Downwind Turning Tendancies

[Hooknswoop 19]

The whole canopys naturally want to turn downwind topic came up again this weekend and got me thinking as to why people think this is true.
Let’s agree on a few things first.
1. Windspeed is the measure of the rate of movement of air across the ground
2. Airspeed is the measure of an object moving through the air.
3. A canopy with a normal forward airspeed of 30 mph will have a 30 mph airspeed regardless of the windspeed.
4. A canopy’s groundspeed will be affected by the windspeed, but it’s airspeed will not.
5. Turbulence can cause a canopy to turn.
6. Jumpers tend to only concentrate on keeping the canopy pointed into the wind on short final (except for short spots).
7. The most turbulence can be found near the ground from interaction between the wind and objects on the ground and thermals are more prevalent near the ground.
Because the only time a jumper is paying very close attention to keeping their canopy pointed into the wind is on short final, at low altitude in the most turbulence, any turn resulting from turbulence will turn the canopy downwind, because any turn when you are into the wind is downwind. On the same principle, if everyone flew short final downwind, any turbulence induced turn would cause the canopy to turn into the wind leading people to believe that canopys naturally weather-vaned into the wind.
A canopy has no idea which way into the wind and downwind are and has no tendencies to settle out on an into the wind heading or a downwind heading. The entire “downwind turning tendencies” is a myth.
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[freefalle 0]

heck if I know but it sounds reasonable to me.

[jceman 1]

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The entire “downwind turning tendencies” is a myth.

OK, then 'splain somethin' to me:
When I was doing groundcrew for Racer's tandem canopy overspeed/weight tests, if this is a myth why did the canopy turn downwind immediatley after opening on every run? Did the lead dummies somehow pull the toggles then reset them in the keepers?

[Hooknswoop 19]

A canopy has no way of knowing which way the wind is blowing. I suppose that if the canopys were opening 180 degrees off heading, you could have dropped them going the other direction and they would have turned into the wind.
If canopys had a tendancy to turn downind, then aircraft would also. They don't. An aicraft flying North in an airmass moving West does not have a tendancy to turn left.
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[Spy38W 0]

Hook,
I've got a question for you, I was an airplane driver before I was a skydiver, so the law of primacy has taught me that an airplane tries to turn into the wind and has been proven through experience, so I'm just wondering how this differentiates from a canopy, is it due to lack of vertical stabilizer (rudder)?
Again, I'm not trying to dispute you, because I'm still a newbie to the "Real Flying"
--
~Captain Cutaway
I can disassemble a rig in less than 5 seconds...

[markbaur 0]

The last time this thread went around, I asked some questions I didn't get answers to. I already know what the pilot-flight instructor crowd says, so my questions are directed to the "parachutes turn downwind" folks, and I'd like to hear what they say.
1. Does the strength of the winds aloft make a difference to the turn rate? That is, will the parachute turn faster in higher winds?
2. Does the turn stop on the downwind heading, or is there some over-rotation, followed by a turn the other direction, with the heading changes getting smaller and smaller until the heading settles down?
3. Do airplanes turn downwind, too? Do gliders also turn downwind? How about helicopters?
Hoping to hear from jceman and allies,
Mark

[jceman 1]

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An aicraft flying North in an airmass moving West does not have a tendancy to turn left.

Horse Hockey!
It's called "weathervaning". The force on the aircraft's vertical stab will tend to turn it into the wind. The thrust of the engine propels the plane North, but put the stab in neutral trim and there will be a tendency to turn left.
Drop a canopy with no pilot, just weight, at any airspeed/altitude and watch which way it goes after deployment. -- the tests I spoke of were with 500# of weight, from 250 feet, static lined at 125 knots, ground winds 15 knots. Canopy opened on line flight and immediately turned downwind every time.

[Hooknswoop 19]

So you are saying that if a a captain of an airliner turns off the heading control part of an automatic pilot, but leaves on the altiude control part and the aircraft is flying into the wind, the aircraft will do a 180, and stabilze on the new heading? And an airliner heading downwind would not need input from an auto pilot or pilot to keep the aicraft pinted downwind? As a pilot, you know from experience this isn't true.
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[Zennie 0]

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Canopy opened on line flight and immediately turned downwind every time.

I guess one possibility (I'm no fluid dynamics expert by any stretch) is that an object generally tends to take a path of least resistance (or greatest stability). If a canopy is flying into the wind, it will encounter more resistance than if it flies downwind, since the nose has a larger profile than the tail. My guess is that if you put it in a wind tunnel, the downwind configuration would create less turbulence over the airfoil than heading upwind. The downwind flight would be a more stable configuration so the canopy, with no other forces acting on it,
would tend to configure itself that way.
This is just my little pea-brained-pseudo-science mind working. Engineers please correct me if I'm wrong.
"Zero Tolerance: the politically correct term for zero thought, zero common sense."

[Hooknswoop 19]

"An aicraft flying North in an airmass moving West does not have a tendancy to turn left.
--------------------------------------------------------------------------------
Horse Hockey!
It's called "weathervaning". The force on the aircraft's vertical stab will tend to turn it into the wind. The thrust of the engine propels the plane North, but put the stab in neutral trim and there will be a tendency to turn left."
The aicraft's ground track would be NE w/ a North heading. The stab controls the pitch axis and has nothing to do w/ left and right, only up and down.
Think of it this way: Put a goldfish in a fish bowl next you in your car. Drive down the freeway, would the fish know that it was travelling 60 mph over the ground? No, it would swim around normally, unaware that it was cruising down the freeway. Wind is an airmass moving across the ground, same as a fish bowl (well not the same, but same concept). A canopy flying through this airmass that is moving across the ground has no idea which way across the ground the airmass is moving and therefore is not influenced by the direction the airmass is moving over the ground.
Drop a canopy with no pilot, just weight, at any airspeed/altitude and watch which way it goes after deployment. -- the tests I spoke of were with 500# of weight, from 250 feet, static lined at 125 knots, ground winds 15 knots. Canopy opened on line flight and immediately turned downwind every time.

[jceman 1]

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the aircraft is flying into the wind, the aircraft will do a 180

No, he said it wants to turn into the wind.
As for a canopy, with no other input, it will be blown with the wind (downwind), the design of the wing with its anhedral, will tend to turn along the direction of movement, hence with the wind (downwind). The higher the wind, the more like it will turn rather than simply drift. This can be proven by simple experimentation, no intelligence is required on the part of the canopy, that is, it doesn't have to "know" which way the wind blows.

[Hooknswoop 19]

That doesn't work, a canopy won't fly backwards in a wind tunnel. Try this: close your eyes under canopy (do a hop and pop w/ no one else in the air) put your hand out and feel the wind in steady-state flight. Turn 180 degrees and let the canopy return to steady state flight. Feel the wind again. It will feel exactly the same. Without looking down, you will have no idea if you are going into with or cross wind.
If you let the canopy fly down wind, any turbulence that turned the canopy would turn in the direction of into the wind.
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[jceman 1]

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The stab controls the pitch axis and has nothing to do w/ left and right, only up and down.

You might try reading my post again, I referred specifically to the VERTICAL stab (or tail), that has absolutely nothing to do with pitch, but controls yaw (left or right). Force coming from the left, working on the stab, will yaw the plane to the left; just as it does a weathervane or a tetrahedron.

[Hooknswoop 19]

Sorry, missed the veeertical part:-). a weather vane and a tetrahedron point into the wind because they are mounted on the ground on bearings.
The vertical stab of an aicraft only feels the relative wind and relative wind has nothing do do with the wind over the ground.
If you took a small remote control boat, and got it going at a constant speed across a smooth river and then locked the rudder, it would hit the other side perpindicular to the river, downstream of where it started. It would not turn up or downstream.
if you were above a solid smooth layer of clouds under canopy, there would be no way for you to deterrmine which way the wind was blowing or how strong. The canopy would not automatically head in any direction.
Same as the goldfish example, the fish has no idea that it is doing 60 mph, it swims happily around it's bowl, it doesn't swim in any particular direction just beacuse the water mass it is swiming in is moving in any particular direction.
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[jceman 1]

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The canopy would not automatically head in any direction.

Cutaway a good canopy. Which way does it goes? Which way does the freebag go? Why do we bother to spot?

[Hooknswoop 19]

Take a look at airliners as they cruise overhead leaving contrails. They fly in a straight line pointed in exactly the opposite direction as their contrail. Now the jet and the contrail may be drifting, but there isn't a crosswind hitting the tail, trying to turn the jet into the wind. The airmass the jet is flying in is moving, same as a fish bowl in a car moving down the freeway.
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[quade 4]

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I've got a question for you, I was an airplane driver before I was a skydiver, so the law of primacy has taught me that an airplane tries to turn into the wind and has been proven through experience . . .

This is true as long as the aircraft in in contact with the surface of the Earth, however, if you believe this is true while the aircraft is flying, then please go back to your CFI and get a full refund. He obviously didn't teach you much.
quade
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[quade 4]

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When I was doing groundcrew for Racer's tandem canopy overspeed/weight tests, if this is a myth why did the canopy turn downwind immediatley after opening on every run?

Were these static line tests and were the jumpruns being made upwind? If so, I think I might have an answer.
quade
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[quade 4]

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Drop a canopy with no pilot, just weight, at any airspeed/altitude and watch which way it goes after deployment. -- the tests I spoke of were with 500# of weight, from 250 feet, static lined at 125 knots, ground winds 15 knots. Canopy opened on line flight and immediately turned downwind every time.

Bingo! EXACTLY as predicted.
How's about this for an explaination?
The static line deploys the parachute but the parachute is asymetrically loaded (just a little bit would be all that it would take). As the canopy slows down from its deployment the suspended weight swings forward along the line of flight and puts slightly more tension on one side of the canopy than the other. The canopy turns and because it was only deployed at 250 ft, you don't see it continue to turn very much -- it simply lands before it can complete a circle.
quade
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[quade 4]

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It's called "weathervaning".

And happens ONLY when in contact with the surface.
quade
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[Hooknswoop 19]

They drift w/ the wind, same as a hot-air balloon, but I don't see where you are going w/ this one. Do you understand the fish bowl concept?
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