Effects of Windspeed on Canopy Direction

[fred 0]

In another thread (and I considered continuing this discussion there, but it seems ripe for its own thread), Hooknswoop and Kris both agreed that:

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A canopy flys EXACTLY the same in wind, as it does in no wind.

... and that it wouldn't naturally turn down-wind.

I'm not saying that I don't believe this, but I'm skeptical. I think it's simplifying the physics and the weather. But I think there are techicalities that make this untrue, and I wish to explore them.

First, I understand completely that a freestanding body in a fluid will eventually (quickly, in skydiving terms) start moving at the same speed as that fluid. I have no idea how long it takes a skydiver to reach this point, but I imagine it's well before pull time.

Assuming that's true, at pull time, the skydiver and the parachute open in this fluid, and are moving at the same speed. Thus, it doesn't matter if this winds are dead still or if they're flowing at 50mph, the canopy will behave the same.

It seems a lot of skydivers don't get this point, but groundspeed has no affect. I think hooknswoop, kris, and others will agree with me up until this point.

But this is where I lose it. Wind does not have a constant speed. Experience on the ground tells me that wind is, in fact, constantly changing speed. Wind gusts, and breezes, and dies down, all within a couple seconds.

For every change in the fluid, the canopy must try to catch up. The canopy has more mass than the surrounding particles, thus, it requires more force to make it slow down or speed up. Certain parts of the canopy will catch more or less wind resistance, and the resultant force from friction will affect it in different ways.

Further, I'd wager that the front of the canopy has more resistance than the back, and thus, will be affected more by changes in windspeed than the back, making the canopy seem to turn downwind.

The possibilities:
(1) The wind at higher altitudes does not change speed as much as that near the ground, thus what I experience while standing in an open field is much different than the wind at opening altitudes. I'm willing to believe this, if anybody can verify this, I'd be much obliged.
(2) The effects of changes in the wind cancel out. That is, wind increasing speed turns the canopy downwind, but it then slows again, turning the canopy back upwind. This is possible, but only if gusts were sustained long enough for the canopy to once again reach equilibrium with the fluid it is traveling through. I tend to think that gusts don't last long enough for this to be true, but again, I don't know.
(3) The effects are negligible. Again, I just don't know enough about wind. In my experience, stronger winds supply stronger gusts (at ground level), if this holds true for higher altitudes, I find it hard to believe that the effect is negligible.
(4) I am right, and it is conceivable that in 3 minutes of descent, changes in windspeed could make the canopy point downwind.

I'm always trying to learn... And I'm certainly no skydiving champ (I would describe my skills as pre-beginner), but I also think I have a pretty firm grasp of physics (except magnetic flux, which has always evaded me), and I feel like a lot of the 'experts' simplify the physics so much that their explanations based on them are inaccurate at best, and sometimes misleading.

I hope I explained that well enough... I'll be ecstatic if somebody can explain to me the fallacy in my logic.

[Hooknswoop 19]

Imagine a submarine in the ocean, submerged. It is turning it's screw (propeller) enough to produce a forward speed of about 5 knots. It is slightly negatively buoyant. There is a 5-knot current. It is facing perpendicular to the current. What direction will it be facing when it hits bottom?

Hook

[fred 0]

Hmm... Let me summarize my argument.

I agree that "A canopy flies EXACTLY the same in wind, as it does in no wind," given one criterion. That wind has to be constant. I don't believe (and I have no knowledge of meteorology), that the real world wind meets this, and I think that the frequent changes in windspeed will have some affect.

I do understand your submarine example 100%, but I think you're missing my point.

[quade 3]

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I don't believe (and I have no knowledge of meteorology), that the real world wind meets this, and I think that the frequent changes in windspeed will have some affect.

So, when the wind increases in a gust you're saying that the canopy would have a tendancy to turn, maybe just slightly, in one direction.
Ok, -maybe- I'll buy that, but then you'd have to buy that when the same gust then decreases that the canopy will then have to change direction in the other direction -- the net effect is that ultimately the canopy still flies in a straight line relative to the wind.

quade -
The World's Most Boring Skydiver

[crazy 0]

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But this is where I lose it. Wind does not have a constant speed. Experience on the ground tells me that wind is, in fact, constantly changing speed. Wind gusts, and breezes, and dies down, all within a couple seconds.

You give the answer:

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(2) The effects of changes in the wind cancel out.

You can even go a bit further: the wind is defined by it's average direction, it's average speed, and one characteristic of the turbulences (max speed of the gusts).
Each gust of wind (faster than average) is, by definition, compensated by a lull (slower than average). Each left crosswind (from the average direction) is compensated by a right crosswind.
When you are moving in the air, you will feel the turbulences if they are strong enough. But you can't tell the direction and the strength of the wind, just from the turbulences, because they cancell out. If you can't tell, then your canopy can't tell either.

However, there might be an explanation why so many people are convinced that a canopy turns downwind: on windy days, you notice much more the distance covered while the canopy is going downwind. Hence, you remember much more the occurences where a canopy is going downwind.

--
Come
Skydive Asia

[Hooknswoop 19]

I didn't specifically address it, but the same analogy applies, as the submarine descends, it encounters eddies, changing currents, etc. These changes (turbulence) can change the direction the submarine (canopy) is pointed, but not in a predictable manner. It will still impact in a random direction.

Here is my belief as to why some people believe an uncontrolled canopy will land facing downwind.:

1) When is the only time, usually, we are concerned with keeping the canopy pointed directly into the wind? A: Downwind spot and landing/final approach.

2) Where is the most turbulence found? A: Near the ground

3) If the canopy is pointed into the wind and flys through turbulence that causes it to turn, which direction does it turn? A: Downwind.

So our perceptions is that when we try to point the canopy into the wind, it keeps trying to turn downwind, hence some people believe canopies have "Down wind turning Tendencies".

I believe that if we landed downwind some people would believe that canopies "Naturally weather-vane into the wind". Because any turbulence we encountered on final approach that caused the canopy to turn would turn the canopy into the wind (any turn from downwind is into the wind).

Hook

[indyz 1]

This is a physics problem, thus I believe that an experiment is in order.

Pull high and, leaving the breaks stowed, turn your canopy to fly north. Take your hands off of the risers and let the canopy fly on it's own. Observe any change in direction of flight. Repeat for the east, south, and west. My hypothesis is that the canopy will not turn, or will turn with some fixed speed regardless of the canopy's orientation into the wind.

[masher 1]

Assuming ideal conditions.... Perfect canopy, perfect body position, even harness/risers, correct brake settings, straight wind (no eddies, turbulance) etc

The canopy won't turn. It will crab in whatever direction the wind is blowing, assuming it's not directly in front or behind.

To induce a turn you need uneven inputs to each side of the canopy, which is what you do when you pull down on a toggle.



Back to real world conditions... anything is possible.

--
Arching is overrated - Marlies

[darkwing 4]

I'm going to gouge my eyes out with a rusty spoon if I see this again. This issue first came to my attention when I started jumping 30 years ago, and it is a recurring theme, both in the skydiving world and the flying world. It makes me crazy.

-- Jeff
My Skydiving History

[dterrick 0]

Gouge not, Mr. Darkwing. If you understand it then you may be able to help those who as yet do not understand.

I'm guessing that back in the days of rounds you all were given a much better grounding in flight mechanics than we students of squares were.

I don't recall seeing a thread on this particular question during my 1.5 years on this forum. I do recall making enquiries to the locals about why on a windy or turbulent day my spirals did not seem to be smooth and even ... I'm confortable with my knowledge but neither ready to explain nor to gouge my eyes out . Hope that feeling passes real soon - I know of very few blind skydivers.

Dave

Life is very short and there's no time for fussing and fighting my friend (Lennon/McCartney)

[JYorkster 0]

I think you touched on a very interesting point that may actually be the answer...

Exaggerate the side view of a canopy in your mind to be a huge triangle, with the pointy end being the tail and having a very wide nose. Even with some forward movement, if place in a fluid perpendicular to the flow of the fluid, there will be more push on the nose than the tail, therefore pushing the nose to the 'downwind' side. On a much smaller scale, the effect is slight, but still present. It would only not affect the heading if the 'triangle' were moving perfectly with or against the flow of the fluid.

If this hypothesis is correct, then it means that canopies due have a 'downwind tendency' but I don't believe it is a great one. Usually unintentional turns under canopy are caused by body position, harness adjustment, etc.

Just a thought.

Rock

[Hooknswoop 19]

I wonder if anything can be gleaned for this discussion from how a "Parafoil" kite flys? It is basically a ram-air parachute, yet it "weather-vanes" into the wind.

Thoughts?

Hook

[riddler 0]

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there will be more push on the nose than the tail, therefore pushing the nose to the 'downwind' side

Physics 111. Force is caused only by acceleration. Therefore, you will only get a "push" if there is a gust in that direction.

When you are in the airstream, you are flowing with it - there is no "push" force because you are going at the same speed as the wind.

[quade 3]

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I wonder if anything can be gleaned for this discussion from how a "Parafoil" kite flys? It is basically a ram-air parachute, yet it "weather-vanes" into the wind.

Uh, the kite is tied to a string and held basically in place.

This would be the difference between a boat that is adrift in a stream and one that is anchored from the bow. Obviously the boat that is anchored by the bow will have a tendance to turn UPSTREAM.

However, the boat that is adrift will have no tendancy to turn upstream or downstream unless acted upon by some uneven forces.

quade -
The World's Most Boring Skydiver

[Hooknswoop 19]

Quote

I wonder if anything can be gleaned for this discussion from how a "Parafoil" kite flys? It is basically a ram-air parachute, yet it "weather-vanes" into the wind.

--------------------------------------------------------------------------------


Uh, the kite is tied to a string and held basically in place.

This would be the difference between a boat that is adrift in a stream and one that is anchored from the bow. Obviously the boat that is anchored by the bow will have a tendance to turn UPSTREAM.

However, the boat that is adrift will have no tendancy to turn upstream or downstream unless acted upon by some uneven forces.

Right, it is anchored, which makes for a bad example, except for the "gust from the side turns the canopy downwind" point. If a "side gust" pushed the nose of the canopy downwind, then a para-foil kite would turn 180 and fly into the ground as it was hit by "side gusts".

We are in total agreem ent that canopies do not have downwind, upwind, or any other "wind" turning "tendancies".

Hook

[billvon 2,427]

>I'm guessing that back in the days of rounds you all were given a
> much better grounding in flight mechanics than we students of
> squares were.

I think the opposite is true. Round jumpers are much better spotters (by neccessity) but by the same token square jumpers understand lift, drag, flaring, and stalls more completely (again, by neccessity.)

[billvon 2,427]

>Imagine a submarine in the ocean, submerged. It is turning it's
> screw (propeller) enough to produce a forward speed of about 5
> knots. It is slightly negatively buoyant. There is a 5-knot current. It
> is facing perpendicular to the current. What direction will it be facing
> when it hits bottom?

Same direction.

Now allow me to throw a monkey wrench in things. The surface current is 10 knots. The current at the bottom is 0 knots. The submarine is moving forward at 5 knots, the current is initally coming from the right side of the submarine. This is more closely related to a parachute's descent, since uppers are usually stronger than ground winds.

Now the submarine descends. As it descends, the current from the right gradually decreases. The submarine, since it cannot change speed instantaneously, sees a very gradual increase in current from the _left._ In reality, there is no current from the left, of course - but since it is drifting 10 knots to the left due to the surface current, a change in the current to 9 knots will be seen as a 1 knot current from the left of the submarine. It it's like most submarines, that will tend to weathervane it to the left. The end result? It turns "downwind."

This is a very slight effect, since wind changes slowly and wind is usually considerably less than the speed of the canopy. It may have an effect, though, if winds are strong, the canopy is free of other influences (a built in turn, turbulence) and there is no steering input.

[riddler 0]

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However, the boat that is adrift will have no tendancy to turn upstream or downstream unless acted upon by some uneven forces.

This was originally explained to me as why a canopy would naturally turn downwind - that person said an unpowered boat would naturally turn downstream. It made sense at the time, but I now see why this is wrong.

I believe that comparing a ram-air canopy to any kind of unpowered vehicle is not a valid analogy. A ram-air canopy, if inflated, always has forward movement, powered by gravity. An open canopy is much more like Hook's example of a submarine that has the propeller turning, even if you can't steer it. Even if you are unconscious, your canopy still has forward power, as long as it is inflated.

[Hooknswoop 19]

I see your point, but would the submarine turn as it is subjected to the decreassing current?

An under-water "gust" wouldn't push the tail of a sub around, it would tilt the sub over a bit, then it would right it's self, but still be pointed in the same direction.

If it does turn, the that would mean a lull after a gust (for a canopy) from the side would turn the canopy back to it's original heading.

With tandems, where I spend some time not controlling the canopy, making the stuednt comfortable, etc, I haven't seen any tendancies to turn any particular direction. This is usually through the altitudes of the greatest change in wind speed, 3,000-5,000 ft-ish. If it is flying straight, regardlesss of direction, it wil continue to fly straight, if we are turning, me or the student un-even in the harness, line trim, etc, it continues to turn.

Hook

[billvon 2,427]

>I see your point, but would the submarine turn as it is subjected to
> the decreassing current?

Yes. Submarines have both pitch and heading stability; they weathervane into the current. Same is true of aircraft. A glider will not, if left alone, gradually turn its tail into the wind.

>If it does turn, the that would mean a lull after a gust (for a canopy)
> from the side would turn the canopy back to it's original heading.

That's true, if the direction of the gusts always averages out. If you are flying perpindicular to the wind, and descending, _and_ the wind decreases with altitude, they will not cancel out. In engineering terms there will be a DC component to the off-axis relative wind.

[riddler 0]

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DC component

Wow - I never thought of applying waveform analysis to the picture. I wonder if a Fourier Analysis could help us detect the anomalies caused by random gusts? j/k Bill - a good explanation for enginerds.

But with my lack of pilot training once again rearing it's ugly head, I have to ask if this is a typical scenario? Right now, I'm visualizing large cubes of air moving over the ground with me in it. As I descend, it's probable that I will hit a few different cubes moving in different directions, due to thermoclines, etc. But how likely is a gradient of force in one cube? It seems more likely that the entire mass will move at the same speed and direction, unless a completely new mass is entered.

[JYorkster 0]

Agreed, but we have already discussed that there are 'gusts' and wind changes. Let's assume, as some have, that the gusts cancel each other out...

It is very unlikely that you are flowing exactly with the airstream as you canopy opens. Wind direction and speed are not constant from 13,500 ft to the ground. So, it will take some time to equalize with the flow of the airstream. During that equalization, the flow may have more of a push on the nose of your canopy. Maybe you tend to turn more downwind, but as soon as you are flowing with the airstream, this turning stops, and you continue on whatever heading you are on. If so, the determining factor about the turn is really how long it takes you to flow with the air in which you inflate your canopy.

Rock

[kallend 1,647]

Quote

>Imagine a submarine in the ocean, submerged. It is turning it's
> screw (propeller) enough to produce a forward speed of about 5
> knots. It is slightly negatively buoyant. There is a 5-knot current. It
> is facing perpendicular to the current. What direction will it be facing
> when it hits bottom?

Same direction.

Now allow me to throw a monkey wrench in things. The surface current is 10 knots. The current at the bottom is 0 knots. The submarine is moving forward at 5 knots, the current is initally coming from the right side of the submarine. This is more closely related to a parachute's descent, since uppers are usually stronger than ground winds.

Now the submarine descends. As it descends, the current from the right gradually decreases. The submarine, since it cannot change speed instantaneously, sees a very gradual increase in current from the _left._ In reality, there is no current from the left, of course - but since it is drifting 10 knots to the left due to the surface current, a change in the current to 9 knots will be seen as a 1 knot current from the left of the submarine. It it's like most submarines, that will tend to weathervane it to the left. The end result? It turns "downwind."

This is a very slight effect, since wind changes slowly and wind is usually considerably less than the speed of the canopy. It may have an effect, though, if winds are strong, the canopy is free of other influences (a built in turn, turbulence) and there is no steering input.

See this thread on rec dot for the same argument:
Clicky

...

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

[markbaur 0]

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Exaggerate the side view of a canopy in your mind to be a huge triangle, with the pointy end being the tail and having a very wide nose. Even with some forward movement, if place in a fluid perpendicular to the flow of the fluid, there will be more push on the nose than the tail, therefore pushing the nose to the 'downwind' side. On a much smaller scale, the effect is slight, but still present. It would only not affect the heading if the 'triangle' were moving perfectly with or against the flow of the fluid

Quite the contrary.
What matters is where the center of pressure is with respect to the center of gravity. Arrows, airplanes, and parachutes have a larger side-surface area aft of the CG, so tend to keep pointed into the relative wind.
You can design a parachute with a more aft CG (and some folks have tried, as a way to reduce the force needed for front riser maneuvers), but it turns out to be more unstable -- the nose tends to collapse more easily.
Mark

[pilotdave 0]

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What matters is where the center of pressure is with respect to the center of gravity. Arrows, airplanes, and parachutes have a larger side-surface area aft of the CG, so tend to keep pointed into the relative wind.

True, but a canopy is shaped very differently from a plane. A fuselage typically has a destabilizing effect on a plane, because usually it narrows toward the tail so in a sideslip or angle of attack, the foreward part creates more drag than the aft part. The total center of pressure is moved back with the vertical tail... something a canopy is missing.

I recently had a homework in my aircraft design class where we had to solve for the rudder deflection needed to keep an aircraft flying straight after an engine failure. The effects of the fuselage and tail and everything else are very important for determining lateral stability of an aircraft. A plane WILL turn into the wind when hit by a crosswind gust. It's not unreasonable to believe a crosswind gust will have SOME effect on a canopy.

I have no problem with the idea that a canopy MIGHT turn in an unsteady crosswind. Anyone ever watched their canopy before it collapses after a crosswind landing? Does it rotate? Which way?

Dave