"just cut 50cms out from the lines, its flying better"

[phoenixlpr 0]

I've heard from some CRW dogs. They have got 50cms cut out from the line-sets of their (CRW)Triatlons. I was told that it was flying and landing better after this fine tuning.

[CornishChris 5]

Let us know how that goes...

CJP

Gods don't kill people. People with Gods kill people

[phoenixlpr 0]

As I told you before they claim the Triatlon flies better after the cut.

[JohnRich 4]

Quote

I've heard from some CRW dogs. They have got 50cms cut out from the line-sets of their (CRW)Triatlons.

And what was the reason for, or advantage of, removing 20" of line length?

[PhreeZone 15]

Short lining a canopy makes it more responsive and twitchier. For rotations and stuff I can see where this is an improvement.

Yesterday is history
And tomorrow is a mystery

Parachutemanuals.com

[bch7773 0]

it would also restrict the canopy once its open, causing a steeper glide path correct?

MB 3528, RB 1182

[FrogNog 1]

Quote

Short lining a canopy makes it more responsive and twitchier. For rotations and stuff I can see where this is an improvement.

I guess it could also be taken as a weird testament to the stability and gentle handling of the factory Triathlon design: "so gentle, you can whack 50 cm off the lines and it still works!".

-=-=-=-=-
Pull.

[darkwing 4]

Quote

I guess it could also be taken as a weird testament to the stability and gentle handling of the factory Triathlon design: "so gentle, you can whack 50 cm off the lines and it still works!".

I suspect that MOST jumpers would hardly notice the difference. I'm not saying there wouldn't be a difference, just that most wouldn't notice.

-- Jeff
My Skydiving History

[faulknerwn 36]

I don't know about that - I've had Lightning 113's with 8 foot lines and 9 foot lines - and it was a HUGE difference.. The 9 foot lines were tremendously floaty compared to the 8. Landed a lot better too...

W

[dorbie 0]

Quote

Quote

I've heard from some CRW dogs. They have got 50cms cut out from the line-sets of their (CRW)Triatlons.

And what was the reason for, or advantage of, removing 20" of line length?

You act like a pendulum under your canopy. The period of that pendulum is shortened if you shorten the lines.

Another way of looking at it is if you move forward a foot relative to your canopy (or your canopy moves back relative to you), the change in angle of attack of the canopy will depend on the line length, the shorter the lines the greater the change, but the initial input to affect that change would be almost identical. Same input very different response, shorter lines causing the greater response.

[pchapman 261]

Simply shortening the lines equal amounts will also change the geometric position of the jumper under the canopy in the fore and aft direction (relative to the canopy), given that the jumper isn't directly under the middle of a canopy with equal length A/B and C/D lines.

This change will happen whether the lines are shortened by equal lengths, or even by equal proportions. (Since the canopy size isn't being scaled at the same time.)

And either way, any cascaded parts of the lines will load up differently as the cascaded section lengths aren't adjusting for other changes in the geometry.

I can't tell anyone exactly what effects line shortening will have in practice, but I wanted to note that there are plenty of subtle geometric changes that will occur, beyond just putting the jumper closer to the canopy.

[riggermick 6]

Quote

Simply shortening the lines equal amounts will also change the geometric position of the jumper under the canopy in the fore and aft direction (relative to the canopy), given that the jumper isn't directly under the middle of a canopy with equal length A/B and C/D lines.



What the hell are you talking about? Due to the physics of gravity the mass will always center itself under the canopy regardless of the line lengths. The trim angle may change if the cascades are altered seperatly but cutting the line groups at the same point at their terminal end won't change the mass's position. What will change is the radial angle of the wing (the curve as viewed from the front/ back) relative to the mass below.

This change will happen whether the lines are shortened by equal lengths, or even by equal proportions. (Since the canopy size isn't being scaled at the same time.)


?????????????? How so?

And either way, any cascaded parts of the lines will load up differently as the cascaded section lengths aren't adjusting for other changes in the geometry.


Not so much, this statement is based on the faulty assertion as stated in the first part of your post.




I can't tell anyone exactly what effects line shortening will have in practice, but I wanted to note that there are plenty of subtle geometric changes that will occur, beyond just putting the jumper closer to the canopy.

Now that I will have to agree with!!



Look I'm not trying to bust your balls here but I think you need to do a little more research in to ram air canopy design before posting your personal view points as fact, there are impressional newer jumpers that read this stuff and hold it as gospel. Sorry for sounding so harsh, I'm not out to pick a fight just trying to set the facts straight.


Mick.

[pchapman 261]

I wrote:
Simply shortening the lines equal amounts will also change the geometric position of the jumper under the canopy in the fore and aft direction (relative to the canopy), given that the jumper isn't

Mick replied:
What the hell are you talking about? Due to the physics of gravity the mass will always center itself under the canopy regardless of the line lengths.

My reply:
Sorry, I wasn't clear on one point, making things very confusing! I hope I can clear up what I mean.

I meant that the simple geometry of the parachute system changed, as would be seen if the canopy were laid on its side on the ground and nailed to the floor. Shortening the lines would move the position of the links fore or aft, and not only closer to the canopy. I wasn't talking about the actual orientation in flight -- which Mick was talking about. But still there can be changes in flight behaviour as I will show.

We are dealing with three effects in a row:
- How much does the shape of the aircraft change?
- How does that affect its orientation when actually in flight?
- How do the orientation and shape changes actually affect flight characteristics?

In flight the mass of the jumper is indeed going to hang pretty much "directly under" the canopy, give or take a little because of the drag of the lines & jumper, and the aerodynamic forces on the parachute canopy (lift, drag, and pitching moment).

(For others to visualize, an extreme example where the pilot is no longer nearly "directly underneath" the canopy can be seen in videos of things like 21 square foot canopies that are not landed. Videos show the jumper pushed well back relative to the canopy, because of the very high speed and thus drag on the jumper.)

The purely geometric issues can be seen in the attached diagram, drawn to scale but with arbitrary lengths and angles.
The canopy starts out with 10 foot front lines, and 13 foot back lines. (For simplicity, the "lines" include the risers.)

[Edit: In the diagram and this post I kept writing "links" but it's clearly the 3-ring suspension point that I mean, where all the lines & risers come together at one point.]

Both are shortened by 3 feet, giving 7 and 10 foot lines. This is a ridiculous amount but makes the principles involved easier to see.

Arcs have been drawn to show where the new lines will intersect, at the "new link location".
The red arrow labelled "1" shows how the lines intersect significantly further back, relative to direction of the lower surface chord of the canopy. The jumper has been moved further back compared to the canopy.

Yes, in flight the jumper's weight will dominate and swing him back "underneath" the canopy again, as Mick said. But that means something else will change: the canopy will be forced to tilt nose up!

If the pitch orientation of the canopy in a normal glide is changed, also affected will be the angle of attack, glide angle, normal (trimmed) airspeed, toggle pressures, stall point, flare ability, and so on.

Shifting the jumpers weight fore or aft is a bit like a hang glider pilot moving himself fore or aft by pulling or pushing on the control bar. Pushing out starts to move the pilot aft, but gravity gets him back "underneath", so the wing pitches up and slows down.

If doing this all relative to the bottom of the canopy seems artificial, then one can just as well look at it in a different way:

Pretend that the diagram shows the canopy as it oriented in normal flight, with gravity straight down. The angles look generally realistic: The jumper is "under" the canopy, and the canopy is angled a little bit nose down from the horizon. Even relative to this in-flight position, one can again see the jumper's position move aft when the lines are shortened. That's shown by red arrow "2". The jumper's weight will tend to swing forward back under the canopy and pitch the canopy nose up.

If the lines are shortened by a given proportion and not a given length, it doesn't fix the problem -- the links again end up at quite a different spot.
For example, the dark green dot on the diagram shows the link location if the lines are shortened by 30%. The front lines become 7 ft long, just like before, and the back lines end up 9.1 ft long.

(If all this is understood, then it is easy to see how the triangles defined by cascaded sections of lines will have geometries that are out of whack, which either mean that some lines would go slack or the canopy will distort in some way to load the lines again.)

I hope all this is adequate to address Mick's concerns.

Overall, I'm saying that there are geometric changes that aren't always obvious, beyond just "putting the jumper closer to the canopy".

Whether small geometric differences will make the canopy fly significantly differently is another matter, which Mick probably can far better address.

This discussion doesn't just apply to line and riser length changes for finely tuned modern CRW or swoop canopies. In the old days one could notice plenty of effect from just adding a single set of extra links to the front or rear risers of a big F-111 canopy.

[riggermick 6]

Quote

I wrote:
Simply shortening the lines equal amounts will also change the geometric position of the jumper under the canopy in the fore and aft direction (relative to the canopy), given that the jumper isn't

Mick replied:
What the hell are you talking about? Due to the physics of gravity the mass will always center itself under the canopy regardless of the line lengths.

My reply:
Sorry, I wasn't clear on one point, making things very confusing! I hope I can clear up what I mean.

I meant that the simple geometry of the parachute system changed, as would be seen if the canopy were laid on its side on the ground and nailed to the floor. Shortening the lines would move the position of the links fore or aft, and not only closer to the canopy. I wasn't talking about the actual orientation in flight -- which Mick was talking about. But still there can be changes in flight behaviour as I will show.

We are dealing with three effects in a row:
- How much does the shape of the aircraft change?
- How does that affect its orientation when actually in flight?
- How do the orientation and shape changes actually affect flight characteristics?

In flight the mass of the jumper is indeed going to hang pretty much "directly under" the canopy, give or take a little because of the drag of the lines & jumper, and the aerodynamic forces on the parachute canopy (lift, drag, and pitching moment).

(For others to visualize, an extreme example where the pilot is no longer nearly "directly underneath" the canopy can be seen in videos of things like 21 square foot canopies that are not landed. Videos show the jumper pushed well back relative to the canopy, because of the very high speed and thus drag on the jumper.)

The purely geometric issues can be seen in the attached diagram, drawn to scale but with arbitrary lengths and angles.
The canopy starts out with 10 foot front lines, and 13 foot back lines. (For simplicity, the "lines" include the risers.)

[Edit: In the diagram and this post I kept writing "links" but it's clearly the 3-ring suspension point that I mean, where all the lines & risers come together at one point.]

Both are shortened by 3 feet, giving 7 and 10 foot lines. This is a ridiculous amount but makes the principles involved easier to see.

Arcs have been drawn to show where the new lines will intersect, at the "new link location".
The red arrow labelled "1" shows how the lines intersect significantly further back, relative to direction of the lower surface chord of the canopy. The jumper has been moved further back compared to the canopy.

Yes, in flight the jumper's weight will dominate and swing him back "underneath" the canopy again, as Mick said. But that means something else will change: the canopy will be forced to tilt nose up!

If the pitch orientation of the canopy in a normal glide is changed, also affected will be the angle of attack, glide angle, normal (trimmed) airspeed, toggle pressures, stall point, flare ability, and so on.

Shifting the jumpers weight fore or aft is a bit like a hang glider pilot moving himself fore or aft by pulling or pushing on the control bar. Pushing out starts to move the pilot aft, but gravity gets him back "underneath", so the wing pitches up and slows down.

If doing this all relative to the bottom of the canopy seems artificial, then one can just as well look at it in a different way:

Pretend that the diagram shows the canopy as it oriented in normal flight, with gravity straight down. The angles look generally realistic: The jumper is "under" the canopy, and the canopy is angled a little bit nose down from the horizon. Even relative to this in-flight position, one can again see the jumper's position move aft when the lines are shortened. That's shown by red arrow "2". The jumper's weight will tend to swing forward back under the canopy and pitch the canopy nose up.

If the lines are shortened by a given proportion and not a given length, it doesn't fix the problem -- the links again end up at quite a different spot.
For example, the dark green dot on the diagram shows the link location if the lines are shortened by 30%. The front lines become 7 ft long, just like before, and the back lines end up 9.1 ft long.

(If all this is understood, then it is easy to see how the triangles defined by cascaded sections of lines will have geometries that are out of whack, which either mean that some lines would go slack or the canopy will distort in some way to load the lines again.)

I hope all this is adequate to address Mick's concerns.

Overall, I'm saying that there are geometric changes that aren't always obvious, beyond just "putting the jumper closer to the canopy".

Whether small geometric differences will make the canopy fly significantly differently is another matter, which Mick probably can far better address.

This discussion doesn't just apply to line and riser length changes for finely tuned modern CRW or swoop canopies. In the old days one could notice plenty of effect from just adding a single set of extra links to the front or rear risers of a big F-111 canopy.

Hi PC,

I see what you are trying to say (and it makes sense on paper), but your arguement presupposes that these devices (canopies) are rigid in nature. Canopies by their very nature are flexable and they will always accomodate the forces of gravity/ mass distribution on any given load path. So while the arguement makes sense in the mathmatical world it doesn't hold true for the real world, but I like the fact that you are THINKING!! About such things. Well done for even theorising about it. It gives me hope for equipment in the future!!

I'm glad you didn't take this as a personal attack but rather as a well reasoned discussion, well done!!



Mick.

[DJL 232]

Quote

I see what you are trying to say (and it makes sense on paper), but your arguement presupposes that these devices (canopies) are rigid in nature.

I understand what he's trying to say. The idea of the wing being a rigid model is valid since it is intended to fly according to its aerodynamic design. Just lopping off 50cm instead of changing line lengths that would maintain that intended airfoil will result in the airfoil being distorted. Let's say you had a canopy with only a front set and back set, both cascaded. The line length is designed according to the proper airfoil. If you just start lopping off a foot at a time the inner cascade becomes more slack and the outer gains tension and your canopy bends into an arc.

As for application to the real world? Sounds like these guys with the tri's aren't having any problems, but I doubt Aerodyne just chops lines on their canopies without accounting for canopy aerodynamics.

"I encourage all awesome dangerous behavior." - Jeffro Fincher

[riggermick 6]

Quote

Quote

I see what you are trying to say (and it makes sense on paper), but your arguement presupposes that these devices (canopies) are rigid in nature.

I understand what he's trying to say. The idea of the wing being a rigid model is valid since it is intended to fly according to its aerodynamic design. Just lopping off 50cm instead of changing line lengths that would maintain that intended airfoil will result in the airfoil being distorted. Let's say you had a canopy with only a front set and back set, both cascaded. The line length is designed according to the proper airfoil. If you just start lopping off a foot at a time the inner cascade becomes more slack and the outer gains tension and your canopy bends into an arc.

As for application to the real world? Sounds like these guys with the tri's aren't having any problems, but I doubt Aerodyne just chops lines on their canopies without accounting for canopy aerodynamics.

OK, I think you are missing the central theme here, it's not about messing with the cascade points and angles of attack it's about shortening the overall line lengths. The foremost change will be side to side!!!!!!!! That is the point where the wing will stop/ start providing lift and become less efficiant. Do the math with a piece of string and ruler, it's pretty basic!

Mick.

[DJL 232]

Yup, it is basic and I was only talking about a slice along one axis, just like PC was in his original post. So you should supplement his statement by adding, "it's not only about messing with the cascade points etc...The foremost change will be side to side." But instead it seemed like you're addressing what he said about one axis by describing it from the other axis. I understand that what the diagram should really show is the canopy curving and the resulting jumper position but like you said, that's difficult to do on paper.

"I encourage all awesome dangerous behavior." - Jeffro Fincher