. When you combine the fact that canopies are unlikely to hold a heading with no input, and that 359 of the 360 possible headings a canopy could fly have a downwind component, than it would appear that canopies always tend to turn downwind with no input.

In reality, canopies turn with no input and the direction of turn and degree of downwind component the new heading will have is just happenstance, and a result of that fact that most headings have a downwind component.

[airdvr 200]

Quote

Quote

Hanging upside down in the harness his canopy eventually headed downwind

That's a bizarre incident to witness but I think you would agree that his thoughts may have been something other than,

"I must remain perfectly still and avoid uneven weighting of the harness, as this would add confounding factors in this attempt at data collection to study wind effects on parachute dynamics!"

I think it was more like "oh shit!"

Adding insult to the incident it was a hot day and after hanging upside down in the tree for about an hour he decided to get out of the harness...fell about 5' and broke his collarbone. Then the fire department damned near hung him trying to get him down.

Funny...he stopped jumping after that.

I have to agree that in a perfect world the canopy doesn't care about wind direction. We all know we don't live there don't we?

Please don't dent the planet.

Destinations by Roxanne

[kallend 1,647]

Quote

Layers and turbulence will do all sorts of things to canopies ... I thought we were discussing constant wind only.

I thought we were discussion reality here.

...

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