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shadeland

Do Canopies Naturally Turn Upwind?

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Very nice explanation.
I think you can see this weathervaning effect happening when planes take off in a crosswind; in that case they also experience a little relative wind from the side.
They move a little bit downwind, but at the same time turn their nose into the wind.
Like in this video: https://www.youtube.com/watch?v=6CBHHBi1aTw


(Or is this effect that you can see purely due to pilot input?)



On the takeoff role, the goal is to keep the aircraft tracking straight down the runway and the wings level, which meams using aileron into the wind and rudder to keep the nose straight. Depending on the strength of the wind, it can feel quite awkward as the wheels skip and hop a bit.

The controls in this case are basically crossed, with the ailerons turning left and the right rudder pedal being used to oopose the aircraft's tendency to turn to the left as a result of weathercocking.

As the aircraft lifts off, the pilot quickly and smoothly returns the aircraft to coordinated flight. This may mean the pilot will turn the heading of the aircraft towards the wind direction, thereby trying to track the aircraft straight over the ground in line with the direction of the runway, but many departure procedures specifiy maintaining runway "heading", so the aircraft longitudinal axis will remain lined up with the runway, and the aircraft will simply drift with the wind.

It should be noted that the weathercocking only happens as a result of an unbalanced force that occurs while the aircraft is attached to the ground. Similar to the effect of a boat on a water. Once the aircraft lifts into the air, there is no longer an unbalanced force, and no longer any weathercocking. (And no tendency to turn into wind or out of wind.)

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>It should be noted that the weathercocking only happens as a result of an
>unbalanced force that occurs while the aircraft is attached to the ground.

Exactly. The aircraft starts out effectively "pointing the wrong direction" in yaw, and the aircraft will weathercock on its own once it is free to do so.

>Similar to the effect of a boat on a water. Once the aircraft lifts into the air, there
>is no longer an unbalanced force, and no longer any weathercocking. (And no
>tendency to turn into wind or out of wind.)

Well, there is a very strong tendency to remain pointed into the wind, due to the vertical stabilizer. If you disturb that (i.e. use the rudder to yaw the plane, then release the rudder) the aircraft will very quickly return to a zero degree yaw difference to the relative wind without any additional control input.

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I'm not sure weathervane is a good way of putting it. An airplane is like a weathervane with a large vertical tail surface. Parachutes don't have that. The only way they change heading is by changing the relative proportion of lift to drag on the right vs the left sides. I have trouble seeing how a cross-wind affects that change other than through the pendulus effect.

In other words, if you didn't have a large mass suspended below the wing, and no vertical stabilizer, what force imbalance would cause a turn?

- Dan G

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>An airplane is like a weathervane with a large vertical tail surface. Parachutes
>don't have that.

Right - but they work exactly the same way. Parachutes have stabilizers that provide yaw stability, but more importantly they are designed so that most of their surface is behind the center of gravity. This imparts yaw stability, and causes the parachute to weathervane into the relative wind. That's why parachutes open and then start flying forward rather than sideways.

>In other words, if you didn't have a large mass suspended below the wing, and
>no vertical stabilizer, what force imbalance would cause a turn?

A B2 bomber has no vertical stabilizer, nor a suspended weight beneath it, and still weathervanes into the wind. So do many other aircraft. (See example below.) They all basically work the same way - the center of pressure is behind the center or gravity, so it tends to turn into the wind.

http://www.simprojects.nl/images/RC_glider6.JPG

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>There is no weathercocking in the air.

Try this the next time you fly -

Kick the rudder pedals in one direction or the other. This will yaw the plane. Then release the pedals, and see if the aircraft weathervanes back into the relative wind.

This is called static stability - an aircraft will tend to return to a zero yaw angle when the controls are released and allowed to go neutral. And that is the effect that causes the downwind tendency of parachutes when descending through a gradual wind shear.

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And that is the effect that causes the downwind tendency of parachutes when descending through a gradual wind shear.



And once it is going downwind it will have a tendency to continue downwind unless there is some imbalance in force - ie brake input or weight shift causing it to continue to turn or wind shear relative to the movement of the canopy through the air mass.

When it is turning to go downwind the force of the wind acting upon the canopy may coming from the front left and as the canopy comes past 90 degrees then comes from the rear left side of the canopy continuing the turn until it is coming from directly behind. There may be some inertia which causes the canopy to travel past this point. The force will be then coming from the rear right causing that inertia to slow to a stop and change the direction to weathercock back towards the downwind.

Eventually these to and fro movements will become less and less and ultimately results in a steady downwind.

In all likelihood there is some imbalance - whether suspended weight or difference in the brake lines and I would say that on a strong wind day the effect is much more noticeable than on a low wind day - as the force of the wind vs the relative movement of the canopy is larger. Vector arithmetic of the wind vs canopy.

Experiment:

Go up on a really windy day, hop and pop at the higher altitude with the stronger winds. Point it into the wind, don't touch the controls and try to avoid putting harness inputs in and over the time see if you're still flying into the wind. I would put money on it that your not.

On a no/low wind day you may find that it would take a lot longer to happen or wouldn't happen at all due to lack of turning force of the wind. The canopy vector + zero vector (or small vector due to little/no wind) = canopy vector or requires a longer time to notice the effect.

The winds at different levels would be different so the downwind direction would change as your descending. In the canoe example, it will be much more noticeable in a strong current than a slow stream but it will eventually happen.

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>We'll I ain't ever seen an airplane turn into an increasing wind from the side.

Sure you have. You have flown from areas with strong winds (not on the nose or tail) into areas with weaker winds (not on the nose or tail.) I only have 600 hours of flight time and it's happened to me, so I'd be amazed if it hasn't happened to you. We adjust to the minor changes of heading that causes all the time - and that's true whether or not we are flying parachutes or airplanes.

However, when you are unconscious, there's no one to correct that effect. In a parachute, the net result is a tendency to turn downwind.

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That's fair enough Okanagan Jumper.

But it might be hard to separate what is a crosswind shear effect vs. more random turbulence vs. own actions from any of the above.

Say I'm flying a light plane and descending into a field surrounded by tall trees, and there's a crosswind from the left, and I'm crabbed left to track along the extended centerline. Down near tree level the airplane bounces. The nose kicks right 5 degrees.

What do I do?

Do I freeze on the controls and wait to see if the nose stays facing that way, and say, "Cool! I've probably hit shear from the trees and in their wind shadow, so the rapid loss of crosswind from the left was like adding relative wind from the right, causing weathervaning to the right. I can't wait to get on dropzone and write about this!"

No... Instead I'm on the controls, quickly using stick and rudder to change the heading to stay or get back lined up with the runway, mutter something about "Damn its bouncy here behind the trees - Knew this landing would be a bitch!" and concentrate on getting right over the runway ready to flare.

So I haven't given the shear a chance to do its thing, but am actively fighting any turbulence / shear / whatever to keep the plane on the desired course.

(Remember I'm also skeptical about how much effect in real life the downwind turning tendency is... but I see no reason to say the effect doesn't exist.)

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I'm on board with Bill's yaw stability explanation regarding the center of gravity vs the center of pressure.

That being said, this tendency to turn downwind is only valid for changes in the wind. If the wind is constant there will be no tendency to turn.

- Dan G

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Okanagan_Jumper

kallend, the relative airflow (relative wind is the less preferred term) is defined as being equal and opposite to the flight path of the aircraft (parachute), so unless the aircraft is moving sideways through the air (not over the ground), there can't be a relative airflow from the side. That would imply that an aircraft could fly with a wing pointed forward and the nose to the side. That would more than likely mean the aircraft was in one heck of a stalled condition.



NEWTON'S FIRST LAW.

Think about it.

You may have thousands of flight hours, but Newton has yet to be proved wrong in non-relativistic or non-quantum situations.
...

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

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DanG

I'm on board with Bill's yaw stability explanation regarding the center of gravity vs the center of pressure.

That being said, this tendency to turn downwind is only valid for changes in the wind. If the wind is constant there will be no tendency to turn.



When was the last time the wind was constant all the way from opening altitude to the ground?
...

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

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I believe we that we all agree that a steady wind won't result in a heading change of a descending parachute.

The theory is now that a wind from let's say the front right side that increases in strength will result in a turn of the canopy to the right. May ask what would happen with a wind from the front right that is decreasing in intensity?

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Okanagan_Jumper

I believe we that we all agree that a steady wind won't result in a heading change of a descending parachute.

The theory is now that a wind from let's say the front right side that increases in strength will result in a turn of the canopy to the right. May ask what would happen with a wind from the front right that is decreasing in intensity?



Explained in great detail in the thread previously linked.

And you never answered this question:

"What happens if you are flying on a heading of 360 in a 150kt wind from 090 and rapidly descend into a layer of 20kt wind from 090? No autopilot, hands off the controls."

Break out your E6B which you probably last used 18,500 flight hours ago and do some vector algebra. And don't forget Newton's 1st Law.
...

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

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Don't need to break out the flight computer because it's going to tell us about path over ground, which I think we aren't talking about. We are interested in the aircraft's response in the air.

Just landed here in Edmonton. On approach we had a moderate headwind from about 45 degrees from the side. If that headwind had suddenly increased and was still from the same direction, what wouldn't have happened is that the aircraft would turn more into that headwind. We would need to manually adjust the heading to compensate for the increased drift in order to stay on the final approach course. Not sure if that helps.

John

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>The theory is now that a wind from let's say the front right side that increases in
>strength will result in a turn of the canopy to the right. May ask what would happen with
>a wind from the front right that is decreasing in intensity?

I assume you are talking about what would happen if a wind from the right side hit the canopy, and then changed again so fast that the canopy did not have time to weathervane into the new wind.

If the change resulted in relative wind changing from 5 degrees to zero degrees directly on the nose - nothing; it would fly straight ahead. The wind change would not be fast enough to affect the canopy.

If the change resulted in relative wind changing from 5 degrees to 355 degrees, then the canopy would weathervane left. ("Decreasing wind from the right" is the same as "increasing wind from the left" from the canopy's point of view.)

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Okanagan_Jumper

Don't need to break out the flight computer because it's going to tell us about path over ground, which I think we aren't talking about. We are interested in the aircraft's response in the air.



Sorry, that is just evasion.

An object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. The path over the ground establishes the motion and speed, and the wind shear sets up the unbalanced force.

Do the math.
...

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

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Agree 100% but just as the unbalanced forces don’t know what the ground is doing or where it’s at, neither does the parachute. There is NO forcing function whatsoever to determine the relative velocity to the ground and to make a parachute turn that way. My original post, which was rebuked by someone that said that it wasn’t relevant. Where I was going with my thought is, if we know that we are heading down wind without input, can we design a system to counteract it. An into wind landing with an unconscious jumper is far more survivable than a down wind landing with no control. It might not be something available now in terms of technology, the accident rate may not warrant the investment. My point was. If it deems necessary, let’s explore the options. GPS wcan determine a ground track and velocity. A 360 degree turn will allow you to determine the winspeed. A controller could do the job of the jumper and land into wind.in an emergency situation. A wind shear can turn a canopy left or right, upwind or downwind.

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You’d be surprised at the mass and inertia of the canopy. It has the mass of the textiles but it also has the included mass of air within the inflated canopy and the apparent mass of the air it’s dragging along with it. These are very real artifacts that change with altitude. The lower you get, the more mass/inertia you gain. Estimate a volume for a canopy and add 0.0023 slugs per foot cubed (sea level air density) to it. The canopy gets pretty massive in a hurry.

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Sincy78

You’d be surprised at the mass and inertia of the canopy. It has the mass of the textiles but it also has the included mass of air within the inflated canopy and the apparent mass of the air it’s dragging along with it. These are very real artifacts that change with altitude. The lower you get, the more mass/inertia you gain. Estimate a volume for a canopy and add 0.0023 slugs per foot cubed (sea level air density) to it. The canopy gets pretty massive in a hurry.




If I did the math right, there is just 1,5 kilogram of air trapped in my canopy at sea level.
I don't see why this is relevant?

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And you do realize the wind at different altitudes can be different? So a 360 turn to determine the wind direction can be irrelevant and completely incorrect as you descend further towards the ground.

The likelihood of this uncontrolled unconscious descent very low. The Idea of a GPS guided reserve also means activation is very low and the idea of a 360 on a small reserve to determine wind direction makes the idea even less palatable.

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>There is NO forcing function whatsoever to determine the relative velocity to the
>ground and to make a parachute turn that way.

That is correct - if the parachute is flying straight and level, or if it is descending in still air.

However, if it is descending in a gradual wind shear (which is usually the case) and is seeing a changing wind direction, it will gradually turn towards where the new headwind is coming from - which, from the parachute's perspective, is downwind.

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