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shadeland

Do Canopies Naturally Turn Upwind?

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SethInMI

***
This was all discussed at great length in the thread to which I provided a link in post #9 of this thread.



You got a laugh out of me, John. In that link you provided, you say (in bold) I wouldn't expect it to be much of an effect Now I could say that physics is unaffected by expectations, but how would that serve the discussion? The correct takeaway for me is to know that you agreed with me, (or that I agreed with you, since you created that post 10+ years ago).

A small effect continuing for a long time can have a large outcome. Just because the rate of turn is small doesn't mean that a large turn won't be achieved after several minutes.
...

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

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billvon

>P.S, I've flown in 150 knot tailwinds up at altitude and the aircraft shows no desire to turn around.

If you rapidly descended from that 150 knot tailwind to a layer of no tailwind, you'd definitely see your aircraft try to turn into the apparent wind.



With all due respect, no.

We practice microburst encountering procedures almost everyone we are in the simulator (every 6 months). In the aircraft I'm currently typed on, the Boeing 737NG, it calls out "WINDSHEAR!" repeatedly when encountering either a rapidly increasing headwind or a rapidly increasing tailwind.

In the situation you describe, a rapidly increasing tailwind, the immediate reaction of the aircraft is a loss of indicated airspeed as noted in the airspeed indicator, a concomitant loss of true airspeed, and the resulting loss of lift , and therefore a loss of aircraft performance (speed altitude) up to and including an aerodynamic stall. We don't ever train to expect a turn "downwind" because it doesn't happen.

Most aircraft, and to a lesser degree, most ram air parachutes, are designed to be inherently stable. For aircraft its pitch stability means it will generally try to return to original speed. Roll stability generally means the aircraft will initially maintain the roll attitude it was left in and gradually end up into an increasing spiral dive (descending turn of increasing speed and bank angle ) until it eventually exceeds design speed and loading leading to structural damage. In a decreasing performance wind shear event, after the nose drops in response to its loss of airspeed and pitch stability, left alone, it would return to its trimmed pitch attitude and airspeed.

I used to fly sailplanes. In heavy convective activity, one wing would occasionally encounter a strong thermal (column of rising air) and thus would result in an imbalance of lift between the left and right wing. The pilot would then have to return the wings to level flight, but there was no tendency for a sailplane to turn "downwind".

I'm just on my way to fly to Honolulu. If we encounter any jet streams in descent I'll try to get some photos.

Cheers

John

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Okanagan_Jumper

... Most aircraft, and to a lesser degree, most ram air parachutes, are designed to be inherently stable. ...



Oh, my, I am interpreting the stability of ram-air parachutes compared to most airplanes completely different than that.

If a (mostly rectangular) ram-air parachute is flying properly, is symmetrical, and is not receiving any control inputs, it is completely stable in "roll" isn't it? (This may not be true of elliptical canopies.)

And isn't it nearly impossible to design enough dihedral angle to an airplane wing to make it stable in roll?

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I recall the conversation with an instructor who just happened to have done a doctorate on Ram Air aerodynamics about a similar issue to do with into wind or downwind jump runs for CRW.

Doing downwind run jumpruns towards the then ground to air camera resulted in the easier ability to keep the formation on heading then when doing into wind jump runs.

In simplistic terms, he stated that the canopy would have a tendency to follow the line of least resistance. In the case of downwind if you tend to turn off the following wind would result in a force which would tend to push back toward that line of least resistance which is downwind. Think to keep a canoe facing straight in a river going with or against the flow.

In the case of doing an into wind jump run the slight turn off heading would result in the winds continuing to push towards that line of least resistance - ie. downwind at which point the momentum of a turn may carry it past but would then tend to then result in it wanting to feather into the back towards downwind.

I know that sounds simplistic and this was assuming no constant user inputs, ie. if you fire a brake it will continue turning in that direction. But it did explain why we found it much easier to keep formations on heading going downwind than into wind. When judging switched to air to air it became a moot point.

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skytribe

I recall the conversation with an instructor who just happened to have done a doctorate on Ram Air aerodynamics about a similar issue to do with into wind or downwind jump runs for CRW.



Hmm, I'm not quite getting his ideas. Within a fairly homogeneous air mass such as up high doing CRW, the canopy shouldn't know whether it is going up or downwind.

A lot of CRW is done with someone trying to keep a formation on heading.

Then the biggest factor is that going downwind, you can see the DZ you are heading towards.
When facing upwind you are typically upwind of the DZ and staring off at the horizon with no easy reference.....

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skytribe

But it did explain why we found it much easier to keep formations on heading going downwind than into wind [when judged from the ground]. When judging switched to air to air it became a moot point.


These two sentences do not go together.

-Mark

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>We don't ever train to expect a turn "downwind" because it doesn't happen.

OK.

Let's say you are flying in your simulator at 130kts and the instructor introduces a wind shear to the side - over the course of 5 seconds the aircraft sees a new 30kt wind coming from the left. (In other words, the aircraft goes from seeing a 130kt airspeed right over the nose to a 133 knot wind hitting the aircraft at a 13 degree angle off the left of the nose.) What will the aircraft do?

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daffes

Not saying that it won't turn, but what's the source of the asymmetrical force being applied?

Plane could just side slide keeping the same heading. If the 13 degree angle is evenly applied across the body of the plane, nothing would change it.



But planes aren't designed that way. They have large vertical tails.
...

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

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pchapman

***I recall the conversation with an instructor who just happened to have done a doctorate on Ram Air aerodynamics about a similar issue to do with into wind or downwind jump runs for CRW.



Hmm, I'm not quite getting his ideas. Within a fairly homogeneous air mass such as up high doing CRW, the canopy shouldn't know whether it is going up or downwind.

A lot of CRW is done with someone trying to keep a formation on heading.

Then the biggest factor is that going downwind, you can see the DZ you are heading towards.
When facing upwind you are typically upwind of the DZ and staring off at the horizon with no easy reference.....


The fact that someone is actively trying to keep something on a heading is true - but if when the formation turns off heading it and control input is taken out it naturally has a tendancy to want to go downwind. Then if your already going downwind then that means the no input state then ends up with a tendancy to want to go downwind.

If your going into wind and the formation is turning off heading and wants to continue turning to line of least resistance then it has a natural tendancy to want to turn 100 degrees.

Try thinking of a canoe facing upstream and if you turn slightly off heading then the canoe will want to turn a go downstream. If youre already going downstream and turn off slightly it will have a tendency to continue to go downstream and will not turn and face upstream stream.

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mark

***But it did explain why we found it much easier to keep formations on heading going downwind than into wind [when judged from the ground]. When judging switched to air to air it became a moot point.


These two sentences do not go together.

-Mark

When you have to keep the formation pointing towards a fixed point on the ground then keeping on heading was important. Turning 90 degrees made judging impossible and resulted in busts.

When using air to air judging the camera person is following the formation when the formation turns they stay behind the formation - so the into wind / down wind run in became less important. The camera was always the same relative to the formation (ie. behind).

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daffes

I see, I guess the same thinki applies to any pitch that would result on the nose being first affected causing torque

.

Nothing to do with that.

The vertical tail causes the nose of a plane to turn into the relative wind. So if the relative wind is coming from the side, the plane will turn towards that side.
...

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

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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.

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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.



I'm with Kallend here: there can be airflow from the side.
What you're thinking of is more steady state conditions. This stuff will be obvious to you when you think about it.

If there's a crosswind and you are flying in that airmass, of course the plane doesn't see the crosswind... it's just drifting with the airmass. But then you descend through a shear layer, and suddenly you get hit by a crosswind. Combine that crosswind and your forward speed, and the relative wind is coming at some sideslip angle (beta). No different than an upgust changing your angle of attack, alpha.

Then what happens depends on all of the plane's stability derivatives and whatnot. Normally the vertical tail provides positive stability in yaw -- the plane 'weathervanes' in simpler terminology -- so the plane yaws into the relative wind, maybe oscillating a bit until settling down back into equilibrium with zero sideslip angle.

The plane will be flying within that new airmass with a different crosswind -- but maybe the nose got kicked off its original heading from hitting that change in crosswind. Instead of flying heading 360 it is now flying 355 or whatever.

A parachute's yaw stability (directional stability) is not that high from the little I've read about it, but in a similar way it might end up pointing a little into the direction of the where the gust came from. Which, if going from higher crosswind up high, to lower crosswind down low, is towards the downwind direction...

========

We all can go in circles here, because as most of us have admitted, we don't know exactly what the size of the effect is. "Only physics knows."

A few people who may be able to estimate the size of the effect those guys who do full aerodynamic simulations of ram air parachutes, for the military or similar, who may want to understand the parachutes used for GPS guided payload drops and stuff like that.

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skytribe

***This might be true for boats but not for planes or parachutes I'm afraid.

Maybe use a submerged submarine as your sample and see if your theory still holds.



And that is based upon what....

Well, for one thing a boat is floating on water. It's, not moving through the air.

When I did my BASE course in May with SRBA, I realized immediately that Tom Aiello understood airflow and canopies as good as a pilot. Perhaps he could explain this stuff better than I could. I'm in the simulator in March or April. If we have time, I'll get the instructor to program in a very rapid shearing wind from the side and fly hands off with no autopilot and film the results.

Cheers

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pchapman



I'm with Kallend here: there can be airflow from the side.
What you're thinking of is more steady state conditions. This stuff will be obvious to you when you think about it.

If there's a crosswind and you are flying in that airmass, of course the plane doesn't see the crosswind... it's just drifting with the airmass. But then you descend through a shear layer, and suddenly you get hit by a crosswind. Combine that crosswind and your forward speed, and the relative wind is coming at some sideslip angle (beta). No different than an upgust changing your angle of attack, alpha.

Then what happens depends on all of the plane's stability derivatives and whatnot. Normally the vertical tail provides positive stability in yaw -- the plane 'weathervanes' in simpler terminology -- so the plane yaws into the relative wind, maybe oscillating a bit until settling down back into equilibrium with zero sideslip angle.

The plane will be flying within that new airmass with a different crosswind -- but maybe the nose got kicked off its original heading from hitting that change in crosswind. Instead of flying heading 360 it is now flying 355 or whatever.

A parachute's yaw stability (directional stability) is not that high from the little I've read about it, but in a similar way it might end up pointing a little into the direction of the where the gust came from. Which, if going from higher crosswind up high, to lower crosswind down low, is towards the downwind direction...
.




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?)

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Quote

But planes aren't designed that way. They have large vertical tails.



But parachutes don't.

I think the only thing that could cause a downwind turn on a parachute is the different masses and drag forces on the canopy vs the jumper's body.

- Dan G

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I think the effect is due to how the two bodies (skydiver vs canopy) react to a sudden wind change. The canopy has low mass and thus low inertia, but high drag. The skydiver is the opposite (high mass and inertia, low drag). The skydiver is going to keep going in a straight line and only be affected lightly by the wing change. The canopy is going to be affected right away by the wind change. The result is the canopy is now slightly downwind of the jumper, and the pendulum effect as the system tries to return to equilibrium causes the turn.

- Dan G

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>Not saying that it won't turn, but what's the source of the asymmetrical force
>being applied?

The tail. Aircraft weathervane into the wind.

>Plane could just side slide keeping the same heading.

So a plane might turn 90 degrees to the relative wind and just keep going in that orientation?

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